IN ALKALINE WATER ELECTROLYSIS · ABSTRACT HYDROGEN BUBBLE CHARACTERIZATION IN ALKALINE WA'I['ER ELECTROLYSIS (Master of Applied Science, 2000) Daniel Lumanauw Department oi Metailurgy

HYDROGEN BUBBLE CHARACTERIZATION

IN ALKALINE WATER ELECTROLYSIS

Daniel Lumanauw

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Gnduate Department of Metailurgy and Materiais Science University of Toronto

Copyright by Daniel Lumanauw 2000

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ABSTRACT

HYDROGEN BUBBLE CHARACTERIZATION IN ALKALINE WA'I['ER ELECTROLYSIS

(Master of Applied Science, 2000)

Daniel Lumanauw

Department oi Metailurgy and Materiais Science

University of Toronto

This thesis investigates the process of bubble evolution in water elec~olysis.

Little experimentd data exists for conditions found in commercial electrolyzers so thar

the pnmary focus was to develop expenmental technologies. which would allow

rneasurements under these conditions.

The project was successful in demonstrating that Image Analysis and Laser

Scattering Particle Size Analysis were useful in analyzing the bubble evolution

phenornena. Hydrogen bubbles were produced from water electrolysis in 1 M &CO3 at

room tempenture and pressure using smooth screen, rough screen, crystalline plate, and

morphous-alloy plate electrodes.

Plate electrodes produced more uniform bubble size distribution than screen type

cathodes. The mean bubble size, for applied current density of 100 to 250 mA/cm2,

decreased in the order of plate electrodes> rough screen > smooth screen electrodes.

Bubble size increases with decrease in electrolyte flownte and increase in current density

(except for the rough screen electrode).

Firstly, 1 would like to thank God for ail opportunities and blessing He has given

to me.

1 would like to say rhank you to my supervison Dr. D. W. Kirk and Dr. S. J.

Thorpe for d l of their guidance, and Stuart Encrgy System Corp., for initiating and

supporting this resexch project.

I would like to thank Dr. D. Rubisov for his technical assistance in the operation

of the particle size analyzer.

I would like to acknowledge the mernben of Electrochemisuy and Surface

Engineering Group: Anson Sinanan. Paulo Borges. and Arthur Pismenny for their

stimulating discussions of research matters relating to my thesis topic.

I give my regards to good friends of mine: Marc Dupere. Johan Susanto, and Rudi

Budiarto.

Finally, would like to thank my family for their many years of supports.

TABLE OF CONTENT

........................................................................................................................ AB STRACT i

.......................................................................... ............................... LIST OF TABLES .... vi

. . .......................................................................................................... LIST OF FIGURES v11

[ . INTRODUCTION

1.1 Rationde for Hydrogen production ........................................................................... 1 3 ............................................................... L . 2 Hydrogen production by water electrolysis -

. .................................................. L 3 Energy efficiency of alkaline water electrolysis 3

................................................................................................................. 1 . 4 Objectives 8

II . BACKGROUND AND LITERATURE SURVEY ........................................................ 9

.......................................................................... 2.1 Gas evolution effects in electrolysis 9

2.1.1 Conductivity of bubble-filled electrolyte ........................................................... 10

2.1.2 Bubble effects on the electrochemical behavior at electrodes ........................... 14

2.1.2.1 Conductivity of the surface bubble layer .................................................. 15

............................................................................ 2.1.2.2 Activation overpotential 15

2.2 Bubble nucleation .................................................................................................... 18

.................................................................................... 7.21. Homogenous nucleation 18

2.2.1.1. Single component system ......................................................................... 18 77 2.2.1.2 Twocomponent system ..............................................................................

2.2.2 Heterogeneous nucleation ................................................................. .. .......... 23

2.2.2.1 The contact angle 0 ............................................................................... 24

2.2.2.2 The geometry of the nucleation site ................... .. ................................ 26

iii

2.2.2.2.1 Gas trapping in cavity .................................... .... ........................ 2 6

2.2.2.2.2 Nucleation from a preexisting gas phase .............................................. 28

3.3 Bubble growth on electrodes ................................................................................... 30

2.3.1 Individuai bubble growth ................................................................................... 30

2.3.2 Growth by coalescence .................................................................................. 32

2.4 Bubble departure ................................................................................................... 33 . . 2.5 Velocity of osing bubblrs ........................................................................................ 3?

2.6 Bubble-bubble interaction in bulk electrolyte ........................................................ 39

..................................................................................................... 3.1 Visualization ce11 44

.................................................................................................. 3.2 Electrode materials 44

3.3 Electrolyte ............................................................................................................... 52

.......................................................................................................... 3.4 Image analysis 52

3 .4.1 Optical rnicroscopy observation ...................................................................... 52

3.4.3 Lnstrumentai appantus .................................................................................. 5 3

3.4.3 Procedure for the image recording .................................................................. 56

3.4.4 Procedure for determination of bubble size distribution ........................... ..,, ....* 56

3.5 Low angle light scattering particle size malysis .................................................... 58

3 . 5. 1 Pnnciples ............................................................................................................ 58

3 S.2 Instrumentai apparatus and procedure for the determination of bubble size

distribution ......................................................................................................... 60

3.5.3.1 IA-PSA cornparison .................................................................................. 61

3.5.1.2 PSA without extemal electrolyte flow ................................................... 61

3.5.2.3 PSA with electrolyte fiow ................... ... ......................................... 69

3.6 Summary .................................................................................................................. 69

AND DISCUSSIONS ............................................................................... 71

4.1 Hydrogen bubble nucleation in 1 M K2C03 ........................................................... 71

4.2 Image andysis .......................................................................................................... 73

................ .......................... 42.1 Optical visualization of hydrogen evolution .... 73

4.2.1.1 Smooth vs . rough screen electrode .......................................................... 73

4.2.1.2 Crystdline vs . arnorphous plate electrode ................................................. 78

4.2.2 Bubble size distribution and mean diameter for different electrode geometries

and current density ............................................................................................ 81

4.3 Low Angle Light Scattering Particle Size Analysis ............................................... 84

4.3.1 Veri fication of the experimental technique ....................................................... 84

4.3.1.1 Cornparison with image andysis ........................ .. ............................ 84

4.3. ! . 3 B-bble-bilbble intenction during tmnrp~gation in the equipmefit .......... 86

4.3.2 Electrolysis with no extemd electrolyte flow ............................................... 89

4.3.2.1 Bubble size distribution for different collector positions .......................... 89

4.3.2.2 Bubble mean diameter for different collector positions and current

..................................................................................................... densities 95

4.3.3 Electrolysis with electrolyte flow ....................................................................... 98

4.3.3 Flow analysis in the electrolysis ce11 .............................................................. 98

4.3.3.2 Bubble size distribution for different electrode geometries .................... 101

4.3.3.2.1 Effect of electrolyte flowrate ........................................................... 107

4.3.3.1.7 Effect of current density ..................................................................... I l l

V . CONCLUSIONS .................................................................................................. 1 17

REFERENCES ......................................................................................................... 119

APPENDK A: BUBBLE SIZE DISTlirSUTION FROM IMAGE ANALYSIS

................................................................................. CALCULATIONS 123

APPENDIX C: iNTERNAL PRESSURE OF A HYDROGEN BUBBLE IN I M

K<03 ................................................. ,. ............................................. 128

APPENDIX D: PROPERTIES OF POPULATION A FOR SCREEN ELECTRODES 129

Table 2.1 Theories of Heterogeneous Effective Conductivity .................................... 20

Table 2.2 Limiting Cavity Conditions ..................... .. ................................................ 38

Table 3.1 Experimental Apparatus and Procedure Summary .................................. 80

Table 4.1 Statisticd Properties of Figure 4.12 ..................................................... 102

Table 4.2 Reynolds Number of the Regions in the Channel without Bubbles ....... 110

Table 4.3 Statistical Values of Population A from Figure 4.18 ................... ............ I 1 1

Table 4.4 Statistical Values of Population B from Figure 4.18 ................................ 114

Table A.1 Recorded Bubble Diameters (in micrometer) €rom a Single Frame ...... 124

Table A.2. Bubble Size Distribution €rom a Single Fmme .................................... 125

Table A.3 Bubble Size Distribution €rom Five Images .............................. ... . . . 136

LIST OF FIGURES

................................................... Figure 1.1 Schematic of alkaiine water electrolysis ce11 4

Figure 1.2 Idealized operating conditions for water electrolysis"" ................................... 5

...... Figure 1.3 Schematic presentation of ce11 voltage as a function of current d e n ~ i t ~ ' ~ ) 7

Figure 2.1 Cornparison of theories predicting the reduced coriductivity of nndom .......................................... dispersions of monosized spheres with experimental data? 12

Figure 2.2 Cornpaison of theories predicting the reduced conducrivity of rmdom dispersions of multisized spheres with experimental data (also included are Maxwell

............................................................................ and P n g r theories for cornparison)'? 13

Figure 2.3 Free energy of a droplet of liquid as a function of its radius ......................... 20

Figure 2.1 A hypothetical spherical cap embryo ............................................................. 25

............................................................... Figure 2.5 The value of contact angle function 25

................................................................. Figure 2.6 Bubble embryo in a conicd cavity 27

Figure 2.7 (a) Advance of liquid sheet over a gas-filled groove. (b) advance of gas- liquid interface over a Iiquid-tilled gro~ve"~'. ................................................................. 27

................................. Figure 2.8 Forces acting on a bubble growing on an e~ectrode'~'. 34

Figure2.9Schematic precoalescence drainage of two approaching bubbfes (1 to DI) ............. ,, ................................................................................................ 40

Figure 2.10 The effect of charge and concentration of ions on interfacial ared5? ........ 41

. l- .......................................................... Figure 3.1 (a) Visualization ce11 configuranon i 45

Figure 3.1 (b) Schematic top projection and cross section of visualization ce11 ............. 46

Figure 3.2 (a) Visuaiization ce11 configuration II ........................................................... 47

Figure 3.2 (b) Schematic top projection and cross section of visuaiization ce11 configuration II ................................................................................................................. 48

Fipre 3 3 Four cathodes gometry (From the left. cathode no . 1: Ni screen. cathode no . 2: Ni plate. cathode no . 3: commercial screen. cathode no . 4: amorphous nickel alloy tape) .............................................................................................................. 50

vii

Figure 3.4 Cathode no. 5: Ni wire geornetry. .............................................................. 5 I

Figure 3.5 (a) A Schematic dnwing of optical microscopy instrumental appmtus. ..... 54

Figure 3.5 (b) Photogaph of optical microscopy instrumental apparatus ...................... 55

Figure 3.6 Image anaiyzer for bubble size distribution measurement. .......................... 57

Figure 3.7 The pnnciple of low angle laser light scattering instnirnent~'~~) ................... 59

Figure 3.8 A schematic drawing of image analysis - particle size analysis cornparison instrumentai appmtus. .................................... ............................................. 62

Figure 3.9 (a) A schematic drawing of PSA without extemal electrolyte fiow instrumental apparatus ..................................................... ................................................. 63

Figure 3.9 (b) Photopph of PSA without extemal electrolyte flow instrumental apparatus. .......................................................................................................................... @

Figure 3.10 Bubble collectors were designed so that after they were clamped to the cathodes, the inlets had a distance of 2.5.5.0, ruid 7.5 cm from the hottom of the cathodes. ...... .................. ............................................................................................. 65

Figure 3.11 (a) A schernatic drawing of PSA with electrolyte flow instrumental apparatus. .......................................................................................................................... 66

Figure 3.11 (b) Photogaph of PSA with electrolyte flow instrumental appantus ........ 67

Figure 3.11 (c) Visudization ceIl and bubble separator in PSA with electrolyte flow instrumental apparatus. .........................................+..................................-............... 68

Figure 4.1 Bubble critical radius as a function of dissolved hydrogen concentration .... 72

Figure 4.2 Hydrogen bubble evolution on a smooth screen electrode at current density of: (a) 1 O mAkm2: (b) 20 rn~lcm'; (c) 30 rnA/cm2; (d) 50 mA/cm2;

2) (e) LOO mAIcm ; and (f) 250 mNcm'. ....... . . ........... .. . .... .. .... .... . . ...*. . .... .* .. . . . . . . .... . ... . ... .. . . -74

Figure 4.3 Schematic of bubble path on a smooth screen electrode: (a) paralle1 to screen electrode and (b) side view projection. ................................................................. 75

Fipre 4.4 Hydrogen bubble evolution on a roua screen electrode at current density of: (a) 10 mA.lcm2; @) 20 m.A/crn2; (c) 30 rn~/crn~; (d) 50 m~/cm';

2) (e) 100 &cm ; and (f) 250 rn~lcrn'. ......................... ............... .................................... 76

Figure 4.5 SEM picture of surface morphology of the rough screen electrode. ............. 77

Figure 4.6 Bubble evolution on a crystailine plate electrode at current density of: ............................ (a) 10 mA/cm2; (b) 20 rn~icrn'; (c) 30 rnA/cm2; and (d) 50 mA/cm2. 79

Figure 3.7 Bubble evolution on a amorphous allloy plate electrode at current density oE (a) 10 m~lcm'; (b) 20 mA/cm2; ( c ) 30 &cm2; and (d) 50 m.A/cm2 ...................... 80

Figure 4.8 Bubble size distribution for different electrode geometries at a current ....................................................................................................... density of 30 m~lcm ' 82

Figure 3.9 Bubble rnean diameter as a function of current density for different ........................................................................................................ electrode geometries. 83

Figure 4.10 Cornparison of bubble size distribution from particle size analysis with . . . .............................................................................. size dtstnbution from image analysis. 85

Figure 4.11 Bubble mean diarneter as a function of connecting pipe length between electrol ysis cell and detector ceIl in the analyzer. ............................................................ 88

Figure 4.12 Hydrogen bubble size distributions in 1 M KrC03 at 50 rnA/cmL for di fferent bubble collector positions from the bottom of smooth screen electrode. .......... 9 1

Figure 4.13 Visualization of bubble-electrolyte system in no external flow electrol yte electrol ysis for di fferent current densi ties. ..................................................... 93

Figure 4.14 Froth structure mode1 suggested by Yianatos er al.. ~ 9 8 6 ' ~ ' . ..................... 94

Figure 1.15 Bubble mean diameter as a function of collector position at current density of 25 and 50 rnNcm2. .......................................................................................... 96

Figure 4.16 Bubble mean diameter as a function of current density for various . * collecter positions. ........................................................................................................... 96

Figure 4.17 Fiow channel geometry in the cathode compartment of ........................................................................................................... the electrolysis cell 99

Figure 4.18 Hydrogen bubble size distributions in 1 M &CO3 at 250 m ~ l c m ' for different electrode geometry with collector position of 50 mm from the bottom and electrolyte flownte of 60 cm/s .................................................................................. 103

Figure 4.19 Schematic of bubble coalescence and departure on a cross section of: (a) horizontal wire of a screen electrode and (b) a plate electrode in an upward flowing electrolyte .................................................................................................................. 105

Figure 4.21 Force balance for a bubble attached on a vertical electrode in an upward flowing fluid. ........................................................................... ......... . ......................... 1 10

Fipre 4.22 Bubble mean diarneter as a function of current density for various electrode geometries at flowrate of 60 cmfs and collecter position of 50 mm. ............. 112

Figure 4.23 Cornparison of bubble growth behavior according equation 4.1 with experimental data from Westerheide and ~ e s t w a t e r ' ~ ~ ' using 1 M sulfuric acid at 130 mA/cm2. .................................. . ................................ ................................. 1 13

Figure 4.24 Theoretical relationship of time with: (3) radius; (b) growth rate; and (c) growth acceleration at various current densities according equation 4.1. ................ 115

Figure A.1. Circled bubbles on image taken from CCD videocamera (1 M KOH, 10 rnNcrn2, room temperature and pressure) ................................................................ 123

Figure A.2 Bubble size distribution histogarn from five images. ................................ 126

Figure C.1 Intemal pressure of a hydrogen bubble as a function of bubble radius. ..... 128

Fipre D.1 Volume percentage of population A from screen electrodes as a function of: (a) electrolyte flowrate and (b) current density. ....................................................... 129

Figure D.2 Bubble mean diameter for populations from screen electrode as a function of: (a) electrolyte nowrate and (b) current density. .................................. 130

= area (m') = effective area (m') = bubble density on electrode surface = concentration of dissolved gas in liquid (mole fraction) = drag coefficient = saturation concentration of dissolved gas in liquid (mole fraction) = bubble diameter (m) = bubble diameter (m) = diffusion coefficient (m2.s'') = Faraday constant (9.64867 x 10" mol-l) = void fraction = contact ange function = (2 + 3 cos 0 - cos38)14 = p v i t y acceleration (9.808 m.s-') = height of the electrolytic ce11 (m) = current (A) = current density ( ~ . m " ) = exchange current density ( ~ . m " ) = rate of nucleation = Boltzman's constant (1.38062 x 1 0 ' ~ J.K-') = electrolyte conductance ratio with the bubbles present venus bubbles absent = number of equivalent = interna1 pressure of a bubble ( ~ . m " ) = partial pressure of gas in a bubble (~.rn") = liquid pressure (Nrn") = pure solvent saturation pressure ( ~ . r n " ) = pressure of liquid vapor ( ~ . m " ) = ideal gas constant (8.3 14341 mol-') = bubble radius (m) = bubble radius at time t (m) = critical ndius for nucleation (m) = bubble departure ndius (m) = Reynolds number = bubble foot ndius (m) = tempenture (K) = time (s) = ûtomic volume (m3.atom-') = steady state single bubble rising velocity (m.s-') = steady state bubble population rising velocity (md) = gas evolving electrode potential (V) = pre-exponentid frequency factor

a C1

B P P E

@r

Y rl f13 %cc

Tl: i7 conc

rln CL PG PL Pv "P

ui Uvap

8 !se

AgV'

AG^"

= vapor coefficient = transfer coefficient [eq. 2.101 = coefficient of degree of supersaturation [eq. 2.341 = Tafel constant 12. i 1 - 2.13 - 2.141 = wedge angle of a groove (rad) = ratio of density ciifference of liquid and gas to density of liquid = reference electrode potentid (V) = surface tension ( ~ . r n - ' ) = vapor coefficient = anode overpotenud (VI = activation overpotential (V) = cathode overpotential (V) = concentration overpotential (V) = ohmic overpotentid (V) = liquid viscosity coefficient (Pa.s) = gas density (kg.m4) = liquid density (kg.m") = vapor density (kgm") = activity coefficient of giis inside a bubble = specific volume of pure solvent (m3) = activity coefficient of vapor inside a bubble = contact angle (rad) = pressure difference between bubble pressure and liquid pressure ( ~ . m " ) = excess pas pressure ( ~ . r n " ) = free energy change per atom associated with the transfer of atom from a vapor

to a liquid droplet (~.atom") = heterogeneous nucleation free energy (I)

AG"^ = homogenous nucleation free energy (J) AG, = maximum free energy barrier in a homogenous nucleadon (J) AG, = free enegy to form an interface (J) AG, = free energy of fonn a phase change (J) AR = electrolyte resistance ratio with the bubbles present venus bubbles absent

the reference electrode (V) A = measured potential between working electrode and reference electrode (V) &aohm = ohmic potential difference between any point on the electrode and

1. INTRODUCTION

1.1 Rationale for Hvdropen production

The primary use of hydrogen at the present time is as an industrial chemical

comrnodity"'. The chemical and petroleurn industries use the majority of hydrogen tu

produce ammonia, the backbone of the fertilizer industry, and for methanol production.

In the metailurgical industries, for example. hydrogen is used in the reduction stage (such

as in nickel production) and is also used to control the O2 content of the atmosphere in a

heat treatment application.

Hydrogen is seen by many people as having a central mle in the future as an

eneqy carrier? Liquid Hz has a significantly higher arnount of energy per unit weight

than any hydrocarbon fuel and has been used for space applications. There is interest in

using hydrogen in automobile technology as it burns with a higher efficiency than

gasoline. It is the cleanest buming fuel and if it is produced from renewable energy

sources such as solar and wind power, it even has a zero emission when it is used in a

fuel cell. Extensive research has been conducted for this application.

For other applications, hydrogen has been used because of its unique physical

properties"'. Due to its low viscosity, hydrogen is used to reduce friction in rotating

matures in electrical power generation systems. Also because of its low density,

hydrogen is used to fil1 weather balloons.

Hydrogen has broad applications and they are being evaluated continuously by

many industries. Hence its indusaial demand has been steadily increasing(? The future

role of hydrogen is even more pmmising as it has the potential to be used as the dominant

fuel especially in the transportation sector.

1.2 Hvdro~en production bv water electrolvsis

For some applications, water electrolysis is the best technology for producing

hydropn. Other competing techniques include s tem reforming of natural gas or

naphtha, partial oxidation of oil, and gasification of coal. When relatively small

quantities of hydrogen are required, on site electrolysis of water may be more economical

than other methods. Production of hydrogen by this method is a simple process with no

moving parts and can be designed as a ponable unit. This technique is very clean,

reliable. and produces more than 99.98 9% purity of hydrogen gas for most commercial

technology. In addition, electrolysis can be linked to renewüble electricity-producing

technologies and hence could become even more important in the future.

The ciiscovery of electrochemical water splitting was made in the year 1800 by

Nicholson and ~arlisle"'. Although an acidic medium was originally used. the current

technology uses an al kaline medium (usually 25-3096 KOH). With this medium.

corrosion is more easily controlled and cheaper construction materials cm be used than in

acidic media.

There are two types of water electrolyzen: monopolar and bipoiar cells.

Monopolar cells have only one polarity on each electrode, either positive or negative.

Conversely, each face of an electrode in bipolar cells has a positive polarity on one side

and negative polarity on the other side. Monopolar cells are simpler systems than bipolar

cells and are easier to manufacture and mintain.

The principle of alkaline water electrolysis for a monopolar tank electrolyzer is

shown schematicdly in Figure 1.1. The electrolytic reactions that occurs on each

electrode are given by:

cathode: 2 H 2 0 + 2 e - + 2OH-+Hz (1.1)

anode : 2 OH- + '/Z 0 2 + HrO + 2 e' ( 1.2)

overail : HzO + H2 + ' h o 2 (1.3)

From the overall reaction, assuming the current efficiency to be LOO %, 2 faradays of

electricity are required to produce 1 mol of H? gas Le. 0.1268 cm3/~ .s of Hr gas at 2S°C

and ! atm.

1.3 Energv efficiencv of alkaline water eiectrolvsis

The Gibbs free energy and the enthalpy change for the overail water

decomposition reaction (reaction 1.3) represent the revenible voltage and themoneutrd

voltage respectively and the values as a function of temperature as shown in Figure 1 .?"".

From thennochemistry, the difference between these two values arises from the entropy

changes and must be balanced by either supplying or removing heat from the system.

The water decomposition reaction is an endothemiic reaction. If the openting cell

voltage is below the themoneutrd voltage (but above the reversible voltage). then the

electrolysis ceIl will absorb heat from the surroundings. Convenely. if the ce11 voltage is

above the thetmoneutrd voltage, then an excess heat will be genented and this causes

energy ineficiency. Thus, it is desirable to opente the ce11 voltage as close as possible to

the themoneutrai voltage.

hydrogen bubble

alkaiine conductive cathode permeable anode electrolyte membrane

Figure 1.1 Schematic of alkaline water electrol ysis cell.

I i f electricity used to make i / hydrogen, waste heat evolved f

thermoneutral potential

i f j electricity and hear used to 1 i make hydrogen 1

' Hydrogen generation i impossible

1 00 200

temperature (OC)

Figure 1.2 Idedized openting conditions for water e~ectrol~sis'~'.

Ce11 voltage is directly proportional to the power consurnption for the process

because the current eficiency for alkaline mter electrolysis is very close to 100%.

Genenlly, this ce11 voltage is composed of:

E=&-~+%+%+r ln ( 1.4)

where &,, refers to the reversible thermodynamic decomposition voltage, q, to the mode

overpotential, q, to the cathode overpotential, and q~ to the interelectrode ohmic drop.

Present day electrolyzers have a ce11 voltage of 1.8 to 2.2 V and operate between

70" to 90" C which means voltage efficiency from 68 to 80 % only '? In order to

increase this efficiency, the voltage sshould be reduced by lowering the overvoltages

experienced (at the cathode and anode) and the interelectrode resistance from the

electrolyte, the membrane, and the bubbles.

The ce11 voltage increases when the process is operated at higher current densities.

as shown schernatically in Figure 1.3'~'. Above a current density of 150 mA./cm2. this

increment is mainly due to the higher bubble population in the electrolyte as the jas

production rate increases Iinearly with current density. Therefore, in order to lower the

intemal ohmic drop, the bubble residence time in the interelectrode gap has to be

minimized.

This residence time is dependent on the chancteristic bubble size as it influences

the drag and buoyancy force of the bubble movement. The characteristic size itself is

dependent on the ce11 configuration (such as ce11 and electrode geometry) and operational

parameten (such as current density, electrolyte flow conditions, temperature and

pressure).

(V vs. SHE)

cell voltage

500

current density (rn~cm')

Figure 1.3 Schematic presentation of ce11 voltage as a function of current density?

1.4 Obiectives

As a result of the importance of gas bubble characteristics on the operation of an

electrolyzer, the following objectives were defined:

1. to construct an experimental apparatus that enable visudization of the bubble

evolution phenornena,

2. to study the effects of current density, electrode geometry, and materids on the

electrolytic hydrogen gas evolution at the microscopie scale,

3. to consuuct a means to directly rneasure bubble size distributions by using laser

scattering anal ysis.

4. to study the bubble-bubble interactions from electrolysis,

5. to determine the effects of electrode geometry and materials, electrolyte flowrate.

and current density on the bubble size distribution.

II. BACKGROUND AND LITERATURE SURVEY

This chapter consists of six sections that provide theoretical background and

related previous research findings. This information gives foundation for discussion of

experimental results as presented in chapter four. The first section descnbes overail

effects of bubble evolution on the electrolyzers. This section shows the importance of

bubble characterization on the effons to improve the performance of electrolyzen and

hence gives an idea for further developrnent of the study.

Electrol ytic bubble chliractenstics are dependent on the physical processes of

bubble evolution on the electrodes surface and in the bulk electrolyte. The following

sections present the bubble nucleation, growth, and departure theones and findings.

Velocity of bubble nsing and bubble-bubble interaction in the bulk electrolyte are

explained for describing the behavior of bubbles in bulk electrolyte.

2.1 Gas evolution effects in electrolvsis

The effects of bubbles on the ce11 performance cm be descnbed by considering

their position in the interelectrode gap. The dispersed bubbles in the bulk electrolyte

decrease its conductivity and alter the macroscopic current distribution of the vertical

elecuode. The bubbles located on or very near to the elecuode surface contribute

significant electrolyte conductivity loss because their population is very crowded at the

gas evolving surface. Their presence on the electrodes also &ers the rnicroscopic current

distribution since their attachment decreases the electrode's effective ares'?

2.1.1 Conductivitv of bubble-fifled electroh :e

In the interelectrode bulk electrolyte, bubbles decrease the fluid volume of the

electrolyte so that the conductivity decreases. Some theones have been developed to

describe this behavior. The conductivity of bubble-filled electrolyte depends on the

volume fraction occupied by the bubbles and the state of bubble aggregation, which

includes their size distribution and position distribution. The theories themselves are still

lirnited as they only relate the conductance ratio with the bubbles present versus bubbles

absent. (Kd, to the void fraction, (0. Some of these relations are presented in Table 2.1.

Table 2.1 Theories of Heterogeneous Effective Conductivity

Investigators ~ a x w e l l ' ~ '

These relationships were derîved for random arrangement of zero-conductance

dispersed spheres in an electrolyte. The equations 2.1 - 2.4 were derived from

monosized spheres and their cornparison with the experimental data'@ is shown in Figure

Meredith and T'obias""

~ruggeman""

Theory (1-f) K m =- (1 + 4)

Eq. (2.1)

8(1 - f )@ - f) Km = (4 + J ) ( f - f)

Km = (1 - f

(2.5)

(2.6)

2.1. It cm be seen that equations of Maxwell and Jeffery follow the data rather well up to

a void fraction of 0.5.

Meanwhile, equations 2.5 and 7.6 were derived from unequai sized spheres. The

equation of Meredith and Tobias used two size fractions while Brugeman's used several

very different size fractions. Cornparison of these equations with experimental data is

shown in Figure 1.1'". After cornparing many theories and some experimental results,

id es'^' pointed out tliat the relationship between size distribution of the dispersed spheres

w.d the conductivity of heterogeneous medium is as follows:

for O c f c O. 1, the conductivity obeys Maxwell's equation and is independent

of the size distribution and the arrangement of the disperse spheres.

for 0.1 c f < 0.6, the bubble size distribution affects the conductivity,

for f > 0.6, the size distribution must be considered.

From some experimental work"". smaller spheres were found to ;ive lower conductivity

at the sarne void fraction for f > 0.3.

The conductivity of bubble filled electrolyte From a real electrolysis sysiem has

been measured and compared to the models given above. On large-scale vertical

electrolyzen, Hine et ai. (14-18) reported that Bniggeman's equation fit their data best.

Another investigator, Sigrist et al.'19' experimented with electrodes evolving bubbles and

concluded that Maxwell's equation described the results well.

Other investigators studied the effects of ceIl design and openting panmeten on

the conduc tivi ty of the bubble-filled electrol yte. From a series of experimental

measurements, Janssen et QL'"' reported a correlation of the ratio of electrolyte resistance

Figure 2.1 Cornparison of theones predicting the reduced conductivity of nndom dispersions of monosized spheres with experimentai data'?

Figure 2.2 Cornparison of theones predicting the reduced conductivity of random dispersions of multisized spheres with expenmental data (also included are Maxwell and Prager theories for ~orn~arison)'~'.

'with' to 'without' bubbles for the electrolyte between the working electrode and ce11

diaphraP, as follows:

AR= K , R ~ " ( I + K , ~ " + K , ~ P ) (2.7)

where h is the height of the electrolytic cell, d is the electrode-membrane distance, Re is

the Reynold number. and the remaining parameters are empiricai constants. Hine et al.""

found that perforated electrodes minimize the ohrnic losses due to surface bubbles and

that the flow condition affects the ce11 voltage significantly.

2.1.2 Bub ble effects on the eiectrochemical behavior a t electrodes

sides'@ considered the effect of bubbles near the electrode by analyzing the

potential measured between a hypothetical gas evolving electrode (V,) and a reference

electrode placed just outside the bubble layer (a,):

aQT=Vw-ar

or in terms of potential componenü.

'@ T = " oivn + + Vc,mc (2.9)

where A@ohm is the ohmic potential difference between any point on the electrode and the

reference electrode. q, is the activation overpotential, and q, is the concentration

overpotential which is negiigible in the case of bubble evolving electrodes.

2.1.2.1 Conductivitv of the surface bubble laver

Bubble evolution on the electrode forces the current to take a longer path around

the bubble and flow through constricted (bubble-bubble contact) areas. Thus as with

bubbles in bulk electrolyte, the conductivity is decreased. Sides and obia as"" found that

by solving Laplace's equation for a single attached bubble on a vertical electrode with

appropriate boundary conditions, the resistance genented is 10% less than in the case of

single bubble in the bulk electrolyte. From experhental meas~rements"~', this behavior

was found for a bubble layer up to a void fraction of 0.4 but at f > 0.5 greater resistances

were found. Furthemore, the conductivity in the region for f c 0.5 is slightly higher than

the value predicted by Mÿrwell's equation for bulk dispersion of gas bubbles.

At high current density (i > 100 rn~lcm') where the evolution of bubble is

vigorous. an appreciable reduction in conductivity due to the bubble layer cm be

estimated (by the equations developed for bulk electrolyte using the actual void fraction

in the layer). Therefore the role of the surface bubble layer as well as the bulk bubbles

must be considered in cell design.

2.1.2.2 Activation overpotential

The electrode's activation overpotential (qact) is usudly descnbed by the Tafel

equation and is given by:

where: cc refers to the transfer coefficient. i and i, refers to applied current density and

exchange current density. The value of a and i, are a function of the nature of the

electrode material, so that Eq. 2.10 at constant temperature can be rearranged to:

where c anand i j are Taîei constants on a gven smooth eiectroàe wirh m a ol A in the

absence of ;as bubbles.

Meanwhile. as the bubbles are genented and becorne attached to the electrode's

surface, the effective area (Aefr) of the electrodes decreases. These bubbles also cause

non-unifomity of the current density distribution dong the elecuodes. Thus, for a gas-

evolving electrode, the TafeI equation should be written as:

- where rl., is an average over

unifonn current distribution.

(2.12)

the distribution of overpotentials accompanying the non-

For expenmentd work, superficial area of the electrode (A) is still generally used

to define the applied current density as describe in equation

between this equation with

where q* is the portion of

density, which accordingly

Eq. 2.12 then should be described as:

the activation overpotential related to

is given by:

2.1 1. The relationship

(2.13)

the nonuniform current

where Aeff is a hnction of applied curent, following the bubble generation according

Faraday's law, and the bubble departure site as will be descnbed later.

2.2 Bubble nucleation

The product of the water splitting reaction via electrolysis is in the form of

dissolved gas in the electrolyte. When its concentration exceeds the supersaturation

limits, a bubble is nucleated. Then the nucleated bubble grows and coalesces with its

bubble neighbon on the electrode's surface. When they reach a certain size, they depart

from the surface. Some of them may interact with each other in the bulk electrolyte.

3.2.1. Homo~enous nucteation

Nucleation is said to be 'homogenous' when it occurs in the bulk of the solution.

away frorn any interface such as walls and particles. Under appropriate tempenture and

pressure conditions. bubbles start to nucleate when the concenuation of dissolved gas in a

solution exceeds its limit of supersaturation.

2.2.1.1. Single component svstem

There is a critical radius where bubbles with this radius or larger grow while

bubbles having radii less than this dimension tend to decay. To explain this, it is helpful

first to examine the case of a spherical liquid &op nucleating from a supersaninted

vapor. When a droplet is formed frorn the vapor, the net free energy change in the

system consists of two contributors: the 'surface' term and 'volume' tem. The surface

term is due to the eneqy required to form a gas-Iiquid interface of a droplet having a

radius r and specific surface free energy y in the form:

AG, = 4m'y

The volume term is due to the gain in free energy as the droplet formed is

thermodynamically more stable than the supersaturated vapor. This is given by:

free-energy change per atom associnted with the transfer of an

iiquid rlropie~ (Le. the hr-energy diîlerençr oerween an acom

in the vapor and liquid phases. assumed to be negative in value), ui is the volume of an

atom in the liquid phase. and r is the radius. Ovenll, the free-energy change may be

written as:

AG = AG, + AG,

Figure 2.3 shows the free energy of a droplet as a function of its radius. The

overail free energy passes through a maximum with increasing size of the particle. The

rüdius corresponding to this maximum free energy requirement is the critical particle

radius, r,. Since the free energy at r, decreases in both direction. the particle will grow

when r > rc and decay when r < r,. The cntical radius relation may be obtained by setting

the derivative of Eq. 2.17 to zero and solving for rc. which becomes:

surface energy

*

Figure 2.3 Free energy of a droplet of liquid as a function of its radius

In the case of bubble nucleation in a boiling system, the bubble formed cm be

considered to represent the lower limit of applicabilicy of the Laplace-Kelvin equation'='

for a sphericai vapor phase with radius of r in a liquid phase, aven by:

equd to the equilibrium vapor phase pressure (Po,,,) and PL is the liquid phase pressure.

Assuming p~ » pv. Eq. 2.18 then can be written as:

where PL is the totai pressure (i.e. applied hydrostatic head in the liquid) md Pb is the

pressure inside the bubble that is equal to the vapor phase pressure in this mono

component system.

The rate of nucleation, (I), or the number of bubbles formed per second per cubic

centimeter of liquid is s h o ~ n ' ~ ' to be proportional to the probability given by:

where Z is the preexponential frequency factor which varies relatively slowly with the

supersatuntion and temperature.

2.2.1.2 Two-com~onent svstem

In the case of bubble formation from supersaturated dissolved gas in a solution,

~olrner'"' showed that the maximum energy barrier for the nucleation is the same as the

one that could be denved for pure vapor-liquid system. However, another

investi;atiod3' showed that the pressure inside the bubble (Pb) is not equal to the

equilibrium vapor phase pressure (Po,,,) but composed of the sum of the partial vapor

pressure of the liquid (P,) and the partial pressure of the dissolved ;as (P,) which are

significantly dependent on the dissolved gas content (C) in the liquid, given by:

where VI. and vz. are the modified activity coefficients of vapor and giis inside the

nucleate bubble respectively (subscripts 1 and 1 refer to a propeny of the solvent and

solute respectively). Cs is the saturation concentration of gas in the liquid, and

where k is the Boltzmann constant. ul is the specific volume of pure solvent, and T is the

temperature.

The critical bubble radius under these conditions can be obtained by substitution

of Eq. 2.22 into Eq. 2.20 which yields:

while

gi ves:

the rate

J = Z

of nucleation can be obtained

- -

substitution of Eq. 2.22 to Eq. 2.31, which

(2.25)

This nucleation theory predicts the maximum attainable limit of supersaturation of

dissolved gas in the solution before homogenous nucleation occurs. Experimentd

re~ults''~' confirm the applicability of this theory for dissolved nitrogen in ethyl ether.

Using this theory. another in~esti~ator"~' calculated that the supenaturation

concentration is one thousand times that of the saturation concentration before hydrogen

bubbles began to nucleate frorn hydrogen dissolved gas in 1 N sulfuric acid. Thus, in

generd, homogenous bubble nucleation is difficult to initiate.

2.2.2 Heterogeneous nucleation

Nucleation that occurs on surfaces is called 'heterogeneous' nucleation. This type

of nucleation is easier than homogenous nucleation. Bubble nucleation in the vicinity of

gas evolving elecuodes requires much less dissolved gas supersaturation than the

concentration for nucleation in the bulk. Thus, it is reasonable to conclude that the

surface decreases the energy b-er for the nucleation process. This decrease c m be

explained by considering two main panmeters, those are contact ange of solid-gas-liquid

interface (0) and the geometry of the nucleation site.

2.2.2.1 The contact angie 9

Consider a vapor embryo at a solid-liquid interface as shown in Figure 2.4.

Assuming the embryo is a part of a sphere (Le. spherical caps) and there is quasi-

equilibrium between the surface forces at the position where the surface of the caps is in

contact with the solid surface. the free energy of the heterogeneouslv nucleated vapor

B AG^") cm be deri~ed"~' as:

where AG^‘'^ is the free energy of a spherical embryo of radius equai to the radius of the

cap and f(8) is given by:

The variation of the ratio of AG^'' to that for AG^^^ is the same as value as the function of

contact angle 0 as shown in Figure 2.5. For 0 = 0'. the liquid completely wets the surface

and AG^^' is the same as AG^^^ because f(0) = I. Conversely, ai 9 of 170' yields AG^" =

0.00017 AG""'. Thus, a surface that has poor wetting characteristics (i.e. 0 -t 1804

should prornote nucleation much more easily than well-wetted surface.

For the case of bubble nucleation from a dissolved gas in a solution, Eq. (2.25)

cm be modified"' to take account of variations in contact angle as:

r 1

Figure 2.1 A hypotheticd sphencal cap embryo.

Figure 2.5 The value of contact angle function.

Small changes in contact mgle in this case, result in large negative values from the

exponential term. Therefore much less supersaturation of dissolved gas is needed to cause

heterogeneous nucleation than is needed for homogeneous nucleation.

2.2.2.2 The geornetrv of the nucieation site

rp. mis pürmeirr has noi k e n iuliy investigated for many compiex surface

geometry shapes. A model has been developed("' for a conical cavity on a flat surface as

first described for the case of nucleate boiling. In this model. a conical shape cavity

geometry is considered as shown in Figure 2.6. Fonunately. this model cm also be

applied to the case of a bubble nucleation process at a gas evolving electrode as they

show a similar mechanism. In nucleate boiling, a bubble is formed due to vaporization of

the liquid phase (one component system) with the temperature gradient 3s its dnving

force. For güs evolving on an eleetrode. the bubble is formed due to supersaturation of

dissolved gas in the liquid (two component system) with its concentration gradient as the

driving force.

2.2.2.2.1 Gas trapping in cavitv

For simplification, consider a two-dimensional grooved surface as shown in

Figure 2.7. As illustnted in Figure 3.7(a), a semi-infinite sheet of liquid is passing

unidirectionally over the groove. If the contact angle, 0, is equal or smaller than the

wedge angle, p, then the advancing liquid front will touch the bottom of the groove so

that the liquid completely fil1 it as the liquid pass through. Conversely, in the condition

when

SOL IO

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -. liquid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 2.6 Bubble embryo in a conical cavity

Figure 2.7 (a) Advance of liquid sheet over a gas-filled groove (-: liquid with minimum contact ange for gas entmpment in the goove) , (b) advance of gas-liquid interface over a Lquid-filled groove (-: liquid with maximum contact angle for Liquid entrapment in the groo~e) '~~' .

the advancing liquid front will strike the opposite wall of the groove and entrap gas in it.

For the case of liquid displacement, Figure 7.7(b) shows that if

< (15-B) (2.30)

then the liquid will not to be removed completely from the groove after it passes through.

From here. the grooves of cavities cm be divided into four types of classes as shown in

Table 2.2.

This preexisting gas in the cavities can be caused by gas entrapment during the

fint introduction of Iiquid ro the surface or by residual gas that is left over after bubble

departure. After the cavities are filled with gas. the nucleus gowth occun at the mouth of

the cavity.

Table 2.2 Limiting Cavity Conditions

2.2.2.2.2 Nucleation from a preeliistin~ gas phase

Class

1 2 3 4

If the cavities fail into class no. i or 3 as described Table 2.2, then the trapped gas

can act as a bubble initiator when its size exceeds the cntical bubble size for nucleation,

However th is remains m e only for a finite penod of time which depends on the diffusion

rate of the trapped gas in the liquid. Thus the initial concentration of dissolved gas in the

Type of cavity steep steep

shallow shallow

Obey inequality Wetting condition

poor well poor well

I

2.29

Y= no Yes no

Trapping ability 2.30

no

YeS yes no

Qas

gas

liquid Iiquid

liquid and the rate of dissolved gas production may also affect the initiai bubble

(30-31) population. This phenornena have been confïrmed by several investigators .

Other researchers studied the effect of surface roughness on bubble nucleation. It

was f~und '~" that for vapor bubble nucleation. the rougher surfaces required lower

superheat to initiate boiling. Similady, for electrolytic hydrogen bubble n~cleation'~~', the

rougher surfaces have a higher mass transfer coefficient so that lower dissolved gas

concentration is needed for the nucleation. The observed increased in nucleation rate on

rough surfaces was hypothesed to be due to a greater size range of cavities existing on the

corner surface so that more cavities with a proper size and shape were available to enuap

the vapor sufficiently. Conversely. polishing narrows the size range so that i t increases

the superheat gradient necessary for nucleate boiling.

2.3 Bub ble a ~ w t h on elecf rodes

2.3.1 Individual bub ble ~rowth

The initial growth of a bubble nucleus (having radius R 2 r,) is driven by the

internai pressure of the hubhle. In an infinite nonviscoils and incompressible liqtiid, the

gowth can be descnbed by a mechanicd energy equation, well known as the Rayleigh

equation'3J' of motion given by:

where p~ is the liquid density. R is the bubble radius at time t. and @ is the ciifference of

intemal pressure inside the bubble to the surrounding liquid. After inteeniion. the

equation generates a rate of bubble growth equation:

From the equation above, it can be seen that a bubble nucleus with radius of r, is

metastable and in mechanicd equilibrium with the surrounding liquid. To initiate growth,

a positive disturbance force is needed by introducing the intemal pressure from the

curvature of the interface and background pressures. Severai simplifications were made

for equation 2.31. In fact at this stage the growth depends strongly on viscous, inertia,

and interfacial forces. The nucleus growth at this stage is slow in the beginning but

accelentes until the supply of new rnoiecuIes limits the growth with this mechanism.

Mass m s f e r of dissolved gas to the gas-liquid interface drives the second stage

of the bubble growth. The bubble growth can be described by solutions of the convective

diffusion equation assuming an initially unifonn supersaturated solution surroundhg the

growing bubble:

where D refers to the diffusion coefficient. E refers to the ratio of the density difference

between liquid and jas to the liquid density, and R refers to the velocity of the bubble

interface (i.e. the nte of bubble growth). The fint stage of bubble growth is small in

cornparison with ihis stage. The error introduced by assuming mass transfer govemed the

entire bubble growth is small.

~criven"" denved a general equütion for diffusion controlled bubble growth

given by:

where p is a coefficient characteristic of the degree of supersaturation. Using a high

speed camen, Westerheide and ~ e s t w a t e r ' ~ ~ ' photogaphed individual electrolytic

hydropn bubbles and found an agreement between their single bubble growth data with

Eq. 1.34. For multipie bubbles, the experimentai growth rate deviates from the theory

because they interfered and coalesced with each other.

2.3.2 Growth bv coaIescence

The density of bubble nucleation sites increases with the applied current density.

When these sites are very close to each other, the surrounding bubbles affect every

individual bubble's growth and even coalesce with each other. This is particularly mie for

a current density higher thm 100 rnA/crn2 in a typical water electrolysis system where

almost the electrode's entire surface becomes active.

Sides and s obi as'"' documented oxygen bubble coalescence on a flat elecvode in

3 wt % KOH solution at 500 mNcm2. They classified three types of bubble coalescence.

The fint type of coalescence occurred between small neighboring bubbles (diameter less

than 10 pm) in much less than 0.0001 S. The second type of observed coalescence was

between medium size bubbles (diameter about 40 pm) with the sunounding smaller

bubbles that are translated ndially across the elecvode surface toward stationary centrai

bubbles. The movement may have been a result of local flows established by continual

coalescence that entrains other bubbles toward the collecter. The last type of coalescence

involved bubbles with a size of 50-100 Fm, where the larger bubble. sliding on the

surface, scavenged other bubbles. This was more obvious on a vertical bubble evolving

electrodes.

2.4 Bubble departure

Bubble size in the bulk electrolyte (before they interact with each other) mainly

depends on their departure size from the electrode. There are five different forces acting

on a growing bubble on a horizontal electrode surface in a no extemal flow liquid, as

shown schematically in Figure 2.8. Drag force (Fd) and surface force (F,) hold the bubble

on the surface, while Iiquid inertia force (F,), pressure force (F,), and buoyancy force (Fb)

pull the bubble away the surface. When the bubble is just about to depart, these iwo

countencting forces component are in balance. or:

Fi + F, = F, + F, + F, (3.35)

Hatton and ~ a I l ' ~ ~ ' , and Beer et ai.'39' have shown that the appropriate equation for each

force cm be describe as:

where Cd is the drag coefficient of the bubble in the liquid. R, and Rd are the bubble base

radius and bubble departure radius respectively. APG is the excess gas pressure, and g is

the gravity acceleration.

With fluid flow, the bubble on the electrode might have a different contact angle

depending on its size, surface roughness, and the magnitude of the extemal force via fluid

motion. This effect is more important in a non-horizontal electrode. The force

equilibnum established by Eq. 2.36 then has to be modified by the introduction of new

t su riace tension

Figure 2.8 Forces acting on a bubble growing on an e lec~ode '~ ' .

important parameters, such as lifting force From liquid shear, retreating and advancing

contact angles as a function of bubble size, and the angle of the electrode to the fluid

flow. A study of bubble diameter on detachment in flowing liquids has ken made my

Hayes and intert ton""' but due to its complexity. so far there is no satisfactory theory

that cm describe the phenornena.

On 3 srnooth electrode surface. the bubble base radius tends to increase as the

bubble grows. But in the case of a rough surface. the morphology can pin the base bubble

and changes the relative contact angle of base bubble to the overall surface. thus it may

affect the bubbie depÿrture radius.

Ibl and ~enczel'"" observed the variation of bubble departure size as a function

of type electrode substrate. Bubbles on a platinurn electrode detached at a larger size than

bubbles on a copper electrode. It was reponed that the difference was due to the number

of nucleation sites. which correspond to the growth of bubble. by codescence and the

difference in the wettability of each surface. which correspond to the adhesion forces of

the bubble to the surface.

Some disagreement remains in the litenture about the effect of current density on

the departing bubble size. Janssen et and Landolt. Acosta, Muller. and obia as'^'

found that the bubbles increased in size with current density, as they observed more

bubble coalescing. Convenely, ~enczel'"~) reponed that from photographed images, the

bubble size decreased with current density and this was in agreement with a theoretical

study conducted by Frumkin and ~abanov'?

Landolt et ~ 1 . ' ~ ) dso found that the bubble size decreased with the electrolyte

flowrate. The hydrodynamics of the electrolyte significantiy affects the mechanical force

balance of a growing bubble. In addition to the flownte, the hydrodynamics is also

affected by the electrode orientation and ce11 configuration '". Most of the expenmental bubble size results were obtained using optical

photognph y method. Lookichev and ~meltzer'"", Bongnaar-Schlenter et n~. '"~ ' and

Jansen et al'''' reponed bubble size distributions using this technique. Another

investigator, Thorpe et al":' reported bubble size distributions from a miniature water

electrolysis ce11 by measurine rise velocity of some nndornly picked bubbles.

2.5 Velocitv of risine bubbles

The rising velocity of a bubble depends on its size and the properties of the liquid.

A small bubble (less than 1 mm diameter) is sphericd, rises rectilinearly, and behaves as

if it was a rigid sphere due to its high intemal pressure'5? The steady state rising velocity

(v,) of this single bubble with circulation flow on its interface in an infinite stagnant

liquid can be predicted from the Stokes Law Jiven by:

where d is the bubble diameter and p is the liquid viscosity coefficient. This velocity is

attained asymptotically after bubble departure from the electrode.

For a population of bubbles, the steady state rising velocity (v,) is affected by the

void fraction of gas in the e~ectrol~te '~~ ' . Generally, if the void fraction is Iower than 2%,

the drag force on the bubbles is lowered due to the velocity distribution around the

interacting bubbles so that v,>v,. Above that fraction in a closed system. the rising

bubbles causes a countefflow in the electrolyte to satisfy the continuity condition so that

the bubble rising velocity is decreased. The drag force in this condition is also increased

due to the velocity gradient in the liquid between the bubbles.

These effects on bubble risinç velocity becorne more important as the gas fraction

increases. ask kas'^' has proposed a theoreticai relationship for uniform. sphencal, and

'rigid' bubbles as follows:

This relationship is in satisfactory agreement with the experirnental data of Richardson

and zaki(55' , given by an empiricd relationship:

2.6 Bubble-bubble interaction in buIk electrolvte

Some bubbles bounce or coalesce with each other when they flow in the bulk

electrolyte. These interactions depend on fluid phenornena both within the electrolyte and

at the bubble surfaces.

The bulk electrolyte motion is responsible for bringin; the rising bubbles into

contact. Smaller bubbles rise more slowly than the bigger ones in the electrolyte flow.

make it possible for them to collide. The extemal flow in the volume surrounding the two

coalescing bubbles will influence the internai drainaje of the film sepanting them. The

drainage of liquid from the region between two approaching bubbles is represented in

Figure 1.9. Local curvature determines the pressure field within the liquid and hence

affects the liquid film drainage rate. The intemal pressure in very small bubbles (dcl mm)

is very high so that there will be very little coalescence between them.

Coalescence between bubbies is sipificantly repressed in ionic solutions. Figure

1.10 shows the effects of some salts on the total interfacial surface area of the bubbles

that was rissumed ro be inversely related to their degree of coalescence on^^'^^'. The

graph shows an increase of interfacial area with respect to the valence of the ionic species

and salt concentration. For example, the surface area at 0.03 M was increased by 300% in

pure water for Alr(S04)3 solution and only by 14% for NaCl solution, indicating less

bubbles coalesced in Alz(S04)3 than in NaCl soIution.

Two theones have been postulated to explain this behavior. Mamcci and

~icodemo"" suggested bat the inhibition was entirely due to the electric repulsive forces

genented by the surface potential. However, the work of ~ m n i k i n ( ~ ~ ) and the published

Figure 2.9 Schematic pre-coalescence drainage of two approaching bubbles (I to m).

Figure 2.10 The effect of charge and concentration of ions on interfacial area?

surface potentid data by Jarvis and s c h e i m d g ) do not support this suggestion.

Zieminski and ~hi t te rnore '~~ ' then postulated the effects of ion-water interactions. Ions

introduced into the water interferes with its fluctuating shon rang order and tends to

make the solution more viscous so this will increase the rigidity of the surface film

between two coalescing bubbles. Sûlts containing small or highly c h q e d ions are strong

structure makers and hence increase the viscosity of the electrolyte so that the

coaiescence is more difficult.

Qualitative and quantitative observations of hydrogen bubble evolution

phenornena in water electrolysis have been studied using two methods: image analysis

(IA) and particle size andysis (PSA). Image anaiysis was pnmarily conducted to study

the effects of current density and electrode geometry on bubble evolution by direct

optical microscopy observation of the electrode surface and to measure bubble size

distribution by means of an image analyzer. Low angle light scattering particle size

analysis was then used to measure the bubble size distribution from electrodes openting

at higher current densities and various electrolyte flow rates.

Four instrumental appantus and procedures have been developed to apply the

methods:

1. optical microscopy apparatus, associated with IA method (section 3.42).

2. LA-PSA cornparison appantus (section 3.5.2.1),

3. PSA with no extemal electrolyte fiow apparatus (section 3.5.3.2),

4. PSA with electrolyte flow apparatus (section 3.5.2.3).

Each apparatus used particular visualization ce11 configurations, electrode materials, and

electrolyte solutions as explained in section 3.1, 3.3, and 3.3 respectively. Al1 of the

measurements in these procedures were conducted at mom temperature and pressure.

The water electrolysis process was studied in a visualization ce11 made from

Plexiglas material that provides a clear view of the electrodes. The ce11 had dimensions of

LOxlOxl8 cm3 and contained two electrodes with diaphragms. Two different

configurations of electrodes and diaphraps placements were applied. Figure 3.l(a)

shows the arrangement (configuration r) for optical microscopy apparatus, for LA-PSA

cornparison apparatus. and for PSA without electrolyte flow apparatus. Figure 3.2(a)

shows the placement (configuration II) for PSA with electrolyte flow apparatus. The top

projections and their cross sections of both confijurations are schematically shown in

Figure 3.1 (b) and 3.2 (b). The cathode cornpartment had dimensions of 2 x 2 ~ 1 8 cm' and

a diaphragm made of Ryton cloth was placed in between the cornpartments to prevent the

mixing of hydrogen and oxygen bubbles.

3.2 Electrode materials

The cathodes had five different geometnes:

1. Ni screen. made of commercial Ni 200 with a wire dimeter of 0.035 cm and a

distance between wire centen of 0.127 cm (cathode l),

2. Ni plate, made of commercial Ni 200 with a thickness of 0.5 mm which was diarnond

polished (3p.m) on both sides (cathode 2),

3. commercial screen, made by a coating process of commercial Ni 100 screen with an

activated coating which produced a rough surface morphology (cathode 3),

Figure 3.1 (a) Visualizaîion ce11 configuration 1.

Figure 3.1 (b) Schematic top projection and cross section of visualization ce11 configuration II.

Figure 3.2 (a) Visualization ce11 configuration II.

10 cm diaphragms

Figure 3.2 (b) Schematic top projection and cmss section of visualization cell configuration II.

amorphous nickel alloy (50 atm% Ni, 25 at.% Co, 20 at.% B, and 5 at.% V), made

from a melt-spinning process, produced in the form of a ribbon with a width of 0.9

cm. The ribbon was diamond polished ( 3 p ) on the wheel-side. The non contact side

had good surface smoothness and was not polished (cathode 4).

Single strand Ni wire, taken from cathode no.! (cathode 5).

The cathodes no. 1-2, and 3 were spot-welded on both sides with Ni 200 plates, to

provide good current distribution, and thrn were coated with a nonconductive epoxy

paint (Amercoat" 90HS), to give an exposed electrode surface of 2x10 cm' on the front

and back face. Cathode no. 4 was clamped with Ni 200 plates and spot-welded on both

top and bottom of the tape. The clamps were spot-welded again to Ni 100 plates on both

sides and coated with the epoxy to give an exposed area of 0.9~10 cm' on the front and

back face. A photograph of the first four cathodes as fabricated is shown in Figure 3.3.

These cathodes were used in dl apparatus configurations except For the iA-PSA

cornparison iippantus. iA-PSA cornparison apparatus used the Ni wire cathode made by

coating the Ni wire with the epoxy to leave an exposed length of 3.5 cm. as illusuated in

Figure 3.1. The exposed Ni wire was placed in a horizontal position within the rniddle of

the cathode cornpartment. For al1 electrodes, three coats of Arnercoata 90HS were

applied to ensure insulative integrity. The anode for dl cathodes was prepared similarly

to the Ni screen (cathode 1).

Figure 3.3 Four cathodes geometry (from the Ieft, cathode no.1: Ni screen, cathode no. 2: Ni plate, cathode no. 3: commercial screen, cathode no. 4: morphous nickel alloy tape).

Figure 3.4 Cathode no. 5: Ni wire geometry.

The electrolyte solutions were prepared by dissolving Analar grade &Co3 in

deionized water. A concentration of L M K2C03 was used for the IA and PSA

measurements, while 0.5 M &Co3 was used for the IA-PSA procedure for cornparison

meûsurements. The experiments were done using &CO3 to avoid caustic attack on the

glass-based material of the sample ce11 wall inside the particle size analyzer. The

electrolyte solution was pre-electrolyzed for 1 hour using the electrodes at 200 &cm2

to satunte the solution with dissolved hydrogen and clean the electrode surfaces.

3.4 Image analvsis

3.4.1 Optical microsco~v observation

The opticai microscopy technique allows direct observation of the bubble

evolution phenornena and its flow path on the electrode for a water electrolysis system.

With calibration, quantitative data for bubble size distribution cm be obtained by means

of an image analyzer.

This technique is most often used for the examination of particles ranging in size

from about 3 to 150 p. The lower resolution limit depends on the wavelength of the

light used, refnctive index different between particle and that of the medium, and the

optical lens configuration. Above the upper b i t , minimal mapification is needed for

observation.

The most severe limitation of this technique is the depth of focus that becornes

shailower as the rnagnification increases, therefore the Mewing area is very Limited at

high mapification. An optimum mapification is needed to give clear visualization of

the bubble field. For the observation of a fast moving bubble, this technique is limited to

the image capture rate. To obtain bubble size distributions, each bubble has to be

measured. Overlappinp bubbles are difficult to isolate.

3.42 Instrumental armaratus

A schematic dnwing and a photopph of the image analysis instrumental

apparatus are shown in Figure 3.5 (a) and (b). The visualization ce11 and the Olympus SZ

II stereomicroscope were placed on top of a bench with vibration darnpers. The

microscope was connected to a SONY SSC-C370PI CCD color videocamera, whose

signal was then sent to a central processor unit with a Rainbow Runner video capture

card and Matrox Millenium II video card. The software used to control the recording

process was VCapture V. 2.0.0.0. Four MR 16 (50watt) lamps were used to illuminate the

ceIl. A Xantrex HPD 30-10 power supply w u used and a multimeter provided an

accurate direct current output. The measurement of bubble diameter was performed using

LEC0 [A-300 1 V 1.20 PC-based image analysis system.

Figure 3.5 (a) A Schematic dnwing of optical microscopy instrumental apparatus.

Note: 1. visudization ce11 2. power supply 3. anode 4. diaphragm 5. cathode

6. microscope 7. bench 8. CCD camera 9. monitor 10. central processor

Figure 3.5 (b) Photogaph of opticd microscopy instrumental apparatus

3.4.3 Procedure for the i m a ~ e recording

The visualization ce11 was placed at the Focal point of the optical microscope to

image both the cathode and the hydrogen bubbles. The bottom section of the elec~ode

was investigated because it had the lowest bubble density. Two illumination larnps were

placed at 13 cm from the focal point on each side at a 15" angle from the electrode plane.

The shutter speed of the CCD camera was set to 1/10,000 second to provide a clear image

of fast moving bubbles. The quality of the images was improved by using the video

capture software. The bubble evolution images were taken at current density of 10, 20,

30, 50, 100 and 250 ~ c m ' at room pressure and temperature. The images were

recorded in mpeg files with a capture rate of 30 frmes per second. individual images

were from single frames of the mpeg file and were saved as a bmp file.

3.4.4 Procedure for determination of bubble size distribution

The bmp files were read into to the image analyzer. The size scale was then

calibrated using the known diameter of the nickel mesh at each microscope magnification

that was used. Bubble diameters were measured by circling each singie bubble on the

image and using the analyzer to record the diameter as illustnted in Figure 3.6. The

diameter data was saved as a text file and then imported into a MS Excel 97 spreadsheet

software. Bubble size distributions were presented in a fom of volume percentage of the

bubble population for a given range of bubble diameter. An example of bubble size

distribution calculation can be seen in Appendix A.

Figure 3.6 Image analyzer for bubble size distribution measurement.

3.5 Low angle light scattering particte size andvsis

The method relies on the fact that when light strikes a particle, some of it is

diffracted and the diffraction angle is invenely proportional to the particle size. The

diffracted light forms a specific angular light intensity pattern, depending on the size of

the particle. The relationship between particle size and the angular distribution of light

flux pattem can be descnbed using Mie t h e ~ r ~ ' ~ ~ * ~ ' ) , which also applies for a population

of particles. Therefore, if the light scattered pattern of a particle population is known, the

particle size distribution theoretically can be obtainrd.

The basic instrument consists of a laser source, a lens train, a sample cell, a

photodetector, and a central processor as shown in Figure 3.7? He-Ne gas lasers with

fixed wavelength of 0.63 pn are commonly used. The laser is focused by a convergent

optical system before and after passinj the sample cell. After the laser passes through the

sample cell, the angular pattem is detected. recorded, and sent to the central processor

where the signai is amplified and deconvoluted based on the Mie theory to yield the

unknown size distribution. The detector is usually a slice of photosensitive silicon with a

nurnber of discrete detectors.

fine particles in suspension

2 mW He - Ne laser

beam expanding optics

unscattered !!@?? multi-elernent

detector

scattered obscuration Iight detector

Figure 3.7 The principle of low angle laser light scattering instrument^'^".

3.5.2 Instrumentai a~~aratus and procedure for the determination of bubble size

distribution

The commercial instrument used in this project was a Malvem Instruments

Mastersizer S. The instrument measures particles within the size range of 0.05-3500 p.

Each measurement takes 2 milliseconds and one data report is taken as an average of one

thousand continuous measurements. The instrument uses Mastersizer-S V2.15 software

that controls the rneasurement and analysis process. Input parameters include the

refractive indices for the material (bubbles) and the medium (electrolyte) needed for the

Mie theory caiculation. A refractive index of 1.00 and 1.33 was used to represent the

index of gaseous hydrogen and aqueous solutions respectively.

The rneasurement was started by aligning the laser in the analyzer sample and

then measunng the background signai for a bubble-free electrolyte. The obscuration (a

measure of the arnount of laser light lost due to the introduction of the bubble population

within the analyzer beam) was regulated in the range of 10 to 30% by controlling the

bubble concentration in the detector cell. When the bubble-filled electrol yte entered the

detector cell, the bubble distribution profile was displayed on the monitor. The initial

bubble size distribution was unstable for about 1 to 10 seconds. The data were recorded

&ter the distribution became stable. At the end of each measurement, the electrode power

was disconnected and the sampling pump was run until no bubbles were detected in the

detector cell. This procedure was performed to prevent bubble accumulation from the

pipe and detector ce11 wail attachent. The measurement was repeated six times at each

same set of conditions to determine repmducibility.

Bubble filled electrolyte was transported from the electrolysis cell to the detector

ce11 using a suction pump upstream of the detector cell. There will be a slight volume

expansion of the measured bubbles due to pressure head difference between the

electrolysis ce11 and the detector cell.

3.5.2.1 IA-PSA cornparison

Electrolysis was conducted at a current density of 300 mNcm2. Figure 3.8 shows

a schematic drawing of the instrumental apparatus. The bubbles were sampled at a

distance 1 cm above the Ni wire cathode using a pipe with a diameter of 0.5 cm with a

liquid capture rate of 3 mUsec. The bubble-filled electrolyte was dmwn to the particle

size analyzer by suction from a pump located &ter the analyzer. The bubble size

distribution wûs detexmined using the image malysis method as described earlier in

section 3.4.3 and 3.4.4.

3.5.2.2 PSA without external etectrolvte flow

Electrolysis was conducted at current densities of 25,50, 75, and 100 mA/cm2. A

schematic drawing and a photograph of the instrumental apparatus are shown in Figure

3.9 (a) and (b). The bubbles were sampled using bubble collectors s h o w in Figure 3.10,

with liquid capture rate of 3 mUs. The bubble collection site could be varied in height

from 2.5,5.0, to 7.5 cm from the bottom of the electrode.

Figure 3.8 A schematic drawing of image analysis - particle size analysis cornparison instrumentai apparatus.

Note: 1. anode 2. diaphragm 3. cathode-bubbles 4. power supply

5. microscope 6. particle size analyzer 7. pump 8. image analyzer

Figure 3.9 (a) A schematic dnwing of PSA without extemal electrolyte instrumental apparatus.

Note: 1. bubble-electrolyte collecter 2. cathode 3. diaphragm 4. anode

5. power supply 6- P U P 7. partide size anaiyzer

flow

Figure 3.9 (b) Photograph of PSA without extemal electrolyte fiow instrumental apparatus.

Figure 3.10 Bubble colIectors were designed so that after they were clamped to the cathodes, the iniets had a distance of 7.5, 5.0, and 7.5 cm from the bottom of the cathodes.

Figure 3.11 (a) A schematic dnwing of PSA with electrolyte flow instrumental apparatus.

Note: 1. bubble separator 2, reservoir 3. bubble-free electrolyte 4. anode 5. permeable membrane

6 . cathode 7. bubble cotlector 8. flowmeter 9. pump 10. particle size andyzer

Figure 3.11 (b) Pho topph of PSA with electrolyte flow instrumental apparatus.

Figure 3.11 (c) Visualization ceIl and bubble separator in PSA with electrolyte flow instrumental appmtus.

3.5.23 PSA with electroivte flow

A schematic dnwing and photogmphs of the instrumental apparatus are shown in

Figure 3.11 (a). (b), and (c). Data were taken at four different current densities: 100, 150,

200, and 250 mNcm2, with linear electrolyte flow rate of 60 cm/s in the cathode

compartment. These measurements were repeated for linear electrolyte flow rates of

52.5, 45.0, 37.5, and 30.0 cmls. Bubbles were sampled using the collector at position 50

mm from the bottorn of the electrode. The voIumetric sampling rate that was used in the

bubble collector was adjusted so that the electrolyte flow rate in the collecton was the

same as in the cathode compartment.

3.6 Summary

Table 3.1 sumarizes the four experimentai apparatus and procedures described

previously for cornparison.

Table 3.1 Experimentiil Apparatus and Procedure Suntinary

Instrumental apparat us

Sc hcniu t ic: Figure

Visudfzation cell configuration (section 3.1)

cornparison I I I l ~ 2 ~ 0 3

IA-opticnl microscopy ( scction 3.4.2) IA-PSA

Cathode (section 3.2)

externd electrolyte flow

Electrolyte (section 3.3)

3.5 (a)

3.8

( section 3.5.2.1) PSA without

elcc t roly te flow (section 3.5.2.3)

I

1

3.9 (a)

(scction 3.5.2.2) PSA with

10.20.30. 1 the boitiim of 50, 100, ihc cuthodes

Current density ( r n ~ / c r n ~ )

1,2,3,4

5

1

3.1 1 (a)

Eleciralyte flow rate in the cathode / compart nien t (cints)

Bubhlc sanipling

I M K2C03

0.5 M 300 1 1 cm übove the 1 O 1

1 ,2,3,4

1 I

bottom of the

1 M K2C03

cu~hodes LOO, 150, 50 mm from 112,3,4 200,250 ilie boitom of

the crithodes

I M K2C03

IV. RESULTS AND DISCUSSION

1

The literature reported in section 7.3.12 indicates that the critical bubble size

radius should be able to be estimated for the experimental conditions used in this work.

Criricd rddius of Duooie nuciei in DuiK eiecuoiyte ciepencis on the concentration of

dissolved hydrogen gas as described in equation 3.24. For a system of hydrogen gas

dissolved in 1 M K2C05 at room ternpenture and pressure. the relationship is presented

in Figure 4.1. Properties of the electrolyte used for the calculation are presented in

Appendix B. Figure 4.1 (a) shows a hyperbolic curve with an asyrnptotic value at a

hydrogen mole fraction of 1.37 n 1 0 ~ ~ . Bubble nucleation is possible within the

concentrition nnge of positive critical radius.

Nucleation at the saturation concentration in the bulk electrolyte (dissolved

hydrogen mole fraction of 1.42 x 10'~) is almost impossible due to the very high value of

the radius (+70 pm). For the electrolyte close to the cathode, a dissolved hydrogen

concentration was reported'63' to be as a mole fraction of 2.15 x 10" at a current density

of LOO m~lcm'. The critical radius for homogeneous nucleation at this concentration is

0.01 pm as shown in Figure 4.1 (b).

With the presence of an electrode surface, the nucleation is thennodynamically

favored compared to bulk homogenous nucleation. In addition, some pi residual may be

left over after bubble departure and would become a nucleus for the next bubble site. This

happens especidly on pits and grooves. As outlined in the literature section, physical

processes on the electrode surface rnainly govern the bubble evolution.

O 1 2 3 4 5

mole fraction of dissolved hydrogen gas x 10'

O 1 2 3 4 5 6 7 9!

mole fraction of dfssolved hydrogen gas x 1@ I

Figure 4.1 Bubble crirical radius as a function of dissolved hydrogen concentration for homogenous nucleation in 1 M KzC03.

1.2 Image analvsis

4.2.1 Optical visualization of hvdroeen evolution

3.2.1.1 Smooth vs. rowh screen electrode

Bubble evolution at different current densities on a smooth screen is illustrated in

Figure 4.2. At current densities of 10 and 20 mNcm2, bubbles tend to be generated at the

screen junctions. The junctions provide cavities that tnp some gas which then serve as

bubble nuclei. As current density increases, more nucleation sites become active on the

wire surface. Therefore, for current densities up to 30 mNcrn2, a decrease in bubble

diameter occurs. This is in agreement with ~enczel"'~', Fnimkin and ~abanov '~ ' .

Bubbles that are genented From the wire surface tend to depw with smaller size than

bubbles frorn screen junctions. From visual observation, the movement of bubble in this

low current density range is schematicdly presented in Figure 4.3. Screen junctions

provide sites for bubble coalescence.

Above 30 mNcm2, the entire surface becomes active. The bubbles are connecting

and overlapping with each other so that it is hard to recognize individuai bubble. At

cumnt density higher than 100 mAkrn2, the screen is completely covered by a high

dynarnic bubble mantle.

Figure 4.4 shows that almost the entire surface of the rough screen is active even

at cumnt density of 10 mA/cm2 due to gas entrapment in the surface rnorphology. A

rnicrognph cross section of this surface is shown in Figure 4.5. The surface was an

Figure 4.2 Hydrogen bubble evolution on a smooth screen electrode at current density of: (a) 10 mA/cm2; (b) 20 mA/cm2; (c) 30 rn~lcrn?; (d) 50 mA/cm2; (e) LOO mAfcm2'; and ( f ) 250 mA/cmZ.

. , - --

I I I

; . ; : ! . # l S I

Figure 4.3 Schemrtic of bubble path on a smooth screen electrode: (a) paralle1 to screen electrode and (b) side view projection.

Figure 4.4 Hydrogen bubble evolution on a rough screen electrode at cumnt density of: (a) 10 rn~lcm'; (b) 20 rn~fcrn'; (c) 30 rnA/cm2; (d) 50 m~fcm'; (e) 100 m~lcm"; and (f) 250 mivcm2.

Figure 1.5 SEM picture of surface morphology of the rough screen electrode.

activated layer coated on a nickel screen. The gmulated coating produces an irregular

porous surface with various cavity geomenies. Once the cavities begin to emit gas

bubbles, a portion of the entrapped gas is carried off with each bubble until eventually the

cavities are filled only with gas. These cavities contain entrapped gas with size p a t e r

than or equal to the critical size so that the bubbles will nucleate and grow spontaneously

when the dissoived gas concentration just exceeds the saturation concentranon.

The roughness also holds the bubbles, so that residence penod on the surface is

longer than on a smooth screen. Some big bubbles (diameter > 150 p) manage to stay

on the surface before they depart to the electrolyte even at a current density higher than

LOO m~/crn'. More variance in bubble size is observed for rough screen electrodes than

for smooth screen electrodes (Figure 4.2 and 4.4).

4.2.1.2 Crvstailine vs. amor~hous plate electrode

Bubble evolution for crystalline and amorphous plate electrodes is shown in

Figure 4.6 and 4.7. Larger bubbles were observed on amorphous plate than crystalline

plate at current density range from 10 to 30 rn~crn'. Bubbles on the an?orphous subsmte

may have a bigger contact angle and hence produce longer bubble foot perimeter that

holds the bigger bubble on the surface. The difference in contact angle may be a result of

stronger hydrophobie characteristic of amorphous alloy than the crystalline substrate.

Sirnilar to screen electrodes, the entire surface becomes active at a current density

above 50 rnA/cm2. At high current density, bubbles on the surface touch each other,

leading to bubble coalescence before deparme. Thus the size of bubbles from crystalline

and amorphous plate electrodes should become sirnilar at higher current densities.

Figure 4.6 Bubble evolution on a crystalline plate electrode at current density of: (a) 10 mA./crn2: (b) 70 mA/crn2; (c) 30 m~lcm'; and (d) 50 mAkrn2.

Figure 4.7 Bubble evolution on a amorphous dloy plate electrode at current density of: (a) 10 rn~lcrn'; (b) 20 rnA/cm2; (c) 30 m~lcm'; and (d) 50 rnA/cm2.

4.2.2 Bubble size distribution and rnean diameter for different electrode geometries

and current densitv

Figure 4.8 shows the bubble size distributions for different electrode geometries at

30 mA/cmL. A screen electrode produces bubble with wider size range than plate

electrodes. At this current density. the role of screen junctions as a larger bubble

genentor is still important.

The mean bubble measurements as a function of current density (from 10 to 30

rn~lcrn') for the electrodes are show in Figure 4.9. Amorphous alloy electrodes produce

the biggest bubble diarneter within the current density range. The size decreases with

current density due to increase in nucleation sites. This technique pmvided size

distributions of bubbles that were still attached on the electrode surface.

The validity of quantitative data produced frorn optical image analysis is limited

for a number of reasons. The measurements cm only be reliably made for current

densities lower than 30 rnNcrn2 due to bubble density. At these current densities the

dynamics of bubble evolution is very variable. The data reproducibility was very limited

and hence genented high error bars for each set of measurements. The method also

requires subjective judgment to differentiate bubbles in a crowded bubble population. In

addition, the number of measured bubbles may not represent the whole bubble population

on the electrode surface. Despite the measurement dificulties, the data illustrates the

order of magnitude of the bubble sizes and shows the trend of bubble size with electrode

geomenies and current density.

1 O0 bubbie diameter {pm)

1 O0 bubble diameter (w)

Figure 1.8 Bubble size distribution for different electrode geometries at a current density of 30 m~/crn'.

amorphous alloy plate x\x-x

smooth screen I I I r

O 10 20 30 40

current density (mNcm2)

Figure 1.9 Bubble rnean diameter as a function of current density for different electrode geometries.

4.3 Low Angle Lieht Scatteriw Particle Size Anaivsis

4.3.1 Verification of the experimental techniaue

Particle size analysis is a new technique to chancterize electrolytic bubbles. The

method is widely used for solid particle chanctenzation of minerals and pigments. It is

required to venfy that the method is applicable for hydrogen bubbles ("a gaseous

particle"). Two approaches were studied to meet this goal. Section 1.3.1.1 reports the

comparison of measured bubble size distribution from this method with results from an

optical rnicroscopic image analysis method. Section 4.3.1.2 investigates the bubble size

stability during transponation from the electrolysis ce11 to the detector cell. in the particle

size analyzer, by v q i n g the length of connecting pipe between the cells.

4.3.1.1 Cornparison with image anahsis

The cornparison of the bubble size distribution from particle size analysis with the

results from image analysis is shown in Figure 4.10. Each bar in the histograms indicates

an average of bubble volume percentage for a given bubble diameter range taken from 5

frames for imap analysis and 6 measurements for particle size andysis. For this

comparison, the electrode used was a single horizontal wire (as shown in Figure 3.4) in

0.5 M &Co3 with a current density of 300 mNcm2. Experimentai details of the

apparatus were descnbed in section 3.5.2.1.

1 Particle size analysis

6.1 6 13.26 28.36 60.87 130.62 280.3:

bubble diameter (pm)

30 Image analysis

bubble diameter (pm)

Figure 4.10 Cornparison of buhble size distribution from particle size analysis with size dishibution from image analysis.

The distribution from the particle size analysis and the image analysis agree

within 2 standard deviations (95.5 96 level of confidence). The mean diameter of the

distribution from particle size analysis and image andysis are 52 and 49 pm respectively.

It is also observed that the distributions have an agreement in their modus size range that

is between 45 and 52 W.

Note that some differences are also observed between the distributions. The

dominant bubble population volumes for particle size analysis and image analysis are 14

and 23 percentage respectively. Bubbles greater than LOO pm are not found in the image

analysis distribution. These are likely the result of an image analysis limitation because

measurements cannot be made for overiapping bubbles. Thus the measurements do not

represent the entire bubble population.

The measurements expenmentall y indicate that the size distri bution measured

from particle size analysis has a good agreement with the measurements from image

ünalysis. The method has greater precision than image analysis and provides a large

number of measurements for data analysis.

4.3.1.2 Bubble-bubble interaction during transportation in the eciuipment

As descnbed in experimental apparatus of PSA (section 3.5.2.2 and 3.5.2.3), the

bubbles from the etectrode (in the electrolysis cell) were transported to the detector ce11

(see Figure 3.7) inside the analyzer. The measured size distribution in the detector ce11

should be the same as the size distribution near the electrode. Therefore, it is desirable

that no bubble-bubble interaction (codescence or breakup) occurs during the

msponation.

When the bubbles flow in the connecting pipe, they may dissolve due to their hi@

intemal pressure or may contact each other due to ciifference in the velocity vectors.

Longer bubble transportation in the pipe gives a greater chance for the bubbles to

dissolve, bounce, codesce, or break, depending on their chernical and physical properties.

Therefore, a test was designed to determine the bubble size stability by varying the path

length to the detector (connecting pipe length).

Figure 4.11 shows that the bubble mean diameten for connecting pipe lengths of

90 to 180 cm are constant. The pipe length of 105 cm was chosen for al1 subsequent

particle size analysis. Under the conditions of the test (current densiiy of 100m~/cm' and

electrolyte flowrate of 60 cmls), the avenge diameter value was 23.61 pm with a

standard deviation of 0.2 1 pin. Coalescence of bubbles in the pipe did not occur due to

the very high intemal bubble pressure (6280 ~ / m ' for the avenge bubble diameter). The

pressure was caiculated using equation 2.22 - 2.24 and is presented in Appendix C. The

bubble interface behaved as a stable rigid solid surface. This result confimis the

applicability of particle size analysis for measunng electrolytic bubble distributions.

Figure 4.11 Bubble rnean diameter as a function of connecting pipe length between electrolysis ce11 and detector ce11 in the analyzer.

1.3.2 Electrolvsis with no e-xternai electrolvte flow

When the electrolyte was not circulated in the electrolysis cell, it was found that

the bubble behavior (size distribution) was very close to the behavior in a flotation

column for minerais separation. The effects of the collector position and current density

on the bubble mean diarneter will be presented in the following section.

4.3.2.1 Bubble size distribution for different collector positions

In a no extemal 1 M K2C03 electrolyte flow, the size distributions of hydrogen

bubble populations at different collector positions (from the bottom of a smooth screen

electrode) at 50 mA./crn2 are presented in Figure 4.12. The distributions at position O and

25 mm from the bottom the electrode (Figure 4.12 (a) and (b) respectively) correspond

closely to a log-normal bubble size distribution with increasing in the distribution spread

and the mean bubble diarneter at the higher position. The log-normal distribution was

confirmed by the linearity of the data plotted against a log normal function. The other two

distributions (Figure 4.12 (c) and (d)) that were measured at position 50 and 75 mm are

bimodai intersecting distributions of two log-normal distributions. Each distribution is a

combination of the same size range log-nomal distribution at lower positions (O and 25

mm) and a new bigpr size range population. The bimodd distributions at these positions

are dmost identical. The summary of statisticai properties from Figure 4.12 is presented

in Table 4.1.

10 100

bubble diameter h m )

10 1 O0

bubble diameter hm)

10 1 O0

bubble diarneter h m )

14

position: 75 mm +total 1

- - -+- - - population 8 i I

10 100

bubble diameter h m )

Figure 4.12 Hydrogen bubble size distributions in I M &Co3 at 50 m~lcrn' for different bubble collecter positions from the bottom of smooth screen electrode.

Table 4.1 Statistical Properties of Figure 1.12

From the observation of the bubble behavior in the cell, bubbles from the lower

Position (mm) Population Volume (%) Modus (pm) Mean (pm) Size range

part of the electrode are seen to travel upward and enter a region where the electrolyte is

mixed by bigger bubbles that are bounced back by the froth layer close to the surface of

the electrolyte. Figure 4.13 visualizes this two-phase system at different current densities.

At a current density of 50 rn~lcm' (Figure 4.13 (c)), i t cm be seen that position O and

25 mm are below the bubble-mixed electrolyte region. while position 50 and 75 mm are

in the region. This 'cloudy' region becomes deeper as the cumnt density increases from

25 to 50 m~/cm'.

i hini) i 206.54 i 443.23 443.23 i 443.23 i 443.23 i 443.23 i

O - 100

70.9 1 67.4 1 9.74 -

The phenornena c m be explained by a froth structure model by Yianatos, e t . a ~ . ' ~ '

that was initinlly developed for a tlotation column froth study. The model shown in

Figure 4.14, divides the system into four sections by the amount of gas holdup. The

sections are bubbling zone; expanded bubble bed: packed bubble bed; and froth zone.

In the bubbling zone. the bubbles move freely upward until they reach an

interface level with an expanded bubble bed. There is almost no collision between

bubbles occurred in the bulk electrolyte. It is sugested that collisions between bubbies

25 -

100 82.6 1 79.26

50 A

44.35 60.87 7 1.30

9.74 - 8.36 -

75 B

55.65 206.54 20 1.37 70.9 1 -

A 44-54 56.56 69.38 7.18 -

B 55.46 206.54 202.00 60.87 -

i

1 bubble collecter

1 positions:

Figure 4.13 Visudization of bubble-electrolyte system in no extternai flow electrolyte electroIysis for different current densities.

froth € 9 > 0.80

pac ked bubble bed E > 0.74

expanded bubble bed E < 0.74

bubbling zone € 9 < 0.20

Figure 4.14 Froth structure mode1 suggested by Yianatos et al., ~986'~'.

when they move upward on the electrode surface leads to only a slight increase in bubble

mean diameter from position O to 25 mm (Figure 4.12 (a) and (b)).

When the bubbles pass the interface, they coalesce with the bubble bed and

genente a shock pressure wave, which promotes additional collisions above the interface.

This phenornenon could cause the second bigger bubble population distribution of the

bimodal distribution at position 50 mm (Figure 4.12 (c)).

At the third section, the packed bubble bed, the fractional liquid content is lower

than 0.76 so that the bubbles loss their momentum to coalesce and they move uniformly

upward to the froth zone. This explains the similarity between the distribution at position

50 with 75 mm at Figure 4.12.

1.3.2.2 Bubble mean diameter for different collector positions and current densities

Most significantly, as illustrated in Figure 4.15, bubble mean size increases in the

regions extending from 25 to 75 mm and 75 to 50 mm for current densities of 75 and 50

m ~ c m ' respectively. These regions correspond to the expanded bubble bed region where

most coalescrnce occurs as explained in the previous section. This is in agreement with

the visual observation of the interface lowering with current density in Figure 4.13. The

bubble mean sizes in the bubbling zone and froth zone are not significantly dependent on

the current density. In the bubble packed region, as explained in the previous section, the

mean bubble size is constant at approximately 143 p.

The effect of current density (gas flow rate, by Faraday Law) on the bubble mean

diameter is shown in Figure 4.16. At position O and 25 mm, the mean bubble diameter

collector position (mm)

Figure 4.15 Bubble mean diarneter as a function of collector position nt cumnt density of 23 and 50 rnNcm2.

O 25 50 75 100 125 150 i current density (mAlcm2) !

Figure 4.16 Bubble mean diameter as a function of current density for various collector positions.

increases slightly with current density. Especially for position O mm, it can be assumed

that the increase is not due to codescence in the bulk electrolyte, but due to the increase

in current density only (this will be explained in section 4.3.3.3.3). The data plotted in

this figure is not complete for higher current density at upper positions due to over

saturation of bubble concentration in the detector ce11 of the parùcle size analyzer (see

section 3.5.1).

4.3.3 Electrolvsis with ekctrolyte fiow

A bubbling zone was observed when bubble-free electrolyte was introduced

upward from the bottom of the cathode compartment. There was no significant

coaiescence between bubbles in the bulk electrolyte. The following sections descnbe how

electrode geometry, electrolyte Rowrate. current density, and collecter position affect the

bubble behrivior.

4.3.3 Flow anatvsis in the electrolvsis ce11

For this type of apparatus. the cathode rectangular electrolyte flow channel (as

described in section 3.5.2.3) is shown in Figure 4.17. Bubble- free electrolyte flowed

îi-om a 1 x 2 cm2 opening on the side-bottom of the channel. The cross section are3 of

channel had a dimension of 2 ?r 2 cm2. The cathode (with dimension 2 x 10 cm') was

placed vertically at 8 cm height from the compartment bottom. The electrolyte linear

velocity in the channel was varied from 0.3 to 0.375. 0.45. 0.525, and 0.6 cm/s

respectively.

The channel can be divided into three flow regions based on the cross section area

and number of phases in the channel. Region I is the bubble-free electrolyte channel

below the electrode. Region II is the channel where the cathode was placed. Region III is

the channel above the electrode where the two-phase flow from region II flow across and

out of the ce11 to the bubble separator (see Figure 3.1 1).

The effective Reynolds number for these regions in the absence of bubbles is

shown in Table 4.2. For thk rectangular cross section, the numbers were found using the

bubble-filled electro l yte

region III (4.5 cm)

region II (1 0 cm)

ded,"O:yt'T 7; flow in

flow out

1

1 bubble collecter i position: I 7 50 mm

\ gas-evolving

electrode

Figure 4.17 Fiow channel geometry in the cathode cornpartment of the electrolysis ce11

Table 4.2 Reynolds Number of the Regions in the Channel without Bubbles

effective flow channel diameter, Deff = [64/(Re)]Dh, where fRe is the friction constant

that is specific for each rectangular width-length mtio and Dh is the hydnulic diameter.

Table 4.2 indicates that the flow was turbulent, because the values were of the order of

thousands. Note that strong flow disturbance was also introduced due to a high

perpendiculx inlet Stream fluctuation.

During the electrolysis, a two-phase flow was genented in region II and III with

oas flowrate as a function of applied current. Void fraction (bubble population) in region C

II increased with height in the ce11 and was dependent on the current on the electrode and

electrolyte flownte. nie discussion covers the flow in region II that is affected by the

flow from region 1.

Linear velocity ( d s ) 0.300 0.375 0.450 0.525 9.600

Effective Reynolds number Region 1, III

6287.978 7859.973 943 1.966 1 1003.96 1 A - 3 4 3 . / 3 3 1 3 C 7 C OCC ,

Region II 3827.586 4784.483 5741.379 6698.276 ?655. ! 73

4.3.3.2 Bubble size distribution for different electrode ~eornetries

Bubble size distributions were measured using smooth screen, rough screen,

crystalline plate, and amorphous alloy plate. In a flowing 1 M &Co3 electrolyte, with a

flowrate of 0.6 d s and a current density of 750 mA/crn2, bubble size distributions for

these electrodes are shown in Figure 4.18. The distributions correspond closely to

bimodal log-normal distribution. Similar distributions were found for each electrode

geometry when the paramerers were varied.

Two non-intersecting distributions were found for screen electrodes while plate

electrodes produced two overlapping distributions. The population with the smaller size

range at each bimodal distribution is referred to 'Population A' and 'Population B' for the

bigger size distribution. The sumrnruy of statistical values of these populations frorn

Figure 4.18 is presented in Table 4.3 and 4.11. Population A occupied smaller than 10

volume percentase in al1 of the experiments conducted. The population properties are

presented in Appendix D. In bubble nurnber percentage, Population A might dominate

the whole bubble population but this bubble would also be susceptible to coalescence

during bubble sepuation due to high coalescence rate with bigger bubbles. Discussion in

the following sections refers to the overdl population propenies.

Table 4.3 Statistical Values of Population A frorn Figure 4.18

Electrode geometry Volume (%)

Smooth screen 1 5-54

Modus (pm)

0.06 - 0.73

Mean (Po

0.29 0.06 - 0.73 4.54 - 38.50 4.54 - 38.50

Size range (pm)

0.30 0.30 20.17 14.2 1

Rough screen Crystalline plate

Amorphous aiioy plate

3.72 1 0.29 5.87 15.40 1.43 1 15.40

bubble diameter (pm)

I 0.01 0.1 1 10 100 I I

bubble diameter (pm)

crystalline plate I

I

0.01 o. 1 1 10 1 O0 1000(

bubble diameter k m )

i amorphous ailoy plate

- - A - - - - - - - - - - - - - - - - - -

total

, - - -+ - - population A ' - - - - - - - - - - - - - - - - - - - A - - - - - - - -

- - -x- - - population 8 - - - - - - - - - - - - - - - - - - * - - - * - - - - - - - - - - - - -

I

1 0.01 0.1 1 10 100 10001 1

bubble diameter (pm) I

Figure 4.18 Hydrogen bubble size distributions in 1 M &Co3 at 250 O c m Z for different electrode geometry with collecter position of 50 mm from the bottom and eiectrolyte Rowraie of 60 c d s .

Table 4.4 Statistical Values of Population B from Figure 4.18

Plate electrodes produce a bubble population with a bigger size than screen

electrodes. For the given parameters in the figure. the mean bubble diarneter for plate

Electrode geometry

Smooth screen Rough screen

Crvstalline lat te

electrodes is approximately LOO Fm, while the bubble mean diarnerer produced by

smooth and rough screen electrodes are 27 and 4 pm respectively. This phenornenon

Volume (c'ro)

94.56 96.29 94.23

rnight have been the result of the locai hydrodyamic forces on the electrode surface

where the bubbles are growing and depming. Figure 4.19 shows a suggested schematic

of bubble interaction and departure on a cross section of a horizontal wire frorn screen

electrode and a vertical plate electrode. Note that at curent densities above 100 mA/cm2,

al1 of the electrode surface is active.

In a flowing electrolyte system, bubble departure size is very dependent on the

relative orientation of electrolyte flow to electrode surface where the bubble is growing.

This is due to the vector balance of forces that hold the bubble to the electrode surface

and forces that pull the bubble from the surface (this wil1 be discussed in the following

section).

The vertical plate electrode (Figure 4.19 (b)) has a unifonn paralle1 Bow-surface

orientation. The electrolyte lifting force push the bubbles at the lower part of the

Modus (pm) 28.36 44.85 1 12.12

Mean (p) 28.39 45.7 1 104.88

Size range (pm) ,

0.99 - 88.91 0.85 - 190.80 20.90 - 206.54

electrolyte flow

electrolyte flow

Figure 4.19 Schematic of bubble coalescence and departure on a cross section of: (a) horizontd wire of a screen electmde and (b) a plate electrode in an upward flowing electrolyte

electrode so that they collide with neighboring bubbles and form bigger size bubbles until

the y reach the departure size.

A horizontal wire from a screen electrode (Figure 4.19 (a)) covers al1 of the

possible relative orientations between the flow and the surface. Smaller bubbles at lower

pan of the electrode are more unstable and tend to move upward dong the surface. This

rnovement ieads to surface coaiescence and hence increases the bubble size on the

surface to the bigpest size at position 12 o'clock. Meanwhile, the pulling force vector

from the upward electrolyte turbulent shear dso increases from position 6 o'clock to 9

o'clock and then decreases again to position 11 o'clock. This pulling force vector is

proportionai with the amount of departed bubbles. Hence wider range of bubble with

smaller relative size is produced from screen than plate electrodes.

4.3.3.2.1 Effect of electrolvte flowrate

The lifting force from electrolyte shear is one of the forces that pull the bubble

away from the electrode. Its magnitude is proportional to the square of electrolyte linear

velocity at the projected bubble area. It cm be predicted. as shown in Figure 4.20, that the

bubble depamire size decreases with increase in electrolyte velocity.

This effect is less significant for screen electrodes than plate electrodes. From the

figure. the linear decrease of bubble mem diameter with electrolyte velocity 60 to 30

cmfs for plate electrodes is 75 Pm, while for screen electrodes is 15 p. This difference

might have been the result of the difference in local hydrodynamic tlow between screen

with plate geornetry (Figure 4.19). Bubbles on a plate electrode expenence a unifonn

effect when subjected to a tlowrate change. In the case of screen electrodes. it is

postulated that the bubbles are not affected in a uniform way. The change in flowrate

less affects the bubbles located distally to the flow direction than the bubble proximally-

located.

Predicting the detachment size of a bubble frorn a vertical electrode in flowing

electrolyte is difficult because the detachment mechanism is well not understood

theoretically. With a truncated spherical bubble mode1 that is attached on the electrode

(as shown in Figure 4.21 (a)), the surface force will have same magnitude as the internai

pressure force of the bubble against the flattened bubble base. Thus, the detachment wiil

rely on the balance of the drag force, and is not likely to be compensated by the inertial

force of a growing bubble. However, the inertial force may expenence a high jump in

magnitude due to coalescence between gowing bubbles on the electrode surface. This is

thought to be the depamire mechanisrn noting that Westerheide and estm mas ter'^^'

I

I electrolyte flowrate (cmls) i

Figure 4.20 Bubble mean diarneter as a function of electrolyte flowrate for various electrode pometnes at c m n t density of 250 rn~lcm' and collector position of 50 mm.

observed a bubble fomed from the coalescence of two large bubbles sometimes jumped

off and retumed for no apparent reason. Also for this sphencal bubble, as shown in

Figure 4.2 1, no vertical force balances the forces that push the bubble upward. Thus the

bubble will move upward and then bounce or coaiesce.

The bubble shape c m be distorted from sphencal shape (as shown in Figure 4.21

(à)) due to fhe surface roughness and fhe duid shear force. In this case, the bubbie

interna1 pressure and surface force will rnainly determine the detachment mechanism.

Also the difference between the receding and advancing surface force (FS., - Fs.J will

balance the lifting force and buoyancy force so that there is a maximum size of bubble

which would be immobile in this circumstances. However. this approach is still in need

of further study, especially in the area of advancing-receding bubble contact angle in a

dense bubble layer in a flowing fluid. Moreover. it should be noted that the surface

tension and contact angle are dependent on surface electrolyte concentration and surface

(65.66) potential . It is also ~peculated'~~' that surface potential generates electrostatic forces

that add to the detachment force balance.

electrolyte flow

1 x - vector balance: I

t I y - vector balance: t

Figure 4.21 Force balance for a bubble attached on a vertical electrode in an upward flowing fluid.

4.3.3.2.2 Effect of current density

Figure 4.17 illustrates that the rnean bubble diameter increases slightly with the

cumnt density (from 100 to 250 mA/cm2) for electrodes with a smooth surface. Plate

electrodes produce bubbles with a Linear increase of 16 pm in bubble mean diameter

when the current density is increased within the above range. Smooth screens produce a

linear increase of 4 pm within the current density mge. The increase in bubble diameter

may be the result of more coalescence on the surface of the electrode.

For current density lower than 100 rnAkrn2. the ~itenture '~~' reported that mass

transfer is the nte detennining of bubble growth (equation 2.34). Some of the dissolved

.as is transferred to the bubble and rest is diffusrd to the bulk electrolyte. =

For current density higher than 100 mNcm2 litenture data from Westerheide and

~es t rnas te r '~~ ' were plotted in Figure 4.13. It is proposed that the current density

determines the rüte of bubble growth and hence influences bubble coaiescence on the

electrode surface. Due to the very high dissolved H2 gas supersaturation (150 times

larger than the solubility at 1 atm, ~hiba ta '~~ ' ) at the dense bubble growing sites, al1

dissolved gas is assumed to be transferred to the growing bubbles. Steady state condition

is required and there is no considention for the effect of the electrolyte Row.

Using Faraday's law and the Ided Ga law with pressure correction in the bubble

by the Kelvin-Laplace equation (eq. 2.20), a relationship between time (t) of the bubbles

on the electrode surface with bubble radius r at current density i can be obtained as

follows :

I l l

Figure 4.22 Bubble mean diameter as a funcdon of current density for various electrode geometries at flownte of 60 cm/s and collecter position of 50 mm.

1 O 0.01 0.02 0.03 0.04 1

i time (s)

Figure 4.23 Cornparison of bubble growth behavior according equation 4.1 with experimental data from Westerheide and ~ e s t w a t e r ' ~ ~ ' using I M sulfuric acid at 130 mA./crn2.

where: n = number of equivalent

F= Faraday constant

F(0) = contact angle function - equation 7.27

b = bubble density on the electrode surface

y = surface tension

PL = bubble surrounding Iiquid pressure

The equation was compared with experimentai data in Figure 4.23. It shows that the

relationship follows the data pretty well. In addition, this equation produces a growth

behavior, R = t" (not R a t"' for bubble growth govemed by mass transfer). The

expenmental results of Darby and que'^^' for an electrolytic bubble growth at

1000rn~/cm' also showed a t'" dependence.

The theoretical growth of hydrogeen bubble in K2COj at room tempenture and

pressure for different current densities using equation 4.1 is given in Figure 4.24. The

growth rate is faster at higher current density. which rneans more frequency of contact

between bubbles that leads to more coalescence on the surface before bubble departure.

This is consistent with the increase in mean bubble diameter with current density as

shown in Figure 4.22 for a smooth electrode.

By using a finite difference method, the derivative of radius function with time

(Figure 4.24 (a)) cm be estimated to obtain the growth n ie and growth acceleration as

shown in Figure 4.24 (b) and (c). The growth rate and the absolute growth acceleration

incretise with current density but decrease with time. From equation 2.35 and 2.36, it cm

be deduced that the drag force and liquid inenia force aiso increase with the current

density.

i O 1 2 3 4 1 Ume (s)

2 3 Ume (s)

t 2 3 4 tfme (s)

Figure 4.24 Theoretical relationship of tirne with: (a) radius; @) growth rate; and (c) growth acceleration at various cunent densities according equation 4.1.

In contrary to the electrodes with smooth surfaces, the bubble mean diarneter

produced from rough screen decreases with increase in curent density (Figure 4.23).

Surface roughness limits the bubble foot to expmd with bubble size so that the surface

tension force is independent of current density while the pressure. inertia, and buoyancy

forces (pulling forces) increase with current density. Thus for rough surfaces, bubbles

escape from the surface at smaller size at higher current density. Lt is also noted that for

these surfaces, the coalescence between bubbles on the surface does not occur as much

due to the lack of bubble mobility.

V. CONCLUSIONS

1. An apparatus to rnicroscopically visualize bubble evolution phenornena was

successfully constructed. This image analysis (TA) technique produced bubble size

distribution data at current densities up to 30 rn~/cm'.

2. For electrolysis in no extemal Row 1 M K2C03 electrolyte, using IA:

at current density lower than 10 m~/cm%bbbls nucleated at sites that caused

gas entrapment, such as screen junctions, pits, and grooves,

at current density higher than 40 w c m 2 bubbles nucleated over the entire

surface,

screen electrodes produced smaller bubble size than plate electrodes,

crystalline plate electrode produced smaller bubble size than morphous dloy

plate electrode.

3. A particle size analysis (PSA) appmtus was successfully used to directly mensure

bubble size distribution. The technique:

was verified for measunng bubble size distribution by cornparison with direct

image anaiysis and by testing of bubble size stability during cransponation from

the electrolysis ce11 to the detector ce11 in the apparatus,

has greater precision than image analysis and provides a large number of

measurements for data analysis.

4. For electrolysis in no extemal flow 1 M &CO3 electrolyte, using PSA:

an interface between bubbling zone and an expanded bubble bed was formed,

bubble population in the bubbling zone had a log-normal bubble size distribution,

bubble population in the expanded bubble bed had a bimodal distribution

comprising distributions from the bubble population at the bubbling zone and a

new bubble population formed by coaiescence in the bulk electrolyte .

5 . For electroiysis in Rowing 1 M K2COs electrolyte at a current density range of 100 to

250 mA/cm2. using PSA:

the hydrogen bubble population had a log-normal size distribution.

for the electrode materials tested:

- plate electrodes produced bigger mean bubble size and narrower size

dis tri butions than screen electrodes,

- rough screen electrodes produced bigger mean bubble size than smooth screen

electrodes,

- crystalline plate electrodes produced same bubble size distribution with

amorphous plate electrodes,

mean bubble size decreased with increase in electrolyte fiowrate and the decrease

was more prominent for plate electrodes than for screen electrodes,

mean bübble size increased with increase in current density for smooth screen,

crystalline plate, and amorphous alloy plate electrodes, but decreased for the

rough screen electrode.

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67. Coehn, A. and Neumann, Hm, 2. Phys., Vol. 20, p. 54, 1923.

68. Darby, R. and Haque, M. S., Chem. Eng. Sci.. vol. 28, p. 1129-1 138, 1973.

APPENDIX A: BUBBLE SIZE DISTRIBUTION FROM IMAGE ANALYSIS

CALCULATIONS

An image taken from the CCD videocamen was imported into the image

analyzer. A length calibntion based on the diameter of the wire (357 pn) as a standard

was used to provide a unit measurement. each bubble diameter was directly measured by

defining and circling each single bubble manually. Figure A.1. shows an image of

circled bubbles generated from a water electrolysis process conducted in I M KOH at

room temperature and pressure. The diameters were recorded in Table A.1 and were

imported into a MS Excel spreadsheet program for further bubble size distribution

analysis.

Figure A.1. Circled bubbles on image taken from CCD videocamera (1 M KOH, 10 mA/cm2, room temperature and pressure).

Table A.l Recorded Bubble Diameters (in micrometer) from a Single Frame.

Bubble size distribution was calculated according to the following steps:

the diameten were sorted in ascending order.

the volume of each bubble wris caiculated,

the total volume of bubbles was calculated (V,, = 230,548,425

the bubbles were grouped in the same size nnges used by Malvem Mastenizers-S

V3.15 softwm for ease of cornparison.

the average volume percentage of bubbles in diameter range of di to di+l was

calculated using the following forrnula:

where ZVd is the total bubble volume in the diameter range.

The cdculation results are presented in Table A.2.

Table A 2 . Bubble Size Distribution from a Single Frame.

1 upper diameter (pm) I

Five frames were used for each set of conditions (electrolyte type and current

density). The calculation results from these frames are shown in Table A.3. Figure A.2

shows the histograrn of these calculation results. The error bars represent two standard

devirttions from the calculated mean.

1 O0

bubble diameter (pn)

Figure A 2 Bubble size distribution histogram f'rom five images.

Table A 3 Bubble Size Distribution from Five Images.

Diam. (m) 8.996

10.4804 12.2096 1 4.2242 O O O O O O O

Frame 4 O O

0.061 647

Frame 3 O O

0.01 6559

Frame 5 O O

0.01 0097

Frame 1 O

0.002521 O

Average 1st-~ev Frame 2 O O

0.051 642

O 0.000504 0.027989

O 0.001127 0.027048

APPENDM B: PROPERTIES OF 1 M K2C03 ELECTROLYTE

Properties 1 M K2C03 (2S°C) 8 M KOH (25 O C ) 8 M KOH (70

Surface tension (~.m-')* 1 7.412 x 10'' 9.609 x 10'' 1 1

t

Saturated vapor pressure (Pa)* 1 3 181.742

* Frorn: "Properties of Aqueous Solutions of Electrolytes", Zaytsev, I., D., and Aseyev, G., G., Ed., CRC Press, 1992.

** From: Anthony, D., "Effects of Cyclic Current Modulation on Cathode Materials for the Hydrogen Evolution Reaction", M. A. Sc. Thesis, Deparnent of Metallurgy and Materids Science. University of Toronto, p. A-4, 1998.

1073.753 11259.01

APPENDIX C: IN'lXRNAL PRESSURE OF A HYDROGEN BUBBLE

1 M &Co3

0.001 0.01 0.1 1 1 O 100

radius (pm)

Figure C.l Interna1 pressure of a hydrogen bubble as a function of bubble radius.

Note: P, = partial pressure of gas in the bubble (kPa) P,, = partial pressure of vapor in the bubble (kPa) Pb = pressure of the bubble = P, + P,, &Pa) PL = pressure of sumounding liquid, assumed to be 1 atm (101.33 kPa)

APPENDUC D: PROPERTIES OF POPULATION A FOR SCREEN

ELECTRODES

srnooth screen A

j 50 1 00 150 200 250 300 1 1

! current density (mNcm2) I l

I

l

1 I

i :

7

Figure D.1 Volume percentage of population A from screen electrodes as a function ok (a) electrolyte flownte (at a current density of 250 rn~lcm') and (b) current density (at a flownte of 60 crnls).

15 225 30 37.5 45 5 2 5 60 67.5 75 '

electrolyte flowrate (cmls)

6 - 5 - 4 -

srnooth screen

tr 3

2 - 1 - O

rough screen

1 Xpopuiation A (smooai screen) ----------.....-- * - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1- - - - - - - - - - - - - - - - - - * - - * - - - - - - - - - - - - - - - - - - - - - - - - - - - . a - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - * - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -----------&-----s-----g-----g-----*-----------

i 15 22.5 30 37.5 45 52.5 60 67.5 75 electrolyte flowate (cmis)

(4

cunent denslty (rnffcm2)

Figure D.2 Bubble mean diameter for populations h m screen electrode as a function oE (a) electrolyte flowrate (at a cumnt density of 250 mA/cm2) and (b) current density (at a flowrate of 60 c d s ) .