Investigation of the Catalytic Activity of Plasma-Treated Fe, Ni, and Co Foil for Water Splitting Nick Lavrov, Olivia Watson.

Investigation of the Catalytic Activity of Plasma-Treated

Fe, Ni, and Co Foil for Water Splitting

Nick Lavrov, Olivia Watson

Introduction• Increasing evidence for climate change,

increasing cost of fossil fuels, and diminishing natural resources motivate today’s alternative energy research.

• An ideal alternative energy would be cost-effective and renewable.

• Our project focuses on storing solar energy in chemical bonds, using the body of research in surface science and electrochemical catalysis as our platform.

2

Introduction• A common mechanism used to store

energy from sunlight is called the water splitting reactionH2O O2 + H2 (E0 =1.23 V)

• Reactive groups called oxides and (oxy)hydroxides can improve the efficiency of oxygen production

• Plasma (ionized gas) can be used to grow thin layers of these materials on metal electrode surfaces

3

Sample Processing Procedure● Measure, cut and clean foils by sonication in

acetone, ethanol, and water, then load and ground sample.

● Turn on water cooling, leak in Ar gas, and ignite plasma to sputter.

● Main treatment:○ Leak in Ar or O2, then H2O○ Heat sample with resistive heating.○ Turn up ion energy and treat for 5-15

minutes.● Break vacuum with N2 gas between runs.

4

Electrochemistry•Electrolyte solution was a solution

of 1M NaHCO3 (NaHCO3 solid

dissolved in DI water).•Sample mounted as the working electrode, platinum coil counter electrode,Ag/AgCl (4 M KCl) reference electrode•Active area of samples marked off using chemically-resistant tape from 3M

5

Results

• For each foil, we implemented some of the following characterization techniques:

Linear Sweep Voltammetry Raman Spectroscopy X-ray Photoelectron Spectroscopy (XPS) Scanning Electron Microscopy (SEM)

6

Linear Sweep Voltammetry• Current at working electrode

is measured while potential between the working electrode and the reference electrode is swept linearly in time.

7

Tafel Plot• Voltammetry data is

converted to fit the Tafel equation.

• From the graph we can extract the Tafel slope and exchange current density parameters.

8

Raman Spectroscopy• “Fingerprinting” method to identify

the unique inelastic scattering of monochromatic light by chemical bonds

• Instrument contains optical microscope, useful for initial assessment of changes in surface appearance

9

X-ray Photoelectron Spectroscopy (XPS)

• Analysis technique that focuses X-ray beam on a surface to eject electrons

• Energy of ejected electron gives information about how tightly it was bound to its atom

• Every atom has a specific corresponding pattern of electrons at various energy levels

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Scanning Electron Microscopy• Special type of microscope that

focuses an electron beam on a surface to give a topographical image of a sample

• SEM has many advantages over light microscopes including: greater depth of field, allow more of a surface image to be in focus at once; very high resolution; very high magnification

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Fe Foil Color Changes• At RT, unchanged• At 200°C and 250°C, gold• At 300°C and 450°C, blue• Tempering colors due to thin film interference of

iron oxide.

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13

1.7 1.75 1.8 1.85 1.9 1.95 20E+00

1E-05

2E-05

3E-05

4E-05

5E-05

6E-05

7E-05

8E-05

0kV, 450C

0.1kV, 150C

BLANK

2kV, 250C

2kV, 250C

1kV, 20C

Iron Linear Voltammetry

Volts vs RHE

Cu

rren

t D

ensi

ty (

A/m

m2)

14

15

SEM: Fe surfaces

50 000x ; 5x5 μm• Very strong morphological differences between untreated Fe and

hematite; hematite grain size ~160 nm.• No obvious morphological difference in magnetite. 16

Untreated Fe Hematite Magnetite

1.6 1.7 1.8 1.9 20E+00

1E-05

2E-05

3E-05

4E-05

5E-05

6E-05

7E-05

8E-05

9E-05

1E-04

1kV

Nickel Linear Voltammetry

Volts vs RHE

Cu

rren

t D

ensi

ty (

A/m

m2)

17

SEM: Ni surfaces

• No obvious morphological differences

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Untreated Ni25 000x 10x10 μm

Treated Ni20 000x 13 x 13 μm

Ni: X-ray Photoelectron Spectroscopy

oxide

XPS data confirms an increasing presence of oxidized Ni with samples treated

with O2/H2O plasma. An increased XPS oxide peak corresponds to a small

cathodic shift in water oxidation onset potential.19

1.4 1.5 1.6 1.7 1.8 1.9 20

0.00002

0.00004

0.00006

0.00008

0.0001

0.00012

0.00014

1.6 kV

1.4 kV

1.2 kV

1.0 kV

0.5 kV

Cobalt Linear Scan (300C)

Volts vs RHE

Cu

rre

nt

De

ns

ity

(A

/mm

2)

20

-6.5 -6 -5.5 -5 -4.5 -4 -3.50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1.6

1.4

1.2

Cobalt Tafel Plot (300C)

Log Current Density

Ov

erp

ote

nti

al (

V)

21

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

-10

-9.5

-9

-8.5

-8

-7.5

-7

-6.5

-6

Exchange current density (I0) vs Ion Energy

Ion Energy (kV)

Lo

g E

xc

ha

ng

e C

urr

en

t D

en

sit

y (

I0)

22

0.4 0.6 0.8 1 1.2 1.4 1.6 1.80

0.05

0.1

0.15

0.2

0.25

Tafel slope (A) vs Ion energy

Ion Energy (kV)

Ta

fel S

lop

e (

V/d

ec

ad

e)

23

Co: Raman Spectroscopy

expected peaks at 191, 470, 510, 608, 675 cm-124

Co: Raman Spectroscopy

Co sample 5Co blank Co sample 4

Images taken at 10x magnification during Raman spectroscopy25

SEM: Co surfaces

There are many scattered grains across the surface of sample 4, compared to sample 5, which has none.

Co sample 425,000 x10 x 10 µm

Co sample 525,000 x10 x 10 µm

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SEM: Co surfaces

There are not any obvious morphological differences between the two Co samples at a higher magnification.

Co sample 450,000 x5 x 5 µm

Co sample 550,000 x5 x 5 µm

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Conclusions

• Plasma treatment seems to be localized to surface and thus too thin to be characterized with Raman spectroscopy

• Plasma treatment is complex, with many parameters to optimize

• Future experiments would involve exploring other regimes of plasma treatment including increasing sputtering time, hydrogen plasma, chamber pressure.

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Appendix

Gen2 Tectra® Plasma Source

● Microwave plasma source with magnets at 0.0875T to enhance plasma by ECR.● Hybrid mode.● One positive grid to accelerate ions to grounded sample by controlling ion energy.● High vacuum chamber kept at 10^(-6) Torr.

FARADAY CUP● Aperture of 3mm^2● Biased to 54V● Connected in series to picoammeter to

measure current at sample location in relation to ion energy and pressure.

● Materials: stainless steel, sapphire ceramic, tantalum wire.

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Ion Beam Characterization ● Variables:

○ Gas: argon, oxygen, water vapor○ Gas mixture○ Partial and total pressure ( 0 - 1.0 mTorr )○ Ion Energy

Ar (mTorr) O2 (mTorr) H2O (mTorr)

1:1 0.46 0.45 0.42

2:3 0.46 0.3 0.75

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Pure Plasma ResultsSputtering Conditions:- 1 kV- 10 minutes- 0.46 mTorr- Room Temperature- Used water contactangle to confirm surface cleaning.

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Mixed Plasma Ion Flux

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