Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable.

Physics E19Interfaces andEnergy Conversion

ZAE BAYERN

Bavarian Centre for Applied Energy Research

Division 1: Technology for Energy Systems and Renewable Energies

Towards an Efficient Conversion of Ethanol in Low Temperature Fuel Cells:

Ethanol Oxidation on Pt/Sn Catalysts and on Alkaline Medium Membrane Electrode Assemblies

Vineet Rao1, Carsten Cremers3, Rainer Bußar 1,2 and Ulrich Stimming1,2

1 Technische Universität München (TUM) , Department of Physics E19, James-Franck-Str.1, D-85748 Garching, Germany

2 Bavarian Center for Applied Energy Research (ZAE Bayern),

Walther-Meißner-Str. 6, D-85748 Garching, Germany

3 New address: Fraunhofer Inst Chem Technol, Dept Appl Electrochem, Pfinztal, Germany.

DPG Frühjahrstagung 2009, Arbeitskreis Energie (AKE)


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Motivation for Direct Fuel Cells (Direct FCs)

• The production of hydrogen from fossil fuels, such as natural gas, is connected with considerable losses in the overall efficiency of fuel cell systems;

• As yet, there is no widespread infrastructure for the distribution and storage of hydrogen;

• The energy density of hydrogen is lower than e.g. methanol or ethanol with respect to volume and weight;

• Ethanol is available as a renewable fuel from biomass;

• Direct fuel cell systems contain fewer components.


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60,0%

65,0%

70,0%

75,0%

80,0%

85,0%

90,0%

95,0%

100,0%

105,0%

h =

DG

0/D

H0

Aspects of Efficiency and Energy Density

• Ethanol is connected with a higher thermodynamic conversion efficiency η as compared to hydrogen;

• The energy density of ethanol is higher to the one of hydrogen.• Ethanol is less toxic than methanol: ‘as save as bear’ (as Bavarians say)


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•CO2 current efficiency for ethanol oxidation as a function of Potential, Temperature and Concentration;

•CO2 current efficiency dependent on intrinsic nature of catalyst experiments with Pt, PtSn and PtRu;

•CO2 current efficiency dependent on the catalyst loading and thus catalyst layer thickness:concept of resident time and active area;

• (CO2 current efficiency on alkaline membrane electrode assemblies.)

Outline of the presentation


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CH3--CH2OH

CH3--CHO

CH3--COOH CH3--COOC2H5

CO2

.CHad .COad

C2H5OH

CH4

Ethanol Oxidation Scheme

(m/z=44, m/z=22 double charged ions)(m/z=15)

(m/z=29, base peak)

Esterification (m/z=43, base peak)(m/z=61)

vacuumto MS

anodeoutlet

teflon discwith holes

detectionzylinder

microporousmembrane

o-ring

DEMS set-up


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DEMS on anodic ethanol oxidation – influence of temperature, potential and concentration on CO2 current efficiency (CCE)

CO2 current efficiency increases significantly with increasing temperature, decreases for anode potentials > 0.5 – 0.6V and decreases with increasing concentration.

0.4 0.5 0.6 0.7 0.8 0.9 1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

CO

2 cu

rre

nt e

ffici

en

cy

potential /V vs. RHE

1M Ethanol 0.1M Ethanol 0.01M Ethanol

T = 60 oC

V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.

This figure shows CO2 current efficiency vs. potential for different temperatures. MEA with Nafion 117 membane.

The anode feed is 0.1 M EtOH at 5 ml / minute.The approximate error limit is : ±10 %. 5 mg / cm2 metal loading using 40 % Pt / C.


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0 200 400 600 800 1000 1200

0.28

0 200 400 600 800 1000 1200

0

4

8

0 200 400 600 800 1000 1200

0

10

200 200 400 600 800 1000 1200

-0.8

-0.4

0 200 400 600 800 1000 1200

050

100150200

m/z = 61

I i / pA

potential(mV/RHE)

m/z = 15

I i / pA

m/z = 29

I i / pA

I i / pA

m/z = 22

I F /

mA

This figure shows CV and MSCV for m / z = 22, 29,15 and 61.The anode feed is 1 M EtOH at 5 ml/minute at 30 0C.scan rate is 1 mV / s.

0 200 400 600 800 1000 1200

3,6

4,2

4,80 200 400 600 800 1000 1200

7,2

7,4

0 200 400 600 800 1000 1200

-3,926

-3,925

0 200 400 600 800 1000 1200

0

50

m/z= 15I i /

pA

potential(mV/RHE)

m/z= 29

I i / p

AI i /

nA

I F /

mA

m/z= 22

This figure shows CV and MSCV for m / z = 22, 29 and 15.The anode feed is 0.1 M EtOH at 5 ml/minute at 30 0C.scan rate is 5 mV / s.

CO2 CO2

CH4

CH3-CHO

Ester

CH3-CHO

CH4


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Effect of catalyst layer thickness or catalyst loading

0.4 0.5 0.6 0.7 0.8 0.90.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

CO2 efficiency at 90 oC

0.1 M EtOH, 5 ml/min flow rate40%Pt/C

CO

2 c

urr

en

t effi

cie

ncy

potential /V vs. RHE

0.20mg/cm2

0.25mg/cm2

0.80mg/cm2

2.45mg/cm2

4.20mg/cm2

8.00 mg/cm2

0 1 2 3 4 5 6 7 8 90,0

0,2

0,4

0,6

0,8 40% Pt/CCO

2 current efficiency

at 90 oC, 0.1MEtOH, 0.6V/RHE

CO

2 c

urr

en

t effi

cie

ncy

Platinum loading(mg/cm2)

Role of resident time and active surface area

Loading increases


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Fuel cell: Convective + diffusive systemC2H5OH+H2O H2



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Resident time: Average time spent by the reactant molecules in the reactor

Active surface area: area where electrochemical reactions can take place

Effect of catalyst layer thickness or catalyst loading


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Anodic ethanol oxidation – Effect of chemical composition of catalyst

Faradic currents for ethanol oxidationare similar at PtSn/C and PtRu/C

At PtRu/C practically no CO2 is formed!



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Acetic acid electro-oxidation on Pt and 20wt%PtSn(7:3)/C

Acetic acid is resistant to electro-oxidation on Pt

0 200 400 600 800 1000 1200

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

curr

en

t(m

A)

Potential /mV vs. RHE

4.3mg/cm2 Pt

2 mg/cm2 20% PtSn(7:3)/C

70 oC0.1M Acetic Acidscan rate:5mV/s

This rules out acetic acid as an intermediate for CO2 formation


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Acetaldehyde electro-oxidation

0 200 400 600 800 1000 1200

0

50

100

150

0 200 400 600 800 1000 1200

0.0

0.5

1.0

1.5

2.0

2.5

3.0

curr

ent(

mA

)

0.1M acetaldehyde

90 oC,5ml/minute5mV/s

40%Pt/C,8mg/cm2 Pt

I m/z

=22

(pA

)

potential /mV vs.RHE

0,5 0,6 0,7 0,8 0,90,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9 0.1M acetaldehyde

90 oC,5ml/minute

40%Pt/C,8mg/cm2 Pt

CO

2 cu

rre

nt e

ffic

ien

cy

potential(V)

CO2 current efficiency

Faradaic current and CO2 current efficiency for acetaldehyde electro-oxidation are high enough to justify acetaldehyde as an intermediate for EOR


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Discussion about mechanism of EtOH oxidation

CH3-CH2-OH

CH3-CHO

CH3-COOH

CHads ,COads

CO2

negligible

86%75%

14%

8mg/cm2 Pt,40%Pt/C, T= 90°C,

0.1M EtOH, 0.1MAcetaldehyde


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• CO2 current efficiency for ethanol oxidation reaction (EOR)

depends strongly on potential, temperature and concentration;

• Catalyst layer thickness and electrochemical active area also affects CO2 current efficiency strongly;

• Intrinsic nature of catalyst is important: PtRu(1:1) exhibits low CO2 formation (CO2-efficiency);

• PtSn(7:1) catalysts shows more complete oxidation;

• In fuel cell active area and resident time is important for the completeness of oxidation;

• (Ethanol oxidation is more complete on alkaline membrane electrode assemblies.)

Conclusions / Summary


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Planned activities

• Identification of a potentially synergy between PtSn and PtRu and thus a structured catalyst layer

• Combination of supported PtRu and PtSn catalysts within a catalyst layer;

• Optimization of flow field geometry depending on catalyst layer structure.

AnodeAnode Cathode

catalyst layer ‚structured‘ catalyst layer with PtRu and PtSn

PtRu/C PtSn/C

or

PtRu/CPtSn/C

Variation of:sequenz of layerscatalyst loading


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• We thank Prof. Dr. Gong-Quan Sun and Dr. Lei Cao, Dalian Institute of Chemical Physics (DICP) in Dalian, PR-China, for providing catalyst samples.

• We acknowledge financial support from Sino-German Center for Science Promotion, Beijing under contract GZ 211 (101/11) and German Research Foundation (DFG) under contract Sti 74/14-1

Acknowledgements

Vielen Dank für Ihr Interesse!

DPG Frühjahrstagung 2009, Arbeitskreis Energie (AKE)

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable.

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