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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)
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
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.
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
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)
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
•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
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
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
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
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.
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
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
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
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
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
Fuel cell: Convective + diffusive systemC2H5OH+H2O H2
V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.
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
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
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!
V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
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
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
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
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
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
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
• 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
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
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
Physics E19Interfaces andEnergy Conversion
ZAE BAYERN
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems and Renewable Energies
• 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)