Transcript Document

Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
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 E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
Motivation for Direct Fuel Cells (Direct FCs)
ZAE BAYERN
• 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 E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Aspects of Efficiency and Energy Density
105,0%
100,0%
0
0
h = D G /D H
95,0%
90,0%
85,0%
80,0%
75,0%
70,0%
65,0%
f
l
Ko
hl
e
ns
to
f
no
Et
ha
l
ha
no
an
Pr
op
ha
n
M
et
M
et
W
as
se
rs
t
of
f
60,0%
• 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 E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Outline of the presentation
•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.)
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Ethanol Oxidation Scheme
teflon disc
with holes
microporous
membrane
CH3--CH2OH
to MS
vacuum
detection
zylinder
CH3--CHO
(m/z=29, base peak)
o-ring
anode
outlet
.CHad .COad
DEMS set-up
C2H5OH
CH3--COOH
CH3--COOC2H5
Esterification
CH4
(m/z=15)
CO2
(m/z=43, base peak)
(m/z=61)
(m/z=44, m/z=22 double charged ions)
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
DEMS on anodic ethanol oxidation – influence of temperature, potential
and concentration on CO2 current efficiency (CCE)
1.0
o
30 C
o
60 C
o
90 C
0.8
1M Ethanol
0.1M Ethanol
0.01M Ethanol
0.7
0.6
CO2 current efficiency
CO2 current efficiency
o
T = 60 C
0.6
0.4
0.2
0.5
0.4
0.3
0.2
0.1
0.0
0.0
0.4
0.5
0.6
0.7
0.8
potential /V vs. RHE
0.9
1.0
0.4
0.5
0.6
0.7
0.8
0.9
1.0
potential /V vs. RHE
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.
CO2 current efficiency increases significantly with increasing temperature, decreases for
anode potentials > 0.5 – 0.6V and decreases with increasing concentration.
V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.
200
150
100
50
0
Ii / pA
400
600
IF / mA
200
800 1000 1200
0
CO2
-3,925
20
0
200
400
600
800 1000 1200
m/z = 29
CH3-CHO
7,4
200
400
600
800 1000 1200
m/z = 15
CH4
4
0
Ii / pA
0
Ii / pA
400
600
800 1000 1200
CO2
m/z= 22
0
0
200
400
600
800 1000 1200
m/z = 61
Ester
0.28
0
200
400
600
400
600
800 1000 1200
potential(mV/RHE)
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.
800 1000 1200
CH3-CHO
7,2
200
400
600
4,8
Ii / pA
0
200
m/z= 29
0
Ii / pA
200
-3,926
10
8
ZAE BAYERN
0
m/z = 22
-0.8
Ii / pA
Division 1: Technology for Energy Systems
and Renewable Energies
50
0
-0.4
Bavarian Centre for Applied Energy Research
Ii / nA
IF / mA
Physics E19
Interfaces and
Energy Conversion
800 1000 1200
m/z= 15
4,2
CH4
3,6
0
200
400
600
800 1000 1200
potential(mV/RHE)
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.
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Effect of catalyst layer thickness or catalyst loading
Loading increases
2
o
0.20mg/cm
2
0.25mg/cm
2
0.80mg/cm
2
2.45mg/cm
2
4.20mg/cm
2
8.00 mg/cm
CO2 efficiency at 90 C
0.1 M EtOH, 5 ml/min flow rate
40%Pt/C
0.8
0.7
0.5
0.4
0.3
0.2
40% Pt/C
o
at 90 C, 0.1MEtOH, 0.6V/RHE
0,6
CO2 current efficiency
CO2 current efficiency
0.6
CO2 current efficiency
0,8
0,4
0,2
0.1
0.0
0,0
0.4
0.5
0.6
0.7
potential /V vs. RHE
0.8
0.9
0
1
2
3
4
5
6
2
Platinum loading(mg/cm )
Role of resident time and active surface area
7
8
9
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Fuel cell: Convective + diffusive system
C2H5OH+H2O
V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.
H2
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Effect of catalyst layer thickness or catalyst loading
Resident time: Average time spent by
the reactant molecules in the reactor
Active surface area: area where
electrochemical reactions can take
place
V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Anodic ethanol oxidation –
Effect of chemical composition of catalyst
Faradic currents for ethanol oxidation
are similar at PtSn/C and PtRu/C
2
Pt unsupported(4.3mg/cm )
2
20 % PtSn/vulcan(2mg/cm )
2
20 % PtRu/vulcan(2.4mg/cm )
0.5
110
o
70 C
0.1M EtOH
100
current(mA)
80
70
60
50
o
70 C
0.1M EtOH
40
30
2
Pt unsupported(4.3mg/cm )
2
20 % PtSn/vulcan(2mg/cm )
2
20 % PtRu/vulcan(2.4mg/cm )
20
10
CO2 current efficiency
0.4
90
0.3
0.2
0.1
0.0
0
0.4
0.5
0.6
0.7
potential /V vs. RHE
0.8
0.9
0.4
0.5
0.6
0.7
0.8
0.9
potential /V vs. RHE
At PtRu/C practically no CO2 is formed!
V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Acetic acid electro-oxidation on Pt and 20wt%PtSn(7:3)/C
2
4.3mg/cm Pt
2
2 mg/cm 20% PtSn(7:3)/C
12
10
8
6
4
current(mA)
2
0
-2
-4
-6
-8
-10
-12
o
70 C
0.1M Acetic Acid
scan rate:5mV/s
-14
-16
-18
0
200
400
600
800
1000
1200
Potential /mV vs. RHE
Acetic acid is resistant to electro-oxidation on Pt
This rules out acetic acid as an intermediate for CO2 formation
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Acetaldehyde electro-oxidation
100
0.1M acetaldehyde
o
90 C,5ml/minute
5mV/s
2
40%Pt/C,8mg/cm Pt
CO2 current efficiency
0,9
0
3.0
0
200
400
600
800
1000
1200
2.5
Im/z =22(pA)
0.1M acetaldehyde
o
90 C,5ml/minute
2
40%Pt/C,8mg/cm Pt
0,8
50
2.0
1.5
CO2 current efficiency
current(mA)
150
0,7
0,6
0,5
0,4
0,3
1.0
0,2
0.5
0.0
0,1
0
200
400
600
potential /mV vs.RHE
800
1000
1200
0,5
0,6
0,7
0,8
0,9
potential(V)
Faradaic current and CO2 current efficiency for acetaldehyde electrooxidation are high enough to justify acetaldehyde as an intermediate for
EOR
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Discussion about mechanism of EtOH oxidation
CH3-CH2-OH
CH3-CHO
CHads ,COads
negligible
14%
CH3-COOH
CO2
75%
86%
8mg/cm2 Pt,40%Pt/C, T= 90°C,
0.1M EtOH, 0.1MAcetaldehyde
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Conclusions / Summary
•
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.)
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Planned activities
Anode
•
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.
Cathode
Anode
PtRu/C PtSn/C PtSn/C PtRu/C
or
catalyst layer
Variation of:
sequenz of layers
catalyst loading
‚structured‘ catalyst layer with PtRu and PtSn
Physics E19
Interfaces and
Energy Conversion
Bavarian Centre for Applied Energy Research
Division 1: Technology for Energy Systems
and Renewable Energies
ZAE BAYERN
Vielen Dank für Ihr
Interesse!
Acknowledgements
• 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
DPG Frühjahrstagung 2009, Arbeitskreis Energie (AKE)