A Novel, Low-Cost, High-Efficiency
Anode Catalyst for Alkaline Fuel Cells
Noramalina Mansora, Rhodri Jervisa, Tom Masona, Christopher Gibbsb, Dan Bretta
a
Centre for CO2 Technology, Chemical Engineering, UCL, Torrington Place, London WC1E 7JE, UK
CMR Fuel Cells, Hartson Mill, Hartson, Cambridge, CB22 7GG
Email: noramalina.mansor.09@ucl.ac.uk
b
INTRODUCTION
LEVICH-KOUTECKY ANALYSIS
The alkaline fuel cell is among the most efficient fuel cells, with a number of
advantages, such as low activation overpotential, small system cost and
potential use of non-precious metals for the cathode. However, the hydrogen
oxidation reaction (HOR) of platinum is several orders of magnitude slower in
alkaline compared to acid electrolytes1. Therefore the use of platinum as an
anode catalyst would require high loadings and thus increasing the cost.
The overall current i is related to ilim according to Levich-Koutecky equation,
from which the kinetic current ik at different potentials can be extracted from
the intercept of i-1 versus ω-1/2:
1 1 1
1
1
= +
= +
1
i ik ilim ik
Kω 2
3.0
400
600
900
1200
1600
2.5
1.0
-1
0.5
-1
4000
3000
0.0
2000
-1.0
0.0
O2
0.1
0.2
0.3
0.4
0.025
Rotating disk-electrode (RDE) is a powerful technique to investigate the
electrocatalytic activity of a fuel cell catalyst under fuel cell relevant
conditions without interference from factors common in fuel cell testing such
as mass transport resistances and water management effects.
From the HOR polarisation curve, Tafel analysis is used to extract two
important kinetic parameters: the exchange current density i0 and the Tafel
slope b.
-0.6
-0.8
Pt/C
CMRanode
-1.0
-1.2
-2
E vs RHE (V)
1000
2000
50
100
150
200
Figure 4: Tafel plot derived from the HOR
polarisation curves
92
97
2.08E-03
2.79E-03
0.51
CMRanode
0.54
Table 1: Summary of Tafel slope, exchange current density and α for Pt/C and CMRanode
CARBON MONOXIDE TOLERANCE
Carbon monoxide (CO) stripping
voltammetry shows that CO binding
energy is stronger for CMRanode.
I (mA)
0.00
-0.05
-0.10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
E vs RHE (V)
Figure 5: Cyclic voltammetry , after CO
saturation, in 1M KOH at room temperature
0.25
0.30
0.35
0.40
)
metal
Figure 9: Electrocataytic activity per mass of
catalyst
550
PEM MEA
AAEM Pt/Pt
AAEM CMR
0.8
0.7
0.6
0.5
0.4
1.0
Activation Region
0.3
0.2
0.8
0.1
Pt/C
0.05
0.20
Single-cell measurement was limited by the formation of carbonates from
contact with carbon dioxide in the air, reacting with hydroxide ions in the
alkaline membrane and thus reducing the conductivity. However, the
activation region on the single-cell polarisation curve is comparable to a PEM
single-cell with a conventional platinum anode.
0.9
Electron transfer
Coefficient, α
0.75 V
0.15
SINGLE-CELL MEASUREMENT
-2.2
Exchange current
density (mA cm-2)
0.70 V
0.10
-1
Figure
8:
Electrocatalytic
activity
per
electrochemical surface area (ECSA) of electrode
1.0
Tafel slope
(mV dec—1)
Pt/C
CMRanode
0.05
Mass Activity (A mg
1.1
E vs RHE (mV)
0.10
0.00
0.00
5000
)
ECSA
1.2
0.4
0.15
4000
0.05
-2
0
0.0
Catalyst
3000
Specific Activity (µA cm
-2.0
E vs RHE (V)
Figure 3: HOR polarisation curves at 1600rpm and
room temperature in 1M KOH electrolyte. Catalyst
loading: 35 µg cm-2
0
-1.8
-2.4
0.2
0.05
0.10
-1.6
-1
0.0
0.050
However, the percentage of surface
sites poisoned by CO is 73% compared
to 100% in platinum. This leads to sites
open for hydrogen adsorption and
subsequent oxidation for CMRanode.
0
100
200
300
400
20
40
500
60
600
80
100
700
450
400
Stream Switched
to Air
350
300
250
200
900
1000
0
-2
20
40
60
80
100
120
140
160
180
200
Time (min)
Current Density (mA cm )
Figure 10: Single-cell polarisation curve of:(i) PEM
MEA: Nafion membrane Pt anode (ii) AAEM Pt/Pt:
Alkaline membrane with Pt anode (iii) AAEM CMR:
Alkaline membrane with CMRanode
Air
N2
150
120
800
500
2
0
-1.4
0.10
Resistivity (mΩ cm )
1
0.045
)
Pt/C
CMRanode
0.00
Potential (V)
log jk (A cm )
-2
j (mA cm )
2
Pt/C
CMRanode
E vs RHE (V)
Figure 2: Basic operations of an alkaline fuel cell
Pt/C
CMRanode
-1/2
0.15
CATHODE
O2 + 2H2O +4e4H2O + 4e-
HOR POLARISATION CURVE
3
0.040
(rpm
Figure 7: Levich-Koutecky plots at 0.025V–
0.125V vs RHE
0.15
Figure 1: Transmission Electron Microscopy
(TEM) image of CMRanode catalyst on
carbon support nanoparticles
0.035
-1/2
ω
The electrocatalytic activity of the catalyst is determined by normalising its
kinetic currents (ik) to its electrochemical surface area (ECSA) and loading on
the electrode:
H2O
ANODE
2H2 + 4OH4H2O + 4e-
0.030
E vs RHE (V)
Figure 6: HOR polarisation curves of CMRanode in
1M KOH at rotation rates 400-1600rpm. Catalyst
loading : 35 µg cm-2.
OH-
H2O
5000
1.5
-0.5
e-
e-
H2
-2
e-
e-
0.025V
0.05V
0.075V
0.1V
0.125V
6000
I (A )
e-
e-
j (mA cm )
We have synthesised a novel and non-platinum commercial anode catalyst
and evaluated its performance in an alkaline environment using
electrochemical and single-cell measurements.
2.0
Figure 11: Change in resistivity of commercial
alkaline membrane on contact with air
CONCLUSION
• A novel, low cost anode catalyst for alkaline fuel cells was synthesised and
characterised using RDE and single-cell measurements.
• Initial testing shows its electrocatalytic activity is equivalent to that of
platinum.
• Future work includes varying catalyst formulation and evaluating its
durability.
Reference
1W. Sheng, H. Gasteiger and Y. Shao-Hern, J. Electrochem. Soc., 157, B1529-B1536 (2010)
Centre for CO2 Technology