Journal of New Materials for Electrochemical Systems 10, 143-145 (2007)
© J. New Mat. Electrochem. Systems
Study of Fuel Cell Corrosion Processes Using Dynamic Hydrogen Reference Electrodes
*
Michael V. Lauritzen, Ping He, Alan P. Young, Shanna Knights, Vesna Colbow, and Paul Beattie
Ballard Power Systems, 9000 Glenlyon Parkway, Burnaby, BC, CANADA
Received: May 22, 2006, Accepted: January 4, 2007
Abstract: Dynamic hydrogen reference electrodes (DHREs) were incorporated into an operating Ballard® Mark 9 fuel cell. Localized
anode and cathode electrode potentials were measured against these references during normal operation, fuel dilution, and anode gas
exchange events. Clear differences were observed between electrode potentials near the anode inlet and anode exhaust. Certain conditions
generated localized corrosive electrochemical potentials in excess of +1.5V (vs. DHRE) on the cathode with a corresponding increase in
CO2 levels measured in the cathode exhaust.
Keywords: PEMFC, Fuel Cell, Corrosion, Reference Electrode, Fuel Starvation
Ballard GDE prior to bonding, and further isolated from the fuel
cell environment by encapsulating the entire assembly with additional polyimide (Figure 1). For the current study, reference electrodes were situated near the fuel inlet and outlet (Figure 2) of the
Ballard Mark 9 unit cell. Redundant DHREs were added on the
opposing membrane faces to provide a measure of stability. H2
was supplied to the fuel cell anode, a N2 / O2 blend to the cathode
(for a low CO2 baseline), and flow rates were measured via Hastings mass-flow meters. Each DHRE was driven by a 100 uA current from a PAR Model 263A Potentiostat / Galvanostat, and localized electrode potentials were measured against the most proximal DHRE cathode using a Eurotherm Model 4250M data recorder. A California Analytical Instruments ZRH infra-red analyzer monitored the cathode exhaust gas for CO2 evolution.
1. INTRODUCTION
Corrosion due to an inhomogeneous anode gas composition (e.g.
cathode to anode air leakage) is a known source of performance
and material degradation in phosphoric acid fuel cells (PAFCs) [13]. Similar corrosion processes have been associated with degradation in PEMFC systems during startup and shutdown procedures[4]. Historically, PAFC fuel cells were studied during air and
fuel starvation events via reversible hydrogen electrodes
(RHEs)[2] located around the perimeter of the MEA and embedded into customized flow field plates. As opposed to previously
reported, the present work demonstrates clear measurement of
these corrosion phenomena, in an operating PEMFC, via DHREs
situated in a non-catalyzed region interior to the active area of an
operating cell[5-7]. The reference electrodes are readily incorporated into any size stack, thus accelerating stress test and operating
protocol development. The DHRE design is based on Giner’s
original concept[8] and reported improvements[9,10] to increase
the stability of the reference potential.
2. EXPERIMENTAL
DHREs were constructed using 0.045 mm Pt and Pd wires (Pd
as the DHRE cathode) and platinized using a 2 mM H2PtCl6 solution in 0.5 M H2SO4. The wires were spaced 4 mm apart and insulated from the gas diffusion electrodes (GDEs) with polyimide
film (0.025 mm). Each reference electrode was applied to the
membrane through a 15 mm window removed from the 300 cm2
Figure 1. Cross-section of reference electrode incorporated into an
MEA, and individual electrode potential monitoring.
*To whom correspondence should be addressed: [email protected]
FAX: 604-453-3782
143
144
Michael V. Lauritzen et al. / J. New Mat. Electrochem. Systems
Table 1. Electrode potentials measured vs. the DHREs for a cell under varying reactant composition.
Cathode (V)
Anode (V)
vs. anode DHRE
vs. cathode DHRE
Cell (V)
Anode
vs. anode DHRE
vs. cathode DHRE
air
H2
0.956
0.978
-0.023
-0.002
0.981
N2
H2
0.072
0.107
-0.034
0.000
0.107
air
N2
1.008
1.011
0.999
1.001
0.009
350
1.6
Ecathode (V), Fuel Stoichiometry
1.4
Fuel Stoich
300
Ecathode In
1.2
250
1.0
Cell Voltage
200
0.8
Ecathode Out
Carbon Loss Rate
Eanode Out
0.6
0.4
150
Carbon Dioxide (ppm)
Gas Composition
Cathode
100
0.2
Eanode In
50
0.0
-0.2
0
20
40
60
80
100
120
0
140
Time (min)
Figure 2. Schematic of completed MEA/reference electrode assembly.
Figure 3. Cell voltage and electrode potential responses to a slow
controlled decrease in fuel stoichiometry.
3. RESULTS AND DISCUSSION
fuel cell that is corroding. With an increase in the fuel flow beyond the critical stoichiometry, it is logical (and has been confirmed with current mapping results, not shown) that the area of
the fuel starved / corroding region would decrease. This delay
observed between the cathode potential and the CO2 evolution (lag
time accounted for), also indicates that the potential measured via
the reference electrode may be highly localized (and perhaps
represents only the catalyst in closest proximity), but further work
is required to determine the extent of the electrode that is visible
using this technique. A somewhat related mechanism explaining
the driving forces for these corrosion phenomena has been proposed in the literature[1], and is adapted and expanded upon here
for clarity (Figure 4).
Under normal operation, the abundant supply of hydrogen and
air to the fuel cell establishes environments of largely H2 on the
anode, O2/N2 on the cathode, with gas crossover rates having little
effect on the local (down-the-channel) reactant composition. As
the fuel supply is reduced to a critical level, regions develop
within the fuel cell (typically near the anode exhaust) where the
local hydrogen concentration falls. This reduces the (Pt-catalyzed)
anode H2/O2 recombination rate, thus allowing local anode O2 and
N2 concentrations to increase (due to their permeating through the
membrane from the cathode). With a continued reduction in the
fuel supply, the anode potentials move from ~0.1V DHRE to approach the open-circuit cathode potential (~1.2 V vs. DHRE) as
the local oxygen content approaches that being supplied via the
cathode (verified by gas chromatography) and the majority of the
current distributes away from the starving region (data not
shown). With the flow field plates being effective conductors, the
cell voltage generated by the “normal” portion of the cell is applied to the fuel starved region (now an air/air system, and acting
more like a resistor due to the lack of H2). As a result, the local
cathode potential is driven to a high value, more specifically, to
approach the sum of the local anode electrochemical potential and
Ex-situ, the DHREs were typically 0.092 ± 0.032 V lower than
an NHE in a 0.5 M H2SO4 solution under air, O2, N2, and H2 environments. In-situ, only a 20 to 35 mV difference was observed for
potentials measured against anode or cathode situated DHREs
(Table 1) at open circuit. Table 1 also shows that, when the cell is
supplied with N2/H2 (cathode/anode), the cell voltage is low
(0.107 V), and the cathode potential is low, due to H2 crossover.
On the other hand, with air/N2 supplied to the cell, O2 diffuses
through the membrane, increases the anode potential to 0.999 V,
and develops the low cell voltage of 0.009 V. In other testing, the
DHRE provided a stable reference point under various current
loads, provided the fuel cell received sufficient reactant humidification.
Incorporating multiple DHREs into a single MEA is particularly
effective in determining when reactants are not supplied to the
fuel cell in sufficient quantity. Figure 3 illustrates the response
(and the change in the cathode CO2 evolution rate) to a slow fuel
supply decrease with a load of 0.018 A cm-2. As the fuel flow
decreases below a critical value, the electrode potentials at the fuel
outlet sharply rise while the inlet potentials and overall cell voltage are largely unaffected. Once the cathode potential exceeds
~1.0V (DHRE) Pt-catalyzed oxidation of the catalyst support accelerates, and CO2 is detected. Following 20 minutes of operation
at the lowest fuel stoichiometry, the fuel flow was slowly increased. A few minutes later, the rate of CO2 evolution is seen to
peak, then decrease, and a few minutes after that, the measured
anode and cathode potentials rapidly return to more normal levels
as the corrosion front retreats towards the fuel outlet and passes
the reference electrode. This result remains consistent with theory, as the CO2 content of the anode exhaust is not simply determined by the electrochemical potential at the location monitored
via the reference electrode, but is also a function of the area of the
Study of Fuel Cell Corrosion Processes Using Dynamic Hydrogen Reference Electrodes / J. New Mat. Electrochem. Systems
4. CONCLUSION
Vcell
1.2
Ecathode
Vcell
Eanode
0
x
C(s) + 2H2O Æ CO2(g) + 4H+ + 4e2H2O Æ O2(g) + 4H+ + 4e-
e-
O2 + 4H+ + 4e- Æ 2H2O
O2/N2
O2/N2
H+
H+
H2
O2/N2
-
e
2 H2 Æ 4H+ + 4e-
4H+ + 4e- Æ 2 H2
O2 + 4H+ + 4e- Æ 2H2O
1.6
1.4
ECathode,
Fuel Out
1.2
1.2
ECathode, Fuel In
1.0
1.0
0.8
0.8
Cell V
0.6
0.6
0.4
0.4
EAnode, Fuel Out
0.2
0.2
EAnode, Fuel In
0.0
0.0
1.0
2.0
Cell Voltage (V)
Electrode Potential (V, vs. DHRE)
Air / H2
1.4
3.0
4.0
5.0
6.0
Time (min)
7.0
8.0
9.0
The authors thank Dr. Stephen Campbell and Christian Tuazon
for insightful discussions and input.
REFERENCES
1.6
Air / N2
DHREs are an effective tool for clearly elucidating the electrochemical phenomena occurring at the electrode level in PEMFCs
due to a variety of fuel cell operational regimes. This has been
demonstrated through a clear match of theory to experimental results in the case of corrosion due to low fuel stoichiometry and
changing anode gas composition. DHREs provide a clear indication
as to the severity of these events much earlier than would be
achieved by lengthy lifetime testing, thus enabling rapid protocol
tuning, accelerated stress test development, and accelerated unit
cell design. The DHRE is compact and practical for everyday use,
does not require customized flow field plate design, liquid electrolytes, or an auxiliary supply of H2, and is stable under a changing
fuel cell environment.
5. ACKNOWLEDGEMENTS
Figure 4. Schematic of the fuel cell environment and electrochemical potentials during a partial fuel starvation.
Air / H2
145
0.0
10.0
Figure 5. Cell voltage and electrode potential responses to a H2 →
N2 anode gas exchange.
the cell voltage (minus losses due to in-plane current flow, polarization effects, etc.). Under these electrolytic conditions, a circular
current can be established in the fuel cell, in addition to the net
through-plane current (or load), with carbon / water electrolysis
occurring at the fuel cell “cathode” (in the fuel starved region) and
proton / oxygen reduction occurring locally at the fuel cell “anode”.
The existence of damaging potentials during startup, shutdown,
or anode gas exchanges, is also readily measured when a cell is
equipped with DHREs. Figure 5 illustrates this for a cell starting
from air/H2, followed by purging of the anode with N2, then a return to the nominal air/H2 state. As in Table 1, when the anode H2
is displaced by N2, the anode potential increases, and the cell voltage falls towards zero. As H2 is reintroduced into the anode, a cell
voltage is rapidly generated at the inlet and is applied along the
entire length of the fuel cell by the flow field plates. The cathode
potential is elevated above the anode potential by this voltage, and
is sufficient to permit Pt-catalyzed carbon oxidation. As the anode
flow field is purged of N2, and replaced by H2, first the anode inlet,
then the anode outlet potentials decrease (with the cathode potentials following suit) and the corrosive potentials are relaxed.
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