Water Activity Dependence of Oxygen Evolution Reaction Catalysts and Carbon Corrosion of Reversal Tolerant Fuel Cell Anodes during Hydrogen Starvation Conditions
Authors: Joo Taigyu , Hu Leiming , Hong Bo Ki , Oh Jong-Gil , Litster Shawn 
Published: February 2020

Before electric vehicles using polymer electrolyte fuel cells (PEFCs) can occupy a significant part of the vehicle fleet, one of their critical vulnerabilities must be resolved: rapid failure that occurs under hydrogen starvation conditions. Local hydrogen starvation within fuel cell system can occur when the hydrogen inlet flow channel is blocked by water and ice, or during the startup/shutdown process [1]. During hydrogen starvation, the fuel-starved anode requires another source of electrons and protons other than the usual hydrogen gas since hydrogen oxidation reaction (HOR) alone cannot support the current imposed by adjacent cells. The starvation eventually increases the anode half-cell potential relative to the cathode potential, and the cell potential is reversed. The high anode half-cell potential will lead to oxidation of the carbon support in the anode. The loss of carbon support in the electrode will decrease the performance of the cell, and the entire stack eventually becomes irreversibly inoperable. One popular mitigation strategy is the use of reversal tolerant anodes (RTAs), which integrates an oxygen evolution reaction (OER) catalyst, such as IrO2, to promote harmless water electrolysis over carbon corrosion [2]. The OER prevents the anode half-cell potential from further increasing, delaying the carbon support from significantly corroding. However, our group’s prior work has shown that RTA durability is significantly impaired under high relative humidity (RH) conditions [2]. Since the hydrogen starvations commonly arise in high RH conditions, it is important to clarify the exact mechanism that leads to the relatively quick RTA deactivation at high RH.
The present work investigates the RTA catalyst deactivation mechanism during hydrogen starvation and the water activity dependence of water electrolysis and carbon corrosion reaction to better understand the relatively quick failure of RTA at high RH. Figure 1a shows the effect of carbon on the OER catalyst during hydrogen starvation, which is simulated by providing humidified nitrogen and applying 1 A cm-2 of current density to the cell. In contrast to the stable anode half-cell potential at 2.2 V by IrO2 alone, the presence of carbon leads to a quick increase of its half-cell potential and then the failure of the cell. This suggests that carbon deactivates the OER catalyst. There are three possible RTA failure mechanisms: (1) the loss of electronic connectivity of the catalysts due to the degradation of the carbon support (2) the isolation of the catalyst from ionomer after severe carbon corrosion (3) the poisoning of the catalyst by carbon oxidation species. To determine which mechanism causes the RTA failure, carbon and IrO2 were separated into two layers to avoid the loss of electronic connectivity and poor ionomer coverage after the carbon support degradation, and tested under hydrogen starvation conditions. As shown in Figure 1b, the carbon-separated IrO2 quickly increases the anode half-cell potential. This indicates that the catalyst deactivates even if the ionomer coverage and electric connectivity are maintained, and the deactivation mechanism is more likely to be the poisoning of the catalyst by carbon oxidation species rather than the loss of electronic or ionic connectivity.
To investigate the water activity dependence of water electrolysis and carbon corrosion reactions, IrO2 and carbon black based membrane electrode assemblies (MEAs) were separately fabricated and tested under hydrogen starvation conditions. Potentials of 1.6, 1.7, and 1.8V, which are relevant to the OER catalyst function in RTAs during cell reversal, were applied to the MEAs under various RH conditions. As shown in Figure 1c and 1d, the catalytic activity of IrO2 measured by the current density response shows a substantially linear dependence on RH, while the rate of carbon corrosion increases exponentially with RH. This linear versus exponential relationship suggests that higher rate of catalyst poisoning at high RHs is correlated with the increased rate of carbon oxidation, resulting in the impaired durability of RTAs.

References

[1] J. Wu, X. Z. Yuan, J. J. Martin, H. Wang, J. Zhang, J. Shen, S. Wu, W. Merida. “A review of PEM fuel cell durability: Degradation Mechanisms and Mitigation Strategies” J. Power Sources 184 (2008) 104-119.
[2] B. K. Hong, P. Mandal, J. Oh, S. Litster. “On the impact of water activity on reversal tolerant fuel cell anode performance and durability” J. Power Sources 328 (2016) 280-288.

Figure 1