(Invited) Updated Catalyst Activity Targets for Performance Parity in Hydroxide Exchange Membrane Fuel Cells

Abstract

The critical importance of power density on fuel cell system cost is well-recognized within the low-temperature fuel cell community. By increasing the required active area of the stack, a low-cost, low-performance component can increase the cost of other major stack components and increase system cost. The cost of platinum-group-metal (PGM) catalysts is a substantial barrier to reaching the long-term U.S. Department of Energy cost target of $30 / kW for automotive proton exchange membrane fuel cell (PEMFC) systems. Hydroxide exchange membrane fuel cells (HEMFCs) offer a possibly pathway to PGM-free catalysis in low-temperature fuel cells, among other potential cost benefits. However, only 35% of PEMFC stack cost is due to PGMs1. Therefore, as a first approximation, we believe the appropriate target for low-PGM or PGM-free HEMFCs is performance parity with state-of-the-art PEMFCs2. Performance parity ensures that catalyst cost savings pass through to the system level, justifying investment in an unproven competitor to the incumbent PEMFC. Although PGM-free catalyst loadings may be relatively unconstrained by economics, transport still places an upper bound on the loading that can be utilized at high current densities. For high catalyst loadings, the most important considerations are ionic conduction through the electrode ionomer and reactant diffusion through the catalyst layer pores. Making reasonable baseline assumptions for effective conductivity and diffusivity, as well as the area-specific resistance, activity targets can be derived for PGM-free catalysts to enable performance-parity with PEMFCs. Comparing reported PGM-free catalysts to these targets, it is clear that progress is needed in both anode and cathode catalysts (Figure 1). Interestingly, the derived target for PGM-free cathode catalysts exceeds the performance of Pt/C considerably. To reach performance parity, any additional mV of overpotential must be compensated by reductions elsewhere. Because the PEMFC anode operates at near-zero overpotential, an unlikely feat for PGM-free catalysts, the HEMFC must compensate for this extra loss at the cathode. In the long-term, the major challenge to achieving high-performance PGM-free HEMFCs is the anode catalyst. Relatively few reports of PGM-free hydrogen oxidation reaction (HOR) catalysts have been made, although many have reported hydrogen evolution reaction (HER) catalysts. The reason for this discrepancy is the tendency of nickel-based catalysts to oxidize and become deactivated at potentials only slightly higher than RHE. In the short-term, the most urgent need for HEMFC catalyst development is a high-performance PGM-free cathode catalyst. On the anode side, PtRu/C offers volumetric activity several times higher than the target for PGM-free catalysts, making low-loading PGM anodes a viable option. Low-loading Pt-alloy cathodes are a less attractive option, given the likely challenges of local oxygen transport and catalyst stability. If only the cathode target can be met, HEMFCs can still reach the important milestone of PEMFC performance parity at equal or lower PGM loading. References B. D. James, J. M. Huya-Kouadio, C. Houchins, and D. A. DeSantis, Mass Production Cost Estimation of Direct H2 PEM Fuel Cell Systems for Transportation Applications: 2017 Update, Arlington, VA, (2017).B. P. Setzler, Z. Zhuang, J. A. Wittkopf, and Y. Yan, Nat. Nanotechnol., 11, 1020–1025 (2016). Figure 1