(Invited) Understanding the Performance Limits of Solid Acid Fuel Cells with Pt Electrocatalysts
Authors: Haile Sossina Mariam , Wang Louis Shen 
Published: February 2024

Solid acid fuel cells (SAFCs) confer unique functional benefits over nearby technologies due to the solid state nature and intermediate temperature operability of the electrolyte. Amongst candidate solid acids, the electrolyte of choice in fuel cell applications is almost exclusively cesium dihydrogen phosphate (CDP), CsH2PO4. The material displays excellent proton conductivity (~2 ∗ 10-2 S/cm) at moderate temperatures (228 – 300 °C) with no detectable electronic conductivity. The elevated temperature of operation relative to polymer electrolyte membrane (PEM) fuel cells (< 90 °C), suggests that the kinetics for the oxygen reduction reaction will be elevated, and that SAFCs should provide higher power densities at reduced catalyst loadings. In practice, however, these advantages have not been realized. State-of-the-art SAFCs produce peak power densities of ~200 mW/cm2 with Pt loadings of 1.75 mg/cm2.1 In comparison, PEMFCs have demonstrated peak power densities >1 W/cm2 with Pt loadings of only 0.1 mg/cm2.2 In this work, we begin to unravel the question of why the anticipated thermal activation of reaction kinetics has not resulted in superior SAFC performance. We measure fundamental microstructural and material parameters of SAFC cathodes, and use these to predict fuel cell polarization curves. We find that, while the exchange current density for the oxygen reduction reaction is high in SAFCs, the Tafel slope is steep. Thus, the fundamental steps in the ORR reaction on Pt follow a different and unfavorable pathway under SAFC conditions than PEMFC conditions.

We further employ the experimentally informed and validated physical model (see figure) to assess the role of microstructural, operational, and material parameters on SAFC polarization curves. The model suggests that a 10-fold increase in active Pt surface area would be required to approach the DOE fuel cell target of 300 mA/cm2 at 0.8 V (even without accounting for voltage drops through the ~ 50 mm thick electrolyte). In turn, this would require a decrease in CDP particle size in the cathode, where CDP serves as the support onto which Pt nanoparticles are deposited, from 500 to 50 nm, and would be accompanied by an untenable increase in Pt loading to over 30 mg/cm2. We further find that the Pt catalytic activity is strongly inhibited by the presence of steam, required to maintain stability of the electrolyte against dehydration. The humidification requirement moreover increases polarization losses at higher temperatures of operation, at which higher steam levels are required for electrolyte stabilization, overcoming the expected benefits of thermally activated catalysis. These insights motivate prioritization of the development and deployment of alternative catalyst materials and advanced electrolytes with greater thermodynamic stability over attempts to modify electrode microstructure, cell architecture, or operational conditions using today’s material set in order to advance SAFC technology towards commercial realization.

1. Papandrew, A. B., Chisholm, C. R. I., Elgammal, R. A., Özer, M. M. & Zecevic, S. K. Advanced Electrodes for Solid Acid Fuel Cells by Platinum Deposition on CsH2PO4. Chemistry of Materials
23, 1659-1667, (2011).

2. Zhao, Z. et al. Graphene-nanopocket-encaged PtCo nanocatalysts for highly durable fuel cell operation under demanding ultralow-Pt-loading conditions. Nat Nanotechnol, (2022).

Figure 1