February 2, 2026

Abstract

The solid state nature and intermediate temperature operability of superprotonic solid acid electrolytes, in particular cesium dihydrogen phosphate (CsH₂PO₄ or CDP), suggest advantages for fuel cell applications. Surprisingly, however, state-of-the-art solid acid fuel cells (SAFCs) operated at 250 °C produce peak power densities that are markedly lower that those of polymer electrolyte membrance fuel cells (PEMFCs) operated at near ambient temperatures and with much lower Pt loadings. Here we measure the fundamental electrochemical characteristics of the oxygen reduction reaction on Pt in SAFC conditions and incorporate the measured parameters into a 1-D model of cathode polarization behavior that enables isolation of the factors limiting SAFC power output. All input parameters to the model, for example, exchange current density, Butler-Volmer kinetic parameters, and microstructural features, are obtained from independent measurements. After validation against experimentally measured polarization curves, the model is used to assess the impact of modifying microstructural, operational, material parameters on fuel cell power output. We find that the Pt catalytic activity is strongly inhibited at the high temperature, high steam conditions employed in SAFCs. Moreover, microstructural modifications to increase the Pt surface area available for reaction so as to reach competitive current densities, though physically viable, would result in unacceptable increases in Pt loading. These insights motivate prioritization of the development and deployment of advanced electrolytes with greater thermodynamic stability and protonic conductivity and alternative catalyst materials with greater activity.

Bio

Sossina M. Haile is a Chaired Full Professor of Materials Science and Engineering at Northwestern University, where she also holds appointments in Applied Physics and (by courtesy) in Chemistry. She earned her S.B. and Ph.D. in Materials Science and Engineering from the Massachusetts Institute of Technology and her M.Sc., also in MSE, from the University of California, Berkeley. Haile’s research broadly encompasses materials for sustainable electrochemical energy technologies, and she has made significant advances in reversible electrochemical cells and solar-driven thermochemical hydrogen production. She directs the DOE Energy Frontier Research Center, Hydrogen in Energy and Information Sciences (HEISs). Amongst her many awards, in 2008 Haile received an American Competitiveness and Innovation (ACI) Fellowship from the U.S. National Science Foundation and in 2025 the Sosman Award of the American Ceramics Society. She is a fellow of the Materials Research Society, the American Ceramics Society, the Electrochemical Society, the Royal Society of Chemistry, the African Academy of Sciences, and the Ethiopian Academy of Sciences. Haile is the Vice President of the Materials Research Society and will serve as its president in 2027. She also serves on the editorial advisory boards of Joule and MRS Energy and Sustainability.