Electronic Transport within Proton-Conducting Ceramics and Its Effect on Faradaic Efficiency of High-Temperature Water Electrolysis for Hydrogen Production

Abstract

Proton conducting ceramics are used as electrolyte materials for protonic ceramic electrolysis cells to produce hydrogen through water electrolysis at high or intermediate temperatures. The widely applied barium-cerium-yttrium-zirconium oxides have been well studied as proton conductors for fuel cell applications, which present a promise in commercialization. In water electrolysis operating conditions electronic leakage within electrolyte can be observed, which reduces the Faradaic efficiency (FE) of hydrogen production. To address this electronic leakage problem, the intrinsic mechanism behind it should be understood. In this work, the defect chemistry in barium cerate and barium zirconate doped with yttrium is comparatively studied in terms of proton, electron and hole defect concentration/conductivity, and respective distribution profile within electrolyte, where electronic and protonic current density are calculated under electrolysis working conditions such as steam concentration, oxygen partial pressure, and over-potential. In addition, the conduction behaviors of three typical proton-conducting ceramics: BaCe0.7Zr0.1Y0.1Yb0.1O3-δ, BaCe0.4Zr0.4Y0.1Yb0.1O3-δ and BaZr0.8Y0.2O3-δ are experimentally investigated under different oxygen partial pressures and temperatures to validate with the prior defect equation derivation. The results indicated that high FE can be achieved under optimized conditions with reduced temperatures, implying numerous research opportunities in materials R&D of protonic ceramic electrochemical cells (PCECs) as well as the feasibility for widespread market adoption.