On the cation distribution in acceptor doped Ba(Zr,Ce)O3-(Zr,Ce)O2 co-ionic composite electrolytes

Abstract

Composite co-ionic ceramic electrolytes that combine proton and oxide ion conductors hold potential for co-electrolysis of CO₂ and H₂O for syngas production due to flexible control of the transport numbers of the two charge carriers. Contrary to purely oxide ion co-electrolysis, co-ionic co-electrolysis embodies the supply of CO₂ and H₂O separately to the negative and positive electrodes, respectively. This study focuses on the development of a chemically stable co-ionic composite electrolyte of an acceptor-doped Ba(Zr,Ce)O₃ proton conducting perovskite phase and an acceptor-doped (Ce,Zr)O₂ oxide ion conducting fluorite phase, annealed at temperatures between 800 and 1600 °C. Comprehensive evaluations of the composites' microstructure, hydration, and conductivity were performed, revealing that annealing temperature and cation selection significantly impact the properties and performance of co-ionic electrolytes. Higher annealing temperatures drive cation redistribution, with the perovskite phase becoming zirconium-rich at its B-site and depleted in acceptor dopants, resulting in diminished hydration and protonic conductivity. Herein, we show that composites pairing cerium-rich fluorite phases (e.g., Ce_0.8Gd_0.2O_1.9, CGO20, or Ce_0.8Y_0.2O_1.9, CYO20) display markedly improved performance. The BaCe_0.8Y_0.2O_2.9 (BCY20)–CYO20 system (1 : 1 weight ratio) achieved the highest conductivity (σ = 0.01 Scm⁻¹ at 650 °C in wet Ar), establishing itself as a promising candidate for co-ionic electrolyte applications in solid oxide electrochemical cells.