Electrochemical characterization and model investigation of BaZr0.625Ce0.2Y0.175O3-δ based proton conductive ceramic cells

In this work, we study the electrochemical properties of BaZr_0.625Ce_0.2Y_0.175O_3-δ (BZCY62), a perovskite oxide applied as electrolyte of proton-conductive ceramic electrolysis cells (PCECs). To investigate the conductivity behavior, electrochemical impedance spectroscopy measurements were performed between 400°C and 600°C at varying molar fractions of O₂ (21-2%), H₂O (35-0.1%), and H₂. The beneficial effect of H₂O on the BZCY62 conductivity emerged (at 600°C, from 0.9 mS cm⁻¹ at 0.1% H₂O to 7.6 mS cm⁻¹ at 35% H₂O) together with the occurrence of a saturation threshold. An equilibrium model was developed to predict the conductivities of the main charge carriers (protons, polarons, and oxygen vacancies). The model included reactions of hydration, polaron formation and consumption via interaction with H₂, and allowed to analyze the conductivity experiments and extract relevant thermodynamic and transport parameters, finding 2.12 × 10⁻⁶ cm² s⁻¹ protons diffusivity at 600°C. Integration of the results of thermogravimetric analyses was required to determine the hydration properties of BZCY62. Electrolyte-supported and electrode-supported button PCECs based on BZCY62 were manufactured for the first time, and tested in steam electrolysis between 500°C and 600°C. The PCECs mounted Ni-BZCY62 fuel electrodes, and BLC-BZCY62 (Ba_0.5La_0.5CoO_3-δ) oxygen electrodes, and were tested with current/voltage and impedance curves by supplying steam/air mixtures up to 30/70 mol/mol and H₂/N₂ mixtures up to 50/50 mol/mol. The electrode-supported PCEC reached 413 mA cm⁻² at 1.4 V and 600°C, in 6% humidified air. Significant impact of the adverse p-type conductivity of BZCY62 on OCV and cell’s performance as a function of the electrolyte thickness was observed.