Robust catalytically-activated LSM-BCZY-based composite steam electrodes for proton ceramic electrolysis cells

Abstract

Backbone electrodes based on an electronic conductor and a protonic conductor show advantages for proton ceramic electrolyzer cells (PCECs). This work, aims to shed further light on the nature of the rate determining steps in the anode operation and improve the reaction rate in high steam pressure electrolysis mode by (i) adjusting their catalytic activity through electrode infiltration with catalytic electronic-conducting nanoparticles; and (ii) electrochemical activation of surface species by applying a net current through the electrode. A composite formed by La_0.8Sr_0.2MnO_3−δ (LSM) and BaCe_0.2Zr_0.7Y_0.1O_3−δ (BCZY27) was deposited on proton-conducting BCZY27 electrolytes and studied in symmetrical cells to investigate the anode microstructure and electrochemical performance. Electrochemical impedance spectroscopy (EIS) measurements were performed in the 800–500 °C range under 3 bar of pressure of wet air (75% of steam). LSM/BCZY27 50/50 vol% showed the best performance with an electrode polarization resistance (R_p) of 6.04 Ω cm² at 700 °C and high steam pressure (0.75 bar of air and 2.25 bar of steam) whereas LSM/BCZY27 60/40 vol% presented a R_p of 18.9 Ω cm². The backbone electrodes were infiltrated using aqueous solutions of metal precursors to boost the electrocatalytic activity towards water splitting and oxygen evolution. The infiltrated cells were fired at 850 °C for 2 h to obtain the desired crystalline nanoparticles (Pr₆O₁₁, CeO₂, ZrO₂ and Pr₆O₁₁–CeO₂) and electrochemically tested under high steam pressures and bias currents to investigate the influence of catalytic activation on surface exchange kinetics. Among the tested catalysts, the lowest electrode polarization resistances (<0.2 Ω cm²) were reached for the Pr₆O₁₁, CeO₂ and Pr₆O₁₁–CeO₂ catalysts at 700 °C by applying current densities ranging from 1.57 to 14.15 mA cm⁻², and the Pr₆O₁₁–CeO₂-activated LSM/BCZY27 electrode exhibited the best performance. Finally, the effect of pO₂ and pH₂O was investigated aiming to characterize the rate limiting processes in the electrodes.