Analysis of the anode reaction of solid oxide electrolyzer cells with BaZr_0.4Ce_0.4Y_0.2O_3-δ electrolytes and Sm_0.5Sr_0.5CoO_3-δ anodes

Anodic reaction pathways in proton-conducting solid oxide electrolyzer cells (H⁺−SOECs) were investigated using electrochemical impedance spectroscopy with a cell structure of Sm_0.5Sr_0.5CoO₃ (anode) | BaZr_0.4Ce_0.4Y_0.2O_3-δ | Pt (cathode). Densely sintered BaZr_0.4Ce_0.4Y_0.2O_3-δ ceramics (>97% relative density) were fabricated by a reactive sintering process with a 2 mol% Zn(NO3)2 additive and were applied as the electrolyte. The impedance spectra were measured while the oxygen (pO2) and water partial pressures (pH2O) in the anode side were systematically varied, which revealed that the SOECs have two polarization resistances at the anode side, one proportional to p_O2^−1/4 and p_H2O⁰ and another insensitive to p_O2 and pH2O. A comparison between the experimental results and elementary step modeling revealed that the actual anode reactions could be described by the reverse mode H⁺-SOFC cathode reactions, and, thus, the elementary steps dominating the anodic polarization resistance were assigned.