Evaluation of Cu-substituted La_1.5Sr_0.5NiO_4+δ as air electrode for CO₂ electrolysis in solid oxide electrolysis cells

Ruddlesden-Popper oxide, Cu-substituted La_1.5Sr_0.5NiO_4+δ series materials (La_1.5Sr_0.5Ni_1-xCu_xO_4+δ; denoted as LSNCu_x; x = 0, 0.1, 0.25, 0.5) are investigated as air electrodes in solid oxide electrolysis cells (SOECs) for electrolysis of CO₂. Room temperature crystal structure, electrical conductivity and oxygen exchange capacity, as well as electrochemical performance of LSNCu_x are comprehensively investigated. Among the series of samples, LSNCu_0.25 half-cell exhibits the lowest polarization resistance value of 0.179 Ω cm² at 800 °C, which decreases by approximately 86.07% compared with that of LSN. In addition, the fuel electrode-supported single cell with LSNCu_0.25 air electrode presents a high current density of 1.2 A cm⁻² at 1.5 V under 30% CO–70% CO₂ condition at 800 °C, which is 207% of LSN (0.58 A cm⁻²) under the same condition. Results show that the impressive catalytic activity for oxygen evolution reaction (OER) is ascribed to the improved electronic conductivity and oxygen exchange capacity. With Cu substitution for Ni-site, the contraction of Ni–O bond in NiO₆ octahedron and increased concentration of charge carries owing to the oxidation of Ni²⁺ to Ni³⁺ are beneficial to the electron conduction. The formation of more interstitial oxygen as ionic compensation also favors the oxygen ion diffusion/exchange and greatly accelerates the charge transfer process. Furthermore, no degradation is observed for the single cell durability test at 750 °C for 50 h, which demonstrates the highly stable performance of LSNCu_0.25 air electrode for electrolysis of CO₂.