(Invited) Chemical Stability and Performance of Rare Earth Nickelate Oxygen Electrodes for Reversible Solid Oxide Cells

Abstract

Composite rare-earth nickelate-rare-earth doped ceria oxygen electrodes have inherent chemical instabilities. Over the last year, we have made progress on achieving chemical stability and high performance using a strategy of heavily doping the ceria phase with rare-earth cations. A higher dopant concentration of a rare-earth cation in the ceria phase ensures thermodynamic stability of the nickelate phase in contact with the ceria phase. In particular, composite oxygen electrodes comprising lanthanum nickelate La2NiO4+δ (LNO) - lanthanum doped ceria (LDC) and neodymium nickelate Nd2NiO4+δ (NNO) - neodymium doped ceria (NDC) have been electrochemically tested in both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes. The performance of these cells have been compared to a composite strontium doped lanthanum manganite (LSM)-8 mol% yttria stabilized zirconia (YSZ) electrode. Both nickelate based electrodes achieve far superior performance compared to the LSM electrodes. We also report impedance measurements from symmetrical cells featuring LNO and NNO electrodes, analyzed using a distribution of relaxation times (DRT) approach. The DRT analysis reveals significant differences in the rate controlling steps in oxygen reduction and incorporation steps between the two materials. Lastly, oxygen surface exchange results for LNO and NNO are also reported which show different oxygen exchange kinetics when measured on dense versus porous materials which have implications for deploying these materials in reversible solid oxide cells.