Titanate Electrodes with Exsolved Ni-Fe Nanocatalysts in H2/H2O and CO/CO2 operation: An Impedance Study
Authors: Geisendorfer Nicholas R. , Barnett Scott A 
Published: July 2021

Operation of solid oxide cells in CO/CO2 fuel atmospheres is important both for renewable energy storage technologies as well as for ongoing, flagship space missions. Nevertheless, the performance of solid oxide cells operating in CO/CO2 atmospheres is generally diminished compared to an identical cell operating in the prototypical H2/H2O atmosphere. Although Ni-cermet electrodes have demonstrated excellent performance for fuel cell and electrolysis operation using H2/H2O gas mixtures, their reduced performance and stability issues when operating in CO/CO2 due to, e.g., the poor coking resistance of Ni, motivates the development of more-stable, alternative electrode materials. Oxide fuel electrodes have been developed partially in response to these constraints, although they possess drawbacks in the form of low electronic conductivity and low surface catalytic activity due to the lack of a metallic phase. However, oxides within the perovskite-family, when doped on the B-site with thermochemically reducible cations such as Ni, Co, etc., undergo an exsolution process whereby B-site dopants are reduced and nucleate as metallic nanoparticles on the oxide surface. These nanoparticles are highly catalytically active and comparatively resistant to degradation compared to similarly sized composite or infiltrated nanocatalysts. Perovskite fuel electrodes with exsolved catalytic nanoparticles have been shown to possess competitive performance compared to conventional Ni-based electrodes, while nevertheless exhibiting higher stability to reoxidation and resistance to coking.

The effect of exsolved catalyst nanoparticles on the performance of perovskite electrodes operating in CO/CO2 fuel atmospheres is not well understood. While several studies report exsolved catalysts improve the performance of electrodes in CO/CO2, the source of improvement is not understood, in particular due to important differences in reaction pathway compared to H2/H2O fuel.

Here we study two recently reported perovskite fuel electrode materials, Sr(Ti0.3Fe0.7)O3-δ (STF) and Sr0.95(Ti0.3Fe0.63Ni0.07)O3-δ (STFN). The latter exhibits the formation of Ni-Fe nanoparticles socketed on the perovskite surface after periods at elevated temperature in reducing atmosphere, as a result of the exsolution of B-site cations from the perovskite. Electrodes are investigated in both symmetric cell and full cell configurations using Hionic electrolyte supports (Nexceris). Electrode reaction kinetics are investigated as a function of temperature and fuel atmosphere in both fuel cell and electrolysis modes using current-voltage measurements and electrochemical impedance spectroscopy. Electrode and catalyst microstructure is examined using scanning electron microscopy. This presentation will examine, for the first time, the performance of STF an STFN fuel electrodes operating across the entire range of CO/CO2 fuel atmospheres, will interpret the effect of exsolved catalyst particles on the performance of these electrodes, and will compare and contrast with observations in H2/H2O atmospheres in order to better understand electrode behavior.