Ni-Substituted Sr(Ti,Fe)O3 SOFC Anodes: Achieving High Performance via Metal Alloy Nanoparticle Exsolution

Abstract

Electrically conducting oxides have been proposed as alternatives to Ni-based cermet anodes for solid oxide fuel cells (SOFCs) to overcome issues such as coking and impurity poisoning, but their electrochemical performance is typically inferior to that of Ni-based cermets. Here we show that a new oxide composition, Sr0.95(Ti0.3Fe0.63Ni0.07)O3−δ, yields anode polarization resistance competitive with Ni cermets, and substantially better than that of the corresponding Ni-free compound, SrTi0.3Fe0.7O3−δ. Exposure to fuel results in exsolution and nucleation of Ni0.5Fe0.5 nanoparticles uniformly dispersed on the Ni-substituted perovskite surface, whereas no nanoparticles are observed on SrTi0.3Fe0.7O3−δ. A general thermodynamic model is developed that quantitatively predicts exsolved nanoparticle composition. The reduction in anode polarization resistance by the nanoparticles, by as much as 4 times, is most pronounced at cell operating temperatures below 800°C and low H2 partial pressures, suggesting that the nanoparticles improve performance by promoting H2 adsorption. Mixed conducting perovskite oxides are proposed as alternatives for solid oxide fuel cell anodes, and in situ metallic nanoparticle precipitation/exsolution is considered to improve their performance. However, understanding of exsolution is still lacking, such as the reason why specific cations exsolve from an oxide anode and what determines the exsolved phase composition under various anode conditions. Here we directly compare Sr(Ti,Fe)O3-based anodes with and without exsolved metallic nanoparticles, showing their impact on electrochemical characteristics especially under low anode hydrogen concentrations and low temperatures. Three different methods are used to quantitatively determine the composition of the exsolved (Ni,Fe) nanoparticles, and for the first time demonstrate that this can be achieved through thermogravimetric analysis. This work is the first to propose a detailed thermodynamic theory predicting the exsolved nanoparticle composition with successful experimental verification. Ni-Fe nanoparticles are observed to exsolve from Ni-substituted Sr0.95(Ti0.3Fe0.63Ni0.07)O3−δ anode. The exsolved nanoparticles act to enhance hydrogen dissociative adsorption, yielding much lower anode polarization resistance and higher cell performance, especially under low pH2 and temperatures, which is comparable with the current Ni-based cermets.