From material design to mechanism study: Nanoscale Ni exsolution on a highly active A-site deficient anode material for solid oxide fuel cells

Abstract

Nonstoichiometric perovskites with active metal nanoparticles exsolved on the surface have shown promising potential in energy and environmental applications ranging from catalysis to power generation. In this work, a novel Sc-based A-site deficient perovskite material La0.4Sr0.4Sc0.9Ni0.1O3−δ (LSSN) is reported as a highly active anode for intermediate-temperature solid oxide fuel cells. The material is designed using both thermodynamical analysis and ab-initio simulations. The drop in Gibbs free energy for Ni in H2 is substantial in comparison to the other elements, and density functional theory simulations indicate that the segregation of Ni towards the surface is energetically favored. Spherical Ni nanoparticles with well-defined boundaries are exsolved on the surface of LSSN after reduction in hydrogen, and the reduced samples show a high electrochemical catalytic activity in symmetric-cell measurements with an area specific resistance as low as 0.055Ωcm2 at 800°C in humid H2. Insights into the exsolution mechanism are also derived from both experiments and analytical modeling. Experimental observations show a patterned particle distribution, which is consistent with heterogeneous nucleation. The particle size evolution is investigated using the strain, reactant, and diffusion limited analytical models. The mechanistic insights gained here can be broadly applied to design more efficient materials capable of exsolution and to control the nanoparticle growth and coverage.