Enhancement of Ni-YSZ Fuel Electrode Performance Via Pressurization and GDC Infiltration

Abstract

Solid oxide cell systems are often designed for operation with a pressurized stack. Although the cell performance is expected to improve with pressurization, the details of how pressure affects the performance of various technologically-relevant electrodes are typically not known. Here we investigate the electrochemical characteristics of Ni-YSZ and GDC-infiltrated Ni-YSZ fuel electrodes in Ni-YSZ-supported cells as a function of total pressure P from 1 to 5 atm in H2/H2O fuel mixtures with humidification of 25%, 50%, and 75% and temperatures of 600˚C and 700˚C. Using electrochemical impedance spectroscopy, the two limiting electrode processes are identified: charge transfer reactions and gas diffusion. The charge transfer resistance is significantly reduced for Ni-YSZ:GDC compared to Ni-YSZ for all conditions, with total polarization resistance RP reduced by 30 - 40%. Fitting the data to a power-law dependence, RP ∝ P−n, yields a power law exponent of n = 0.28 for Ni-YSZ and 0.36 for Ni-YSZ:GDC (at 600˚C) and n = 0.32 for Ni-YSZ and 0.39 (at 700˚C). That is, GDC infiltration improved electrode performance more at higher pressure. Increasing the total pressure from 1 to 5 atm results in a 42% and 47% reduction in RP for infiltrated electrodes at 600˚C and 700˚C; these values are averaged for all humidities. Increasing humidity from 25 to 50% at 1 atm resulted in a ~26% reduction in total RP. The Ni-YSZ:GDC electrode at 5 atm had a RP value 63% - 65% lower than that of the Ni-YSZ electrode at 1 atm, a very substantial combined effect. The impact of pressurization on overall cell area-specific resistance is assessed based on the present data combined with prior measurements of oxygen electrode pressurization effects.