Evaluation Of SrSc_0.175Nb_0.025Co_0.8O_3-δ Perovskite As A Cathode For Proton-Conducting Solid Oxide Fuel Cells

Solid oxide fuel cells based on proton conducting electrolytes(H+-SOFCs)show competitive advantages of the low activation energy and the high ionic conductivity of electrolyte at lower temperatures compared to SOFCs based on oxygen ion conducting electrolytes(O2--SOFCs).This high ionic conductivity of electrolyte at low temperatures allows H+-SOFCs to operate at intermediate-to-low temperatures(400-700 ℃),which minimizes the thermal degradation of SOFCs components and reduces the longer duration of start-up and shut-down procedures.However,when the H+-SOFC operates at<750 ℃,it has some challenges for cathode mateirals,becuase in terms of the lower electrocatalytic activity of cathode materials for oxygen reduction reaction.In addition,different to O2--SOFCs,water is formed at the cathode side of H+-SOFCs.This formed water vapor could involve some specific requirements for the cathode materials in H+-SOFCs.Firstly,SrSc0.175Nb0.025Co0.8O3-δ(SSNC)was synthesized for evaluation as a cathode material in H+-SOFCs based on a BaZr0.1Ce0.7Y0.2O3-δ(BZCY)electrolyte.The chemical compatibility and stability of the SSNC cathode with the BZCY electrolyte in humid air were studied.In addition,the electrochemical behavior of the SSNC cathode on the BZCY electrolyte was investigated using SSNC/BZCY/SSNC symmetrical cells at 600 ℃ in dry air and humidified air at various H2O partial pressures.Promising electrocatalytic activity was observed for the SSNC cathode in humidified air.The area specific resistance obtained on symmetrical cells at 600 ℃ in a 10%H2O-air atmosphere was 0.26 Ω cm2.A promising peak power density of 498 nW cm2 was obtained using an anode-supported cell at 700 ℃.Secondly,more detail analysis on the SSNC/BZCY/SSNC symmetric cells in simulated air atmosphere with various humidities was applied.It was found that ohmic resistance was significantly reduced after water was introduced,which was consistent with typical hydration behavior of the BZCY proton conducting electrolyte.At 650 ℃,oxygen adsorption,oxygen reduction and oxygen ion diffusion rates were accelerated after water was introduced.At 550 ℃,oxygen adsorption rates were increased.At 550 and 450 ℃,oxygen ion diffusion rates were reduced.Single cells with the configuration of NiO+BZCY/BZCY/SSNC were further tested for opreation stability in circulating wet and dry air and demonstrated to have excellent stability.Thirdly,we aimed to develop high performance and durable H+-SOFCs cathode by doping Ba2+into the Sr-site of the SSNC perovskite oxide.With a BSSNC cathode,a proton conducting SOFCs achieved good performance at a temperature of 700 ℃with a power density of 633 mW cm-2·However,the performance of single cell decreased with time,probably due to the agglomeration of cathode particles and the coverage of produced water on the active surface.To improve the durability of the H+-SOFCs,it is critical to minimize the cathode particle agglomeration and remove the produced water effectively.Simultaneously,the influence of moisture on the catalytic activity of BSSNC cathode using a BSSNC/BZCY/BSSNC symmetric cell was investigated by electrochemical impedance spectroscopy(EIS)at 600 ℃.The results indicated that the resistance in the low-frequency range was the rate-limiting step of the diffusion processes in the dry air,while the resistance in the medium-frequency range became the rate-limiting step of oxygen reduction in the moist air.