Evaluation of Ag–NiO mixture as a cathode contact material for solid oxide fuel cell applications

Abstract

Various compositions of mixture of silver (Ag) and nickel oxide (NiO) powders are studied for use as a cathode contact material in solid oxide fuel cell stacks. The addition of NiO is intended to mitigate Ag evaporation and maintain low resistivity of the contact material during stack operation. It is found that the mixture in the form of pellets consisting of 20 mol % Ag and 80 mol % NiO can yield acceptable area specific resistance (ASR) 2.24 mΩ cm² at 800°C with reduced weight loss. However, in the steel/Ag–NiO paste/steel assemblies prepared with 30–50 mol % Ag in mixed powders, each paste containing 64 wt % mixed powder, high ASR values of ∼100–200 mΩ cm² after 1552 h thermal exposure at 800°C are observed. The low resistivity of the pellet with 20 mol % Ag is attributed to the formation of effective conducting pathways by complete sintering of the densely packed Ag particles through the pressing process in producing the pellets. For the steel/Ag–NiO paste/steel assemblies, Ag particles are scattered within the Ag–NiO layer and cannot readily get sintered. It is found that the resistance of Ag–NiO mixture critically depends on the mass concentration of Ag powder.