Sheet Resistance in Thin Film Solid Oxide Fuel Cell Model Cathodes: A Case Study on Circular Bi1-xSrxFeO3-δ Microelectrodes

Abstract

The potential solid oxide fuel cell cathode material Bi1-xSrxFeO3-δ exhibits low electrical conductivities (1-12 S cm-1 at 750°C) and is therefore especially susceptible for significant sheet resistance effects when investigated in thin film form. Impedance spectra of circular thin film microelectrodes of Bi1-xSrxFeO3-δ (x = 0.2, 0.5 and 0.8) show remarkable features such as an increase in apparent ohmic resistance (high-frequency intercept) and the appearance of a semicircle at intermediate frequencies, due unlikely to an interfacial resistance contribution. To understand the origin of these features, a 2D empirical numerical model developed by Lynch et al. to account for sheet resistance in thin electrode films was applied to simulate the experimental spectra. All spectral features were reproduced qualitatively and, in most cases, nearly quantitatively indicating high sheet resistance to be the reason for the observed peculiarities. In the presence of a lateral electrical potential gradient, the apparent chemical capacitance strongly underestimates the real material properties, whereas the electrode surface resistance remains largely unaffected.