Theory, modelling and performance measurement of unitised regenerative fuel cells

Unitised Regenerative Fuel Cells (URFCs) based on Proton Exchange Membrane (PEM) technology provide a promising opportunity for reducing the cost of the hydrogen subsystem used in renewable-energy hydrogen systems for remote area power supply. A general theoretical relationship between cell potential and current density of a single-cell URFC operating in both fuel-cell and electrolyser modes is derived using the Butler–Volmer equation for both oxygen and hydrogen electrodes, and accounting for membrane resistance and mass transport losses. Modifying the standard Butler–Volmer equation with a denominator term containing two additional ‘saturation’ parameters to reflect mass transport constraints generates voltage–current curves that are much closer to experimentally obtained polarisation curves in both modes. The theoretical relationship is used to construct a computer model based on Excel and Visual Basic to generate voltage–current curves in both electrolyser and fuel cell modes for URFCs with a range of membrane electrode assembly characteristics. Hence the influence of key factors such as exchange current densities and charge transfer coefficients on cell performance is analysed. Experimental results for voltage–current curves from singe-cell URFCs with a number of different oxygen-side catalysts are reported, and compared to the theoretically modelled curves. Generally values have been found for exchange current densities, charge transfer coefficients, and saturation current densities that give a close fit between the empirical and theoretically generated curves. The values found conform well to expectations based on the catalyst loadings, in partial confirmation of the validity of the modelling approach. The model thus promises to be a useful tool in identifying electrodes with materials and structures, together with optimal catalyst types and loadings that will improve URFC performance.