Comparative analysis of the electroactive area of Pt/C PEMFC electrodes in liquid and solid polymer contact by underpotential hydrogen adsorption/desorption

Because of the different experimental conditions found in literature for the measurement of the electroactive area of Pt/C electrodes of proton exchange membrane fuel cells (PEMFC) by means of underpotential hydrogen adsorption (H_UPD) voltammetry, specially concerning sweep rate and temperature, it was found necessary to perform an analysis of these parameters. With this aim, the electroactive area of PEMFC electrodes has been measured by means of H_UPD voltammetry at different sweep rates and temperatures, in liquid electrolyte and solid polymer contact. Both configurations show that H_UPD adsorption and desorption charges are strongly dependent on sweep rate voltage and temperature. The most common behaviour observed is a maximum in H_UPD desorption charge, typically in the 100–10 mV s⁻¹ sweep rate range, whereas H_UPD adsorption charge shows continuous increase with decreasing sweep rate. The decrease of desorption charge at low sweep rates is attributed to adsorbing species related with carbon support reactivity. These processes are also responsible for the increase in desorption H_UPD charge at low sweep rate. At high sweep rate, both adsorption and desorption H_UPD charges decrease due to limiting diffusion of protons through the microporous electrode. As a consequence, it is found that the closest approximation to the real electroactive area (i.e. the area accessible to protons) corresponds to the maximum in the H_UPD desorption charge in the range of 10–100 mV s⁻¹ sweep rate. The influence of measuring temperature is also tested in the range 25 °C–80 °C. A dependence of the adsorption and desorption hydrogen charges is found, due to thermodynamic and kinetics factors. We observe that the processes competing with hydrogen adsorption, i.e. generation and adsorption of carbon species are enhanced with temperature, so a low measuring temperature is found as most appropriate.