Comparing the thin-film rotating disk electrode and the ultramicroelectrode with cavity techniques to study carbon-supported platinum for proton exchange membrane fuel cell applications
Résumé
Active layers elaborated from commercial carbon-supported platinum nanoparticles (10 wt% Pt/C, E-TEK) have been characterized from cyclic voltammetry using either the porous rotating disk electrode (RDE) or the ultramicroelectrode with cavity (UMEC) techniques. The electrochemical properties of the platinum-based active layers were compared for two classical fuel cell catalysts benchmark measurements: the platinum specific area determined from hydrogen adsorption/desorption or CO-stripping coulometry and the evaluation of kinetics parameters for oxygen reduction reaction. Whereas the porous RDE technique requires the prior preparation of an ink composed of a binder (Nafion®) and non negligible amounts of active material (at least 10 mg Pt/C powder), the UMEC is compatible with the elaboration of Nafion®-free active layers, only composed of very small amounts of Pt/C (a few nanograms). Both the UMEC and the porous RDE techniques require the correction from the oxygen diffusion in the active layer for high ORR overpotentials (E < 0.8 V vs. NHE). At low ORR overpotentials (E > 0.8 V vs. NHE) only the UMEC requires a correction from the oxygen diffusion in the active layer, easily calculated using the macro-homogeneous model. The so-corrected ORR kinetic current densities are compared with that determined from other techniques of the literature, among which the thin-film RDE. Last, the comparison between UMEC including or not Nafion® and porous RDE showed the non-negligible effect of the presence of Nafion® in carbon-supported active layers, inducing modification of the platinum nanoparticles electrocatalytic activity for both CO-stripping and ORR. Thanks to its great advantages (low amount of Pt/C used, absence of binder and facile control of the quantity of active material in the cavity, shorter time of experiment, higher reproducibility) the ultra-microelectrode with cavity is a promising method to characterize fuel cell electrocatalysts. It enables Pt/C catalysts easy specific area measurements and ORR kinetics parameters determination without using any additional binder. The benefit of UMECs has been validated for the characterisation of commercial fresh or used anode active layer materials, which could not be attempted accurately using the classical RDE technique because of their high Nafion® loading.