Kinetics of the Hydrogen Oxidation/Evolution Reaction on Polycrystalline Platinum in Alkaline Electrolyte Reaction Order with Respect to Hydrogen Pressure

The hydrogen oxidation and evolution reactions (HOR, HER) are of key importance to the development of novel alkaline membrane fuel cells and electrolyzers, which feature a potential cost advantage over their acid-operating counterparts. However, their mechanism remains poorly understood even on the most catalytically-active platinum surfaces, for which such a fundamental parameter as the reactions' order with respect to the hydrogen concentration is still unknown. With this motivation, we have performed rotating disc electrode measurements on polycrystalline Pt in 0.1 M NaOH with different hydrogen partial pressures (between 10 and 100 kPa H2), from which we have derived a reaction order of 1/2. The latter value has important implications in the procedure to follow in order to derive kinetic currents free of diffusion contributions. More precisely, the HOR currents must be corrected for the diffusion overpotential and converted into a mass-transport free kinetic current using a modified version of the Koutecky-Levich equation that takes into consideration this non-unit reaction order, while the HER side only needs to be ohmically compensated due to the impossibility to supersaturate the electrolye with H2. Most importantly, our results point at a mechanism consisting of a dissociative adsorption (Tafel) reaction combined with a one-electron transfer (Volmer) rate-determining step, in terms consistent with the well-established view of the hydrogen-bonding strength as the main HOR/HER activity descriptor.