Hydrogen reaction at open circuit in alkaline media on Pt in a gas-diffusion electrode

Hydrogen reaction on Pt-catalyzed carbon-polytetrafluoroethylene gas-diffusion electrode in KOH solutions at concentrations in the range 1.0–6.0 mol dm−3, temperatures from 5.0 to 50.0 °C and H2 partial pressures between 0.1 and 1.0 atm has been studied by electrochemical impedance spectroscopy at open circuit potential. The experimental impedance diagrams, with two time constants, have been explained by means of an equivalent circuit considering the electrode porosity and the kinetics of hydrogen reaction. No significant diffusion effects have been detected in the Nyquist diagrams up to about the mHz. The dependence of the exchange current density on the OH− ion concentration and hydrogen partial pressure has been interpreted on the basis of a Tafel–Volmer mechanism, with the Tafel reaction as the slowest step, in agreement with previous work in which kinetic data were obtained from linear sweep voltammetry. The apparent activation energies determined from the Arrhenius plots of the exchange current densities correspond to the hydrogen reaction on Pt(1 0 0) faces, in agreement with a cubo-octahedral structure of the Pt nanoparticles.