Overpotentials and reaction mechanism in electrochemical hydrogen pumps

The overpotentials generated in an electrochemical hydrogen pump (EHP) are experimentally separated using a hydrogen reference electrode. An intentionally thick polymer electrolyte membrane and a low-Pt-catalyst-loaded layer are embedded in the EHP cell for high accuracy in separation. The results show that the non-ohmic overpotential of the cathode is larger than that of the anode. Electrochemical impedance spectroscopy (EIS) is employed to analyze the non-ohmic overpotential in detail, and different features are observed in the Nyquist plots for the anode and the cathode, where the cathode spectra exhibits charge transfer rate-limiting features and the anode spectra displays mass transfer rate-limiting features. These different characteristics are theoretically clarified by the classical Volmer–Heyrovsky–Tafel mechanism. The hydrogen evolution reaction (HER) in the cathode is dominated by the Volmer–Heyrovsky route, and the hydrogen oxidation reaction (HOR) in the anode is dominated by the Volmer–Tafel route. Due to the slow reaction rate imposed by the Volmer–Heyrovsky route, the cathode HER displays high non-ohmic overpotential. With increasing cathode pressure, the rate of HER is increased due to the increasing coverage of the hydrogen adsorbed on the surface of the catalyst.