Kinetic Implication from Temperature Effect on Hydrogen Evolution Reaction at Ag Electrode

Hydrogen evolution reaction (HER) at polycrystalline silver electrode in 0.1 mol/L HClO4 solution is investigated by cyclic voltammetry in the temperature range of 278-333 K. We found that at electrode potential phi<PZC (potential of zero charge), the apparent activation energy E-a,E-app decreases with phi, while pre-exponential factor A remains nearly unchanged, which conforms well the prediction from Butler-Volmer equation. In contrast, with phi negative shifts from the onset potential for HER to the potential of zero charge (PZC approximate to-0.4 V), both E-a,E-app and A for HER increase (e.g., E-a,E-app increases from 24 kJ/mol to 32 kJ/mol). The increase in E-a,E-app and A with negative shift in phi from -0.25 V to PZC is explained by the increases of both internal energy change and entropy change from reactants to the transition states, which is correlated with the change in the hydrogen bond network during HER. The positive entropy effects overcompensate the adverse effect from the increase in the activation energy, which leads to a net increase in HER current with the activation energy negative shift from the onset potential of HER to PZC. It is pointed out that entropy change may contribute greatly to the kinetics for electrode reaction which involves the transfer of electron and proton, such as HER.