Hydrogen Evolution on Diamond Electrodes by the Volmer-Heyrovsky Mechanism

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Abstract
Hydrogen evolution on (111) diamond electrode surfaces was studied by hybrid density functional theory with cluster models. The reductive hydronium ion discharge in the Volmer-Heyrovsky (V-H) mechanism to form H2H2 was found to be activated when there was either a bond-inserted interstitial hydrogen defect or a substitutional boron defect in the near subsurface region. Activation energies for the Heyrovsky reaction were calculated for the doped and undoped surfaces as functions of electrode potential, ranging between 0 and −3.2V−3.2V , using a constrained variation theory. It is concluded that weakly adsorbed H associated with near subsurface substitutional boron defects catalyze the hydrogen evolution activity that has been observed for boron-doped diamond electrodes. Following the electron transfer, we found no barrier for forming H2H2 , irrespective of whether the defect was hydrogen or boron. A barrier was found for hydrogen evolution on defect-free diamond. Calculated activation energies for the Volmer reaction step were lower than for the Heyrovsky step, which indicates the Heyrovsky step is rate limiting. An important conclusion from this work is that the boron doping of diamond not only provides conductivity, but, we propose, also gives rise to a weakly bonded hydrogen that participates in the V-H mechanism of hydrogen evolution.

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