Enhancing the Understanding of Hydrogen Evolution and Oxidation Reactions on Pt(111) through Ab Initio Simulation of Electrode/Electrolyte Kinetics

The hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) play an important role in hydrogen-based energy conversion. However, the sluggish kinetics in alkaline media has raised debates on the relevant mechanism, especially on the role of surface hydroxyl (OH*). With the potential-related free energy profiles obtained with density functional theory calculations, the full pH range transient kinetics simulation of HER/HOR polarization curves on Pt(111) agrees well with experimental observations. Studying model systems with varying metal-OH* binding energies confirms that the current near the HOR onset potential is contributed from the pathway through OH^- rather than OH*, suggesting that OH* is unlikely an effective activity descriptor for HOR. The degree of rate control analyses reveal that, while acidic current is controlled solely by the Tafel step, alkaline current is controlled jointly by Tafel and Volmer steps, as the Volmer barrier is considerably increased in alkaline conditions. Finally, based on a model study, we draw up a scheme of reducing the overpotential of alkaline HER/HOR by accelerating the Tafel step.