Highly active hydrogen evolution reaction (HER) catalysts formed by energetic Ptn cluster deposition: Deposition dynamics and the HER mechanism

Mass- and energy-selected Ptn+ (n ≤ 7) clusters were deposited at variable energies on highly oriented pyrolytic graphite (HOPG) to create highly active hydrogen evolution reaction (HER) electrocatalysts with clusters stabilized by interaction with carbon defects and subplanted Pt atoms. HER mass activities for the Ptn/HOPG electrodes were between 2 and >10 times higher than that for the surface monolayer of bulk Pt, and also substantially higher than those for Ptn deposited on several other supports. Thus, high activity is a property of the Pt-C structures formed by Ptn – HOPG impacts, rather than simply being due to high Pt surface availability. The physical properties of the Ptn/HOPG electrodes were probed by X-ray photoelectron spectroscopy, low energy He+ ion scattering, and a variety of electron microscopy methods. Born-Oppenheimer molecular dynamic (BOMD) trajectories were used to simulate the Ptn – HOPG impacts, revealing the types of structures formed at different energies, then DFT was used to simulate different HER pathways for the most important Ptn-carbon structures. Experiment and theory lead to the following mechanism. For low deposition energies, the Ptn deposit onto the HOPG surface with sticking probability below unity, and aggregate at defects and steps. With increasing deposition energy, the sticking probability initially decreases, then approaches unity as subplantation and carbon defect creation increase the Pt-surface binding. The rate-limiting barriers for HER at the structures formed in the trajectories were found to be low and weakly dependent on the Ptn size, consistent with the observation of high HER activities that were weakly size dependent. The activity of the surface atoms in Ptn/HOPG were highest for small covalently-bonded Pt-C structures created at high deposition energies. The activity of the larger aggregated structures form at low energies was lower, but still substantially higher than the surface monolayer of bulk Pt. The catalysts were stable in repeated potential cycling, unless the upper potential ranged into oxidizing conditions. In that case, electrodes with Pt all on the surface were still stable, but electrodes containing subplanted Pt became more active as Pt emerged into electrolyte-accessible surface sites.