Highly Active Hydrogen Evolution Reaction (HER) Catalysts Formed by Energetic Pt_n Cluster Deposition: Deposition Dynamics and the HER Mechanism

Mass-selected Ptn+ (n ≤ 7) were deposited at variable energies on highly oriented pyrolytic graphite (HOPG), creating highly active hydrogen evolution reaction (HER) electrocatalysts. HER mass activities were ∼2 to >10 times higher than those for the surface atoms in bulk Pt and for Ptn deposited on several other supports. Thus, high activity reflects the Pt-C structures formed by energetic Ptn─HOPG impacts, in addition to high Pt surface availability. The Ptn/HOPG electrodes were probed by X-ray photoelectron spectroscopy, low energy ion scattering, and electron microscopy. Born-Oppenheimer molecular dynamics (BOMD) was used to simulate Ptn─HOPG impacts, revealing the types of structures formed at different energies, then DFT was used to probe their most important HER pathways. For low deposition energies, the Ptn deposit onto the HOPG surface with subunit sticking probability, aggregating at defects. With increasing deposition energy, the sticking probability initially decreases, then rises to unity as subplantation and defect creation allow formation of strongly bonded platinum-carbon structures. Barriers for HER on these structures were found to be low and weakly dependent on Ptn size, consistent with experiment. The activities were highest for small covalently bonded Pt-C structures created at high deposition energies. The larger aggregated structures formed at low energies were less active, but still substantially better than the bulk Pt surface monolayer. The catalysts were stable in repeated potential cycling at reducing potentials, but electrodes containing subplanted Pt became more active when scanned to oxidizing potentials, due to emergence of subplanted Pt onto the surface.