Electrocatalytic Hydrogen Evolution at Full Atomic Utilization over ITO-Supported Sub-nano-Pt_n Clusters: High, Size-Dependent Activity Controlled by Fluxional Pt Hydride Species

Abstract

A combination of density functional theory (DFT) and experiments with atomically size-selected Pt n clusters deposited on indium-tin oxide (ITO) electrodes was used to examine the effects of applied potential and Pt n size on the electrocatalytic activity of Pt n (n = 1, 4, 7, and 8) for the hydrogen evolution reaction (HER). Activity is found to be negligible for isolated Pt atoms on ITO, increasing rapidly with Pt n size such that Pt7/ITO and Pt8/ITO have roughly double the activity per Pt atom compared to atoms in the surface layer of polycrystalline Pt. Both the DFT and experiment find that hydrogen under-potential deposition (Hupd) results in Pt n /ITO (n = 4, 7, and 8) adsorbing ∼2H atoms/Pt atom at the HER threshold potential, equal to ca. double the Hupd observed for Pt bulk or nanoparticles. The cluster catalysts under electrocatalytic conditions are hence best described as a Pt hydride compound, significantly departing from a metallic Pt cluster. The exception is Pt1/ITO, where H adsorption at the HER threshold potential is energetically unfavorable. The theory combines global optimization with grand canonical approaches for the influence of potential, uncovering the fact that several metastable structures contribute to the HER, changing with the applied potential. It is hence critical to include reactions of the ensemble of energetically accessible Pt n H x /ITO structures to correctly predict the activity vs Pt n size and applied potential. For the small clusters, spillover of Hads from the clusters to the ITO support is significant, resulting in a competing channel for loss of Hads, particularly at slow potential scan rates.

Hydrogen Evolution on Electrode‐Supported Ptn Clusters: Ensemble of Hydride States Governs the Size Dependent Reactivity

“…Here we compare the HER mass activities for Pt n /FTO with the activity of Pt poly , and with that for Pt n supported on indium tin oxide (ITO), [40] measured under identical conditions. Figure 7 compares the mass activities for Pt n /FTO and Pt n /ITO ( n =1, 4, 7, 8) at a potential of −0.027 V vs. RHE, corresponding to an overpotential η =0.05 V, based on currents measured in the CVs in Figure 2a, and analogous CVs reported for Pt n /ITO [40] .…”

“…Here we compare the HER mass activities for Pt n /FTO with the activity of Pt poly , and with that for Pt n supported on indium tin oxide (ITO), [40] measured under identical conditions. Figure 7 compares the mass activities for Pt n /FTO and Pt n /ITO ( n =1, 4, 7, 8) at a potential of −0.027 V vs. RHE, corresponding to an overpotential η =0.05 V, based on currents measured in the CVs in Figure 2a, and analogous CVs reported for Pt n /ITO [40] . The size dependences are generally similar, but all four Pt n /FTO samples were significantly more active than the corresponding Pt n /ITO at η =0.05 V. For Pt 1 , the activity on FTO, although small, was more than double that on ITO, and for the larger, more active clusters, the FTO support is more active by ≈20 %.…”

Electrochemical Oxidation of Methane to Methanol on Electrodeposited Transition Metal Oxides

“…Convergence in the adsorption energies and recovery of the bulk magnetic ordering of the center layers was obtained with a minimum thickness of four Co oxide layers and 30 Å of vacuum. To complete their octahedral coordination shell, each surface Co atom was liganded with one hydroxyl group, which would be expected in electrochemical conditions under liquid water. ,− All model slabs show two identical surfaces and present an inversion symmetry to cancel any dipole moment in the perpendicular direction. Only the outer atoms were allowed to relax during the surface geometry relaxations; the inside atoms (in a 2 Å large intermediate layer) and cell parameters remained fixed at the values established for the bulk β-CoOOH.…”

Elucidation of the Active Site for the Oxygen Evolution Reaction on a Single Pt Atom Supported on Indium Tin Oxide

“…Single-atom catalysts (SACs), with their potential maximized atom utilization efficiency, have attracted significant attention as a result of their high catalytic activity and selectivity. , These catalysts have been applied to a large range of reactions, including CO oxidation, − hydrogenation, dehydrogenation, , and other electrocatalytic reactions. − For the expensive and in-demand noble metal catalysts, like Ir, Pd, and Pt, SACs represent an ultimate dispersion of the metal with all of the atoms exposed − and, if activity remains high compared to nanoparticle catalysts, an optimal utilization and cost. Additionally, SACs might open up particular reaction pathways, different from that of larger particles, and are well-suited for a molecular-level understanding of the active-site structure and reaction mechanism by utilizing a variety of element-specific analytical tools supplemented by in situ / operando characterization and coupled with computational modeling. ,− There is now strong evidence that SACs are not static catalysts but that they respond to reaction conditions by adapting their set of ligands and their mode of interaction with the support, making the determination of the active site challenging .…”