(326a) Highly Active Hydrogen Evolution Catalysts Based On Monolayer Pt On Tungsten Carbide

For the electrolysis of water in acidic solutions, the best hydrogen evolution reaction (HER) catalysts are precious metals such as Pt, but their prohibitively high costs are major hurdles for the widespread production of H2 by this means. One approach for overcoming this challenge is to disperse small amounts of the active precious metal on a low-cost, stable support or co-catalyst. In this work, we investigate the lower limits of Pt loading for the HER using sub-monolayer (ML) to multilayer amounts of Pt on low-cost tungsten monocarbide (WC) and its lower carbide, W2C. Using a combination of experimental and theoretical methods, we show that Pt-modified carbides, when engineered correctly, can exhibit HER activity comparable to bulk Pt. Tungsten carbides are attractive support materials for ML Pt catalysts because they exhibit Pt-like catalytic properties,[1] display good stability in acidic solutions,[2] and have demonstrated synergistic effects when used as a co-catalyst with Pt.[3] The similar catalytic properties of WC and Pt have often been attributed to similarities in the electronic properties of the two materials, an explanation that further suggests that ML Pt supported on WC might also show catalytic properties that are similar to bulk Pt. To test this hypothesis for Pt-tungsten carbide surfaces, we have calculated their hydrogen binding energies (HBE), a useful descriptor of a surface's electronic properties, using density functional theory (DFT). As expected, the DFT-calculated HBEs for Pt-WC and Pt-W2C surfaces are very similar to Pt. Given the strong correlation between HBEs and the HER activity of various surfaces, these DFT results suggest that ML Pt-Tungsten carbide surfaces, like bulk Pt, should be very active HER catalysts. To verify DFT predictions, single-phase WC and W2C thin films were synthesized using magnetron sputtering. The phase purity of these films was verified by X-ray photoelectron spectroscopy, symmetric X-ray diffraction (XRD), and glancing incidence XRD. Sub-ML to multi-ML amounts of Pt were deposited on the WC and W2C substrates by thermal evaporation, a process that has been shown to proceed in a layer-by-layer growth mechanism up to 2 ML.[4] The HER activities of all tungsten carbide and Pt-modified carbide surfaces were tested in 0.5 M H2SO4 using linear sweep voltammetry. The activity of the Pt-modified films was seen to increase significantly for sub-ML amounts of Pt, and approach that of bulk Pt at 1 ML coverage as predicted by DFT results. A plot of the experimental HER exchange current densities versus the DFT-calculated HBEs for each surface produces a volcano relationship, which is in good agreement with literature.[5] These experimental and theoretical results suggest that bulk Pt HER catalysts can be replaced with ML Pt-tungsten carbide, a development representing a large reduction in Pt loading and associated Pt costs. Ongoing work is focused on analyzing the long-term stability of these Pt-tungsten carbide surfaces in H2SO4.

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