Elucidating the synergistic mechanism of nickel-molybdenum electrocatalysts for the hydrogen evolution reaction
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unctional Oxides Research Letter
Elucidating the synergistic mechanism of nickel–molybdenum electrocatalysts for the hydrogen evolution reaction Ian S. McKay†, Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA Jay A. Schwalbe†, Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA; SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, CA 94305, USA Emmett D. Goodman, and Joshua J. Willis, Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA Arun Majumdar, Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA Matteo Cargnello, Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA; SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, CA 94305, USA Address all correspondence to Matteo Cargnello at [email protected] (Received 22 April 2016; accepted 2 August 2016)
Abstract Nickel–molybdenum (Ni–Mo) materials are widely used functional oxide catalysts for the hydrogen evolution reaction. In this work, we investigate the high activity of Ni–Mo by depositing size-controlled Ni nanocrystals (NCs) onto Mo substrates. We observe a synergistic increase in catalytic activity that does not scale with the Ni–Mo interface length. This evidence points to a bulk electronic interaction of the two metals that is separate from the mechanism of enhancement seen in conventionally co-deposited Ni–Mo electrocatalysts. In addition to elucidating the catalytic behavior of the Ni–Mo system, this work offers a general NC-based paradigm for investigating fundamental interactions and synergistic effects in electrocatalytic materials.
Introduction Hydrogen is a promising energy vector, a valuable industrial building block, and a key reagent for the thermochemical conversion of CO2 into fuels and chemicals.[1,2] The challenge is to produce it sustainably and cost-effectively. To date, renewable means of H2 production such as water electrolysis have failed to achieve cost-parity with fossil fuel-based steam methane reforming and account for 100%) is too large to be due to oxidation of the particles alone, although this effect would certainly contribute. Based on the lattice parameters and crystal structures of Ni and NiO,[28] total oxidation of the particles should increase the large NC apparent diameters by maximum 20%. Sintering also fails to explain the observed size increase; as shown in Fig. 2(a), multiple layers of particles on a glassy carbon substrate appear the same size as identical particles viewed in TEM. This observation is consistent with previous work showing that fast annealing to remove ligands does not cause sintering in the NC layers.[25] The different point spread functions of the SEM and TEM optics were also considered as potential explanations for the observed size disparity, but the ligand-coated NCs on TEM grids have approximately the same apparent size when viewed in either instrument. Ultimately, by tilting the substrat
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