Article pubs.acs.org/JPCC
High Elastic Strain Directly Tunes the Hydrogen Evolution Reaction on Tungsten Carbide Kai Yan, Seok Ki Kim, Alireza Khorshidi, Pradeep R. Guduru,* and Andrew A. Peterson* School of Engineering, Brown University, 184 Hope Street, Providence, Rhode Island 02912, United States S Supporting Information *
ABSTRACT: Elastic strain provides a direct means to tune a material’s electronic structure from both computational and experimental vantage points and can thus provide insights into surface reactivity via changes induced by electronic structure shifts. Here we investigate the role of elastic strain on the catalytic activity of tungsten carbide (WC) in the hydrogen evolution reaction. WC makes an interesting material for such investigations as it is an inherently promising catalyst that can sustain larger elastic strains (e.g., −1.4 to 1.4%) than common transition-metal catalysts, such as Pt or Ni (e.g., −0.4 to 0.4%). On the basis of density functional theory calculations, a compressive uniaxial strain is expected to cause weakening of the surface−hydrogen interaction of 10−15 meV per percent strain, while a tensile strain is calculated to strengthen the surface−hydrogen interaction by a similar magnitude. Sabatier analysis suggests that weakening of the surface-hydrogen interaction would enhance catalysis. We prepared 20 nm thin films of WC supported on thick polymer substrates and mechanically subjected them to uniaxial tensile and compressive loading, while the films catalyze hydrogen evolution in an electrochemical cell. We report a systematic shift in the hydrogen evolution sweeps of cyclic voltammetry measurements: Compressive strain increases the activity, and tensile strain has the opposite effect. The magnitude of the shift was measured to be 10−20 mV per 1% strain, which agrees well with the computations and corresponds to 5−10% of the difference in the overpotentials of WC and Pt. These results were further substantiated through chronoamperometry measurements and highlight how strain can be used to systematically improve catalytic activity. or porous structures) has gained significant growth.24−26 Michalsky et al.10 has suggested that WC sits on the strongbinding regime of the hydrogen-evolution volcano plot (i.e., to the left of the volcano peak) and that it exhibits a stronger sensitivity to hydrogen coverage than similar transition metals. This would suggest that a weakening of the hydrogen adsorption energy could increase the catalytic activity. Lattice strain is capable of directly tuning adsorbate binding energy on a catalyst surface, thus changing catalytic activity.12,27−30 Recently, the effects of lattice strain have been cleanly separated from preparation effects (e.g., ligand effects12,28,31 or artifacts of dealloying14−16,32) by applying varying levels of mechanical loading to a single sample consisting of a transition-metal catalyst.33−38 However, in addition to its interesting catalytic properties described above, WC can sustain larger elastic strains than the common transition-metal catalysts employed in previous studies.39−41 In the current work, we explore the role of strain on WC HER catalysts, both through electronic-structure calculations of the surface-hydrogen bond strength and via the direct application of mechanical strain. We anticipate that these findings will be useful in the future design of improved WC electrocatalysts.
1. INTRODUCTION Hydrogen can enable the wider use of renewable energy sources: It can be used directly as an energy storage medium or as a chemical reactant for the reduction of other species, such as CO2/CO, N2, or biomass.1−4 Efficient electrocatalytic hydrogen production is a key technology for converting renewable electricity into H2 and therefore reducing dependence on fossil fuels.5−10 As a result, the development of inexpensive catalysts for the electrochemical hydrogen evolution reaction (HER), that is, water splitting, has attracted a great deal of research effort in recent decades. Some recent studies have focused on the generation of lattice strain on Pt- or Pd-derived bimetallic catalysts to influence the surface electronic structure and thus the catalytic properties.11−14 However, it has been recognized that large-scale water splitting for hydrogen production would require less expensive, noble-metal-free catalysts. As a result of this research direction, several classes of low-cost HER catalysts have been developed, which include materials such as carbides, sulfides, and nitrides.15−21 Over the last few decades, starting with the discovery that tungsten carbide (WC) possesses dband electronic states similar to those of Pt and can have similar catalytic properties,22 it has emerged as a promising HER catalyst. In a recent investigation, Wirth et al.23 showed that the overpotential of WC toward HER is only ∼200 mV greater than that of Pt. The design and synthesis of efficient WC materials with a variety of microstructures (e.g., microspheres © XXXX American Chemical Society
Received: January 10, 2017 Revised: February 15, 2017
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DOI: 10.1021/acs.jpcc.7b00281 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C 2H* → H 2 + *
2. METHODS We sputtered WC onto a polymeric substrate in a planar geometry such that the WC would inherit the strain response of the substrate. (See ref 38 for a discussion of the transfer of strain from the substrate to a thin metal film.) This composite electrode was bolted into a universal mechanical testing machine, which allowed us to precisely apply various tensile and compressive loads to the electrode, via the polymeric substrate, and the composite electrode was submerged in a custom-fabricated three-electrode cell to facilitate these experiments. Electronic structure calculations were undertaken in density functional theory (DFT) with the assumption that the WC inherited the substrate’s response. Full details follow. 2.1. Electronic Structure Calculations. DFT calculations were performed to evaluate the effect of uniaxial strain on the hydrogen binding energy for the WC surface. The planewave basis set implemented in the DACAPO code27,42 was used with a planewave cutoff energy of 400 eV and density cutoff energy of 500 eV. Inner electrons were represented by Vanderbilt ultrasoft pseudopotentials.43,44 The exchange-correlation interaction was parametrized with the revised Perdew−Burke− Ernzerhof (RPBE) functional45 within a generalized gradient approximation (GGA). (RPBE can be more accurate than PBE for the calculation of binding energies on transition-metal surfaces.27,46,47) The convergence criteria for the electron density between ionic steps was 1 × 10−4 eV for all calculations. To improve electronic convergence, the occupation of Kohn− Sham states was smeared according to a Fermi−Dirac distribution with a thermal energy kBT = 0.1 eV, with standard extrapolation of electronic energies to kBT = 0 eV. The bulk and surface slab calculations were performed using (8 × 8 × 8) and (8 × 8 × 1) k-point samplings, respectively, with standard dipole corrections. During geometry optimizations, all forces were converged to be
