Geometric and electronic modulation of fcc NiCo alloy by Group-Ⅵ B metal doping to accelerate hydrogen evolution reaction in acidic and alkaline media

Abstract

Bimetallic nickel–cobalt (Ni-Co) alloy has been regarded as an emerging class of material for the electrochemical hydrogen evolution reaction (HER). However, it operates at an overpotential significantly above the thermodynamic requirement. Here we present a facile one-step electrochemical reduction approach to incorporate fcc NiCo alloy with ⅥB group transition metals (M−NiCo, M = chromium (Cr), molybdenum (Mo), or tungsten (W)). This method enables to control the induction time of protrusion growth (τ) to produce hierarchical structure by imposing high current density (-0.6 A cm−2) and strong electric field on the cathode. Herein, W-NiCo holey nanotower arrays show tremendously enhanced HER activity compared with pristine NiCo alloy, which provides an overpotential of 109.2 mV (reduced by 58.7%) to achieve a current density of −10 mA cm−2, with a Tafel slope of 110.3 mV dec-1 in alkaline medium. The same law applies to the acid electrolyte with an overpotential of 78.5 mV (reduced by 66.1%) and low Tafel slop of 56.6 mV dec-1. Based on the experimental and theory analysis, We find W-doping can promote more active sites exposed, tune the electron configuration, moderate the gibbs free energy of the H intermediate, and accelerate the desorption of H2. It makes the rate-determining step change from Volmer to Heyrovsky reaction and finally surmount the limit of sluggish dynamics. The novel route provides an efficient doping control on Ni-based hierarchical alloys via 4f-orbital electronic and geometric structure engineering, which suggests an environmentally friendly method for energy-related process in practical applications.