Tuning the hydrogen evolution activity of β-Mo₂C nanoparticles via control of their growth conditions

The use of water electrocatalysis for hydrogen production is a promising, sustainable and greenhouse-gas-free process to develop disruptive renewable energy technologies. Transition metal carbides, in particular β-phase Mo₂C, are garnering increased attention as hydrogen evolution reaction (HER) catalysts due to their favourable synthesis conditions, stability and high catalytic efficiency. We use a thermodynamic approach in conjunction with density functional theory and a kinetic model of exchange current density to systematically study the HER activity of β-Mo₂C under different experimental conditions. We show that the (011) surface has the highest HER activity, which is rationalized by its lack of strong Mo-based hydrogen adsorption sites. Thus, the HER efficiency of β-Mo₂C can be tuned using nanoparticles (NPs) that expose larger fractions of this termination. We give definite maps between NP morphologies and experimental synthesis conditions, and show that the control of the carbon chemical potential during synthesis can expose up to 90% of the (011) surface, while ambient H₂ has little effect on the NP morphology. The "volcano" plot shows that under these optimum conditions, the NP exchange current density is ∼10^-5 A cm^-2, that is only slightly smaller than that of Pt (111).