N-Doped graphene-coated molybdenum carbide nanoparticles as highly efficient electrocatalysts for the hydrogen evolution reaction

Abstract

In our efforts to explore promising substitutes for Pt-based electrocatalysts for the hydrogen evolution reaction (HER), a new type of molybdenum carbide nanoparticle coated with graphene shells with nitrogen dopants (abbr. MoC_x@C-1) is prepared from an entangled polyoxometalate-encapsulated coordination polymer (PECP), [Zn(bimbp)₂]₃[PMo₁₂O₄₀]₂·2H₂O (PECP-1) (bimbp = 4,4′-bis(imidazolyl)biphenyl) via the annealing and etching processes. The synergistic effects between highly dispersive MoC_x particles, graphene coatings and N dopants in MoC_x@C-1 lead to remarkable HER performance in acidic media with a very positive onset potential close to that of commercial 20% Pt/C catalysts, a low Tafel slope of 56 mV dec⁻¹, a high exchange current density of 0.27 mA cm⁻², and superior long-term cycle stability. In particular, MoC_x@C-1 exhibiting an overpotential of 79 mV at a current density of 10 mA cm⁻² represents one of the currently best reported MoC_x-based HER electrocatalysts in acidic media. Such performance is also better than that of uncoated MoC_x-2 nanoparticles prepared by carburizing another PECP [Bu₄N][Zn₃(bimb)₄Cl(MoO₄)][PMo^VMo^VI₁₁O₄₀]·4H₂O (PECP-2) (bimb = 1,4-bis(1-imidazolyl)benzene). This work provides a new feasible route to prepare nanostructured hybrids composed of transition metal carbides, graphene and N dopants with higher HER activity and stability.