Mixed close-packed cobalt molybdenum nitrides as non-noble metal electrocatalysts for the hydrogen evolution reaction

A two-step solid-state reaction for preparing cobalt molybdenum nitride with a nanoscale morphology has been used to produce a highly active and stable electrocatalyst for the hydrogen evolution reaction (HER) under acidic conditions that achieves an iR-corrected current density of 10 mA cm ^-2 at -0.20 V vs RHE at low catalyst loadings of 0.24 mg/cm ² in rotating disk experiments under a H₂ atmosphere. Neutron powder diffraction and pair distribution function (PDF) studies have been used to overcome the insensitivity of X-ray diffraction data to different transition-metal nitride structural polytypes and show that this cobalt molybdenum nitride crystallizes in space group P6₃/mmc with lattice parameters of a = 2.85176(2) Å and c = 10.9862(3) Å and a formula of Co_0.6Mo_1.4N₂. This space group results from the four-layered stacking sequence of a mixed close-packed structure with alternating layers of transition metals in octahedral and trigonal prismatic coordination and is a structure type for which HER activity has not previously been reported. Based on the accurate bond distances obtained from time-of-flight neutron diffraction data, it is determined that the octahedral sites contain a mixture of divalent Co and trivalent Mo, while the trigonal prismatic sites contain Mo in a higher oxidation state. X-ray photoelectron spectroscopy (XPS) studies confirm that at the sample surface nitrogen is present and N-H moieties are abundant.