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/cm2 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 Co0.6Mo1.4N2. 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.