Abstract
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 iRcorrected
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.
