Films
of CoP have been electrochemically synthesized, characterized, and
evaluated for performance as a catalyst for the hydrogen-evolution
reaction (HER). The film was synthesized by cathodic deposition from
a boric acid solution of Co2+ and H2PO2
– on copper substrates followed by operando remediation of exogenous contaminants. The films were characterized
structurally and compositionally by scanning-electron microscopy,
energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy,
and Raman spectrophotometry. The catalytic activity was evaluated
by cyclic voltammetry and chronopotentiometry. Surface characterization
prior to electrocatalysis indicated that the film consisted of micrometer-sized
spherical clusters located randomly and loosely on a slightly roughened
surface. The composition of both the clusters and surface consisted
of cobalt in the metallic, phosphide, and amorphous-oxide forms (CoO·Co2O3) and of phosphorus as phosphide and orthophosphate.
The orthophosphate species, produced by air-oxidation, were eliminated
upon HER electrocatalysis in sulfuric acid. The operando film purification yielded a functional electrocatalyst with a Co:P
stoichiometric ratio of 1:1. After the HER, the surface was densely
packed with micrometer-sized, mesa-like particles whose tops were
flat and smooth. The CoP eletrodeposit exhibited an 85 mV overvoltage
(η) for the HER at a current density of 10 mA cm–2 and was stable under operation in highly acidic solution, with an
increase in η of 18 mV after 24 h of continuous operation. The
comparative HER catalytic performance of CoP, film or nanoparticles,
is as follows: ηPt < ηCoP film = ηCoP NP, ηNi2P < ηCoSe2
< ηMoS2
< ηMoSe2
.
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