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
.
A solar-driven CO2 reduction
(CO2R) cell
was constructed, consisting of a tandem GaAs/InGaP/TiO2/Ni photoanode in 1.0 M KOH(aq) (pH = 13.7) to facilitate the oxygen-evolution
reaction (OER), a Pd/C nanoparticle-coated Ti mesh cathode in 2.8
M KHCO3(aq) (pH = 8.0) to perform the CO2R reaction,
and a bipolar membrane to allow for steady-state operation of the
catholyte and anolyte at different bulk pH values. At the operational
current density of 8.5 mA cm–2, in 2.8 M KHCO3(aq), the cathode exhibited <100 mV overpotential and >94%
Faradaic efficiency for the reduction of 1 atm of CO2(g)
to formate. The anode exhibited a 320 ± 7 mV overpotential for
the OER in 1.0 M KOH(aq), and the bipolar membrane exhibited ∼480
mV voltage loss with minimal product crossovers and >90 and >95%
selectivity
for protons and hydroxide ions, respectively. The bipolar membrane
facilitated coupling between two electrodes and electrolytes, one
for the CO2R reaction and one for the OER, that typically
operate at mutually different pH values and produced a lower total
cell overvoltage than known single-electrolyte CO2R systems
while exhibiting ∼10% solar-to-fuels energy-conversion efficiency.
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