CoP as an Acid-Stable Active Electrocatalyst for the Hydrogen-Evolution Reaction: Electrochemical Synthesis, Interfacial Characterization and Performance Evaluation

Abstract

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 .

Operando Spectroscopic Analysis of CoP Films Electrocatalyzing the Hydrogen-Evolution Reaction

“…20 Moreover, the XPS analysis as well as prior energy-dispersive X-ray spectroscopic (EDS) analysis, indicated a Co:P elemental ratio in large excess of the 1:1 ratio expected for stoichiometric CoP, in accord with expectations that a substantial amount of Co species is not in the reduced phosphide state. 20 The Raman data presented herein are thus consistent with the prior XPS and EDS analyses. The same as-deposited electrocatalyst film was then immersed in 0.500 M H2SO4(aq) and an in-situ Raman spectrum was collected without an applied bias (open-circuit conditions; Figure 2b).…”

“…[15][16][17][18][19] Electrodeposited CoP films effect the HER at a current density of -10 mA cm -2 with - < 100 mV in acidic solution (pH < 1). 20 Characterization of this material, as well as related metal phosphides, has relied principally upon ex-situ techniques that typically involve analysis in vacuum or laboratory ambients. Herein, electrodeposited CoP films have been investigated under in-situ and operando conditions using Raman spectroscopy and Co and P K-edge X-ray absorption spectroscopy (XAS), including analysis of the resulting X-ray absorption nearedge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data.…”

Electrodeposition and Characterization of CoP Compounds Produced from the Hydrophilic Room-Temperature Ionic Liquid 1-Butyl-1-Methylpyrrolidinium Dicyanamide

“…Previous studies indicate that CoP could be formed via a chemical reaction of Co + P → CoP where elemental Co and P are produced during the electrodeposition. 11,32 Relevant to the CoP, an indirect mechanism for NiP formation has been reported in which H 2 PO 2 − is reduced to phosphine (PH 3 ) which then reacts with Ni 2+ to form NiP. 33 However, this mechanism favors in a strongly acidic condition that is different from the condition in our study.…”

Highly branched cobalt phosphide nanostructures for hydrogen-evolution electrocatalysis

“…Indeed, comparable HER activity has been observed by several groups for CoP materials that span a range of morphologies, synthetic protocols, accessible surface areas, and support materials. [4][5][6][7][8][9][10][11] A key nding of this work is that the activities are largely the same, regardless of morphology or preparation method, underscoring the high intrinsic activity of CoP. The intrinsic HER activity of CoP makes it a highly viable candidate for practical applications, regardless , along with Ti foil and Pt mesh electrodes for comparison.…”