Structure and Valency of a Cobalt−Phosphate Water Oxidation Catalyst Determined by in Situ X-ray Spectroscopy

TLDR

A cobalt‑phosphate water oxidation catalyst (Co−Pi) formed by electrodeposition from aqueous phosphate and Co²⁺ solutions has been investigated by in situ X‑ray absorption spectroscopy, revealing that its CoO₆ octahedra form molecular‑size clusters rather than the extended planes found in cobaltates. The study obtained Co−Pi spectra at catalytic potentials and open circuit for films of two thicknesses, and, using EXAFS fits and comparison with Co oxide spectra, proposed that the catalyst consists of edge‑sharing CoO₆ octahedra forming Co‑oxo/hydroxo clusters. EXAFS revealed bis‑oxo/hydroxo‑bridged Co subunits forming higher‑nuclearity clusters, with larger clusters in thicker films; XANES and electrochemical data showed Co valency >3 during catalysis at 1.25 V, while open‑circuit reduction—whose rate depends on cluster size—was evidenced by a negative edge‑energy shift.

Abstract

A water oxidation catalyst generated via electrodeposition from aqueous solutions containing phosphate and Co2+ (Co−Pi) has been studied by in situ X-ray absorption spectroscopy. Spectra were obtained for Co−Pi films of two different thicknesses at an applied potential supporting water oxidation catalysis and at open circuit. Extended X-ray absorption fine structure (EXAFS) spectra indicate the presence of bis-oxo/hydroxo-bridged Co subunits incorporated into higher nuclearity clusters in Co−Pi. The average cluster nuclearity is greater in a relatively thick film (∼40−50 nmol Co ions/cm2) deposited at 1.25 V vs NHE than in an extremely thin film (∼3 nmol Co ions/cm2) deposited at 1.1 V. X-ray absorption near edge structure (XANES) spectra and electrochemical data support a Co valency greater than 3 for both Co−Pi samples when catalyzing water oxidation at 1.25 V. Upon switching to open circuit, Co−Pi undergoes a continuous reduction due to residual water oxidation catalysis, as indicated by the negative shift of the edge energy. The rate of reduction depends on the average cluster size. On the basis of structural parameters extracted from fits to the EXAFS data of Co−Pi with two different thicknesses and comparisons with EXAFS spectra of Co oxide compounds, a model is proposed wherein the Co oxo/hydroxo clusters of Co−Pi are composed of edge-sharing CoO6 octahedra, the structural motif found in cobaltates. Whereas cobaltates contain extended planes of CoO6 octahedra, the Co−Pi clusters are of molecular dimensions.