Abstract
Here we report an in situ electron paramagnetic resonance (EPR) study of a low-cost, high-stability cobalt oxide electrodeposited material (Co-Pi) that oxidizes water at neutral pH and low over-potential, representing a promising system for future large-scale water splitting applications. Using CW X-band EPR we can follow the film formation from a Co(NO3)2 solution in phosphate buffer and quantify Co uptake into the catalytic film. As deposited, the film shows predominantly a Co(II) EPR signal, which converts into a Co(IV) signal as the electrode potential is increased. A purpose-built spectroelectrochemical cell allowed us to quantify the extent of Co(II) to Co(IV) conversion as a function of potential bias under operating conditions. Consistent with its role as an intermediate, Co(IV) is formed at potentials commensurate with electrocatalytic O2 evolution (+1.2 V, vs. SHE). The EPR resonance position of the Co(IV) species shifts to higher fields as the potential is increased above 1.2 V. Such a shift of the Co(IV) signal may be assigned to changes in the local Co structure, displaying a more distorted ligand field or more ligand radical character, suggesting it is this subset of sites that represents the catalytically ‘active’ component. The described spectroelectrochemical approach provides new information on catalyst function and reaction pathways of water oxidation.
Highlights
The Nocera catalyst [1], a self-healing [2], cobalt/phosphate derivative (Co-Pi), formed in situ from a CoII solution under oxygen evolution potentials represents a promising system for future large-scale water splitting applications coupled to the production of hydrogen as fuel [3]
The measurement of catalysts interfaced with conducting electrodes poses a series of challenges when using electron paramagnetic resonance (EPR) spectroscopy
EPR measurements are conducted in EPR resonators that separate the magnetic field component (B1 ), inducing the EPR signal, from the electric component (E1 ) of the electromagnetic radiation
Summary
The Nocera catalyst [1], a self-healing [2], cobalt/phosphate derivative (Co-Pi), formed in situ from a CoII solution under oxygen evolution potentials represents a promising system for future large-scale water splitting applications coupled to the production of hydrogen as fuel [3]. Co(IV) species in Co-oxide films, on the contrary, exhibit rhombic g-tensors with significantly larger g-anisotropy as compared to their Co(III) Co(IV) counterparts This discrepancy was rationalized by a much more localized electron spin density on one Co center [33]. More reliable quantitative information on catalyst structures and oxidation states can be extracted from in situ EPR measurements on intact films on electrodes with applied potential. Such measurements require dedicated spectroelectrochemical cells that can be inserted in an EPR resonator. Possible structural changes in the local Co structure inducing these shifts of the EPR resonance are discussed
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