In Situ EPR Characterization of a Cobalt Oxide Water Oxidation Catalyst at Neutral pH

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.

Principles of Water Electrolysis and Recent Progress in Cobalt‐, Nickel‐, and Iron‐Based Oxides for the Oxygen Evolution Reaction

“…The mechanism of OER by surface cobalt oxyhydroxide is foreseen to involve the combination of two lattice oxygen atoms to produce the Co‐superoxide intermediate and the release of dioxygen from the Co‐superoxide as the rate‐determining step (Figure 7 c). [24b] The formation of Co IV species as an intermediate on the surface of cobalt oxide has been also confirmed by an in situ electron paramagnetic resonance (EPR) study, whereby Co II EPR signal was converted into a Co IV signal at higher electrode potential [92] …”

“…[24b] The formation of Co IV species as an intermediate on the surface of cobalt oxide has been also confirmed by an in situ electron paramagnetic resonance (EPR) study, whereby Co II EPR signal was converted into a Co IV signal at higher electrode potential. [92] Co 3 O 4 spinel has the advantage that it can incorporate other di-and trivalent cations, thus the intrinsic properties can be easily adjusted. The integration of a second metal such as Mg, Ni, Fe, and Cu has been shown to improve the OER activity of Co 3 O 4 .…”

Surface-Promoted Evolution of Ru-bda Coordination Oligomers Boosts the Efficiency of Water Oxidation Molecular Anodes

“…We used an EPR spectroelectrochemical cell 37 to follow the paramagnetic signature of 10 and 10′(H 2 O) 2 as well as their derived species immobilized on the electrode surface of a carbon paper ( 10@C-paper ), at different oxidation states. The main spectra obtained are displayed in Figure 8 .…”

Quantum computing for neutrino-nucleus scattering

“…These estimates put a full computation of neutrino scattering off Ar at the same complexity level as factorizing a 1024-bit integer (cf. [35,41]) and possibly out of reach to near term NISQ devices. In the next section we explore possible improvements to this estimate using qubitization.…”