Parameterization of Water Electrooxidation Catalyzed by Metal Oxides Using Fourier Transformed Alternating Current Voltammetry

Abstract

Detection and quantification of redox transformations involved in water oxidation electrocatalysis is often not possible using conventional techniques. Herein, use of large amplitude Fourier transformed ac voltammetry and comprehensive analysis of the higher harmonics has enabled us to access the redox processes responsible for catalysis. An examination of the voltammetric data for water oxidation in borate buffered solutions (pH 9.2) at electrodes functionalized with systematically varied low loadings of cobalt (CoO), manganese (MnO), and nickel oxides (NiO) has been undertaken, and extensive experiment-simulation comparisons have been introduced for the first time. Analysis shows that a single redox process controls the rate of catalysis for Co and Mn oxides, while two electron transfer events contribute in the Ni case. We apply a "molecular catalysis" model that couples a redox transformation of a surface-confined species (effective reversible potential, E) to a catalytic reaction with a substrate in solution (pseudo-first-order rate constant, k), accounts for the important role of a Brønsted base, and mimics the experimental behavior. The analysis revealed that E values for CoO, MnO, and NiO lie within the range 1.9-2.1 V vs reversible hydrogen electrode, and k varies from 2 × 10 to 4 × 10 s. The k values are much higher than reported for any water electrooxidation catalyst before. The E values provide a guide for in situ spectroscopic characterization of the active states involved in catalysis by metal oxides.

Evolution of Oxygen–Metal Electron Transfer and Metal Electronic States During Manganese Oxide Catalyzed Water Oxidation Revealed with In Situ Soft X‐Ray Spectroscopy

“…To reiterate,X AS indicates that the highest Mn oxidation state of IV is reached at 1.45 V ( Figure 2), whereas the RIXS charge transfer peak intensity increases until the application of 1.75 V ( Figure 3). [4]), which is consistent with the stabilization of the RIXS data upon reaching these potentials ( Figure 3; Supporting Information, Figure S9). [4] Therein, the last step for the oxidation of MnO x prior to the onset of water oxidation occurs at about 1.5 Vand is not directly involved in catalysis.B ased on the PFY spectra, this transition can now be related to the complete transformation of the catalyst into birnessite, which is the catalytically active phase of MnO x .A tt he same time,t he ac voltammetric signal directly coupled to the oxidation of H 2 Oi se xpected in the 1.70-1.75 Vr ange (compare water oxidation activity in Figure 1a nd extrapolation of data in Figure 5b from Ref.…”

“…These saturation values correlate well with the potentials of redox processes for MnO x electrocatalysts detected by Fourier transformed alternating current voltammetry. Moreover,t he aggregate of the XAS/RIXS data obtained herein and the outcomes of the preceding mechanistic study [4] indicate that Mn IV is the highest oxidation state involved in water oxidation electrocatalysis. [4]), which is consistent with the stabilization of the RIXS data upon reaching these potentials ( Figure 3; Supporting Information, Figure S9).…”

“…An important feature of the present work is the use of af ully parameterized model of MnO x -catalyzed H 2 Oo xidation [4] to define the potential needed to stabilize the catalyst in its putative active state (see Supporting Information, Figure S1). Potentials examined in all previous studies were less positive than the critical value of about 2Vvs.the reversible hydrogen electrode (RHE;a ll potentials are given vs.t his reference hereinafter).…”

“…Measurements at > 2Vare needed to exclude or reveal the involvement of higher oxidation states of Mn in the catalytic mechanism of water electrooxidation. 1.0 V. Functionalization of the electrode with MnO x suppresses these signals and produces av oltammetric curve dominated by processes associated with the water oxidation catalyst:1)a combination of at least two redox transformations of Mn within the potential range 0.8-1.4 V [4] and 2) the electrocatalytic oxidation of H 2 Oa tp otentials > 1.6 V. Am ismatch between the anodic and cathodic counterparts of the voltam-mogram within this catalytic region is interpreted in terms of mass-transport limitations for the [H 2 BO 3 ] À /[B(OH) 4 ] À conjugate base as defined by the experimental conditions employed, namely no convection and comparatively low borate concentration. Films of MnO x were deposited by oxidative electrodeposition at an applied potential of 1.95 Vo nA ucoated Si 3 N 4 membranes from aqueous Mn 2+ solutions buffered with borate (0.1m,p H9.2).…”

“…[4] Importantly,t he extent of orbital hybridization detected by in situ RIXS and quantified via theoretical simulations suggests that the ligand-metal charge transfer is associated with the formation of an O À species. 1.45 V; 2) there is no evidence supporting involvement of manganese species with oxidation states higher than IV to the catalytic mechanism; and 3) the degree of hybridization of Mn 3d and O2porbitals increases with positive potentials up to 1.75 Vand thereby O À Mn charge transfer becomes increasingly facile.T he latter potential correlates with the potential of the rate-determining electron-transfer step.…”