Redox-State Kinetics in Water-Oxidation IrO_x Electrocatalysts Measured by Operando Spectroelectrochemistry

Abstract

Hydrous iridium oxides (IrO x ) are the best oxygen evolution electrocatalysts available for operation in acidic environments. In this study, we employ time-resolved operando spectroelectrochemistry to investigate the redox-state kinetics of IrO x electrocatalyst films for both water and hydrogen peroxide oxidation. Three different redox species involving Ir3+, Ir3.x+, Ir4+, and Ir4.y+ are identified spectroscopically, and their concentrations are quantified as a function of applied potential. The generation of Ir4.y+ states is found to be the potential-determining step for catalytic water oxidation, while H2O2 oxidation is observed to be driven by the generation of Ir4+ states. The reaction kinetics for water oxidation, determined from the optical signal decays at open circuit, accelerates from ∼20 to <0.5 s with increasing applied potential above 1.3 V versus reversible hydrogen electrode [i.e., turnover frequencies (TOFs) per active Ir state increasing from 0.05 to 2 s–1]. In contrast, the reaction kinetics for H2O2 is found to be almost independent of the applied potential (increasing from 0.1 to 0.3 s–1 over a wider potential window), indicative of a first-order reaction mechanism. These spectroelectrochemical data quantify the increase of both the density of active Ir4.y+ states and the TOFs of these states with applied positive potential, resulting in the observed sharp turn on of catalytic water oxidation current. We reconcile these data with the broader literature while providing a unique kinetic insight into IrO x electrocatalytic reaction mechanisms, indicating a first-order reaction mechanism for H2O2 oxidation driven by Ir4+ states and a higher-order reaction mechanism involving the cooperative interaction of multiple Ir4.y+ states for water oxidation.

Spectroelectrochemistry of Water Oxidation Kinetics in Molecular versus Heterogeneous Oxide Iridium Electrocatalysts

“…A new feature appeared at 800 nm between 1.13 and 1.43 V RHE , and above 1.43 V RHE , absorption changes were primarily detected below 500 nm. These features were deconvolved (Figures B and S3–S7) by fitting a model which assumes three additive contributions to the absorption, each linearly proportional to the concentration of redox states, and where the concentration of redox states formed at each potential follows a Gaussian distribution (SI, Equations S2–7), as reported in our previous work . The deconvoluted spectroelectrochemistry results are shown in Figure B, where the normalized concentration distributions and differential absorption corresponding to the three redox transitions detected in Ir Molecular are compared to those in IrO x .…”

“…When immobilized on high-surface area mesoporous indium tin oxide (mesoITO), Ir Molecular can reach turnover frequencies around 7.9 s –1 at an overpotential of 520 mV, and its ligands remain intact for at least 11 h at pH 2.6 under 1.55 V RHE ,, In our previous work on IrO x electrodes, spectroelectrochemical techniques were applied to identify three redox transitions, attributed following the literature to the oxidation of Ir 3+ to Ir 3.x+ , Ir 3.x+ to Ir 4+ , and Ir 4+ to Ir 4.y+ , where Ir 3.x+ and Ir 4.y+ represent mixed valence states including Ir 3+ and Ir 4+ , and oxidized Ir 4+ respectively. − The final redox transition results in the formation of oxygenated species, *O, whose OER kinetics can also be probed using time-resolved spectroelectrochemistry . Herein, we extend this spectroelectrochemical analysis to the dimeric complex Ir Molecular immobilized on mesoITO.…”

“…Herein, we extend this spectroelectrochemical analysis to the dimeric complex Ir Molecular immobilized on mesoITO. By comparing the molecular redox states and their kinetics to those of electrodeposited IrO x films, we aim to understand the impact on OER activity of the specific coordination environment of iridium centers in Ir Molecular as well as the potential impact of co-operative effects between Ir centers present in the heterogeneous IrO x film.…”

“…The redox chemistry of Ir Molecular on mesoITO was analyzed further spectroelectrochemically, following procedures we have previously reported for IrO x (Figures S2–S3). Its UV–vis absorption spectrum was measured as a function of applied potential with 5 mV increments in aqueous 0.1 M HClO 4 electrolyte at pH 1.2 (Figure A). For the potential range 0.78–1.08 V RHE , an absorption band at 590 nm dominated the spectrum.…”

Performance of Insoluble IrO2 Anode for Sewage Sludge Cake Electrodehydration Application with Respect to Operation Conditions

“…Dehydration of sewage sludge cake (SSC) has been studied based on mechanical, thermal, and electrodehydration (ED) methods [7,8,13,14]. Water in SSCs can be divided into four categories: (i) free water, (ii) surface water, (iii) interstitial water, and (iv) bound water [15,16].…”