Reactive Electrophilic OI- Species Evidenced in High-Performance Iridium Oxohydroxide Water Oxidation Electrocatalysts

Abstract

Although quasi-amorphous iridium oxohydroxides have been identified repeatedly as superior electrocatalysts for the oxygen evolution reaction (OER), an exact description of the performance-relevant species has remained a challenge. In this context, we report the characterization of hydrothermally prepared iridium(III/IV) oxohydroxides that exhibit exceptional OER performances. Holes in the O 2p states of the iridium(III/IV) oxohydroxides result in reactive O^I- species, which are identified by characteristic near-edge X-ray absorption fine structure (NEXAFS) features. A prototypical titration reaction with CO as a probe molecule shows that these O^I- species are highly susceptible to nucleophilic attack at room temperature. Similarly to the preactivated oxygen involved in the biological OER in photosystem II, the electrophilic O^I- species evidenced in the iridium(III/IV) oxohydroxides are suggested to be precursors to species involved in the O-O bond formation during the electrocatalytic OER. The CO titration also highlights a link between the OER performance and the surface/subsurface mobility of the O^I- species. Thus, the superior electrocatalytic properties of the iridium (III/IV) oxohydroxides are explained by their ability to accommodate preactivated electrophilic O^I- species that can migrate within the lattice.

Keywords: electrochemistry; energy storage; iridium; oxygen; water splitting.

Figure 1

The OER activity reflected by the current density j at 350 mV overpotential (blue bars) and OER stability indicated by stable operation at 15 mA cm⁻² (red bars) tend to worsen for the MW samples as the base/Ir ratio increases. The performances are compared with those of two commercial benchmarks (right).

Figure 2

The selected‐area electron diffraction (SAED) patterns and radial profiles for MW 5 (a) under low electron dose and (b) with high electron dose show the complete transformation of the initial iridium oxohydroxide to metallic iridium after beam irradiation.

Figure 3

The Raman spectra of the MW‐prepared iridium oxohydroxides show distinctive features in the $\tilde{\nu }$ =300–750 cm⁻¹ range. The average bulk oxidation states indicated on the right were quantified through H₂‐TPR.

Figure 4

DFT‐calculated structure of a bis‐μ‐oxo Ir^IV trimer. Blue H, red O, and purple Ir.

Figure 5

DFT‐calculated structure of a reduced Ir^III/IV trimer. Blue H, red O, and purple Ir.

Figure 6

The XPS spectra of (a) MW_5 and (b) MW_100 in the Ir 4f region for 450 eV kinetic energy show the contributions from Ir^III and Ir^IV species. The average oxidation state calculated from the fit is indicated in the top‐right corner.

Figure 7

NEXAFS spectra for the MW‐prepared iridium(III/IV) oxohydroxides and the reference samples SA‐IrO₂ and AA‐IrO_x.

Figure 8

(a) The gas‐stream composition for the test of one of our best OER catalysts, MW_5. During the initial switch from 100 % He to 1 % CO/He (100 mL min⁻¹), a clear transient CO₂ signal was observed, and this indicates that CO was oxidized by a finite oxygen source originating from the iridium oxohydroxide. Once no more CO₂ evolution was detected, the reactor was purged with He, and the sample was again subjected to a 1 % CO/He stream. During this second titration, no CO₂ evolution was detected (blue CO₂ signal, Figure 8 a), and this result confirms the stoichiometric nature of the reaction. The procedure was repeated for every compound (see Figure S9). For each sample, we confirmed the irreversible nature of the reaction observed as transient CO₂ evolution during the initial switch to 1 % CO/He.

Figure 9

The CO₂‐related peak appearing after CO adsorption on MW_5 during the warming procedure from liquid‐nitrogen temperature to RT was fitted and integrated with a Gaussian fit model (shown for the spectra obtained at 20 °C in the inset). The determined CO₂ peak areas are reported as a function of T for the first (black squares) and second temperature increases (red dots).

Figure 10

CO‐treated MW_5 and pristine MW_5 were tested (a) for OER activity through LSV and (b) for OER stability through CP for electrode loadings of 20 μg_Ir cm⁻².

Figure 11

Our results suggest an OER scheme involving O−O bond formation through the nucleophilic attack of the catalytic O^I− species in the iridium(III/IV) oxohydroxides under the OER potential.