Unravelling the effects of active site densities and energetics on the water oxidation activity of iridium oxides

Abstract

Understanding what controls the reaction rate on iridium-based catalysts is central to designing more active and stable electrocatalysts for the water oxidation reaction in proton exchange membrane (PEM) electrolysers. Here, we quantify the densities of redox active centres and probe their binding strengths on amorphous IrOx and rutile IrO2 using a combination of operando time-resolved optical spectroscopy, X-ray absorption spectroscopy (XAS) and time of flight secondary ion mass spectrometry (TOF-SIMs). Firstly, our results show that although IrOx exhibits an order of magnitude higher geometry current density compared to IrO2, the intrinsic rates of reaction per active state, on IrOx and IrO2 are comparable at a given potential. Secondly, we establish a quantitative experimental correlation between the intrinsic rate of water oxidation and the energetics of the active states. We use density functional theory (DFT) based models to provide a molecular scale interpretation of our data. We find that the *O species formed at water oxidation potentials have repulsive adsorbate-adsorbate interactions, and thus increasing their coverage weakens their binding and promotes the rate-determining O-O bond formation. Finally, we provide insights into how the intrinsic water oxidation kinetics can be increased by optimising both the binding energy and the interaction strength of the catalytically active states.

Role of Electrolyte pH on Water Oxidation for Iridium Oxides

“…The IrO x films consist of 100−200 nm diameter nanoparticles and are XRD amorphous. 42 Cyclic voltammograms measured on these films (Figure 1A) show peaks at ∼0.9 V RHE and ∼1.22 V RHE in acidic conditions (0.1 M HClO 4 , pH ∼ 1.0) and at ∼0.68 V RHE and ∼1.03 V RHE in alkaline conditions (0.1 M KOH, pH ∼ 12.9). These two peaks are attributed to deprotonation of *H 2 O at a coordinatively unsaturated (CUS) Ir site (H 2 O (cus) →*OH (cus) + H + + e − ) and deprotonation of the bridging oxygen (*OH (cus) + *H b → *OH (cus) + H + + e − + *), respectively, based on our previous work.…”

“…On increasing the potential in the acidic electrolyte, broad absorption bands at ∼650, ∼800, and ∼500 nm are observed at potential ranges of 0.6 V RHE −1.0 V RHE , 1.0 V RHE −1.3 V RHE , and >1.3 V RHE , respectively, in agreement with our previous reports. 42,43 Optical spectroscopy measurements in an alkaline electrolyte show three similar absorption bands with increasing potential. However, the potential range for the formation of these spectral features are lower, 0.5 V RHE −0.8 V RHE , 0.8 V RHE −1.15 V RHE , and >1.15 V RHE , respectively (Figure 2B).…”

“…pH-Dependent Redox and OER Activity. Hydrous, amorphous iridium oxide films were prepared by electrodeposition on FTO substrates, using the same method described in our previous work 42 (see Methods section in the SI). The IrO x films consist of 100−200 nm diameter nanoparticles and are XRD amorphous.…”

“…12 To analyze the concentration of each redox state as a function of potential, we deconvoluted the total absorption using a linear combination fitting method, reported in our previous work (see Supporting note 1 for deconvolution details). 42 The three distinct spectral shapes used for the deconvolution can be obtained for both acid and alkaline conditions by using differential analysis. We note that the spectral shapes of the three states in acid and base are similar, with absorption bands in the alkaline electrolyte slightly shifted to higher wavelengths, The onset of redox processes at less positive potentials in the alkaline KOH electrolyte compared to the acidic HClO 4 electrolyte was further confirmed using in situ X-ray absorption spectroscopy (XAS).…”

“…We find that the U vs θ data cannot be fitted using a simple Langmuir isotherm (Figures S16 and S17), which assumes no interaction between adsorbates, as also demonstrated in our previous work in acidic electrolytes. 42 Instead, the electroadsorption isotherms can be modeled using a Frumkin isotherm, with R 2 as high as ∼0.99 (Figures 2D,E, S16 and S17). We note that a similar trend of Frumkin interaction parameters was also observed on rutile iridium oxide in our previous work, 42 indicating that although IrO x is a highly porous catalyst, its redox transition processes are similar to the dense rutile iridium oxide film and is thus most likely related to the adsorption process of oxygenated species, as opposed to a slower, battery-like intercalation process, where the reaction is limited by solid-state diffusion.…”