A current challenge faced in water electrolysis is the development of structure–activity relationships for understanding and improving IrOx-based catalysts for the oxygen evolution reaction (OER). We report a simple and scalable modified Adams fusion method for preparing highly OER active, chlorine–free iridium oxide nanoparticles of various size and shape. The applied approach allows for the effects of particle size, morphology, and the nature of the surface species on the OER activity of IrO2 to be investigated. Iridium oxide synthesized at 350 °C from Ir­(acac)3, consisting of 1.7 ± 0.4 nm particles with a specific surface area of 150 m2 g–1, shows the highest OER activity (E = 1.499 ± 0.003 V at 10 A gox –1). Operando X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) studies indicate the presence of iridium hydroxo (Ir–OH) surface species, which are strongly linked to the OER activity. Preparation of larger IrO2 particles using higher temperatures results in a change of the particle morphology from spherical to rod-shaped particles. A decrease of the intrinsic OER activity was associated with the predominant termination of the rod-shape particles by highly ordered (110) facets in addition to limited diffusion within mesoporous features.

The utilization and development of efficient water electrolyzers for hydrogen production is currently limited due to the sluggish kinetics of the anodic processthe oxygen evolution reaction (OER). Moreover, state of the art OER catalysts contain high amounts of expensive and low-abundance noble metals such as Ru and Ir, limiting their large-scale industrial utilization. Therefore, the development of low-cost, highly active, and stable OER catalysts is a key requirement toward the implementation of a hydrogen-based economy. We have developed a synthetic approach to high-surface-area chlorine-free iridium oxide nanoparticles dispersed in titania (IrO2-TiO2), which is a highly active and stable OER catalyst in acidic media. IrO2-TiO2 was prepared in one step in molten NaNO3 (Adams fusion method) and consists of ca. 1–2 nm IrO2 particles distributed in a matrix of titania nanoparticles with an overall surface area of 245 m2 g–1. This material contains 40 molM % of iridium and demonstrates improved OER activity and stability in comparison to the commercial benchmark catalyst and state of the art high-surface-area IrO2. Ex situ characterization of the catalyst indicates the presence of iridium hydroxo surface species, which were previously associated with the high OER activity. Operando X-ray absorption studies demonstrate the evolution of the surface species as a function of the applied potential, suggesting the conversion of the initial hydroxo surface layer to the oxo-terminated surface via anodic oxidation (OER regime).

Proton exchange membrane water electrolysis (PEMWE) is a promising technology for electricity-to-fuel conversion which allows for direct production of hydrogen from water. One of the key problems limiting widespread implementation of PEMWE into energy systems is the sluggish kinetics of the anodic process: the oxygen evolution reaction (OER). Additionally, state-of-the-art OER materials contain large amounts of low abundant noble metals (Ru, Ir), and therefore, development of low-cost, highly active and stable OER catalysts remains an important challenge. We developed a synthetic approach to the iridium pyrochlores−complex oxides of iridium with reduced content of the noble metal as compared to IrO 2 . The materials were synthesized from molten sodium nitrate (Adams fusion method) at moderate temperatures (500−575 °C) and consist of highly crystalline iridium pyrochlore nanoparticles with surface areas of up to 40 m 2 g −1 , which is a significant improvement compared to the traditional high temperature solid-state synthesis. Electrochemical measurements in acidic media showed that yttrium and bismuth pyrochlore catalysts possess high OER activity approaching the activity of state-of-the-art IrO 2 nanoparticles. High intrinsic activities and stability behavior of yttrium iridium catalysts were correlated with the formation of the highly active IrO x surface layer due to leaching of the Y 3+ cations into the electrolyte solution, revealed both experimentally and computationally using density functional theory calculations.

MXenes, a recently discovered family of two-dimensional (2D) materials, are promising catalysts and supports for applications in heterogeneous catalysis; however, the thermal stability of MXenes and their surface chemistry are not fully explored. Here, we report that 2D molybdenum carbide Mo2CT x remains stable and shows no appreciable sintering up to ca. 550–600 °C in a reducing environment, as assessed by a combined in situ X-ray absorption near-edge spectroscopy (XANES) and powder X-ray diffraction (XRD) study during a temperature-programmed reduction (TPR) experiment. At higher temperatures, the passivating oxo, hydroxy, and fluoro groups defunctionalize the molybdenum-terminated surface, inducing a transformation to bulk β-Mo2C that is complete at ca. 730 °C. We demonstrate that Mo2CT x is a highly stable and active catalyst for the water–gas shift reaction with a selectivity >99% toward CO2 and H2 at 500 °C. The conversion of carbon monoxide on Mo2CT x starts to decline at temperatures that are associated with the decrease of the interlayer distance between the carbide sheets, as determined by the XRD-probed TPR, indicative of increasing mass transfer limitations at these conditions. Our results provide an insight into the thermal stability and reducibility of Mo2CT x and serve as a guideline for its future catalytic applications.

Extensive investigations in understanding the functional mechanisms of metal oxides behind oxygen evolution have been carried out since an electrolyzer has demonstrated promising possibilities as a device to produce hydrogen for electrochemical energy conversion systems. In particular, perovskite oxides are reputable for high activity toward the oxygen evolution reaction (OER). Here, we revisited the list of active perovskite oxides constructed based on theoretical oxygen binding energies of reaction intermediates to the catalyst surface. From this list, Ru-based perovskites, i.e. SrRuO 3 and LaRuO 3 , have been predicted as active perovskites to exhibit a particularly high OER activity. We report on the stability of nanoscaled SrRuO 3 perovskite prepared by a simple and scalable flame synthesis method. Attempts to obtain LaRuO 3 were made; however, its DFT calculated phase diagram suggests that its perovskite phase is not thermodynamically stable, which supports our experimental results such that only a mixture of different La−Ru−O phases has been obtained. Nanoscaled SrRuO 3 is evaluated for its electrochemical activity with a particular emphasis pointed toward stability in both alkaline and acidic media. Through conjoining electrochemical methods, operando X-ray absorption spectroscopy (XAS), and theoretical calculations, we show that SrRuO 3 exhibits trivial activity toward OER that decreases promptly. The loss in activity is rationalized through DFT based computations, which corroboratively suggest the poor chemical stability of both selected perovskites. Regardless of the predicted theoretical OER activity, the intrinsic instability strongly suggests that Sr-and La-based ruthenium oxides are not viable catalysts for OER in aqueous media. This further suggests that their activities are independent of their binding energies between intermediates and catalyst surface but rather closely associated with material dissolution. We highlight that understanding the origin of stability under a real operating environment is absolutely essential for the design of a sustainable electrocatalyst with optimal balance between activity and stability.