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Hydrogen production from solar energy is currently considered the best alternative to fossil fuels. Thus, materials enabling efficient and sustainable energy conversion and storage need to be developed. Iridium is still the only material used in proton exchange membrane electrolyzers that efficiently catalyze hydrogen evolution counter‐reaction, namely, the oxygen evolution reaction (OER) for electrochemical water splitting in acidic media. With no practical alternatives that can sustain the harsh reaction conditions, new approaches need to be developed to increase the utilization of this scarce metal. Hereby, a carbon–ceramic nanocomposite material is investigated, where Ir nanoparticles and nanoflakes of titanium oxynitride (TiONx) are deposited on the surface of reduced graphene oxide nanoribbons (rGONRs). OER performance is shown to be dependent on the mutual distribution of the Ir–TiONx–rGONR phases and in the best case leads up to 30 times higher activity relative to the commercial IrO2 benchmark. Adjusting the domains of different chemical nature within the same hybrid nanocomposite material through the formation of heterojunctions is shown to boost OER performance. This work demonstrates how fine‐tuning of morphology, composition, and particle distribution of the carbon–ceramic catalytic material can introduce a strong synergistic effect on OER activity and stability of iridium. [ABSTRACT FROM AUTHOR]

The rational design of electrochemical oxygen evolution reaction (OER) electrocatalyst is essential for the development of efficient and sustainable electrochemical energy conversion, storage and electrolysis applications. One of the remaining limitations of the low‐temperature electrolyzers is the large amounts of highly scarce and expensive iridium used as the OER electrocatalysts. This could be solved by applying much smaller amounts of iridium on efficient and stable support. Here we present a very promising functionality of titanium oxynitride (TiONx) high‐surface‐area support that effectively disperses the iridium nanoparticles, exhibits good intrinsic electrical conductivity and stability and thus promises efficient reduction of the noble‐metal loading in electrolyzers gas diffusion electrodes. The new nanocomposite made of approximately 3 nm‐sized iridium nanoparticles finely dispersed on TiONx support is produced using a novel synthetic route. Extensive characterization shows that the new composites exhibit an electronic interaction with the support and, ultimately, a high OER performance in acidic media. [ABSTRACT FROM AUTHOR]

B The Front Cover b shows titanium oxynitride nanoribbons with finely dispersed iridium nanoparticles as an oxygen evolution reaction electrocatalyst. In their Full Paper, Nejc Hodnik, Miran Gaberscek and co-workers describe how Ir nanoparticles dispersed on the titanium oxynitride nanoribbon show promise to be used as proton exchange membrane electrolysis anode electrocatalyst. Electrocatalysis, PEM electrolyzers, oxygen evolution reaction, titanium oxynitride, stability. [Extracted from the article]

Water splitting in acidic media is a sustainable and efficient new way to produce hydrogen fuel. However, the main bottleneck preventing the wider use of electrolyzers is still the oxygen evolution reaction (OER). Currently, the best electrocatalysts for OER are Ir-based materials, but the mechanistic details are still not fully understood. In this work, we investigate the similarities and differences in the OER mechanism of three different Ir-based catalysts, namely Vulcan carbon-supported and unsupported metallic Ir and rutile IrO 2 nanoparticles, and the process of Ir activation. For this purpose, we use large amplitude AC Voltammetry to distinguish between capacitive and faradaic processes. To quantify the data, we also use a mechanistic fit of the resolved harmonics. We show that all catalysts share the same OER mechanism but require different amounts of activation cycles. We have found three intrinsic properties, which can adequately describe a material for electrocatalysis. First is the activation threshold number (ATN), second is the current normalized to the number of active sites (intrinsic current), and lastly the mass normalized active site density. It was found that Ir on Vulcan has an intrinsic activity at least 3 times higher than the other two materials under consideration, as well as having the highest active site density. From the model fits we can also gain further insight into the mechanistic details for each material. For metallic Ir samples, the rate-limiting step is shown to be water adsorption, whereas for IrO 2 the bottleneck is the inherently slower kinetics. This also indicates that IrO 2 does not produce the same IrOH as Ir metal and therefore has different intrinsic properties. This study provides new insights into the intrinsic properties of different Ir nanocatalysts and highlights a general approach to studying the reaction mechanisms and intrinsic properties of electrocatalysts. [ABSTRACT FROM AUTHOR]

Iridium-based particles, regarded as the most promising proton exchange membrane electrolyzer electrocatalysts, were investigated by transmission electron microscopy and by coupling of an electrochemical flow cell (EFC) with online inductively coupled plasma mass spectrometry. Additionally, studies using a thin-film rotating disc electrode, identical location transmission and scanning electron microscopy, as well as X-ray absorption spectroscopy have been performed. Extremely sensitive online time-and potential-resolved electrochemical dissolution profiles revealed that Ir particles dissolve well below oxygen evolution reaction (OER) potentials, presumably induced by Ir surface oxidation and reduction processes, also referred to as transient dissolution. Overall, thermally prepared rutile-type IrO2 particles are substantially more stable and less active in comparison to as-prepared metallic and electrochemically pretreated (E-Ir) analogues. Interestingly, under OER-relevant conditions, E-Ir particles exhibit superior stability and activity owing to the altered corrosion mechanism, where the formation of unstable Ir(>IV) species is hindered. Due to the enhanced and lasting OER performance, electrochemically pre-oxidized E-Ir particles may be considered as the electrocatalyst of choice for an improved low-temperature electrochemical hydrogen production device, namely a proton exchange membrane electrolyzer. [ABSTRACT FROM AUTHOR]