The hydrogen evolution reaction (HER) is an important energy conversion process that underpins many clean energy technologies including water splitting. Herein, we report for the first time the application of two-dimensional (2D) layered transition metal carbides, MXenes, as electrocatalysts for the HER. Our computational screening study of 2D layered M 2 XT x (M = metal; X = (C, N); and T x = surface functional groups) predicts Mo 2 CT x to be an active catalyst candidate for the HER. We synthesized both Mo 2 CT x and Ti 2 CT x MXenes, and in agreement with our theoretical predictions, Mo 2 CT x was found to exhibit far higher HER activity than Ti 2 CT x . Theory suggests that the basal planes of Mo 2 CT x are catalytically active toward the HER, unlike in the case of widely studied MoS 2 , in which only the edge sites of the 2H phase are active. This work paves the way for the development of novel 2D layered materials that can be applied in a multitude of other clean energy reactions for a sustainable energy future.

Atomic layer deposition (ALD) provides a promising route for depositing uniform thin coatings of electrocatalysts useful in many technologies, including the splitting of water. For materials such as NiO x that readily form hydrous oxides, however, the smooth, compact films deposited by ALD may result in higher overpotentials due to low catalyst surface area compared to other deposition methods. Here, the use of ALD–NiO thin films as oxygen evolution reaction (OER) electrocatalysts is explored. Thin films of crystalline ALD­–NiO are deposited and OER activity is tested using cyclic voltammetry (CV). Fe incorporated from the electrolyte can increase the activity of NiO, and it is shown that the turnover frequency (TOF) increases tenfold by going from an Fe‐poor to Fe‐rich KOH electrolyte. Applying a potential exfoliates the NiO, increasing the number of electrochemically accessible Ni sites. Interestingly, by X‐ray photoelectron spectroscopy (XPS) and CV, it is found that an Fe‐rich electrolyte reduces the amount of restructuring and oxidation is found. It is shown that a high surface area, high TOF catalyst may be created by using a two‐step process in which the sample is sequentially conditioned in Fe‐poor then Fe‐rich KOH. This work highlights the importance of pretreatment on catalytic activity for compact NiO films deposited by ALD.

Enhanced catalysis for electrochemical oxygen evolution is essential for the efficacy of many renewable energy technologies, including water electrolyzers and metal-air batteries. Recently, Au supports have been shown to enhance the activity of many 3d transition metal-oxide thin films for the oxygen evolution reaction (OER) in alkaline media. Herein, we translate the beneficial impact of Au supports to high surface area, device-ready core-shell nanoparticles consisting of a Au-core and a metal-oxide shell (Au@MO where M = Ni, Co, Fe, and CoFe). Through a systematic evaluation, we establish trends in performance and illustrate the universal activity enhancement when employing the Au-core in the 3d transition metal-oxide nanoparticles. The highest activity particles, Au@CoFeO, demonstrate an overpotential of 328 ± 3 mV over a 2 h stability test at 10 mA cm, illustrating that strategically coupling Au support and mixed metal-oxide effects in a core-shell nanoparticle morphology is a promising avenue to achieve device-ready, high-performance OER catalysts.

Iridium-based mixed metal oxide phases have been shown as promising electrocatalysts for the oxygen evolution reaction (OER) because of their ability to stabilize unique Ir-based surface sites with modulated properties and improved activity. Herein, the effect of crystal structure on the OER activity of Sr iridate particles is explored. Phase-pure Sr 4 IrO 6 , Sr 2 IrO 4 , and SrIrO 3 micrometer-scale particles show high activity toward the OER with the electrode area-based geometric activity increasing in the order of IrO x < SrIrO 3 < Sr 4 IrO 6 < Sr 2 IrO 4 at a constant Ir mass electrode loading. Particularly, Sr 2 IrO 4 displays superior activity and stability compared to commercial Ir/ C (Premetek) nanoparticles, including more than an order of magnitude improvement in the catalyst surface area normalized specific activity. This translated to a similar Irbased mass activity for Sr 2 IrO 4 despite significantly larger average particles sizes (0.1−3 μm for Sr 2 IrO 4 versus 2−3 nm for Ir/C) and a 40-fold improvement in Ir-based mass activity in comparison to IrO x particles synthesized by a similar thermochemical procedure. During electrochemical testing of the Sr iridate materials, initial Sr leaching results in the formation of a stable Ir-rich catalyst surface with a modified electronic environment compared to Ir-only materials, potentially leading to enhanced OER activity. The superior intrinsic activity of Sr iridates illustrates the ability of surface-leached crystalline materials to stabilize high activity surface sites capable of significantly improving catalyst performance toward economical OER-based technologies.