Developing efficient and stable earth-abundant electrocatalysts for acidic oxygen evolution reaction is the bottleneck for water splitting using proton exchange membrane electrolyzers. Here, we show that nanocrystalline CeO2 in a Co3O4/CeO2 nanocomposite can modify the redox properties of Co3O4 and enhances its intrinsic oxygen evolution reaction activity, and combine electrochemical and structural characterizations including kinetic isotope effect, pH- and temperature-dependence, in situ Raman and ex situ X-ray absorption spectroscopy analyses to understand the origin. The local bonding environment of Co3O4 can be modified after the introduction of nanocrystalline CeO2, which allows the CoIII species to be easily oxidized into catalytically active CoIV species, bypassing the potential-determining surface reconstruction process. Co3O4/CeO2 displays a comparable stability to Co3O4 thus breaks the activity/stability tradeoff. This work not only establishes an efficient earth-abundant catalysts for acidic oxygen evolution reaction, but also provides strategies for designing more active catalysts for other reactions.

Electrochemical synthesis of hydrogen peroxide (H2O2) in acidic solution can enable the electro-Fenton process for decentralized environmental remediation, but robust and inexpensive electrocatalysts for the selective two-electron oxygen reduction reaction...

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We synthesize EuSn2As2, the first exfoliatable layered magnetic Zintl–Klemm phase that features van der Waals bonding between neighboring layers.

The discovery of new families of exfoliatable 2D crystals that have diverse sets of electronic, optical, and spin-orbit coupling properties, enables the realization of unique physical phenomena in these few-atom thick building blocks and in proximity to other materials. Herein, using NaSn2As2 as a model system, we demonstrate that layered Zintl phases having the stoichiometry ATt2Pn2 (A = Group 1 or 2 element, Tt = Group 14 tetrel element and Pn = Group 15 pnictogen element) and feature networks separated by van der Waals gaps can be readily exfoliated with both mechanical and liquid-phase methods. We identified the symmetries of the Raman active modes of the bulk crystals via polarized Raman spectroscopy. The bulk and mechanically exfoliated NaSn2As2 samples are resistant towards oxidation, with only the top surface oxidizing in ambient conditions over a couple of days, while the liquid-exfoliated samples oxidize much more quickly in ambient conditions. Employing angle-resolved photoemission spectroscopy (ARPES), density functional theory (DFT), and transport on bulk and exfoliated samples, we show that NaSn2As2 is a highly conducting 2D semimetal, with resistivities on the order of 10-6 {\Omega} m. Due to peculiarities in the band structure, the dominating p-type carriers at low temperature are nearly compensated by the opening of n-type conduction channels as temperature increases. This work further expands the family of exfoliatable 2D materials to layered van der Waals Zintl phases, opening up opportunities in electronics and spintronics

On-site electrochemical production of hydrogen peroxide (H2O2), an oxidant and disinfectant with growing demand, could be realized through the selective two-electron oxygen reduction reaction (2e– ORR), but the widespread adoption of this method depends on robust and efficient electrocatalysts. Current catalysts have been limited by cost or toxicity and lack well-defined structures that can facilitate systematic tuning of activity and selectivity. Here, we demonstrate a series of CuCo2–x Ni x S4 (0 ≤ x ≤ 1.2) thiospinel catalysts for 2e– ORR with variable compositions that can be synthesized via hydrothermal conversion. Rotating ring disk electrode measurements show that these catalysts have high selectivity for 2e– ORR (>60%) and that their activity can be improved by increasing the nickel content without compromising selectivity. An acid treatment step is critical prior to employing the optimized CuCo0.8Ni1.2S4 catalyst for bulk electrosynthesis of H2O2 in 0.05 M H2SO4 solution. Various structural analyses, including synchrotron X-ray spectroscopy, confirm that the catalysts retain the spinel structure after acid treatment and H2O2 electrosynthesis. The acid treatment likely leaches the soluble copper species from the as-synthesized catalysts that would catalyze an electro-Fenton process to consume H2O2, generate hydroxyl radicals, and therefore prevent the accumulation of H2O2. This work demonstrates a general strategy for systematic tuning of metal compound catalysts for practical H2O2 electrosynthesis and facile generation of hydroxyl radicals.

The practical electrosynthesis of hydrogen peroxide (H 2 O 2 ) is hindered by the lack of inexpensive and efficient catalysts for the two-electron oxygen reduction reaction (2e − ORR) in neutral electrolytes. Here, we show that Ni 3 HAB 2 (HAB = hexaaminobenzene), a two-dimensional metal organic framework (MOF), is a selective and active 2e − ORR catalyst in buffered neutral electrolytes with a linker-based redox feature that dynamically affects the ORR behaviors. Rotating ring-disk electrode measurements reveal that Ni 3 HAB 2 has high selectivity for 2e − ORR (>80% at 0.6 V vs RHE) but lower Faradaic efficiency due to this linker redox process. Operando X-ray absorption spectroscopy measurements reveal that under argon gas the charging of the organic linkers causes a dynamic Ni oxidation state, but in O 2 -saturated conditions, the electronic and physical structures of Ni 3 HAB 2 change little and oxygen-containing species strongly adsorb at potentials more cathodic than the reduction potential of the organic linker (E redox ∼ 0.3 V vs RHE). We hypothesize that a primary 2e − ORR mechanism occurs directly on the organic linkers (rather than the Ni) when E > E redox , but when E < E redox , H 2 O 2 production can also occur through Ni-mediated linker discharge. By operating the bulk electrosynthesis at a low overpotential (0.4 V vs RHE), up to 662 ppm of H 2 O 2 can be produced in a buffered neutral solution in an H-cell due to minimized strong adsorption of oxygenates. This work demonstrates the potential of conductive MOF catalysts for 2e − ORR and the importance of understanding catalytic active sites under electrochemical operation.

Hydrogen peroxide (H 2 O 2 ) is a powerful oxidant with many applications, but its chemical production is unsustainable and unsafe. Decentralized electrosynthesis of H 2 O 2 via the selective two-electron oxygen reduction reaction (2e − ORR) is attractive, which demands active, selective, stable, and cost-effective electrocatalysts in acidic and neutral solutions where H 2 O 2 is stable. Metal compounds are an emerging class of 2e − ORR catalysts with diverse and tunable structural motifs for optimizing H 2 O 2 electrosynthesis, yet they remain underexplored, with poorly understood structure−property relationships. This Focus Review summarizes the recent computational and experimental developments in metal-compound-based acidic and neutral 2e − ORR catalysts, and the resultant mechanistic understanding and catalyst design rules for guiding future catalyst discoveries. The many fundamental and practical factors at the reaction, catalyst, electrode, and device levels that impact H 2 O 2 electrosynthesis are systematically discussed. Metal-compound-based acidic 2e − ORR catalysts can also enable efficient electro-Fenton processes for environmental remediation and biomass valorization.