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Designing efficient catalyst for the oxygen evolution reaction (OER) is of importance for energy conversion devices. The anionic redox allows formation of O-O bonds and offers higher OER activity than the conventional metal sites. Here, we successfully prepare LiNiO2 with a dominant 3d8L configuration (L is a hole at O 2p) under high oxygen pressure, and achieve a double ligand holes 3d8L2 under OER since one electron removal occurs at O 2p orbitals for NiIII oxides. LiNiO2 exhibits super-efficient OER activity among LiMO2, RMO3 (M = transition metal, R = rare earth) and other unary 3d catalysts. Multiple in situ/operando spectroscopies reveal NiIII→NiIV transition together with Li-removal during OER. Our theory indicates that NiIV (3d8L2) leads to direct O-O coupling between lattice oxygen and *O intermediates accelerating the OER activity. These findings highlight a new way to design the lattice oxygen redox with enough ligand holes created in OER process. Lattice-oxygen redox is pivotal for high oxygen evolution reaction (OER) activity. Here, LiNiO2, a unary 3d-transition metal oxide catalyst, exhibits superefficient activity during the OER due to the creation of double O 2p holes states, according to operando XAS, XRD, and Raman spectroscopy observations. [ABSTRACT FROM AUTHOR]

The production of ecologically compatible fuels by electrochemical water splitting is highly desirable for modern industry. The Zhang-Rice singlet is well known for the superconductivity of high-temperature superconductors cuprate, but is rarely known for an electrochemical catalyst. Herein, we observe two steps of surface reconstruction from initial catalytic inactive Cu1+ in hydrogen treated Cu2O to Cu2+ state and further to catalytic active Zhang-Rice singlet state during the oxygen evolution reaction for water splitting. The hydrogen treated Cu2O catalyst exhibits a superior catalytic activity and stability for water splitting and is an efficient rival of other 3d-transition-metal catalysts. Multiple operando spectroscopies indicate that Zhang-Rice singlet is real active species, since it appears only under oxygen evolution reaction condition. This work provides an insight in developing an electrochemical catalyst from catalytically inactive materials and improves understanding of the mechanism of a Cu-based catalyst for water oxidation. The Zhang-Rice singlet is known for the superconductivity of high-temperature superconductors cuprate but is rarely studied for an electrochemical catalyst. Here, the authors use operando spectroscopic tools and observe Cu active site evolves into high-valent CuO4 geometry with Cu3+ active species, so-called Zhang-Rice singlet state, during oxygen evolution reaction. [ABSTRACT FROM AUTHOR]

Urea oxidation reaction (UOR) with a low equilibrium potential offers a promising route to replace the oxygen evolution reaction for energy‐saving hydrogen generation. However, the overpotential of the UOR is still high due to the complicated 6e− transfer process and adsorption/desorption of intermediate products. Herein, utilizing a cation exchange strategy, Ni‐doped CuO nanoarrays grown on 3D Cu foam are synthesized. Notably, Ni‐CuO NAs/CF requires a low potential of 1.366 V versus a reversible hydrogen electrode to drive a current density of 100 mA cm−2, outperforming various benchmark electrocatalysts and maintaining robust stability in alkaline media. Theoretical and experimental studies reveal that Ni as the driving force center can effectively enhance the urea adsorption and stabilize CO*/NH* intermediates toward the UOR. These findings suggest a new direction for constructing nanostructures and modulating electronic structures, ultimately developing promising Cu‐based electrode catalysts. [ABSTRACT FROM AUTHOR]

Oxygen evolution reaction (OER) is a key half-reaction in many electrochemical transformations, and efficient electrocatalysts are critical to improve its kinetics which is typically sluggish due to its multielectron-transfer nature. Perovskite oxides are a popular category of OER catalysts, while their activity remains insufficient under the conventional adsorbate evolution reaction scheme where scaling relations limit activity enhancement. The lattice oxygen-mediated mechanism (LOM) has been recently reported to overcome such scaling relations and boost the OER catalysis over several doped perovskite catalysts. However, direct evidence supporting the LOM participation is still very little because the doping strategy applied would introduce additional active sites that may mask the real reaction mechanism. Herein, a dopant-free, cation deficiency manipulation strategy to tailor the bulk diffusion properties of perovskites without affecting their surface properties is reported, providing a perfect platform for studying the contribution of LOM to OER catalysis. Further optimizing the A-site deficiency achieves a perovskite candidate with excellent intrinsic OER activity, which also demonstrates outstanding performance in rechargeable Zn-air batteries and water electrolyzers. These findings not only corroborate the key role of LOM in OER electrocatalysis, but also provide an effective way for the rational design of better catalyst materials for clean energy technologies. [ABSTRACT FROM AUTHOR]

The synthesis of transition metal oxides is typically time‐ and energy‐consuming. Recently, fast sintering methods have demonstrated great potential in reducing ceramic sintering time and energy use, improving the commercial prospects of these materials. In this article, a quenched ultrafast high‐temperature sintering (qUHS) technique is developed to sinter metastable brownmillerite SrCoO2.5 (SCO) in less than a minute. Surprisingly, SCO fabricated by qUHS shows higher activity for the oxygen evolution reaction (OER) than solid‐state‐reaction‐synthesized SCO. Comparing samples produced by these two techniques, the increased OER performance of SCO qUHS is likely due to the synergistic combination of surface Co chemical state, higher mesoporosity, and enhanced hydroxyl ion (OH–) adsorption. This work demonstrates the potential of qUHS for producing high‐performance electrocatalysts and provides detailed insights into the impact of ultrafast sintering on the materials' physical properties and electrocatalytic activity. [ABSTRACT FROM AUTHOR]

Oxygen evolution reaction (OER) is a vital electrochemical process for various energy conversion and fuel production technologies. Co/Ni‐rich perovskite oxides are extensively studied as promising alternatives to precious‐metal catalysts; however, low‐cost and earth‐abundant iron (Fe)‐rich perovskites are rarely investigated to date due to their poor activity and durability. This study reports an Fe‐rich Sr0.95Ce0.05Fe0.9Ni0.1O3−δ (SCFN) perovskite oxide with minor Ce/Ni co‐doping in A/B sites as a high‐performance OER electrocatalyst. Impressively, SCFN shows more than an order of magnitude enhancement in mass‐specific activity compared to the SrFeO3−δ (SF) parent oxide, and delivers an attractive small overpotential of 340 mV at 10 mA cm−2, outperforming many Co/Ni‐rich perovskite oxides ever reported. Additionally, SCFN displays robust operational durability with negligible activity loss under alkaline OER conditions. The increased activity and stability of SCFN can be ascribed to co‐doping‐induced synergistic promotion between structural and electronic modulation, where Ce doping facilitates the formation of a 3D corner‐sharing cubic structure and Ni doping gives rise to strong electronic interactions between active sites, which is key to achieving a highly active long‐life catalyst. Importantly, this strategy is universal and can be extended to other Fe‐based parent perovskite oxides with high structural diversity. [ABSTRACT FROM AUTHOR]

Identifying real active sites and understanding the mechanism of oxygen evolution reaction (OER) are still a big challenge today for developing efficient electrochemical catalysts in renewable energy technologies. Here, using a combined in situ/operando experiments and theory, the catalytic mechanism of the ordered OER active Co and Ir ions in Sr2CoIrO6−δ is studied, which exhibits an unprecedented low overpotential 210 mV to achieve 10 mA cm–2, ranking the highest performance among perovskite‐based solid‐state catalysts. Operando X‐ray absorption spectroscopies as a function of applied voltage indicates that Ir4+ ion is gradually converted into extremely high‐valence Ir5+/6+, while the part of Co3+ ion is transferred into Co4+ under OER process. Density functional theory calculations explicitly reveal the order Co‐O‐Ir network as an origin of ultrahigh OER activity. The work opens a promising path to overcome the sluggish kinetics of OER bottleneck for water splitting via proper arrangements of the multi‐active sites in catalyst. [ABSTRACT FROM AUTHOR]

Replacing the anodic oxygen evolution reaction (OER) with a thermodynamically favorable ethanol oxidation reaction (EOR) is regarded as a promising approach to simultaneously realize energy‐saving H2 evolution and high‐value chemical production. Herein, the single‐atom In‐doped subnanometer Pt nanowires (SA In‐Pt NWs) as high‐performance electrocatalysts for both the hydrogen evolution reaction (HER) and EOR under universal pH conditions is designed. The SA In‐Pt NWs/C can be employed to integrate HER with EOR to avoid the large overpotential caused by sluggish OER, which requires a smaller voltage of 0.62 V to reach 10 mA cm–2 compared with that of water splitting (2.07 V). The reaction also exhibits a high faradaic efficiency of over 93% in upgrading ethanol to valuable acetate in the anodic cell. Mechanistic investigations indicate that the combination of the ultrathin 1D morphology and single‐atom In decoration provides the maximum number of active sites and effectively activates Pt atoms for catalysis. Density functional theory calculations further demonstrate that doped In can effectively promote the HER, while also promoting the conversion of ethanol to acetate. Moreover, through the use of SA In‐Pt NWs/C as electrocatalysts, many other alcohols can also be employed as anodic feedstock to achieve coupled electrolysis. [ABSTRACT FROM AUTHOR]

Perovskite‐based oxides have emerged as promising oxygen evolution reaction (OER) electrocatalysts. The performance is closely related to the lattice, electronic, and defect structure of the oxides, which determine surface and bulk properties and consequent catalytic activity and durability. Further, interfacial interactions between phases in a nanocomposite may affect bulk transportation and surface adsorption properties in a similar manner to phase doping except without solubility limits. Herein, we report the development of a single/double perovskite nanohybrid with limited surface self‐reconstruction capability as an OER electrocatalyst. Such superior performance arises from a structure that maintains high crystallinity post OER catalysis, in addition to forming an amorphous layer following the self‐reconstruction of a single perovskite structure during the OER process. In situ X‐ray absorption near edge structure spectroscopy and high‐resolution synchrotron‐based X‐ray diffraction reveal an amorphization process in the hybrid single/double perovskite oxide system that is limited in comparison to single perovskite amorphization, ensuring high catalytic activity. [ABSTRACT FROM AUTHOR]

The ability to determine the electronic structure of catalysts during electrochemical reactions is highly important for identification of the active sites and the reaction mechanism. Here we successfully applied soft X-ray spectroscopy to follow in operando the valence and spin state of the Co ions in Li2Co2O4 under oxygen evolution reaction (OER) conditions. We have observed that a substantial fraction of the Co ions undergo a voltage-dependent and time-dependent valence state transition from Co3+ to Co4+ accompanied by spontaneous delithiation, whereas the edge-shared Co–O network and spin state of the Co ions remain unchanged. Density functional theory calculations indicate that the highly oxidized Co4+ site, rather than the Co3+ site or the oxygen vacancy site, is mainly responsible for the high OER activity. Determining catalyst electronic structures during electrochemical reactions is crucial to understand mechanisms. Here authors perform in operando soft X-ray spectroscopy on a cobalt oxide catalyst during O2 evolution and observe voltage and time-dependent valence state transitions. [ABSTRACT FROM AUTHOR]

Double perovskites have emerged as efficient candidates for catalyzing the electrochemical oxygen evolution reaction (OER). Smart control of the composition of a B‐site ordered double perovskite can lead to improved catalytic performance. By adopting a facile co‐doping strategy, the OER‐active elements are simultaneously introduced into the B‐site and B′‐site of a B‐site‐ordered double perovskite (A2BB′O6), leading to an enhancement of the exposed reactive sites and an optimum surface chemical state. As a result, a model system built from the substitution of Co for Mo and Fe in the Sr2FeMoO6−δ double perovskite (with a composition of Sr2Fe0.8Co0.2Mo0.6Co0.4O6−δ) shows significantly enhanced OER activity in alkaline media compared with the host material, requiring an overpotential of 345 mV to reach a 10 mA cm−2 current density (catalyst loading≈0.232 mgcat cm−2GEO) and a cell voltage of 1.57 V to afford the same current density for the overall water splitting when coupled with a Pt/C cathode (catalyst loading≈2 mg cm−2). It also demonstrates excellent electrochemical stability. The generalizability of the compositional control methodology has also been demonstrated in double perovskites incorporating transition metals other than Co (e.g., Ni). [ABSTRACT FROM AUTHOR]

Keywords: doping; electrocatalysis; perovskites; oxygen evolution reaction; water splitting B The Cover Feature b shows how oxygen evolution reaction (OER)-active elements are simultaneously introduced into the B-site and B'-site of a B-site-ordered double perovskite (A SB 2 sb BB'O SB 6 sb ), leading to an enhancement in the exposed reactive sites and an optimum surface chemical state. [Extracted from the article]

An effective strategy using d -block metal hybridization was proposed to induce electronic interaction and promote the synergistic effect, aiming to enhance the OER performance of cost-effective spinel LiMn 2 O 4. Indeed, we deployed a series of LiNi x Mn 2- x O 4 catalysts substituted with Ni, yielding the increase of Mn valence states from Mn3.5+ to Mn4+. The LiNi 0.5 Mn 1.5 O 4 catalyst achieved an excellent OER performance in alkaline media, with an overpotential reduction more than 320 mV at 10 mA cm−2 compared to the unsubstituted LiMn 2 O 4 , as well as the electrochemical stability. It was shown to be superior to the most reported Mn-based oxides OER electrocatalysts. Theoretical calculations demonstrated that Ni doped holes in the Mn site, leading to an increase in Mn4+, and that the shared covalent network of Ni 3 d -O 2 p -Mn 3 d optimized an easier desorption of intermediates. These results pave a new example for modulating the chemisorption strength, thus enabling activating the activity and stability of OER process for cost-effective electrocatalysts based on spinel Mn oxides. [Display omitted] • The d -block metal hybridization induced the synergistic effect in LiNi 0.5 Mn 1.5 O 4. • The site-to-site electron transfer was enhanced through the Ni-O-Mn network. • The adsorption energy of intermediates was optimized, yielding the improved activity and stability towards OER. [ABSTRACT FROM AUTHOR]

Abstract The oxygen evolution reaction (OER) plays a crucial role in the application of water splitting, which is a highly competitive option for a sustainable energy future. Thus, it is vital to design highly active and durable electrocatalyst for OER. Herein a hybrid with the nominal composition of Ba 2 Co 1.5 Mo 0.25 Nb 0.25 O 6-δ (denoted as BC1.5 MN) electrocatalyst consisting of both double perovskite and single perovskite structures is synthesized by a solid-state reaction method. When tested as an electrocatalyst for OER, the BC1.5 MN electrocatalyst requires a current density of 10 mA cm−2 at an overpotential of 400 mV, an onset overpotential of 260 mV, and a Tafel slope of 70 mV dec−1, which are superior to that of precious metal oxide IrO 2 catalyst. Chronoamperometric and cyclic voltammetry studies demonstrate that the BC1.5 MN electrocatalyst has outstanding durability in alkaline solution. The synergistic effect between multi-active sites derived from a single/double perovskite hybrid structure results in one of the most active perovskite-based OER electrocatalysts in alkaline solution. Graphical abstract Synopsis: The synergistic effect between multi-active sites derived from a single/double perovskite hybrid structure lead to the extraordinary OER performance. Image 1 [ABSTRACT FROM AUTHOR]

• Mo-doping is responsible for an enhanced OER activity of the exsoluted catalyst Co 0.76 Fe 0.18 Mo 0.06 O 1.35. • Mo-doping enhanced the oxidation state of the exsolved Co 0.76 Fe 0.18 Mo 0.06 O 1.35 before and after OER. • The O-K SXAS spectra revealed no La ions at the surface layers, and confirmed the formation of oxyhydroxide. Developing and understanding efficient catalysts for anodic oxygen evolution reaction (OER) is critical for the electrochemical production of H 2 for energy storage and transformation. Herein, a Co 0.76 Fe 0.18 Mo 0.06 O 1.35 catalyst with a spinel structure (CFMO-800) was prepared from Mo-doped LaCo 0.8 Fe 0.2 O 3 perovskites (LCF) by a simple exsolution method. The CFMO-800 catalyst exhibited a low potential of 1.48 V (vs. RHE) at a current density of 10 mA cm−2 in 1 M KOH surpassing all spinel and perovskite Co/Fe oxide catalysts reported. The surface-sensitive soft X-ray absorption spectroscopies (SXAS) at the Co-L 2,3 and the O-K edges revealed an unexpected increase in the Co valence state of CFMO-800 upon doped with the high-valence Mo and the clear formation of Co/Fe (oxyhydr)oxides on the surface layers of the exsolved catalyst. This study presents a strategy for the surface reconstruction by combining the exsolution and the high-valence ion doping to develop highly active electrochemical catalysts. [ABSTRACT FROM AUTHOR]

Based on that high-valence cubic perovskite can keep crystalline while Ruddlesden-Popper perovskites tend to become amorphous after oxygen evolution reaction (OER), a unique electrochemistry-include crystalline-amorphous phase on La 0.33 SrCo 0.5 Fe 0.5 O x nanofiber is develpoed as a superior catalyst for OER. La 0.33 SrCo 0.5 Fe 0.5 O x nanofiber triggers an exceptional lattice-oxygen activation mechanism to exhibit a top-level OER performance. [Display omitted] • Nano-sized crystalline-amorphous combination triggers lattice-oxygen activation mechanism to boost oxygen-evolving performance. • La 0.33 SrCo 0.5 Fe 0.5 O x - nanofiber delivers ultra-low overpotential of 260 mV and 240 mV to achieve 10 mA cm−2 in 0.1 M KOH and 1 M KOH, respectively. • Unique crystalline-amorphous combination can be artificially modulated by initial doping level and sintering temperature. The development of crystalline-amorphous phase for oxygen-evolving reaction (OER) in water splitting is lagging, and the underlying catalysis mechanism is still unknown. Here, we can facilely construct electrochemistry-induced crystalline-amorphous phase in hybrid La 0.33 SrCo 0.5 Fe 0.5 O x (H-LSCF) nanofibers based on the behavior of high-valence cubic perovskites and Ruddlesden-Popper perovskites after the OER, where such unique combination can be artificially modulated. This nano-sized combination surpasses its pure-phase counterparts and exhibits ultra-low overpotentials of 260 mV and 240 mV at 10 mA cm−2 in 0.1 M and 1 M KOH, respectively, which can be ascribed to their favorable solid-liquid contact, higher Co valence and stronger Co-O covalency to accelerate OER electron transport. The beneficial changes in various surface properties of H-LSCF ensure its stability for 80 h and 60 h in 0.1 M and 1 M KOH, respectively. In -situ 18O isotope-labelled experiments directly reveal that such combination can trigger exceptional lattice-oxygen activation mechanism. [ABSTRACT FROM AUTHOR]

S-modified FeOOH electrocatalysts were prepared via a facile hydrothermal method at 120 °C. The electrocatalyst exhibited an outstanding OER performance with an overpotential of 254 mV at 50 mA·cm−2. The effect of S on the catalytic process was attributed to the formation of (Fe-S)-O*, which reduces the adsorption energy of OH* and increase that of O*, resulting in a smaller ΔG of the rate-determining step. [Display omitted] • S-modified FeOOH was facilely prepared with outstanding OER activity. • Fe4+ improves OER performance. • The (Fe-S)-O* bond was first reported. Surface modification of electrocatalysts is a convenient strategy for addressing the drawbacks arising from the sluggish kinetics of the oxygen evolution reaction (OER). However, the preparation of electrocatalysts usually involves complex procedures, rendering investigation of the catalytic mechanism difficult. In this study, S-modified FeOOH electrocatalysts were prepared via a facile hydrothermal method at 120 °C. The electrocatalyst exhibited an outstanding OER performance with an overpotential of 254 mV at 50 mA·cm−2. Morphological, structural, and electrochemical characterizations revealed that Fe ion is the most important active center. The effect of S on the catalytic process was investigated based on the steric hindrance for OH* adsorption, changes in ΔG of the rate-determining step, and coordination mode of the active species. This is the first report on the formation of an (Fe-S)-O* bond that plays a key role in enhancing the electrochemical performance. [ABSTRACT FROM AUTHOR]