Superexchange Effects on Oxygen Reduction Activity of Edge-Sharing [Cox Mn1-x O6 ] Octahedra in Spinel Oxide

Synergistic engineering of oxygen vacancies, cation vacancies, and surface acidity in MOFs-derived Co-Mn spinel oxides via acid etching: A pathway to enhanced toluene oxidation performance

MOFs-derived spinel oxides hold great potential for the catalytic oxidation of VOCs, but their low intrinsic activity and unmanageable surface defects limit practical applications. Herein, this study proposes a method for oxalic acid etching MIL-101(CoMn) to synergistically regulate oxygen vacancies, cation vacancies, and surface acidity for enhanced toluene oxidation. Oxalic acid selectively dissolves Co2 +, leading to the generation of cobalt vacancies. While cobalt vacancies can facilitate the migration or detachment of oxygen from the lattice, thus contributing to the formation of oxygen vacancies. The coexistence of oxygen vacancies and cobalt vacancies, with exposed surface atoms and unsaturated metal centers, enhances surface acidity. Importantly, mild oxalic acid converts strong acid sites into medium-strength ones, improving toluene adsorption and reducing carbon deposition. Meanwhile, acid etching reconfigures surface morphology to transform spherical into lamellar structures with higher specific surface area and edge atomic density. DFT calculations confirm that oxygen vacancies and cobalt vacancies optimize electronic structures of Co and Mn, boosting electron exchange and redox properties. In-situ DRIFTS reveals that this synergistic modulation improves the generation and consumption of intermediates. Hence, MIL-101(CoMn)/O shows higher activity and stability with 90 % toluene conversion at 223 °C, lower than CoMn (293 °C) and MIL-101(CoMn) (267 °C). This work offers insights for designing efficient MOFs-derived spinel oxides in environmental remediation.

Nitrogen-doping-induced electron spin polarization activates scandium oxide as high-performance zinc-air battery cathode

Platinum (Pt) is the most active catalyst for the oxygen reduction reaction (ORR). However, the scarcity, high cost, and susceptibility to deactivation of Pt constrain its large-scale applications. Transition metal oxide (TMO) materials have emerged as promising alternatives due to their abundant availability and catalytic potential. Herein, we report a dissolution-and-carbonization strategy to synthesize a carbon-supported nitrogen-doped Sc2O3 catalyst (N-Sc2O3/C). Nitrogen doping significantly enhances the conductivity of the otherwise poor-conductivity Sc2O3, transforming it into a superior ORR catalyst. The synthesized N-Sc2O3/C exhibits remarkable ORR performance in 0.1 M KOH, achieving a half-wave potential of 0.92 V, which is 55 mV higher than the state-of-the-art commercial Pt/C (0.87 V). Moreover, as a cathode for a zinc-air battery, N-Sc2O3/C achieves a peak power density of 150.7 mW cm-2 and a specific capacity of 766.4 mAh gZn-1. Density functional theory calculations reveal that nitrogen doping induces electron spin polarization within Sc2O3, narrowing the bandgap. This enhanced electronic structure improves conductivity and optimizes the adsorption of oxygen intermediates, thereby facilitating the ORR process. Our study demonstrates that nitrogen doping activates the wide-bandgap Sc2O3 semiconductor, converting it into a highly efficient ORR electrocatalyst and highlighting the potential of wide-bandgap TMO materials in energy applications.

Acid-etched defect engineering in spinel CoMn2O4: Synergizing oxygen and cation vacancies to unlock high NH3-SCR performance

Spinel oxides hold considerable potential for NH3-SCR reaction, but poor inherent activity and untreatable surface defects restrict their practical application. Herein, we proposed a strategy to synergistically modulate oxygen vacancies and cation vacancies by acid etching in spinel CoMn2O4. Nitric acid selectively solubilizes weakly coordinated Co2+ to produce cobalt vacancies, while the destabilization of O2- coordinated to Co2+ may induce the formation of oxygen vacancies. Oxygen vacancies contribute to the generation of reactive oxygen species to facilitate NO reduction, while cobalt vacancies increase the density of acid sites to improve the adsorption and activation of NO and NH3. DFT calculations reveal that oxygen vacancies and cobalt vacancies optimize the electronic structure, especially the electron density and energy level distribution near the Co and Mn sites, enhancing the electron exchange and redox properties. In-situ DRIFTS analysis proves that this cooperative regulation improves NH3-SCR performance by facilitating the adsorption and conversion of intermediates. Therefore, CoMn2O4/N0.10 shows high efficiency and anti-poisoning capability at a wide operating temperature, with above 90 % NO conversion from 165 to 400 °C. These findings present new insights into designing spinel oxide with high wide-temperature NH3-SCR activity, expected to break the limitated operating temperature.

Spin Effects in Optimizing Electrochemical Applications

Efficient electrocatalyst development is crucial for addressing global energy challenges, and recent advances have highlighted the significant role of electron spin-a fundamental property of electrons-in influencing catalytic processes. Regulating the spin states of active sites has emerged as a powerful strategy to enhance catalytic performance. In response to growing interest in spin-induced electrocatalysis, this review offers a comprehensive examination of the impact of spin states on electrocatalytic activity. We explore various strategies for modulating spin states, review state-of-the-art techniques for spin state characterization, and elucidate the mechanisms by which spin effects enhance catalytic efficiency. Additionally, we discuss future research directions, emphasizing the potential of spin regulation to drive innovation in electrocatalyst design and application. This review aims to provide a foundational understanding of spin effects in electrocatalysis, guiding future efforts in the rational design of high-performance catalysts.

The Doping of Al3+ at the Tetrahedral Site of Spinel Mn3O4 for Electrocatalytic Water Oxidation

Spinel oxides have attracted much attention for electrocatalytic water oxidation. Specially, the Mn-based spinel structures merits fundamental investigation, as Mn is involved in water oxidation in natural photosynthesis. Herein, Al-doped Mn3O4 spinel electrocatalyst was prepared for water oxidation. The Al cations selectively occupy the tetrahedral sites of Mn3O4 spinel structure, leaving Mn(III) at the catalytically critical octahedral sites. The substitution of Al3+ regulated the electronic structure of Mn3O4, induced lattice distortion and produced higher concentration of oxygen vacancy to promote the water oxidation performance.

Rational Regulation of High-Entropy Perovskite Oxides through Hole Doping for Efficient Oxygen Electrocatalysis

Due to the high configuration entropy, unique atomic arrangement, and electronic structures, high-entropy materials are being actively pursued as bifunctional catalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable zinc-air batteries (ZABs). However, a relevant strategy to enhance the catalytic activity of high-entropy materials is still lacking. Herein, a hole doping strategy has been employed to enable the high-entropy perovskite La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 to effectively catalyze the ORR and OER. Hole doping experiments rely on the substitution of Sr2+ for La3+. The optimized La0.7Sr0.3(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 displays remarkable activity for the ORR and the OER, with a low potential difference of 0.880 V between the half-wave potential of the ORR and the OER potential at 10 mA cm-2, exceeding the majority of perovskite bifunctional catalysts. Further analysis of the electronic structures reveals that hole doping could regulate the eg-orbital filling of the transition-metal cations in high-entropy perovskites to an ideal position and thereby generate many highly active sites to promote the redox activity of oxygen. The assembled rechargeable ZAB with the targeted high-entropy perovskite as the cathode affords a specific capacity of 774.5 mAh gZn-1 under 10 mA cm-2 and durability for a period of 300 cycles, comparable to that of the 20%Pt/C + RuO2 ZAB. This work offers an important approach for the advancement of efficient high-entropy perovskites for ZABs.

Triggering Oxygen Redox Cycles in Nickel Ferrite by Octahedral Geometry Engineering for Enhancing Oxygen Evolution

Spinel-type nickel ferrite (NixFe3-xO4, x≤1) is a widely used electrocatalyst for the oxygen evolution reaction (OER). Due to the lower hybridization of metal-d and oxygen-p orbitals, the OER process on NixFe3-xO4 follows the sluggish adsorbate evolution mechanism (AEM). Generally, activating the lattice oxygen to trigger the lattice-oxygen-mediated mechanism (LOM) can enhance the OER activity. Herein, to trigger the LOM pathway while maintaining high stability, iron foam (IF)-supported Ni0.75Fe2.25O4 (NiFeO) with geometrical defects of [NiO6] (nickel cation coordinated with six oxygen anions) units and higher ratio of Fe to Ni cations in octahedral sites (d-NiFeHRO/IF) is prepared by ion-exchanging with polar aprotic solvent followed by annealing. As a result, as-synthesized d-NiFeHRO/IF exhibits excellent activity (at 295 mV overpotential to achieve 100 mA cm-2), fast kinetics (Tafel slope of only 34.6 mV dec-1), and high stability (maintaining a current density of 100 mA cm-2 over 130 h) for the OER. The theoretical calculations reveal that the construction of octahedral defect in NixFe3-xO4 enhances the overlap of Fe-d and O-p orbitals, which can activate the lattice oxygen. Therefore, increasing the ratio of Fe to Ni will further improve the lattice oxygen redox activity, and thus trigger the fast LOM pathway, ultimately facilitating the OER process.

In Situ Manipulation of Surface Spin Configurations for Enhanced Performance in Oxygen Evolution Reactions

In situ studies of the relationship between surface spin configurations and spin-related electrocatalytic reactions are crucial for understanding how magnetic catalysts enhance oxygen evolution reaction (OER) performance under magnetic fields. In this work, 2D Fe7Se8 nanosheets with rich surface spin configurations are synthesized via chemical vapor deposition. In situ magnetic force microscopy and Raman spectroscopy reveal that a 200 mT magnetic field eliminates spin-disordered domain walls, forming a spin-ordered single-domain structure, which lowers the OER energy barrier, as confirmed by theoretical calculations. Electrochemical tests show that under a 200 mT magnetic field, the OER overpotential of multidomain Fe7Se8 nanosheets at 10 mA cm-2 decreases from 346 mV to 259 mV, while the magnetic field has minimal effect on single-domain nanosheets. These findings highlight the critical role of spin configurations in enhancing electrocatalytic performance, offering new insights into the design of magnetic catalysts for industrial applications.

Screening of Silver-Based Single-Atom Alloy Catalysts for NO Electroreduction to NH3 by DFT Calculations and Machine Learning

Exploring NO reduction reaction (NORR) electrocatalysts with high activity and selectivity toward NH3 is essential for both NO removal and NH3 synthesis. Due to their superior electrocatalytic activities, single-atom alloy (SAA) catalysts have attracted considerable attention. However, the exploration of SAAs is hindered by a lack of fast yet reliable prediction of catalytic performance. To address this problem, we comprehensively screened a series of transition-metal atom doped Ag-based SAAs. This screening process involves regression machine learning (ML) algorithms and a compressed-sensing data-analytics approach parameterized with density-functional inputs. The results demonstrate that Cu/Ag and Zn/Ag can efficiently activate and hydrogenate NO with small Φmax(η), a grand-canonical adaptation of the Gmax(η) descriptor, and exhibit higher affinity to NO over H adatoms to suppress the competing hydrogen evolution reaction. The NH3 selectivity is mainly determined by the s orbitals of the doped single-atom near the Fermi level. The catalytic activity of SAAs is highly correlated with the local environment of the active site. We further quantified the relationship between the intrinsic features of these active sites and Φmax(η). Our work clarifies the mechanism of NORR to NH3 and offers a design principle to guide the screen of highly active SAA catalysts.

Mn─O Covalency as a Lever for Na⁺ Intercalation Kinetics: The Role of Oxygen Edge-Sharing Co Octahedral Sites in MnO₂

The strategic enhancement of manganese-oxygen (Mn─O) covalency is a promising approach to improve the intercalation kinetics of sodium ions (Na⁺) in manganese dioxide (MnO2). In this study, an augmenting Mn─O covalency in MnO2 by strategically incorporating cobalt at oxygen edge-sharing Co octahedral sites is focused on. Both experimental results and density functional theory (DFT) calculations reveal an increased electron polarization from oxygen to manganese, surpassing that directed toward cobalt, thereby facilitating enhanced electron transfer and strengthening covalency. The synthesized Co-MnO2 material exhibits outstanding electrochemical performance, demonstrating a superior specific capacitance of 388 F g-1 at 1 A g-1 and maintaining 97.21% capacity retention after 12000 cycles. Additionally, an asymmetric supercapacitor constructed using Co-MnO2 achieved a high energy density of 35 Wh kg-1 at a power density of 1000 W kg-1, underscoring the efficacy of this material in practical applications. This work highlights the critical role of transition metal-oxygen interactions in optimizing electrode materials and introduces a robust approach to enhance the functional properties of manganese oxides, thereby advancing high-performance energy storage technologies.

Construction of Surface Ruoct─O─Cooct Units With Optimized Cooct Spin States for Enhanced Oxygen Reduction and Evolution

The introduction of noble metal into spinel structure is an effective strategy to develop efficient oxygen evolution/reduction reaction (OER/ORR) catalysts. Herein, surface Cooct is substituted by Ruoct in Rux-Mn0.5Co2.5-xO4/NCNTs by ion-exchange, where presence of Ruoct─O─Cooct unit facilitates electron transfer. This strong electron coupling effect leads downward shift in d-band center and a narrowing of d-p bandgap. The increased charge density of Cooct bridged with Ruoct dioxygen optimizes adsorption of oxygen intermediates (*OH) and occupation of electrons in eg-orbital octahedral. The measured ORR/OER voltage difference is only 0.71 V. The peak power density of assembled zinc-air battery reaches 148.8 mW h cm-2, and energy density at 100 mA cm-2 reaches 813.6 mA h gZn -1, approaching a theoretical value of 820 mA h gZn -1. The catalyst demonstrates stable operation for over 500 h at 10 mA cm-2 and over 200 h at 50 mA cm-2. This work provides new insights to guide fabrication of advanced oxygen electrocatalysts.

Spin-dependent electrocatalysis

The shift towards sustainable energy requires efficient electrochemical conversion technologies, emphasizing the crucial need for robust electrocatalyst design. Recent findings reveal that the efficiency of some electrocatalytic reactions is spin-dependent, with spin configuration dictating performance. Consequently, understanding the spin's role and controlling it in electrocatalysts is important. This review succinctly outlines recent investigations into spin-dependent electrocatalysis, stressing its importance in energy conversion. It begins with an introduction to spin-related features, discusses characterization techniques for identifying spin configurations, and explores strategies for fine-tuning them. At the end, the article provides insights into future research directions, aiming to reveal more unknown fundamentals of spin-dependent electrocatalysis and encourage further exploration in spin-related research and applications.

Fe, N-Inducing Interfacial Electron Redistribution in NiCo Spinel on Biomass-Derived Carbon for Bi-functional Oxygen Conversion

Herein, an interfacial electron redistribution is proposed to boost the activity of carbon-supported spinel NiCo2O4 catalyst toward oxygen conversion via Fe, N-doping strategy. Fe-doping into octahedron induces a redistribution of electrons between Co and Ni atoms on NiCo1.8Fe0.2O4@N-carbon. The increased electron density of Co promotes the coordination of water to Co sites and further dissociation. The generation of proton from water improves the overall activity for the oxygen reduction reaction (ORR). The increased electron density of Ni facilitates the generation of oxygen vacancies. The Ni-VO-Fe structure accelerates the deprotonation of *OOH to improve the activity toward oxygen evolution reaction (OER). N-doping modulates the electron density of carbon to form active sites for the adsorption and protonation of oxygen species. Fir wood-derived carbon endows catalyst with an integral structure to enable outstanding electrocatalytic performance. The NiCo1.8Fe0.2O4@N-carbon express high half-wave potential up to 0.86 V in ORR and low overpotential of 270 mV at 10 mA cm-2 in OER. The zinc-air batteries (ZABs) assembled with the as-prepared catalyst achieve long-term cycle stability (over 2000 cycles) with peak power density (180 mWcm-2). Fe, N-doping strategy drives the catalysis of biomass-derived carbon-based catalysts to the highest level for the oxygen conversion in ZABs.

Mineral Phase Reconfiguration Enables the High Enzyme-like Activity of Vermiculite for Antibacterial Application

Phyllosilicates-based nanomaterials, particularly iron-rich vermiculite (VMT), have wide applications in biomedicine. However, the lack of effective methods to activate the functional layer covered by the external inert layer limits their future applications. Herein, we report a mineral phase reconfiguration strategy to prepare novel nanozymes by a molten salt method. The peroxidase-like activity of the VMT reconfiguration nanozyme is 10 times that of VMT, due to the electronic structure change of iron in VMT. Density-functional theory calculations confirmed that the upward shifted d-band center of the VMT reconfiguration nanozyme promoted the adsorption of H2O2 on the active iron sites and significantly elongated the O-O bond lengths. The reconfiguration nanozyme exhibited nearly 100% antibacterial activity toward Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), much higher than that of VMT (E. coli 10%, S. aureus 21%). This work provides new insights for the rational design of efficient bioactive phyllosilicates-based nanozyme.

Electrocatalytic Mechanism of Defect in Spinels for Water and Organics Oxidation

Spinels display promising electrocatalytic ability for oxygen evolution reaction (OER) and organics oxidation reaction because of flexible structure, tunable component, and multifold valence. Unfortunately, limited exposure of active sites, poor electronic conductivity, and low intrinsic ability make the electrocatalytic performance of spinels unsatisfactory. Defect engineering is an effective method to enhance the intrinsic ability of electrocatalysts. Herein, the recent advances in defect spinels for OER and organics electrooxidation are reviewed. The defect types that exist in spinels are first introduced. Then the catalytic mechanism and dynamic evolution of defect spinels during the electrochemical process are summarized in detail. Finally, the challenges of defect spinel electrocatalysts are brought up. This review aims to deepen the understanding about the role and evolution of defects in spinel for electrochemical water/organics oxidation and provide a significant reference for the design of efficient defect spinel electrocatalysts.

Recent advances of bifunctional catalysts for zinc air batteries with stability considerations: from selecting materials to reconstruction

With the growing depletion of traditional fossil energy resources and ongoing enhanced awareness of environmental protection, research on electrochemical energy storage techniques like zinc-air batteries is receiving close attention. A significant amount of work on bifunctional catalysts is devoted to improving OER and ORR reaction performance to pave the way for the commercialization of new batteries. Although most traditional energy storage systems perform very well, their durability in practical applications is receiving less attention, with issues such as carbon corrosion, reconstruction during the OER process, and degradation, which can seriously impact long-term use. To be able to design bifunctional materials in a bottom-up approach, a summary of different kinds of carbon materials and transition metal-based materials will be of assistance in selecting a suitable and highly active catalyst from the extensive existing non-precious materials database. Also, the modulation of current carbon materials, aimed at increasing defects and vacancies in carbon and electron distribution in metal-N-C is introduced to attain improved ORR performance of porous materials with fast mass and air transfer. Finally, the reconstruction of catalysts is introduced. The review concludes with comprehensive recommendations for obtaining high-performance and highly-durable catalysts.

Designing Oxide Catalysts for Oxygen Electrocatalysis: Insights from Mechanism to Application

The electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are fundamental processes in a range of energy conversion devices such as fuel cells and metal-air batteries. ORR and OER both have significant activation barriers, which severely limit the overall performance of energy conversion devices that utilize ORR/OER. Meanwhile, ORR is another very important electrochemical reaction involving oxygen that has been widely investigated. ORR occurs in aqueous solutions via two pathways: the direct 4-electron reduction or 2-electron reduction pathways from O2 to water (H2O) or from O2 to hydrogen peroxide (H2O2). Noble metal electrocatalysts are often used to catalyze OER and ORR, despite the fact that noble metal electrocatalysts have certain intrinsic limitations, such as low storage. Thus, it is urgent to develop more active and stable low-cost electrocatalysts, especially for severe environments (e.g., acidic media). Theoretically, an ideal oxygen electrocatalyst should provide adequate binding to oxygen species. Transition metals not belonging to the platinum group metal-based oxides are a low-cost substance that could give a d orbital for oxygen species binding. As a result, transition metal oxides are regarded as a substitute for typical precious metal oxygen electrocatalysts. However, the development of oxide catalysts for oxygen reduction and oxygen evolution reactions still faces significant challenges, e.g., catalytic activity, stability, cost, and reaction mechanism. We discuss the fundamental principles underlying the design of oxide catalysts, including the influence of crystal structure, and electronic structure on their performance. We also discuss the challenges associated with developing oxide catalysts and the potential strategies to overcome these challenges.

Coordination Chemistry of Large-Sized Yttrium Single-Atom Catalysts for Oxygen Reduction Reaction

Although being transition metals, the Fenton-inactive group 3-4 elements (Sc, Y, La, Ti, Zr, and Hf) can easily lose all the outermost s and d electrons, leaving behind ionic sites with nearly empty outermost orbitals that are stable but inactive for oxygen involved catalysis. Here, it is demonstrated that the dynamic coordination network can turn these commonly inactive ionic sites into platinum-like catalytic centers for the oxygen reduction reaction (ORR). Using density functional theory calculations, a macrocyclic ligand coordinated yttrium single-atom (YN₄ ) moiety is identified, which is originally ORR inactive because of the too strong binding of hydroxyl intermediate, while it can be activated by an axial ligand X through the covalency competition between YX and YOH bonds. Strikingly, it is also found that the binding force of the axially coordinated ligand is an effective descriptor, and the chlorine ligand is screened out with an optimal binding force that behaves self-adaptively to facilitate each ORR intermediate steps by dynamically changing its YCl covalency. These experiments validate that the as-designed YN₄ -Cl moieties embedded within the carbon framework exhibit a high half-wave potential (E_1/2 = 0.85 V) in alkaline media, the same as that of the commercial Pt/C catalyst .

Oxidation State Engineering in Octahedral Ni by Anchored Sulfate to Boost Intrinsic Oxygen Evolution Activity

Promoting the electron occupancy of active sites to unity is an effective method to enhance the oxygen evolution reaction (OER) performance of spinel oxides, but it remains a great challenge. Here, an in situ approach is developed to modify the valence state of octahedral Ni cations in NiFe₂O₄ inverse spinel via surface sulfates (SO₄^2-). Different from previous studies, SO₄^2- is directly anchored on the spinel surface instead of forming from uncontrolled conversion or surface reconstruction. Experiment and theoretical calculations reveal the precise adsorption sites and spatial arrangement for SO₄^2- species. As a main promoting factor, surface SO₄^2- effectively converts the crystal field stable Ni state (t_2g⁶e_g²) to the near-unity e_g electron state (t_2g⁶e_g¹). Moreover, the inevitable oxygen vacancies (V_o) further optimize the energy barrier of the potential-determining step (from OH* to O*). This co-modification strategy enhances turnover frequency-based electrocatalytic activity about two orders higher than the control sample without surface sulfates. This work may provide insight into the OER activity enhancement mechanism by the oxyanion groups.

Understanding of Oxygen Redox in the Oxygen Evolution Reaction

The electron-transfer process during the oxygen evolution reaction (OER) often either proceeds solely via a metal redox chemistry (adsorbate evolution mechanism (AEM), with metal bands around the Fermi level) or an oxygen redox chemistry (lattice oxygen oxidation mechanism (LOM), with oxygen bands around the Fermi level). Unlike the AEM, the LOM involves oxygen redox chemistry instead of metal redox, which leads to the formation of a direct oxygen-oxygen (OO) bond. As a result, such a process is able to bypass the rate-determining step, that is, OO bonding, in AEM, which highlights the critical advantage of LOM as compared to the conventional AEM. Thus, it has been well reported that LOM-based catalysts are able to demonstrate higher OER activities as compared to AEM-based catalysts. Here, a comprehensive understanding of the oxygen redox in LOM and all documented and possible characterization techniques that can be used to identify the oxygen redox are reviewed. This review will interpret the origins of oxygen redox in the reported LOM-based electrocatalysts and the underlying science of LOM-induced surface reconstruction in transition metal oxides. Finally, perspectives on the future development of LOM electrocatalysts are also provided.

Magnetic Field Manipulation of Tetrahedral Units in Spinel Oxides for Boosting Water Oxidation

Magnetic field enhanced electrocatalysis has recently emerged as a promising strategy for the development of a viable and sustainable hydrogen economy via water oxidation. Generally, the effects of magnetic field enhanced electrocatalysis are complex including magnetothermal, magnetohydrodynamic and spin selectivity effects. However, the exploration of magnetic field effect on the structure regulation of electrocatalyst is still unclear whereas is also essential for underpinning the mechanism of magnetic enhancement on the electrocatalytic oxygen evolution reaction (OER) process. Here, it is identified that in a mixed NiFe₂ O₄ (NFO), a large magnetic field can force the Ni²⁺ cations to migrate from the octahedral (O_h ) sites to tetrahedral (T_d ) sites. As a result, the magnetized NFO electrocatalyst (NFO-M) shows a two-fold higher current density than that of the pristine NFO in alkaline electrolytes. The OER enhancement of NFO is also observed at 1 T (NFO@1T) under an operando magnetic field. Our first-principles calculations further confirm the mechanism of magnetic field driven structure regulation and resultant OER enhancement. These findings provide a strategy of manipulating tetrahedral units of spinel oxides by a magnetic field on boosting OER performance.

Porous spinel-type transition metal oxide nanostructures as emergent electrocatalysts for oxygen reduction reactions

Porous spinel-type transition metal oxide (PS-TMO) nanocatalysts comprising two kinds of metal (denoted as A_xB_3-xO₄, where A, B = Co, Ni, Zn, Mn, Fe, V, Sm, Li, and Zn) have emerged as promising electrocatalysts for oxygen reduction reactions (ORRs) in energy conversion and storage systems (ECSS). This is due to the unique catalytic merits of PS-TMOs (such as p-type conductivity, optical transparency, semiconductivity, multiple valence states of their oxides, and rich active sites) and porous morphologies with great surface area, low density, abundant transportation paths for intermediate species, maximized atom utilization and quick charge mobility. In addition, PS-TMOs nanocatalysts are easily prepared in high yield from Earth-abundant and inexpensive metal precursors that meet sustainability requirements and practical applications. Owing to the continued developments in the rational synthesis of PS-TMOs nanocatalysts for ORRs, it is utterly imperative to provide timely updates and highlight new advances in this research area. This review emphasizes recent research advances in engineering the morphologies and compositions of PS-TMOs nanocatalysts in addition to their mechanisms, to decipher their structure-activity relationships. Also, the ORR mechanisms and fundamentals are discussed, along with the current barriers and future outlook for developing the next generation of PS-TMOs nanocatalysts for large-scale ECSS.

Electron delocalization triggers nonradical Fenton-like catalysis over spinel oxides

Nonradical Fenton-like catalysis offers opportunities to overcome the low efficiency and secondary pollution limitations of existing advanced oxidation decontamination technologies, but realizing this on transition metal spinel oxide catalysts remains challenging due to insufficient understanding of their catalytic mechanisms. Here, we explore the origins of catalytic selectivity of Fe-Mn spinel oxide and identify electron delocalization of the surface metal active site as the key driver of its nonradical catalysis. Through fine-tuning the crystal geometry to trigger Fe-Mn superexchange interaction at the spinel octahedra, ZnFeMnO₄ with high-degree electron delocalization of the Mn-O unit was created to enable near 100% nonradical activation of peroxymonosulfate (PMS) at unprecedented utilization efficiency. The resulting surface-bound PMS* complex can efficiently oxidize electron-rich pollutants with extraordinary degradation activity, selectivity, and good environmental robustness to favor water decontamination applications. Our work provides a molecule-level understanding of the catalytic selectivity and bimetallic interactions of Fe-Mn spinel oxides, which may guide the design of low-cost spinel oxides for more selective and efficient decontamination applications.

Artificially steering electrocatalytic oxygen evolution reaction mechanism by regulating oxygen defect contents in perovskites

The regulation of mechanism on the electrocatalysis process with multiple reaction pathways is more efficient and essential than conventional material engineering for the enhancement of catalyst performance. Here, by using oxygen evolution reaction (OER) as a model, which has an adsorbate evolution mechanism (AEM) and a lattice oxygen oxidation mechanism (LOM), we demonstrate a general strategy for steering the two mechanisms on various La_xSr_1-xCoO_3-δ. By delicately controlling the oxygen defect contents, the dominant OER mechanism on La_xSr_1-xCoO_3-δ can be arbitrarily transformed between AEM-LOM-AEM accompanied by a volcano-type activity variation trend. Experimental and computational evidence explicitly reveal that the phenomenon is due to the fact that the increased oxygen defects alter the lattice oxygen activity with a volcano-type trend and preserve the Co⁰ state for preferably OER. Therefore, we achieve the co-optimization between the activity and stability of catalysts by altering the mechanism rather than a specific design of catalysts.

Electrochemistry in Magnetic Fields

Developing new strategies to advance the fundamental understanding of electrochemistry is crucial to mitigating multiple contemporary technological challenges. In this regard, magnetoelectrochemistry offers many strategic advantages in controlling and understanding electrochemical reactions that might be tricky to regulate in conventional electrochemical fields. However, the topic is highly interdisciplinary, combining concepts from electrochemistry, hydrodynamics, and magnetism with experimental outcomes that are sometimes unexpected. In this Review, we survey recent advances in using a magnetic field in different electrochemical applications organized by the effect of the generated forces on fundamental electrochemical principles and focus on how the magnetic field leads to the observed results. Finally, we discuss the challenges that remain to be addressed to establish robust applications capable of meeting present needs.

Tailoring Oxygen Reduction Reaction Pathway on Spinel Oxides via Surficial Geometrical-Site Occupation Modification Driven by the Oxygen Evolution Reaction

The oxygen reduction reaction (ORR) has been demonstrated as a critical technology for both energy conversion technologies and hydrogen peroxide intermediate production. Herein, an in situ oxygen evolution reaction (OER) surface evolution strategy is applied for changing the surface structure of MnCo₂ O₄ oxide with tetrahedral and octahedral cations vacancies to realize reaction pathway switching from 2e^- ORR and 4e^- ORR. Interestingly, the as-synthesized MnCo₂ O₄ -pristine (MnCo₂ O₄ -P) with the highest surficial Mn/Co octahedron occupation favors two electrons reaction routes exhibiting high H₂ O₂ selectivity (≈80% and reaches nearly 100% at 0.75 V vs RHE); after surface atoms reconstruction, MnCo₂ O₄ -activation (MnCo₂ O₄ -A) with the largest Mn/Co tetrahedron occupation present excellent ORR performance through the four-electron pathway with an ultrahigh onset potential and half-wave potential of 0.78 and 0.92 V, ideal mass activity (MA), and turnover frequencies (TOF) values. Density functional theory (DFT) calculations reveal the concurrent modulations of both Co and Mn by the surface reconstructions, which improve the electroactivity of MnCo₂ O₄ -A toward the 4e^- pathway. This work provides a new perspective to building correlation of OER activation-ORR property, bringing detailed understating for reaction route transformation, and thus guiding the development of certain electrocatalysts with specific purposes.

Metalloid-Cluster Ligands Enabling Stable and Active FeN4 -Ten Motifs for the Oxygen Reduction Reaction

In nature, the oxygen reduction reaction (ORR) is catalyzed by cytochrome P450 (CYP) enzymes containing heme iron centers with an axial thiolate ligand (FeN₄ -S), which are among the most finely developed catalysts by natural selection. However, the exceptional ORR activity and selectivity of CYP enzymes originate from their non-rigid and self-adaptive coordination network with molecular ligands, which sacrifices the stability of the active motifs under electrochemical reaction conditions. Here, a design strategy to circumvent this dilemma by incorporating Fe-N₄ motifs into carbon matrices instead of the protein scaffold and replacing the axial molecular thiolate ligand with a stable tellurium cluster (Te_n ) is demonstrated. Theoretical calculations indicate a moderate interaction between Fe 3d and Te 5p orbitals once n > 2, allowing the FeTe bond to dynamically change its strength to adaptively facilitate the intermediate steps during the ORR process, which renders FeN₄ -Te_n active sites with superior ORR activity. This adaptive behavior mimics the conformational dynamics of an enzyme during the reaction, but retains the stability nature as a heterogeneous catalyst. The experiments validate that the as-designed catalyst with a characterized FeN₄ -Te_n structure outperforms the commercial Pt/C catalyst both on activity and stability.

Facet-Defined Strain-Free Spinel Oxide for Oxygen Reduction

Exposing facet and surface strain are critical factors affecting catalytic performance but unraveling the composition-dependent activity on specific facets under strain-controlled environment is still challenging due to the synthetic difficulties. Herein, we achieved a (001) facet-defined Co-Mn spinel oxide surface with different surface compositions using epitaxial growth on Co₃O₄ nanocube template. We adopted composition gradient synthesis to relieve the strain layer by layer, minimizing the surface strain effect on catalytic activity. In this system, experimental and calculational analyses of model oxygen reduction reaction (ORR) activity reveals a volcano-like trend with Mn/Co ratios because of an adequate charge transfer from octahedral-Mn to neighboring Co. Co_0.5Mn_0.5 as an optimized Mn/Co ratio exhibits both outstanding ORR activity (0.894 V vs RHE in 1 M KOH) and stability (2% activity loss against chronoamperometry). By controlling facet and strain, this study provides a well-defined platform for investigating composition-structure-activity relationships in electrocatalytic processes.

Oxygen Evolution Reaction in Energy Conversion and Storage: Design Strategies Under and Beyond the Energy Scaling Relationship

The oxygen evolution reaction (OER) is the essential module in energy conversion and storage devices such as electrolyzer, rechargeable metal-air batteries and regenerative fuel cells. The adsorption energy scaling relations between the reaction intermediates, however, impose a large intrinsic overpotential and sluggish reaction kinetics on OER catalysts. Developing advanced electrocatalysts with high activity and stability based on non-noble metal materials is still a grand challenge. Central to the rational design of novel and high-efficiency catalysts is the development and understanding of quantitative structure-activity relationships, which correlate the catalytic activities with structural and electronic descriptors. This paper comprehensively reviews the benchmark descriptors for OER electrolysis, aiming to give an in-depth understanding on the origins of the electrocatalytic activity of the OER and further contribute to building the theory of electrocatalysis. Meanwhile, the cutting-edge research frontiers for proposing new OER paradigms and crucial strategies to circumvent the scaling relationship are also summarized. Challenges, opportunities and perspectives are discussed, intending to shed some light on the rational design concepts and advance the development of more efficient catalysts for enhancing OER performance.

Structural Insights into Multi-Metal Spinel Oxide Nanoparticles for Boosting Oxygen Reduction Electrocatalysis

Multi-metal oxide (MMO) materials have significant potential to facilitate various demanding reactions by providing additional degrees of freedom in catalyst design. However, a fundamental understanding of the (electro)catalytic activity of MMOs is limited because of the intrinsic complexity of their multi-element nature. Additional complexities arise when MMO catalysts have crystalline structures with two different metal site occupancies, such as the spinel structure, which makes it more challenging to investigate the origin of the (electro)catalytic activity of MMOs. Here, uniform-sized multi-metal spinel oxide nanoparticles composed of Mn, Co, and Fe as model MMO electrocatalysts are synthesized and the contributions of each element to the structural flexibility of the spinel oxides are systematically studied, which boosts the electrocatalytic oxygen reduction reaction (ORR) activity. Detailed crystal and electronic structure characterizations combined with electrochemical and computational studies reveal that the incorporation of Co not only increases the preferential octahedral site occupancy, but also modifies the electronic state of the ORR-active Mn site to enhance the intrinsic ORR activity. As a result, nanoparticles of the optimized catalyst, Co_0.25 Mn_0.75 Fe_2.0 -MMO, exhibit a half-wave potential of 0.904 V (versus RHE) and mass activity of 46.9 A g_oxide ^-1 (at 0.9 V versus RHE) with promising stability.

Boosting Oxygen Reduction Activity of Manganese Oxide Through Strain Effect Caused By Ion Insertion

Transition-metal oxides with a strain effect have attracted immense interest as cathode materials for fuel cells. However, owing to the introduction of heterostructures, substrates, or a large number of defects during the synthesis of strain-bearing catalysts, not only is the structure-activity relationship complicated but also their performance is mediocre. In this study, a mode of strain introduction is reported. Transition-metal ions with different electronegativities are intercalated into the cryptomelane-type manganese oxide octahedral molecular sieves (OMS-2) structure with K ions as the template, resulting in the octahedral structural distortion of MnO₆ and producing strains of different degrees. Experimental studies reveal that Ni-OMS-2 with a high compressive strain (4.12%) exhibits superior oxygen reduction performance with a half-wave potential (0.825 V vs RHE) greater than those of other reported manganese-based oxides. This result is related to the increase in the covalence of MnO₆ octahedral configuration and shifting down of the e_g band center caused by the higher compression strain. This research avoids the introduction of new chemical bonds in the main structure, weakens the effect of e_g electron filling number, and emphasizes the pure strain effect. This concept can be extended to other transition-metal-oxide catalysts.

Cation-Tuning Induced d-Band Center Modulation on Co-based Spinel Oxide for Rechargeable Zn-Air Batteries

Atomic substitutions at the tetrahedral site (A Td ) could theoretically achieve an efficient optimization of the charge at the octahedral site (B Oh ) through the A Td -O-B Oh interactions in the spinel oxides (AB2O4). However, the precise control and adjustment of the spinel oxides are still challenging owing to the complexity of their crystal structure. In this work, we demonstrate a simple solvent method to tailor the structures of spinel oxides and further use the spinel oxide composites (ACo2O4/NCNTs, A = Mn, Co, Ni, Cu, Zn) for oxygen electrocatalysis. And the optimized MnCo2O4/NCNTs exhibit high activity and excellent durability for oxygen reduction/evolution reactions. Remarkably, the rechargeable liquid Zn-air battery equipped the MnCo2O4/NCNTs cathode affords a specific capacity of 827 mAh gZn-1 with high power density of 74.63 mW cm-2 and no voltage degradation after 300 cycles at a high charging-discharging rate (5 mA cm-2). The density functional theory (DFT) calculations reveal that the substitution could regulate the ratio of Co3+/Co2+ and thereby lead to the electronic structure modulated accompanied with the movement of d-band center. The tetrahedral and octahedral sites interact through the Mn-O-Co, the Co3+ Oh of MnCo2O4 with the optimal charge structure allows more suitable binding interaction between the active center and the oxygenated species, resulting in superior oxygen electrocatalytic performance. This work not only proves the influence of the charge modulation mechanism on the oxygen catalysis process but also provides novel strategies for the subsequent design of other oxygen catalysis materials.

Reconstruction of spinel Co3O4 by inert Zn2+ towards enhanced oxygen catalytic activity

Zn_xCo_3-xO₄ (0 ≤ x≤ 1) coupled with nitrogen-doped hollow porous carbon spheres exhibits a superior oxygen catalytic activity. A Zn-air battery using Zn_0.6Co_2.4O₄/NHCS as a cathodic catalyst affords a high-power density (130 mW cm^-2) and excellent stability. The effect of reconstruction of catalytically active Co ions induced by Zn is well-investigated.

Doping Independent Work Function and Stable Band Gap of Spinel Ferrites with Tunable Plasmonic and Magnetic Properties

Tuning optical or magnetic properties of nanoparticles, by addition of impurities, for specific applications is usually achieved at the cost of band gap and work function reduction. Additionally, conventional strategies to develop nanoparticles with a large band gap also encounter problems of phase separation and poor crystallinity at high alloying degree. Addressing the aforementioned trade-offs, here we report Ni-Zn nanoferrites with energy band gap (E_g) of ≈3.20 eV and a work function of ≈5.88 eV. While changes in the magnetoplasmonic properties of the Ni-Zn ferrite were successfully achieved with the incorporation of bismuth ions at different concentrations, there was no alteration of the band gap and work function in the developed Ni-Zn ferrite. This suggests that with the addition of minute impurities to ferrites, independent of their changes in the band gap and work function, one can tune their magnetic and optical properties, which is desired in a wide range of applications such as nanobiosensing, nanoparticle based catalysis, and renewable energy generation using nanotechnology.

Main Descriptors To Correlate Structures with the Performances of Electrocatalysts

Traditional trial and error approaches to search for hydrogen/oxygen redox catalysts with high activity and stability are typically tedious and inefficient. There is an urgent need to identify the most important parameters that determine the catalytic performance and so enable the development of design strategies for catalysts. In the past decades, several descriptors have been developed to unravel structure-performance relationships. This Minireview summarizes reactivity descriptors in electrocatalysis including adsorption energy descriptors involving reaction intermediates, electronic descriptors represented by a d-band center, structural descriptors, and universal descriptors, and discusses their merits/limitations. Understanding the trends in electrocatalytic performance and predicting promising catalytic materials using reactivity descriptors should enable the rational construction of catalysts. Artificial intelligence and machine learning have also been adopted to discover new and advanced descriptors. Finally, linear scaling relationships are analyzed and several strategies proposed to circumvent the established scaling relationships and overcome the constraints imposed on the catalytic performance.

Atomic Sulfur Filling Oxygen Vacancies Optimizes H Absorption and Boosts the Hydrogen Evolution Reaction in Alkaline Media

The hydrogen evolution reaction (HER) usually has sluggish kinetics in alkaline solution due to the difficulty in forming binding protons. Herein we report an electrocatalyst in which sulfur atoms are doping in the oxygen vacancies (V_O ) of inverse spinel NiFe₂ O₄ (S-NiFe₂ O₄ ) to create active sites with enhanced electron transfer capability. This electrocatalyst has an ultralow overpotential of 61 mV at the current density of 10 mA cm^-2 and long-term stability of 60 h at 1.0 Acm^-2 in 1.0 M KOH media. In situ Raman spectroscopy revealed that S sites adsorb hydrogen adatom (H*) and in situ form S-H*, which favor the production of hydrogen and boosts HER in alkaline solution. DFT calculations further verified that S introduction lowered the energy barrier of H₂ O dissociation. Both experimental and theoretical investigations confirmed S atoms are active sites of the S-NiFe₂ O₄ .

Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment

The proton exchange membrane (PEM) water electrolysis is one of the most promising hydrogen production techniques. The oxygen evolution reaction (OER) occurring at the anode dominates the overall efficiency. Developing active and robust electrocatalysts for OER in acid is a longstanding challenge for PEM water electrolyzers. Most catalysts show unsatisfied stability under strong acidic and oxidative conditions. Such a stability challenge also leads to difficulties for a better understanding of mechanisms. This review aims to provide the current progress on understanding of OER mechanisms in acid, analyze the promising strategies to enhance both activity and stability, and summarize the state-of-the-art catalysts for OER in acid. First, the prevailing OER mechanisms are reviewed to establish the physicochemical structure-activity relationships for guiding the design of highly efficient OER electrocatalysts in acid with stable performance. The reported approaches to improve the activity, from macroview to microview, are then discussed. To analyze the problem of instability, the key factors affecting catalyst stability are summarized and the surface reconstruction is discussed. Various noble-metal-based OER catalysts and the current progress of non-noble-metal-based catalysts are reviewed. Finally, the challenges and perspectives for the development of active and robust OER catalysts in acid are discussed.

Intermediate Sr2Co1.5Fe0.5O6-δ Tetragonal Structure between Perovskite and Brownmillerite as a Model Catalyst with Layered Oxygen Deficiency for Enhanced Electrochemical Water Oxidation

The generation of hydrogen in an environmentally benign way is highly essential to meet future energy demands. However, in the process of splitting water electrochemically, sluggish kinetics of the oxygen evolution reaction (OER) curtails its applicability, as it drags energy input. Herein, we synthesized Sr-Co-Fe-O oxides to optimize their OER activity by varying the Co/Fe ratio. Among them, Sr2Co1.5Fe0.5O6-δ exhibited the best OER catalytic activity in the series, with an overpotential of 318 mV at 10 mA cm-2 and Tafel slope of 44.8 mV dec-1. High-resolution neutron powder diffraction analysis identified an intermediate structure between the perovskite and brownmillerite, with alternating layers of disorderly orientated oxygen-deficient tetrahedra and fully stoichiometric octahedra. The unique stacking of tetrahedral and octahedral units facilitates desired interactions between the electrode surface and electrolyte. Theoretical calculations revealed that increased covalency of Co 3d and O 2p in Sr2Co1.5Fe0.5O6-δ oxide is another primary contributor to its augmented water oxidation ability. As a model for developing catalysts with such an intermediate structure, the synergetic effect of oxygen vacancy and hybridization between Co 3d and O 2p assured the Sr2Co1.5Fe0.5O6-δ oxide as a better catalyst for its enhanced OER activity.

Single-phase perovskite oxide with super-exchange induced atomic-scale synergistic active centers enables ultrafast hydrogen evolution

The state-of-the-art active HER catalysts in acid media (e.g., Pt) generally lose considerable catalytic performance in alkaline media mainly due to the additional water dissociation step. To address this issue, synergistic hybrid catalysts are always designed by coupling them with metal (hydro)oxides. However, such hybrid systems usually suffer from long reaction path, high cost and complex preparation methods. Here, we discover a single-phase HER catalyst, SrTi_0.7Ru_0.3O_3-δ (STRO) perovskite oxide highlighted with an unusual super-exchange effect, which exhibits excellent HER performance in alkaline media via atomic-scale synergistic active centers. With insights from first-principles calculations, the intrinsically synergistic interplays between multiple active centers in STRO are uncovered to accurately catalyze different elementary steps of alkaline HER; namely, the Ti sites facilitates nearly-barrierless water dissociation, Ru sites function favorably for OH* desorption, and non-metal oxygen sites (i.e., oxygen vacancies/lattice oxygen) promotes optimal H* adsorption and H₂ desorption.

Efficient electrocatalysts are crucial for the sustainable hydrogen production as an alternative clean fuel. Here, the authors explore a single-phase perovskite oxide as a high-performance hydrogen evolution electrocatalyst via super-exchange induced atomic scale synergistic active sites.

Solid-solution hexagonal Ni0.5Co0.5Se nanoflakes toward boosted oxygen evolution reaction

The oxygen evolution reaction (OER) with sluggish kinetics is a bottleneck for the large-scale application of water electrolysis. Herein, solid-solution hexagonal Ni_0.5Co_0.5Se nanoflakes are designed and successfully synthesized via a facile hydrothermal method with a much lower overpotential of 216 mV at 10 mA cm^-2 and a Tafel slope of 37.08 mV dec^-1.