In-situ surface reconstruction of Co-based imidazole zeolite framework by Mo etching for superior water oxidation

Although zeolitic imidazolate frameworks (ZIFs) possess the merits of orderly porosity, high permeability, and easy functionalization, the transformation of ZIFs into the real active species and the promotion of the catalytic efficiency and stability are still challenging. Herein, CoMo-based three-dimensional (3D) hollow nanocages composed of interconnected nanosheets are fabricated by in-situ etching metal-organic framework (ZIF-67) under the aid of MoO42-. X-ray photoelectron spectroscopy (XPS) and in-situ Raman confirm that Mo leaching can accelerate surface reconstruction and generate CoOOH active sites after continuous oxidation. Benefiting from the nanostructure and electronic properties after surface reconstruction, the engineered CoMo-30 exhibits the lowest overpotential of 280 mV at 30 mA cm-2 and robust stability over 110 h in 1 M KOH media for oxygen evolution reaction (OER), which significantly surpasses the other counterparts and commercial RuO2. Density functional theory (DFT) calculations indicate that CoMo-30 has a lower free energy of *O → *OOH as rate determining step (RDS), suggesting that CoOOH sites play a crucial role in enhancing the activity and kinetics of OER. This work provides valuable insights into the rational design of hollow structures and the structure-composition-activity relationship during the electrochemical reaction process.

Self-Reconstruction of High Entropy Alloys for Efficient Alkaline Hydrogen Evolution

Alkaline water (H2O) electrolysis is currently a commercialized green hydrogen (H2) production technology, yet the unsatisfactory hydrogen evolution reaction (HER) performance severely limits its energy conversion efficiency and cost reduction. Herein, PtRu2.9Fe0.15Co1.5Ni1.3 high entropy alloys (HEAs) is synthesized and subsequently exploited electrochemically induced structural oxidation processes to construct self-reconfigurable HEAs, as an efficient alkaline HER catalyst. The optimized self-reconstructed PtRu2.9Fe0.15Co1.5Ni1.3 HEAs with the HEAs and cobalt rutheniate interface (HEAs-Co2RuO4) exhibits excellent alkaline HER performance, requiring just 11.8 mV to obtain a current density (j) of 10 mA cm-2 in 1 m KOH. And the j on HEAs-Co2RuO4 is 41.8 mA cm-2 at 0.07 VRHE, 2.0 and 6.1 times higher than PtRu2.9Fe0.15Co1.5Ni1.3 HEAs and 20% Pt/C. Mechanism studies reveal that the improved alkaline HER performance of HEAs-Co2RuO4 is due to the formation of HEAs-Co2RuO4, which significantly shrinks the Helmholtz layer, provides a new fast material transport channel, boosts H2O adsorption, and reduces hydrogen adsorption, and thus accelerates the alkaline HER. This research not only throws new light on the self-reconstruction of catalysts but also provides guidance for the rational design of efficient electrocatalysts.

Regulating the Local Spin States in Spinel Oxides to Promote the Activity of Li-CO2 Batteries

Due to the high energy barrier, slow reaction kinetics, and complex reaction environments of Li-CO2 batteries, the development of durable and efficient catalysts is essential. Transition metal oxides are promising for their availability, stability, and 3d electronic features, with spin states playing an important role in CO2 activation. In this study, the local spin states are regulated by incorporating Ni into Co3O4 and its impact on activity in Li-CO2 batteries is explored. The results show that Ni atoms with high spin states in Ni0.1Co2.9O4 facilitate electron transfer from the catalyst to the unoccupied orbitals of CO2, providing sufficient active sites for the nucleation and growth of small Li2CO3 crystals. These small crystals have a low decomposition barrier, leading to improved battery efficiency. Therefore, Ni0.1Co2.9O4 shows superior catalytic performance with an overpotential of 0.72 V and an energy efficiency of ≈70% after 500 h. This work provides insights into the relationship between spin states and CO2 reactions, highlighting a promising avenue for developing high-performance metal-CO2 batteries.

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.

Mn Doping at High-Activity Octahedral Vacancies of γ-Fe2O3 for Oxygen Reduction Reaction Electrocatalysis in Metal-Air Batteries

γ-Fe2O3 with the intrinsic cation vacancies is an ideal substrate for heteroatom doping into the highly active octahedral sites in spinel oxide catalysts. However, it is still a challenge to confirm the vacancy location of γ-Fe2O3 through experiments and obtain enhanced catalytic performance by preferential occupation of octahedral sites for heteroatom doping. Here, a Mn-doped γ-Fe2O3 incorporated with carbon nanotubes catalyst was developed to successfully achieve preferential doping into highly active octahedral sites by employing γ-Fe2O3 as the precursor. Further, the vacancy in γ-Fe2O3 was only located on octahedral sites rather than tetrahedral ones, which was first proved by direct experimental evidence through the clarification doping sites of Mn. Notably, the catalyst shows outstanding activity towards oxygen reduction reaction with the half-wave potential of 0.82 V and 0.64 V vs. reversible hydrogen electrode in alkaline and neutral electrolytes, respectively, as well as the maximum power density of 179 mWcm-2 and 403 mWcm-2 for Mg-air batteries and Al-air batteries, respectively. It could be attributed to the synergistic effect of the doping Mn on octahedral sites and the substrate γ-Fe2O3 along with the modification of the adsorption/desorption properties for oxygen-containing intermediates as well as the optimization of the reaction energy barriers.

"Seaweed Structure" design for solid gel electrolyte with hydroxide ion conductivity enabling flexible zinc air batteries

The construction of solid-state electrolytes for flexible zinc-air batteries is extremely challenging. A flexible and highly conductive solid electrolyte designed with a "seaweed structure" is reported in this work. Sodium alginate serves as the backbone to form a robust network structure, and the grafted quaternary ammonium groups provide channels for rapid ion transport, achieving excellent flexibility and hydroxide conductivity. The conductivity of the modified electrolyte membrane (QASA) is 5.23 × 10-2 S cm-1 at room temperature and reaches up to 8.51 × 10-2 S cm-1 at 75 °C. In the QASA based battery, bending at any angle is realized, and the power density is up to 57.28 mW cm-2. This work provides a new way to prepare high conductivity, green solid-state zinc-air batteries, and opens up a research line of thought for flexible energy storage materials.

Tuning Octahedron Sites of CoV2O4 via Cationic Competition for Efficient Oxygen Evolution Reaction

Doping transition metal oxide spinels with metal ions represents a significant strategy for optimizing the electronic structure of electrocatalysts. Herein, a bimetallic Fe and Ru doping strategy to fine-tune the crystal structure of CoV2O4 spinel for highly enhanced oxygen evolution reaction (OER) is presented performance. The incorporation of Fe and Ru is observed at octahedral sites within the CoV2O4 structure, effectively modulating the electronic configuration of Co. Density functional theory calculations have confirmed that Fe acts as a novel reactive site, replacing V. Additionally, the synergistic effect of Fe, Co, and Ru effectively optimizes the Gibbs free energy of the intermediate species, reduces the reaction energy barrier, and accelerates the kinetics toward OER. As expected, the best-performing CoVFe0.5Ru0.5O4 displays a low overpotential of 240 mV (@10 mA cm-2) and a remarkably low Tafel slope of 38.9 mV dec-1, surpassing that of commercial RuO2. Moreover, it demonstrates outstanding long-term durability lasting for 72 h. This study provides valuable insights for the design of highly active polymetallic spinel electrocatalysts for energy conversion applications.

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.

Enhancing the Electrocatalytic Oxidation of 5-Hydroxymethylfurfural Through Cascade Structure Tuning for Highly Stable Biomass Upgrading

Electrocatalytic 5-hydroxymethylfurfural oxidation reaction (HMFOR) provides a promising strategy to convert biomass derivative to high-value-added chemicals. Herein, a cascade strategy is proposed to construct Pd-NiCo2O4 electrocatalyst by Pd loading on Ni-doped Co3O4 and for highly active and stable synergistic HMF oxidation. An elevated current density of 800 mA cm-2 can be achieved at 1.5 V, and both Faradaic efficiency and yield of 2,5-furandicarboxylic acid remained close to 100% over 10 consecutive electrolysis. Experimental and theoretical results unveil that the introduction of Pd atoms can modulate the local electronic structure of Ni/Co, which not only balances the competitive adsorption of HMF and OH- species, but also promote the active Ni3+ species formation, inducing high indirect oxidation activity. We have also discovered that Ni incorporation facilitates the Co2+ pre-oxidation and electrophilic OH* generation to contribute direct oxidation process. This work provides a new approach to design advanced electrocatalyst for biomass upgrading.

Lattice Oxygen Activation through Deep Oxidation of Co4N by Jahn-Teller-Active Dopants for Improved Electrocatalytic Oxygen Evolution

Triggering the lattice oxygen oxidation mechanism is crucial for improving oxygen evolution reaction (OER) performance, because it could bypass the scaling relation limitation associated with the conventional adsorbate evolution mechanism through the direct formation of oxygen-oxygen bond. High-valence transition metal sites are favorable for activating the lattice oxygen, but the deep oxidation of pre-catalysts suffers from a high thermodynamic barrier. Here, taking advantage of the Jahn-Teller (J-T) distortion induced structural instability, we incorporate high-spin Mn3+ (t 2 g 3 e g 1 ${{t}_{2g}^{3}{e}_{g}^{1}}$ ) dopant into Co4N. Mn dopants enable a surface structural transformation from Co4N to CoOOH, and finally to CoO2, as observed by various in situ spectroscopic investigations. Furthermore, the reconstructed surface on Mn-doped Co4N triggers the lattice oxygen activation, as evidenced experimentally by pH-dependent OER, tetramethylammonium cation adsorption and online electrochemical mass spectrometry measurements of 18O-labelled catalysts. In general, this work not only offers the introducing J-T effect approach to regulate the structural transition, but also provides an understanding about the influence of the catalyst's electronic configuration on determining the reaction route, which may inspire the design of more efficient catalysts with activated lattice oxygen.

Optimizing the intermediates adsorbability and revealing the dynamic reconstruction of Co6Fe3S8 solid solution for bifunctional water splitting

Co9S8 has been extensively studied as a promising catalyst for water electrolysis. Doping Co9S8 with Fe improves its oxygen evolution reaction (OER) performance by regulating the catalyst self-reconfigurability and enhancing the absorption capacity of OER intermediates. However, the poor alkaline hydrogen evolution reaction (HER) properties of Co9S8 limit its application in bifunctional water splitting. Herein, we combined Fe doping and sulfur vacancy engineering to synergistically enhance the bifunctional water-splitting performance of Co9S8. The as-synthesized Co6Fe3S8 catalyst exhibited excellent OER and HER characteristics with low overpotentials of 250 and 84 mV, respectively. It also resulted in the low Tafel slopes of 135 mV dec-1 for the OER and 114 mV dec-1 for the HER. A two-electrode electrolytic cell with Co6Fe3S8 used as both the cathode and anode produced a current density of 10 mA cm-2 at a low voltage of only 1.48 V, maintaining high stability for 100 h. The results of in/ex-situ experiments indicated that the OER process induced electrochemical reconfiguration, forming CoOOH/FeOOH active species on the catalyst surface to enhance its OER performance. Density functional theory (DFT) simulations revealed that Fe doping and the presence of unsaturated coordination metal sites in Co6Fe3S8 promoted H2O and H* adsorption for the HER. The findings of this study can help develop a strategy for designing highly efficient bifunctional water splitting electrocatalysts.

Bifunctional ligand Co metal-organic framework derived heterostructured Co-based nanocomposites as oxygen electrocatalysts toward rechargeable zinc-air batteries

Rational construction of efficient and robust bifunctional oxygen electrocatalysts is key but challenging for the widespread application of rechargeable zinc-air batteries (ZABs). Herein, bifunctional ligand Co metal-organic frameworks were first explored to fabricate a hybrid of heterostructured CoOx/Co nanoparticles anchored on a carbon substrate rich in CoNx sites (CoOx/Co@CoNC) via a one-step pyrolysis method. Such a unique heterostructure provides abundant CoNx and CoOx/Co active sites to drive oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively. Besides, their positive synergies facilitate electron transfer and optimize charge/mass transportation. Consequently, the obtained CoOx/Co@CoNC exhibits a superior ORR activity with a higher half-wave potential of 0.88 V than Pt/C (0.83 V vs. RHE), and a comparable OER performance with an overpotential of 346 mV at 10 mA cm-2 to the commercial RuO2. The assembled ZAB using CoOx/Co@CoNC as a cathode catalyst displays a maximum power density of 168.4 mW cm-2, and excellent charge-discharge cyclability over 250 h at 5 mA cm-2. This work highlights the great potential of heterostructures in oxygen electrocatalysis and provides a new pathway for designing efficient bifunctional oxygen catalysts toward rechargeable ZABs.

Defect engineered ternary metal spinel-type Ni-Fe-Co oxide as bifunctional electrocatalyst for overall electrochemical water splitting

Transition metal spinel oxides were engineered with active elements as bifunctional water splitting electrocatalysts to deliver superior intrinsic activity, stability, and improved conductivity to support green hydrogen production. In this study, we reported the ternary metal Ni-Fe-Co spinel oxide electrocatalysts prepared by defect engineering strategy with rich and deficient Na+ ions, termed NFCO-Na and NFCO, which suggest the formation of defects with Na+ forming tensile strain. The Na-rich NiFeCoO4 spinel oxide reveals lattice expansion, resulting in the formation of a defective crystal structure, suggesting higher electrocatalytic active sites. The spherical NFCO-Na electrocatalysts exhibit lower OER and HER overpotentials of 248 mV and 153 mV at 10 mA cm-2 and smaller Tafel slope values of about 78 mV dec-1 and 129 mV dec-1, respectively. Notably, the bifunctional NFCO-Na electrocatalyst requires a minimum cell voltage of about 1.67 V to drive a current density of 10 mA cm-2. The present work highlights the significant electrochemical activity of defect-engineered ternary metal oxides, which can be further upgraded as highly active electrocatalysts for water splitting applications.

Mesoporous Electrocatalysts with p-n Heterojunctions for Efficient Electroreduction of CO2 and N2 to Urea

The electrocatalytic synthesis of high-value-added urea by activating N2 and CO2 is a green synthesis technology that has achieved carbon neutrality. However, the chemical adsorption and C-N coupling ability of N2 and CO2 on the surface of the catalyst are generally poor, greatly limiting the improvement of electrocatalytic activity and selectivity in electrocatalytic urea synthesis. Herein, novel hierarchical mesoporous CeO2/Co3O4 heterostructures are fabricated, and at an ultralow applied voltage of -0.2 V, the urea yield rate reaches 5.81 mmol g-1 h-1, with a corresponding Faraday efficiency of 30.05%. The hierarchical mesoporous material effectively reduces the mass transfer resistance of reactants and intermediates, making it easier for them to access active centers. The emerging space-charge regions at the heterointerface generate local electrophilic and nucleophilic regions, facilitating CO2 targeted adsorption in the electrophilic region and activation to produce *CO intermediates and N2 targeted adsorption in the nucleophilic region and activation to generate *N ═ N* intermediates. Then, the electrons in the σ orbitals of *N ═ N* intermediates can be easily accepted by the empty eg orbitals of Co3+ in CeO2/Co3O4, which presents a low-spin state (LS: t2g6eg0). Subsequently, *CO couples with *N ═ N* to produce the key intermediate *NCON*. Interestingly, it was discovered through in situ Raman spectroscopy that the CeO2/Co3O4 catalyst has a reversible spinel structure before and after the electrocatalytic reaction, which is due to the surface reconstruction of the catalyst during the electrocatalytic reaction process, producing amorphous active cobalt oxides, which is beneficial for improving electrocatalytic activity.

Efficient Homolytic Cleavage of H2O2 on Hydroxyl-Enriched Spinel CuFe2O4 with Dual Lewis Acid Sites

Traditional H2O2 cleavage mediated by macroscopic electron transfer (MET) not only has low utilization of H2O2, but also sacrifices the stability of catalysts. We present a non-redox hydroxyl-enriched spinel (CuFe2O4) catalyst with dual Lewis acid sites to realize the homolytic cleavage of H2O2. The results of systematic experiments, in situ characterizations, and theoretical calculations confirm that tetrahedral Cu sites with optimal Lewis acidity and strong electron delocalization can synergistically elongate the O-O bonds (1.47 Å → 1.87 Å) in collaboration with adjacent bridging hydroxyl (another Lewis acid site). As a result, the free energy of H2O2 homolytic cleavage is decreased (1.28 eV → 0.98 eV). H2O2 can be efficiently split into ⋅OH induced by hydroxyl-enriched CuFe2O4 without MET, which greatly improves the catalyst stability and the H2O2 utilization (65.2 %, nearly 2 times than traditional catalysts). The system assembled with hydroxyl-enriched CuFe2O4 and H2O2 affords exceptional performance for organic pollutant elimination. The scale-up experiment using a continuous flow reactor realizes long-term stability (up to 600 mL), confirming the tremendous potential of hydroxyl-enriched CuFe2O4 for practical 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.

Design Principles and Mechanistic Understandings of Non-Noble-Metal Bifunctional Electrocatalysts for Zinc-Air Batteries

Zinc-air batteries (ZABs) are promising energy storage systems because of high theoretical energy density, safety, low cost, and abundance of zinc. However, the slow multi-step reaction of oxygen and heavy reliance on noble-metal catalysts hinder the practical applications of ZABs. Therefore, feasible and advanced non-noble-metal electrocatalysts for air cathodes need to be identified to promote the oxygen catalytic reaction. In this review, we initially introduced the advancement of ZABs in the past two decades and provided an overview of key developments in this field. Then, we discussed the working mechanism and the design of bifunctional electrocatalysts from the perspective of morphology design, crystal structure tuning, interface strategy, and atomic engineering. We also included theoretical studies, machine learning, and advanced characterization technologies to provide a comprehensive understanding of the structure-performance relationship of electrocatalysts and the reaction pathways of the oxygen redox reactions. Finally, we discussed the challenges and prospects related to designing advanced non-noble-metal bifunctional electrocatalysts for ZABs.

Spin-Magnetic Effect of d-π Conjugation Polymer Enhanced O-H Cleavage in Water Oxidation

A deep understanding of the mechanism for the spin-magnetic effect on O-H cleavage is crucial for the development of new catalysts for water oxidation. Herein, we designed and synthesized the crystalline Fe-DABDT and Co-DABDT (DABDT = 2,5-diaminobenzene-1,4-dithiol) and optimized an effective magnetic moment to explore the role of the spin-magnetic effect in the regulation of water oxidation activity. It can be found that the OER activity of the catalyst is positively correlated with its effective magnetic moment. Under the external magnetic field, Fe-DABDT with more spin single electrons has a stronger spin-magnetic response to water oxidation than Fe/Co-DABDT and Co-DABDT. The increase in OER current of Fe-DABDT is nearly 2 times higher than that of Co-DABDT. Experimental and density functional theory studies show that magnetized Fe sites could realize nucleophilic reaction, accelerate the polarization of electron spin states, and promote the polar decomposition of O-H and the formation of the O-O bond. This study provides mechanistic insight into the spin-magnetic effect of oxygen evolution reaction and further understanding of the spin origin of catalytic activity, which is expected to improve the energy efficiency of hydrogen production.

Spin-state Conversion by Asymmetrical Orbital Hybridization in Ni-doped Co3 O4 to Boost Singlet Oxygen Generation for Microbial Disinfection

Singlet oxygen (1 O2 ) plays a significant role in environmental and biomedical disinfection fields. Electrocatalytic processes hold great potential for 1 O2 generation, but remain challenging. Herein, a facile Ni doping converted spin-state transition approach is reported for boosting 1 O2 production. Magnetic analysis and theoretical calculations reveal that Ni occupied at the octahedral site of Co3 O4 can effectively induce a low-to-high spin-state transition. The high-spin Ni-Co3 O4 generate appropriate binding strength and enhance electron transfer between the Co centers with oxygen intermediates, thereby improving the catalytic activity of Ni-Co3 O4 for effective generating 1 O2 . In neutral conditions, 1×106  CFU mL-1 Gram-negative ESBL-producing Escherichia coli (E. coli) could be inactivated by Ni-Co3 O4 system within 5 min. Further antibacterial mechanisms indicate that 1 O2 can lead to cell membrane damage and DNA degradation so as to irreversible cell death. Additionally, the developed Ni-Co3 O4 system can effectively inactivate bacteria from wastewater and bioaerosols. This work provides an effective strategy for designing high-spin electrocatalysis to boost 1 O2 generation for disinfection process.

Corner-Sharing Tetrahedrally Coordinated W-V Dual Active Sites on Cu2 V2 O7 for Photoelectrochemical Water Oxidation

The sluggish four-electron oxygen evolving reaction is one of the key limitations of photoelectrochemical water decomposition. Optimizing the binding of active sites to oxygen in water and promoting the conversion of *O to *OOH are the key to enhancing oxygen evolution reaction. In this work, W-doped Cu2 V2 O7 (CVO) constructs corner-sharing tetrahedrally coordinated W-V dual active sites to induce the generation of electron deficiency active centers, promote the adsorption of ─OH, and accelerate the transformation of *O to *OOH for water splitting. The photocurrent obtained by the W-modified CVO photoanode is 0.97 mA cm-2 at 1.23 V versus RHE, which is much superior to that of the reported CVO. Experimental and theoretical results show that the excellent catalytic performance may be attributed to the formation of synergistic dual active sites between W and V atoms, and the introduction of W ions reduces the charge migration distance and prolongs the lifetime of photogenerated carriers. Meanwhile, the electronic structure in the center of the d-band is modulated, which leads to the redistribution of the electron density in CVO and lowers the energy barrier for the conversion of the rate-limiting step *O to *OOH.

Iron-doping-induced formation of Ni-Co-O nanotubes as efficient bifunctional electrodes

The rational design of earth-abundant and efficient electrocatalysts to replace precious metal-based materials is highly anticipated for overall water splitting. Herein, NiCo2O4 electrocatalysts with different Fe doping amounts (Fex-NCO, x = 1, 2, 3) were synthesized by a low-temperature chemical method. It was interesting to find that the doping of Fe induced the formation of NiCo2O4 nanotube arrays by modulating the Fe content. The Fe3-NCO electrode with a nanotube structure and rich oxygen vacancies exhibited exceptional electrocatalytic activities for the hydrogen evolution reaction (97 mV, 10 mA cm-2) and oxygen evolution reaction (188.4 mV, 10 mA cm-2). DFT calculations revealed that Fe promoted the modulation of the electronic structure, which played a crucial role in optimizing the reaction intermediates and altered the energy level of the d band center, and as a result, enhanced the water dissociation ability. Additionally, a low cell voltage of 1.56 V (10 mA cm-2) was realized for water splitting based on an as-fabricated Fe-doped NiCo2O4 nanotube array bifunctional electrode.

Cation Concavities Induced d-Band Electronic Modulation on Co/FeOx Nanostructure to Activate Molecular and Interfacial Oxygen for CO Oxidation

Cobalt-based catalysts have been identified for effective CO oxidation, but their activity is limited by molecular O2 and interfacial oxygen passivation at low temperatures. Optimization of the d-band structure of the cobalt center is an effective method to enhance the dissociation of oxygen species. Here, we developed a novel Co/FeOx catalyst based on selective cationic deposition to anchor Co cations at the defect site of FeOx, which exhibited superior intrinsic low-temperature activity (100%, 115 °C) compared to that of Pt/Co3O4 (100%, 140 °C) and La/Co2O3 (100%, 150 °C). In contrast to catalysts with oxygen defects, the cationic Fe defect in Co/FeOx showed an exceptional ability to accept electrons from the Co 3d orbital, resulting in significant electron delocalization at the Co sites. The Co/FeOx catalyst exhibited a remarkable turnover frequency of 178.6 per Co site per second, which is 2.3 times higher than that of most previously reported Co-based catalysts. The d-band center is shifted upward by electron redistribution effects, which promotes the breaking of the antibonding orbital *π of the O═O bond. In addition, the controllable regulation of the Fe-Ov-Co oxygen defect sites enlarges the Fe-O bond from 1.97 to 2.02 Å to activate the lattice oxygen. Moreover, compared to CoxFe3-xO4, Co/FeOx has a lower energy barrier for CO oxidation, which significantly accelerates the rate-determining step, *COO formation. This study demonstrates the feasibility of modulating the d-band structure to enhance O2 molecular and interfacial lattice oxygen activation.

Vacancy Defects Inductive Effect of Asymmetrically Coordinated Single-Atom Fe─N3 S1 Active Sites for Robust Electrocatalytic Oxygen Reduction with High Turnover Frequency and Mass Activity

The development of facile, efficient synthesis method to construct low-cost and high-performance single-atom catalysts (SACs) for oxygen reduction reaction (ORR) is extremely important, yet still challenging. Herein, an atomically dispersed N, S co-doped carbon with abundant vacancy defects (NSC-vd) anchored Fe single atoms (SAs) is reported and a vacancy defects inductive effect is proposed for promoting electrocatalytic ORR. The optimized catalyst featured of stable Fe─N3 S1 active sites exhibits excellent ORR activity with high turnover frequency and mass activity. In situ Raman, attenuated total reflectance surface enhanced infrared absorption spectroscopy reveal the Fe─N3 S1 active sites exhibit different kinetic mechanisms in acidic and alkaline solutions. Operando X-ray absorption spectra reveal the ORR activity of Fe SAs/NSC-vd catalyst in different electrolyte is closely related to the coordination structure. Theoretical calculation reveals the upshifted d band center of Fe─N3 S1 active sites facilitates the adsorption of O2 and accelerates the kinetics process of *OH reduction. The abundant vacancy defects around the Fe─N3 S1 active sites balance the OOH* formation and *OH reduction, thus synergetically promoting the electrocatalytic ORR process.

Tailoring Fe0 Nanoparticles via Lattice Engineering for Environmental Remediation

Lattice engineering of nanomaterials holds promise in simultaneously regulating their geometric and electronic effects to promote their performance. However, local microenvironment engineering of Fe0 nanoparticles (nFe0) for efficient and selective environmental remediation is still in its infancy and lacks deep understanding. Here, we present the design principles and characterization techniques of lattice-doped nFe0 from the point of view of microenvironment chemistry at both atomic and elemental levels, revealing their crystalline structure, electronic effects, and physicochemical properties. We summarize the current knowledge about the impacts of doping nonmetal p-block elements, transition-metal d-block elements, and hybrid elements into nFe0 crystals on their local coordination environment, which largely determines their structure-property-activity relationships. The materials' reactivity-selectivity trade-off can be altered via facile and feasible approaches, e.g., controlling doping elements' amounts, types, and speciation. We also discuss the remaining challenges and future outlooks of using lattice-doped nFe0 materials in real applications. This perspective provides an intuitive interpretation for the rational design of lattice-doped nFe0, which is conducive to real practice for efficient and selective environmental remediation.

Advanced Architectures of Air Electrodes in Zinc-Air Batteries and Hydrogen Fuel Cells

The air electrode is an essential component of air-demanding energy storage/conversion devices, such as zinc-air batteries (ZABs) and hydrogen fuel cells (HFCs), which determines the output power and stability of the devices. Despite atom-level modulation in catalyst design being recently achieved, the air electrodes have received much less attention, causing a stagnation in the development of air-demanding equipment. Herein, the evolution of air electrodes for ZABs and HFCs from the early stages to current requirements is reviewed. In addition, the operation mechanism and the corresponding electrocatalytic mechanisms of ZABs are summarized. In particular, by clarifying the air electrode interfaces of ZABs at different scales, several approaches to improve the air electrode in rechargeable ZABs are reviewed, including innovative electrode structures and bifunctional oxygen catalysts. Afterward, the operating mechanisms of proton-exchange-membrane fuel cells (PEMFCs) and anion-exchange-membrane fuel cells (AEMFCs) are explained. Subsequently, the strategies employed to enhance the efficiency of the membrane electrode assembly (MEA) in PEMFCs and AEMFCs, respectively, are highlighted and discussed in detail. Last, the prospects for air electrodes in ZABs and HFCs are considered by discussing the main challenges. The aim of this review is to facilitate the industrialization of ZABs and HFCs.

Spin state modulation on dual Fe center by adjacent Ni sites enabling the boosted activities and ultra-long stability in Zn-air batteries

Breakthrough in developing cost-effective Fe-based catalysts with superior oxygen reduction reaction (ORR) activities and ultra-long-term stability for application in Zn-air batteries (ZABs) remain a priority but still full of challenges. Herein, the neighboring NiN4 single-metal-atom and Fe2N5 dual-metal-atoms on the N-doped hollow carbon sphere (Fe/Ni-NHCS) were deliberately constructed as the efficient and robust ORR catalyst for ZABs. Both theory calculations and magnetic measurements demonstrate that the introduction of NiN4 provides a significant role on optimizing the electron spin state of Fe2N5 sites and reducing the energy barrier for the adsorption and conversion of the oxygen-containing intermediates, enabling the Fe/Ni-NHCS with excellent ORR performance and ultralow byproduct HO2- yield (0.5%). Impressively, the ZABs driven by Fe/Ni-NHCS exhibit unprecedented long-term rechargeable stability over 1200 h. This work paves a new venue to manipulate the spin state of active sites for simultaneously achieving superior catalytic activities and ultra-long-term stability in energy conversion technologies.

Tailoring Oxygen Reduction Reaction Kinetics on Perovskite Oxides via Oxygen Vacancies for Low-Temperature and Knittable Zinc-Air Batteries

High kinetics oxygen reduction reaction (ORR) electrocatalysts under low temperature are critical and highly desired for temperature-tolerant energy conversion and storage devices, but remain insufficiently investigated. Herein, oxygen vacancy-rich porous perovskite oxide (CaMnO3 ) nanofibers coated with reduced graphene oxide coating (V-CMO/rGO) are developed as the air electrode catalyst for low-temperature and knittable Zn-air batteries. V-CMO/rGO exhibits top-level ORR activity among perovskite oxides and shows impressive kinetics under low temperature. Experimental and theoretical calculation results reveal that the synergistic effect between metal atoms and oxygen vacancies, as well as the accelerated kinetics and enhanced electric conductivity and mass transfer over the rGO coated nanofiber 3D network contribute to the enhanced catalytic activity. The desorption of ORR intermediate is promoted by the regulated electron filling. The V-CMO/rGO drives knittable and flexible Zn-air batteries under a low temperature of -40 °C with high peak power density of 56 mW cm-2 and long cycle life of over 80 h. This study provides insight of kinetically active catalyst and facilitates the ZABs application in harsh environment.

Cold plasma synthesis of phosphorus-doped CoFe2O4 with oxygen vacancies for enhanced OER activity

Spinel-type metal oxides are promising electrocatalysts for the oxygen evolution reaction (OER) due to their unique electronic structure and low cost. Herein, we induced oxygen vacancies and doped phosphorus into CoFe2O4 using cold plasma. The abundant oxygen vacancies enhanced hydrophilicity and modified the electronic structure of CoFe2O4, while the phosphorus doping formed numerous new active centers. The doped P and formed FeP promoted the charge transfer and improved the conductivity of the catalyst. The phosphorus-doped CoFe2O4 exhibited exceptional OER activity with an overpotential of 180 mV at 10 mA cm-2 and a Tafel slope of 65.8 mV dec-1 in an alkaline electrolyte. DFT calculations confirmed that phosphorus doping can improve the charge distribution near the Fermi level and optimize the d-band center position.

A First-Principles Study of Regulating Spin States of MoSi2N4 Supported Single-Atom Catalysts Via Doping Strategy for Enhancing Electrochemical Nitrogen Fixation Activity

Regulating the spin states of catalysts to enhance activity is fascinating but challenging. Herein, by using first-principles calculations, single transition-metal (TM) atoms Mo, Re, and Os embedded in nitrogen vacancy of the MoSi2N4 monolayer (TM1/VN-MoSi2N4) were screened out as potential catalysts for electrochemical nitrogen reduction reaction to ammonia. Our findings suggest that the spin states of these active centers can be precisely and gradually tuned through a simple doping strategy. Additionally, doping one O atom into the Mo1/VN-MoSi2N4 system as an example significantly improves catalytic activity. The spin state of Mo1 transitions from high to intermediate while simultaneously breaking the C3v symmetry of the supported atom. These factors synergistically lead to better orbital overlap between the catalyst and intermediates, facilitating subsequent protonation processes and overall catalytic activity. This work provides novel insight into designing, precisely controlling, and revisiting the spin-related catalytic performance in heterogeneous catalysis.

Edge-Sharing Octahedrally Coordinated NiFe Dual Active Sites on ZnFe2 O4 for Photoelectrochemical Water Oxidation

The structural properties of octahedral sites (B_Oh ) in spinel oxides (AB₂ O₄ ) play vital roles in the electrochemical performance of oxygen-related reactions. However, the precise manipulation of AB₂ O₄ remains challenging due to the complexity of their crystal structure. Here, a simple and versatile molten-salt-mediated strategy is reported to introduce Ni²⁺ in B_oh sites intentionally on the surface of zinc ferrite (ZnFe₂ O₄ , ZFO) to promote the active sites for photoelectrochemical (PEC) water splitting. The as-created photoanode (ZFO-MSNi) shows a remarkable cathodic shift of ≈ 450 mV (turn-on voltage of ≈ 0.6 V_RHE ) as well as three times the 1-sun photocurrent density at 1.23 V_RHE for PEC water oxidation in comparison with bare ZFO. A comprehensive structural characterization clearly reveals the local structure of the introduced Ni²⁺ in ZFO-MSNi. Fewer surface trapping states are observed while the precisely introduced Ni²⁺ and associated neighboring Fe^(3-σ)+ (0<σ<1) sites unite in an edge-sharing octahedral configuration to function as NiFe dual active sites for PEC water oxidation. Moreover, open circuit potential measurements and rapid-scan voltammetry investigation give further insight into the enhanced PEC performance. Overall, this work displays a versatile strategy to regulate the surface active sites of photoelectrodes for increasing performance in PEC solar energy conversion systems.

FeCo alloy entrapped in N-doped graphitic carbon nanotubes-on-nanosheets prepared by coordination-induced pyrolysis for oxygen reduction reaction and rechargeable Zn-air battery

Oxygen reduction reaction (ORR) on cathode severely suffers from sluggish kinetics in zinc-air batteries. Therefore, substantial efforts have been made to prepare advanced electrocatalysts for facilitating the ORR. Herein, we synthesized FeCo alloyed nanocrystals entrapped in N-doped graphitic carbon nanotubes on nanosheets (FeCo-N-GCTSs) by 8-aminoquinoline coordination-induced pyrolysis, whose morphology, structures, and property were characterized in details. Impressively, the obtained FeCo-N-GCTSs catalyst showed a positive onset potential (E_onset = 1.06 V) and half-wave potential (E_1/2 = 0.88 V), revealing excellent ORR activity. Further, the FeCo-N-GCTSs assembled zinc-air battery displayed the maximum power density of 133 mW cm^-2 and negligible gap change in the discharge-charge voltage plot over 288 h (ca. 864 cycles) at 5 mA cm^-2, outperforming the Pt/C + RuO₂ based counterpart. This work provides a facile route for construction of high-efficiency, durable and low-cost nanocatalysts for the ORR in fuel cells and rechargeable Zn-air batteries.

Chromium-doped inverse spinel electrocatalysts with optimal orbital occupancy for facilitating reaction kinetics of lithium-oxygen batteries

Tailored electrocatalysts that can modulate their electronic structure are highly desirable to facilitate the reaction kinetics of oxygen evolution reaction (OER) and oxidation reduction reaction (ORR) in lithium-oxygen batteries (LOB). Although octahedron predominant inverse spinels (e.g., CoFe₂O₄) have been proposed as promising candidates for catalytic reactions, their performance has remained unsatisfactory. Herein, the chromium (Cr) doped CoFe₂O₄ nanoflowers (Cr-CoFe₂O₄) are elaborately constructed on nickel foam as a bifunctional electrocatalyst that drastically improves the performance of LOB. The results show that the partially oxidized Cr⁶⁺ stabilizes the cobalt (Co) sites at high-valence and regulates the electronic structure of Co sites, facilitating the oxygen redox kinetics of LOB due to their strong electron-withdrawing capability. Moreover, DFT calculations and ultraviolet photoelectron spectrometer (UPS) results consistently demonstrate that Cr doping optimizes the e_g electron filling state of the active octahedral Co sites, significantly improves the covalency of Co-O bonds, and enhances the degree of Co 3d-O 2p hybrids. As a result, Cr-CoFe₂O₄ catalyzed LOB can achieve low overpotential (0.48 V), high discharge capacity (22030 mA h g^-1) and long-term cycling durability (over 500 cycles at 300 mA g^-1). This work promotes the oxygen redox reaction and accelerates the electron transfer between Co ions and oxygen-containing intermediates, highlighting the potential of Cr-CoFe₂O₄ nanoflowers as bifunctional electrocatalysts for LOB.

Highly-dispersed NiFe alloys in-situ anchored on outer surface of Co, N co‑doped carbon nanotubes with enhanced stability for oxygen electrocatalysis

Transition metal alloys have emerged as promising catalysts for oxygen reduction/evolution reactions (ORR/OER) because of their intermetallic synergy and tunable redox properties. However, for alloy nanoparticles, it is quite challenging to suppress the self-aggregation and promote the bifunctional activity. Anchoring alloys in heteroatoms-doped carbon matrix with excellent electro-conductibility is a powerful strategy to form strongly-coupled alloy-carbon nanohybrids. Here, highly-dispersed NiFe alloys are evenly in-situ anchored on the surface of Co, N co-doped carbon nanotubes (NiFe/Co-N@CNTs) via a gravity-guided chemical vapor deposition and self-assembly strategy. Stably-structured NiFe/Co-N@CNTs possesses a tubular skeleton with diameters of 80-100 nm and a hydrophilic surface. For ORR, half-wave potential of NiFe/Co-N@CNTs (0.87 V vs RHE) is higher than that of Pt/C (0.85 V). Strong synergies between NiFe alloys and Co-N_x species facilitate the charge transfer on one-dimensional conductive structure to boost the 4e^- ORR kinetics. For OER, NiFe/Co-N@CNTs has a lower overpotential (300 mV) than RuO₂ (400 mV) at 10 mA cm^-2 due to in-situ formation of highly-active NiOOH/FeOOH species (as indicated by in-situ X-ray diffraction) at the catalytic sites on NiFe alloy. Rechargeable Zn-air battery (ZAB) with NiFe/Co-N@CNTs-based air-cathode exhibits promising open-circuit potential (1.52 V) and charge-discharge cycling stability (350 h). This alloy-carbon integrating strategy is meaningful for promoting dispersion, activity and stability of non-noble metal alloys for oxygen electrocatalysis.

The built-in electric field across FeN/Fe3N interface for efficient electrochemical reduction of CO2 to CO

Nanostructured metal-nitrides have attracted tremendous interest as a new generation of catalysts for electroreduction of CO₂, but these structures have limited activity and stability in the reduction condition. Herein, we report a method of fabricating FeN/Fe₃N nanoparticles with FeN/Fe₃N interface exposed on the NP surface for efficient electrochemical CO₂ reduction reaction (CO₂RR). The FeN/Fe₃N interface is populated with Fe-N₄ and Fe-N₂ coordination sites respectively that show the desired catalysis synergy to enhance the reduction of CO₂ to CO. The CO Faraday efficiency reaches 98% at -0.4 V vs. reversible hydrogen electrode, and the FE stays stable from -0.4 to -0.9 V during the 100 h electrolysis time period. This FeN/Fe₃N synergy arises from electron transfer from Fe₃N to FeN and the preferred CO₂ adsorption and reduction to *COOH on FeN. Our study demonstrates a reliable interface control strategy to improve catalytic efficiency of the Fe-N structure for CO₂RR.

Moiré Superlattice Structure in Two-Dimensional Catalysts: Synthesis, Property and Activity

Two-dimensional (2D) layered materials have been widely used as catalysts due to their high specific surface area, large fraction of uncoordinated surface atoms, and high charge carrier mobility. Moiré superlattice emerges in 2D layered materials with twist angle or lattice mismatch. By manipulating the moiré superlattice structure, 2D layered materials present modulated electronic band structure, topological edge states, and unconventional superconductivity which are tightly associated with the performance of catalysts. Hence, engineering moiré superlattice structures are proposed to be an important technology in modifying 2D layered materials for improved catalytic properties. However, currently, the investigation of moiré superlattice structure in a catalytic application is still in its infancy. This perspective starts with the discussion of structural features and fabrication strategy of 2D materials with moiré superlattice structure. Afterward, the catalytic applications, including electrocatalytic and photocatalytic applications, are summarized. In particular, the promotion mechanism of the catalytic performance caused by the moiré superlattice structure is proposed. Finally, the perspective is concluded by outlining the remaining challenges and possible solutions for the future development of 2D materials with moiré superlattice structure towards the catalytic applications.

Preparation of "Co-Nx Carbon Net" Protected CoFe Alloy on Carbon Nanotubes as an Efficient Bifunctional Electrocatalyst in Zn-Air Batteries

Herein, Co/Fe bimetallic hydroxide nanosheets (Co₃Fe₂ BMHs) were densely deposited on polypyrrole nanotubes (PPy NTs), followed by the successive coating of polydopamine (PDA) and zeolitic imidazolate frameworks (ZIF)-67 to form a composite catalyst precursor. Then, Co₃Fe₂ BMHs, PPy NTs, and ZIF-67/PDA in this precursor were calcined into Co₂Fe alloy nanoparticles, nitrogen-doped carbon NTs (NCNTs), and a Co-N_x activated carbon net, respectively, which constituted a novel composite catalyst. In this composite catalyst, the high-density Co₂Fe alloy nanoparticles are highly dispersed on the NCNT and coated by the Co-N_x activated carbon net. The Co-N_x activated carbon net protects the alloy particles from agglomerating during calcination and from being corroded by the electrolyte. Moreover, the experimental results demonstrated that the calcination temperature and chemical components of the catalyst precursors greatly affect the morphology, structure, composition, and ultimately electrocatalytic activity of the calcined products. The obtained optimum catalyst material exhibited significant electrocatalytic effects on both the oxygen reduction reaction and oxygen evolution reaction with a small ΔE of 0.715 V. The Zn-air battery utilizing this material as the air electrode catalyst showed a power density of 235.5 mW cm^-2, an energy density of 1073.5 Wh kg^-1, and a round-trip efficiency of 62.3% after 1000 cycles, superior to the benchmark battery based on the mixed commercial catalyst of Pt/C and RuO₂. An all-solid-state battery was also assembled to confirm the practical application prospect of the prepared composite material as the air electrode catalyst. More importantly, both experimental data and density functional theory calculations verified that the superior bifunctional catalytic activity was mainly attributed to the synergy between the Co-N_x activated carbon net and Co₂Fe alloy.

Cation-Tuning Engineering on Metal Oxides for Oxygen Electrocatalysis

Cation-tuning engineering has become a new frontier in altering the electronic structure of electrocatalysts, which has been employed to enhance their electrochemical performance. Significant efforts have been made to promote the electrochemical performance of transition metal-based materials during oxygen electrocatalysis and related energy devices such as Zn-air batteries. Herein, the advantages of cation-tuning engineering, including cation vacancies/defects and cation doping, in the modification of the electronic structure of transition metal oxide catalysts are discussed. Additionally, practical applications of the cation-tuning engineering strategy are reviewed in detail with a special emphasis on oxygen reduction reaction and oxygen evolution reaction. Lastly, challenges and future opportunities in this field are also proposed.

Activating C-H Bonds by Tuning Fe Sites and an Interfacial Effect for Enhanced Methanol Oxidation

The interaction mechanism between the reacting species and the active site of α-Fe₂ O₃ -based photoanodes in photoelectrochemical methanol conversion reaction is still ambiguous. Herein, a simple two-step strategy is demonstrated to fabricate a porous α-Fe₂ O₃ /CoFe₂ O₄ heterojunction for the methanol conversion reaction. The influence of the electronic structure of active site and interfacial effect on the reaction are investigated by constructing two different FeO₆ octahedral configurations and heterogeneous structures. The optimal sample ZnFeCo-2 affords high photocurrent density of 1.17 mA cm^-2 at 0.5 V vs Ag/AgCl, which is 3.2 times than that of ZnFe (0.37 mA cm^-2 ). Meanwhile, the ZnFeCo-2 also exhibits 97.8% Faraday efficiency of CH₃ OH to HCHO, and long-term stability over 40 h. Furthermore, density functional theory calculations reveal that the heterostructured α-Fe₂ O₃ /CoFe₂ O₄ with favorable electron transfer effectively lowers methanol adsorption, C-H bond activation, and HCHO desorption energy relative to the pristine α-Fe₂ O₃ , resulting in excellent methanol conversion efficiency.

Coupling Transition Metal Catalysts with Ir for Enhanced Electrochemical Water Splitting Activity

Developing advanced electrocatalysts is of great significance for boosting electrochemical water splitting to produce hydrogen. The electrocatalytic activity of a catalyst is associated with the surface/interface, geometric structure, and electronic properties. Coupling Ir with transition metal compounds is an effective strategy to improve their electrocatalytic performance. In this review, we summarize the recent progress of Ir coupled transition metal compound catalysts for the application in driving electrochemical water splitting. The significant role of Ir played in the promotion of electrocatalytic performance is firstly illustrated. Then, the applications of Ir-based catalysts in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are comprehensively discussed, with an emphasis on correlating the structure-function relationships. Lastly, the challenges and future directions for the fabrication of more advanced Ir coupled electrocatalysts are also presented.

Orbital Occupancy and Spin Polarization: From Mechanistic Study to Rational Design of Transition Metal-Based Electrocatalysts toward Energy Applications

Over the past few decades, development of electrocatalysts for energy applications has extensively transitioned from trial-and-error methodologies to more rational and directed designs at the atomic levels via either nanogeometric optimization or modulating electronic properties of active sites. Regarding the modulation of electronic properties, nonprecious transition metal-based materials have been attracting large interest due to the capability of versatile tuning d-electron configurations expressed through the flexible orbital occupancy and various possible degrees of spin polarization. Herein, recent advances in tailoring electronic properties of the transition-metal atoms for intrinsically enhanced electrocatalytic performances are reviewed. We start with discussions on how orbital occupancy and spin polarization can govern the essential atomic level processes, including the transport of electron charge and spin in bulk, reactive species adsorption on the catalytic surface, and the electron transfer between catalytic centers and adsorbed species as well as reaction mechanisms. Subsequently, different techniques currently adopted in tuning electronic structures are discussed with particular emphasis on theoretical rationale and recent practical achievements. We also highlight the promises of the recently established computational design approaches in developing electrocatalysts for energy applications. Lastly, the discussion is concluded with perspectives on current challenges and future opportunities. We hope this review will present the beauty of the structure-activity relationships in catalysis sciences and contribute to advance the rational development of electrocatalysts for energy conversion applications.

Emerging single-atom iron catalysts for advanced catalytic systems

Due to the elusive structure-function relationship, traditional nanocatalysts always yield limited catalytic activity and selectivity, making them practically difficult to replace natural enzymes in wide industrial and biomedical applications. Accordingly, single-atom catalysts (SACs), defined as catalysts containing atomically dispersed active sites on a support material, strikingly show the highest atomic utilization and drastically boosted catalytic performances to functionally mimic or even outperform natural enzymes. The molecular characteristics of SACs (e.g., unique metal-support interactions and precisely located metal sites), especially single-atom iron catalysts (Fe-SACs) that have a similar catalytic structure to the catalytically active center of metalloprotease, enable the accurate identification of active centers in catalytic reactions, which afford ample opportunity for unraveling the structure-function relationship of Fe-SACs. In this review, we present an overview of the recent advances of support materials for anchoring an atomic dispersion of Fe. Subsequently, we highlight the structural designability of support materials as two sides of the same coin. Moreover, the applications described herein illustrate the utility of Fe-SACs in a broad scope of industrially and biologically important reactions. Finally, we present an outlook of the major challenges and opportunities remaining for the successful combination of single Fe atoms and catalysts.

Identifying and Interpreting Geometric Configuration-Dependent Activity of Spinel Catalysts for Water Reduction

The catalytic activity of transition metal-based catalysts is overwhelmingly dependent on the geometric configuration. Identification and interpretation of different geometric configurations' contributions to catalytic activity plays a pivotal role in catalytic performance elevation. Spinel structured AB₂X₄, consisting of tetrahedral (A²⁺-X)_Td and octahedral (B³⁺-X)_Oh geometric configurations, is a prototypical category of multi-geometric-configuration featured catalysts. However, it is still under debate about the predominant geometric configuration responsible for spinel catalyst activity, and the mechanistic origin of specific activity discrepancy among varied geometric configurations also remains ambiguous. Herein, Co_Td²⁺ and Co_Oh³⁺ in Co₃O₄ are replaced by catalytically inert Zn²⁺ and Al³⁺ to yield ZnCo₂O₄ and CoAl₂O₄, respectively, thus ensuring the manipulable exposure of monotypic active configurations. By means of pulse voltammetry and in situ extended X-ray absorption fine structure, (Co³⁺-O)_Oh is identified to be dominant for alkaline HER. In-depth theoretical investigation in combination with X-ray absorption spectroscopy further interprets the synergistic effect between Co and O sites in (Co³⁺-O)_Oh configuration on water reduction kinetics upon both water dissociation and hydrogen desorption steps. Furthermore, specific facet dependence of catalytic activity is also deciphered based on precise facet exposure identification and serial theoretical analysis. This work unambiguously figures out the subtle geometric configuration dependence of spinel catalyst activity for water reduction and highlights the synergistic relationship among different components confined in geometric configuration, thereby shedding new light on the rational design of advanced catalysts from the atomic level of geometric configuration optimization.

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.

Double-Faced Atomic-Level Engineering of Hollow Carbon Nanofibers as Free-Standing Bifunctional Oxygen Electrocatalysts for Flexible Zn-Air Battery

Flexible solid-state zinc-air batteries (ZABs) with low cost, excellent safety, and high energy density has been considered as one of ideal power sources for portable and wearable electronic devices, while their practical applications are still hindered by the kinetically sluggish cathodic oxygen reduction and oxygen evolution reactions (ORR/OER). Herein, a Janus-structured flexible free-standing bifunctional oxygen electrocatalyst, with OER-active O, N co-coordinated Ni single atoms and ORR-active Co₃O₄@Co_1-xS nanosheet arrays being separately integrated at the inner and outer walls of flexible hollow carbon nanofibers (Ni-SAs/HCNFs/Co-NAs), is reported. Benefiting from the sophisticated topological structure and atomic-level-designed chemical compositions, Ni-SAs/HCNFs/Co-NAs exhibits outstanding bifunctional catalytic activity with the ΔE index of 0.65 V, representing the current state-of-the-art flexible free-standing bifunctional ORR/OER electrocatalyst. Impressively, the Ni-SAs/HCNFs/Co-NAs-based liquid ZAB show a high open-circuit potential (1.45 V), high capacity (808 mAh g^-1 Zn), and extremely long life (over 200 h at 10 mA cm^-2), and the assembled flexible all-solid-state ZABs have excellent cycle stability (over 80 h). This work provides an efficient strategy for developing high-performance bifunctional ORR/OER electrocatalysts for commercial applications.

Regulating Magnetic Behavior of Fe in Hematene by Defects to Improve Oxygen Evolution Reaction

Two-dimensional hematene is earth-abundant and exhibits easy modulation with unique electronic properties, suggesting a promising role as an electrocatalyst for oxygen evolution reaction (OER). In this Letter, we propose a strategy to regulate the magnetic behavior of Fe atoms in hematene by introducing structural defects to enhance its OER activity. Hematene is proved to be thermodynamically stable at electrolyte pH values ≥3 under the OER working potential according to the Pourbaix diagram. Among all the defective structures, the most stable DV_Fe1-O defect exhibits superior OER activity, which originates from the unique spin state of the active Fe atom. We further propose a novel descriptor of the magnetic moment difference on active Fe to efficiently evaluate the OER activity. We believe that our strategy of combining the defect modulation in the geometric structure and spin-state control in the electronic configuration could provide a guideline to design highly active Fe-based electrocatalysts.