Atomically Dispersed Binary Co-Ni Sites in Nitrogen-Doped Hollow Carbon Nanocubes for Reversible Oxygen Reduction and Evolution

Active site engineering of single-atom carbonaceous electrocatalysts for the oxygen reduction reaction

The electrocatalytic oxygen reduction reaction (ORR) is the vital process at the cathode of next-generation electrochemical storage and conversion technologies, such as metal-air batteries and fuel cells. Single-metal-atom and nitrogen co-doped carbonaceous electrocatalysts (M-N-C) have emerged as attractive alternatives to noble-metal platinum for catalyzing the kinetically sluggish ORR due to their high electrical conductivity, large surface area, and structural tunability at the atomic level, however, their application is limited by the low intrinsic activity of the metal-nitrogen coordination sites (M-N x ) and inferior site density. In this Perspective, we summarize the recent progress and milestones relating to the active site engineering of single atom carbonous electrocatalysts for enhancing the ORR activity. Particular emphasis is placed on the emerging strategies for regulating the electronic structure of the single metal site and populating the site density. In addition, challenges and perspectives are provided regarding the future development of single atom carbonous electrocatalysts for the ORR and their utilization in practical use.

Insights into N-Coordinated Bimetallic Site Synergy during NO Selective Catalytic Reduction by CO

The nature of the synergistic effect in bimetallic catalysts remains a challenging issue, due to the difficulty in understanding the adjacent interaction between dual metals at the atomic level. Herein, a CuFe-N/C catalyst featuring diatomic metal-nitrogen sites was prepared through a sequential ion exchange strategy and applied for NO selective catalytic reduction by CO (CO-SCR). The bimetallic CuFe-N/C catalyst exhibits high N₂ selectivity with a NO conversion efficiency of nearly 100% over a wide temperature range from 225 to 400 °C, significantly higher than that of its single-component counterparts. The synergistic effect of bimetallic Cu-Fe sites is well revealed using the combined in situ FTIR technique and DFT calculations. Bifunctional Cu-Fe sites are demonstrated not only to provide two different preferential adsorption centers for the CO molecule and ONNO intermediate but also to achieve a complete electron cycle for efficient interfacial electron transfer upon ONNO uptake. The unique electron transfer mechanism stemmed from 4s-3d-type electron coupling, and different 3d shell fillings of Cu (3d¹⁰) and Fe (3d⁶) atoms are presented. These fundamental insights pave the way for the understanding of N-coordinated bimetallic site synergy and rational design of highly active atomic-scale metal catalysts for SCR applications.

Structural Design Strategy and Active Site Regulation of High-Efficient Bifunctional Oxygen Reaction Electrocatalysts for Zn-Air Battery

Zinc-air batteries (ZABs) exhibit high energy density as well as flexibility, safety, and portability, thereby fulfilling the requirements of power batteries and consumer batteries. However, the limited efficiency and stability are still the significant challenge. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are two crucial cathode reactions in ZABs. Development of bifunctional ORR/OER catalysts with high efficiency and well stability is critical to improve the performance of ZABs. In this review, the ORR and OER mechanisms are first explained. Further, the design principles of ORR/OER electrocatalysts are discussed in terms of atomic adjustment mechanism and structural design in conjunction with the latest reported in situ characterization techniques, which provide useful insights on the ORR/OER mechanisms of the catalyst. The improvement in the energy efficiency, stability, and environmental adaptability of the new hybrid ZAB by the inclusion of additional reaction, including the introduction of transition-metal redox couples in the cathode and the addition of modifiers in the electrolyte to change the OER pathway, is also summarized. Finally, current challenges and future research directions are presented.

Atomically Dispersed Co2 -N6 and Fe-N4 Costructures Boost Oxygen Reduction Reaction in Both Alkaline and Acidic Media

Polynary transition-metal atom catalysts are promising to supersede platinum (Pt)-based catalysts for oxygen reduction reaction (ORR). Regulating the local configuration of atomic catalysts is the key to catalyst performance enhancement. Different from the previously reported single-atom or dual-atom configurations, a new type of ternary-atom catalyst, which consists of atomically dispersed, nitrogen-coordinated Co-Co dimers, and Fe single sites (i.e., Co₂ -N₆ and Fe-N₄ structures) that are coanchored on highly graphitized carbon supports is developed. This unique atomic ORR catalyst outperforms the catalysts with only Co₂ -N₆ or Fe-N₄ sites in both alkaline and acid conditions. Density functional theory calculations clearly unravels the synergistic effect of the Co₂ -N₆ and Fe-N₄ sites, which can induce higher filling degree of Fe-d orbitals and favors the binding capability to *OH intermediates (the rate determining step). This ternary-atom catalyst may be a promising alternative to Pt to drive the cathodic ORR in zinc-air batteries.

Double shelled hollow CoS2@MoS2@NiS2 polyhedron as advanced trifunctional electrocatalyst for zinc-air battery and self-powered overall water splitting

Electrocatalysts play important role in various energy conversion and storage devices. The catalytic performance of electrocatalysts can be enhanced through the increasement of intrinsic catalytic activity by optimizing electronic structure and the improvement of exposed active sites by designing proper nanostructures. In this work, CoS₂@MoS₂@NiS₂ nano polyhedron with double-shelled structure was prepared using metal organic framework as a precursor. Due to the rational integration of multifunctional active center, the strong electronic interaction of the various component, the high electrochemical surface area and shortened mass transport induced by the special structure, CoS₂@MoS₂@NiS₂ exhibits high catalytic activity for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Specifically, low overpotentials of 156 and 200 mV was achieved to deliver a current density of 10 mA cm^-2 for HER and OER, and a high half-wave potential of 0.80 V was observed for ORR. More importantly, the Zn-air battery assembled by CoS₂@MoS₂@NiS₂ exhibits a high-power density of 80.28 mW cm^-2 and could effectively drive overall water splitting. This work provides a new platform for designing multifunctional catalysts with high activity for energy conversion and storage.

Strategies for Developing Transition Metal Phosphides in Electrochemical Water Splitting

Electrochemical water splitting involving hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is a greatly promising technology to generate sustainable and renewable energy resources, which relies on the exploration regarding the design of electrocatalysts with high efficiency, high stability, and low cost. Transition metal phosphides (TMPs), as nonprecious metallic electrocatalysts, have been extensively investigated and proved to be high-efficient electrocatalysts in both HER and OER. In this minireview, a general overview of recent progress in developing high-performance TMP electrocatalysts for electrochemical water splitting has been presented. Design strategies including composition engineering by element doping, hybridization, and tuning the molar ratio, structure engineering by porous structures, nanoarray structures, and amorphous structures, and surface/interface engineering by tuning surface wetting states, facet control, and novel substrate are summarized. Key scientific problems and prospective research directions are also briefly discussed.

Moving beyond bimetallic-alloy to single-atom dimer atomic-interface for all-pH hydrogen evolution

Single-atom-catalysts (SACs) afford a fascinating activity with respect to other nanomaterials for hydrogen evolution reaction (HER), yet the simplicity of single-atom center limits its further modification and utilization. Obtaining bimetallic single-atom-dimer (SAD) structures can reform the electronic structure of SACs with added atomic-level synergistic effect, further improving HER kinetics beyond SACs. However, the synthesis and identification of such SAD structure remains conceptually challenging. Herein, systematic first-principle screening reveals that the synergistic interaction at the NiCo-SAD atomic interface can upshift the d-band center, thereby, facilitate rapid water-dissociation and optimal proton adsorption, accelerating alkaline/acidic HER kinetics. Inspired by theoretical predictions, we develop a facile strategy to obtain NiCo-SAD on N-doped carbon (NiCo-SAD-NC) via in-situ trapping of metal ions followed by pyrolysis with precisely controlled N-moieties. X-ray absorption spectroscopy indicates the emergence of Ni-Co coordination at the atomic-level. The obtained NiCo-SAD-NC exhibits exceptional pH-universal HER-activity, demanding only 54.7 and 61 mV overpotentials at −10 mA cm⁻² in acidic and alkaline media, respectively. This work provides a facile synthetic strategy for SAD catalysts and sheds light on the fundamentals of structure-activity relationships for future applications.

While single, dispersed atoms enable efficient atomic utilization, controllably preparing single-atom dimers remains challenging. Here, authors prepare nickel-cobalt single-atom dimers as high-performance pH-universal H₂ evolution electrocatalysts.

Single-Atom-like B-N3 Sites in Ordered Macroporous Carbon for Efficient Oxygen Reduction Reaction

On the premise of cleanliness and stability, improving the catalytic efficiency for the oxygen reduction reaction in the electrode reaction of fuel cells and metal-air batteries is of vital importance. Studies have shown that heteroatom doping and structural optimization are efficient strategies. Herein, a single-atom-like B-N₃ configuration in carbon is designed for efficient oxygen reduction reaction catalysis inspired by the extensively studied transition metal M-N_x sites, which is supported on the ordered macroporous carbon prepared by utilizing a hydrogen-bonded organic framework as carbon and nitrogen sources and SiO₂ spheres as a template. The co-doping of B/N and ordered macroporous structures promote the metal-free material high oxygen reduction catalytic performance in alkaline media. DFT calculations reveal that the B-N₃ structure played a key role in enhancing the oxygen reduction activity by providing rich favorable *OOH and *OH adsorption sites on the B center. The promoted formation of *OH/*OOH intermediates accelerated the electrocatalyst reaction. This study provides new insights into the design of single-atom-like nonmetallic ORR electrocatalysts and synthesis of ordered macroporous carbons based on hydrogen-bonded organic frameworks.

Structure Defect Tuning of Metal-Organic Frameworks as a Nanozyme Regulatory Strategy for Selective Online Electrochemical Analysis of Uric Acid

Nanozymes have been designed to address the limitations of high cost and poor stability involving natural enzymes in analytical applications. However, the catalytic efficiency of the nanozyme still needs to be improved so that it can meet the selectivity and stability requirements of accurate biomolecule analysis. Here, we presented structure defects of metal-organic frameworks (MOFs) as a tuning strategy to regulate the catalytic efficiency of artificial nanozymes and investigated the roles of defects on the catalytic activity of oxidase-like MOFs. Structural defects were introduced into a novel Co-containing zeolitic imidazolate framework with gradually loosened morphology (ZIF-L-Co) by doping cysteine (Cys). It was found that with the increase in defect degree, the properties of materials such as ascorbate oxidase-like, glutathione oxidase-like, and laccase-like were obviously enhanced by over 5, 2, and 3 times, respectively. In-depth structural investigations indicate that the doping of sulfur inducing structural defects which may destroy the equilibrium state between cobalt and nitrogen in 2-methylimidazole and distort the crystal lattice, thereby enhancing the adsorption of oxygen and thus promoting the oxidase-like activity. The ZIF-L-Co-10 mg with enhanced ascorbate oxidase- and laccase-like activity was loaded into a microreactor and integrated into an online electrochemical system (OECS) in the upstream of the detector. This nanozyme-based microreactor can completely remove ascorbic acid, dopamine, and 3,4-dihydroxyphenylacetic acid which are the main interference toward uric acid (UA) electrochemical measurement, and the ZIF-L-Co-10 mg Cys-based OECS system is capable of continuously capturing UA change in rat brain following ischemia-reperfusion injury. Structure defect tuning of ZIF-L-Co not only provides a new regulatory strategy for artificial nanozyme activity but also provides a critical chemical platform for the investigation of UA-related brain function and brain diseases.

Substrate strain tunes operando geometric distortion and oxygen reduction activity of CuN2C2 single-atom sites

Single-atom catalysts are becoming increasingly significant to numerous energy conversion reactions. However, their rational design and construction remain quite challenging due to the poorly understood structure–function relationship. Here we demonstrate the dynamic behavior of CuN₂C₂ site during operando oxygen reduction reaction, revealing a substrate-strain tuned geometry distortion of active sites and its correlation with the activity. Our best CuN₂C₂ site, on carbon nanotube with 8 nm diameter, delivers a sixfold activity promotion relative to graphene. Density functional theory and X-ray absorption spectroscopy reveal that reasonable substrate strain allows the optimized distortion, where Cu bonds strongly with the oxygen species while maintaining intimate coordination with C/N atoms. The optimized distortion facilitates the electron transfer from Cu to the adsorbed O, greatly boosting the oxygen reduction activity. This work uncovers the structure–function relationship of single-atom catalysts in terms of carbon substrate, and provides guidance to their future design and activity promotion.

The rational design of single-atom catalysts is challenging. This work reveals a substrate-strain tuned geometry distortion of CuN₂C₂ single-atom site, which greatly boosts oxygen reduction activity by facilitating electron transfer to adsorbed O.

MOF-derived CoNi,CoO,NiO@N-C bifunctional oxygen electrocatalysts for liquid and all-solid-state Zn-air batteries

Metal-organic framework (MOF)-derived carbon composites with embedded metal alloy/metal oxides have attracted much attention due to their low-cost and excellent electrochemical reactivity. However, the drawback of this concept is the severe carbon evaporation during their synthesis, resulting in a reduction of active sites and catalytic durability. To solve this problem, this study proposes the possibility of using Ketjen black (KB) to replenish the carbon content. Impressively, MOF-derived bimetal and oxide N-doped porous carbon (CoNi-CoO-NiO@NC-800) exhibits extremely high catalytic activity with an oxygen reduction reaction (half-wave potential: 0.83 V) and oxygen evolution reaction (overpotential: 352 mV @ 10 mA cm^-2) potential gap of 0.75 V due to the virtue of the hierarchically porous structure and sufficient active sites. By combining theoretical studies, a strong synergetic coupling of the CoNi dual metal is proposed in decreasing the overall reaction barriers and promoting the reversible oxygen reactions. Moreover, the assembled liquid- and all-solid-state Zn-air batteries (ZABs) with the bifunctional catalyst as the air cathode demonstrate superior discharging (223 mW cm^-2 at 310 mA cm^-2) and charging-discharging performance and long lifetime (450 cycles, 75 h). This work will provide guidance for the rational design of metal/carbon hybrid catalysts and cut-price reproducible energy systems.

Toward Multicomponent Single-Atom Catalysis for Efficient Electrochemical Energy Conversion

Single-atom catalysts (SACs) have recently emerged as the ultimate solution for overcoming the limitations of traditional catalysts by bridging the gap between homogeneous and heterogeneous catalysts. Atomically dispersed identical active sites enable a maximal atom utilization efficiency, high activity, and selectivity toward the wide range of electrochemical reactions, superior structural robustness, and stability over nanoparticles due to strong atomic covalent bonding with supports. Mononuclear active sites of SACs can be further adjusted by engineering with multicomponent elements, such as introducing dual-metal active sites or additional neighbor atoms, and SACs can be regarded as multicomponent SACs if the surroundings of the active sites or the active sites themselves consist of multiple atomic elements. Multicomponent engineering offers an increased combinational diversity in SACs and unprecedented routes to exceed the theoretical catalytic performance limitations imposed by single-component scaling relationships for adsorption and transition state energies of reactions. The precisely designed structures of multicomponent SACs are expected to be responsible for the synergistic optimization of the overall electrocatalytic performance by beneficially modulating the electronic structure, the nature of orbital filling, the binding energy of reaction intermediates, the reaction pathways, and the local structural transformations. This Review demonstrates these synergistic effects of multicomponent SACs by highlighting representative breakthroughs on electrochemical conversion reactions, which might mitigate the global energy crisis of high dependency on fossil fuels. General synthesis methods and characterization techniques for SACs are also introduced. Then, the perspective on challenges and future directions in the research of SACs is briefly summarized. We believe that careful tailoring of multicomponent active sites is one of the most promising approaches to unleash the full potential of SACs and reach the superior catalytic activity, selectivity, and stability at the same time, which makes SACs promising candidates for electrocatalysts in various energy conversion reactions.

Effect of Coordination Environment Surrounding a Single Pt Site on the Liquid-Phase Aerobic Oxidation of 5-Hydroxymethylfurfural

As the frontier in heterogeneous catalyst, a monomer and positively charged active sites in the single-atom catalyst (SAC), anchored by high electronegative N, O, S, P, etc., atoms, may not be active for the multispecies (O₂, substrates, intermediates, solvent etc.) involved liquid-phase aerobic oxidation. Here, with catalytic, aerobic oxidation of 5-hydroxymethylfurfural as an example, Pt SAC (Pt₁-N₄) was synthesized and tested first. With commercial Pt/C (Pt loading of 5 wt %) as a benchmark, 2,5-furandicarboxylic acid (FDCA) yield of 97.6% was obtained. Pt SAC (0.56 wt %) gave a much lower FDCA yield (28.8%). By changing the coordination atoms from highly electronegative N to low electronegative Co atoms, the prepared Pt single-atom alloy (SAA, Pt₁-Co₃) catalyst with ultralow Pt loading (0.06 wt %) gave a much high FDCA yield (99.6%). Density functional theory (DFT) calculations indicated that positively charged Pt sites (+0.712e) in Pt₁-N₄ almost lost the capability for oxygen adsorption and activation, as well as the adsorption for the key intermediate. In Pt₁-Co₃ SAA, the central negatively charged Pt atom (-0.446e) facilitated the adsorption of the key intermediate; meanwhile, the nearby Co atoms around the Pt atom constituted the O₂-preferred adsorption/activation sites. This work shows the difference between the SAC with NPs and the SAA during liquid-phase oxidation of HMF and gives a useful guide in the future single-atom catalyst design in other related reactions.

Synergistic Effect of Active Sites of Double-Atom Catalysts for Nitrogen Reduction Reaction

Nitrogen fixation to produce ammonia is a vital process since nitrogen is an essential element for the human body. Industrial nitrogen fixation mainly relies on the Haber-Bosch process. However, this process requires huge energy consumption and leads to pollution emission. In this study, the behaviors of intermediates in the nitrogen reduction reaction (NRR) are investigated for fifteen double-atom catalysts (DACs) through density functional theory calculations, revealing that under the synergistic effect of active sites on appropriate DACs, intermediates can be adsorbed through different configurations according to the activity improvement needs. VFe-N-C shows the best catalytic activity for electrochemical NRR with a limiting potential of -0.36 V vs. the reversible hydrogen electrode. The proposed synergistic effect of active sites on DACs for NRR could provide a new method for design of NRR catalysts.

Hollow nitrogen-doped carbon nanospheres as cathode catalysts to enhance oxygen reduction reaction in microbial fuel cells treating wastewater

Hollow nanospheres play a pivotal role in the electro-catalytic oxygen reduction reaction (ORR), which is a crucial step in microbial fuel cell (MFC) device. Herein, the hollow nitrogen-doped carbon nanospheres (HNCNS) were synthesized with the sacrifice of silica coated carbon nanospheres (CNS@SiO₂) as template. HNCNS remarkably enhanced the ORR activity compared to the solid carbon and solid silica spheres. By tuning calcination temperature (800-1100 °C), the surface chemistry properties of HNCNS were effectively regulated. The optimal HNCNS-1000 catalyst which was calcined at 1000 °C exhibited the highest ORR activity in neutral media with the onset potential of 0.255 V and half-wave potential of -0.006 V (vs. Ag/AgCl). Single chamber MFC (SCMFC) assembled with HNCNS-1000 cathode unveiled comparable activity to a conventional Pt/C reference. It showed the highest maximum power density of 1307 ± 26 mW/m², excellent output stability of 5.8% decline within 680 h, chemical oxygen demand (COD) removal of 94.0 ± 0.3% and coulombic efficiency (CE) of 7.9 ± 0.9%. These excellent results were attributed to a cooperative effect of the optimized surface properties (e.g., structural defects, relative content of pyrrolic nitrogen and specific surface area) and the formation of hollow nanosphere structure. Furthermore, the positive linear relationship of the structural defects and pyrrolic nitrogen species with the maximum power generation in SCMFC were clearly elucidated. This study demonstrated that the cost effective HNCNS-1000 was a promising alternative to commercial Pt/C catalyst for practical application in MFCs treating wastewater. Our result revealed the effectiveness of MFC fabricated with HNCNS-1000 cathode catalyst in terms of power generation and wastewater treatment.

Rational Design of Highly Stable and Active MXene-Based Bifunctional ORR/OER Double-Atom Catalysts

Designing highly active and bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts has attracted great interest toward metal-air batteries. Herein, an efficient solution to the search for MXene-based bifunctional catalysts is proposed by introducing non-noble metals such as Fe/Co/Ni at the surfaces. These results indicate that the ultrahigh activities in Ni1/Ni2- and Fe1/Ni2-modified MXene-based double-atom catalysts (DACs) for bifunctional ORR/OER are better than those of well-known unifunctional catalysts with low overpotentials, such as Pt(111) for the ORR and IrO₂ (110) for the OER. Strain can profoundly regulate the catalytic activities of MXene-based DACs, providing a novel pathway for tunable catalytic behavior in flexible MXenes. An electrochemical model, based on density functional theory and theoretical polarization curves, is proposed to reveal the underlying mechanisms, in agreement with experimental results. Electronic structure analyses indicate that the excellent catalytic activities in the MXene-based DACs are attributed to the electron-capturing capability and synergistic interactions between Fe/Co/Ni adsorbents and MXene substrate. These findings not only reveal promising candidates for MXene-based bifunctional ORR/OER catalysts but also provide new theoretical insights into rationally designing noble-metal-free bifunctional DACs.

Encapsulating Cobalt Nanoparticles in Interconnected N-Doped Hollow Carbon Nanofibers with Enriched CoNC Moiety for Enhanced Oxygen Electrocatalysis in Zn-Air Batteries

Rational design of bifunctional efficient electrocatalysts for both oxygen reduction (ORR) and oxygen evolution reactions (OER) is desirable-while highly challenging-for development of rechargeable metal-air batteries. Herein, an efficient bifunctional electrocatalyst is designed and fabricated by encapsulating Co nanoparticles in interconnected N-doped hollow porous carbon nanofibers (designated as Co@N-C/PCNF) using an ultrafast high-temperature shock technology. Benefiting from the synergistic effect and intrinsic activity of the CoNC moiety, as well as porous structure of carbon nanofibers, the Co@N-C/PCNF composite shows high bifunctional electrocatalytic activities for both OER (289 mV at 10 mA cm-2 ) and ORR (half-wave potential of 0.85 V). The CoNC moiety in the composite can modulate the local environmental and electrical structure of the catalysts, thus optimizing the adsorption/desorption kinetics and decreasing the reaction barriers for promoting the reversible oxygen electrocatalysis. Co@N-C/PCNF-based aqueous Zn-air batteries (AZAB) provide high power density of 292 mW cm-2 , and the assembled flexible ZAB can power wearable devices.

Recent progress of electrocatalysts for oxygen reduction in fuel cells

Oxygen reduction reaction (ORR) has gradually been in the limelight in recent years because of its great application potential for fuel cells and rechargeable metal-air batteries. Therefore, significant issues are increasingly focused on developing effective and economical ORR electrocatalysts. This review begins with the reaction mechanisms and theoretical calculations of ORR in acidic and alkaline media. The latest reports and challenges in ORR electrocatalysis are traced. Most importantly, the latest advances in the development of ORR electrocatalysts are presented in detail, including platinum group metal (PGM), transition metal, and carbon-based electrocatalysts with various nanostructures. Furthermore, the development prospects and challenges of ORR electrocatalysts are speculated and discussed. These insights would help to formulate the design guidelines for highly-active ORR electrocatalysts and affect future research to obtain new knowledge for ORR mechanisms.

Fe-Ni Alloy Nanoclusters Anchored on Carbon Aerogels as High-Efficiency Oxygen Electrocatalysts in Rechargeable Zn-Air Batteries

In this work, Fe-Ni alloy nanoclusters (Fe-Ni ANCs) anchored on N, S co-doped carbon aerogel (Fe-Ni ANC@NSCA catalysts) are successfully prepared by the optimal pyrolysis of polyaniline (PANI) aerogels derived from the freeze-drying of PANI hydrogel obtained by the polymerization of aniline monomers in the co-presence of tannic acid (TA), Fe³⁺ , and Ni²⁺ ions. In addition, the optimal molar ratio of the TA, Fe³⁺ , and Ni²⁺ ions for synthesis of Fe-Ni ANC@NSCA catalysts are 1:2:5, which can guarantee the formation of carbon aerogel composed of quasi-2D porous carbon sheets and the formation of high-density Fe-Ni ANCs with an ultrasmall size between 2 to 2.8 nm. These Fe-Ni ANCs consisting of N₄ -Fe-O-Ni-N₄ moiety are proposed as a new type of active species for the first time, to the best of the authors' knowledge. Thanks to their unique features, the Fe-Ni ANC@NSCA catalysts show excellent performance in oxygen reduction reaction with a half-wave potential (E_1/2 ) of 0.891 V and oxygen evolution reaction (260 mV @ 10 mA cm^-2 ) in alkaline media as bifunctional catalysts, which are better than the state-of-the-art commercial Pt/C catalysts and RuO₂ catalysts. Moreover, Zn-air battery assembled with the Fe-Ni ANC@NSCA catalysts also shows a remarkable performance and exceptionally high stability over 500 h at 5 mA cm^-2 .

Emerging Dual-Atomic-Site Catalysts for Efficient Energy Catalysis

Atomically dispersed metal catalysts with well-defined structures have been the research hotspot in heterogeneous catalysis because of their high atomic utilization efficiency, outstanding activity, and selectivity. Dual-atomic-site catalysts (DASCs), as an extension of single-atom catalysts (SACs), have recently drawn surging attention. The DASCs possess higher metal loading, more sophisticated and flexible active sites, offering more chance for achieving better catalytic performance, compared with SACs. In this review, recent advances on how to design new DASCs for enhancing energy catalysis will be highlighted. It will start with the classification of marriage of two kinds of single-atom active sites, homonuclear DASCs and heteronuclear DASCs according to the configuration of active sites. Then, the state-of-the-art characterization techniques for DASCs will be discussed. Different synthetic methods and catalytic applications of the DASCs in various reactions, including oxygen reduction reaction, carbon dioxide reduction reaction, carbon monoxide oxidation reaction, and others will be followed. Finally, the major challenges and perspectives of DASCs will be provided.

Atomically Structural Regulations of Carbon-Based Single-Atom Catalysts for Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO₂ RR) converting CO₂ into value-added chemicals and fuels to realize carbon recycling is a solution to the problem of renewable energy shortage and environmental pollution. Among all the catalysts, the carbon-based single-atom catalysts (SACs) with isolated metal atoms immobilized on conductive carbon substrates have shown significant potential toward CO₂ RR, which intrigues researchers to explore high-performance SACs for fuel and chemical production by CO₂ RR. Especially, regulating the coordination structures of the metal centers and the microenvironments of the substrates in carbon-based SACs has emerged as an effective strategy for the tailoring of their CO₂ RR catalytic performance. In this review, the current in situ/operando study techniques and the fundamental parameters for CO₂ RR performance are first briefly presented. Furthermore, the recent advances in synthetic strategies which regulate the atomic structures of the carbon-based SACs, including heteroatom coordination, coordination numbers, diatomic metal centers, and the microenvironments of substrates are summarized. In particular, the structure-performance relationship of the SACs toward CO₂ RR is highlighted. Finally, the inevitable challenges for SACs are outlined and further research directions toward CO₂ RR are presented from the perspectives.

Fabrication, functionalization and advanced applications of magnetic hollow materials in confined catalysis and environmental remediation

Magnetic hollow-structured functional hybrid materials with unique architectures and preeminent properties have always been an area of extensive research. They represent a subtle collaboration of hollow architecture, mesoporous nanostructure and magnetic character. Owing to the merits of a large void space, low density, high specific surface area, well-defined active sites and facile magnetic recovery, these materials present promising application projections in numerous fields, such as drug delivery, adsorption, storage, catalysis and many others. In this review, recent progress in the design, synthesis, functionalization and applications of magnetic hollow-meso/nanostructured materials are discussed. The first part of the review has been dedicated to the preparation and functionalization of the materials. The synthetic protocols have been broadly classified into template-assisted and template-free methods and major trends in their synthesis have been elaborated in detail. Furthermore, the benefits and drawbacks of each method are compared. The later part summarizes the application aspects of confined catalysis in organic transformations and environmental remediation such as degradation of organic pollutants, dyes and antibiotics and adsorption of heavy metal ions. Finally, an outlook of future directions in this research field is highlighted.

Orbital coupling of hetero-diatomic nickel-iron site for bifunctional electrocatalysis of CO2 reduction and oxygen evolution

While inheriting the exceptional merits of single atom catalysts, diatomic site catalysts (DASCs) utilize two adjacent atomic metal species for their complementary functionalities and synergistic actions. Herein, a DASC consisting of nickel-iron hetero-diatomic pairs anchored on nitrogen-doped graphene is synthesized. It exhibits extraordinary electrocatalytic activities and stability for both CO2 reduction reaction (CO2RR) and oxygen evolution reaction (OER). Furthermore, the rechargeable Zn-CO2 battery equipped with such bifunctional catalyst shows high Faradaic efficiency and outstanding rechargeability. The in-depth experimental and theoretical analyses reveal the orbital coupling between the catalytic iron center and the adjacent nickel atom, which leads to alteration in orbital energy level, unique electronic states, higher oxidation state of iron, and weakened binding strength to the reaction intermediates, thus boosted CO2RR and OER performance. This work provides critical insights to rational design, working mechanism, and application of hetero-DASCs.

Engineering the Coordination Sphere of Isolated Active Sites to Explore the Intrinsic Activity in Single-Atom Catalysts

Reducing the dimensions of metallic nanoparticles to isolated, single atom has attracted considerable attention in heterogeneous catalysis, because it significantly improves atomic utilization and often leads to distinct catalytic performance. Through extensive research, it has been recognized that the local coordination environment of single atoms has an important influence on their electronic structures and catalytic behaviors. In this review, we summarize a series of representative systems of single-atom catalysts, discussing their preparation, characterization, and structure-property relationship, with an emphasis on the correlation between the coordination spheres of isolated reactive centers and their intrinsic catalytic activities. We also share our perspectives on the current challenges and future research promises in the development of single-atom catalysis. With this article, we aim to highlight the possibility of finely tuning the catalytic performances by engineering the coordination spheres of single-atom sites and provide new insights into the further development for this emerging research field.

MOFs-Derived Carbon-Based Metal Catalysts for Energy-Related Electrocatalysis

Electrochemical devices, as renewable and clean energy systems, display a great potential to meet the sustainable development in the future. However, well-designed and highly efficient electrocatalysts are the technological dilemmas that retard their practical applications. Metal-organic frameworks (MOFs) derived electrocatalysts exhibit tunable structure and intriguing activity and have received intensive investigation in recent years. In this review, the recent progress of MOFs-derived carbon-based single atoms (SAs) and metal nanoparticles (NPs) catalysts for energy-related electrocatalysis is summarized. The effects of synthesis strategy, coordination environment, morphology, and composition on the catalytic activity are highlighted. Furthermore, these SAs and metal NPs catalysts for the applications of electrocatalysis (hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, and nitrogen reduction reaction) are overviewed. Finally, some current challenges and foresighted ideas for MOFs-derived carbon-based metal electrocatalysts are presented.

Highly Curved Nanostructure-Coated Co, N-Doped Carbon Materials for Oxygen Electrocatalysis

Nitrogen-doped graphene could catalyze the electrochemical reduction and evolution of oxygen, but unfortunately suffers from sluggish catalytic kinetics. Herein, for the first time, we report an onion-like carbon coated Co, N-doped carbon (OLC/Co-N-C) material, which possesses multilayers of highly curved nanostructures that form mesoporous architectures. These unique nanospheres are produced when surfactant micelles are introduced to synthesis precursors. Owing to the combined electronic effect and nanostructuring effect, our OLC/Co-N-C materials exhibit high bifunctional oxygen reduction/evolution reaction (ORR/OER) activity, showing a promising application in rechargeable Zn-air batteries. Experimental results are rationalized by theoretical calculations, showing that the curvature of graphitic carbon plays a vital role in promoting activities of meta-carbon atoms near graphitic N and ortho/meta carbon atoms close to pyridinic N.

Dual-atom Pt heterogeneous catalyst with excellent catalytic performances for the selective hydrogenation and epoxidation

Atomically monodispersed heterogeneous catalysts with uniform active sites and high atom utilization efficiency are ideal heterogeneous catalytic materials. Designing such type of catalysts, however, remains a formidable challenge. Herein, using a wet-chemical method, we successfully achieved a mesoporous graphitic carbon nitride (mpg-C3N4) supported dual-atom Pt2 catalyst, which exhibited excellent catalytic performance for the highly selective hydrogenation of nitrobenzene to aniline. The conversion of ˃99% is significantly superior to the corresponding values of mpg-C3N4-supported single Pt atoms and ultra-small Pt nanoparticles (~2 nm). First-principles calculations revealed that the excellent and unique catalytic performance of the Pt2 species originates from the facile H2 dissociation induced by the diatomic characteristics of Pt and the easy desorption of the aniline product. The produced Pt2/mpg-C3N4 samples are versatile and can be applied in catalyzing other important reactions, such as the selective hydrogenation of benzaldehyde and the epoxidation of styrene.

Single-atom catalysts for high-energy rechargeable batteries

Clean and sustainable electrochemical energy storage has attracted extensive attention. It remains a great challenge to achieve next-generation rechargeable battery systems with high energy density, good rate capability, excellent cycling stability, efficient active material utilization, and high coulombic efficiency. Many catalysts have been explored to promote electrochemical reactions during the charge and discharge process. Among reported catalysts, single-atom catalysts (SACs) have attracted extensive attention due to their maximum atom utilization efficiency, homogenous active centres, and unique reaction mechanisms. In this perspective, we summarize the recent advances of the synthesis methods for SACs and highlight the recent progress of SACs for a new generation of rechargeable batteries, including lithium/sodium metal batteries, lithium/sodium-sulfur batteries, lithium-oxygen batteries, and zinc-air batteries. The challenges and perspectives for the future development of SACs are discussed to shed light on the future research of SACs for boosting the performances of rechargeable batteries.

Platinum quantum dots enhance electrocatalytic activity of bamboo-like nitrogen doped carbon nanotubes embedding Co-MnO nanoparticles for methanol/ethanol oxidation

Interaction of multi-active components can effectively maximize the overall catalytic ability of alcohol fuel cells. Herein, the self-assembled nitrogen doped carbon nanotubes (NCNTs) containing Co-MnO composite (Co-MnO/NCNTs) are successfully synthesized using dihydrodiamine as carbon and nitrogen source through one-step synthesis. In order to further improve the catalytic activity of Co-MnO/NCNTs for alcohol oxidation, small amounts of platinum quantum dots are uniformly loaded on Co-MnO/NCNTs formation of quaternary hybrid (named Pt/Co-MnO/NCNTs) during microwave reduction stage. Notably, the prepared Pt/Co-MnO/NCNTs hybrids possess the excellent methanol and ethanol oxidation mass current density of 1775.4 and 1112.8 mA mg^-1 in alkaline condition, which are 3.6 and 2.25 times higher than that of Pt/C catalysts, respectively. The current density of ethanol catalytic oxidation is lower than that of methanol, which may be due to the partial oxidation of acetyl (the intermediate product of ethanol) on the Pt (1 1 1) crystal surface. More importantly, CO oxidation experiments reveal that strong electronic synergistic effect between MnO and Pt quantum dot can greatly improve the CO anti-poisoning ability. Another significant advantage of Pt/Co-MnO/NCNTs is that low platinum loading leads to low cost effective, which demonstrates that the modification non-noble metal catalysts with a few noble metals quantum dots is a promising choice to mass produce high performance catalyst with remarkably boosting electrocatalytic activity for alcohol oxidation.

Tuning the Spin Density of Cobalt Single-Atom Catalysts for Efficient Oxygen Evolution

Single-atom catalysts (SACs) with magnetic elements as the active center have been widely exploited for efficient electrochemical conversions. Understanding the catalytic role of spin, and thus modulating the spin density of a single-atom center, is of profound fundamental interest and technological impact. Here, we synthesized ferromagnetic single Co atom catalysts on TaS₂ monolayers (Co₁/TaS₂) as a model system to explore the spin-activity correlation for the oxygen evolution reaction (OER). A single Co atom adsorbed at the hollow site (Co_HS) with spin-polarized electronic states serves as the active site for OER, whose spin density can be regulated by its neighboring single Co site via tuning the Co loading. Both experimental and theoretical results reveal the spin density-dependent OER activity that an optimal spin density of Co_HS can be achieved with a neighboring hetero-single Co_Ta site (substitution of Ta by Co) for a superior OER performance, in contrast to a homo-single Co_HS site, which creates an excessive spin density over vicinal Co_HS. An optimized spin density of Co_HS results in an optimal binding energy of oxygen species for the OER. Establishing the spin-activity correlation in SACs may create a descriptor for designing efficient magnetic SACs for renewable energy conversions.

Three-phase composites of NiFe2O4/Ni@C nanoparticles derived from metal-organic frameworks as electrocatalysts for the oxygen evolution reaction

Composite electrocatalysts of carbon and metals or metal compounds with homogeneous active sites can be obtained through the carbonization of metal organic framework (MOF) materials under inert atmosphere. In this work, a three-phase composite electrocatalysts NiFe₂O₄/Ni@C were prepared via pyrolysis from self-assembled MOF nanosheets aggregates. The excellent electrocatalytic activity of the obtained electrocatalysts with various Ni:Fe ratios is demonstrated. Especially, the NiFe₂O₄/Ni@C sample with the mole ratio of Ni:Fe = 1:1 can use the overpotential (η) of 330 and 423 mV to drive 10 and 50 mA cm^-2 respectively. After 80 000 s/22 h, the current density could retained 90% of the initial current density. The excellent activity and stability of the electrocatalysts are attributed to nickel and iron ions with uniform dispersion at atomic level in the NiFe₂O₄ phase and the synergistic effect of nickel and NiFe₂O₄ nanoparticles with amorphous carbon atoms or nanoparticles around.

Controlled Synthesis and Structure Engineering of Transition Metal-based Nanomaterials for Oxygen and Hydrogen Electrocatalysis in Zinc-Air Battery and Water-Splitting Devices

Electrocatalytic energy conversion plays a crucial role in realizing energy storage and utilization. Clean energy technologies such as water electrolysis, fuel cells, and metal-air batteries heavily depend on a series of electrochemical redox reactions occurring on the catalysts surface. Therefore, developing efficient electrocatalysts is conducive to remarkably improved performance of these devices. Among numerous studies, transition metal-based nanomaterials (TMNs) have been considered as promising catalysts by virtue of their abundant reserves, low cost, and well-designed active sites. This Minireview is focused on the typical clean electrochemical reactions: hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction. Recent efforts to optimize the external morphology and the internal electronic structure of TMNs are described, and beginning with single-component TMNs, the active sites are clarified, and strategies for exposing more active sites are discussed. The summary about multi-component TMNs demonstrates the complementary advantages of integrating functional compositions. A general introduction of single-atom TMNs is provided to deepen the understanding of the catalytic process at an atomic scale. Finally, current challenges and development trends of TMNs in clean energy devices are summarized.

Co and Pt Dual-Single-Atoms with Oxygen-Coordinated Co-O-Pt Dimer Sites for Ultrahigh Photocatalytic Hydrogen Evolution Efficiency

The platinum single-atom-catalyst is verified as a very successful route to approach the size limit of Pt catalysts, while how to further improve the catalytic efficiency of Pt is a fundamental scientific question and is challenging because the size issue of Pt is approached at the ultimate ceiling as single atoms. Here, a new route for further improving Pt catalytic efficiency by cobalt (Co) and Pt dual-single-atoms on titanium dioxide (TiO₂ ) surfaces, which contains a fraction of nonbonding oxygen-coordinated Co-O-Pt dimers, is reported. These Co-Pt dimer sites originate from loading high-density Pt single-atoms and Co single-atoms, with them anchoring randomly on the TiO₂ substrate. This dual-single-atom catalyst yields 13.4% dimer sites and exhibits an ultrahigh and stable photocatalytic activity with a rate of 43.467 mmol g^-1 h^-1 and external quantum efficiency of ≈83.4% at 365 nm. This activity far exceeds those of equal amounts of Pt single-atom and typical Pt clustered catalysts by 1.92 and 1.64 times, respectively. The enhancement mechanism relies on the oxygen-coordinated Co-O-Pt dimer coupling, which can mutually optimize the electronic states of both Pt and Co sites to weaken H* binding. Namely, the "mute" Co single-atom is activated by Pt single-atom and the activity of the Pt atom is further enhanced through the dimer interaction. This strategy of nonbonding interactive dimer sites and the oxygen-mediated catalytic mechanisms provide emerging rich opportunities for greatly improving the catalytic efficiency and developing novel catalysts with creating new electronic states.

Cooperativity in supported metal single atom catalysis

The discussion concerning cooperativity in supported single-atom (SA) catalysis is often limited to the metal-support interaction, which is certainly important, but which is not the only lever for modifying the catalytic performance. Indeed, if the interaction between the SA and the support, which can be seen as a solid ligand presenting its own specificities that fix the first coordination sphere of the metal, plays a central role as in homogeneous catalysis, other factors can strongly contribute to modification of the activity, selectivity and stability of SAs. Therefore, in this mini-review, we briefly summarize the importance of the support (oxide, carbon or a second metal) in SA photo- electro- and thermal-catalysis (support-assisted operation), and concentrate on other types of cooperativities that in some cases enable previously impossible reaction pathways on supported metal SAs. This includes topics that are not specific to SA catalysis, such as metal-ligand or heterobimetallic cooperativity, and cooperativity which is SA-specific such as nanoparticle-SA or mixed-valence SA cooperativity.

Metal-organic frameworks-derived hollow dodecahedral carbon combined with FeNx moieties and ruthenium nanoparticles as cathode electrocatalyst for lithium oxygen batteries

Owing to their high energy density, lithium-oxygen batteries (LOBs) have been drawn great attention as one of the promising electrochemical energy sources. However, the sluggish kinetics of oxygen reduction/evolution reaction (ORR/OER) hamper the widespread application of LOBs. Herein, an elaborate designed catalysts which are constructed by FeN_x moieties dispersed on the network-like hollow dodecahedral carbon and then decorated with Ru nanoparticles (FeN_x-HDC@Ru). Since the homogeneously dispersed FeN_x moieties could promote ORR performance, and the Ru nanoparticles could facilitate OER capability, the FeN_x-HDC@Ru nanocomposites used as cathode catalysts can significantly improve LOBs performance. A lower discharge and charge overpotentials of 0.15 V and 0.78 V can be detected in the first cycle, respectively, and an excellent cycle performance of 90 cycles at 200 mA g^-1 and 89 cycles at 500 mA g^-1 can be demonstrated. Herein, the charge transfer kinetics has been enhanced with the internal network-like hollow structure and a low impedance Li₂O₂/catalysts contact interface could be earned by the constructed Ru nanoparticles, these factors would lead to an efficient acceleration to the formation and decomposition of Li₂O₂ during discharge and charge process.

Self-Healing and Anti-CO2 Hydrogels for Flexible Solid-State Zinc-Air Batteries

Flexible solid-state zinc-air batteries (ZABs) generally suffer from poor electrolyte/electrode contact and mechanical degradation in practical applications. In addition, CO₂ corrosion is also a common issue for ZABs with alkaline electrolyte. Herein, we report a thermoreversible alkaline hydrogel electrolyte that can simultaneously solve the aforementioned problems. Through a simple cooling process, the hydrogel electrolyte transforms from solid state to liquid state that can not only restore the deformed electrolyte layer to its original state but also rebuild intimate contact between electrode and electrolyte. Moreover, the ZAB based on this hydrogel electrolyte exhibits an unprecedented anti-CO₂ property. As a result, such a battery shows almost 2.5 times discharge duration than that of ZAB based on liquid electrolyte.