Mechanistic insight into the active centers of single/dual-atom Ni/Fe-based oxygen electrocatalysts

Understanding the Dynamic Aggregation in Single-Atom Catalysis

The dynamic response of single-atom catalysts to a reactive environment is an increasingly significant topic for understanding the reaction mechanism at the molecular level. In particular, single atoms may experience dynamic aggregation into clusters or nanoparticles driven by thermodynamic or kinetic factors. Herein, the inherent mechanistic nuances that determine the dynamic profile during the reaction will be uncovered, including the intrinsic stability and site-migration barrier of single atoms, external stimuli (temperature, voltage, and adsorbates), and the influence of catalyst support. Such dynamic aggregation can be beneficial or deleterious on the catalytic performance depending on the optimal initial state. Those examples will be highlighted where in situ formed clusters, rather than single atoms, serve as catalytically active sites for improved catalytic performance. This is followed by the introduction of operando techniques to understand the structural evolution. Finally, the emerging strategies via confinement and defect-engineering to regulate dynamic aggregation will be briefly discussed.

Catalytic Oxidation of Benzene over Atomic Active Site AgNi/BCN Catalysts at Room Temperature

Benzene is the typical volatile organic compound (VOC) of indoor and outdoor air pollution, which harms human health and the environment. Due to the stability of their aromatic structure, the catalytic oxidation of benzene rings in an environment without an external energy input is difficult. In this study, the efficient degradation of benzene at room temperature was achieved by constructing Ag and Ni bimetallic active site catalysts (AgNi/BCN) supported on boron-carbon-nitrogen aerogel. The atomic-scale Ag and Ni are uniformly dispersed on the catalyst surface and form Ag/Ni-C/N bonds with C and N, which were conducive to the catalytic oxidation of benzene at room temperature. Further catalytic reaction mechanisms indicate that benzene reacted with ·OH to produce R·, which reacted with O2 to regenerate ·OH. Under the strong oxidation of ·OH, benzene was oxidized to form alcohols, carboxylic acids, and eventually CO2 and H2O. This study not only significantly reduces the energy consumption of VOC catalytic oxidation, but also improves the safety of VOC treatment, providing new ideas for the low energy consumption and green development of VOC treatment.

Well-defined diatomic catalysis for photosynthesis of C2H4 from CO2

Owing to the specific electronic-redistribution and spatial proximity, diatomic catalysts (DACs) have been identified as principal interest for efficient photoconversion of CO2 into C2H4. However, the predominant bottom-up strategy for DACs synthesis has critically constrained the development of highly ordered DACs due to the random distribution of heteronuclear atoms, which hinders the optimization of catalytic performance and the exploration of actual reaction mechanism. Here, an up-bottom ion-cutting architecture is proposed to fabricate the well-defined DACs, and the superior spatial proximity of CuAu diatomics (DAs) decorated TiO2 (CuAu-DAs-TiO2) is successfully constructed due to the compact heteroatomic spacing (2-3 Å). Owing to the profoundly low C-C coupling energy barrier of CuAu-DAs-TiO2, a considerable C2H4 production with superior sustainability is achieved. Our discovery inspires a novel up-bottom strategy for the fabrication of well-defined DACs to motivate optimization of catalytic performance and distinct deduction of heteroatom synergistically catalytic mechanism.

Current Status and Perspectives of Dual-Atom Catalysts Towards Sustainable Energy Utilization

The exploration of sustainable energy utilization requires the implementation of advanced electrochemical devices for efficient energy conversion and storage, which are enabled by the usage of cost-effective, high-performance electrocatalysts. Currently, heterogeneous atomically dispersed catalysts are considered as potential candidates for a wide range of applications. Compared to conventional catalysts, atomically dispersed metal atoms in carbon-based catalysts have more unsaturated coordination sites, quantum size effect, and strong metal-support interactions, resulting in exceptional catalytic activity. Of these, dual-atomic catalysts (DACs) have attracted extensive attention due to the additional synergistic effect between two adjacent metal atoms. DACs have the advantages of full active site exposure, high selectivity, theoretical 100% atom utilization, and the ability to break the scaling relationship of adsorption free energy on active sites. In this review, we summarize recent research advancement of DACs, which includes (1) the comprehensive understanding of the synergy between atomic pairs; (2) the synthesis of DACs; (3) characterization methods, especially aberration-corrected scanning transmission electron microscopy and synchrotron spectroscopy; and (4) electrochemical energy-related applications. The last part focuses on great potential for the electrochemical catalysis of energy-related small molecules, such as oxygen reduction reaction, CO2 reduction reaction, hydrogen evolution reaction, and N2 reduction reaction. The future research challenges and opportunities are also raised in prospective section.

Tailoring C─N Containing Compounds into Carbon Nanomaterials with Tunable Morphologies for Electrocatalytic Applications

Carbon materials with unique sp2 -hybridization are extensively researched for catalytic applications due to their excellent conductivity and tunable physicochemical properties. However, the development of economic approaches to tailoring carbon materials into desired morphologies remains a challenge. Herein, a convenient "bottom-up" strategy by pyrolysis of graphitic carbon nitride (g-C3 N4 ) (or other carbon/nitrogen (C, N)-enriched compounds) together with selected metal salts and molecules is reported for the construction of different carbon-based catalysts with tunable morphologies, including carbon nano-balls, carbon nanotubes, nitrogen/sulfur (S, N) doped-carbon nanosheets, and single-atom catalysts, supported by carbon layers. The catalysts are systematically investigated through various microscopic, spectroscopic, and diffraction methods and they demonstrate promising and broad applications in electrocatalysis such as in the oxygen reduction reaction and water splitting. Mechanistic monitoring of the synthesis process through online thermogravimetric-gas chromatography-mass spectrometry measurements indicates that the release of C─N-related moieties, such as dicyan, plays a key role in the growth of carbon products. This enables to successfully predict other widely available precursor compounds beyond g-C3 N4 such as caffeine, melamine, and urea. This work develops a novel and economic strategy to generate morphologically diverse carbon-based catalysts and provides new, essential insights into the growth mechanism of carbon nanomaterials syntheses.

Unlocking Bimetallic Active Centers via Heterostructure Engineering for Exceptional Phosphate Electrosorption: Internal Electric Field-Induced Electronic Structure Reconstruction

Development of electrode materials exhibiting exceptional phosphate removal performance represents a promising strategy to mitigate eutrophication and meet ever-stricter stringent emission standards. Herein, we precisely designed a novel LaCeOx heterostructure-decorated hierarchical carbon composite (L8C2PC) for high-efficiency phosphate electrosorption. This approach establishes an internal electric field within the LaCeOx heterostructure, where the electrons transfer from Ce atoms to neighboring La atoms through superexchange interactions in La-O-Ce coordination units. The modulatory heterostructure endows a positive shift of the d band of La sites and the increase of electron density at Fermi level, promoting stronger orbital overlap and binding interactions. The introduction of oxygen vacancies during the in situ nucleation process reduces the kinetic barrier for phosphate-ion migration and supplies additional active centers. Moreover, the hierarchical carbon framework ensures electrical double-layer capacitance for phosphate storage and interconnected ion migration channels. Such synergistically multiple active centers grant the L8C2PC electrode with high-efficiency record in phosphate electrosorption. As expected, the L8C2PC electrode demonstrates the highest removal capability among the reported electrode materials with a saturation capacity of 401.31 mg P g-1 and a dynamic capacity of 91.83 mg P g-1 at 1.2 V. This electrochemical system also performs well in the dephosphorization in natural water samples with low concentration that enable effluent concentration to meet the first-class discharge standard for China (0.5 mg P L-1). This study advances efficient dephosphorization techniques to a new level and offers a deep understanding of the internal electric field that regulates metal orbitals and electron densities in heterostructure engineering.

Harnessing the electronic structure of active metals to lower the overpotential of the electrocatalytic oxygen evolution reaction

Despite substantial advancements in the field of the electrocatalytic oxygen evolution reaction (OER), the efficiency of earth-abundant electrocatalysts remains far from ideal. The difficulty stems from the complex nature of the catalytic system, which limits our fundamental understanding of the process and thus the possibility of a rational improvement of performance. Herein, we shed light on the role played by the tunable 3d configuration of the metal centers in determining the OER catalytic activity by combining electrochemical and spectroscopic measurements with an experimentally validated computational protocol. One-dimensional coordination polymers based on Fe, Co and Ni held together by an oxonato linker were selected as a case study because of their well-defined electronic and geometric structure in the active site, which can be straightforwardly correlated with their catalytic activity. Novel heterobimetallic coordination polymers were also considered, in order to shed light on the cooperativity effects of different metals. Our results demonstrate the fundamental importance of electronic structure effects such as metal spin and oxidation state evolutions along the reaction profile to modulate ligand binding energies and increase catalyst efficiency. We demonstrated that these effects could in principle be exploited to reduce the overpotential of the electrocatalytic OER below its theoretical limit, and we provide basic principles for the development of coordination polymers with a tailored electronic structure and activity.

Mo-doping heterojunction: interfacial engineering in an efficient electrocatalyst for superior simulated seawater hydrogen evolution

Exploring economical, efficient, and stable electrocatalysts for the seawater hydrogen evolution reaction (HER) is highly desirable but is challenging. In this study, a Mo cation doped Ni0.85Se/MoSe2 heterostructural electrocatalyst, Mox-Ni0.85Se/MoSe2, was successfully prepared by simultaneously doping Mo cations into the Ni0.85Se lattice (Mox-Ni0.85Se) and growing atomic MoSe2 nanosheets epitaxially at the edge of the Mox-Ni0.85Se. Such an Mox-Ni0.85Se/MoSe2 catalyst requires only 110 mV to drive current densities of 10 mA cm-2 in alkaline simulated seawater, and shows almost no obvious degradation after 80 h at 20 mA cm-2. The experimental results, combined with the density functional theory calculations, reveal that the Mox-Ni0.85Se/MoSe2 heterostructure will generate an interfacial electric field to facilitate the electron transfer, thus reducing the water dissociation barrier. Significantly, the heteroatomic Mo-doping in the Ni0.85Se can regulate the local electronic configuration of the Mox-Ni0.85Se/MoSe2 heterostructure catalyst by altering the coordination environment and orbital hybridization, thereby weakening the bonding interaction between the Cl and Se/Mo. This synergistic effect for the Mox-Ni0.85Se/MoSe2 heterostructure will simultaneously enhance the catalytic activity and durability, without poisoning or corrosion of the chloride ions.

g-C3N4 promoted MOF-derived Fe single atoms anchored on N-doped hierarchically porous carbon for high-performance Zn-air batteries

Exploring efficient, easy-to-manufacture, and inexpensive bifunctional electrocatalysts with abundant accessible active sites is crucial for rechargeable zinc-air batteries (ZABs). Herein, we report the strategy consisting of the space confinement and pore-making engineering to fabricate single-atom catalyst enriched with Fe-N4 sites anchored on N-doped hierarchically porous carbon (Fe-NC-C3N4). The optimized Fe-NC-C3N4 exhibits excellent oxygen reduction/evolution reaction (ORR/OER) activities with a half-wave potential (E1/2) of 0.90 V vs. RHE and a promising low overpotential of 0.305 V vs. RHE at 10 mA·cm-2 in alkaline electrolyte. These superior catalytic activities are attributed to the combined effect between the atomic active sites and the well-balanced micro-meso-macropore structures. The homemade liquid Zn-air battery (ZAB) assembled with Fe-NC-C3N4 catalyst displays a power density of 133.59 mW·cm-2 and a significant energy density of 882.58 mAh·g-1, exceeding those of the equipment with commercial Pt/C-RuO2 (56.82 mW·cm-2 and 643.87 mAh·g-1, respectively). Particularly, the corresponding flexible wearable ZAB manifests outstanding foldability and cyclical stability. This work opens a new perspective for the structural design of single-atom catalysts in the energy storage and conversion area.

Recent Progress in Hot Spot Regulated Strategies for Catalysts Applied in Li-CO2 Batteries

As a high energy density power system, lithium-carbon dioxide (Li-CO2 ) batteries play an important role in addressing the fossil fuel crisis issues and alleviating the greenhouse effect. However, the sluggish transformation kinetic of CO2 and the difficult decomposition of discharge products impede the achievement of large capacity, small overpotential, and long life span of the batteries, which require exploring efficient catalysts to resolve these problems. In this review, the main focus is on the hot spot regulation strategies of the catalysts, which include the modulation of the active sites, the designing of microstructure, and the construction of composition. The recent progress of promising catalysis with hot spot regulated strategies is systematically addressed. Critical challenges are also presented and perspectives to provide useful guidance for the rational design of highly efficient catalysts for practical advanced Li-CO2 batteries are proposed.

Bifunctional diatomic site catalysts supported by β12-borophene for efficient oxygen evolution and reduction reactions

Efficient bifunctional catalysts for oxygen evolution and reduction reactions (OERs/ORRs) are of great importance for sustainable and renewable clean energy, especially for metal-air batteries. Herein, we investigated β12-borophene with double-hole sites capped with 3d transition metal atoms to explore its catalyst performance for hydrogen evolution reactions (HERs), OERs and ORRs. It was found that the borophene is a good platform for diatomic site catalysts (DASCs) due to their advantage of stability over the corresponding single-atom catalysts (SACs) or clusters. The HER performance of DASCs on β12-BM was further improved compared to the SAC case. Furthermore, the supported FeNi DASC exhibited good catalytic performance for both OERs and ORRs, the overpotentials for which were 0.43 and 0.55 V, respectively, better than those of the corresponding supported Ni or Fe SAC due to synergistic effects. We herein propose a novel descriptor involving the Bader charges of coordinated atoms explicitly, behaving much better than the d-band center and integrated crystal orbital Hamilton population (-ICOHP) for DASCs. The synergistic effect of Fe-Ni pairs balanced the too strong binding of OH and further activated OH to achieve better catalytic performance. The results of this study can provide theoretical guidance for the design of efficient bifunctional electrocatalysts.

Non-Interacting Ni and Fe Dual-Atom Pair Sites in N-Doped Carbon Catalysts for Efficient Concentrating Solar-Driven Photothermal CO2 Reduction

Solar-to-chemical energy conversion under weak solar irradiation is generally difficult to meet the heat demand of CO2 reduction. Herein, a new concentrated solar-driven photothermal system coupling a dual-metal single-atom catalyst (DSAC) with adjacent Ni-N4 and Fe-N4 pair sites is designed for boosting gas-solid CO2 reduction with H2 O under simulated solar irradiation, even under ambient sunlight. As expected, the (Ni, Fe)-N-C DSAC exhibits a superior photothermal catalytic performance for CO2 reduction to CO (86.16 μmol g-1  h-1 ), CH4 (135.35 μmol g-1  h-1 ) and CH3 OH (59.81 μmol g-1  h-1 ), which are equivalent to 1.70-fold, 1.27-fold and 1.23-fold higher than those of the Fe-N-C catalyst, respectively. Based on theoretical simulations, the Fermi level and d-band center of Fe atom is efficiently regulated in non-interacting Ni and Fe dual-atom pair sites with electronic interaction through electron orbital hybridization on (Ni, Fe)-N-C DSAC. Crucially, the distance between adjacent Ni and Fe atoms of the Ni-N-N-Fe configuration means that the additional Ni atom as a new active site contributes to the main *COOH and *HCO3 dissociation to optimize the corresponding energy barriers in the reaction process, leading to specific dual reaction pathways (COOH and HCO3 pathways) for solar-driven photothermal CO2 reduction to initial CO production.

Neighboring Platinum Atomic Sites Activate Platinum-Cobalt Nanoclusters as High-Performance ORR/OER/HER Electrocatalysts

The rational design of efficient and multifunctional electrocatalysts for energy conversion devices is one of the major challenges for clean and renewable energy transition. Herein, the local electronic structure of cobalt-platinum nanoclusters is regulated by adjacent platinum atomic site encapsulated in N-doped hollow carbon nanotubes (PtSA -PtCo NCs/N-CNTs) by pyrolysis of melamine-orientation-induced zeolite imidazole metal-organic frameworks (ZIF-67) with thimbleful platinum doping. The introduction of melamine can reactivate adjacent carbon atoms and initiate the oriented growth of nitrogen-doped carbon nanotubes. The systematic analysis suggests the significant role of thimbleful neighboring low-coordinated Pt─N2 in altering the localized electronic structure of PtCo nanoclusters. The optimized PtSA -PtCo NCs/N-CNTs-900 exhibit excellent hydrogen evolution reaction (HER)/oxygen evolution reaction (OER)/oxygen reduction reaction (ORR)/ catalytic performance reaching the current density of 10 mA cm-2 in 1 m KOH under the low 47 (HER) and 252 mV (OER) overpotentials, and a high half-wave potential of 0.86 and 0.89 V (ORR) in 0.1 m KOH and 0.1 m HClO4 , respectively. Remarkably, the PtSA -PtCo NC/N-CNT-900 also presents outstanding catalytic performances toward water splitting and rechargeable Zn-air batteries. The theoretical calculations reveal that optimal regulation of the electronic structure of PtCo nanoclusters by thimbleful neighboring Pt atomic reduces the reaction energy barrier in electrochemical process, facilitating the ORR/OER/HER performance.

Inter-Metal Interaction of Dual-Atom Catalysts in Heterogeneous Catalysis

Dual-atom catalysts (DACs) have been a new frontier in heterogeneous catalysis due to their unique intrinsic properties. The synergy between dual atoms provides flexible active sites, promising to enhance performance and even catalyze more complex reactions. However, precisely regulating active site structure and uncovering dual-atom metal interaction remain grand challenges. In this review, we clarify the significance of the inter-metal interaction of DACs based on the understanding of active center structures. Three diatomic configurations are elaborated, including isolated dual single-atom, N/O-bridged dual-atom, and direct dual-metal bonding interaction. Subsequently, the up-to-date progress in heterogeneous oxidation reactions, hydrogenation/dehydrogenation reactions, electrocatalytic reactions, and photocatalytic reactions are summarized. The structure-activity relationship between DACs and catalytic performance is then discussed at an atomic level. Finally, the challenges and future directions to engineer the structure of DACs are discussed. This review will offer new prospects for the rational design of efficient DACs toward heterogeneous catalysis.

Charge transfer mechanism through S-scheme heterojunction in in-situ synthesized TiO2/Fe-doped hydroxyapatite for improved photodegradation of xanthate

The heterojunction structure of the photocatalyst composite, which necessitates a robust interface and sufficient contact areas, holds the key to obtaining high charge carrier migration efficiency. Here, a novel composite, TiO2 nanoparticles/Fe-doped hydroxyapatite (TONPs/FH_CS), is fabricated using a two-step synthetic technique, in which FH_CS is synthesized from artificial converter slag enriched with Fe and Ca. The unique nanorod@plate structure of FH_CS enables the uniform immobilization of TONPs onto FH_CS. Thereby, an n-n type heterojunction exhibits a highly intimate Ti-O-Fe heterointerface. Kelvin probe testing demonstrates the formation of an interfacial electric field oriented from FH_CS to TONPs, which serves as the driving force for interfacial electron transfer through the Ti-O-Fe channels. The photoacoustic signals provide information on electron trap levels and densities, indicating the formation of the electron transfer channels. •O2- and •OH species are responsible for being the active species in this system. A photoexcited carrier transfer pathway exhibiting an S-scheme mechanism with high separation efficiency significantly enhances the utilization of charge carriers in each phase. Thus, improved xanthate degradation has been achieved using a heterojunction containing a photocatalyst derived from industrial solid waste. This work demonstrates the significant potential of steel-making byproduct utilization in industrial wastewater treatment.

Dual Atom Catalysts for Energy and Environmental Applications

The pursuit of high metal utilization in heterogeneous catalysis has triggered the burgeoning interest of various atomically dispersed catalysts. Our aim in this review is to assess key recent findings in the synthesis, characterization, structure-property relationship and computational studies of dual-atom catalysts (DACs), which cover the full spectrum of applications in thermocatalysis, electrocatalysis and photocatalysis. In particular, combination of qualitative and quantitative characterization with cooperation with DFT insights, synergies and superiorities of DACs compare to counterparts, high-throughput catalyst exploration and screening with machine-learning algorithms are highlighted. Undoubtably, it would be wise to expect more fascinating developments in the field of DACs as tunable catalysts.

Single-Atom and Dual-Atom Electrocatalysts: Synthesis and Applications

Distinguishing themselves from nanostructured catalysts, single-atom catalysts (SACs) typically consist of positively charged single metal and coordination atoms without any metal-metal bonds. Dual-atom catalysts (DACs) have emerged as extended family members of SACs in recent years. Both SACs and DACs possess characteristics that combine both homogeneous and heterogeneous catalysis, offering advantages such as uniform active sites and adjustable interactions with ligands, while also inheriting the high stability and recyclability associated with heterogeneous catalyst systems. They offer numerous advantages and are extensively utilized in the field of electrocatalysis, so they have emerged as one of the most prominent material research platforms in the direction of electrocatalysis. This review provides a comprehensive review of SACs and DACs in the field of electrocatalysis: encompassing economic production, elucidating electrocatalytic reaction pathways and associated mechanisms, uncovering structure-performance relationships, and addressing major challenges and opportunities within this domain. Our objective is to present novel ideas for developing advanced synthesis strategies, precisely controlling the microstructure of catalytic active sites, establishing accurate structure-activity relationships, unraveling potential mechanisms underlying electrocatalytic reactions, identifying more efficient reaction paths, and enhancing overall performance in electrocatalytic reactions.

Dual-Active-Sites Single-Atom Catalysts for Advanced Applications

Dual-Active-Sites Single-Atom catalysts (DASs SACs) are not only the improvement of SACs but also the expansion of dual-atom catalysts. The DASs SACs contains dual active sites, one of which is a single atomic active site, and the other active site can be a single atom or other type of active site, endowing DASs SACs with excellent catalytic performance and a wide range of applications. The DASs SACs are categorized into seven types, including the neighboring mono metallic DASs SACs, bonded DASs SACs, non-bonded DASs SACs, bridged DASs SACs, asymmetric DASs SACs, metal and nonmetal combined DASs SACs and space separated DASs SACs. Based on the above classification, the general methods for the preparation of DASs SACs are comprehensively described, especially their structural characteristics are discussed in detail. Meanwhile, the in-depth assessments of DASs SACs for variety applications including electrocatalysis, thermocatalysis and photocatalysis are provided, as well as their unique catalytic mechanism are addressed. Moreover, the prospects and challenges for DASs SACs and related applications are highlighted. The authors believe the great expectations for DASs SACs, and this review will provide novel conceptual and methodological perspectives and exciting opportunities for further development and application of DASs SACs.

Bifunctional Single Atom Catalysts for Rechargeable Zinc-Air Batteries: From Dynamic Mechanism to Rational Design

Ever-growing demands for rechargeable zinc-air batteries (ZABs) call for efficient bifunctional electrocatalysts. Among various electrocatalysts, single atom catalysts (SACs) have received increasing attention due to the merits of high atom utilization, structural tunability, and remarkable activity. Rational design of bifunctional SACs relies heavily on an in-depth understanding of reaction mechanisms, especially dynamic evolution under electrochemical conditions. This requires a systematic study in dynamic mechanisms to replace current trial and error modes. Herein, fundamental understanding of dynamic oxygen reduction reaction and oxygen evolution reaction mechanisms for SACs is first presented combining in situ and/or operando characterizations and theoretical calculations. By highlighting structure-performance relationships, rational regulation strategies are particularly proposed to facilitate the design of efficient bifunctional SACs. Furthermore, future perspectives and challenges are discussed. This review provides a thorough understanding of dynamic mechanisms and regulation strategies for bifunctional SACs, which are expected to pave the avenue for exploring optimum single atom bifunctional oxygen catalysts and effective ZABs.

Advances in Transition-Metal-Based Dual-Atom Oxygen Electrocatalysts

Oxygen electrocatalysis has aroused considerable interest over the past years because of the new energy technologies boom in hydrogen energy and metal-air battery. However, due to the sluggish kinetic of the four-electron transfer process in oxygen reduction reaction and oxygen evolution reaction, the electro-catalysts are urgently needed to accelerate the oxygen electrocatalysis. Benefit from the high atom utilization efficiency, unprecedentedly high catalytic activity, and selectivity, single-atom catalysts (SACs) are considered the most promising candidate to replace the traditional Pt-group-metal catalysts. Compared with SACs, the dual-atom catalysts (DACs) are attracting more attraction including higher metal loading, more versatile active sites, and excellent catalytic activity. Therefore, it is essential to explore the new universal methods approaching to the preparation, characterization, and to elucidate the catalytic mechanisms of the DACs. In this review, several general synthetic strategies and structural characterization methods of DACs are introduced and the involved oxygen catalytic mechanisms are discussed. Moreover, the state-of-the-art electrocatalytic applications including fuel cells, metal-air batteries, and water splitting have been sorted out at present. The authors hope this review has given some insights and inspiration to the researches about DACs in electro-catalysis.

Understanding the Bifunctional Trends of Fe-Based Binary Single-Atom Catalysts

Binary single-atom catalysts (BSACs) have demonstrated fascinating activities compared to single atom catalysts (SACs) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Notably, Fe SACs is one of the most promising ORR electrocatalysts, and further revealing the synergistic effects between Fe and other 3d transition metals (M) for FeM BSACs are very important to enhance bifunctional performance. Herein, density functional theory (DFT) calculations are first adapted to demonstrate the role of various transition metals on the bifunctional activity of Fe sites, and a notable volcano relationship is established through the generally accepted adsorption free energy that ΔG* OH for ORR, and ΔG* O -ΔG* OH for OER, respectively. Further, ten of the atomically dispersed FeM anchored on nitrogen-carbon support (FeM-NC) are successfully synthesized with typical atomic dispersion by a facile movable type printing method. The experimental data confirms the bifunctional activity diversity of FeM-NC between the early- and late- transition metals, agrees very well with the DFT results. More importantly, the optimal FeCu-NC shows the expected performance with high ORR and OER activity, thereby, the assembled rechargeable zinc-air battery delivers a high power density of 231 mW cm-2 , and an impressive stability that can be stably operated over 300 h.

Synergy of dual-atom catalysts deviated from the scaling relationship for oxygen evolution reaction

Dual-atom catalysts, particularly those with heteronuclear active sites, have the potential to outperform the well-established single-atom catalysts for oxygen evolution reaction, but the underlying mechanistic understanding is still lacking. Herein, a large-scale density functional theory is employed to explore the feasibility of *O-*O coupling mechanism, which can circumvent the scaling relationship with improving the catalytic performance of N-doped graphene supported Fe-, Co-, Ni-, and Cu-containing heteronuclear dual-atom catalysts, namely, M'M@NC. Based on the constructed activity maps, a rationally designed descriptor can be obtained to predict homonuclear catalysts. Seven heteronuclear and four homonuclear dual-atom catalysts possess high activities that outperform the minimum theoretical overpotential. The chemical and structural origin in favor of *O-*O coupling mechanism thus leading to enhanced reaction activity have been revealed. This work not only provides additional insights into the fundamental understanding of reaction mechanisms, but also offers a guideline for the accelerated discovery of efficient catalysts.

Potential-dependent transition of reaction mechanisms for oxygen evolution on layered double hydroxides

Oxygen evolution reaction (OER) is of crucial importance to sustainable energy and environmental engineering, and layered double hydroxides (LDHs) are among the most active catalysts for OER in alkaline conditions, but the reaction mechanism for OER on LDHs remains controversial. Distinctive types of reaction mechanisms have been proposed for the O-O coupling in OER, yet they compose a coupled reaction network with competing kinetics dependent on applied potentials. Herein, we combine grand-canonical methods and micro-kinetic modeling to unravel that the nature of dominant mechanism for OER on LDHs transitions among distinctive types as a function of applied potential, and this arises from the interplay among applied potential and competing kinetics in the coupled reaction network. The theory-predicted overpotentials, Tafel slopes, and findings are in agreement with the observations of experiments including isotope labelling. Thus, we establish a computational methodology to identify and elucidate the potential-dependent mechanisms for electrochemical reactions.

A novel peroxymonosulfate activation process by single-atom iron catalyst from waste biomass for efficient singlet oxygen-mediated degradation of organic pollutants

Single-atom dispersed catalysts (SACs) have gained considerable attention in organic contaminants remediation due to their superior reactivity and stability. However, the complex and costly synthesis processes limit their practical applications in environmental protection. Herein, a facile and cost-effective single-atom iron catalyst (Fe-SA/NC) anchored on nitrogen-doped porous carbon was first fabricated by using waste biomass as a carbon source. The Fe-SA/NC catalyst exhibited outstanding performance with a high turnover frequency of 1.72 min^-1 toward antibiotics degradation via peroxymonosulfate activation. ECOSAR program and algae growth experiments demonstrated that the byproducts produced during the sulfamethoxazole degradation process were not detrimental to the aquatic environment. Radical quenching and electron paramagnetic resonance experiments revealed that Fe-SA/NC remarkably promoted ¹O₂ production in PMS-assisted reaction, and thus ¹O₂ contributed as much as 78.77% to sulfamethoxazole degradation. As indicated by experiment and density functional theory (DFT) calculations, FeN₂O₂ configuration serves as the active site. DFT calculations further presented the most rational generation route of ¹O₂ as PMS→OH* →O* →¹O₂. We also designed Fe-SA/NC embedded spherical pellets for contaminants elimination at the device level. This study offers new insights into the synthesis of SACs from waste biomass and their practical application in environmental remediation.

Rational design of bifunctional hydroxide/sulfide heterostructured catalyst for efficient electrochemical seawater splitting

Heterostructure engineering is one of the most promising strategies for efficient water splitting by electrocatalysts. However, it remains challenging to design heterostructured catalysts to achieve the desired goals in both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in seawater splitting. Here, particulate heterostructures of FeCoNi hydroxide/sulfide supported on nickel foams were prepared by hydrothermal methods to achieve a high-performance bifunctional catalyst. The synthesized FeCoNi hydroxide/sulfide exhibited excellent electrocatalytic performance, requiring an overpotential of 195 mV for OER and 76 mV for HER to achieve a current density of 10 mA cm^-2 while showing excellent stability. The catalyst maintains its excellent performance even in artificial or natural seawater with high salinity, which is a harsh environment. When applied directly to a water splitting system, the catalyst achieves a current density of 10 mA cm^-2 at only 1.5 V (1.57 V in alkaline seawater). The FeCoNi hydroxide/sulfide heterostructure is an excellent electrocatalytic bifunctional catalyst due to compositional modulation, systematic charge transfer optimization, improved intermediates adsorption, and increased electrocatalytic active sites and the synergistic effect of the heterostructure.

Atomically Dispersed Dual-Metal Sites Showing Unique Reactivity and Dynamism for Electrocatalysis

The real structure and in situ evolution of catalysts under working conditions are of paramount importance, especially for bifunctional electrocatalysis. Here, we report asymmetric structural evolution and dynamic hydrogen-bonding promotion mechanism of an atomically dispersed electrocatalyst. Pyrolysis of Co/Ni-doped MAF-4/ZIF-8 yielded nitrogen-doped porous carbons functionalized by atomically dispersed Co-Ni dual-metal sites with an unprecedented N8V4 structure, which can serve as an efficient bifunctional electrocatalyst for overall water splitting. More importantly, the electrocatalyst showed remarkable activation behavior due to the in situ oxidation of the carbon substrate to form C-OH groups. Density functional theory calculations suggested that the flexible C-OH groups can form reversible hydrogen bonds with the oxygen evolution reaction intermediates, giving a bridge between elementary reactions to break the conventional scaling relationship.

Regulating the electronic structure through charge redistribution in dense single-atom catalysts for enhanced alkene epoxidation

Inter-site interaction in densely populated single-atom catalysts has been demonstrated to have a crucial role in regulating the electronic structure of metal atoms, and consequently their catalytic performances. We herein report a general and facile strategy for the synthesis of several densely populated single-atom catalysts. Taking cobalt as an example, we further produce a series of Co single-atom catalysts with varying loadings to investigate the influence of density on regulating the electronic structure and catalytic performance in alkene epoxidation with O₂. Interestingly, the turnover frequency and mass-specific activity are significantly enhanced by 10 times and 30 times with increasing Co loading from 5.4 wt% to 21.2 wt% in trans-stilbene epoxidation, respectively. Further theoretical studies reveal that the electronic structure of densely populated Co atoms is altered through charge redistribution, resulting in less Bader charger and higher d-band center, which are demonstrated to be more beneficial for the activation of O₂ and trans-stilbene. The present study demonstrates a new finding about the site interaction in densely populated single-atom catalysts, shedding insight on how density affects the electronic structure and catalytic performance for alkene epoxidation.

Cooperation between Dual Metal Atoms and Nanoclusters Enhances Activity and Stability for Oxygen Reduction and Evolution

We have achieved the synthesis of dual-metal single atoms and atomic clusters that co-anchor on a highly graphitic carbon support. The catalyst comprises Ni₄ (and Fe₄) nanoclusters located adjacent to the corresponding NiN₄ (and FeN₄) single-atom sites, which is verified by systematic X-ray absorption characterization and density functional theory calculations. A distinct cooperation between Fe₄ (Ni₄) nanoclusters and the corresponding FeN₄ (NiN₄) atomic sites optimizes the adsorption energy of reaction intermediates and reduces the energy barrier of the potential-determining steps. This catalyst exhibits enhanced oxygen reduction and evolution activity and long-cycle stability compared to counterparts without nanoclusters and commercial Pt/C. The fabricated Zn-air batteries deliver a high power density and long-term cyclability, demonstrating their prospects in energy storage device applications.

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

Heterogeneous bimetallic catalysts have broad applications in industrial processes, but achieving a fundamental understanding on the nature of the active sites in bimetallic catalysts at the atomic and molecular level is very challenging due to the structural complexity of the bimetallic catalysts. Comparing the structural features and the catalytic performances of different bimetallic entities will favor the formation of a unified understanding of the structure-reactivity relationships in heterogeneous bimetallic catalysts and thereby facilitate the upgrading of the current bimetallic catalysts. In this review, we will discuss the geometric and electronic structures of three representative types of bimetallic catalysts (bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles) and then summarize the synthesis methodologies and characterization techniques for different bimetallic entities, with emphasis on the recent progress made in the past decade. The catalytic applications of supported bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles for a series of important reactions are discussed. Finally, we will discuss the future research directions of catalysis based on supported bimetallic catalysts and, more generally, the prospective developments of heterogeneous catalysis in both fundamental research and practical applications.

Self-catalyzed Co, N-doped carbon nanotubes-grafted hollow carbon polyhedrons as efficient trifunctional electrocatalysts for zinc-air batteries and self-powered overall water splitting

It is still essential and challenging to explore inexpensive and versatile electrocatalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), for the development of rechargeable zinc-air batteries (ZABs) and overall water splitting. Herein, a rambutan-like trifunctional electrocatalyst is fabricated by re-growth of secondary zeolitic imidazole frameworks (ZIFs) on ZIF-8-derived ZnO and the following carbonization treatment. Co nanoparticles (NPs) are encapsulated into N-doped carbon nanotubes (NCNT) grafted N-enriched hollow carbon (NHC) polyhedrons to form the Co-NCNT@NHC catalyst. The strong synergy between the N-doped carbon matrix and Co NPs endows Co-NCNT@NHC with trifunctional catalytic activity. The Co-NCNT@NHC displays a half-wave potential of 0.88 V versus RHE for ORR in alkaline electrolyte, an overpotential of 300 mV at 20 mA cm^-2 for OER, and an overpotential of 180 mV at 10 mA cm^-2 for HER. Impressively, a water electrolyzer is successfully powered by two rechargeable ZABs in series, with Co-NCNT@NHC as the 'all-in-one' electrocatalyst. These findings are inspiring for the rational fabrication of high-performance and multifunctional electrocatalysts intended for the practical application of integrated energy-related systems.

Direct Observation of Transition Metal Ions Evolving into Single Atoms: Formation and Transformation of Nanoparticle Intermediates

Understanding the dynamical evolution from metal ions to single atoms is of great importance to the rational development of synthesis strategies for single atom catalysts (SACs) against metal sintering during pyrolysis. Herein, an in situ observation is disclosed that the formation of SACs is ascertained as a two-step process. There is initially metal sintering into nanoparticles (NPs) (500-600 °C), followed by the conversion of NPs into metal single atoms (Fe, Co, Ni, Cu SAs) at higher temperature (700-800 °C). Theoretical calculations together with control experiments based on Cu unveil that the ion-to-NP conversion can arise from the carbon reduction, and NP-to-SA conversion being steered by generating more thermodynamically stable Cu-N4 configuration instead of Cu NPs. Based on the evidenced mechanism, a two-step pyrolysis strategy to access Cu SACs is developed, which exhibits excellent ORR performance.

Improved Electronic Structure from Spin-State Reconstruction of a Heteronuclear Fe-Co Diatomic Pair to Boost the Fenton-like Reaction

Dual-atom catalysts (DACs) are promising candidates for various catalytic reactions, including electrocatalysis, chemical synthesis, and environmental remediation. However, the high-activity origin and mechanism underlying intrinsic activity enhancement remain elusive, especially for the Fenton-like reaction. Herein, we systematically compared the catalytic performance of dual-atom FeCo-N/C with its single-atom counterparts by activating peroxymonosulfate (PMS) for pollutant abatement. The unusual spin-state reconstruction on FeCo-N/C is demonstrated to effectively improve the electronic structure of Fe and Co in the d orbital and enhance the PMS activation efficiency. Accordingly, the dual-atom FeCo-N/C with an intermediate-spin state remarkably boosts the Fenton-like reaction by almost 1 order of magnitude compared with low-spin Co-N/C and high-spin Fe-N/C. Moreover, the established dual-atom-activated PMS system also exhibits excellent stability and robust resistance against harsh conditions. Combined theoretical calculations reveal that unlike unitary Co atom or Fe atom transferring electrons to the PMS molecule, the Fe atom of FeCo-N/C provides extra electrons to the neighboring Co atom and positively shifts the d band of the Co center, thereby optimizing the PMS adsorption and decomposition into a unique high-valent Fe^IV-O-Co^IV species via a low-energy barrier pathway. This work advances a conceptually novel mechanistic understanding of the enhanced catalytic activity of DACs in Fenton-like reactions and helps to expand the application of DACs in various catalytic reactions.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Insight into the Mechanism for Catalytic Activity of the Oxygen/Hydrogen Evolution Reaction on a Dual-Site Catalyst

The dual-site catalysts consisting of two adjacent single-atom sites on graphene have exhibited promising catalytic activity of the electrochemical oxygen/hydrogen evolution reaction (OER/HER). However, the electrochemical mechanisms of the OER/HER on dual-site catalysts have still been ambiguous. In this work, we employed density functional theory calculations to study the catalytic activity of the OER/HER with a O-O (H-H) direct coupling mechanism on dual-site catalysts. Specifically, these element steps should be classified into two categories: a step evolving proton-coupled electron transfer (PCET step) that needs to be driven by electrode potential and a step without PCET (non-PCET step) that occurs naturally under mild conditions. Our calculated results show that both the maximal free energy change (ΔG_Max) contributed by the PCET step and the activity barrier (E_a) of the non-PCET step must be examined to evaluate the catalytic activity of the OER/HER on the dual site. Importantly, it is a basically inevitable negative relationship between ΔG_Max and E_a, which would play a critical role in guiding the rational design of effective dual-site catalysts for electrochemical reactions.

Dual Metal Site Fe Single Atom Catalyst with Improved Stability in Acidic Conditions

Dual atom catalysts (DACs) not only retain uniform active sites and high atomic utilization efficiency as the single atom catalysts, but the two adjacent metal sites also cooperate and play a synergistic role to achieve additional benefits. However, the relationships connecting their dual-site synergistic effects on catalytic performance are not well rationalized due to limited pairs available from experiments. Herein, Fe/M dual sites supported by nitrogen doped carbon (Fe/M-N-C whereby M from 3 d–5 d electron containing transition metals) have been screened as an oxygen reduction reaction (ORR) catalyst. The results show that the absorption strength of ORR intermediates on four nitrogen coordinated metals is weaker than the three coordinated metals, which promotes favourable ORR activities. As a result, we recommended FeIr, FeRh, FeRu and FeOs as promising ORR catalysts. Ab initio molecular dynamic (AIMD) simulations suggest Fe/M-N-C (M = Ir, Rh, Ru and Os) catalysts with encouraging structural stability at room temperature. Furthermore, it is found that the nitrogen atoms in-between metals are vulnerable sites for proton attacking, yet the protonation process demands high energy, even under O2 atmosphere, which underlines good tolerance under acidic conditions. This work provides a broad understanding of Fe based catalyst and a new direction for catalytic design.

Role of the Support Effects in Single-Atom Catalysts

In recent years, single-atom catalysts (SACs) have received a significant amount of attention due to their high atomic utilization, low cost, high reaction activity, and selectivity for multiple catalytic reactions. Unfortunately, the high surface free energy of single atoms leads them easily migrated and aggregated. Therefore, support materials play an important role in the preparation and catalytic performance of SACs. Aiming at understanding the relationship between support materials and the catalytic performance of SACs, the support effects in SACs are introduced and reviewed herein. Moreover, special emphasis is placed on exploring the influence of the type and structure of supports on SAC catalytic performance through advanced characterization and theoretical research. Future research directions for support materials are also proposed, providing some insight into the design of SACs with high efficiency and high loading.

Partially Nitrided Ni Nanoclusters Achieve Energy-Efficient Electrocatalytic CO2 Reduction to CO at Ultralow Overpotential

Electrocatalytic CO₂ reduction reaction (CO₂ RR) offers a promising strategy to lower CO₂ emission while producing value-added chemicals. A great challenge facing CO₂ RR is how to improve energy efficiency by reducing overpotentials. Herein, partially nitrided Ni nanoclusters (NiN_x ) immobilized on N-doped carbon nanotubes (NCNT) for CO₂ RR are reported, which achieves the lowest onset overpotential of 16 mV for CO₂ -to-CO and the highest cathode energy efficiency of 86.9% with CO Faraday efficiency >99.0% to date. Interestingly, NiN_x /NCNT affords a CO generation rate of 43.0 mol g^-1  h^-1 at a low potential of -0.572 V (vs RHE). DFT calculations reveal that the NiN_x nanoclusters favor *COOH formation with lower Gibbs free energy than isolated Ni single-atom, hence lowering CO₂ RR overpotential. As NiN_x /NCNT is applied to a membrane electrode assembly system coupled with oxygen evolution reaction, a cell voltage of only 2.13 V is required to reach 100 mA cm^-2 , with total energy efficiency of 62.2%.

Transformation mechanism of high-valence metal sites for the optimization of Co- and Ni-based OER catalysts in an alkaline environment: recent progress and perspectives

As an important semi-reaction process in electrocatalysis, oxygen evolution reaction (OER) is closely associated with electrochemical hydrogen production, CO₂ electroreduction, electrochemical ammonia synthesis and other reactions, which provide electrons and protons for the related applications. Considering their fundamental mechanism, metastable high-valence metal sites have been identified as real, efficient OER catalytic sites from the recent observation by in situ characterization technology. Herein, we review the transformation mechanism of high-valence metal sites in the OER process, particularly transition metal materials (Co- and Ni-based). In particular, research progress in the transformation process and role of high-valence metal sites to optimize OER performance is summarized. The key challenges and prospects of the design of high-efficiency OER catalysts based on the above-mentioned mechanism and some new in situ characterizations are also discussed.

Long-Range Interactions in Diatomic Catalysts Boosting Electrocatalysis

The simultaneous presence of two active metal centres in diatomic catalysts (DACs) leads to the occurrence of specific interactions between active sites. Such interactions, referred to as long-range interactions (LRIs), play an important role in determining the rate and selectivity of a reaction. The optimal combination of metal centres must be determined to achieve the targeted efficiency. To date, various types of DACs have been synthesised and applied in electrochemistry. However, LRIs have not been systematically summarised. Herein, the regulation, mechanism, and electrocatalytic applications of LRIs are comprehensively summarised and discussed. In addition to the basic information above, the challenges, opportunities, and future development of LRIs in DACs are proposed in order to present an overall view and reference for future research.

Unique amorphous/crystalline heterophase coupling for an efficient oxygen evolution reaction

Designing amorphous/crystalline heterophase catalysts is still in the initial stage, and the study of amorphous/crystalline heterophase and carbon-free catalysts has not yet been realized. Herein, we report a unique amorphous/crystalline heterophase catalyst consisting of NiFe alloy nanoparticles (NPs) supported on Ti₄O₇ (NiFe/Ti₄O₇) for the first time, which is achieved by a heterophase supporting strategy of dual heat treatment. Surprisingly, the amorphous/crystalline heterophase is flexibly composed of amorphous and crystalline phases of alloy NPs and Ti₄O₇. The heterophase coupling endows the catalyst with a low overpotential (256 mV at 10 mA cm^-2), a small Tafel slope (47 mV dec^-1) and excellent endurance stability (over 100 h) in 1 M KOH electrolyte, which already outperforms commercial RuO₂ (338 mV and 113 mV dec^-1) and exceeds most reported representative carbon-based and titanium-based non-precious metal catalysts. The density functional theory (DFT) calculations and experimental results reveal that the unique amorphous/crystalline heterophase coupling in NiFe/Ti₄O₇ results in electron transfer between the alloy NPs and Ti₄O₇, allowing more catalytically active sites and faster interfacial electron transfer dynamics. This work provides insights into the synthesis of amorphous/crystalline heterophase catalysts and can be generalized to the heterophase coupling of other transition metal-based electrocatalysts.

Hydrogen Peroxide Assisted Electrooxidation of Benzene to Phenol over Bifunctional Ni-(O-C2 )4 Sites

Direct electrocatalytic oxidation of benzene has been regarded as a promising approach for achieving high-value phenol product, but remaining a huge challenge. Here an oxygen-coordinated nickel single-atom catalyst (Ni-O-C) is reported with bifunctional electrocatalytic activities toward the two-electron oxygen reduction reaction (2e- ORR) to H2 O2 and H2 O2 -assisted benzene oxidation to phenol. The Ni-(O-C2 )4 sites in Ni-O-C ar proven to be the catalytic active centers for bifunctional 2e- ORR and H2 O2 -assisted benzene oxidation processes. As a result, Ni-O-C can afford a benzene conversion as high as 96.4 ± 3.6% with a phenol selectivity of 100% and a Faradaic efficiency (FE) of 80.2 ± 3.2% with the help of H2 O2 in 0.1 m KOH electrolyte at 1.5 V (vs RHE). A proof of concept experiment with Ni-O-C concurrently as cathode and anode in a single electrochemical cell demonstrates a benzene conversion of 33.4 ± 2.2% with a phenol selectivity of 100% and a FE of 44.8 ± 3.0% at 10 mA cm-2 .

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.

Effect of ZrS2 load single/dual-atom catalysts on the hydrogen evolution reaction: A first-principles study

The hydrogen evolution effect of ZrS₂ carrier loaded with transition metal single-atom (SA) was explored by first-principles method. ZrS₂ was constructed with transition metal single-atom and dual-atom. The structure-activity relationship of supported single-atom catalysts was described by electronic properties and hydrogen evolution kinetics. The results show that the ZrS₂ carrier-loaded atomic-level catalysts are more likely to occur in acidic environments, where the Mo SA load has a higher hydrogen precipitation capacity than the Pt SA. In the case of dual-atom adsorption, most of the hydrogen reduction processes are higher than that of single atom loading, which indicates that the outer orbital hybridization is more likely to lead to the interfacial charge recombination of the catalyst. Thereinto, Ni/Pt @ZrS₂ has the lowest Gibbs free energy (0.08 eV), and the synergistic effect of transition metals induces the deviation of the center of the d-band from the Fermi level and improves the dissociation ability of H ions. The design provides a new catalytic model for the HER and provides some ideas for understanding the two-site catalysis.

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.

Optimized NiFe-Based Coordination Polymer Catalysts: Sulfur-Tuning and Operando Monitoring of Water Oxidation

In-depth insights into the structure-activity relationships and complex reaction mechanisms of oxygen evolution reaction (OER) electrocatalysts are indispensable to efficiently generate clean hydrogen through water electrolysis. We introduce a convenient and effective sulfur heteroatom tuning strategy to optimize the performance of active Ni and Fe centers embedded into coordination polymer (CP) catalysts. Operando monitoring then provided the mechanistic understanding as to how exactly our facile sulfur engineering of Ni/Fe-CPs optimizes the local electronic structure of their active centers to facilitate dioxygen formation. The high OER activity of our optimized S-R-NiFe-CPs outperforms the most recent NiFe-based OER electrocatalysts. Specifically, we start from oxygen-deprived O_d-R-NiFe-CPs and transform them into highly active Ni/Fe-CPs with tailored sulfur coordination environments and anionic deficiencies. Our operando X-ray absorption spectroscopy analyses reveal that sulfur introduction into our designed S-R-NiFe-CPs facilitates the formation of crucial highly oxidized Ni⁴⁺ and Fe⁴⁺ species, which generate oxygen-bridged Ni^IV-O-Fe^IV moieties that act as the true OER active intermediates. The advantage of our sulfur-doping strategy for enhanced OER is evident from comparison with sulfur-free O_d-R-NiFe-CPs, where the formation of essential high-valent OER intermediates is hindered. Moreover, we propose a dual-site mechanism pathway, which is backed up with a combination of pH-dependent performance data and DFT calculations. Computational results support the benefits of sulfur modulation, where a lower energy barrier enables O-O bond formation atop the S-Ni^IV-O-Fe^IV-O moieties. Our convenient anionic tuning strategy facilitates the formation of active oxygen-bridged metal motifs and can thus promote the design of flexible and low-cost OER electrocatalysts.

Real-Space Identification of Non-Noble Single Atomic Catalytic Sites within Metal-Coordinated Supramolecular Networks

With regard to the development of single atom catalysts (SACs), non-noble metal-organic layers combine a large functional variability with cost efficiency. Here, we characterize reacted layers of melamine and melem molecules on a Cu(111) surface by noncontact atomic force microscopy (nc-AFM), X-ray photoelectron spectroscopy (XPS) and ab initio simulations. Upon deposition on the substrate and subsequent heat treatments in ultrahigh vacuum (UHV), these precursors undergo a stepwise dehydrogenation. After full dehydrogenation of the amino groups, the molecular units lie flat and are strongly chemisorbed on the copper substrate. We observe a particularly extreme interaction of the dehydrogenated nitrogen atoms with single copper atoms located at intermolecular sites. In agreement with the nc-AFM measurements performed with an O-terminated copper tip on these triazine- and heptazine-based copper nitride structures, our ab initio simulations confirm a pronounced interaction of oxygen species at these N-Cu-N sites. To investigate the related functional properties of our samples regarding the oxygen reduction reaction (ORR), we developed an electrochemical setup for cyclic voltammetry experiments performed at ambient pressure within a drop of electrolyte in a controlled O₂ or N₂ environment. Both copper nitride structures show a robust activity in irreversibly catalyzing the reduction of oxygen. The activity is assigned to the intermolecular N-Cu-N sites of the triazine- and heptazine-based copper nitrides or corresponding oxygenated versions (N-CuO-N, N-CuO₂-N). By combining nc-AFM characterization on the atomic scale with a direct electrochemical proof of performance, our work provides fundamental insights about active sites in a technologically highly relevant reaction.