Boosting Oxygen Electrocatalytic Activity of Fe-N-C Catalysts by Phosphorus Incorporation

Atomic-level engineering Ni-N2O2 interfacial structure for enhanced CO2 electrocatalytic reduction efficiency

The precise atomic-scale preparation of single-atomic active sites with unique coordination structures in electrocatalysts for the carbon dioxide reduction reaction (CO2RR), coupled with the elucidation of their mechanisms at the atomic level, remains a formidable challenge. In this manuscript, a simple one-pot synthesis method was adopted to successfully synthesize an O-doped Ni single-atom catalyst (Ni-NOG), characterized by a distinct Ni-N2O2 symmetric coordination structure. The incorporation of Ni-O bonds alters the electronic configuration of the catalyst's central atoms within the catalyst, thereby boosting both the catalytic selectivity and efficiency during CO2RR. The synthesized electrocatalyst exhibited outstanding performance in the CO2RR process, achieving a Faraday efficiency (FE) of 97.4 % at a potential of -0.8315 V versus to reversible hydrogen electrode (vs. RHE). Furthermore, the selectivity remained consistently above 95 % throughout a 98-hour stability test, surpassing the performance of most advanced catalysts currently available. Theoretical simulations demonstrate that the Ni-N2O2 symmetric coordination structure shows a small activation barrier in the rate-limiting step, favoring the swift generation of intermediate species and demonstrating robust catalytic activity. This work not only offers a straightforward and approach method for the preparation of single-atom catalysts but also clarifies the pivotal role of O-element doping within the coordination environment in enhancing catalyst performance.

Engineering Triple Phase Interface and Axial Coordination Design of Single-Atom Electrocatalysts for Rechargeable Zn─air Batteries

Bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable for rechargeable Zn─air batteries (rZABs). Herein, a space optimized 3D heterostructure Co-N-C@MoS2 catalyst with Co single atom and Co cluster sites is developed by pyrolysis of ZIF-67 and in situ grown ultrathin MoS2 nanosheets. The introduced MoS2 not only has abundant defective structures, but also regulates the Co electronic distribution, thus introducing additional active sites and enhancing Co-Nx activity. In addition, the MoS2 modification leads to an appropriate increase in hydrophilicity which can make a stable liquid/gas/solid triple phase interface, facilitating the approachability of electrolytes into the porous channels and promotes the mass transfer through ensuring a favorite contact among the catalyst, electrolyte and reactants and enhancing utility of active reaction sites. Comprehensive analysis and theoretical simulation indicate that the enhancement of activity stems from the axial coordination of Co cluster over Co single-atom active sites to regulate local electronic structure, thereby optimizing the adsorption of ORR intermediates and enhancing the catalytic activity. Compared with the commercial Pt/C and IrO2, the structurally optimized Co-N-C@MoS2 catalyst displays exceptional bifunctional electrocatalytic activity and long-time stability toward both OER and ORR. Moreover, the Co-N-C@MoS2 catalyst exhibits higher peak power density and superior stability in liquid and flexible ZABs compared to the commercial Pt/C + IrO2 catalyst.

Efficient Oxygen Reduction Catalysis on Fe4 Cluster Site Facilitated by Adjacent Single Atom

The inherent sluggish kinetics of the conventional four-electron transfer pathway fundamentally limits the oxygen reduction reaction (ORR) efficiency. While electronic structure modulation offers potential solutions, developing effective catalytic regulation strategies remains challenging due to elusive structure-activity correlations. In this study, Fe4 cluster sites are engineered with dual parallel electron transfer channels that enable concurrent O─O bond cleavage and dual oxygen atom protonation. This unique configuration facilitates an optimized two-step double electron transfer mechanism, significantly enhancing ORR kinetics. Synergistic Mn single atom sites, strategically positioned as electron reservoirs, substantially elevate the electron density of Fe4 clusters while reinforcing Fe─N coordination bonds through charge redistribution. Remarkably, the spatial configuration of Fe4 clusters at the support periphery minimizes steric confinement effects, allowing simultaneous product desorption and oxygen adsorption - a critical advantage for sustaining continuous catalytic cycles. Through combined experimental and theoretical analyses, it is demonstrated that this dual-channel electron transport system effectively reduces activation barriers for elementary steps while accelerating charge transfer kinetics. This fundamental study establishes a new paradigm for designing high-performance ORR catalysts through multi-site collaborative engineering and reaction pathway optimization.

Heteroatom sulfur-doping in single-atom FeNC catalysts for durable oxygen reduction performance in zinc-air batteries

Heteroatom doping into the transition metal-based catalysts is an effective strategy to improve the oxygen reduction reaction (ORR) kinetics. Herein, we proposed a one-step, soft template assisted, and green method for the synthesis of Sulfur (S) doped single atom FeNC catalyst. XAFS demonstrated that the Fe active sites in the FeNSC were more likely to possess the Fe-N4 configuration. Density functional theory (DFT) calculations revealed the effect of S-doping into the single atom Fe-N4 symmetric structure, resulting in the delocalization of 3d electrons and asymmetric structure for the single atom FeNSC. The energy barrier of the rate-determining step decreased from 0.535 eV (for FeNC) to 0.474 eV for the FeNSC structure, indicating the possible good catalytic activity of the FeNSC catalyst. The following experiments demonstrated that the FeNSC catalyst showed an excellent ORR performance in both acidic medium with a half wave potential (E1/2) of 0.81 V vs. RHE and basic medium with an E1/2 value of 0.93 V vs. RHE. The high ORR performance is validated by assembling a homemade Zinc-air battery (ZAB) using the single atom FeNSC as a cathode, showing a high power density of 240 mW cm-2. The synthesized single-atom FeNSC catalysts outperformed the state-of-the-art 20 % Pt/C catalyst. The combination of physical characterization, experimental results, and DFT calculations unveiled exceptional improvements in the ORR activity through the incorporation of the S atom into the Fe-N4 matrix. Our findings offer a pathway towards sustainable energy solutions, driving innovation in the field of green energy technologies.

Atomically dispersed rare earth dysprosium-nitrogen-carbon for boosting oxygen reduction reaction

Transition metal-nitrogen-carbon (MNC) based on 3d metal atoms as promising non-precious metal catalysts have been extensively exploited for oxygen reduction reaction (ORR), but MNC with 4f rare earth metals have been largely ignored, most likely due to their large atomic radii that are difficult to coordinate with N dopants using conventional precursors. Herein, atomically dispersed dysprosium-nitrogen-carbon (DyNC) nanosheets were developed via the pyrolysis of anitrogen-containing chelate compound of 2, 4, 6-Tri (2-pyridyl) 1, 3, 5-triazine (TPTZ) ligand with Dy3+ under the assistance of molten NaCl. The as-synthesized DyNC features specific moieties of single Dy atom coordinated by N and O as active sites for ORR, displaying excellent performance. The half-wave potentials of 0.77 V and 0.88 V in acidic and alkaline media respectively are superior to those of iron-nitrogen-carbon (FeNC) synthesized using the same method. Meanwhile, a practical zinc-air battery verifies the ORR activity of DyNC with a maximum power output of 216 mW cm-2, which is even better than the commercial platinum on carbon catalyst (Pt/C) under the same loading.In addition, theoretical calculations verify that compared to the classic FeN4 moiety,the DyN4O1 exhibits a lower overpotential of 0.570 V, demonstrating that it possesses more significant catalytic performance for ORR. This work provides the inspiration of developing non-precious metal electrocatalysts with atomic 4f rare earth metals for ORR.

Integrated Two-in-one Strategy for Efficient Neutral Hydrogen Peroxide Electrosynthesis via Phosphorous Doping in 2D Mesoporous Carbon Carriers

Herein, we demonstrate a two-in-one strategy for efficient neutral electrosynthesis of H2O2 via two-electron oxygen reduction reaction (2e- ORR), achieved by synergistically fine-modulating both the local microenvironment and electronic structure of indium (In) single atom (SA) sites. Through a series of finite elemental simulations and experimental analysis, we highlight the significant impact of phosphorous (P) doping on an optimized 2D mesoporous carbon carrier, which fosters a favorable microenvironment by improving the mass transfer and O2 enrichment, subsequently leading to an increased local pH levels. Consequently, an outstanding 2e- ORR performance is observed in neutral electrolytes, achieving over 95 % selectivity for H2O2 across a broad voltage range of 0.1 to 0.5 V vs RHE. In a flow cell, the production rate of H2O2 exceeds 22.54 mol gcat -1 h-1 while maintaining high stability at industrial-level current densities. These results are comparable to, if not better than, those achieved under alkaline conditions. Further analysis, both experimental and theoretical, indicates that the P dopant occupies the second coordination sphere of the In SA, which shows optimized OOH* binding strength for an enhanced 2e- ORR kinetic.

Tailoring the Surface Curvature of the Supporting Carbon to Tune the d-Band Center of Fe-N-C Single-Atom Catalysts for Zinc-Urea-Air Batteries

The catalytic activities of the Fe-N-C single-atom catalysts (SACs) are associated with the varying atomic interactions through its characteristic coordination geometry. Yet, modulation of the surface curvature of carbon acting as a supporting body has not been investigated. Herein, we report the superior catalytic activity for the oxygen reduction reaction (ORR) and enhanced performance for urea oxidation reaction (UOR) of single Fe atoms anchored on a highly curved N-doped carbon dodecahedron with concave morphology (Fe SA/NhcC). Theoretical calculations and in situ spectroscopy disclose that the curvature of the carbon support helps to shorten the bond length of Fe-N, spatially redistributing the charges around the Fe and thereby lowering the d-band center toward optimal adsorption for oxygenated species. The Fe SA/NhcC catalyst displays an ultrahigh half-wave potential of 0.926 V for ORR and a small potential difference of 0.686 V for bifunctional ORR/UOR. A rechargeable Zn-urea-air battery with the Fe SA/NhcC cathode displays robust discharge durability, excellent cycling lifespan and higher energy efficiency compared to conventional Zn-air batteries. This work provides new insight into promoting the catalytic activity of SACs through varying the surface curvature of the supporting carbon, tailoring geometric configuration and electronic states of SACs.

Tuning Transition Metal 3d Spin state on Single-atom Catalysts for Selective Electrochemical CO2 Reduction

Tuning transition metal spin states potentially offers a powerful means to control electrocatalyst activity. However, implementing such a strategy in electrochemical CO2 reduction (CO2R) is challenging since rational design rules have yet to be elucidated. Here we show how the addition of P dopants to a ferromagnetic element (Fe, Co, and Ni) single-atom catalyst (SAC) can shift its spin state. For instance, with Fe SAC, P dopants enable a switch from low spin state (dx2- y2 0, dz2 0, dxz 2, dyz 1, dxy 2) in Fe-N4 to high spin state (dx2-y2 0, dxz 1, dyz 1, dz2 1, dxy 2) in Fe-N3-P. This is studied using a suite of characterization efforts, including X-ray absorption spectroscopy (XAS), electron spin resonance (ESR) spectroscopy, and superconducting quantum interference device (SQUID) measurements. When used for CO2R, the SAC with Fe-N3-P active sites yields > 90% Faradaic efficiency to CO over a wide potential window of ≈530 mV and a maximum CO partial current density of ≈600 mA cm-2. Density functional theory calculations reveal that high spin state Fe3+ exhibits enhanced electron back donation via the dxz/dyz-π* bond, which enhances *COOH adsorption and promotes CO formation. Taken together, the results show how the SAC spin state can be intentionally tuned to boost CO2R performance.

Quantifying Asymmetric Coordination to Correlate with Oxygen Reduction Activity in Fe-Based Single-Atom Catalysts

Precisely manipulating asymmetric coordination configurations and examining electronic effects enable to tunethe intrinsic oxygen reduction reaction (ORR) activity of single-atom catalysts (SACs). However, the lackof a definite relationship between coordination asymmetry and catalytic activity makes the rational design of SACs ambiguous. Here, we propose a concept of "asymmetry degree" to quantify asymmetric coordination configurations and assess the effectiveness of active moieties in Fe-based SACs. A theoretical framework is established, elucidating the volcanic relationship between asymmetry degree and ORR activity by constructing a series of Fe-based SAC models doped with non-metal atoms (B, P, S, Se, and Te) in the first or second coordination sphere, which aligns with Sabatier principle. The predicted ORR activity of Fe asymmetric active moieties is then experimentally validated using asymmetry degree. The combined computational and experimental results suggest that single-atom moiety with a moderate asymmetry degree exhibits optimal intrinsic ORR activity, because breaking the square-planar symmetry of FeN4 can alter the electronic population of the Fe 3d-orbital, thereby optimizing the adsorption-desorption strength of intermediates and thus enhancing the intrinsic ORR activity. This fundamental understanding of catalytic activity from geometric and electronic aspects offers a rational guidance to design high-performance SACs with asymmetric configurations.

Decrypting Synergy of Alloy & Metal Nanoparticles Within Nitrogen-Doped Carbon Nanosheets for Zn-Air Batteries with Ultralong Cycling Stability

The exploration of efficient, robust, and low-cost bifunctional electrocatalysts to drive the commercial application of Zn-air batteries (ZABs) is of great significance but still remains a challenge. Herein, a 1D coordination polymer (1D-CP) derived FeNi alloy & Co nanoparticles (NPs) co-implanted N-doped carbon nanosheets (FNC/NCS) is judiciously crafted and employed as a high-performance electrocatalyst for ultralong lifetime ZABs. The key to this strategy is the leveraging of metal-coordinated melamine to direct the pyrolysis of 1D-CP, enabling the in situ formation of well-dispersed FeNi alloy and Co NPs within the carbon matrix. The resulting FNC/NCS exhibits prominent oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity with a small overall oxygen potential difference (ΔE = 0.68 V). Density functional theory (DFT) simulation demonstrates that the synergistic effect between FeNi alloy and Co NPs can reduce energy barriers, promote electron transfer, and optimize the formation of crucial intermediates, thereby largely boost ORR/OER activity of FNC/NCS. The FNC/NCS-assembled ZABs possess high specific capacity, large power density, and ultralong cycling life in both aqueous (> 3300 h) and solid-state (150 h) electrolytes. This work provides a viable strategy for 1D-CP-derived bifunctional electrocatalysts and dissects the synergistic effect between different metal species, affording significant guidance for the development of renewable energy materials.

Construction of Asymmetric Fe-N3P1 Sites on Freestanding Nitrogen/Phosphorus Co-Doped Carbon Nanofibers for Boosting Oxygen Electrocatalysis and Zinc-Air Batteries

The construction of freestanding carbon nanofiber membrane with single-atomic metal active sites and interconnected microchannels as air electrodes is vital for boosting the performance of zinc-air batteries (ZABs). Herein, single-atomic Fe sites is prepared on freestanding hierarchical nitrogen/phosphorus co-doped carbon nanofibers (Fe SACs@PNCNFs) by loading Fe-doped zeolitic imidazolate framework-8 with leaf-like structures on electrospun polyacrylonitrile (PAN) nanofibers with subsequent multi-step pyrolysis in the presence of sodium monophosphate, which are confirmed to be in the form of Fe-N3P1 by X-ray adsorption spectra. The asymmetric N/P coordinated Fe sites is theoretically demonstrated to boost the ORR performance with a half-wave potential of 0.89 V due to the weakened *O adsorption while stabilizing *OOH adsorption arising from the increased charge density of Fe sites compared to symmetric N coordinated Fe sites with Fe-N4. Moreover, when liquid and quasi-solid ZABs are assembled, excellent battery performance is also achieved with peak power density of 163 and 72 mW cm-2 as well as good stability for more than 190 and 65 h, respectively.

Symmetry Breaking of FeN4 Moiety via Edge Defects for Acidic Oxygen Reduction Reaction

Fe-N-C catalysts, with a planar D4h symmetric FeN4 structure, show promising as noble metal-free oxygen reduction reaction catalysts. Nonetheless, the highly symmetric structure restricts the effective manipulation of its geometric and electronic structures, impeding further enhancements in oxygen reduction reaction performance. Here, a high proportion of asymmetric edge-carbon was successfully introduced into Fe-N-C catalysts through morphology engineering, enabling the precise modulation of the FeN4 active site. Electrochemical experimental results demonstrate that FeN4@porous carbon (FeN4@PC), featuring enriched asymmetric edge-FeN4 active sites, exhibits higher acidic oxygen reduction reaction catalytic activity compared to FeN4@flaky carbon (FeN4@FC), where symmetric FeN4 is primarily distributed within the basal-plane. Synchrotron X-ray absorption spectra, X-ray emission spectra, and theoretical calculations indicate that the enhanced oxygen reduction reaction catalytic activity of FeN4@PC is attributed to the higher oxidation state of Fe species in the edge structure of FeN4@PC. This finding paves the way for controlling the local geometric and electronic structures of single-atom active sites, leading to the development of novel and efficient Fe-N-C catalysts.

Precisely Tailoring the Second Coordination Sphere of a Cobalt Single-Atom Catalyst for Selective Hydrogenation of Halogenated Nitroarenes

The development of highly efficient and cost-effective nonprecious metal catalysts for the selective hydrogenation of halogenated nitroarenes is very appealing yet challenging. Here, we demonstrate that the hydrogenation activity and selectivity of Co single-atom catalyst (SAC) can be tuned by tailoring the structure of second coordination sphere via P doping. As revealed by synchrotron radiation-based X-ray absorption spectroscopy characterizations, such a P doping on N-coordinated Co SAC results in the unsymmetric Co-N4P1 coordination structure. With a combination of experimental characterizations and theoretical simulations, we find that tailoring the second coordination sphere can greatly improve H2 dissociation and product desorption. As a result, the Co-N4P1 SAC exhibits superior activity, selectivity and stability for the hydrogenation of halogenated nitroarenes to corresponding amines (20 examples, >99 % yields) at 80 °C under 0.5 MPa H2 pressure, significantly outperforming most heterogeneous catalysts reported in the literature. We expect that this work opens a new avenue for the design of highly efficient nonprecious metal SACs for important hydrogenation reactions.

Atomic-Level Tin Regulation for High-Performance Zinc-Air Batteries

The trade-off between the performances of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) presents a challenge in designing high-performance aqueous rechargeable zinc-air batteries (a-r-ZABs) due to sluggish kinetics and differing reaction requirements. Accurate control of the atomic and electronic structures is crucial for the rational design of efficient bifunctional oxygen electrocatalysts. Herein, we designed a Sn-Co/RuO2 trimetallic oxide utilizing dual-active sites and tin (Sn) regulation strategy by dispersing Co (for ORR) and auxiliary Sn into the near-surface and surface of RuO2 (for OER) to enhance both ORR and OER performances. Both theoretical calculations and advanced dynamic monitoring experiments revealed that the auxiliary Sn effectively regulated the atomic/electronic environment of Ru and Co dual-active sites, which optimized the *OOH/*OH adsorption behavior and promoted the release of the final products, thus breaking the reaction limits. Therefore, the as-designed Sn-Co/RuO2 catalysts exhibited superb bifunctional performance with an oxygen potential difference (ΔE) of 0.628 V and negligible activity degradation after 200,000 (ORR) or 20,000 (OER) CV cycles. The a-r-ZABs based on the Sn-Co/RuO2 catalyst exhibited a higher performance at a wide temperature range of -30 to 65 °C. They demonstrated an ultralong lifespan of 138 days (20,000 cycles) at 5 mA cm-2, 39.7 times higher than that of Pt/C + IrO2 coupled catalysts at a low temperature of -20 °C. Additionally, they maintained an initial power density of 85.8% after long-term tests, significantly outperforming previously reported catalysts. More importantly, the a-r-ZABs also showed excellent stability of 766.45 h (about 4598 cycles) at a high current density of 10 mA cm-2.

Atomically dispersed iron sites from eco-friendly microbial mycelium as highly efficient hydrogenation catalyst

Iron, one of the most abundant elements on earth and an essential element for living organisms, plays a crucial role in our daily metabolism. In the field of catalysis, the development of high-performance catalysts based on less toxic iron element is also of significant importance for green chemistry and a sustainable future. To construct Fe-based heterogeneous catalysts with excellent hydrogenation performance, precise modulation of the atomic coordination structure is a key strategy for enhancing catalytic activity. In this study, we present an in-situ coating method for applying a zeolitic imidazolate framework (ZIF) onto the surface of fungal hyphae. The asymmetric coordination structure of Fe1-N3P1 was precisely tailored by utilizing the phosphorus source from the fungus and the nitrogen source in the ZIFs. Detailed characterizations and density functional theory calculations revealed that the incorporation of ZIFs not only increased the specific surface area of catalysts, but also facilitated the dispersion of Fe2P nanoparticles into the Fe1-N3P1 center, making the lowest reaction energy barrier and resulting in the best performance for nitrobenzene hydrogenation when compared to the Fe2P nanoparticles and clusters. This research introduces a novel design concept for constructing asymmetric monoatomic configuration based on the inherent characteristics of natural microorganisms and the exogenous porous coordination polymers.

MOF-derived Carbon-Based Materials for Energy-Related Applications

New carbon-based materials (CMs) are recommended as attractively active materials due to their diverse nanostructures and unique electron transport pathways, demonstrating great potential for highly efficient energy storage applications, electrocatalysis, and beyond. Among these newly reported CMs, metal-organic framework (MOF)-derived CMs have achieved impressive development momentum based on their high specific surface areas, tunable porosity, and flexible structural-functional integration. However, obstacles regarding the integrity of porous structures, the complexity of preparation processes, and the precise control of active components hinder the regulation of precise interface engineering in CMs. In this context, this review systematically summarizes the latest advances in tailored types, processing strategies, and energy-related applications of MOF-derived CMs and focuses on the structure-activity relationship of metal-free carbon, metal-doped carbon, and metallide-doped carbon. Particularly, the intrinsic correlation and evolutionary behavior between the synergistic interaction of micro/nanostructures and active species with electrochemical performances are emphasized. Finally, unique insights and perspectives on the latest relevant research are presented, and the future development prospects and challenges of MOF-derived CMs are discussed, providing valuable guidance to boost high-performance electrochemical electrodes for a broader range of application fields.

Clarifying the Active Structure and Reaction Mechanism of Atomically Dispersed Metal and Nonmetal Sites with Enhanced Activity for Oxygen Reduction Reaction

Atomically dispersed transition metal (ADTM) catalysts are widely implemented in energy conversion reactions, while the similar properties of TMs make it difficult to continuously improve the activity of ADTMs via tuning the composition of metals. Introducing nonmetal sites into ADTMs may help to effectively modulate the electronic structure of metals and significantly improve the activity. However, it is difficult to achieve the co-existence of ADTMs with nonmetal atoms and clarify their synergistic effect on the catalytic mechanism. Therefore, elucidating the active sites within atomically dispersed metal-nonmetal materials and unveiling catalytic mechanism is highly important. Herein, a novel hybrid catalyst, with coexistence of Co single-atoms and Co─Se dual-atom sites (Co─Se/Co/NC), is successfully synthesized and exhibits remarkable performance for oxygen reduction reaction (ORR). Theoretical results demonstrate that the Se sites can effectively modulate the charge redistribution at Co active sites. Furthermore, the synergistic effect between Co single-atom sites and Co─Se dual-atom sites can further adjust the d-band center, optimize the adsorption/desorption behavior of intermediates, and finally accelerate the ORR kinetics. This work has clearly clarified the reaction mechanism and shows the great potential of atomically dispersed metal-nonmetal nanomaterials for energy conversion and storage applications.

Symmetry Breaking in Rationally Designed Copper Oxide Electrocatalyst Boosts the Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) kinetics is critically dependent on the precise modulation of the interactions between the key oxygen intermediates and catalytic active sites. Herein, a novel electrocatalyst is reported, featuring nitrogen-doped carbon-supported ultra-small copper oxide nanoparticles with the broken-symmetry C4v coordination filed sites, achieved by a mild γ-ray radiation-induced method. The as-synthesized catalyst exhibits an excellent ORR activity with a half-wave potential of 0.873 V and shows no obvious decay over 50 h durability in alkaline solution. This superior catalytic activity is further corroborated by the high-performance in both primary and rechargeable Zn-air batteries with an ultrahigh-peak-power density (255.4 mW cm-2) and robust cycling stability. The experimental characterizations and density functional theory calculations show that the surface Cu atoms are configured in a compressed octahedron coordination. This geometric arrangement interacts with the key intermediate OH*, facilitating localized charge transfer and thereby weakening the Cu─O bond, which promotes the efficient transformation of OH* to OH- and the subsequent desorption, and markedly accelerates kinetics of the rate-determining step in the reaction. This study provides new insights for developing the utilization of γ-ray radiation chemistry to construct high-performance metal oxide-based catalysts with broken symmetry toward ORR.

Atomically Dispersed Fe1Mo1 Dual Sites for Enhanced Electrocatalytic Nitrogen Reduction

The electrocatalytic nitrogen reduction reaction (eNRR) is an attractive strategy for the green and distributed production of ammonia (NH3); however, it suffers from weak N2 adsorption and a high energy barrier of hydrogenation. Atomically dispersed metal dual-site catalysts with an optimized electronic structure and exceptional catalytic activity are expected to be competent for knotty hydrogenation reactions including the eNRR. Inspired by the bimetallic FeMo cofactor in biological nitrogenase, herein, an atomically dispersed Fe1Mo1 dual site anchored in nitrogen-doped carbon is proposed to induce a favorable electronic structure and binding energy. The as-prepared electrocatalyst (FeMo-NC) presents a maximum NH3 yield rate of 1.07 mg h-1 mgmetal-1 together with a Faradaic efficiency of 21.7% at -0.25 V vs RHE, outperforming many reported atomically dispersed non-noble metal electrocatalysts. Further density functional theory (DFT) calculations reveal that the Fe1Mo1 dual site activates *N2 most strongly via a side-on adsorption configuration and optimizes the binding energy of eNRR intermediates, thus lowering the limiting barrier during the overall hydrogenation and promoting NH3 generation.

Synergistic effect of Fe-Ru alloy and Fe-N-C sites on oxygen reduction reaction

In the pursuit of optimizing Fe-N-C catalysts for the oxygen reduction reaction (ORR), the incorporation of alloy nanoparticles has emerged as a prominent strategy. In this work, we effectively synthesized the FeRu-NC catalyst by anchoring Fe-Ru alloy nanoparticles and FeN4 single atom sites onto carbon nanotubes. The FeRu-NC catalyst exhibits significantly enhanced ORR activity and long-term stability, with a high half-wave potential of 0.89 V (vs. RHE) in alkaline conditions, and the half-wave potential remains nearly unchanged after 5000 cycles. The zinc-air battery (ZAB) assembled with FeRu-NC demonstrates a power density of 169.1 mW cm-2, surpassing that of commercial Pt/C. Density functional theory (DFT) calculations reveal that the synergistic interaction between the Fe-Ru alloy and FeN4 single atoms alters the electronic structure and facilitates charge transfer at the FeN4 sites, thereby modulating the adsorption and desorption of ORR intermediates. This enhancement in catalytic activity for the ORR process underscores the potential of this approach for refining M-N-C catalysts, providing novel insights into their optimization strategies.

Arrayed metal phosphide heterostructure by Fe doping for robust overall water splitting

Transition metal phosphides (TMPs) show promise in water electrolysis due to their electronic structures, which activate hydrogen/oxygen reaction intermediates. However, TMPs face limitations in catalytic efficiency due to insufficient active sites, poor conductivity, and multiple intermediate steps in water electrolysis. Here, we synthesize a highly efficient bifunctional self-supported electrocatalyst, which consists of an N-doped carbon shell anchored on Fe-doped CoP/Co2P arrays on nickel foam (NC@Fe-CoxP/NF) using hydrothermal and phosphorization techniques. Experimental and theoretical results indicate that the modified morphology, with increased active site density and a tunable electronic structure induced by Fe doping in the CoP/Co2P heterostructure, leads to superior water electrolysis performance. The resulting NC@Fe0.1-CoP/Co2P/NF catalyst exhibits overpotentials of 122 mV for the hydrogen evolution reaction (HER) and 270 mV for the oxygen evolution reaction (OER) at 100 mA cm-2. Furthermore, using NC@Fe0.1-CoP/Co2P/NF as both the cathode and anode in an alkaline electrolyzer enables the cell system to achieve 100 mA cm-2 at a voltage of 1.70 V, while maintaining long-term catalytic durability. This work may pave the way for designing self-supported, highly efficient electrocatalysts for practical water electrolysis applications.

Copper dual-doping strategy of porous carbon nanofibers and nickel fluoride nanorods as bi-functional oxygen electrocatalysis for effective zinc-air batteries

Designing and developing efficient, low-cost bi-functional oxygen electrocatalysts is essential for effective zinc-air batteries. In this study, we propose a copper dual-doping strategy, which involves doping both porous carbon nanofibers (PCNFs) and nickel fluoride nanoparticles with copper alone, successfully preparing copper-doped nickel fluoride (NiF2) nanorods and copper nanoparticles co-modified PCNFs (Cu@NiF2/Cu-PCNFs) as an efficient bi-functional oxygen electrocatalyst. When copper is doped into the PCNFs in the form of metallic nanoparticles, the doped elemental copper can improve the electronic conductivity of composite materials to accelerate electron conduction. Meanwhile, the copper doping for NiF2 can significantly promote the transformation of nickel fluoride nanoparticles into nanorod structures, thus increasing the electrochemical active surface area and enhancing mass diffusion. The Cu-doped NiF2 nanorods also possess an optimized electronic structure, including a more negative d-band center, smaller bandgap width and lower reaction energy barrier. Under the synergistic effect of these advantages, the obtained Cu@NiF2/Cu-PCNFs exhibit outstanding bi-functional catalytic performances, with a low overpotential of 0.68 V and a peak power density of 222 mW cm-2 in zinc-air batteries (ZABs) and stable cycling for 800 h. This work proposes a one-step way based on the dual-doping strategy, providing important guidance for designing and developing efficient catalysts with well-designed architectures for high-performance ZABs.

Identification of true active sites in N-doped carbon-supported Fe2P nanoparticles toward oxygen reduction reaction

Transition metal-based nanoparticles (NPs) are emerging as potential alternatives to platinum for catalyzing the oxygen reduction reaction (ORR) in zinc-air batteries (ZAB). However, the simultaneous coexistence of single-atom moieties in the preparation of NPs is inevitable, and the structural complexity of catalysts poses a great challenge to identifying the true active site. Herein, by employing in situ and ex situ XAS analysis, we demonstrate the coexistence of single-atom moieties and iron phosphide NPs in the N, P co-doped porous carbon (in short, Fe-N4-Fe2P NPs/NPC), and identify that ORR predominantly proceeds via the atomic-dispersed Fe-N4 sites, while the presence of Fe2P NPs exerts an inhibitory effect by decreasing the site utilization and impeding mass transfer of reactants. The single-atom catalyst Fe-N4/NPC displays a half-wave potential of 0.873 V, surpassing both Fe-N4-Fe2P NPs/NPC (0.858 V) and commercial Pt/C (0.842 V) in alkaline condition. In addition, the ZAB based on Fe-N4/NPC achieves a peak power density of 140.3 mW cm-2, outperforming that of Pt/C-based ZAB (91.8 mW cm-2) and exhibits excellent long-term stability. This study provides insight into the identification of true active sites of supported ORR catalysts and offers an approach for developing highly efficient, nonprecious metal-based catalysts for high-energy-density metal-air batteries.

Modulation of the Second-Beyond Coordination Structure in Single-Atom Electrocatalysts for Confirmed Promotion of Ammonia Synthesis

Although microenvironments surrounding single-atom catalysts (SACs) have been widely demonstrated to have a remarkable effect on their catalytic performances, it remains unclear whether the local structure beyond the secondary coordination shells works as well or not. Herein, we employed a series of metal-organic frameworks (MOFs) with well-defined and tunable second-beyond coordination spheres as model SAC electrocatalysts to discuss the influence of long-distance structure on the ammonia synthesis from nitrate, which were synthesized and denoted as Cu12-NDI-X (X = NMe2, H, F). It is first experimentally confirmed that the remote substitution of function groups beyond the secondary coordination sphere can remarkably affect the activity of ammonia synthesis. Meanwhile, the -H endowed Cu12-NND-H exhibits a superior ammonia yield (35.1 mg·h-1·mgcat-1) and FE (98.7%) to those modified with -NMe2 and -F, which also shows good stability at 100 mA·cm-2. The remarkable promotion of the modulated second-beyond coordination structure is unraveled to result from the adjustable d-band center of the Cu active site leading to promoted adsorption of the NO3- and protonation of key intermediates. Encouraged by its extraordinary ammonia yield, we employed the Cu12-NND-H electrode as a cathode to assemble one rechargeable Zn-nitrate battery that exhibits an impressive power density of 34.0 mW·cm-2, demonstrating its promising application in energy conversion and storage.

Enhancing Oxygen Reduction Reaction of Single-Atom Catalysts by Structure Tuning

Deciphering the fine structure has always been a crucial approach to unlocking the distinct advantages of high activity, selectivity, and stability in single-atom catalysts (SACs). However, the complex system and unclear catalytic mechanism have obscured the significance of exploring the fine structure. Therefore, we endeavored to develop a three-component strategy to enhance oxygen reduction reaction (ORR), delving deep into the profound implications of the fine structure, focusing on central atoms, coordinating atoms, and environmental atoms. Firstly, the mechanism by which the chemical state and element type of central atoms influence catalytic performance is discussed. Secondly, the significance of coordinating atoms in SACs is analyzed, considering both the number and type. Lastly, the impact of environmental atoms in SACs is reviewed, encompassing existence state and atomic structure. Thorough analysis and summarization of how the fine structure of SACs influences the ORR have the potential to offer valuable insights for the accurate design and construction of SACs.

Tailoring O-Monodentate Adsorption of CO2 Initiates C-N Coupling for Efficient Urea Electrosynthesis with Ultrahigh Carbon Atom Economy

The thermodynamically and kinetically sluggish electrocatalytic C-N coupling from CO2 and NO3 - is inert to initially take place while typically occurring after CO2 protonation, which severely dwindles urea efficiency and carbon atom economy. Herein, we report a single O-philic adsorption strategy to facilitate initial C-N coupling of *OCO and subsequent protonation over dual-metal hetero-single-atoms in N2-Fe-(N-B)2-Cu-N2 coordination mode (FeN4/B2CuN2@NC), which greatly inhibits the formation of C-containing byproducts and facilitates urea electrosynthesis in an unprecedented C-selectivity of 97.1 % with urea yield of 2072.5 μg h-1 mgcat. -1 and 71.9 % Faradaic efficiency, outperforming state-of-the-art electrodes. The carbon-directed antibonding interaction with Cu-B is elaborated to benefit single O-philic adsorption of CO2 rather than conventional C-end or bridging O,O-end adsorption modes, which can accelerate the kinetics of initiated C-N coupling and protonation. Theoretical results indicate that the O-monodentate adsorption pathway benefits the thermodynamics of the C-N coupling of *OCO with *NO2 and the protonation rate-determining step, which markedly inhibits CO2 direct protonation. This oriented strategy of manipulating reactant adsorption patterns to initiate a specific step is universal to moderate oxophilic transition metals and offers a kinetic-enhanced path for multiple conversion processes.

Oxygen Reduction Reaction Coupled Electro-Oxidation for Highly-Efficient and Sustainable Water Treatment

Electro-oxidation (EO) technology demonstrates significant potential in wastewater treatment. However, the high energy consumption has become a pivotal constraint hindering its large-scale implementation. Herein, we design an EO and 4-electron oxygen reduction reaction coupled system (EO-4eORR) to replace the traditional EO and hydrogen evolution reaction (HER) coupled system (EO-HER). The theoretical cathodic potential of the electrolytic reactor is tuned from 0 V (vs. RHE) in HER to 1.23 V (vs. RHE) in 4eORR, which greatly decreases the required operation voltage of the reactor. Moreover, we demonstrate that convection can improve the mass transfer of oxygen and organic pollutants in the reaction system, leading to low cathodic polarization and high pollutant removal rate. Compared with traditional EO-HER system, the energy consumption of the EO-4eORR system under air aeration for 95 % total organic carbon (TOC) removal is greatly decreased to 2.61 kWh/kgTOC (only consider the electrolyzer energy consumption), which is superior to previously reported EO-based water treatment systems. The reported results in this study offer a new technical mode for development of highly efficient and sustainable EO-based treatment systems to remove organic pollutants in waste water.

Bamboo-Like Carbon Nanotube-Encapsulated Fe2C Nanoparticles Activate Confined Fe2O3 Nanoclusters Via d-p-d Orbital Coupling for Alkaline Oxygen Evolution Reaction

The efficient anion exchange membrane water electrolysis is challenging with low cell voltage and long-term stability at large current density, due to the unstable anodic oxygen evolution reaction (OER). Fe-based electrocatalysts are potential candidates for the anodic OER. In Fe-based materials, iron oxides always show better stability in alkaline solution but lower OER activity. However, the catalysts in previous study are difficult to continuously and effectively activate iron oxides supported on carbon during electrocatalysis. Herein, a new class of electrocatalyst: bamboo-like carbon nanotubes (B-CNT)-encapsulated Fe2C nanoparticles (NPs) supported Fe2O3 nanoclusters (NCs), named Fe2O3/B-CNT@Fe2C is reported. Theoretical calculations and experimental results reveal that B-CNT-encapsulate Fe2C NPs activate Fe2O3 NCs by the d-p-d orbital coupling, thereby weakening the adsorption of OOH* intermediate during OER process. The electrolyzer based on the electrocatalyst requires only 1.48 V to reach 1.0 A cm-2 and shows a long-term stability at 1.0 A cm-2 for 1600 h, comparable to the best-reported values for the anion exchange membrane water electrolyzer (AEMWE).

Breaking the Mutual-Constraint of Bifunctional Oxygen Electrocatalysis via Direct O─O Coupling on High-Valence Ir Single-Atom on MnOx

Insufficient bifunctional activity of electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is the major obstruction to the application of rechargeable metal-air batteries. The primary reason is the mutual constraint of ORR and OER mechanism, involving the same oxygen-containing intermediates and demonstrating the scaling limitations of the adsorption energies. Herein, it is reported a high-valence Ir single atom anchored on manganese oxide (IrSA-MnOx) bifunctional catalyst showing independent pathways for ORR and OER, i.e., associated 4e- pathway on high-valence Ir site for ORR and a novel chemical-activated concerted mechanism for OER, where a distinct spontaneous chemical activation process triggers direct O─O coupling. The IrSA-MnOx therefore delivers outstanding bifunctional activities with remarkably low potential difference (0.635 V) between OER potential at 10 mA cm-2 and ORR half-wave potential in alkaline solution. This work breaks the scaling limitations and provides a new avenue to design efficient and multifunctional electrocatalysts.

In situ visualization of interfacial processes at nanoscale in non-alkaline Zn-air batteries

Zn-air batteries (ZABs) present high energy density and high safety but suffer from low oxygen reaction reversibility and dendrite growth at Zn electrode in alkaline electrolytes. Non-alkaline electrolytes have been considered recently for improving the interfacial processes in ZABs. However, the dynamic evolution and reaction mechanisms regulated by electrolytes at both the positive and Zn negative electrodes remain elusive. Herein, using in situ atomic force microscopy, we disclose that thin ZnO2 nanosheets deposit in non-alkaline electrolyte during discharge, followed by the formation of low-modulus products encircled around them. During recharge, the nanosheets are completely decomposed, revealing the favorable reversibility of the O2/ZnO2 chemistry. The circular outlines with low-modulus, composed of C = C and ZnCO3, are left which play a key role in promoting the oxygen reduction reaction (ORR) during the subsequent cycles. In addition, in situ optical microscopy shows that Zn can be uniformly dissolved and deposited in non-alkaline electrolyte, with the formation of homogeneous solid electrolyte interphase. Our work provides straightforward evidence and in-depth understanding of the interfacial reactions at both electrode interfaces in non-alkaline electrolyte, which can inspire strategies of interfacial engineering and material design of advanced ZABs.

Axial Coordination Engineering on Fe-N-C Materials for Oxygen Reduction: Insights from Theory

Axial coordination engineering has emerged as an effective strategy to regulate the catalytic performance of metal-N-C materials for oxygen reduction reaction (ORR). However, the ORR mechanism and activity changes of their active centers modified by axial ligands are still unclear. Here, a comprehensive investigation of the ORR on a series of FeN4-L moieties (L stands for an axial ligand) is performed using advanced density functional theory (DFT) calculations. The axial ligand has a substantial effect on the electronic structure and catalytic activity of the FeN4 center. Specially, FeN4-C6H5 is screened as a promising active moiety with superior ORR activity, as further revealed by constant-potential calculations and kinetic analysis. The enhanced activity is attributed to the weakened *OH adsorption caused by the altered electronic structure. Moreover, microkinetic modeling shows that at pH=1, FeN4-C6H5 possesses an impressive theoretical half-wave potential of ~1.01 V, superior to the pristine Fe-N-C catalysts (~0.88 V) calculated at the same level. These findings advance the understanding of the ORR mechanism of FeN4-L and provide guidance for optimizing the ORR performance of single-metal-atom catalysts.

Electronic Structure Regulated Carbon-Based Single-Atom Catalysts for Highly Efficient and Stable Electrocatalysis

Single-atom-catalysts (SACs) with atomically dispersed sites on carbon substrates have attained great advancements in electrocatalysis regarding maximum atomic utilization, unique chemical properties, and high catalytic performance. Precisely regulating the electronic structure of single-atom sites offers a rational strategy to optimize reaction processes associated with the activation of reactive intermediates with enhanced electrocatalytic activities of SACs. Although several approaches are proposed in terms of charge transfer, band structure, orbital occupancy, and the spin state, the principles for how electronic structure controls the intrinsic electrocatalytic activity of SACs have not been sufficiently investigated. Herein, strategies for regulating the electronic structure of carbon-based SACs are first summarized, including nonmetal heteroatom doping, coordination number regulating, defect engineering, strain designing, and dual-metal-sites scheming. Second, the impacts of electronic structure on the activation behaviors of reactive intermediates and the electrocatalytic activities of water splitting, oxygen reduction reaction, and CO2/N2 electroreduction reactions are thoroughly discussed. The electronic structure-performance relationships are meticulously understood by combining key characterization techniques with density functional theory (DFT) calculations. Finally, a conclusion of this paper and insights into the challenges and future prospects in this field are proposed. This review highlights the understanding of electronic structure-correlated electrocatalytic activity for SACs and guides their progress in electrochemical applications.

Conductive Polymer Hydrogel-Derived 3D Nanostructures for Energy and Environmental Electrocatalysis

Renewable energy and advanced water treatment technologies hold profound significance for driving sustainable development in modern society. Given the environmental friendliness and high efficiency of electrocatalysis processes, great expectations are placed on their applications in energy and water-related fields. However, the electrocatalysis is limited by the selectivity, activity, and durability of the electrocatalytic reactions. Hydrogels, with their hierarchical porous structure, compositional and structural tunability, and ease of functionalization, are bringing surprising advances in advanced energy and environment. Hydrogel catalysts, inheriting the advantages of hydrogel materials, hold promise for achieving significant breakthroughs in electrochemical performance. Here, the latest advancements in energy and environmental electrocatalytic fields are summarized based on the 3D nanostructured hydrogel catalysts. In addition, future potentials and challenges of continuing research on hydrogel materials for energy and environment are discussed.

A Metal-Sulfur-Carbon Catalyst Mimicking the Two-Component Architecture of Nitrogenase

The production of ammonia (NH3) from nitrogen sources involves competitive adsorption of different intermediates and multiple electron and proton transfers, presenting grand challenges in catalyst design. In nature nitrogenases reduce dinitrogen to NH3 using two component proteins, in which electrons and protons are delivered from Fe protein to the active site in MoFe protein for transfer to the bound N2. We draw inspiration from this structural enzymology, and design a two-component metal-sulfur-carbon (M-S-C) catalyst composed of sulfur-doped carbon-supported ruthenium (Ru) single atoms (SAs) and nanoparticles (NPs) for the electrochemical reduction of nitrate (NO3 -) to NH3. The catalyst demonstrates a remarkable NH3 yield rate of ~37 mg L-1 h-1 and a Faradaic efficiency of ~97 % for over 200 hours, outperforming those consisting solely of SAs or NPs, and even surpassing most reported electrocatalysts. Our experimental and theoretical investigations reveal the critical role of Ru SAs with the coordination of S in promoting the formation of the HONO intermediate and the subsequent reduction reaction over the NP-surface nearby. Such process results in a more energetically accessible pathway for NO3 - reduction on Ru NPs co-existing with SAs. This study proves a better understanding of how M-S-Cs act as a synthetic nitrogenase mimic during ammonia synthesis, and contributes to the future mechanism-based catalyst design.

Rational design of Fe, N co-doped porous carbon derived from conjugated microporous polymer as an electrocatalytic platform for oxygen reduction reaction

Porous iron-nitrogen-doped carbons (FeNC) offer a great platform for construction of cathodic oxygen reduction reaction (ORR) catalysts in fuel cells. However, challenges still remain regarding with the collapse of carbon-skeleton during pyrolysis, uneven distribution of active sites and aggregation of metal atoms. In this work, we synthesized Fe, N co-doped conjugated microporous polymer (FeN-CMP) through a facile bottom-up strategy using 1,3,5-triethynylbenzene and iron-chelated 3,8-dibromo-1,10-phenanthroline as monomers, ensuring the uniform coordination of N with Fe element in network. Then, the resulting FeN-CMP was treated by pyrolysis without structural collapse to obtain porous FeNC electrocatalyst for ORR. The most active catalyst was fabricated under 900 °C, which exhibits remarkable ORR activity in alkaline medium with half-wave potential of 0.796 V (18 mV and 105 mV positive deviation from the commercial Pt/C catalyst and post-doping catalyst), high selectivity with nearly 4e- transfer process and excellent methanol tolerance. Our study first developed porous FeNC electrocatalysts derived from Fe, N-anchoring CMPs based on pre-functionalization of monomers, which exhibits great potential as an alternative to commercial Pt/C catalyst for ORR, and provides a feasible strategy of developing multi-atoms doping catalysts for energy storage and conversion as well as heterogeneous catalysis.

High-density asymmetric iron dual-atom sites for efficient and stable electrochemical water oxidation

Double-atom catalysts (DACs) have opened distinctive paradigms in the field of rapidly developing atomic catalysis owing to their great potential for promoting catalytic performance in various reaction systems. However, increasing the loading and extending the service life of metal active centres represents a considerable challenge for the efficient utilization of DACs. Here, we rationally design asymmetric nitrogen, sulfur-coordinated diatomic iron centres on highly defective nitrogen-doped carbon nanosheets (denoted A-Fe2S1N5/SNC, A: asymmetric), which possess the atomic configuration of the N2S1Fe-FeN3 moiety. The abundant defects and low-electronegativity heteroatoms in the carbon-based framework endow A-Fe2S1N5/SNC with a high loading of 6.72 wt%. Furthermore, A-Fe2S1N5/SNC has a low overpotential of 193 mV for the oxygen evolution reaction (OER) at 10 mA cm-2, outperforming commercial RuO2 catalysts. In addition, A-Fe2S1N5/SNC exhibits extraordinary stability, maintaining > 97% activity for over 2000 hours during the OER process. This work provides a practical scheme for simultaneously balancing the activity and stability of DACs towards electrocatalysis applications.

Second-shell modulation on porphyrin-like Pt single atom catalysts for boosting oxygen reduction reaction

The first coordination shell is considered crucial in determining the performance of single atom catalysts (SACs), but the significance of the second coordination shell has been overlooked. In this study, we developed a post-doping strategy to realize predictable and controlled modulation on the second coordination shell. By incorporating a P atom into the second coordination shell of a porphyrin-like Pt SAC, the charge density at the Fermi level of Pt single atom increases, enhancing its intrinsic activity. Moreover, the P atom shows stronger adsorption towards large size anions (ClO4 -) than Pt atoms, preventing the Pt site poisoning in acid. As a result, the Pt-N4P-C catalyst exhibits significantly higher activity than the Pt-N4-C catalyst. It even outperforms commercial Pt/C (20 wt% Pt) with a Pt content of only 0.22 wt% in both alkaline and acidic solutions. This work indicates the second coordination shell modulation also greatly impacts the performance of SACs.

Construction of an Axial Charge Transfer Channel Between Single-Atom Fe Sites and Nitrogen-Doped Carbon Supports for Boosting Oxygen Reduction

The introduction of axial-coordinated heteroatoms in Fe─N─C single-atom catalysts enables the significant enhancement of their oxygen reduction reaction (ORR) performance. However, the interaction relationship between the axial-coordinated heteroatoms and their carbon supports is still unclear. In this work, a gas phase surface treatment method is proposed to prepare a series of X─Fe─N─C (X = O, P, and S) single-atom catalysts with axial X-coordination on graphitic-N-rich carbon supports. Synchrotron-based X-ray absorption near-edge structure spectra and X-ray photoelectron spectroscopy indicate the formation of an axial charge transfer channel between the graphitic-N-rich carbon supports and single-atom Fe sites by axial O atoms in O─Fe─N─C. As a result, the O─Fe─N─C exhibits excellent ORR performance with a half-wave potential of 0.905 V versus RHE and a high specific capacity of 884 mAh g-1 for zinc-air battery, which is superior to other X─Fe─N─C catalysts without axial charge transfer and the commercial Pt/C catalyst. This work not only demonstrates a general synthesis strategy for the preparation of single-atom catalysts with axial-coordinated heteroatoms, but also presents insights into the interaction between single-atom active sites and doped carbon supports.

Fe-N co-doped carbon nanofibers with Fe3C decoration for water activation induced oxygen reduction reaction

Proton activity at the electrified interface is central to the kinetics of proton-coupled electron transfer (PCET) reactions in electrocatalytic oxygen reduction reaction (ORR). Here, we construct an efficient Fe3C water activation site in Fe-N co-doped carbon nanofibers (Fe3C-Fe1/CNT) using an electrospinning-pyrolysis-etching strategy to improve interfacial hydrogen bonding interactions with oxygen intermediates during ORR. In situ Fourier transform infrared spectroscopy and density functional theory studies identified delocalized electrons as key to water activation kinetics. Specifically, the strong electronic perturbation of the Fe-N4 sites by Fe3C disrupts the symmetric electron density distribution, allowing more free electrons to activate the dissociation of interfacial water, thereby promoting hydrogen bond formation. This process ultimately controls the PCET kinetics for enhanced ORR. The Fe3C-Fe1/CNT catalyst demonstrates a half-wave potential of 0.83 V in acidic media and 0.91 V in alkaline media, along with strong performance in H2-O2 fuel cells and Al-air batteries.

Main-group element-boosted oxygen electrocatalysis of Cu-N-C sites for zinc-air battery with cycling over 5000 h

Developing highly active and durable air cathode catalysts is crucial yet challenging for rechargeable zinc-air batteries. Herein, a size-adjustable, flexible, and self-standing carbon membrane catalyst encapsulating adjacent Cu/Na dual-atom sites is prepared using a solution blow spinning technique combined with a pyrolysis strategy. The intrinsic activity of the Cu-N4 site is boosted by the neighboring Na-containing functional group, which enhances O2 adsorption and optimizes the rate-determining step of O2 activation (*O2 → *OOH) during the oxygen reduction reaction process. Meanwhile, the Cu-N4 sites are encapsulated within carbon nanofibers and anchored by the carbon matrix to form a C2-Cu-N4 configuration, thereby reinforcing the stability of the Cu centers. Moreover, the introduction of Na-containing functional groups on the carbon atoms significantly reduces the positive charge on their outer shell C atoms, rendering the carbon skeletons less susceptible to corrosion by oxygen species and further preventing the dissolution of Cu centers. Under these multi-type regulations, the zinc-air battery with Cu/Na-carbon membrane catalyst as the air cathode demonstrates long-term discharge/charge cycle stability of over 5000 h. This considerable stability improvement represents a critical step towards developing Cu-N4 active sites modified with the neighboring main-group metal-containing functional groups to overcome the durability barriers of zinc-air batteries for future practical applications.

Combination of nanoparticles with single-metal sites synergistically boosts co-catalyzed formic acid dehydrogenation

The development of hydrogen technologies is at the heart of a green economy. As prerequisite for implementation of hydrogen storage, active and stable catalysts for (de)hydrogenation reactions are needed. So far, the use of precious metals associated with expensive costs dominates in this area. Herein, we present a new class of lower-cost Co-based catalysts (Co-SAs/NPs@NC) in which highly distributed single-metal sites are synergistically combined with small defined nanoparticles allowing efficient formic acid dehydrogenation. The optimal material with atomically dispersed CoN2C2 units and encapsulated 7-8 nm nanoparticles achieves an excellent gas yield of 1403.8 mL·g-1·h-1 using propylene carbonate as solvent, with no activity loss after 5 cycles, which is 15 times higher than that of the commercial Pd/C. In situ analytic experiments show that Co-SAs/NPs@NC enhances the adsorption and activation of the key intermediate monodentate HCOO*, thereby facilitating the following C-H bond breaking, compared to related single metal atom and nanoparticle catalysts. Theoretical calculations show that the integration of cobalt nanoparticles elevates the d-band center of the Co single atoms as the active center, which consequently enhances the coupling of the carbonyl O of the HCOO* intermediate to the Co centers, thereby lowering the energy barrier.

Ni-doping optimized d-band center in bifunctional Fe2O3 modified by bamboo-like NCNTs as a cathode material for Zn-air batteries

During the development of Zn-air batteries, designing an affordable, efficient and stable electrocatalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) presents a great challenge. Fe2O3 exhibits ORR and OER activities, but when used as a cathode material in Zn-air batteries, its activity requires further improvement. To achieve this goal, Ni is doped into Fe2O3 hexagonal nanorods, derived from a metal-organic framework (MOF) precursor, and further modified by N-doped carbon nanotubes. In ORR, its half-wave potential achieves 0.946 and 0.716 V in alkaline and neutral electrolytes, respectively. In OER, it requires 388 mV to obtain 10 mA cm-2 in an alkaline electrolyte. As illustrated by theoretical calculation, Ni-doping raises the d-band center of Fe2O3, which enhances its adsorption towards relevant oxygen species in electrocatalysis. This improves its ORR and OER activities. Based on these merits, the Zn-air battery is assembled with an alkaline electrolyte. At 10 mA cm-2, its specific capacity and energy density reach 819.8 mA h g-1 and 960.1 W h kg-1, respectively. This battery remains stable after a long time of charge and discharge. In neutral electrolytes, its promising discharge performance is also well retained. This work develops an effective approach to improve ORR and OER activities of Fe2O3-based cathode materials in Zn-air batteries.

Materials Containing Single-, Di-, Tri-, and Multi-Metal Atoms Bonded to C, N, S, P, B, and O Species as Advanced Catalysts for Energy, Sensor, and Biomedical Applications

Modifying the coordination or local environments of single-, di-, tri-, and multi-metal atom (SMA/DMA/TMA/MMA)-based materials is one of the best strategies for increasing the catalytic activities, selectivity, and long-term durability of these materials. Advanced sheet materials supported by metal atom-based materials have become a critical topic in the fields of renewable energy conversion systems, storage devices, sensors, and biomedicine owing to the maximum atom utilization efficiency, precisely located metal centers, specific electron configurations, unique reactivity, and precise chemical tunability. Several sheet materials offer excellent support for metal atom-based materials and are attractive for applications in energy, sensors, and medical research, such as in oxygen reduction, oxygen production, hydrogen generation, fuel production, selective chemical detection, and enzymatic reactions. The strong metal-metal and metal-carbon with metal-heteroatom (i.e., N, S, P, B, and O) bonds stabilize and optimize the electronic structures of the metal atoms due to strong interfacial interactions, yielding excellent catalytic activities. These materials provide excellent models for understanding the fundamental problems with multistep chemical reactions. This review summarizes the substrate structure-activity relationship of metal atom-based materials with different active sites based on experimental and theoretical data. Additionally, the new synthesis procedures, physicochemical characterizations, and energy and biomedical applications are discussed. Finally, the remaining challenges in developing efficient SMA/DMA/TMA/MMA-based materials are presented.

Strong interactions through the highly polar "Early-Late" metal-metal bonds enable single-atom catalysts good durability and superior bifunctional ORR/OER activity

Simultaneously enhancing the durability and catalytic performance of metal-nitrogen-carbon (M-Nx-C) single-atom catalysts is critical to boost oxygen electrocatalysis for energy conversion and storage, yet it remains a grand challenge. Herein, through the combination of early and late metals, we proposed to enhance the stability and tune the catalytic activity of M-Nx-C SACs in oxygen electrocatalysis by their strong interaction with the M2'C-type MXene substrate. Our density functional theory (DFT) computations revealed that the strong interaction between "early-late" metal-metal bonds significantly improves thermal and electrochemical stability. Due to considerable charge transfer and shift of the d-band center, the electronic properties of these SACs can be extensively modified, thereby optimizing their adsorption strength with oxygenated intermediates and achieving eight promising bifunctional catalysts for ORR/OER with low overpotentials. More importantly, the constant-potential analysis demonstrated the excellent bifunctional activity of SACs supported on MXene substrate across a broad pH range, especially in strongly alkaline media with record-low overpotentials. Further machine learning analysis shows that the d-band center, the charge of the active site, and the work function of the formed heterojunctions are critical to revealing the ORR/OER activity origin. Our results underscore the vast potential of strong interactions between different metal species in enhancing the durability and catalytic performance of SACs.

Bifunctional Oxygen-Defect Bismuth Catalyst toward Concerted Production of H2O2 with over 150% Cell Faradaic Efficiency in Continuously Flowing Paired-Electrosynthesis System

The electrosynthesis of hydrogen peroxide (H2O2) from O2 or H2O via the two-electron (2e-) oxygen reduction (2e- ORR) or water oxidation (2e- WOR) reaction provides a green and sustainable alternative to the traditional anthraquinone process. Herein, a paired-electrosynthesis tactic is reported for concerted H2O2 production at a high rate by coupling the 2e- ORR and 2e- WOR, in which the bifunctional oxygen-vacancy-enriched Bi2O3 nanorods (Ov-Bi2O3-EO), obtained through electrochemically oxidative reconstruction of Bi-based metal-organic framework (Bi-MOF) nanorod precursor, are used as both efficient anodic and cathodic electrocatalysts, achieving concurrent H2O2 production at both electrodes with high Faradaic efficiencies. Specifically, the coupled 2e- ORR//2e- WOR electrolysis system based on such distinctive oxygen-defect Bi catalyst displays excellent performance for the paired-electrosynthesis of H2O2, delivering a remarkable cell Faradaic efficiency of 154.8% and an ultrahigh H2O2 production rate of 4.3 mmol h-1 cm-2. Experiments combined with theoretical analysis reveal the crucial role of oxygen vacancies in optimizing the adsorption of intermediates associated with the selective two-electron reaction pathways, thereby improving the activity and selectivity of the 2e- reaction processes at both electrodes. This work establishes a new paradigm for developing advanced electrocatalysts and designing novel paired-electrolysis systems for scalable and sustainable H2O2 electrosynthesis.

Iron-impregnated cellulosic carbon as an effective electrocatalyst for seawater oxidation

The quest for cost effective but active electrocatalysts for water oxidation is at the forefront of research towards hydrogen economy. In this regard, bamboo as biomass derived N-doped cellulosic carbon has shown potential electrocatalytic performance towards water oxidation. The impregnation of optimum metallic Fe boosts the performance further, achieving an overpotential value of 238 mV at a benchmark current density of 10 mA cm-2. Owing to its promising OER performances in alkaline freshwater, the electrocatalyst was further explored in alkaline saline water and alkaline real seawater, exhibiting overpotentials of 272 mV and 280 mV, respectively, to reach 10 mA cm-2 current density. Most importantly, the protective graphitic multilayer surrounding the metallic Fe allowed the electrocatalyst to demonstrate excellent durability over 30 h even at a high current density in alkaline real seawater electrolyte.

p-d Orbital Hybridization Induced by Asymmetrical FeSn Dual Atom Sites Promotes the Oxygen Reduction Reaction

With more flexible active sites and intermetal interaction, dual-atom catalysts (DACs) have emerged as a new frontier in various electrocatalytic reactions. Constructing a typical p-d orbital hybridization between p-block and d-block metal atoms may bring new avenues for manipulating the electronic properties and thus boosting the electrocatalytic activities. Herein, we report a distinctive heteronuclear dual-metal atom catalyst with asymmetrical FeSn dual atom sites embedded on a two-dimensional C2N nanosheet (FeSn-C2N), which displays excellent oxygen reduction reaction (ORR) performance with a half-wave potential of 0.914 V in an alkaline electrolyte. Theoretical calculations further unveil the powerful p-d orbital hybridization between p-block stannum and d-block ferrum in FeSn dual atom sites, which triggers electron delocalization and lowers the energy barrier of *OH protonation, consequently enhancing the ORR activity. In addition, the FeSn-C2N-based Zn-air battery provides a high maximum power density (265.5 mW cm-2) and a high specific capacity (754.6 mA h g-1). Consequently, this work validates the immense potential of p-d orbital hybridization along dual-metal atom catalysts and provides new perception into the logical design of heteronuclear DACs.

Inhibiting Demetalation of Fe─N─C via Mn Sites for Efficient Oxygen Reduction Reaction in Zinc-Air Batteries

Demetalation caused by the electrochemical dissolution of metallic Fe atoms is a major challenge for the practical application of Fe─N─C catalysts. Herein, an efficient single metallic Mn active site is constructed to improve the strength of the Fe─N bond, inhibiting the demetalation effect of Fe─N─C. Mn acts as an electron donor inducing more delocalized electrons to reduce the oxidation state of Fe by increasing the electron density, thereby enhancing the Fe─N bond and inhibiting the electrochemical dissolution of Fe. The oxygen reduction reaction pathway for the dissociation of Fe─Mn dual sites can overcome the high energy barriers to direct O─O bond dissociation and modulate the electronic states of Fe─N4 sites. The resulting FeMn─N─C exhibits excellent ORR activity with a high half-wave potential of 0.92 V in alkaline electrolytes. FeMn─N─C as a cathode catalyst for Zn-air batteries has a cycle stability of 700 h at 25 °C and a long cycle stability of more than 210 h under extremely cold conditions at -40 °C. These findings contribute to the development of efficient and stable metal-nitrogen-carbon catalysts for various energy devices.

Designing asymmetrical TMN4 sites via phosphorus or sulfur dual coordination as high-performance electrocatalysts for oxygen evolution reaction

The development ofhighly efficient oxygen evolution reaction (OER) catalysts based on more cost-effective and earth-abundant elements is of great significance and still faces a huge challenge. In this work, a series of transition metal (TM)embedding a newly-defined monolayer carbon nitride phase is theoretically profiled and constructed as a catalytic platform for OER studies. Typically, a four-step screening strategy was proposed to rapidly identified high performance candidates and the coordination structure and catalytic performance relationship was thoroughly analyzed. Moreover, the eliminating criterion was established to condenses valid range based on the Gibbs free energy of OH*. Our results reveal that the as-constructed 2FeCN/P exhibits superior activity toward OER with an ultralow overpotential of 0.25 V, at the same time, the established 3FeCN/S configuration performed well as abifunctional OER/ORR electrocatalysis with extremely low overpotential ηOER/ηORR of 0.26/0.48 V. Overall, this work provides an effective framework for screening advanced OER catalysts, which can also be extended to other complex multistep catalytic reactions.

Planar Chlorination Engineering: A Strategy of Completely Breaking the Geometric Symmetry of Fe-N4 Site for Boosting Oxygen Electroreduction

Introducing asymmetric elements and breaking the geometric symmetry of traditional metal-N4 site for boosting oxygen reduction reaction (ORR) are meaningful and challenging. Herein, the planar chlorination engineering of Fe-N4 site is first proposed for remarkably improving the ORR activity. The Fe-N4/CNCl catalyst with broken symmetry exhibits a half-wave potential (E1/2) of 0.917 V versus RHE, 49 and 72 mV higher than those of traditional Fe-N4/CN and commercial 20 wt% Pt/C catalysts. The Fe-N4/CNCl catalyst also has excellent stability for 25 000 cycles and good methanol tolerance ability. For Zn-air battery test, the Fe-N4/CNCl catalyst has the maximum power density of 228 mW cm-2 and outstanding stability during 150 h charge-discharge test, as the promising substitute of Pt-based catalysts in energy storage and conversion devices. The density functional theory calculation demonstrates that the adjacent C─Cl bond effectively breaks the symmetry of Fe-N4 site, downward shifts the d-band center of Fe, facilitates the reduction and release of OH*, and remarkably lowers the energy barrier of rate-determining step. This work reveals the enormous potential of planar chlorination engineering for boosting the ORR activity of traditional metal-N4 site by thoroughly breaking their geometric symmetry.