Unraveling the pH-Dependent Oxygen Reduction Performance on Single-Atom Catalysts: From Single- to Dual-Sabatier Optima

Engineering the regulation strategy of active sites to explore the intrinsic mechanism over single‑atom catalysts in electrocatalysis

The development of efficient and sustainable energy sources is a crucial strategy for addressing energy and environmental crises, with a particular focus on high-performance catalysts. Single-atom catalysts (SACs) have attracted significant attention because of their exceptionally high atom utilization efficiency and outstanding selectivity, offering broad application prospects in energy development and chemical production. This review systematically summarizes the latest research progress on SACs in five key electrochemical reactions: hydrogen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, and oxygen evolution reaction. Initially, a brief overview of the current understanding of electrocatalytic active sites in SACs is provided. Subsequently, the electrocatalytic mechanisms of these reactions are discussed. Emphasis is placed on various modification strategies for SAC surface-active sites, including coordination environment regulation, electronic structure modulation, support structure regulation, the introduction of structural defects, and multifunctional site design, all aimed at enhancing electrocatalytic performance. This review comprehensively examines SAC deactivation and poisoning mechanisms, highlighting the importance of stability enhancement for practical applications. It also explores the integration of density functional theory calculations and machine learning to elucidate the fundamental principles of catalyst design and performance optimization. Furthermore, various synthesis strategies for industrial-scale production are summarized, providing insights into commercialization. Finally, perspectives on future research directions for SACs are highlighted, including synthesis strategies, deeper insights into active sites, the application of artificial intelligence tools, and standardized testing and performance requirements.

A covalent organic framework with asymmetric coordination for the oxygen reduction reaction: a computational study

Through constant-potential first-principles calculations coupled with microkinetic modelling, a covalent organic framework with an asymmetric CoN2O2 moiety is identified to exhibit remarkable oxygen reduction activity, for which the theoretical half-wave potential is 0.98 V. Electronic structure analysis reveals that the asymmetric coordination environment effectively modulates charge redistribution at the active site, thereby enhancing the activation of oxygen species.

Rational Dual-Atom Design to Boost Oxygen Reduction Reaction on Iron-Based Electrocatalysts

The oxygen reduction reaction (ORR) is critical for energy conversion technologies like fuel cells and metal-air batteries. However, advancing efficient and stable ORR catalysts remains a significant challenge. Iron-based single-atom catalysts (Fe SACs) have emerged as promising alternatives to precious metals. However, their catalytic performance and stability remain constrained. Introducing a second metal (M) to construct Fe─M dual-atom catalysts (Fe─M DACs) is an effective strategy to enhance the performance of Fe SACs. This review provides a comprehensive overview of the recent advancements in Fe-based DACs for ORR. It begins by examining the structural advantages of Fe─M DACs from the perspectives of electronic structure and reaction pathways. Next, the precise synthetic strategies for DACs are discussed, and the structure-performance relationships are explored, highlighting the role of the second metal in improving catalytic activity and stability. The review also covers in situ characterization techniques for real-time observation of catalytic dynamics and reaction intermediates. Finally, future directions for Fe─M DACs are proposed, emphasizing the integration of advanced experimental strategies with theoretical simulations as well as artificial intelligence/machine learning to design highly active and stable ORR catalysts, aiming to expand the application of Fe─M DACs in energy conversion and storage technologies.

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.

Data-driven discovery of single-atom catalysts for CO2 reduction considering the pH-dependency at the reversible hydrogen electrode scale

The electrochemical carbon dioxide reduction reaction (CO2RR) represents a promising approach to mitigating climate change and addressing energy challenges by converting CO2 into value-added chemicals. Among various CO2RR products, CO is attractive due to its economic viability and industrial relevance. By integrating large-scale data mining (with 939 experimental performance data), we reveal that the catalytic performance of d-block transition metal-based single-atom catalysts (SACs) for CO2RR is influenced not only by the coordination environment but also significantly by pH. However, the unified model that could accurately depict the pH-dependent CO2RR to CO activity of d-block SACs is urgently needed. Herein, we conducted pH-dependent microkinetic modeling based upon density functional theory calculations and pH-electric field coupled microkinetic modeling to analyze CO2RR performance of 101 SACs. Our data-driven screening identifies 12 high-performance SACs with promising CO selectivity across different pH conditions, primarily based on Fe, Cu, and Ni centers. We establish a scaling relation between key intermediates (*COOH and *CO) and analyze their adsorption behaviors under varying electrochemical conditions. Furthermore, our pH-dependent microkinetic modeling reveals the critical role of electric field effects in determining catalytic performance, aligning well with experimental turnover frequency values. Most importantly, our theoretical model accurately captures the pH-dependent performance of CO2RR-to-CO on d-block SACs, which is experimentally validated and serves as a general theoretical framework for the rational design of high-performance CO2RR catalysts. Based on this model, we identify a series of promising M-N-C catalysts, providing a universal design principle for optimizing CO2-to-CO conversion.

Recent Progress in Oxygen Reduction Reaction Toward Hydrogen Peroxide Electrosynthesis and Cooperative Coupling of Anodic Reactions

Electrosynthesis of hydrogen peroxide (H2O2) via two-electron oxygen reduction reaction (2e- ORR) is a promising alternative to the anthraquinone oxidation process. To improve the overall energy efficiency and economic viability of this catalytic process, one pathway is to develop advanced catalysts to decrease the overpotential at the cathode, and the other is to couple 2e- ORR with certain anodic reactions to decrease the full cell voltage while producing valuable chemicals on both electrodes. The catalytic performance of a 2e- ORR catalyst depends not only on the material itself but also on the environmental factors. Developing promising electrocatalysts with high 2e- ORR selectivity and activity is a prerequisite for efficient H2O2 electrosynthesis, while coupling appropriate anodic reactions with 2e- ORR would further enhance the overall reaction efficiency. Considering this, here a comprehensive review is presented on the latest progress of the state-of-the-art catalysts of 2e- ORR in different media, the microenvironmental modulation mechanisms beyond catalyst design, as well as electrocatalytic system coupling 2e- ORR with various anodic oxidation reactions. This review also presents new insights regarding the existing challenges and opportunities within this rapidly advancing field, along with viewpoints on the future development of H2O2 electrosynthesis and the construction of green energy roadmaps.

Electric-Double-Layer Mechanism of Surface Oxophilicity in Regulating the Alkaline Hydrogen Electrocatalytic Kinetics

Regulating the surface oxophilicity of the electrocatalyst is known as an efficient strategy to mitigate the order-of-magnitude kinetic slowdown of hydrogen electrocatalysis in a base, which is of great scientific and technological significance. So far, its mechanistic origin remains mainly ascribed to the bifunctional or electronic effects that revolve around the catalyst-intermediate interactions and is under extensive debate. In addition, the understanding from the perspective of interfacial electric-double-layer (EDL) structures, which should also strongly depend on the electrode property, is still lacking. Here, by decorating a Pt electrode with Mo, Ru, Rh, and Au metal atoms to tune surface oxophilicity and systematically combining electrochemical activity tests, in situ surface-enhanced infrared absorption spectroscopy, density functional theory calculation, and ab initio molecular dynamics simulation, we found that there exist consistent volcano-type relationships between *OH adsorption strength and alkaline hydrogen evolution activity, the stretching/bending vibration information on interfacial water, and the potential of zero charge (PZC) of the electrode. This demonstrates that the origin of surface oxophilicity in impacting the alkaline hydrogen electrocatalytic activity lies in its modification toward the electrode PZC, which thereby dictates the electric field strength, rigidity, and hydrogen bonding network structure in EDL and ultimately governs the interfacial proton transfer kinetics. These findings emphasize the importance of focusing on electrocatalytic interface structures to understand electrode property-dependent reaction kinetics.

Space Charge, Modulating the Catalytic Activity of Single-Atom Metal Catalysts

Potential-induced electrode charging is a prerequisite to initiate electrochemical reactions at the electrode-electrolyte interface. The 'interface space charge' could dramatically alter the reaction environment and the charge density of the active site, both of which potentially affect the electrochemical activity. However, our understanding of the electrocatalytic role of space charge has been limited. Here, we separately modulate the amount of space charge (characterized by the areal density, σ) with maintaining the electrochemical potential for the oxygen reduction reaction (ORR) at the same level, by exploiting the unique structural feature of MeNC. We reveal that changes in σ control the ORR activity, which is computationally explained by the inductive polarization of the charge density at the active sites, affecting their turnover rates. To guide catalyst design including the space charge effect, we develop a new descriptor, explaining the activity trend in various metal centers and pH conditions using a single volcano. These findings offer fresh insights into the role of space charge in electrocatalysis, providing a new framework for optimizing catalyst design and performance.

Mesoporous Single-Crystalline Particles as Robust and Efficient Acidic Oxygen Evolution Catalysts

The scarcity of iridium (Ir) limits its widespread use in acidic oxygen evolution reaction (OER). Herein, mesoporous single-crystalline spinel Co3O4 with atomically dispersed low-valence-state Ir has been developed to enable Ir's efficient and stable utilization. The surface Pourbaix diagram suggests that under acidic OER conditions, O* fully covers both Co3O4(111) and (110) surfaces, passivating Co sites but enhancing Co3O4's structural stability, a benefit further improved by Ir doping. Mesopores offer numerous loading sites for Ir single atoms (13.8 wt %), which activate the originally O*-passivated Co3O4(111) surface by creating high-intrinsic-activity Co-Ir bridge sites; meanwhile, Ir and Co leaching rates are reduced to about 1/4 and 1/5, respectively, compared to conventional Ir/Co3O4 catalysts. Our catalyst exhibits a low η10 of 248 mV for over 100 h, showcasing its potential in water electrolysis.

Electron Donor-Acceptor Activated Anti-Fenton Property for the Ultradurable Oxygen Reduction Reaction

Iron-nitrogen-carbon (Fe-N-C) materials are recognized as an effective category of catalysts that do not contain platinum (Pt) for the oxygen reduction reaction (ORR). Nonetheless, the long-term stability and effectiveness of these materials are significantly hindered by the dissolution and oxidation of Fe atoms. Microstructural engineering of Fe-N-C is a viable approach to enhancing ORR activity and stability. Herein, CuN5-single-atom nanozymes (SAzyme)-assisted Fe-N5 catalysts (SA-Fe-N5) were developed by introducing single-atom Cu to enhance Fe-N-C catalyst ORR performance. Electrochemical assessments indicated that SA-Fe-N5 exhibited excellent ORR activity in alkaline solutions, with a half-wave potential and a diffusion-limited current density similar to that of commercial Pt/C. Calculations based on density functional theory indicated that a single copper atom can function as an electron donor, enhancing the electron density at the iron sites. This modification improves the adsorption and desorption energies for intermediates involved in the ORR process, ultimately boosting the ORR performance of the single-atom Fe-N5 catalyst. Moreover, the introduction of the Cu site can be regarded as a catalase single-atom nanozyme (CAT-SAzyme), facilitating the decomposition of the byproduct H2O2 to H2O and thereby enhancing the anti-Fenton activity during the ORR process. Notably, as a cathode catalyst in a zinc-air battery, SA-Fe-N5 demonstrated an impressive power density of 217.8 mW cm-2 alongside a current density of 257.3 mA cm-2.

Understanding the effects of adduct functionalization on C60 nanocages for the hydrogen evolution reaction

In this work, we use experimental and theoretical techniques to study the origin of the boosted hydrogen evolution reaction (HER) catalytic activity of two pyridyl-pyrrolidine functionalized C60 fullerenes. Notably, the mono-(pyridyl-pyrrolidine) penta-adduct of C60 has exhibited a remarkable HER catalytic activity as a metal-free catalyst, delivering an overpotential (η10) of 75 mV vs. RHE and a very low onset potential of -45 mV vs. RHE. This work addresses fundamental questions about how functionalization on C60 changes the electron density on fullerene cages for high-performance HER electrocatalysis.

Hydrogen Binding Energy Is Insufficient for Describing Hydrogen Evolution on Single-Atom Catalysts

The design principles for metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) in the hydrogen evolution reaction (HER) have been extensively studied. Yet, consensus remains elusive, hindering advancements in hydrogen energy technologies. Although the hydrogen binding energy (ΔGH*) has long been used as a key HER descriptor during the past two decades, originating from the activity volcano of metallic surfaces, its applicability to HER SACs has been met with significant controversy. Herein, we investigate the effects of HO*/O* poisoning and H* coverage on SACs with varied metal centers and coordination environments using pH-dependent surface Pourbaix diagrams at the reversible hydrogen electrode (RHE) scale and microkinetic modeling. Our findings reveal that HO* poisoning, realistic H* adsorption strengths at active metal sites, and the potential HER activity at the coordinating N-sites are crucial factors that should be considered for accurate descriptor development. Experimental validation using a series of M-phthalocyanine/CNT catalysts (M = Co, Ni, Cu) confirms the theoretical predictions, with excellent agreement in exchange current densities and the role of N-sites in Ni/Cu-phthalocyanine/CNT catalysts. This work provides answers to a long-lasting debate on HER descriptors by establishing ΔGH* and ΔGHO* as a combined HER descriptor for SACs, offering new guidelines for catalyst design.

Spin polarization regulation of Fe-N4 by Fe3 atomic clusters for highly active oxygen reduction reaction

The Fe-N4 motif is regarded as a leading non-precious metal catalyst for the oxygen reduction reaction (ORR) with the potential to replace platinum (Pt), yet achieving or surpassing the performance of Pt-based catalysts remains a significant challenge. In this study, we introduce a modification strategy employing homogeneous few-atom Fe3 cluster to regulate the spin polarization of Fe-N4. Experimental research and theoretical calculations show that the incorporation of the Fe3 cluster significantly enhances the adsorption of Fe-N4 motif toward OH ligands, leading to a structural transformation from a square-planar field (Fe-N4) to a square-pyramid field structure (Fe(OH) -N4). This structural transformation reduces the spin polarization of 3dxz, 3dyz, and 3dz2 orbitals of Fe-N4, resulting in a decrease in unpaired electrons within 3d orbitals. As a result, this modulation leads to moderate adsorption/desorption energies of reaction intermediates, thereby facilitating the ORR process. Moreover, the in-situ spectroscopy confirms that the desorption of OH* on Fe3/Fe(OH) -NC motif is more favorable compared to atomic Fe-NC, indicating a lower energy barrier for ORR. Consequently, the Fe3/Fe-NC catalyst demonstrates outstanding ORR performance with a half-wave potential of 0.836 V vs. reversible hydrogen electrode (RHE) in 0.1 mol L-1 HClO4 solution and 0.936 V vs. RHE in 0.1 mol L-1 KOH solution, even surpassing commercial Pt/C catalyst. It also exhibits excellent Zn-air battery efficiency. Our study introduces a novel approach to modulating the electronic structure of single atoms catalysts by leveraging the robust interaction between single atoms and atomic clusters.

Acidic and Alkaline pH Controlled Oxygen Reduction Reaction Pathway over Co-N4C Catalyst

Enhanced oxygen reduction reaction (ORR) kinetics and selectivity are crucial to advance energy technologies like fuel cells and metal-air batteries. Single-atom catalysts (SACs) with M-N4/C structure have been recognized to be highly effective for ORR. However, the lack of a comprehensive understanding of the mechanistic differences in the activity under acidic and alkaline environments is limiting the full potential of the energy devices. Here, a porous SAC is synthesized where a cobalt atom is coordinated with doped nitrogen in a graphene framework (pCo-N4C). The resulting pCo-N4C catalyst demonstrates a direct 4e- ORR process and exhibits kinetics comparable to the state-of-the-art (Pt/C) catalyst. Its higher activity in an acidic electrolyte is attributed to the tuned porosity-induced hydrophobicity. However, the pCo-N4C catalyst displays a difference in ORR activity in 0.1 m HClO4 and 0.1 m KOH, with onset potentials of 0.82 V and 0.91 V versus RHE, respectively. This notable activity difference in acidic and alkaline media is due to the protonation of coordinated nitrogen, restricted proton coupled electron transfer (PCET) at the electrode/electrolyte interface. The effect of pH over the catalytic activity is further verified by Ab-initio molecular dynamics (AIMD) simulations using density functional theory (DFT) calculations.

Construction of PdCu Alloy Decorated on the N-Doped Carbon Aerogel as a Highly Active Electrocatalyst for Enhanced Oxygen Reduction Reaction

Fuel cells/zinc-air cells represent a transformative technology for clean energy conversion, offering substantial environmental benefits and exceptional theoretical efficiency. However, the high cost and limited durability of platinum-based catalysts for the sluggish oxygen reduction reaction (ORR) at the cathode severely restrict their scalability and practical application. To address these critical challenges, this study explores a groundbreaking approach to developing ORR catalysts with enhanced performance and reduced costs. We present a novel Pd3Cu alloy, innovatively modified with N-doped carbon aerogels, synthesized via a simple self-assembly and freeze-drying method. The three-dimensional carbon aerogel-based porous structures provide diffusion channels for oxygen molecules, excellent electrical conductivity, and abundant ORR reaction sites. The Pd3Cu@2NC-20% aerogel exhibits a remarkable enhancement in ORR activity, achieving a half-wave potential of 0.925 V, a limiting current density of 6.12 mA/cm2, and excellent long-term stability. Density functional theory (DFT) calculations reveal that electrons tend to transfer from the Pd atoms to the neighboring *O, leading to an increase in the negative charge around the *O. This, in turn, weakens the interaction between the catalyst surface and the *O and optimizes the elementary steps of the ORR process.

Why Do Weak-Binding M-N-C Single-Atom Catalysts Possess Anomalously High Oxygen Reduction Activity?

Single-atom catalysts (SACs) with metal-nitrogen-carbon (M-N-C) structures are widely recognized as promising candidates in oxygen reduction reactions (ORR). According to the classical Sabatier principle, optimal 3d metal catalysts, such as Fe/Co-N-C, achieve superior catalytic performance due to the moderate binding strength. However, the substantial ORR activity demonstrated by weakly binding M-N-C catalysts such as Ni/Cu-N-C challenges current understandings, emphasizing the need to explore new underlying mechanisms. In this work, we integrated a pH-field coupled microkinetic model with detailed experimental electron state analyses to verify a novel key step in the ORR reaction pathway of weak-binding SACs─the oxygen adsorption at the metal-nitrogen bridge site. This step significantly altered the adsorption scaling relations, electric field responses, and solvation effects, further impacting the key kinetic reaction barrier from HOO* to O* and pH-dependent performance. Synchrotron spectra analysis further provides evidence for the new weak-binding M-N-C model, showing an increase in electron density on the antibonding π orbitals of N atoms in weak-binding M-N-C catalysts and confirming the presence of N-O bonds. These findings redefine the understanding of weak-binding M-N-C catalyst behavior, opening up new perspectives for their application in clean energy.

Divergent Activity Shifts of Tin-Based Catalysts for Electrochemical CO2 Reduction: pH-Dependent Behavior of Single-Atom Versus Polyatomic Structures

Tin (Sn)-based catalysts have been widely studied for electrochemical CO2 reduction reaction (CO2RR) to produce formic acid, but the intricate influence of the structural sensitivity in single-atom Sn (e.g., Sn-N-C) and polyatomic Sn (e.g., SnOx and SnSx; x=1,2) on their pH-dependent performance remains enigmatic. Herein, we integrate large-scale data mining (with >2,300 CO2RR catalysts from available experimental literature during the past decade), ab initio computations, machine learning force field accelerated molecular dynamic simulations, and pH-field coupled modelling to unravel their pH dependence. We reveal a fascinating contrast: the electric field response of the binding strength of *OCHO on Sn-N4-C and polyatomic Sn exhibits opposite behaviors due to their differing dipole moment changes upon *OCHO formation. Such response leads to an intriguing opposite pH-dependent volcano evolution for Sn-N4-C and polyatomic Sn. Subsequent experimental validations of turnover frequency and current density under both neutral and alkaline conditions well aligned with our theoretical predictions. Most importantly, our analysis suggests the necessity of distinct optimization strategies for *OCHO binding energy on different types of Sn-based catalysts.

Well-Defined PtCo@Pt Core-Shell Nanodendrite Electrocatalyst for Highly Durable Oxygen Reduction Reaction

The rational design of efficient electrocatalysts with controllable structure and composition is crucial for enhancing the lifetime and cost-effectiveness of oxygen reduction reaction (ORR). PtCo nanocrystals have gained attention due to their exceptional activity, yet suffer from stability issues in acidic media. Herein, an active and highly stable electrocatalyst is developed, namely 3D Pt7Co3@Pt core-shell nanodendrites (NDs), which are formed through the self-assembly of small Pt nanoparticles (≈6 nm). This unique structure significantly improves the ORR with an enhanced mass activity (MA) of 0.54 A mgPt -1, surpassing that of the commercial Pt/C (com-Pt/C) catalyst by three fold (0.17 A mgPt -1). The well-organized dendritic morphology, along with the Pt-rich shell, contributes significantly to the observed high catalytic activity and superior stability for acidic ORR, which exhibit a loss of 2.1% in MA and, impressively, an increase of 12.0% in specific activity (SA) after an accelerated durability test (ADT) of 40,000 potential-scanning cycles. This work offers insights for improving the design of highly stable Pt-based electrocatalysts for acidic ORR.

Dynamic evolution of metal-nitrogen-codoped carbon catalysts in electrocatalytic reactions

Atomic metal-nitrogen-codoped carbon (M-N-C) catalysts are highly efficient for various electrocatalytic reactions because of their high atomic utilization efficiency. However, the high surface energy of M-N-C catalysts often results in stability issues in electrochemical reactions. Therefore, understanding the stability and dynamic evolution of M-N-C catalysts is crucial for elucidating the active centers and the composition/structure-activity relationship. This review summarizes the factors affecting the durability of atomic catalysts in electrochemical reactions and discusses possible changes in catalysts during these electrochemical processes. Finally, advanced characterization techniques are described, with a focus on tracking the dynamic evolution of M-N-C catalysts during electrocatalysis. This review offers insights into the rational optimization of M-N-C electrocatalysts and provides a framework for linking their composition and structure with their catalytic activity in future research.

Interfacial Water Orientation in Neutral Oxygen Catalysis for Reversible Ampere-Scale Zinc-Air Batteries

The neutral oxygen catalysis is an electrochemical reaction of the utmost importance in energy generation, storage application, and chemical synthesis. However, the restricted availability of protons poses a challenge to achieving kinetically favorable oxygen catalytic reactions. Here, we alter the interfacial water orientation by adjusting the Brønsted acidity at the catalyst surface, to break the proton transfer limitation of neutral oxygen electrocatalysis. An unexpected role of water molecules in improving the activity of neutral oxygen catalysis is revealed, namely, increasing the H-down configuration water in electric double layers rather than merely affecting the energy barriers for reaction limiting steps. The proposed porous nanofibers with atomically dispersed MnN3 exhibit record-breaking activity (EORR@1/2/EOER@10 mA = 0.85/1.65 V vs. RHE) and reversibility (2500 h), outperforming all previously reported neutral catalysts and rivaling conventional alkaline systems. In particular, practical ampere-scale zinc-air batteries (ZABs) stack are constructed with a capacity of 5.93 Ah and can stably operate under 1.0 A and 1.0 Ah conditions, demonstrating broad application prospects. This work provides a novel and feasible perspective for designing neutral oxygen electrocatalysts and reveals the future commercial potential in mobile power supply and large-scale energy storage.

Are transition metal phthalocyanines active for urea synthesis via electrocatalytic coupling of CO2 and N2?

Electrocatalytic coupling of CO2 and N2 to synthesize urea presents a promising approach to address global energy and environmental challenges. Despite the potential, developing an efficient catalyst capable of activating both CO2 and N2 while suppressing side reactions remains a significant challenge. Recent studies have indicated that CuPc and CoPc exhibit notable activity in this process. Herein, we report a theoretical analysis of the catalytic performance of 3d-5d transition metal phthalocyanines (MPcs) in the electrocatalytic urea synthesis reaction. Our findings reveal that MPcs generally exhibit limited activity due to the poor competitiveness of N2 for adsorption sites and the high energy barrier associated with CO-N2 coupling, which hinders their ability to compete with CO reduction and/or N2 reduction pathways. Furthermore, the coupling between CO and NH2* is either insufficient for N2 reduction or is outcompeted by ammonia formation. We propose that enhancing N2 adsorption could facilitate C-N coupling, offering a potential strategy for the design of single-atom catalysts aimed at improving urea synthesis efficiency.

Activating the Mn Single Atomic Center for an Efficient Actual Active Site of the Oxygen Reduction Reaction by Spin-State Regulation

The ligand engineering for single-atom catalysts (SACs) is considered a cutting-edge strategy to tailor their electrocatalytic activity. However, the fundamental reasons underlying the reaction mechanism and the contemplation for which the actual active site for the catalytic reaction depends on the pyrrolic and pyridinic N ligand structure remain to be fully understood. Herein, we first reveal the relationship between the oxygen reduction reaction (ORR) activity and the N ligand structure for the manganese (Mn) single atomic site by the precisely regulated pyrrolic and pyridinic N4 coordination environment. Experimental and theoretical analyses reveal that the long Mn-N distance in Mn-pyrrolic N4 enables a high spin state of the Mn center, which is beneficial to reduce the adsorption strength of oxygen intermediates by the high filling state in antibond orbitals, thereby activating the Mn single atomic site to achieve a half-wave potential of 0.896 V vs RHE with outstanding stability in acidic media. This work provides a new fundamental insight into understanding the ORR catalytic origin of Mn SACs and the rational design strategy of SACs for various electrocatalytic reactions.

Fe/Fe3C particles encapsulated in hollow carbon nanoboxes for high performance zinc-air batteries

Zinc-air batteries are recognized for their environmental friendliness and high energy density; however, the slow kinetics of the oxygen reduction reaction (ORR) at the air electrode hinder their commercial viability. The research focuses on synthesizing cubic hollow carbon structures derived from Metal-Organic Frameworks (MOFs), which enhance catalytic performance through improved conductivity and mass transfer. The resulting Fe/Fe3C/HCNB catalyst demonstrates a half-wave potential of 0.826 V for ORR and achieves a peak power density of 274 mW cm-2 in zinc-air batteries, surpassing commercial Pt/C catalysts. Electrochemical impedance spectroscopy reveals that the hollow structure enhances hydrophilicity and reduces solution resistance, facilitating greater active site engagement in electrochemical reactions. The study concludes that the unique structural features of Fe/Fe3C/HCNB significantly improve discharge performance and stability, positioning it as a promising alternative for zinc-air battery applications.

Refining Metal-Free Carbon Nanoreactors through Electronic and Geometric Comodification for Boosted H2O2 Electrosynthesis toward Efficient Water Decontamination

Hydrogen peroxide (H2O2) electrosynthesis using metal-free carbon materials via the 2e- oxygen reduction pathway has sparked considerable research interest. However, the scalable preparation of carbon electrocatalysts to achieve satisfactory H2O2 yield in acidic media remains a grand challenge. Here, we present the design of a carbon nanoreactor series that integrates precise O/N codoping alongside well-regulated geometric structures targeting high-efficiency electrosynthesis of H2O2. Theoretical computations reveal that strategic N/O codoping facilitates partial electron transfer from C sites to O sites, realizing electronic rearrangement that optimizes C-site adsorption of *OOH. Concurrently, the O-O bond in *OOH is strengthened by charge transfer from antibonding to π-orbitals, stabilizing the O-O bond and preventing its dissociation. The carbon nanoreactor with a hollow bowl geometry also facilitates the mass transport of O2 and H2O2, achieving an H2O2 selectivity of 96% in acidic media. Furthermore, a flow cell integrated with the refined nanoreactor catalyst achieves an impressive H2O2 production rate of 2942.4 mg L-1 h-1, coupled with stable operation of nearly 80 h, surpassing the state-of-the-art metal-free analogs. The feasibility of the electro-synthesized H2O2 is further demonstrated to be highly efficient in wastewater remediation.

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.

Spin-Polarized PdCu-Fe3O4 In-Plane Heterostructures with Tandem Catalytic Mechanism for Oxygen Reduction Catalysis

Alloying has significantly upgraded the oxygen reduction reaction (ORR) of Pd-based catalysts through regulating the thermodynamics of oxygenated intermediates. However, the unsatisfactory activation ability of Pd-based alloys toward O2 molecules limits further improvement of ORR kinetics. Herein, the precise synthesis of nanosheet assemblies of spin-polarized PdCu-Fe3O4 in-plane heterostructures for drastically activating O2 molecules and boosting ORR kinetics is reported. It is demonstrated that the deliberate-engineered in-plane heterostructures not only tailor the d-band center of Pd sites with weakened adsorption of oxygenated intermediates but also endow electrophilic Fe sites with strong ability to activate O2 molecules, which make PdCu-Fe3O4 in-plane heterostructures exhibit the highest ORR specific activity among the state-of-art Pd-based catalysts so far. In situ electrochemical spectroscopy and theoretical investigations reveal a tandem catalytic mechanism on PdCu-Fe3O4─Fe sites that initially activate molecular O2 and generate oxygenated intermediates being transferred to Pd sites to finish the subsequent proton-coupled electron transfer steps.

In Situ Carbon Thermal Reduction to Enrich Sulfur-Vacancy in Nickel Disulfide Cathode for Efficient Synthesizing Hydrogen Peroxide

Transition metal catalysts are widely used in the 2e- ORR due to their cost-effectiveness. However, they often encounter issues related to low activity. Defect engineering are used on developing highly active catalysts, which can effectively modify active sites and promote electron transfer. Here, carbon-coated Ni3S2 (Ni3S2@C), where the additional sulfur vacancies (VS) is prepared induced by the carbon layer is coupled with active nickel sites. Through in situ and ex situ experiments combined with DFT calculations, it is demonstrated that the carbon layer can regulate the quantity of VS in Ni3S2. Materials with a higher concentration of VS exhibit enhanced 2e- ORR activity and higher H2O2 selectivity. In situ Raman spectroscopy confirms that Ni serves as the key active site in this catalyst. DFT calculations indicate that the OOH binding energy (ΔG) decreases with an increase in the number of VS, favoring the protonation of *OOH to generate H2O2. Upon performance testing, the average H2O2 selectivity is 92.3%, with the highest yield reaching up to 3860 mmol gcat-1 h-1. It is noteworthy that Ni3S2@C exhibits high stability, with only a slight decrease in 2e- pathway selectivity after 5000 cycles of ADT.

Inhibiting carbon corrosion of cobalt-nitrogen-carbon materials via Mn sites for highly durable oxygen reduction reaction in acidic media

Cobalt-nitrogen-carbon (CoNxC) materials are regarded as promising low-cost electrocatalysts for the oxygen reduction reaction (ORR). However, their susceptibility to deactivation and poor stability in acidic media limits their practical applications. In this study, we develop cobalt (Co) and manganese (Mn) embedded in nitrogen-doped carbon (CoMnNxC) dual-site catalysts by incorporating Mn into CoNxC and leverage a synergistic dual-catalysis strategy to optimize both activity and stability. The dynamic evolution of *OOH intermediate on the catalyst surface is monitored via in situ Raman spectroscopy, confirming that Mn introduction modulates the reaction pathway. Due to electron transfer from Mn to the Co-Nx center in CoMnNxC, *OOH activation on the surface is enhanced, and the two-electron ORR process is inhibited. Consequently, the CoMnNxC catalyst exhibits excellent ORR activity (E1/2 = 0.76 V vs. reversible hydrogen electrode) and a very low hydrogen peroxide (H2O2) yield (<2.9 %) in acidic electrolyte. Additionally, the dynamic evolution of *OH on the Mn-Nx site confirms that Mn-Nx can serve as a potential catalytic site for the hydrogen peroxide reduction reaction (HPRR), facilitating H2O2 decomposition. Differential electrochemical mass spectrometry (DEMS) demonstrates that this parallel catalytic pathway effectively weaks the oxidative corrosion of H2O2 on the carbon carrier. The results indicate that the negative half-wave potential shift of CoMnNxC catalysts in acidic electrolyte after 10,000 accelerated durability tests (ADT) is only 11 mV. The synergistic dual-catalytic strategy proposed in this work offers a novel approach for designing high-efficiency and stable transition metal-nitrogen-carbon catalysts.

Surface coverage and reconstruction analyses bridge the correlation between structure and activity for electrocatalysis

Electrocatalysis is key to realizing a sustainable future for our society. However, the complex interface between electrocatalysts and electrolytes presents an ongoing challenge in electrocatalysis, hindering the accurate identification of effective/authentic structure-activity relationships and determination of favourable reaction mechanisms. Surface coverage and reconstruction analyses of electrocatalysts are important to address each conjecture and/or conflicting viewpoint on surface-active phases and their corresponding electrocatalytic origin, i.e., so-called structure-activity relationships. In this review, we emphasize the importance of surface states in electrocatalysis experimentally and theoretically, providing guidelines for research practices in discovering promising electrocatalysts. Then, we summarize some recent progress of how surface states determine the adsorption strengths and reaction mechanisms of occurring electrocatalytic reactions, exemplified in the electrochemical oxygen evolution reaction, oxygen reduction reaction, nitrogen reduction reaction, CO2 reduction reaction, CO2 and N2 co-reductions, and hydrogen evolution reaction. Finally, the review proposes deep insights into the in situ study of surface states, their efficient building and the application of surface Pourbaix diagrams. This review will accelerate the development of electrocatalysts and electrocatalysis theory by arousing broad consensus on the significance of surface states.

Carbon-anchoring synthesis of Pt1Ni1@Pt/C core-shell catalysts for stable oxygen reduction reaction

Proton-exchange-membrane fuel cells demand highly efficient catalysts for the oxygen reduction reaction, and core-shell structures are known for maximizing precious metal utilization. Here, we reported a controllable "carbon defect anchoring" strategy to prepare Pt1Ni1@Pt/C core-shell nanoparticles with an average size of ~2.6 nm on an in-situ transformed defective carbon support. The strong Pt-C interaction effectively inhibits nanoparticle migration or aggregation, even after undergoing stability tests over 70,000 potential cycles, resulting in only 1.6% degradation. The stable Pt1Ni1@Pt/C catalysts have high oxygen reduction reaction mass activity and specific activity that reach 1.424 ± 0.019 A/mgPt and 1.554 ± 0.027 mA/cmPt2 at 0.9 V, respectively, attributed to the optimal compressive strain. The experimental results are generally consistent with the theoretical predictions made by our comprehensive microkinetic model which incorporates essential kinetics and thermodynamics of oxygen reduction reaction. The consistent results obtained in our study provide compelling evidence for the high accuracy and reliability of our model. This work highlights the synergy between theory-guided catalyst design and appropriate synthetic methodologies to translate the theory into practice, offering valuable insights for future catalyst development.

Structure-Stability Relation of Single-Atom Catalysts under Operating Conditions of CO2 Reduction

Single-atom catalysts (SACs) have exhibited exceptional atomic efficiency and catalytic performance in various reactions but suffer poor stability. Understanding the structure-stability relation is the prerequisite for stability optimization but has been rarely explored due to complexity of the degradation process and reaction environments. Herein, we successfully established the structure-stability relation of N-doped carbon-supports SACs (MN4 SACs) under working conditions of CO2 reduction, by using advanced constant-potential density functional theory calculations. Systematic mechanism investigation that considered different factors identifies the key role of initial hydrogen adsorption on the coordination N atom in catalytic stability, where the feasibility of the adsorption eventually determines the leaching of the metal atom. On this basis, a simple descriptor consisting of electron number and electronegativity is constructed, realizing accurate and rapid prediction of the stability of SACs. Furthermore, strategies via modifying the local geometric structure to improve the stability without changing the active centers are proposed accordingly, which are supported by related experiments. These findings fill the current void in understanding SAC stability under practical working conditions, potentially advancing the widespread application of SACs in sustainable energy conversion systems.

Dual-Metal-Site Metal-Organic Frameworks for Oxygen Reduction: The Crucial Role of Environmental Species Covering on the Secondary Site

Macrocycle-based dual-metal-site metal-organic frameworks emerge as promising catalysts whose activity can be conveniently manipulated via metal node modification. However, how the metal node affects catalysis remains unclear. Herein, using first-principles calculations, we provide new mechanistic insight into dual-metal-site catalysis, where the recently synthesized M1-CoOAPc materials (M1 = Co, Ni, Cu; OAPc = octaaminophthalocyanine) are adopted for demonstration. The modeling results explain experimental measurements of Ni- and Cu-CoOAPc for facilitating oxygen reduction while highlighting a contradiction between the theoretical and experimental activity of Co-CoOAPc. Remarkably, this contradiction is attributed to the inherent H2O adsorption on Co nodes, which is usually neglected in dual-metal-site studies. We expand M1-CoOAPc with other metal nodes and find that Fe-CoOAPc (involving *H2O on the Fe nodes) exhibits a desirable theoretical half-wave potential of 0.82 V, as revealed from constant-potential and microkinetic modeling. This work improves the understanding of dual-metal-site catalysis by uncovering the impact of environmental species covering on the secondary site.

Diatomic Iron with a Pseudo-Phthalocyanine Coordination Environment for Highly Efficient Oxygen Reduction over 150,000 Cycles

Atomically dispersed Fe-N-C catalysts emerged as promising alternatives to commercial Pt/C for the oxygen reduction reaction. However, the majority of Fe-N-C catalysts showed unsatisfactory activity and durability due to their inferior O-O bond-breaking capability and rapid Fe demetallization. Herein, we create a pseudo-phthalocyanine environment coordinated diatomic iron (Fe2-pPc) catalyst by grafting the core domain of iron phthalocyanine (Fe-Nα-Cα-Nβ) onto defective carbon. In situ characterizations and theoretical calculation confirm that Fe2-pPc follows the fast-kinetic dissociative pathway, whereby Fe2-pPc triggers bridge-mode oxygen adsorption and catalyzes direct O-O radical cleavage. Compared to traditional Fe-N-C and FePc-based catalysts exhibiting superoxo-like oxygen adsorption and an *OOH-involved pathway, Fe2-pPc delivers a superior half-wave potential of 0.92 V. Furthermore, the ultrastrong Nα-Cα bonds in the pPc environment endow the diatomic iron active center with high tolerance for reaction-induced geometric stress, leading to significantly promoted resistance to demetallization. Upon an unprecedented harsh accelerated degradation test of 150,000 cycles, Fe2-pPc experiences negligible Fe loss and an extremely small activity decay of 17 mV, being the most robust candidate among previously reported Fe-N-C catalysts. Zinc-air batteries employing Fe2-pPc exhibit a power density of 255 mW cm-2 and excellent operation stability beyond 440 h. This work brings new insights into the design of atomically precise metallic catalysts.

Selective oxygen reduction reaction: mechanism understanding, catalyst design and practical application

The oxygen reduction reaction (ORR) is a key component for many clean energy technologies and other industrial processes. However, the low selectivity and the sluggish reaction kinetics of ORR catalysts have hampered the energy conversion efficiency and real application of these new technologies mentioned before. Recently, tremendous efforts have been made in mechanism understanding, electrocatalyst development and system design. Here, a comprehensive and critical review is provided to present the recent advances in the field of the electrocatalytic ORR. The two-electron and four-electron transfer catalytic mechanisms and key evaluation parameters of the ORR are discussed first. Then, the up-to-date synthetic strategies and in situ characterization techniques for ORR electrocatalysts are systematically summarized. Lastly, a brief overview of various renewable energy conversion devices and systems involving the ORR, including fuel cells, metal-air batteries, production of hydrogen peroxide and other chemical synthesis processes, along with some challenges and opportunities, is presented.

Advancing electrocatalytic reactions through mapping key intermediates to active sites via descriptors

Descriptors play a crucial role in electrocatalysis as they can provide valuable insights into the electrochemical performance of energy conversion and storage processes. They allow for the understanding of different catalytic activities and enable the prediction of better catalysts without relying on the time-consuming trial-and-error approaches. Hence, this comprehensive review focuses on highlighting the significant advancements in commonly used descriptors for critical electrocatalytic reactions. First, the fundamental reaction processes and key intermediates involved in several electrocatalytic reactions are summarized. Subsequently, three types of descriptors are classified and introduced based on different reactions and catalysts. These include d-band center descriptors, readily accessible intrinsic property descriptors, and spin-related descriptors, all of which contribute to a profound understanding of catalytic behavior. Furthermore, multi-type descriptors that collectively determine the catalytic performance are also summarized. Finally, we discuss the future of descriptors, envisioning their potential to integrate multiple factors, broaden application scopes, and synergize with artificial intelligence for more efficient catalyst design and discovery.

Synergistic dual sites of Zn-Mg on hierarchical porous carbon as an advanced oxygen reduction electrocatalyst for Zn-air batteries

The development of cost-effective and high-performance non-noble metal catalysts for the oxygen reduction reaction (ORR) holds substantial promise for real-world applications. Introducing a secondary metal to design bimetallic sites enables effective modulation of a metal-nitrogen-carbon (M-N-C) catalyst's electronic structure, providing new opportunities for enhancing ORR activity and stability. Here, we successfully synthesized an innovative hierarchical porous carbon material with dual sites of Zn and Mg (Zn/Mg-N-C) using polymeric ionic liquids (PILs) as precursors and SBA-15 as a template through a bottom-up approach. The hierarchical porous structure and optimized Zn-Mg bimetallic catalytic centers enable Zn/Mg-N-C to exhibit a half-wave potential of 0.89 V, excellent stability, and good methanol tolerance in 0.1 M KOH solution. Theoretical calculations indicated that the Zn-Mg bimetallic sites in Zn/Mg-N-C effectively lowered the ORR energy barrier. Furthermore, the Zn-air batteries assembled based on Zn/Mg-N-C demonstrated an outstanding peak power density (298.7 mW cm-2) and superior cycling stability. This work provides a method for designing and synthesizing bimetallic site catalysts for advanced catalysis.

Synthesis of Metal-Nitrogen-Carbon Electrocatalysts with Atomically Regulated Nitrogen-Doped Polycyclic Aromatic Hydrocarbons

Tuning the active site structure of metal-nitrogen-carbon electrocatalysts has recently attracted increasing interest. Herein, we report a bottom-up synthesis strategy in which atomically regulated N-doped polycyclic aromatic hydrocarbons (N-PAHs) of NxC42-x (x = 1, 2, 3, 4) were used as ligands to allow tuning of the active site's structures of M-Nx and establish correlations between the structures and electrocatalytic properties. Based on the synthesis process, detailed characterization, and DFT calculation results, active structures of Nx-Fe1-Nx in Fe1-Nx/RGO catalysts were constructed. The results demonstrated that the extra uncoordinated N atoms around the Fe1-N4 moieties disrupted the π-conjugated NxC42-x ligands, which led to more localized electronic state in the Fe1-N4 moieties and superior catalytic performance. Especially, the Fe1-N4/RGO exhibited optimized performance for ORR with E1/2 increasing by 80 mV and Jk at 0.85 V improved 18 times (compared with Fe1-N1/RGO). This synthesis strategy utilizing N-PAHs holds significant promise for enhancing the controllability of metal-nitrogen-carbon electrocatalyst preparation.

Reversible Hydrogen Electrode (RHE) Scale Dependent Surface Pourbaix Diagram at Different pH

In the analysis of electrocatalysis mechanisms and the design of catalysts, the effect of electrochemistry-induced surface coverage is a critical consideration that should not be overlooked. The surface Pourbaix diagram emerges as a fundamental tool in this context, providing essential insights into the surface coverage of adsorbates generated via electrochemical potential-driven water activation. A classic surface Pourbaix diagram considers the pH effects by correcting the free energy of H+ ions by the concentration-dependent term: -kBT ln(10) × pH, which is independent of the reversible hydrogen electrode (RHE) scale. However, this is sometimes inconsistent with the experimentally observed potential-dependent surface coverage at an RHE scale, especially under high-pH conditions. Here, we derived the pH-dependent surface Pourbaix diagram at an RHE scale by considering the energetics computed by density functional theory with the Bayesian Error Estimation Functional with van der Waals corrections (BEEF-vdW), the electric field effects, the derived adsorption-induced dipole moment and polarizability, and the potential of zero-charge. Using Pt(111) as the typical example, we found that the surface coverage predicted by the proposed RHE-dependent surface Pourbaix diagram can significantly minimize the discrepancy between theory and experimental observations, especially under neutral-alkaline, moderate-potential conditions. This work provides a new methodology and establishes guidelines for the precise analysis of the surface coverage prior to the evaluation of the activity of an electrocatalyst.

Benchmarking pH-field coupled microkinetic modeling against oxygen reduction in large-scale Fe-azaphthalocyanine catalysts

Molecular metal-nitrogen-carbon (M-N-C) catalysts with well-defined structures and metal-coordination environments exhibit distinct structural properties and excellent electrocatalytic performance, notably in the oxygen reduction reaction (ORR) for fuel cells. Metal-doped azaphthalocyanine (AzPc) catalysts, a variant of molecular M-N-Cs, can be structured with unique long stretching functional groups, which make them have a geometry far from a two-dimensional geometry when loaded onto a carbon substrate, similar to a "dancer" on a stage, and this significantly affects their ORR efficiency at different pH levels. However, linking structural properties to performance is challenging, requiring comprehensive microkinetic modeling, substantial computational resources, and a combination of theoretical and experimental validation. Herein, we conducted pH-dependent microkinetic modeling based upon ab initio calculations and electric field-pH coupled simulations to analyze the pH-dependent ORR performance of carbon-supported Fe-AzPcs with varying surrounding functional groups. In particular, this study incorporates large molecular structures with complex long-chain "dancing patterns", each featuring >650 atoms, to analyze their performance in the ORR. Comparison with experimental ORR data shows that pH-field coupled microkinetic modeling closely matches the observed ORR efficiency at various pH levels in Fe-AzPc catalysts. Our results also indicate that assessing charge transfer at the Fe-site, where the Fe atom typically loses around 1.3 electrons, could be a practical approach for screening appropriate surrounding functional groups for the ORR. This study provides a direct benchmarking analysis for the microkinetic model to identify effective M-N-C catalysts for the ORR under various pH conditions.

Confined Co@NCNTs as highly efficient catalysts for activating peroxymonosulfate: free radical and non-radical co-catalytic mechanisms

A series of transition metal (Co, Ni, Fe) nanoparticles were confined in N-doped carbon nanotubes (NCNTs) prepared (Co@NCNTs, Ni@NCNTs, and Fe@NCNTs) by the polymerization method. The structure and composition of catalysts were well characterized. The catalytic activity of catalysts for activating peroxymonosulfate (PMS) was conducted via acid orange 7 (AO7) degradation. Among the catalysts, Co@NCNTs performed the best catalytic activity. Additionally, Co@NCNTs performed good catalytic activity in pH values of 2.39-10.98. Cl- and SO42- played a promoting roles in AO7 degradation. NO3- presented a weak effect on the catalytic performance of Co@NCNTs, while HCO3- and CO32- significantly suppressed the catalytic performance of Co@NCNTs. Both non-radical (1O2 and electron transfer) and free-radical (·OH and SO4·-) pathways were detected in the Co@NCNTs/PMS system. Notably, 1O2 was identified to be the main active specie in this study. The catalytic activity of Co@NCNTs gradually decreased after cycle reuse of Co@NCNTs. Finally, the toxicity of the AO7 degradation solution in the study was evaluated by Chlorella pyrenoidosa.