Efficient Electrocatalyst For Oxygen Reduction Reaction Research Articles

Nitrogen, Fluorine, and Phosphorus Tri‐Doped Porous Carbon with High Electrical Conductivity as an Excellent Metal‐Free Electrocatalyst for Oxygen Reduction Reaction

AbstractToday's oxygen reduction reaction (ORR) depends on the precious metal Pt‐based catalysts, limiting the large commercialization of promising energy conversion technologies such as proton exchange membrane fuel cells (PEMFCs) and metal‐air batteries. Therefore, the rational design of low‐cost and highly efficient electrocatalysts for ORR is strongly desired. Herein, we report a porous carbon doped with nitrogen, fluorine, and phosphorus (N, F, P tri‐doped carbon) prepared via only a heating process using low‐cost glycine, ammonium fluoride, and phytic acid as precursors. X‐ray photoelectron spectroscopy (XPS) analysis revealed the existence of pyridinic‐N and the co‐existence of graphitic‐N and oxidized graphitic‐P, which can be active sites, with semi‐ionic C−F bonds highly boosting ORR activity. Remarkably, the resultant N, F, P tri‐doped carbon exhibited outstanding activity for the ORR with an onset potential comparable to that of commercial 20 wt % Pt/C catalyst, the half‐wave potential superior to that of the Pt/C catalyst, and the electron transfer number close to 4.

Engineering peripheral S-doped atomic Fe-N4 in defect-rich porous carbon nanoshells for durable oxygen reduction reaction and Zn-air batteries

Iron-nitrogen-carbon (Fe-N-C) single-atom materials have emerged as the most promising catalysts for the oxygen reduction reaction (ORR), which yet suffers from inadequate stability arising from the attack of ·OH and HO2· radicals generated by H2O2. In this study, density functional theory (DFT) calculations reveal that the presence of peripheral S atoms can regulate the electronic structure of the Fe center and suppress the formation of H2O2 to enhance the catalyst's durability. Thereafter, atomic Fe-N4 with peripheral S doping is intricately engineered in defect-rich porous carbon nanoshells (Fe-NS-C) through an adsorption-pyrolysis method. The incorporation of S not only optimizes the coordination microenvironment of Fe-N4 but also induces a larger specific surface area and richer defects. The Fe-NS-C catalyst displays remarkable ORR performance, featuring a half-wave potential (E1/2) of 0.90 V. Its low H2O2 yield (below 1.1 %) and excellent stability (10 mV day in E1/2 after 10,000 cycles) further underscore the superiority of Fe-NS-C. When employed as the cathode for Zn-air battery, Fe-NS-C achieves an impressive peak power density of 230.1 mW cm−2 and stable charge/discharge potentials over an extended duration of 150 h. This study presents an effective strategy for engineering efficient and durable single-atom electrocatalysts for ORR and Zn-air batteries.

Modulating Spin of Atomic Manganese Center for High-Performance Oxygen Reduction Reaction.

Single atom catalysts (SACs) are promising non-precious catalysts for oxygen reduction reaction (ORR). Unfortunately, the ORR SACs usually suffer from unsatisfactory activity and in particular poor stability. Herein, we report atomically dispersed manganese (Mn) embedded on nitrogen and sulfur co-doped graphene as an efficient and robust electrocatalyst for ORR in alkaline electrolyte, realizing a half-wave potential (E1/2) of 0.883 V vs. reversible hydrogen electrode (RHE) with negligible activity degradation after 40,000 cyclic voltammetry (CV) cycles in 0.1 M KOH. Introducing sulfur (S) to form Mn-S coordination changes the spin state of single Mn atom from high-spin to low-spin, which effectively optimizes the oxygen intermediates adsorption over the single Mn atomic sites and thus greatly improves the ORR activity.

Modulating Spin of Atomic Manganese Center for High‐Performance Oxygen Reduction Reaction

Single atom catalysts (SACs) are promising non‐precious catalysts for oxygen reduction reaction (ORR). Unfortunately, the ORR SACs usually suffer from unsatisfactory activity and in particular poor stability. Herein, we report atomically dispersed manganese (Mn) embedded on nitrogen and sulfur co‐doped graphene as an efficient and robust electrocatalyst for ORR in alkaline electrolyte, realizing a half‐wave potential (E1/2) of 0.883 V vs. reversible hydrogen electrode (RHE) with negligible activity degradation after 40,000 cyclic voltammetry (CV) cycles in 0.1 M KOH. Introducing sulfur (S) to form Mn‐S coordination changes the spin state of single Mn atom from high‐spin to low‐spin, which effectively optimizes the oxygen intermediates adsorption over the single Mn atomic sites and thus greatly improves the ORR activity.

Designing Highly Efficient Electrocatalyst for ORR and OER Based on Nb2CO2 MXene: The Role of Transition Metals and N-Doping Content.

Finding efficient and stable electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is imperative for advancing zinc-air batteries. Herein, the effect of transition metal anchored on Nb2CO2 with different N content to form TM-Nx-Nb2CO2 on the catalytic activity of ORR and OER is investigated by density functional theory. Among all the designed TM-Nx-Nb2CO2, Pt-N12.50%-Nb2CO2, Pt-N37.50%-Nb2CO2, Pt-N50.00%-Nb2CO2, Pd-N68.75%-Nb2CO2, and Pd-N100%-Nb2CO2 are excellent ORR electrocatalysts with ηORR values of 0.38, 0.36, 0.38, 0.38, and 0.34 V, respectively. Rh-Nb2CO2, Rh-N12.50%-Nb2CO2, Rh-N31.25%-Nb2CO2, Rh-N37.50%-Nb2CO2, Rh-N50.00%-Nb2CO2, Pt-N50.00%-Nb2CO2, Rh-N68.75%-Nb2CO2, and Rh-N81.25%-Nb2CO2 are excellent OER electrocatalysts with ηOER values of 0.33, 0.37, 0.34, 0.36, 0.37, 0.34, 0.38, and 0.33 V, respectively. Notably, Rh-Nb2CO2 and Pt-N50.00%-Nb2CO2 exhibit outstanding ORR and OER bifunctional catalytic activity with potential gap values of 0.80 and 0.72 V, respectively, which are higher than the activities of most reported bifunctional catalysts. Furthermore, electronic structure analysis indicates that the moderate adsorption strength of oxygen-containing intermediates on active centers is crucial for achieving highly active bifunctional catalysts for ORR and OER. This study provides a strategy for the design of novel ORR and OER catalysts using 2D MXene materials.

Designing highly efficient electrocatalyst for ORR and OER of transition metals doped g-C9N7: A density functional theory study

The application of fuel cells and metal-air batteries is limited by the rate of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Thus, the aim of this study is to find an efficient electrocatalyst to accelerate the reaction rate by DFT method. The structures of VIIB, VIII, IB, and IIB groups transition metals doped g-C9N7 as superb bifunctional electrocatalysts for ORR and OER are probed. The results show that Fe-, Pd-, and Au–C9N7 possess the superb ORR performance with the ORR overpotentials values of 0.35, 0.40, and 0.39 V, which gives them better performance than Pt(111). Moreover, Rh-, Pd-, and Pt–C9N7 have better OER performance with the OER overpotentials values of 0.30, 0.57, and 0.43 V, which makes them comparable to RuO2(110) and IrO2(110). Finally, the calculated bifunctional index (BI) results show that Pd–C9N7 can be used as an excellent bifunctional electrocatalyst because it has a minimum BI value of 0.97 V. This research has the potential to provide valuable theoretical direction into the use of M − C9N7 as electrocatalysts for both the ORR and OER.

In-situ construction of ionic liquid salt-derived graphene-like dual-heteroatoms doping engineering as efficient metal-free electrocatalyst for oxygen reduction reaction

Metal-free heteroatom-doped carbon catalysts can effectively eliminate the degradation and pollution problems caused by metal dissolution. However, single heteroatom doping often restricts the regulation of geometric and electronic structures for oxygen reduction reaction (ORR) electrocatalysts. Herein, a dual atomic doping engineering to optimize the electronic structure and local coordination environment of ionic liquid salt-derived graphene-like host material as an efficient metal-free electrocatalyst for ORR. The resultant PNG catalyst with a typical nanosheets morphology with a high specific surface area and unique hierarchical porous structure. Benefiting from the uniform doping of P, N heteroatoms, graphene-like structure, optimized electronic structure and coordination environment, thus-prepared PNG-2 exhibits outstanding ORR activity, which the onset potential and half-potential are negatively shifted by around 70 mV and 46 mV, respectively, as compared to the commercial Pt/C (20 wt.%) for ORR. Furthermore, the PNG-2 exhibits the higher diffusion limiting current density, superior long-term stability as well as excellent resistance to methanol oxidation than Pt/C. Experimental studies and density functional theory (DFT) calculations reveal that the enhanced chemisorption of oxygen species (*OOH, *O and *OH) onto the P and N sites of the PNG catalysts can effectively accelerate the charge transfer kinetics and promote a favorable ORR performance.

CuFe nanoparticles coupled Cu–Nx for enhancing oxygen reduction reaction

For the large-scale development of zinc-air batteries (ZABs), designing efficient, and cost-effective oxygen reduction reaction (ORR) electrocatalysts is crucial yet challenging. Herein, CuFe nanoparticles coupled with Cu–Nx active sites co-anchored on 2D nitrogen–doped porous carbon nanosheets (CuFe@Cu–N–C) were obtained by the pyrolysis of Cu–MOF–74, nitrogen source, and ferric salt. The 2D porous carbon structure and the coupling effect between CuFe nanoparticles and Cu–Nx active sites endowed the CuFe@Cu–N–C electrocatalyst with excellent ORR electrocatalytic performance. Consequently, the CuFe@Cu–N–C electrocatalyst exhibited enhanced ORR activity with a higher half–wave potential than Cu–N–C, surpassing even that of the commercial 20 % Pt/C by 21 mV in alkaline media. Additionally, the CuFe@Cu–N–C electrocatalyst exhibited superior ORR stability to commercial 20 % Pt/C. Primary ZABs assembled with the CuFe@Cu–N–C as the cathode ORR electrocatalyst also showed excellent electrochemical properties. The work shows that the prepared CuFe@Cu–N–C electrocatalyst can serve as an ideal alternative to Pt-based catalysts for ZABs.

Study of the Suitability of Corncob Biochar as Electrocatalyst for Zn–Air Batteries

There has been a recent increasing interest in Zn–air batteries as an alternative to Li-ion batteries. Zn–air batteries possess some significant advantages; however, there are still problems to solve, especially related to the tuning of the properties of the air–cathode which should carry an inexpensive but efficient bifunctional oxygen reduction (ORR) and oxygen evolution (OER) reaction electrocatalyst. Biochar can be an alternative, since it is a material of low cost, it exhibits electric conductivity, and it can be used as support for transition metal ions. Although there is a significant number of publications on biochars, there is a lack of data about biochar from raw biomass rich in hemicellulose, and biochar with a small number of heteroatoms, in order to report the pristine activity of the carbon phase. In this work, activated biochar has been made by using corncobs. The biomass was first dried and minced into small pieces and pyrolyzed. Then, it was mixed with KOH and pyrolyzed for a second time. The final product was characterized by various techniques and its electroactivity as a cathode was determined. Physicochemical characterization revealed that the biochar had a hierarchical pore structure, moderate surface area of 92 m2 g−1, carbon phase with a relatively low sp2/sp3 ratio close to one, and a limited amount of N and S, but a high number of oxygen groups. The graphitization was not complete while the biochar had an ordered structure and contained significant O species. This biochar was used as an electrocatalyst for ORR and OER in Zn–air batteries where it demonstrated a satisfactory performance. More specifically, it reached an open-circuit voltage of about 1.4 V, which was stable over a period of several hours, with a short-circuit current density of 142 mA cm−2 and a maximum power density of 55 mW cm−2. Charge–discharge cycling of the battery was achieved between 1.2 and 2.1 V for a constant current of 10 mA. These data show that corncob biochar demonstrated good performance as an electrocatalyst in Zn–air batteries, despite its low specific surface and low sp2/sp3 ratio, owing to its rich oxygen sites, thus showing that electrocatalysis is a complex phenomenon and can be served by biochars of various origins.

Fullerene-metalloporphyrin co-crystal as efficient oxygen reduction reaction electrocatalyst precursor for Zn-air batteries

Fullerene-metalloporphyrin co-crystal as efficient oxygen reduction reaction electrocatalyst precursor for Zn-air batteries

Elaborate construction of honeycomb-like porous carbon with embedment of N-doped carbon nanotubes as efficient electrocatalyst for zinc-air battery

Devising the synthesis strategy and uncovering the evolution mechanism of carbon-based materials are critical to the development of efficient oxygen reduction reaction (ORR) electrocatalysts for renewable energy storage systems. This study reports a hierarchical ORR electrocatalyst, Co@N-CNTs/3DHC, which features ultrafine cobalt nanoparticles embedded in nitrogen-doped carbon nanotubes (Co@N-CNTs) and Co@N-CNTs firmly anchored in the nano-chambers of a three-dimensional honeycomb-like porous carbon (3DHC). The synthesis strategy of Co@N-CNTs/3DHC involves the initial construction of a precursor, followed by controllable pyrolysis step. Based on detailed analysis of both evolved gases and carbonized products of precursors, the evolution mechanism of Co@N-CNTs/3DHC is specifically proposed. Prompted by the electron reconfiguration, extensive reaction interface and fast mass-transfer pathway, the as-synthesized hierarchical Co@N-CNTs/3DHC displays superior ORR activity in alkaline media (0.88 V of half-wave potential) compared to commercial Pt/C. Impressively, zinc-air battery equipped with Co@N-CNTs/3DHC cathode presents outstanding discharge voltage and exceptional stability. This work highlights a unique perspective on the synthesis strategy and growth mechanism of Co@N-CNTs@3DHC toward creation of hierarchical 3D carbon-based composite for advanced electrochemical energy devices.

Multiscale Regulation of Ordered PtCu Intermetallic Electrocatalyst for Highly Durable Oxygen Reduction Reaction.

Transforming the Pt-M alloy into an ordered intermetallic is an effective strategy to improve the electrocatalytic activity and stability toward the oxygen reduction reaction (ORR). However, the synthesis of nanosized intermetallics remains challenging. Herein, we report an efficient ORR electrocatalyst, consisting of a monodisperse nanosized PtCu intermetallic on hollow mesoporous carbon spheres (HMCS). As predicted by theoretical calculations, PtCu intermetallics exhibit beneficial electronic structure, with a low theoretical overpotential of 0.33 V and enhanced Cu stability. Resulting from the multiscale modulation of catalyst structure, the O-PtCu/HMCS catalyst delivers a high mass activity of 2.73 A cm-2Pt at 0.9 V and remarkable stability. Identical location transmission electron microscopy (IL-TEM) investigations demonstrate that the rate of carbon corrosion is alleviated on HMCS, which contributes to the long-term durability. This work provides a promising design strategy for an ORR electrocatalyst, and the IL-TEM investigations offer new perspectives for the performance enhancement mechanism.

Impact of Heat Treatment on the Oxygen Reduction Reaction Performance of PtCo/C(N) Electrocatalyst in PEM Fuel Cell

AbstractThe synthesis of efficient and stable Pt alloy catalysts is a major challenge for the commercialization of PEMFC. Herein, we report a PtCo/C(N) electrocatalyst prepared by Na2CO3‐assisted urea liquid‐phase deposition strategy as an efficient cathodic oxygen reduction reaction electrocatalyst, and test its activity and stability in a rotating disc electrode and a single‐cell. The membrane electrode prepared with PtCo/C(N)‐800 °C catalyst has an output voltage of 0.652 V at 2 A/cm2 and a maximum power density of 1.501 W/cm2, which are 36 mV and 81 W/cm2 higher than those of commercial Pt/C catalyst, respectively. In addition, the mass activity and half‐wave potential of PtCo/C(N)‐800 °C are 2.44 times and 50 mV higher than those of commercial Pt/C, respectively. After 5000 voltammetric cycles, its mass activity and half‐wave potential only lose 35 A/gPt and 9 mV, respectively. XPS results show that the binding energy of Pt 4f is positively shifted relative to Pt/C‐TKK, and the d‐band center of Pt decreases, leading to weak chemical interaction between the oxygen‐containing intermediate and the surface of the electrocatalyst, which is conducive to the improvement of the ORR activity and stability of the catalyst.

High-Density CoSe2 Sites Embedded within 2D Porous N-Doped Carbon for High-Performance Oxygen Reduction Reaction Electrocatalysis.

Designing and fabricating efficient and stable nonprecious metal-based oxygen reduction reaction (ORR) electrocatalysts is a pressing and challenging task for the pursuit of sustainable new energy devices. Herein, porous P-CoSe2@NC electrocatalysts with high-density carbon-coated CoSe2 sites were successfully fabricated based on a pyridyl-porphyrinic metal-organic framework (Co-TPyP MOF) via a molten salt-assisted synthesis method. The hierarchical pore and N-doping carbon substrate of P-CoSe2@NC promotes mass transfer and electron-transfer efficiency, which is beneficial to maximize CoSe2 site utilization. Well-designed P-CoSe2@NC exhibits efficient ORR catalytic activity with a high half-wave potential of 0.863 V and excellent catalytic stability. Meanwhile, rechargeable aqueous primary/quasi-solid-state ZABs based on a P-CoSe2@NC air cathode show a high peak power density and exceptional operating stability, catering to the demands of practical applications. The qualified performance and structure stability of the electrocatalytic system may be mainly attributed to the protection of the CoSe2 nanoparticle by the coated carbon layer. Given the rational design of the structure and the component of the electrocatalyst with enhanced ORR activity, we believe that this work has provided a reliable pathway to the development of high-performance transition-metal chalcogenides for energy-storage and -conversion devices.

Highly dispersed Zn-N, S co-doped carbon for highly efficient electrocatalytic oxygen reduction

Supportive atomic Zn electrocatalysts exhibit excellent stability for oxygen reduction reaction (ORR) due to their fully occupied d orbitals of Zn, but their catalytic activity is not satisfactory. Whereas heteroatom doping can anchor the Zn atoms to achieve higher Zn loading; and additionally serve as auxiliary sites to enhance the catalytic activity of Zn sites. Herein, we present a method for the synthesis of N, S co-doped carbon with highly dispersed Zn (named Zn-NSC). The ORR activity of the Zn-NSC catalysts is optimized by adjusting the pyrolysis temperature and the atomic structure of the dopant molecules. The optimized Zn-NSC demonstrates significantly enhanced ORR electrocatalytic activity, this is attributable to its highly co-doping of N and S heteroatoms that are adjacent to Zn atoms. Specifically, the Zn-NSC3 catalyst exhibits remarkable ORR characteristics with excellent onset voltage (0.97 V vs RHE) and half-wave potential (0.87 V vs RHE), surpassing those of Pt/C (0.97 V and 0.85 V, respectively). Additionally, the Zn-NSC3 can be utilized as efficient cathode for Al-air batteries, achieving a high power density of 119.4 mW cm−2 and satisfactory discharge stability. This work proposes an approach for synthesizing low-cost and efficient ORR electrocatalysts for Al-air battery applications.

General Pyrolysis for High-Loading Transition Metal Single Atoms on 2D-Nitro-Oxygeneous Carbon as Efficient ORR Electrocatalysts.

Single-atom catalysts (SACs) possess the potential to involve the merits of both homogeneous and heterogeneous catalysts altogether and thus have gained considerable attention. However, the large-scale synthesis of SACs with rich isolate-metal sites by simple and low-cost strategies has remained challenging. In this work, we report a facile one-step pyrolysis that automatically produces SACs with high metal loading (5.2-15.9 wt %) supported on two-dimensional nitro-oxygenated carbon (M1-2D-NOC) without using any solvents and sacrificial templates. The method is also generic to various transition metals and can be scaled up to several grams based on the capacity of the containers and furnaces. The high density of active sites with N/O coordination geometry endows them with impressive catalytic activities and stability, as demonstrated in the oxygen reduction reaction (ORR). For example, Fe1-2D-NOC exhibits an onset potential of 0.985 V vs RHE, a half-wave potential of 0.826 V, and a Tafel slope of -40.860 mV/dec. Combining the theoretical and experimental studies, the high ORR activity could be attributed its unique FeO-N3O structure, which facilitates effective charge transfer between the surface and the intermediates along the reaction, and uniform dispersion of this active site on thin 2D nanocarbon supports that maximize the exposure to the reactants.

Asymmetric Atomic Tin Catalysts with Tailored p‐Orbital Electron Structure for Ultra‐Efficient Oxygen Reduction

AbstractAtomically dispersed transition metal–nitrogen–carbon (M–N–C) catalysts guide by the d‐band center theory have been extensively studied for oxygen reduction reaction (ORR) in various energy conversion and storage processes. However, asymmetric p‐block metal single‐atom catalysts (SACs) toward ORR have rarely been reported, and the origin of their catalytic activity is still unclear. Here, an asymmetric N, O coordinated Sn SAC is developed as an efficient ORR electrocatalyst. Remarkably, the optimized Sn SAC (e.g., Sn–N/O–C) exhibit outstanding ORR performance with a half‐wave potential of 0.910 V in alkaline media, outperforming most state‐of‐the‐art ORR catalysts. More importantly, the Sn–N/O–C possesses a long‐term durability in both alkaline and acidic electrolytes. Besides, Zn–air batteries based on the Sn–N/O–C cathode also show a higher energy density (254 mW cm‐2) than that of their reported M–N–C counterparts. Theoretical calculations suggest that the asymmetric N, O coordinated atomic Sn sites have a stronger binding interaction with O2 and better charge transfer ability compared with the symmetric SnN4 sites, thereby facilitating the ORR process. This work provides a nitrogen‐, oxygen‐coordinated engineering strategy for the rational design of highly active and durable carbon‐based catalysts with atomic p‐block metal sites for ORR and beyond.

Exploration of metal-free 2D electrocatalysts toward the oxygen electroreduction.

The advancement of economical and readily available electrocatalysts for the oxygen reduction reaction (ORR) holds paramount importance in the advancement of fuel cells and metal-air batteries. Recently, 2D non-metallic materials have obtained substantial attention as viable alternatives for ORR catalysts due to their manifold advantages, encompassing low cost, ample availability, substantial surface-to-volume ratio, high conductivity, exceptional durability, and competitive activity. The augmented ORR performances observed in metal-free 2D materials typically arise from heteroatom doping, defects, or the formation of heterostructures. Here, the authors delve into the realm of electrocatalysts for the ORR, pivoting around metal-free 2D materials. Initially, the merits of metal-free 2D materials are explored and the reaction mechanism of the ORR is dissected. Subsequently, a comprehensive survey of diverse metal-free 2D materials is presented, tracing their evolutionary journey from fundamental concepts to pragmatic applications in the context of ORR. Substantial importance is given on the exploration of various strategies for enhancing metal-free 2D materials and assessing their impact on inherent material performance, including electronic properties. Finally, the challenges and future prospects that lie ahead for metal-free 2D materials are underscored, as they aspire to serve as efficient ORR electrocatalysts.

Computational Assistance in the Design of Efficient Oxygen Reduction Reaction Electrocatalysts

AbstractRecently, theoretical calculations have played an irreplaceable role in the discovery of electrocatalysts as a relatively time‐efficient, cost‐effective, and predictable method. More importantly, theoretical calculations can help screen out the best active reaction sites and modulate them accordingly, which is beneficial for the development of efficient catalysts. In this concept, we focus on the important role of theoretical calculations in determining catalytic active sites and understanding reaction mechanisms, emphasizing its importance in assisting the design and synthesis of efficient oxygen reduction reaction catalysts. Finally, we provide an outlook on the challenges and future development trend in this field.

Ordered mesoporous carbon with binary CoFe atomic species for highly efficient oxygen reduction electrocatalysis.

The exploration of powerful, efficient and precious metal-free electrocatalysts for facilitating the sluggish kinetics of the oxygen reduction reaction (ORR) is a crucial endeavor in the development and application of energy conversion and storage devices. Herein, we have rationally designed and synthesized bimetallic CoFe species consisting of CoFe nanoparticles and atomically dispersed dual atoms anchored on an ordered mesoporous carbon matrix (CoFe/NC) as highly efficient ORR electrocatalysts. The pyrolyzation temperature for CoFe/NC plays a vital role in regulating the morphology and composition of both the carbon matrix and CoFe species. The optimized CoFe/NC-750 exhibits a favorable ORR performance in 0.1 M KOH with a high half-wave potential (E1/2) of 0.87 V vs. RHE, excellent tolerance to methanol and remarkable durability (no obvious decrease in E1/2 value after 3000 cycles), all of which are superior to the performance of commercial Pt/C. Experimental measurements and density functional theory (DFT) calculations reveal that the improved ORR performance of CoFe/NC-750 is mainly attributed to the electronic structure of atomically dispersed Fe active sites modulated by the surrounding CoFe alloys and Co single atoms, which accelerates the dissociation and reduction of intermediate OH* species and promotes the ORR process.