Emerging Carbon-Based Catalysts for the Oxygen Reduction Reaction: Insights into Mechanisms and Applications

Abstract

The oxygen reduction reaction (ORR), as a key electrode process in fuel cells and metal-air batteries, plays a pivotal role in advancing clean energy technologies. However, the slow kinetics and high overpotential of the ORR significantly limit the efficiency of these energy devices. Therefore, the development of efficient, stable, and cost-effective ORR catalysts has become a central focus of current research. Carbon-based catalysts, with their excellent conductivity, chemical stability, and tunable structural features, have emerged as promising alternatives to traditional precious metal catalysts. Nevertheless, challenges remain in the design of active sites, the tuning of electronic structures, and the large-scale synthesis of carbon-based catalysts. This review systematically introduces the fundamental mechanisms and key factors influencing the ORR, providing an analysis of the critical variables that affect catalyst performance. Furthermore, it summarizes several common methods for synthesizing carbon-based catalysts, including pyrolysis, deposition, and ball milling. Following this, the review categorizes and discusses the latest advancements in metal-free carbon-based catalysts, single-atom and dual-atom catalysts, as well as metal-based nanoparticle catalysts, with a particular focus on their mechanisms for enhancing the ORR performance. Finally, the current state of research on carbon-based ORR catalysts is summarized, and future development directions are proposed, emphasizing the optimization of active sites, improvements in catalyst stability, and potential strategies for large-scale applications.

1. Introduction

With the increasing environmental pollution problems and the depletion of fossil fuels, the daily living environment of people is facing significant challenges, so it is urgent to seek green and clean energy to alleviate this situation [1,2,3,4]. Fuel cells can directly convert the chemical energy of fuels into electrical energy through electrochemical redox reactions, without being limited by the Carnot cycle, as they do not require a combustion process [5]. This results in very high energy efficiency. Zinc–air batteries (ZABs), with their high specific energy, large capacity, stable discharge performance, and low cost, have attracted widespread attention [6,7]. The Oxygen reduction reaction (ORR) reaction occurs at the cathode of a ZABs, undergoes a two-electron (2 e⁻) path to first generate intermediate products such as HO₂⁻ and then reduces to water, or undergoes a four-electron (4 e⁻) path to directly generate OH⁻ [8,9,10]. ORR takes place on the cathode, but the large discharge overpotential results in low energy utilization of the ZABs, which limits its application in real life [11,12]. Therefore, an efficient ORR electrocatalyst is utilized to reduce the energy barrier of the ORR, reduce the polarization problem in the reaction process, and improve the current density in the discharge process.

In order to accelerate the dynamic process of the ORR, it is most important to develop efficient oxygen reduction catalysts. Among many metal catalysts, precious metals such as Pt, Pd, and Ir show excellent performance and play an important role in the ORR process [13,14,15]. However, the lower storage capacity, higher energy cost and lower stability of precious metals hinder their application in actual production [16,17]. At present, researchers mainly use two strategies to solve this problem: (1) reduce the size of the noble metal catalyst and increase the atomic utilization to reduce the amount [18,19]; and (2) develop other non-precious metal catalysts to replace the status of precious metals in oxygen reduction catalysis [20,21,22]. In addition, doping platinum atoms into the structure of ORR electrocatalysts or creating nanoparticles with unique defect architectures can significantly enhance their catalytic activity. Platinum doping can improve the electronic properties of the catalyst, facilitating efficient oxygen reduction reactions [23]. Additionally, the introduction of special defects, such as vacancies or edge sites, within nanoparticles, creates active sites that can further optimize the catalytic process by lowering activation energies or enhancing charge transfer [24]. These modifications contribute to improved ORR performance and stability by promoting more favorable interaction with reactants. Carbon materials are widely used in electrocatalytic processes due to their advantages such as abundant raw materials, low price, relative stability and good electrical conductivity. However, the pure carbon material has a stable structure, contains fewer ORR active sites, and exhibits lower catalytic activity.

Recent studies have shown that breaking the complete π bond structure in the carbon matrix can effectively improve the catalytic performance of the carbon material [25,26]. Therefore, researchers modify the electronic structure of carbon materials by manufacturing defects, doping heteroatoms and changing the composition to improve the electrocatalytic properties of carbon materials [27]. When the complete sp² structure is broken, undoped pure carbon materials can also exhibit excellent oxygen reduction activity, so that edge-plane pyrolytic graphite exhibits higher electrode kinetics than base-plane pyrolytic graphite [28,29]. The concept of heteroatom doping refers to replacing some carbon atoms in the carbon matrix with other doping atoms, thereby modifying the carbon material to change its surface charge and electrochemical properties [30,31,32]. In recent years, studies have shown that N-doped carbon materials such as N-doped carbon nanotube, N-doped graphene, N-doped porous carbon, etc. exhibited better electrocatalytic efficiency and stability than Pt/C in the ORR [33]. In order to obtain better catalytic performance, other non-nitrogen atoms (S/P/B/halogens) are also employed to modify the carbon material [34,35,36,37]. Since the Co-N-C catalyst prepared by Co phthalocyanine showed excellent catalytic activity in 1964, the M-N-C structure is considered to be one of the most effective ORR catalysts to replace Pt/C [38]. M-Nx active sites usually exhibit high four-electron (4 e⁻) selectivity and extremely high catalytic activity. In addition to single metal atom-doped carbon materials, bimetallic catalysts have also shown significant improvements in oxygen reduction performance, with the combination of two metals creating synergistic effects that enhance catalytic efficiency [39]. Furthermore, metal oxides, sulfides, and phosphides also exhibit higher oxygen reduction activity, adding more options for developing efficient electrocatalysts [40,41,42].

In this paper, the research progress of an ORR catalyst based on carbon is reviewed. Subsequently, the effects of the existence of defects, non-metallic heteroatom doping and single metal-atom doping on the catalytic activity of carbon matrix materials are briefly summarized, as shown in Figure 1. Finally, based on the current research progress and results, we summarize the problems in the ORR, and put forward simple views and opinions on the future development direction.

2. The Mechanisms and Influencing Factors for ORR

2.1. Mechanisms for ORR

Exploring the ORR is of great significance to the study of metal–air batteries. Normally, oxygen in the air diffuses to the gas–liquid interface of the metal–air battery and reaches the surface of the catalyst, where ORR occurs [43]. For different types of catalysts, there are two types of adsorption for oxygen molecules: bidentate adsorption (both oxygen atoms of the oxygen molecule are coordinated with the catalyst) and end-on adsorption (an oxygen atom in an oxygen molecule coordinates with the catalyst and is perpendicular to the surface). In these two cases, oxygen molecules are partially reduced and completely reduced, which involves a slow two-electron process and an efficient four-electron process, respectively [44].

Take the alkaline environment as an example, as shown in Figure 2. In the process of four electrons, oxygen molecules are completely reduced. Corresponding to the process K₁ in the figure, oxygen molecules react directly with water to form OH⁻ [45,46]. The electrode reaction equation is

O₂ + 2 H₂O + 4 e⁻→4 OH⁻ (0.401 V vs. SHE)

(1)

In the two-electron process, oxygen molecules are partially reduced through two consecutive two-electron reactions [47,48]. First, O₂ reacts with water to produce hydrogen peroxide ions, corresponding to the K₂ process in the figure. Subsequently, hydrogen peroxide ions continue to react with water to form OH⁻, corresponding to the K₃ process in the figure. In the K₂K₃ process, two electrons participate in the reaction, and 2 + 2 electron transfer is realized to generate OH⁻. The electrode reaction equation is

O₂ + 2 H₂O + 2 e⁻→3 OH⁻ (−0.076 V vs. SHE)

(2)

HO₂⁻ + H₂O + 2 e⁻→3 OH⁻ (−0.878 V vs. SHE)

(3)

However, hydrogen peroxide ions obtained in the K₂ process can be directly dispersed into the solution, corresponding to the K₅ process in the figure, or disproportionated with water to produce O₂, corresponding to the K₄ process in the figure [49]. The electrode reaction formula is as follows:

2 HO₂⁻→2 HO⁻ + O₂

In 2004, Nørskov et al. calculated the specific ORR steps of O₂ molecules in the alkaline environment through density functional theory, as follows [50]:

O₂ + *→O₂*

O₂* + H₂O + e⁻→HOO* + OH⁻

HOO* + e⁻→O* + OH⁻

O* + H₂O + e⁻→HO* + OH⁻

HO* + e⁻→OH⁻ + *

where * represents the ORR active center of the catalyst. It can be seen from the reaction formula that the reaction is more complex and there are many intermediate products. For conventional catalysts, the efficient four-electron reaction pathway and continuous two-electron reaction pathway are carried out simultaneously. At present, the continuous two-electron reaction is considered an excellent method to produce hydrogen peroxide [51,52]. However, in the case of oxygen reduction, where two-electron reactions reduce energy conversion efficiency and produce intermediates that are harmful to catalysis, researchers choose to reduce oxygen molecules by a complete four-electron reaction.

2.2. Influencing Factors for ORR

The structure and surface-electron distribution of a material have great influence on its electrochemical properties. By modifying and adjusting the electron distribution on the surface, conventional carbon-based materials can also obtain oxygen reduction activity. Graphene has been widely applied in the preparation of ORR catalysts, due to its high specific surface area and good electrical conductivity [53]. In addition, other carbon-based precursors with layered porous structure, rich surface defects and doping of different heteroatoms are also the focus of research [54].

Excellent electrical conductivity can accelerate the transfer of electrons, which is one of the most important factors of excellent electrocatalyst materials. As one of the metal-organic frameworks (MOFs), ZIF-8 has a high specific surface area and abundant pores, and is widely used as the base material for supporting the single-atomic method [55,56]. However, due to its extremely low conductivity, ZIF-8 needs to be carbonized at high temperature, which greatly limits its application in the field of electrocatalysis.

High specific surface area and layered porous structure can improve the contact area with oxygen molecules, enable oxygen to contact more active sites, and improve catalytic efficiency [57]. A large number of holes or defects can lead to excessive contact resistance and reduce the conductivity of the matrix. Therefore, in order to obtain excellent catalytic performance of the substrate, it is necessary to regulate the content of holes and defects to maintain good conductivity. For example, high-temperature treatment is frequently used to improve the electrical conductivity of the substrate. However, the matrix materials with different defect contents can be obtained by pyrolysis at different temperatures [58]. Therefore, the optimal conditions can be selected by controlling the sintering temperature so that the catalyst has high electrical conductivity and appropriate defect content to regulate the electrochemical performance of the catalyst.

Defects in carbon matrix materials are considered to be highly efficient oxygen-reduction active centers [59,60]. The existence of defects can change the uniform distribution of charge on the surface of carbon materials, improve the charge density, and make oxygen molecules more easily adsorbed on the surface. Heteroatom doping can also play the same role. The introduction of N atoms can redistribute the charge on the surface of carbon materials [61,62]. At the same time, after the combination of N and metal atoms, a M-N_X active site is formed, which shows very high oxygen reduction reactivity in theory and experiment and becomes one of the most popular oxygen-reduction catalysts at present.

The durability of an ORR electrocatalyst depends on several factors related to the stability of both the active sites and the overall material structure. Stability of active sites: over time, many electrocatalysts suffer from active-site degradation due to sintering, leaching, or oxidation. Single-atom catalysts, for example, require a stable bonding environment to prevent atom migration or aggregation [63]. The incorporation of metal atoms into a robust carbon support (e.g., Zr or Hf atoms into the magnesium lattice) can provide increased stability, helping to maintain active sites over extended periods of use [64]. Carbon support stability: the carbon matrix plays an essential role in stabilizing active sites. However, under harsh operating conditions, carbon supports can undergo oxidation or degradation [65]. Therefore, the use of highly stable carbon supports, such as nitrogen-doped or graphitic carbon, can enhance the catalyst’s durability.

For carbon matrix materials, it is an important measure to improve the specific surface area and porosity of the matrix as much as possible on the premise of good conductivity. In addition, defects and the introduction of heteroatoms can improve the density of the active center of oxygen reduction and increase the catalytic performance of the material effectively.

2.3. Key Metrics for ORR

In assessing and comparing the performance of electrocatalysts for the ORR, several critical metrics offer valuable insights into efficiency, reaction kinetics, and stability. The following outlines these key parameters and highlights their role as benchmarks for evaluating electrocatalyst performance.

The onset potential is the potential at which the ORR current begins to rise noticeably, marking the start of the ORR process. A higher onset potential indicates a more favorable catalytic activity, as the electrocatalyst requires less energy to initiate the reaction. This metric is essential for comparing catalysts’ efficiencies, where a positive shift in onset potential generally correlates with a better ORR catalyst.

The half-wave potential is the potential at which the current density reaches half of its maximum value. This parameter is a critical indicator of catalytic activity in ORR, often serving as a more reliable benchmark than onset potential due to its representation of catalytic performance throughout the reaction. Higher E_1/2 values are generally associated with more effective ORR electrocatalysts.

Current density reflects the rate of the ORR at a given potential, typically measured at the peak or in a specified potential range. High current densities indicate that the electrocatalyst supports a higher reaction rate, demonstrating enhanced catalytic activity. When comparing catalysts, current density at specific potentials helps determine the capacity of the catalyst to facilitate the ORR under working conditions.

The Tafel slope, derived from Tafel plots (logarithmic plots of current density vs. overpotential), provides insights into the reaction kinetics. Lower Tafel slopes indicate faster ORR kinetics and a lower overpotential required to drive the reaction. This metric allows researchers to infer the reaction mechanisms and compare the kinetic efficiency of different catalysts.

The electron transfer number, often determined using the rotating ring-disk electrode method, indicates the number of electrons transferred per oxygen molecule during the ORR. Ideally, efficient ORR follows a four-electron transfer pathway, reducing O₂ directly to H₂O. Electron transfer numbers close to four reflect a more selective and efficient catalytic process, minimizing byproducts like hydrogen peroxide, which are less desirable.

Stability refers to the catalyst’s ability to maintain its performance over prolonged usage, and is often tested through continuous cycling or chronoamperometry at a fixed potential. High stability is crucial for practical applications, as electrocatalysts in fuel cells or metal–air batteries must operate over extended periods without significant performance degradation. Durability comparisons are critical for evaluating the practical applicability of different catalysts.

Each of these metrics serves a distinct purpose in assessing ORR electrocatalysts. These parameters combined provide a comprehensive assessment, helping researchers compare and select catalysts with optimal performance characteristics for ORR applications.

3. Synthesis Methods of Carbon-Based Catalysts

The synthesis method plays a crucial role in determining the physicochemical properties and catalytic performance of electrocatalysts. During the synthesis process, the electrochemical performance of the material can be further modulated by adjusting heteroatoms, defects or vacancies, porous structures, and nanomorphology. This section discusses several common synthesis methods for carbon-based catalysts (Figure 3), including pyrolysis, chemical vapor deposition (CVD), atomic layer deposition (ALD), and ball milling. The performance of catalysts synthesized by different methods is summarized in Table 1.

3.1. Pyrolysis

Pyrolysis refers to the thermal decomposition of organic materials in the absence of oxygen or in a low-oxygen environment, typically at high temperatures (Figure 3a). The pyrolysis process involves decomposing a carbon precursor at high temperatures under a gaseous atmosphere (such as Ar and NH₃), where low-melting-point substances volatilize, leaving behind a carbon-framework structure [66,67]. During pyrolysis, the doping of target atoms can be achieved by mixing them with the carbon precursor and performing heat treatment in a specific atmosphere. In the synthesis of electrocatalysts, pyrolysis is often employed to transform metal precursors or nitrogen-containing compounds into catalytically active carbon structures, or to incorporate metal/non-metal elements into carbon supports to enhance catalytic performance. Overall, the preparation of carbon-based catalysts via pyrolysis is simple, has a short processing time, and is easy to scale up. Moreover, by selecting different carbon precursors and using various templates, the structure and morphology of the resulting carbon-based catalysts can be tailored, thereby adjusting their electrochemical performance.

Carbon-based catalysts can be derived from various precursors, such as metal–organic frameworks (MOFs), biomass materials, polymers, and more. The choice of precursor plays a crucial role in determining the structure and catalytic activity of carbon-based electrocatalysts. Different precursors affect the formation of the carbon matrix, surface properties, and active sites. For example, conductive polymers like polyaniline and polypyrrole, when pyrolyzed, yield high surface-area and porous carbon structures, enhancing catalytic performance. Biomass-derived precursors offer high porosity, which improves the diffusion of reactants, increasing catalyst activity [68,69,70]. Additionally, the incorporation of dopants such as nitrogen, sulfur, or oxygen from the precursor influences the electronic properties of the carbon material, promoting higher catalytic efficiency [71]. Nitrogen doping, for instance, introduces active sites like pyridinic-N, which enhance ORR activity. The precursor’s chemical stability and its behavior during pyrolysis also impact the durability and long-term performance of the catalyst. Among these materials, MOFs have garnered significant attention in recent years, due to their unique porous structures and diverse compositions [72]. Wang and co-workers successfully anchored atomically dispersed iron atoms and neighboring iron clusters onto N-doped layered porous hollow carbon, named FeSA/AC@HNC, using a bimetallic ligand zinc-based ZIF-8 [73]. The microstructure of the bimetallic zinc zeolitic imidazolate framework (ZIF-8-AT) evolved into a microscale spherical structure composed of regularly ordered solid nanosheets. Upon pyrolysis of the synthesized ZIF-8-AT, zinc ions volatilized at high temperatures, resulting in hollow nitrogen-doped carbon (HNC). This hollow structure reduces the diffusion distance of substances between the solid and liquid phases, accelerates reaction kinetics, and exposes previously buried active sites within the carbon substrate to the electrolyte, increasing the density of accessible active sites. The FeSA/AC@HNC catalyst was formed by immobilizing iron clusters and single atoms using abundant nitrogen sources and micropores. In addition, at high temperatures, magnesium can reduce CO₂ to produce oxygen-deficient carbon, which can be used as a carbon precursor. Peng and co-workers employed a two-step ball milling and sintering method, where during the ball milling process, recovered magnesium metal waste was mixed with a Zr metal precursor, allowing Zr to dissolve uniformly in the Mg matrix, laying the foundation for the preparation of single-atom catalysts [74]. Subsequently, during the sintering process under a CO₂/NH₃ mixed atmosphere, Mg reduces CO₂ at high temperatures, forming oxygen-deficient carbon materials, while NH₃ serves as a nitrogen source, creating defects and/or vacancies on the carbon substrate during pyrolysis. These defects and vacancies serve as excellent anchoring sites, or directly encapsulate atomically dispersed Zr on N/O-doped graphene. After acid washing to remove by-products, an axially oxygen-coordinated ZrN₄O structure catalyst was obtained. This method not only achieves the recycling of magnesium metal waste, but also successfully prepares a carbon-supported axially oxygen-coordinated ZrN₄O structure catalyst.

The evolution of active-site structures in ORR electrocatalysts during pyrolysis plays a crucial role in determining their catalytic performance. Recent studies highlight the transformation of Fe species in Fe/N/C systems, where the formation of Fe–N₄ sites is influenced by the size of FeO_x nanoparticles [75]. Smaller nanoparticles facilitate Fe–N₄ formation, while larger ones hinder it, leading to the reconstruction of carbon layers. As the temperature increases, oxygen binds to the Fe sites as a fifth ligand [76]. Additionally, the pyrolysis process can be divided into stages, where the formation of amorphous carbon and the dispersion of metal species are followed by graphitization and the formation of metal–nitrogen moieties [77]. Re-pyrolysis has been found to optimize the distribution of Fe-N_x sites, enhancing electrocatalytic activity without forming undesirable crystalline phases [78].

3.2. Deposition

Deposition techniques, including chemical vapor deposition (CVD) and atomic layer deposition (ALD), are powerful methods for creating high-quality carbon-based catalysts with precise control over composition and structure (Figure 3b,c). CVD involves the decomposition of volatile precursors on a substrate to form thin films of carbon or carbon-containing compounds. This method allows for the incorporation of various dopants and the formation of uniform, well-defined structures. ALD, on the other hand, is a layer-by-layer deposition technique that provides atomic-level precision in controlling the thickness and composition of the deposited films. Both methods are highly versatile and can be used to create catalysts with tailored properties for specific electrochemical applications.

In practical production, the reaction time of CVD is crucial. By adjusting the reaction time, one can control the film thickness, defect density, and uniformity of film growth [79,80]. Proper reaction time also contributes to energy savings and improved production efficiency. Additionally, controlling the temperature of the CVD reaction is key to achieving the desired deposition rate and film quality. The temperature directly affects the diffusion rate of reactant molecules to the substrate surface and the quality of the deposited film. Higher temperatures generally lead to faster growth rates, but an excessively rapid reaction may result in poor film quality. Due to the ordered nature of the CVD process, the slowest reaction step can significantly impact the entire deposition process. Xu and co-workers found that the evolution pathway of Mn-N₄ sites is similar to that of Fe-N₄ and Co-N₄ sites, i.e., as the pyrolysis temperature increases, the Mn precursor transforms into MnO_x, and then into Mn-N₄ [79]. However, the transition temperature from MnO_x to Mn-N₄ is much higher than that for Fe-N₄ and Co-N₄ sites, hindering the selective formation of Mn-N₄ sites. This discovery led to the development of a novel Mn-N-C catalyst synthesized via the CVD method, where Mn-N₄ sites are formed through demetallation without involving MnO_x intermediates, thereby suppressing the formation of MnO_x sites. The ORR activity of the Mn-N-C catalyst synthesized by the CVD method is significantly higher than that of Mn-N-C catalysts synthesized by the traditional pyrolysis method using a mixture of Mn, N, and C precursors.

ALD is a vapor-phase catalyst synthesis technique for thin film growth and has emerged as an alternative method for synthesizing heterogeneous catalysts. Like CVD, ALD operates on the principle of alternating introduction of two precursor vapors that undergo “self-limiting” reactions at the molecular level on the surface of the support, enabling precise deposition of the target material [81,82]. By varying the number of deposition cycles, sequence, and types of precursors, atomic-level fine control over the structure of the catalyst active sites can be achieved, providing researchers with a new “bottom-up” strategy for the precise and controllable synthesis of catalysts. Lu and co-workers developed a new strategy for synthesizing bimetallic nanoparticles using ALD [83]. By using lower deposition temperatures and appropriate reactants, they were able to add a second metal component to the surface of supported monometallic nanoparticles, resulting in atomically controlled bimetallic nanoparticles. The study found that at lower temperatures, the metal substrate promotes nucleation and ALD growth of the metal precursor on its surface, whereas metal oxides are typically inert and cannot react with the metal precursor or initiate nucleation at low temperatures.

3.3. Ball Milling

Ball milling was initially a cost-effective method for reducing material size, and it can be used to synthesize various functionalized carbon materials (Figure 3d). During the ball milling process, the milling medium layers upon rotation of the grinding jar, causing high-energy collisions between the sample mixture and steel or ceramic balls. The kinetic energy of the moving balls is imparted to the sample itself, breaking chemical bonds and reducing particle size while creating new surfaces [84,85]. Ball milling can be performed in wet or dry modes, and several factors, such as rotation speed, mill temperature, ball size, and load, influence the milling efficiency. The graphite C-C bonds in carbon materials can be mechanochemically cleaved by ball milling, thereby introducing heteroatoms or functional groups during the milling process [86,87].

Chemical doping of graphene with heteroatoms, such as N and B, is one of the most feasible methods to regulate its electronic properties. Doped graphene exhibits n-type or p-type behavior, significantly altering its electronic characteristics [88]. In this context, nitrogen-doped graphene nanosheets have attracted considerable attention due to their excellent performance in various energy and electronic devices, including fuel cells, batteries, and field-effect transistors. Traditionally, nitrogen-doped graphene is typically produced through complex processes and/or chemical reagents containing undesirable components. However, nitrogen gas (N₂), the most abundant component in the atmosphere, can be considered an ideal nitrogen doping material. Recently, it has been reported that nitrogen can be successfully fixed at the edges of graphene nanosheets by dry ball milling graphite in a nitrogen atmosphere, despite N₂ being generally considered an inert diatomic gas due to its strong triple bond [89]. During the ball milling process, the mechanochemical cleavage of graphite C-C bonds generates active carbon, which can readily react with N₂. However, heteroatom dopants have been limited to non-metal elements such as nitrogen, boron, sulfur, and phosphorus. These materials do not fully meet the broad commercial demands of practical applications in terms of performance, cost, and stability. Interestingly, the single-atom catalysts (including iron, cobalt, nickel, and copper) can be prepared using a top-down approach through ball milling wear [85,90]. In this method, bulk metal sources are directly atomized and loaded onto a carbon framework. By adjusting the degree of wear, the amount of loaded metal can be controlled. No synthetic chemicals, solvents, or even water are used in this process, and no by-products or waste are generated. The potential reaction mechanism involves only the in situ mechanochemical forces creating defects in the carbon framework, followed by the loading and stabilization of isolated atomized metal atoms.

Figure 3. Schematic illustration of some common synthetic methods for C@NPMs: (a) pyrolysis; (b) CVD; (c) ALD; (d) ball milling. Copyright 2020, Wiley-VCH [91].

Figure 3. Schematic illustration of some common synthetic methods for C@NPMs: (a) pyrolysis; (b) CVD; (c) ALD; (d) ball milling. Copyright 2020, Wiley-VCH [91].

3.4. Waste-Derived Carbon

The development of waste-derived carbon precursors as ORR electrocatalysts has emerged as a promising strategy for advancing sustainable energy conversion technologies while addressing waste management [92]. Waste-derived carbon materials present significant advantages in terms of sustainability, cost-efficiency, and large-scale availability. Biomass sources are rich in carbon, oxygen, and nitrogen. Through carbonization processes, these materials transform into highly porous carbon frameworks with tunable surface area, electrical conductivity, and catalytic active sites, essential for enhancing ORR performance [93]. For example, nitrogen-doped carbon materials derived from proteins or chitin exhibit improve ORR activity by creating defects and active nitrogen species that facilitate oxygen molecule adsorption and electron transfer [94]. Additionally, functionalizing these carbon precursors with metals (such as Fe, Co, or Mn) during synthesis enhances their catalytic properties by forming metal–nitrogen–carbon (M-N-C) active sites, which are recognized for their high ORR selectivity and durability [95].

The transformation of cigarette butts into Fe-N_x-C materials has shown strong potential for ORR applications in both acidic and alkaline media [96]. Through pyrolysis and subsequent KOH activation with iron phthalocyanine (FePc) (Figure 4a,b), researchers have created an Fe-N_x-C electrocatalyst with a high half-wave potential of 0.89 V vs. RHE in alkaline media, rivaling Pt-based catalysts for alkaline fuel cells. Parallel studies on biomass waste, particularly plant residues, have yielded high-performance, metal-free bifunctional catalysts for both ORR and oxygen evolution reaction (OER) [97]. A standout example, BRC_AC8502, a plant-residue-derived catalyst, exhibits excellent ORR and OER performance, with its 3D hierarchical structure, nitrogen functionalities, oxygen vacancies, and carbon defects contributing to its high activity (Figure 4c–e). This catalyst even outperformed commercial Pt/C-RuO₂ when used in a ZABs. Other biomass sources, such as litchi skins, have been converted into ORR and hydrogen evolution reaction (HER) electrocatalysts by combining pyrolyzed carbon with Fe, Ni, and Co [98]. Among these, Fe at 600 °C excelled in ORR, achieving an onset potential near 0.9 V. Lignin-derived activated carbon (LAC), another biomass-derived material, has also been used with Fe and Mn phthalocyanine for M-N-C catalysts, showing robust performance in both acidic and alkaline media, with the bimetallic L_FeMn variant achieving a high onset potential of ~0.942 V (Figure 4f) [99]. Nitrogen-doped porous carbon derived from corncobs has shown tunable nitrogen doping and pore structures (Figure 4g,h), optimizing ORR performance with an onset potential of 0.97 V in alkaline media [100]. In conclusion, waste-derived carbon precursors present a viable and sustainable alternative for ORR electrocatalysis. By leveraging diverse waste streams and innovative processing methods, researchers are developing high-performance carbon-based catalysts that not only enhance energy conversion technologies, but also contribute to environmental sustainability. Continued research and development in this field are essential for realizing the full potential of waste-derived materials in the quest for efficient and sustainable energy solutions.

Table 1. Performance comparison of different electrocatalysts synthesized via different routes.

Catalysts	EOnset	E1/2	pH	Methods	Reference
FeSA/AC@HNC	0.88 V	0.78 V 0.90 V	0.5 M H2SO4 0.1 M KOH	Pyrolysis	[73]
Zr-N/O-C	1.000 V	0.910 V	0.1 M KOH	Pyrolysis	[74]
MnNC-CVD	~0.9 V	—	0.5 M H2SO4	CVD	[79]
N-DC/G	0.78 V	—	0.1 M KOH	waste-derived carbon	[94]
N-PCN	—	−0.154 V	0.1 M KOH	waste-derived carbon	[95]
CIGPF_450	0.77 V 0.95 V	0.63 V 0.89 V	0.5 M H2SO4 0.1 M KOH	waste-derived carbon	[96]
Fe-Co 600	~0.9 V	~0.81 V	0.1 M KOH	waste-derived carbon	[98]
L_Fe	0.94 V 0.84 V	0.87 V 0.77 V	0.1 M KOH 0.5 M H2SO4	waste-derived carbon	[99]
CC1U	0.97 V	0.70 V	0.1 M KOH	waste-derived carbon	[100]

Table 1. Performance comparison of different electrocatalysts synthesized via different routes.

Catalysts	EOnset	E1/2	pH	Methods	Reference
FeSA/AC@HNC	0.88 V	0.78 V 0.90 V	0.5 M H2SO4 0.1 M KOH	Pyrolysis	[73]
Zr-N/O-C	1.000 V	0.910 V	0.1 M KOH	Pyrolysis	[74]
MnNC-CVD	~0.9 V	—	0.5 M H2SO4	CVD	[79]
N-DC/G	0.78 V	—	0.1 M KOH	waste-derived carbon	[94]
N-PCN	—	−0.154 V	0.1 M KOH	waste-derived carbon	[95]
CIGPF_450	0.77 V 0.95 V	0.63 V 0.89 V	0.5 M H2SO4 0.1 M KOH	waste-derived carbon	[96]
Fe-Co 600	~0.9 V	~0.81 V	0.1 M KOH	waste-derived carbon	[98]
L_Fe	0.94 V 0.84 V	0.87 V 0.77 V	0.1 M KOH 0.5 M H2SO4	waste-derived carbon	[99]
CC1U	0.97 V	0.70 V	0.1 M KOH	waste-derived carbon	[100]

4. Carbon-Based ORR Catalysts

ORR catalysts are essential for energy conversion technologies such as fuel cells and metal–air batteries. Among various types of ORR catalysts, carbon-based materials have gained significant attention due to their cost-effectiveness, chemical stability, and tunable electronic properties. This section provides an overview of carbon-based ORR catalysts, categorized into metal-free catalysts, metal single-atom/dual-atom catalysts, and metal nanoparticle catalysts, with their performance summarized in Table 2.

4.1. Metal-Free Catalysts

Carbon-based catalysts have emerged as a significant area of research in the field of ORR due to their abundant resources, excellent chemical stability, and low cost. Compared to traditional noble metal catalysts, carbon-based catalysts offer substantial advantages in terms of environmental friendliness and economic viability. By tuning the defects in the carbon material itself and introducing non-metallic atom modifications, the catalytic activity of these materials can be significantly enhanced.

4.1.1. Defect Doping

Carbon materials exist in various structures, such as graphite formed by sp²-hybridized carbon atoms and diamond formed by sp³-hybridized carbon atoms [101]. Diamond-structured carbon materials possess high hardness and poor conductivity, making them unsuitable as electrocatalysts. On the other hand, sp²-hybridized carbons have multiple allotropes, and due to significant differences in structure, their physical and chemical properties vary greatly. However, sp²-hybridized carbon materials generally exhibit high chemical stability and low electrochemical activity. Recent studies have shown that breaking the intact conjugated π-bonds on the surface of carbon materials to create active electrochemical sites is an effective strategy to enhance ORR performance [102,103]. As shown in Figure 5a, Zigzag sites (carbene-like) and armchair sites (carbyne-like) on the carbon substrate surface, which contain unpaired electrons in carbon atoms, are the most active sites in defect-free carbon substrates and are favorable for improving ORR performance [104]. Theoretical calculations have indicated that smaller graphene sheets, due to higher edge content, exhibit greater ORR activity, with oxygen-containing functional groups such as -COOH, -CHO, -C=O, -C-O-C-, and -OH often present near the edges (Figure 5b). Unlike armchair edges, zigzag edges and their associated oxygen-containing functional groups exhibit lower barriers in ORR, demonstrating superior activity (Figure 5c). Baek’s research demonstrates that ball milling can effectively reduce the size of graphene [105]. As shown in Figure 5d, by introducing different gases during the ball milling process, a series of edge-selectively functionalized graphene nanoplatelets (EFGnPs) with various functional groups can be obtained. The results indicate that the edge polarity of EFGnPs without heteroatom doping plays a significant role in modulating ORR efficiency. The order of electrocatalytic activity is as follows: SGnP > CSGnP > CGnP > HGnP > pristine graphite. Even without heteroatom doping, graphene with polar edges can still exhibit excellent ORR activity. Jing’s report introduced a novel ORR catalyst based on graphene nanoribbons (GNRs) loaded with graphene quantum dots (GQDs), synthesized via a one-step synchronous reduction reaction [106]. This catalyst features numerous surface/edge defects as active sites, along with efficient charge transfer between closely contacted GQDs and GNRs, resulting in observed GQD-GNR hybrids with higher limiting current density and lower overpotential than platinum, with selectivity and stability in alkaline media comparable to the best carbon-based ORR catalysts reported to date. In addition, the pyrolysis of MgO precursors can produce carbon nanocages rich in numerous pores, edges, and positive topological defects, exhibiting superior oxygen-reduction performance in an alkaline environment compared to N-doped and B-doped carbon materials (Figure 5e–g) [103]. Both experimental results and theoretical studies have shown that pentagon and zigzag edges have lower ORR barriers, indicating that the presence of intrinsic defects significantly contributes to ORR activity.

4.1.2. Heteroatom Doping

Introducing defects can disrupt the uniform electronic distribution on the surface of sp²-hybridized carbon materials. However, the formation of intrinsic defects such as vacancies is complex, leading more researchers to focus on heteroatom doping, including atoms such as nitrogen (N), phosphorus (P), sulfur (S), boron (B), and halogens [107,108]. Due to the differences in atomic size and electronegativity, doping carbon materials with these heteroatoms redistributes the electronic structure of adjacent carbon atoms, thereby altering their electrochemical properties. In addition to the type of doping atom, the doping content and position also significantly influence the ORR activity of the catalyst. Nitrogen atoms are widely studied, due to their similar atomic radius to carbon (N/C = 0.74 Å/0.77 Å) and relatively high electronegativity difference (N/C = 3.04/2.55), which allows them to be easily incorporated into the carbon matrix, leading to surface charge redistribution [30,109,110]. As shown in Figure 6a, N atoms are doped into the carbon matrix in three forms: pyridinic N, pyrrolic N, and graphitic N. Among these, pyridinic N, with the lowest bond energy (398.2 eV), is typically located at edges or vacancies, where it bonds with two carbon atoms and contributes a p-electron, making it an effective ORR active center [11,111,112]. Research indicates that, compared to undoped carbon nanotubes, more electrons are transferred to nitrogen atoms in the nitrogen-doped nanotubes, leading to a significant positive charge distribution around the carbon atoms adjacent to nitrogen (Figure 6b,c) [113]. This suggests that nitrogen doping promotes charge redistribution within the carbon nanotubes. The electron-deficient carbon atoms near the nitrogen atoms enhance the chemisorption of O₂, thereby improving the ORR electrocatalytic activity of NCNTs. Besides nitrogen, doping with boron, sulfur, phosphorus, and halogen atoms can also effectively enhance the ORR activity of carbon materials. Boron atoms, with an atomic radius similar to carbon (B/C = 0.82 Å/0.77 Å), are typically doped into carbon-based materials using CVD. Additionally, direct pyrolysis of B₄C can produce boron-doped graphene, which exhibits ORR activity comparable to Pt/C [114]. Experimental results demonstrate that multi-atomic doping can alter the bond lengths between atoms, thereby better regulating the electron spin density and surface structure of carbon-based materials. Among the various multi-atomic doping methods, N atoms still play a significant role. Fan and colleagues synthesized a pyridinic N-B pair and, in situ, formed a crystalline graphene nanoribbon (GN)/amorphous carbon hierarchical structure [115]. In this structure, the carbon atoms adjacent to B atoms exhibit electron enrichment, while those near nitrogen atoms are charge-depleted. As a result, a certain concentration of B atom doping disrupts the linear proportionality in carbon materials (Figure 6d). The carbon atoms adjacent to B atoms in the pyridinic N-B pair become the primary active sites for appropriate OH/OOH adsorption, due to charge transfer. Additionally, as shown in Figure 6e–g, Yao and colleagues developed a three-dimensional hierarchical porous carbon aerogel featuring atomic S-C and N-S-C defects as active sites for the ORR [116]. Compared to S and C atoms, N atoms, with their higher electronegativity (3.04 eV), play a crucial role in electron-withdrawing processes, leading to the formation of high electron-density regions. In the S-D-G (S-defect graphene) structure, the introduction of more electronegative N atoms results in increased charge transfer. Furthermore, N-S-D-G (N-modified S-defect graphene) exhibits a relatively narrow band gap, with the valence band shifting closer to the Fermi level, thereby reducing the reaction overpotential.

4.2. Metal Single-Atom/Dual-Atom Catalysts

Metal single-atom and dual-atom catalysts (SACs and DACs) have emerged as a research frontier in the field of carbon-based catalysts, due to their efficient atomic utilization and unique electronic structures. These catalysts achieve outstanding catalytic performance in the ORR by dispersing metal atoms or atom pairs on the surface or within the carbon materials.

4.2.1. Single-Atom Catalysts

SACs refer to catalysts where individual metal atoms are dispersed at the atomic level on carbon-based materials. These metal atoms are typically stabilized by coordinating with atoms within the carbon material, such as nitrogen, forming M-N_x structures that create highly dispersed active sites. The prominent advantages of SACs include their 100% atomic utilization and unique electronic structures, which can significantly enhance the selectivity and activity of catalytic reactions [117,118]. The optimization of SAC structures can be divided into two key aspects: coordination chemistry environment and the regulation of the local carbon structure. (1) Coordination Chemistry Environment. The coordination chemistry environment of SACs has a significant impact on their electrochemical performance. The coordination environment directly affects the electronic structure of the central atom, including its valence state, spin, and band structure, thereby influencing the adsorption energy of reaction intermediates and ultimately determining the catalyst’s activity, reaction pathways, and selectivity. Some SACs with an M-N₄ structure exhibit excellent electrocatalytic potential for the ORR. However, the M-N₄ catalysts from subgroup-IVB (M = Ti, Zr, Hf) demonstrate strong adsorption of *OH intermediates, resulting in lower ORR activity. As shown in Figure 7a,b, Zhao et al. proposed that by introducing an axial oxygen ligand, the inactive subgroup-IVB M-N₄ catalysts could be converted into active M-N₄O catalysts [64]. Theoretical and experimental studies have shown that the axial oxygen ligand can regulate the competition between metal–ligand covalency and metal–intermediate covalency. This regulation affects the d-p orbital hybridization between the metal site and intermediates, and optimizes the adsorption strength of the metal site to the reaction intermediate, thereby accelerating the ORR (Figure 7c). (2) Regulating local carbon structure to enhance ORR catalytic activity. Modifying the carbon atom structure near the MN₄ sites (Figure 7d) can directly influence the catalytic activity and stability. Carbon vacancies can enhance the catalytic activity of Fe-N-C catalysts [20]. The rate-determining step (RDS) for ORR on Fe-N-C is the reduction of OH* to form the final product, so weakening the adsorption of OH* can improve catalytic activity. As shown in Figure 7e, by considering six possible single-carbon vacancies and calculating the Gibbs free-energy change (ΔG) of defected Fe-N₄ during the 4 e⁻ ORR process, it was found that at A, B, C, and F sites, the adsorption of ORR intermediates strengthens, stabilizing the adsorbed OH*, indicating that a higher ΔG is required to remove the OH group. In contrast, Fe-N-C with E site defects showed weaker adsorption of ORR intermediates, resulting in superior ORR activity compared to defect-free Fe-N-C (Figure 7f). Additionally, different carbon structures are produced after the pyrolysis of various carbon precursors. For instance, compared to the CoN₄ sites in Co-N-C catalysts derived from polyaniline pyrolysis, CoN₂₊₂ sites (formed by two adjacent chair-shaped graphite-carbon edge sites) generated by ZIF-8 pyrolysis exhibit higher ORR catalytic activity and 4 e⁻ selectivity (Figure 7g,h) [119]. DFT theoretical calculations show that altering one of the reaction step’s free energy in the ORR catalytic process at CoN₂₊₂ sites causes the reaction to become exothermic at 0.73 V. Moreover, the OOH dissociation energy at CoN₂₊₂ sites (0.69 eV) is lower than that at CoN₄ sites (1.11 eV), giving the CoN₂₊₂ sites an advantage in reaction kinetics.

4.2.2. Dual-Atom Catalysts

DACs refer to catalysts in which two metal atoms, either identical or different, are dispersed and coordinated on carbon-based materials. This structure leverages the interaction between the two metal atoms to create a synergistic catalytic effect, thereby enhancing the catalyst’s performance. Compared to SACs, DACs possess a more complex electronic structure, offering more active sites and higher catalytic activity. The dual metal atoms can be classified into two types: those with identical nuclei (A₂) and those with different nuclei (AB). When two atoms are embedded adjacent to each other on the support, they may interact, thereby tuning the electronic structure of the dual-atom active sites [120,121]. Homonuclear DACs consist of two identical and adjacent metal atoms fixed on a substrate, where the chemical valence states of the two metal atoms can be either identical or different. Due to the interaction between the two metal centers, DACs composed of identical metal atoms exhibit characteristics distinct from SACs, and the interaction between the two active sites is beneficial for optimizing catalytic kinetics. Xie et al. developed a planar Fe₂N₆ structure to achieve efficient oxygen reduction in PEMFCs [122]. As shown in Figure 8a, compared to the sample at open-circuit voltage, the Fe-Fe bond significantly contracts at 1.0 V vs. RHE. This demonstrates that in the planar Fe₂N₆ structure, two adjacent Fe atoms each bind to an oxygen atom from the oxidative intermediate, forming bidentate adsorption. This reduces the Fe-Fe bond distance, promoting the dissociation of oxidative intermediates (Figure 8b). This phenomenon directly proves that the adjacent iron atoms in the planar Fe₂N₆ structure can synergistically regulate the adsorption and accelerating of ORR kinetics. Additionally, the planar Fe₂N₆ follows a unique ORR redox transition from the initial state Ox-Fe³⁺-Fe²⁺ to the final state Fe²⁺-Fe²⁺, with the cyclic structure shown in Figure 8c. Compared to homonuclear A₂-type DACs, the introduction of a second metal typically modulates the electronic configuration of the active sites on a large scale through charge redistribution, bringing catalytic activity closer to the peak of the volcano plot. Yan et al. reported the synthesis of a carbon-based catalyst with dual Fe-Co atomic sites, where Fe and Co atoms are coordinated with N and O atoms, respectively, and connected via bridging N and O atoms (FeCo-N₃O₃@C) (Figure 8d–g) [123]. Benefiting from the unique structure and strong interaction of the FeCo-N₃O₃ dual atoms, the Janus FeCo-N₃O₃@C catalyst exhibited a half-wave potential of 0.936 V, significantly outperforming commercial Pt/C and other SACs. DFT calculations revealed charge transfer between the adjacent Fe and Co atoms, with the synergistic effect of the FeN₃ and CoO₃ groups significantly altering the charge distribution at the metal active sites, optimizing the adsorption and desorption of reaction intermediates, thereby effectively enhancing ORR performance. In addition to combining two different metal atoms, the combination of metal and non-metal atoms, which often have distinct physical and chemical properties, can also play different roles in catalytic reactions. Zhai et al. successfully synthesized an Fe-Se co-modified nitrogen-doped carbon material by introducing selenium atoms into an Fe-N_xC single-atom catalyst using selenium dioxide as the selenium source (Figure 8h–j) [124]. The Se atoms, with their larger atomic size, rich d-electrons, and high polarizability, have multiple impacts on the Fe-NC material. Besides providing new Se active sites, they can effectively modulate the Fe charge distribution and spin state, thereby enhancing ORR activity. Coleman et al. synthesized a carbon-supported PtCu catalyst by galvanically displacing nanoporous copper with Pt [125]. The PtCu/C catalyst exhibited a reduced affinity for OH adsorption (OH_ads) in both acidic and alkaline electrolytes, resulting in 2–3 times-higher ORR activity compared to Pt/C.

4.3. Metal-Based Nanoparticle Catalysts

Carbon-supported metal-based nanoparticle catalysts represent a diverse and effective category of catalysts with broad applications in various chemical processes. These catalysts include both carbon-supported metal nanoparticles and carbon-supported metal oxides, nitrides, sulfides, and phosphides.

4.3.1. Carbon-Supported Metal Nanoparticles

Due to the unique interaction between the metal and the carbon matrix, metal nanoparticles dispersed on high surface-area and highly conductive carbon support the benefit from enhanced catalytic activity and stability. This synergy promotes efficient electron transfer and reduces the tendency of metal nanoparticles to aggregate, thereby improving the catalyst’s longevity. Further enhancement of catalytic activity can be achieved by designing and synthesizing different nanostructures, such as core-shell, hollow, and layered structures, and by tuning the interface between the metal and the carbon support. Platinum (Pt), palladium (Pd), and other precious metal nanoparticles, in particular, were among the earliest studied ORR catalysts, demonstrating excellent ORR performance. As shown in Figure 9a, Huang et al. synthesized Pd(0) nanoparticles by reducing atomically dispersed Pd(II) and then loading them onto hydrogen-substituted nitrogen-doped graphdiyne (Pd/N-HsGY) [126]. The unique sp²-hybridized carbon atoms in HsGY exhibit significant external charge compensation, which helps maintain the loaded Pd nanoparticles in a zero-valent state (Figure 9b,c). The synthesized Pd/N-HsGY demonstrates exceptional ORR activity, outperforming commercial Pd/C and Pt/C catalysts (Figure 9d). Bimetallic alloy nanoparticles, due to their unique electronic and geometric structures, exhibit distinctly different properties compared to monometallic materials, particularly alloy nanoparticles composed of platinum group metals and transition metals supported on carbon. These catalysts have been extensively studied as electrocatalysts for fuel cells, due to their high activity and stability at lower platinum content. Platinum alloys with ordered atomic crystal structures, such as L1₂ and L1₀, can significantly enhance electrocatalytic activity and stability by increasing d-d orbital electron interactions between platinum and transition metals. Hyeon et al. directly synthesized catalysts with high loading and highly dispersed MPt (M = Fe, Co, or Ni) alloy nanoparticles on various carbon supports (Figure 9e,f) [127]. Among them, the L1₀-FePt catalyst supported on reduced graphene oxide (37 wt%-FePt/rGO) exhibits excellent ORR activity and stability (Figure 9g). Additionally, carbon support can endow non-metal oxides, sulfides, and phosphides with high conductivity and large surface area, compensating for their inherent disadvantages as ORR catalysts. While Co₃O₄ nanoparticles alone do not exhibit ORR activity, when combined with N-doped reduced mildly oxidized graphene oxide, they display high catalytic performance comparable to Pt/C, with significantly better durability and stability, indicating the importance of the synergistic coupling effect between Co₃O₄ and graphene in enhancing catalyst performance [128]. Beyond graphene, Co-MOF embedded in carbon cloth and then pyrolyzed yields NC-Co₃O₄/CC, which also demonstrates excellent OER and ORR activity (Figure 9h–k) [129]. Co-based ZIF-67 can serve as a precursor, which after growing a CoS shell on its surface and subsequent pyrolysis, results in Co-C@Co₉S₈ DSNCs with a double-shell structure. The synergistic effect of the double-shell structure, where the permeable outer Co₉S₈ shell facilitates mass transfer and prevents agglomeration and deactivation of the active Co-C centers in the hollow interior during catalysis, leads to high ORR activity and durability. Furthermore, a Co_1-xS/RGO hybrid catalyst, prepared through a two-step process of low-temperature solid-state reaction and high-temperature annealing, shows excellent ORR catalytic performance. The electrochemical coupling between RGO and Co_1-xS nanoparticles enhances the catalyst’s conductivity and controls the Co_1-xS particle size, preventing agglomeration and ensuring the catalyst’s catalytic activity. Metal phosphide carbon materials can be obtained by pyrolyzing metal inorganic salts with phosphorus-containing precursors. For example, mixing phytic acid with FeCl₃ allows the phosphate groups in phytic acid to react with Fe³⁺ to form ferric phytate, which upon pyrolysis yields Fe-P/C catalysts [130]. Compared to PA-900, which lacks iron, the Fe-containing Fe-P/C exhibits superior ORR activity, with the presence of Fe-P and P-C bonds being key to the catalyst’s high performance.

4.3.2. Synergistic Effect of Metal Single Atom and Nanoparticle

The synergistic interaction between metal single atoms and nanoparticles is a promising strategy for enhancing catalytic performance, particularly in the ORR. This approach leverages the unique properties of metal single atoms, which offer high atomic utilization and distinctive electronic structures, alongside metal nanoparticles, which provide a high density of active sites and robust catalytic activity. The combination of these two components can be categorized into two types: combinations involving single atoms and nanoparticles of the same metal, and combinations involving single atoms and nanoparticles of different metals. In systems where the single atoms and nanoparticles are of the same metal, single atoms can enhance catalytic activity by altering the electronic environment of the nanoparticles, thereby improving reaction kinetics and selectivity. For example, as shown in Figure 10a, Deng et al. synthesized a series of M-N-C catalysts (M = Co, Fe, Cu) containing metal single atoms and nanoparticles using a novel cyclodextrin-based MOF material as a precursor [131]. DFT calculations revealed that the strong interaction between Co nanoparticles and atomic Co-N₄ sites increases the density of d-electrons near the Fermi level, reducing the reaction energy barrier, thus optimizing the rapid adsorption/desorption capability of reaction intermediates and enhancing ORR kinetics. Han et al. proposed that disrupting the symmetric electronic distribution of single-atom Fe centers is an effective strategy to improve the intrinsic activity of the ORR [132]. Therefore, they used a soft-template self-sacrificial pyrolysis method to prepare asymmetric N, P-coordinated single-atom Fe sites supported on porous carbon nanosheets with Fe₂P nanoclusters/nanoparticles (Fe₂P_ac-FeN₃P₁) (Figure 10b,c). The introduction of P coordination not only disrupts the symmetric charge distribution of the original FeN₄ sites, but also, due to the weaker electronegativity of P relative to N, decreases the number of electrons lost by the Fe active center in FeN₃P₁. In contrast, the introduction of Fe₂P_ac increases the number of electrons lost by the Fe active center in Fe₂P_ac-FeN₃P₁ (Figure 10d). Additionally, the synergistic effect of P coordination and Fe₂P leads to a balanced electron distribution at the single-atom Fe sites, optimizing the adsorption strength of *O and *OH, which is beneficial for improving ORR performance. On the other hand, when single atoms and nanoparticles are composed of different metals, the synergistic effects can be even more pronounced. Heterogeneous metal interactions can induce electron charge redistribution, optimize the binding energies of reaction intermediates, and create new active sites with enhanced catalytic performance. Compared to single-metal systems, these bimetallic systems typically exhibit superior catalytic performance. Wu et al. reported that MnN₄ sites can induce a strong coupling effect between Pt clusters and the carbon support (Figure 10g) [133]. This potential synergistic interaction can enhance the chemical bonding between Pt clusters and the Mn-NC support, improve catalyst stability, and weaken the adsorption strength of OH on the Pt surface, thereby enhancing the intrinsic ORR activity. Zhao et al., using density functional theory (DFT) and machine learning (ML), revealed the collective effects among FeN₄OH sites, CeN₄OH motifs, Fe nanoparticles (NPs), and Fe-CeO₂ NPs [134]. Among them, Fe_SAs+NPsCe_SAs+Fe-ONPs/NC exhibited the lowest energy barrier among all candidate catalysts, with a high half-wave potential of 0.948 V, making it one of the best performing non-precious-metal ORR catalysts reported to date (Figure 10h).

Table 2. Performance comparison of different catalysts.

Catalysts	EOnset	E1/2	pH	Reference
SGnP	0.81 V	—	0.1 M KOH	[105]
GQD-GNR	—	~0.805 V	0.1 M KOH	[106]
BNC-1	0.876 V	0.812 V	0.1 M KOH	[103]
NSCA-700-1000	—	0.85 V 0.76 V 0.76 V	0.1 M KOH 0.5 M H2SO4 0.1 M HClO4	[116]
s-Hf-N/O-C	1.050 V	0.920 V	0.1 M KOH	[64]
Co-N-C@F127	0.93 V	0.84 V	0.5 M H2SO4	[119]
Fe2N6	—	0.84 V	0.5 M H2SO4	[122]
FeCo-N3O3@C	—	0.936 V	0.1 M KOH	[123]
Fe1Se1-NC	1.0 V 0.88 V	0.88 V 0.74 V	0.1 M KOH 0.5 M H2SO4	[124]
PtCu/C	~0.90 V ~0.87 V	—	0.1 M KOH 0.1 M HClO4	[125]
Pd/N-HsGY	0.96 V	0.849 V	0.1 M KOH	[126]
37 wt%-FePt/rGO	—	~0.92 V	0.1 M HClO4	[127]
Co3O4/N-rmGO	0.88 V	0.83 V	0.1 M KOH	[128]
NC-CO3O4-90	0.91 V	0.87 V	1 M KOH	[129]
Fe-P-900	0.95 V 0.84 V	—	0.1 M KOH 0.1 M HClO4	[130]
Fe@C-FeNC	~1.025 V	0.917 V	0.1 M KOH	[131]
FePNC	—	0.76 V 0.90 V	0.5 M H2SO4 0.1 M KOH	[132]
Pt@MnSA-NC	—	0.915 V	0.1 M HClO4	[133]
FeSAs+NPsCeSAs+Fe−ONPs/NC	—	0.948 V	0.1 M KOH	[134]

Table 2. Performance comparison of different catalysts.

Catalysts	EOnset	E1/2	pH	Reference
SGnP	0.81 V	—	0.1 M KOH	[105]
GQD-GNR	—	~0.805 V	0.1 M KOH	[106]
BNC-1	0.876 V	0.812 V	0.1 M KOH	[103]
NSCA-700-1000	—	0.85 V 0.76 V 0.76 V	0.1 M KOH 0.5 M H2SO4 0.1 M HClO4	[116]
s-Hf-N/O-C	1.050 V	0.920 V	0.1 M KOH	[64]
Co-N-C@F127	0.93 V	0.84 V	0.5 M H2SO4	[119]
Fe2N6	—	0.84 V	0.5 M H2SO4	[122]
FeCo-N3O3@C	—	0.936 V	0.1 M KOH	[123]
Fe1Se1-NC	1.0 V 0.88 V	0.88 V 0.74 V	0.1 M KOH 0.5 M H2SO4	[124]
PtCu/C	~0.90 V ~0.87 V	—	0.1 M KOH 0.1 M HClO4	[125]
Pd/N-HsGY	0.96 V	0.849 V	0.1 M KOH	[126]
37 wt%-FePt/rGO	—	~0.92 V	0.1 M HClO4	[127]
Co3O4/N-rmGO	0.88 V	0.83 V	0.1 M KOH	[128]
NC-CO3O4-90	0.91 V	0.87 V	1 M KOH	[129]
Fe-P-900	0.95 V 0.84 V	—	0.1 M KOH 0.1 M HClO4	[130]
Fe@C-FeNC	~1.025 V	0.917 V	0.1 M KOH	[131]
FePNC	—	0.76 V 0.90 V	0.5 M H2SO4 0.1 M KOH	[132]
Pt@MnSA-NC	—	0.915 V	0.1 M HClO4	[133]
FeSAs+NPsCeSAs+Fe−ONPs/NC	—	0.948 V	0.1 M KOH	[134]

5. Summary and Outlook

The development of efficient ORR catalysts is of great significance for the large-scale application of green and clean energy. This paper reviews the performance and progress of carbon-based materials in ORR catalysts in recent years and proposes reaction mechanisms that can guide the design and preparation of future catalysts. Enhancing the concentration of defects within carbon materials or doping heteroatoms has proven to be effective in improving the ORR catalytic activity of carbon-based catalysts, making it a viable strategy for modifying carbon materials to achieve high electrochemical performance. However, preparing carbon material defects at the atomic level remains highly uncontrollable. While edge and vacancy defects can now be successfully designed and synthesized, the controlled synthesis of topological defects still faces significant challenges. The synergistic coupling between heteroatoms, as well as between defects and heteroatoms, can further reduce the free energy and overpotential of the ORR, demonstrating electrocatalytic performance that surpasses Pt/C. However, for carbon matrices, the direct doping of one or more heteroatoms significantly affects the carbon matrix, disrupting its order and leading to reduced conductivity. Therefore, finding a simple way to balance the relationship between heteroatom doping concentration and catalyst conductivity is of great importance. In addition, reducing metal particle sizes and developing single-atom catalysts to improve metal utilization and active-site concentration has also proven to be a powerful approach to preparing efficient ORR catalysts. However, due to the high surface energy of metal single atoms, they tend to aggregate during preparation and catalysis. Moreover, the pyrolysis and subsequent acid leaching processes can lead to the collapse of active sites, thereby reducing catalyst performance. Therefore, further improving the stability and catalytic activity of metal single atoms is a major challenge that single-atom catalysts need to overcome. Additionally, the analysis of single-atom catalysts requires characterization at the atomic level, which necessitates the use of advanced equipment such as HAADF-STEM and synchrotron radiation. However, these instruments are extremely expensive and not accessible to ordinary researchers, further limiting the development of metal single-atom catalysts.

In light of the current challenges, this paper presents prospects for the future development of carbon-based catalysts: (1) correctly understanding the ORR mechanism and identifying active sites is key to researching and developing efficient catalysts, and it is essential to use theoretical calculations to guide catalyst preparation. (2) Fully understanding the relationship between heteroatom doping, catalyst structure, and the conductivity of carbon materials is crucial to allow for reasonable planning before catalyst preparation. (3) Developing more effective methods beyond pyrolysis for preparing metal single atoms is necessary to avoid the loss of active sites during the preparation process.