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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 10: 1443 - 1455

DOI: https://doi.org/10.7536/PC240212

综述

Advanced Design Strategies for High-Performance Electrocatalysts Based on Oxygen Reduction Reaction towards H2O2 Synthesis

  • Yani Ding ,
  • Wei Zhou , * ,
  • Jihui Gao , *
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  • School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
*e-mail: (Wei Zhou);
(Jihui Gao)

Received date: 2024-02-20

  Revised date: 2024-07-07

  Online published: 2024-09-15

Supported by

National Natural Science Foundation of China(52106237)

Open Project of State Key Laboratory of Urban Water Resource and Environment，Harbin Institute of Technology(HC202331)

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Abstract

Hydrogen peroxide (H2O2), as an important chemical raw material in the fields of environment, chemical, and energy, has become an emerging candidate in promoting energy transformation and green development of the chemical industry due to its characteristics of green environmental protection and strong sustainability. At present, over 95% of H2O2 worldwide is synthesized through the anthraquinone oxidation (AO process), which mainly involves the hydrogenation and oxidation process of anthraquinone molecules in Ni or Pd catalysts and organic solvents. However, the AO process also brings in additional costs and poses risks such as flammability and explosion during transportation, high energy consumption, and waste generation. Oxygen reduction reaction (ORR) towards H2O2 synthesis provides an economical, efficient, and harmless alternative process for the in-situ synthesis of green reagents under mild conditions. However, ORR towards H2O2 synthesis mainly faces two major challenges: low reaction selectivity and slow reaction kinetics, which lead to generally low H2O2 yield and Faraday efficiency, hindering further industrial applications. As the core of electrocatalytic reactions, the surface physicochemical properties of electrocatalysts are usually closely related to the catalytic process, directly affecting the adsorption and desorption of reaction species, thereby further affecting the overall reaction thermodynamics and kinetics. Therefore, developing electrocatalysts with high activity, high selectivity, and good stability, is the key to further improving the catalytic activity and energy conversion efficiency. Based on this, this review systematically summarizes the advanced design strategies of high-performance electrocatalysts in the H2O2 synthesis through ORR in recent years. The synthesis strategies and control mechanisms of advanced electrocatalysts are summarized and sorted out from four aspects: electronic structure control, geometric structure control, surface morphology control, and atomization active site design. Prospects and suggestions are also proposed for the design direction and application prospects of ORR electrocatalysts, which are beneficial for achieving precise control of intermediate adsorption and desorption behavior in reaction rate-determining steps, and constructing interface conditions for efficient energy and mass transfer of reaction species.

Contents

1 Introduction

2 Oxygen reduction reaction fundamental mechanism

3 Electronic structure regulation strategies

3.1 Chemical doping engineering

3.2 Defect construction engineering

4 Geometric structure regulation strategies

4.1 Size regulation

4.2 Pore/interlayer structure regulation

4.3 Surface morphology regulation

5 Surface modification and functionalization strategies

6 Atomic level active site design strategies

6.1 Metal active centers regulation

6.2 Local coordination domain regulation

7 Conclusion and outlook

Key words： oxygen reduction reaction; hydrogen peroxide; electronic structure regulation; geometric structure regulation; surface functionalization modification

Cite this article

Yani Ding , Wei Zhou , Jihui Gao . Advanced Design Strategies for High-Performance Electrocatalysts Based on Oxygen Reduction Reaction towards H2O2 Synthesis[J]. Progress in Chemistry, 2024 , 36(10) : 1443 -1455 . DOI: 10.7536/PC240212

1 Introduction

Hydrogen peroxide (H2O2) as an important chemical raw material in the fields of environment, chemical industry, and energy, has become a new force driving the transformation of energy and the green development of the chemical industry due to its characteristics of being environmentally friendly and highly sustainable. In the environmental field, H2O2, with its strong oxidizing power and non-toxic reduction products, is widely used in advanced oxidation processes (AOPs) for wastewater treatment and in the pulp and paper industry[1]; in the field of chemical synthesis, H2O2, thanks to its mild reaction conditions and high selectivity, serves as an important oxidant in organic synthesis reactions such as hydroxylation[2] and oxyhalogenation[3]; and in the energy sector, H2O2 is regarded as one of the ideal choices for fuel cell hydrogen sources and rocket propellants due to its high calorific value and excellent energy conversion efficiency, and it is seen as a promising oxidant and energy carrier in renewable energy conversion technologies[4]. Currently, the synthesis methods for H2O2 mainly include: direct catalytic synthesis from hydrogen and oxygen[5], anthraquinone oxidation method (AO method)[6], and oxygen cathode reduction reaction method[7,8]. Globally, over 95% of H2O2 is synthesized via the anthraquinone method, which primarily involves the hydrogenation and oxidation processes of anthraquinone molecules in the presence of Ni or Pd catalysts and organic solvents[9]. The anthraquinone method can produce H2O2 with a mass fraction exceeding 70 wt%, but the transportation process adds extra costs and risks such as flammability and explosiveness, and also leads to issues like high energy consumption, expensive costs, and the generation of large amounts of waste during production[10].
Electrocatalytic O2 cathode reduction (ORR) for H2O2 synthesis provides an economical, efficient, and harmless alternative process capable of maximizing the in-situ synthesis of "green" reagents under mild conditions while reducing costs associated with storage and transportation. This approach offers lower energy consumption, higher selectivity, better tunability, and sustainability. Additionally, synthesizing H2O2 in fuel cells has the potential to recover energy released during the electrolysis process, offering low-carbon production and energy storage potential, thus providing new solutions for large-scale and distributed production of H2O2[11]. However, the ORR-based H2O2 synthesis technology still faces challenges such as low selectivity and slow reaction kinetics, leading to generally low H2O2 yield and Faraday efficiency, which have become key constraints for its large-scale application. As the core of electrocatalytic reactions, the electronic structure and surface physicochemical properties of electrocatalysts are closely related to the catalytic process, directly affecting the adsorption and desorption of reactant species[12], thereby influencing the thermodynamics and kinetics of the 2-electron ORR. Therefore, developing electrocatalysts with high activity, high selectivity, and long-term stability, to precisely control the adsorption and desorption behavior of rate-determining step intermediates and construct interfaces that facilitate efficient mass and charge transfer, is crucial for further enhancing the catalytic activity and energy conversion efficiency of ORR for H2O2 synthesis. Based on this, this paper systematically summarizes and organizes the synthesis strategies and regulation mechanisms of advanced electrocatalysts from four aspects: electronic structure regulation, geometric structure regulation, surface morphology regulation, and atomic-level active site design. Finally, this paper proposes prospects and suggestions for the design direction and application prospects of ORR electrocatalysts.

2 Fundamental Principles of Oxygen Electroreduction Reaction

The oxygen cathode reduction reaction can typically proceed through two pathways: (1) the two-electron oxygen reduction pathway (2eORR). The adsorbed oxygen molecule first generates *OOH through a one-step proton-electron coupled transfer (CPET) step, and then *OOH combines with another proton-electron via a CPET step to produce H2O2 (Equations 1~2); (2) the four-electron oxygen reduction pathway (4eORR). Compared to 2eORR, the main difference lies in the tendency of the O—O bond in the ORR intermediate *OOH to break. After undergoing a CPET step, the O—O bond in *OOH breaks to form *O and H2O; in addition, *O can continue to gain electrons through a CPET step to form *OH, which can then be reduced to H2O by gaining more electrons (Equations 3~6)[13].
(1) two-electron oxygen reduction pathway.
O2 + * + H+ + e- → *OOH
*OOH + H+ + e- → H2O2
(2) four-electron oxygen reduction pathway.
O2 + * + H+ + e- → *OOH
*OOH + H+ + e- → *O + H2O
*O + H+ + e- → *OH
*OH + H+ + e- → H2O
from the above reaction pathways, it is known that the selectivity of ORR mainly depends on the binding strength between the intermediate *OOH and the surface of the electrocatalyst, which will greatly affect whether the O—O bond breaks or remains, thus leading to different reaction pathways and products. Based on the classic binding energy-theoretical limit potential volcano plot theory proposed by Nørskov et al.[14], for different metal-based catalysts, the binding energy of *OH also varies. Considering the linear relationship among the binding energies of *O, *OH, and *OOH, this further influences the binding energy between the catalyst surface and *OOH. On the left side of the volcano plot, Pd and Pt (111) have a strong binding energy with *OH, which usually leads to an increase in ORR activity; therefore, in the 4eORR pathway, Pd and Pt (111) are very close to the apex of the volcano plot, meaning they possess the optimal theoretical limit potential (Figure 1a). However, in the 2eORR pathway, too strong a binding energy with oxygen-containing intermediates can make it difficult for *OOH to desorb, instead favoring the breaking of the O—O bond, which is not conducive to the formation of H2O2 (Figure 1b). In contrast, metals like Au, which have relatively weaker binding energies, due to having a more suitable binding energy with *OOH, are more likely to undergo CPET steps and convert to H2O2, improving ORR selectivity; however, at the same time, weaker binding energy also results in higher activation energy barriers for oxygen molecules or other oxygen-containing intermediates, making it impossible for Au to reach the top of the theoretical limit potential of the volcano plot in the 2eORR pathway. Therefore, how to use rational catalyst design strategies to ensure that reaction intermediates have appropriate adsorption/desorption energies at the electrode interface is key to further improving the rate-limiting steps and reaction energy barriers.
图1 经典ORR结合能-理论极限电位火山图[14]。（a）在四电子反应路径中不同金属*OH结合能与ORR理论极限电位的关系，其中蓝色实线表示强结合*OH区域，绿色实线表示弱结合*OOH区域；（b）在二电子反应路径中不同金属*OH结合能与ORR理论极限电位的关系，其中紫色实线表示强结合*OOH区域，绿色实线表示弱结合*OOH区域

Fig. 1 The classical volcano plot with binding energy and theoretical limiting potential[14]. Copyright 2018 American Chemical Society. (a) The relationship between the binding energy of different metal *OH and the theoretical limit potential of ORR in the four electron reaction pathway, where the blue solid line represents the strong binding *OH region and the green solid line represents the weak binding *OOH region; (b) The relationship between the binding energy of different metal *OH and the theoretical limit potential of ORR in the two electron reaction pathway, where the purple solid line represents the strong binding *OOH region and the green solid line represents the weak binding *OOH region

3 Regulation of Electronic Structure of Electrocatalysts

In the 2eORR process, there is usually an irreconcilable contradiction between activity and selectivity. Typically, metal catalysts represented by Pd, Pt, and Ni generally have high activity, but the overly strong binding energy often makes it difficult for oxygen-containing intermediates to desorb, leading to low 2eORR selectivity. Metal catalysts represented by Au, Hg, and Ag typically exhibit higher 2eORR selectivity, but their weaker binding energy is not conducive to the adsorption of oxygen molecules, thereby reducing ORR activity. The electronic structure effect can influence the interaction between the catalytic interface and reaction intermediates by regulating the band or surface electron density of states, thus balancing the adsorption and desorption processes of *OOH to bring the theoretical limit potential as close as possible to the apex of the volcano plot, which becomes a key to improving the selectivity and activity of the 2eORR reaction. Currently, chemical doping and defect construction have been proven to be effective means of regulating the electronic structure of catalysts[15].

3.1 Chemical Doping Engineering

By introducing noble metals, pre-transition metals/non-3d transition metals, and non-metal elements, it is possible to significantly regulate the electronic structure and band structure of electrocatalysts, thereby affecting the overall reaction kinetics and thermodynamics. Among these, noble metal elements are expected to provide new active sites, while non-3d transition metals can easily optimize the Gibbs free energy for the formation of electrocatalytic intermediates. Non-metal elements, with their typically high electronegativity, influence interfacial charge transfer. These chemical doping methods can all effectively regulate the overall electronic structure of the electrocatalyst, inducing strong synergistic effects or constructing new reactive sites, thus improving reaction kinetics and thermodynamics.
For precious metal catalysts, the multi-component alloying strategy has been proven to increase the density of states (DOS) near the Fermi level, narrow the band gap, and shift the d-band center position of the metal catalyst. In particular, combining metals with high selectivity (Au, Ag, Hg) with those with high activity (Pd, Pt) can optimize the adsorption of key ORR intermediates, thereby achieving both high selectivity and high activity[10]. For example, Siahrostami et al.[11] constructed a core-shell structured PtHg4 nanoparticle modified by Hg, where the isolated atoms of the active metal Pt are surrounded by the more inert element Hg. Rotating disk electrode tests showed that PtHg4 could achieve over 95% H2O2 selectivity at 0.2 ~ 0.4 V vs. RHE and still exhibit stable activity after 8000 cycles between 0.05 ~ 0.8 V vs. RHE; theoretical calculations further confirmed that the theoretical limit potential of PtHg4 is closer to the top of the volcano plot. Pizzutilo et al.[16] prepared Au1−xPdx nanoalloys with variable Pd content on Vulcan XC-72, and the results showed that increasing the Pd concentration to 8% could lead to nearly 95% electrocatalytic H2O2 production selectivity. Density functional theory (DFT) calculations indicated that the presence of Pd enhances O2 molecule adsorption, while the presence of Au helps avoid the breaking of the O—O bond, further regulating the adsorption of oxygen intermediates. Zhao et al.[17] prepared bimetallic PdAu nanoframes through a solution etching method, which exhibited optimal 2eORR activity and selectivity when the Au/Pd atomic ratio reached 9, showing a relatively positive onset potential (about 0.56 V vs. RHE) in 0.1 M HClO4 solution and an H2O2 yield exceeding 90% over a wide potential window (Figure 2a). Theoretical calculations found that incorporating Pd atoms into the PdAu (111) crystal face could stabilize *OOH adsorption through direct Pd—O bonds, thus enhancing the 2eORR activity.
图2 电催化剂的电子结构调控策略促进2eORR合成H2O2。（a）Au（111）和PdAu（111）上的2eORR途径的DFT模拟[17]；（b）碳纳米管（CNTs）修饰的亚磷酸镍钴（NiCo−Phi）构型[18]；（c）氮掺杂碳基材料反应前后的碳K边和氮K边XANES光谱及不同氮掺杂构型、含量对于ORR反应路径的影响[23];（d）不同碳基缺陷构型对于2电子及4电子ORR火山图的影响[24]

Fig. 2 The electronic structure control strategy of electrocatalysts promotes the H2O2 synthesis. (a) DFT simulation of the 2eORR pathway on Au(111) and PdAu(111) [17]. Copyright 2021 American Chemical Society. (b) Carbon nanotubes (CNTs) modified nickel cobalt phosphite (NiCo Phi) configuration[18]. Copyright 2020 Wiley VCH GmbH. (c) The carbon K edge and nitrogen K edge XANES spectra of nitrogen doped carbon based materials before and after reaction, as well as the influence of different nitrogen doping configurations and contents on the ORR reaction pathway[23]. Copyright 2020 Wiley VCH GmbH. (d) The influence of different carbon-based defects on the 2-electron and 4-electron ORR volcano diagrams. Adapted with permission from [24]. Copyright 2018 American Chemical Society

Transition metal-based catalysts have attracted increasing attention from researchers in recent years due to their relatively low cost and good electrocatalytic activity. Conventional alloying strategies include constructing Ni-M (M = Co, Cu, Zn, Fe) structures, etc.[18,19]. Liu et al.[20] designed a bimetallic organic framework alloy (NiZn-MOF) catalyst based on NiZn, where, under the coordination of the Zn site, the high-valent Ni(2+δ)+ site was proven to be the primary active center for 2eORR. This dual-ion synergistic effect induced the optimization of the electronic structure at the metal sites, thereby promoting *OOH generation. In experiments, it showed up to 90% H2O2 selectivity, with mass activity and turnover frequency reaching 38.4 A·gmetal-1 and 42.0 h-1 at 0.25 V vs. RHE, respectively. Additionally, doping with non-metal elements (such as O, S, P, Se, etc.) has also been shown to effectively regulate the electronic structure. Sun et al.[21] prepared NiSe2, which exhibited excellent 2eORR catalytic properties, with an onset potential of about 0.6 V vs. RHE, and could achieve nearly 90% high selectivity in the 0 ~ 0.4 V vs. RHE range, with the synthesized H2O2 concentration reaching 988 ppm, surpassing most transition metal chalcogenides (Figure 2b). Furthermore, Xia et al.[22] developed a Chevrel phase chalcogenide-based Ni2Mo6S8 catalyst that could simultaneously activate the ligand effect of Ni, ensemble effect, and spatial confinement effect of Ni, Mo, and S in the active motif of the catalyst, where Ni metal atoms contributed the main ORR active sites. Meanwhile, co-doping with non-metal atoms could optimize the local electronic structure, making the *OOH binding energy closer to the top of the volcano plot, ultimately achieving a Faraday efficiency of up to 98% for H2O2, with a yield of approximately 90 mmol H2O2 gcat-1·h-1.
Non-metal catalysts, represented by carbon-based materials, are considered ideal alternatives to metal-based electrocatalysts due to their abundant resources, green and low-cost characteristics, excellent conductivity, and high catalytic activity. Since carbon atoms typically form σ bonds in sp2 hybridization, with the remaining pz orbitals overlapping to form delocalized large π bonds, this stable valence bond structure results in relatively weak catalytic activity for pure carbon materials[25]. Therefore, it is necessary to utilize the electronegativity differences brought about by heteroatom doping to modulate the charge/spin distribution on the original sp2 hybridized carbon structures, thereby optimizing the chemical adsorption of intermediates and enhancing intrinsic activity[26]. Heteroatom doping generally includes single doping, co-doping, and multiple doping. For single doping, non-metal elements with sp hybridized orbitals are primarily used, such as N, B, S, P, F, O, etc. For example, Li et al.[23] prepared N-FLG-8 with a nitrogen content as high as 19.2 at%, which exhibited over 95% H2O2 selectivity within a wide potential range of 0.30 ~ 0.70 V vs. RHE and provided a stable current density exceeding 20 mA·cm-2 in gas diffusion electrodes, where the proportion of pyrrolic nitrogen was found to be positively correlated with H2O2 selectivity, attributed to the adsorption of *OOH intermediates on carbon atoms near pyrrolic nitrogen causing heterocyclic distortion, thus optimizing *OOH intermediate adsorption (Figure 2c). Ri et al.[27] successfully prepared B-doped rGO catalysts that showed 95%~98.6% high selectivity for 2eORR, and when the current density was 200 mA·cm-2, the H2O2 yield reached up to 95.63  mg·cm-2·h-1; theoretical calculations indicated that isolated B atoms (BC2O-B and BC3-B) in B-doped rGO could serve as additional 2eORR active sites, thereby reducing the overall reaction free energy barrier. Chen et al.[28] prepared an oxygen-doped carbon nanosheet OCNS900 catalyst through template-assisted pyrolysis, achieving a high mass activity of 14.5 A g-1 at a low overpotential of 0.75 V vs. RHE, while exhibiting over 90% H2O2 selectivity between 0.45~0.6 V. DFT calculations and chemical titration strategies revealed the activity contributions of C=O, C—OH, and COOH groups in 2eORR, with C=O at defect edges being recognized as the most active reaction site, optimizing intermediate chemical adsorption to enhance intrinsic activity, combining high activity and selectivity; in the study by Lu et al.[29], carbon atoms adjacent to —COOH and C—O—C oxygen functional groups were also confirmed to be the main active sites for 2eORR.
In addition, compared to single heteroatom doping, the use of more than one type of heteroatom for doping graphitic carbon materials has proven to be a more effective strategy for enhancing the catalytic performance of CMFCs, due to the synergistic effects produced by the electronic interactions between different heteroatom dopants[30]. Typically, Song et al.[31] prepared carboxylated hexagonal boron nitride/graphene (h-BN/G) heterojunctions using activated carbon as the support, demonstrating 95% H2O2 selectivity and 95% Faraday efficiency, with an H2O2 production rate as high as 13.4 mol·g-1·h-1. It was found that the carboxylated h-BN/G configuration could promote oxygen adsorption, while stabilizing key intermediates (*OOH and *HOOH), and having a lower reaction energy barrier in the desorption step of *H2O2.

3.2 Defect Construction Engineering

Defect engineering typically refers to the introduction of surface or bulk defects in catalysts to regulate the coordination environment of active sites. Therefore, by introducing point defects, surface defects, or bulk defects, it is also possible to effectively regulate the coordination environment of active sites or provide new reactive sites, which can significantly alter the electronic structure and chemical properties of the catalyst, while optimizing electron transfer and the adsorption free energy of intermediates (*OH, *O, and *OOH)[32], and is considered another efficient method for enhancing the catalytic activity of catalysts.
In metal-based/metal oxide catalysts, defect sites in solid-phase catalysts typically signify distortions in the crystal structure, including vacancy defects, phase defects, substitutional defects, and dislocation defects[33]. Zhou et al.[34] successfully introduced cationic vacancies into nickel phosphide Ni2−xP-VNi through a cation engineering strategy, demonstrating high selectivity and activity under full pH range conditions, where the molar fraction of H2O2 exceeded 95%, and the Faraday efficiency was over 90%. The results indicated that cationic vacancies (VNi) could effectively induce optimization of geometric and electronic structures, thereby promoting the adjustment of oxygen adsorption patterns to an "end-on" configuration, making it easier for the 2eORR pathway to occur. Gao et al.[35], on the other hand, modified Fe2O3 via facet engineering and oxygen vacancy engineering. DFT calculations showed that the exposed (001) facets had intrinsic selectivity for H2O2 production, and oxygen vacancies could trigger high reactivity, providing sites for O2 adsorption and protonation, stabilizing *OOH intermediates, and preventing O bond cleavage, greatly resolving the contradiction between 2eORR activity and selectivity. Electrochemical performance tests revealed that α-Fe2O3 single crystals exhibited H2O2 selectivity above 88% in weakly acidic, neutral, and alkaline electrolytes, with an H2O2 yield of 454 mmol·gcat-1·h-1 at 0.1 V vs. RHE.
In non-metallic carbon-based catalysts, intrinsic defects can often directly serve as potential ORR active sites. By introducing edge defects (armchair and zigzag), point defects (vacancies and holes), or topological defects (such as pentagons, heptagons, Stone-Wales defects, and their combinations), the surface charge state of the carbon-based catalyst can be altered, thereby regulating the adsorption free energy of key intermediates and reducing the bandgap[36]. For example, Chen et al.[24] prepared sp2-defect-rich carbon-based materials that, under alkaline conditions, exhibited an onset potential close to the thermodynamic equilibrium potential (0.7 V) with H2O2 selectivity exceeding 70%. It was confirmed that some carbon atoms at the intersection of pentagonal and heptagonal defects had stronger *OOH adsorption energy, thus approaching the top of the thermodynamic volcano plot (Figure 2d). In certain cases, the presence of intrinsic defects can even surpass the influence of heteroatoms in determining the selectivity and activity of ORR. For instance, Kim et al.[37] prepared a series of nitrogen-doped reduced graphene oxide (NrGO) with different nitrogen contents and forms. The final Pourbaix diagram still indicated that the dominant catalytic activity came from the more abundant sp2 carbon sites adjacent to sp3 carbon regions.

4 Geometric Structure Regulation of Electrocatalysts

In addition to enhancing the intrinsic activity of a single active site by regulating the electronic structure of the catalyst, the catalytic performance of electrocatalysts can also be further optimized by adjusting the geometric structure of the catalyst. Currently, measures such as size effects, pore/interlayer structure regulation, and surface morphology control can be employed to increase the number/density of exposed active sites on electrode materials, adjust the adsorption mode of oxygen at the active sites, or improve the mass transfer conditions at the electrode/electrolyte interface, thereby significantly enhancing the activity and selectivity of the ORR.

4.1 Size Control

The size effect generally refers to the impact of catalyst particle size on catalytic reactions, and it has been found that the selectivity of H2O2 is closely related to the size of the catalyst nanoparticles. In metal catalysts, smaller catalyst particle sizes have been found to promote end-to-end adsorption of O2, thereby increasing the selectivity of H2O2[38]. Typically, Weber et al.[39] deposited Ptn clusters of different sizes on indium tin oxide films, and their studies showed that the selectivity of H2O2 strongly depends on the size of the Ptn clusters. As the size of the Pt clusters decreased from 14 nm to 5 nm, the ratio of H2O2 to H2O continuously increased to 0.5 (Figure 3c). Yang et al.[40] also observed a similar phenomenon, where the selectivity of H2O2 gradually increased as the size of Pt nanoparticles decreased and the distance between particles increased. This is attributed to the fact that smaller particles have a more negative zero total charge potential, which can enhance the adsorption of reactive species such as *OH; when the inter-particle distance in the nanoarray approaches or exceeds ten times the particle size, the diffusion rate of reactants or intermediates/products to and from the particles is also expected to increase significantly.
图3 电催化剂的几何结构调控策略促进2eORR合成H2O2。（a）碳涂层调控了O2在Pt表面的吸附模式，其中末端吸附更有利于H2O2的生成[44]；（b）CoSe2中不同层间距离对2eORR合成的反应能垒、理论极限电位及中间体吸附能的影响[45]；（c）不同Ptn团簇尺寸对于ORR合成的H2O2/H2O比例的影响[39]；（d）碳纳米管尖端结构活性位点的形成机制[46]

Fig. 3 The geometric structure control strategy of electrocatalysts promotes the H2O2 synthesis. (a) The carbon coating modified on the surface of Pt allows O2 to undergo end configuration adsorption on the Pt surface, thereby facilitating the generation of H2O2[44]. Copyright 2014 American Chemical Society. (b) The influence of different interlayer distances in CoSe2 on the reaction energy barrier, theoretical limit potential, and intermediate adsorption energy of 2eORR synthesis[45]. Copyright 2021 Wiley VCH GmbH. (c) The effect of different Ptn cluster sizes on the H2O2/H2O ratio in ORR synthesis[39]. Copyright 2015 American Chemical Society. (d) The formation mechanism of active sites in the tip structure of carbon nanotubes[46]. Copyright 2023 Wiley VCH GmbH

Moreover, a similar size effect can also be observed in carbon-based materials, but it is more often reflected in the correlation with ORR activity. Deng et al.[41]prepared graphene with particle sizes ranging from 20 to 100 nm using a ball milling method and found that the smaller the graphene crystal size, the higher the ORR limiting diffusion current density. Byambasuren et al.[42]prepared nitrogen-doped carbon-based catalysts with an average particle size range of 20, 45, and 75 μm, discovering that the reduction in the size of carbon-based catalyst particles leads to an increase in surface area and pore volume, which is beneficial for the mass transfer of reactants, thereby achieving higher activity in the limiting current region. Sun et al.[43]used a ball milling method to prepare a series of amorphous carbon clusters rich in carboxyl oxygen-containing functional groups and of different sizes, finding that smaller-sized amorphous carbon lattices with abundant edges are conducive to enhancing 2eORR activity, while basal plane carbon atoms in small-sized carbon planes doped with ether are the most favorable sites for H2O2selectivity.

4.2 Pore/Interlayer Structure Regulation

Constructing pore/interlayer structures typically provides more surface area, increasing the contact area between the catalyst and reactants, or influencing the diffusion and molecular adsorption of reactants, thereby affecting the reaction rate and selectivity. For two-dimensional materials, interlayer spacing is one of the important characteristics of the interlayer structure. Zhang et al.[45] combined theoretical analysis and experimental exploration to investigate the effect of interlayer distance on the adsorption of *OOH intermediates and the thermodynamics of the reaction for the two-dimensional material CoSe2. First, through DFT calculations, it was found that Co sites with different interlayer distances in CoSe2 are all located on the left side of the ORR volcano plot, indicating a strong *OOH binding energy. When the interlayer distance of CoSe2 is increased from 0 Å to 2 Å, CoSe2 exhibits the highest electron density and an enhancement in the antibonding orbital filling of Co, leading to weakened *OOH adsorption, thus bringing it closer to the optimal binding strength on the volcano plot; further increasing the interlayer distance enhances *OOH adsorption, making it more prone to cleavage into *O species. Therefore, the interlayer distance of CoSe2 needs to be controlled around 2 Å to achieve the most suitable *OOH binding energy. Subsequently, using ethylenediamine (DETA) molecules as a soft template, a sc-CoSe2 structure with an interlayer spacing of 2.2 Å was synthesized, achieving over 95% H2O2 selectivity in a wide potential range of 0 ~ 0.6 V vs. RHE. In situ X-ray absorption fine structure spectroscopy further confirmed that the strong interlayer coupling of sc-CoSe2 altered the surface electronic structure, weakening *OOH adsorption, and further preventing O—O bond breaking, which is favorable for H2O2 synthesis.
For three-dimensional materials, such as the porous structures of carbon-based materials, different pore structures (micropores, mesopores, and macropores) have been proven to play varying regulatory roles in the reaction or mass transfer stages of catalytic processes[30]. Generally speaking, abundant micropore structures possess high specific surface areas, capable of exposing more active sites; however, due to the longer residence time of generated H2O2 within the micropores, it is more likely to gain electrons and be further reduced to H2O[47]. Macropore structures offer relatively superior mass transfer conditions, strong accessibility to reactants, and facilitate the diffusion of products, thus enhancing catalytic activity, although the density of reactive sites may be limited by the specific surface area[48]. In comparison, mesoporous structures can combine the advantages of both micropores and macropores. Therefore, constructing a hierarchical pore structure with micropores, mesopores, and macropores has become a consensus in the design of porous carbon-based catalysts. Typically, Liu et al.[49] synthesized hierarchically porous carbon HPC via a two-step hydrothermal method, which exhibited over 90% H2O2 selectivity at pH = 1 ~ 4, with the highest H2O2 generation rate reaching 395.7 mmol·h-1·g-1. Wu et al.[50] further enhanced the catalytic activity of amorphous nickel oxide NiOx by constructing a mesoporous carbon nanosheet carrier, achieving nearly 90% selectivity within a wide potential range of 0.15 ~ 0.60 V vs. RHE, and promoting the adsorption of *OOH intermediates in a terminal mode, thereby favoring the 2eORR process.

4.3 Surface Morphology Control

Typically, from one-dimensional quantum dots to two-dimensional nanowires and nanosheets, and then to three-dimensional nanotubes and porous structured catalysts, the wide variation in molecular symmetry and size provides an ideal platform[30] for designing various high-performance carbon material catalysts. Different surface morphologies also have a significant impact on electron transfer and the rapid mass transport of reactants and products. For example, Yang et al.[51] used an aerosol-assisted assembly method to develop a novel carbon-nanotube-graphene hybrid nanostructure, where the integration of carbon nanotubes and graphene in the hybrid nanospheres can synergistically enhance electronic conductivity and increase specific surface area, thereby greatly improving the activity of ORR.
In addition, the surface morphology of the catalyst can be effectively modulated to control the adsorption mode of oxygen. Choi et al[44] synthesized a carbon-coated catalyst on commercial Pt/C catalysts via acetylene chemical vapor deposition (CVD) (see Figure 3a), successfully inducing the adsorption mode of oxygen molecules on the Pt surface from a "bridging mode" to an "end-on mode" by eliminating accessible Pt ensemble sites, which was proven to alleviate the O—O bond breaking tendency in *OOH and enhance ORR selectivity and activity. The experimental results showed that this carbon-coated Pt catalyst exhibited an onset potential of 0.7 V vs. RHE and 41% H2O2 selectivity; moreover, due to the protective effect of the amorphous carbon layer, it prevented the leaching and loss of Pt particles under acidic operating conditions, significantly improving reaction stability.
Constructing cutting-edge structures has also been found to be more conducive to subsequent heteroatom modifications, thereby further improving the 2eORR reaction pathway. Long et al.[46] constructed a rich array of carbon nanotube tips (Fig. 3d) by controlling the ball-milling time, thus enhancing the enrichment of N, S heteroatoms at the short nanotube tips. This made N, S-TCNTs exhibit a more positive onset potential of 0.78 V vs. RHE and up to 94.5% H2O2 selectivity (0.56 V vs. RHE), which is attributed to moderate and medium *OOH adsorption energy, avoiding overly strong or weak *OOH adsorption, thus favoring the formation of H2O2.

5 Surface Modification and Functionalization of Electrocatalysts

When the electrode comes into contact with the electrolyte solution, a double-layer structure mainly composed of closely stacked ion clouds is formed due to electrostatic attraction, and this structure also has a significant impact on the kinetic and thermodynamic characteristics of electrochemical reactions. The double-layer structure is closely related to factors such as the ionic properties of the electrolyte solution and the composition at the electrolyte/electrocatalyst interface. Therefore, in addition to directly designing the active sites of the catalyst, chemical modification or alteration of the catalyst surface using organic small molecules or groups such as ionic liquids and surfactants can also regulate the adsorption behavior of reaction intermediates through weak intermolecular interactions or hydrogen bond networks in the double layer, thereby influencing the reaction pathways and selectivity.
Considering the charged nature of some catalysts and reaction intermediates, modifying the catalyst surface with ionic surfactants is expected to construct an interfacial built-in electric field to further influence the catalytic process. Wang et al.[52] proposed modifying the surface of carbon black with a layer of cationic surfactant hexadecyltrimethylammonium bromide (CTAB) (Figure 4a), which can utilize the in-situ Coulombic effect produced by the interfacial electric field to allow for the timely desorption of generated H2O2, preventing its further reduction, thereby achieving an H2O2 selectivity exceeding 95% within the potential window of 0.80 V vs. RHE. Chen et al.[53] further discussed the applicability of CTAB modification in supramolecular catalysts, using cobalt phthalocyanine as a model electrocatalyst, successfully constructing a self-assembled monolayer of single-tailed cationic surfactants, significantly reducing the charge transfer resistance and promoting the oxygen uptake rate, ultimately increasing the H2O2 selectivity from 60% to 93%, with intrinsic activity improved by more than three times.
图4 电催化剂的表面修饰功能化策略促进2eORR合成H2O2。（a）在碳基催化剂表面修饰CTAB表面活性剂能够促进H2O2的解吸过程[52]；（b）利用离子液体修饰乙炔黑以提高H2O2选择性，其中离子液体中烷基链的延长能够影响碳电极的疏水性[55]

Fig. 4 The geometric structure control strategy of electrocatalysts promotes the H2O2 synthesis. (a) Surface modification of CTAB surfactants on carbon-based catalysts can promote the desorption process of H2O2[52]. Copyright 2020 Elsevier Inc. (b) Acetylene black-modified with ionic liquids could improve H2O2 selectivity, and the extension of alkyl chains in ionic liquids can affect the hydrophobicity of carbon electrodes[55]. Copyright 2020 Elsevier B.V

In addition, the activity of protons at the electrode interface also greatly affects the proton-coupled electron transfer (CPET) step. Therefore, for multi-proton electron coupling transfer processes represented by 2eORR, adjusting the proton activity and hydrogen bond structure at the charged interface is expected to further improve the ORR reaction kinetics. Wang et al.[54] successfully regulated the local proton activity on Pt and Au surfaces using a series of protic ionic liquids (all with tributylmethylphosphonium dicyanamide as the anion). It was found that when the pKa values of the proton cations in the ionic liquid and the reaction intermediates in the rate-limiting step of ORR are similar, the ORR activity can be maximally enhanced. In situ infrared characterization and proton vibrational wave function calculations showed that there is a stronger hydrogen bond between the proton cation and the ORR intermediate, which, while keeping the activation free energy of the reaction basically unchanged, increases the proton tunneling dynamics by about 10 to 1000 times, thus achieving a higher ORR exchange current density.
Ionic liquid modification has also been proven to alter the hydrophobicity of electrocatalysts to enhance H2O2 selectivity. Liu et al.[55] used ionic liquids to modify acetylene black and found that the extension of alkyl chains in ionic liquids could influence the hydrophobicity of carbon electrodes (Figure 4b), thus preventing them from being flooded or occupying active sites, increasing H2O2 selectivity from 48% to 90%. The addition of the surfactant CTAB further promoted the desorption of H2O2 at the electrode interface, with this IL@AB-CTAB system exhibiting up to 99.5% H2O2 selectivity and stability exceeding 100 h. Wang et al.[56] also utilized the organic molecule 1-octadecanethiol (ODT) to modify hollow mesoporous carbon spheres, forming a superhydrophobic interface that provided an O2-enriched surface layer at catalytic active sites, maintaining 94.6% H2O2 selectivity over a wide potential range, with a yield of 838.8 mmol·gcat−1·h−1.

6 Atomic-Level Active Site Design

In traditional heterogeneous metal-based catalysts, due to the widespread non-uniform aggregation or clustering of metal atoms, some active sites are insufficiently exposed, leading to low atomic utilization and limiting their electrocatalytic performance[57]. Since Weber et al.[39] demonstrated that reducing the size of catalyst particles could significantly increase ORR activity, researchers have gradually developed a series of atomic-level active site design strategies in recent years, including single-atom catalyst (SACs) design and supramolecular catalyst design, achieving atomic-level dispersion of metal active sites and making atomic utilization approach 100%. The metal atom center-heteroatom/organic ligand structure (M—N/O—C, M = Fe, Co, Mn, Cu, Ni, etc.) as a typical SACs structure, not only reduces synthesis costs but also possesses high stability and catalytic activity. In particular, thanks to the atomic-level dispersed active sites in SACs, oxygen molecules tend to adsorb terminally on SACs, avoiding μ-peroxo bridged adsorption, thus alleviating O—O bond cleavage and improving H2O2 selectivity[58,59]. Additionally, the simplicity and homogeneity of such atomic-level active site catalyst structures also facilitate precise identification of active sites, especially through adjusting the metal center element, ligand structure, and substrate to optimize the electronic structure and adsorption properties of the active sites, thereby further enabling precise control of the reaction pathways[60].

6.1 Metal Active Center Regulation

In atomic-level active site electrocatalysts, the enhancement of ORR activity mainly stems from the redistribution of charges between the metal active centers and the surrounding coordinating atoms, which can effectively modulate and optimize the adsorption energy of reaction intermediates in the rate-determining step, thereby improving the thermodynamics and kinetics of the reaction[61], thus optimizing the electronic structure by selecting appropriate central metal atoms is the most direct and effective method for regulating catalytic performance[62].
For SACs with a coordination structure based on the basal plane of graphite carbon, Sun et al.[63] comprehensively compared the catalytic performance of M—N—C type SACs with Mn, Fe, Co, Ni, and Cu as metal active centers. Among them, Co—N—C exhibited over 80% H2O2 selectivity, achieving a high H2O2 production rate of up to 4.33 mol·gcat-1·h-1 at a current density of 50 mA·cm-2. Theoretical calculations showed that Co—N—C has the most suitable binding energy with *OH, and its theoretical limiting potential is also closer to the top of the volcano plot for 2eORR; Gao et al.[58] systematically studied the changes in binding energy between the above-mentioned metal active centers and ORR intermediates. As the number of valence electrons in M increases (from Mn to Cu), the antibonding states generated by the coupling between the d orbitals of M atoms and the 2p orbitals of the bonding O atom in intermediates move downward in energy (Figure 5a). Therefore, the binding energies of *OOH, *O, and *OH with M atoms gradually weaken. Among them, Ni—N—C and Cu—N—C have too weak binding energy with O2, thus increasing the energy barrier for oxygen activation and proton-electron coupled transfer; Fe—N—C and Mn—N—C have too strong binding energy with O2, leading to a significant increase in the energy barrier for the conversion step from OOH to H2O2; in comparison, Co—N—C has the most suitable binding energy with oxygen-containing intermediates, with the lowest reaction free energy barrier, and shows more than 90% H2O2 Faraday efficiency at 0.6 V vs. RHE, with catalytic activity that can last for 10 hours without decay.
图5 电催化剂的原子级活性位点策略促进2eORR合成H2O2。（a）不同金属活性中心对于M—N—C型单原子催化剂d带中心及自由能垒的影响[58]；（b）Co—N—C单原子催化剂中的第一配位域及第二配位域，其中掺杂氧原子及修饰C—O—C含氧官能团有利于2eORR反应能垒的降低[67]；不同金属活性中心对于M@PC-N4型超分子催化剂（c）2eORR火山图及（d）理论极限电位的影响[64]

Fig. 5 The geometric structure control strategy of electrocatalysts promotes the H2O2 synthesis. (a) The influence of different metal active centers on the d-band center and free energy barrier of M—N—C type single atom catalysts[58]. Copyright 2020 Elsevier Inc. (b) The first and second coordination domains in Co—N—C single atom catalysts, where doping oxygen atoms and modifying C—O—C oxygen-containing functional groups are beneficial for reducing the energy barrier of the 2eORR reaction[67]. Copyright 2021 American Chemical Society. The influence of different metal active centers on the (c) 2eORR volcano plot and (d) theoretical limiting potential of M@PC-N4 supramolecular catalyst[64]. Copyright 2019 American Chemical Society

For supramolecular catalysts with large molecular rings such as titanium phthalocyanine or porphyrin as coordination structures, Guo et al[64] based on spin-polarized DFT calculations and thermodynamic analysis, explored the impact of a series of metal active centers (M = Ni, Cu, Zn, Pd, Ag, Pt, Au) on the ORR activity of typical metal-titanium phthalocyanine structures M@Pc-N4 (Figure 5c and Figure 5d). The results showed that phthalocyanine with Zn atom as the center (Zn@PcN4) could enhance *OOH adsorption while inhibiting *O adsorption, thereby weakening the 4eORR pathway, exhibiting the lowest 2eORR overpotential of about 0.15 V. Lee et al[65] comparatively calculated the ∆G*OOH and theoretical limit potential for cobalt phthalocyanine (CoPc), iron phthalocyanine (FePc), and nickel phthalocyanine (NiPc). The results indicated that CoPc was closer to the optimal ∆G*OOH (4.11 eV) and had the lowest H2O2 overpotential (120 mV). Zhao et al[66] first screened 32 metal porphyrin structures with different metal centers through theoretical calculations, finding that metal centers with more positive charges generally have stronger *OOH adsorption strength. Among them, *OOH has the most suitable adsorption strength on Co porphyrin structures, where the metal center is strongly anchored by nitrogen atoms mainly through electron coupling between N 2p orbitals and Co 3d orbitals, which was confirmed to have the lowest theoretical overpotential (0.04 V). Subsequently, hydrogen-bonded cobalt porphyrin framework PFC-72-Co was prepared using molecular self-assembly technology. Electrochemical tests showed that the Co porphyrin structure had a high ORR onset potential of about 0.68 V vs. RHE in 0.1 M HClO4 and over 90% H2O2 selectivity.

6.2 Local Coordination Environment Regulation

Although the activity of atomic-level electrocatalysts mainly originates from metal active sites, the local coordination environment (including the first coordination domain primarily consisting of heteroatoms and vacancies directly involved in coordination, and the second coordination domain mainly characterized by functional group modifications[67]) also influences the reaction pathways and activation energy. On one hand, charge transfer and redistribution between the metal atom and surrounding coordination atoms can alter the electron density of the active center, thereby affecting the interaction strength with reactant molecules, including the formation of O—H bonds and the breaking of O—O bonds, and reducing the reaction energy barrier[61]. On the other hand, the adsorption and desorption processes of reactants/intermediates induced by the coordination domain can enhance the catalyst surface's ability to capture target reaction species, promoting the stable existence of reaction intermediates, and thus providing additional active sites for the ORR process.
For typical Co—N—C structures, adjusting the coordination atom morphology in the first coordination sphere and introducing defects or oxygen-containing functional groups in the second coordination sphere has been shown to directly affect the adsorption and desorption process of intermediates. Tang et al[67] explored the impact of introducing O atoms in the first coordination sphere and modifying oxygen-containing functional groups in the second coordination sphere on ORR selectivity (Figure 5b). As O coordination replaces some N coordination in the first coordination sphere, the d-band center of Co continuously shifts downward, weakening the binding strength of *OOH on the central Co atom. This causes the active site to shift to the C atom adjacent to the O coordination, thereby regulating selectivity; at the same time, the introduction of C—O—C and C=O in the second coordination sphere can use steric hindrance to tilt the adsorbed *OOH towards the Co center, thus passivating the O—O bond and activating the C—O bond, further enhancing the activity and selectivity of 2eORR. Chen et al[69] found that when the N atom in the first coordination sphere is a pyrrolic nitrogen, Co—N4—C tends more towards 2eORR, while when the coordinating N atom is a pyridinic nitrogen, it tends more towards 4eORR. This is attributed to the fact that after adsorbing *OOH, the pyrrolic CoN4 exhibits a high spin state, which promotes the desorption of *OOH to produce H2O2. Xiao et al[70] induced the formation of a hyper-coordinated N4Ni1O2 structure by introducing carboxyl oxygen-containing functional groups (OCNT) on multi-walled carbon nanotubes, with the two O atoms coming from the carboxyl groups in OCNT. This six-coordinate Ni SACs catalyst can achieve over 90% H2O2 selectivity and operate continuously for 24 hours under conditions up to 300 mA·cm−2.
For supramolecular catalysts such as porphyrins/titanocenes, the electronic distribution of the metal center can typically be significantly modulated by introducing heteroatoms or functional groups in the second coordination sphere. Typically, Liu et al[68] investigated the impact of different β-substituents (H, Et, Br, and F) on the electronic structure of cobalt porphyrins, where the electron-donating ethyl substituent (Et) could shift the Co-dz2 orbital center upwards, resulting in the most negative intermediate binding energy and the strongest O2 affinity; conversely, the electron-withdrawing Br and F substituents lowered the d-band center of Co, weakening the binding strength between cobalt porphyrin and intermediates, thereby inhibiting O2 dissociation and favoring H2O2 formation. Subsequently, through the synthesis of CoPorX/CNT structures (X = H, Et, Br, and F), it was experimentally confirmed that the ORR activity trend followed H ≈ F > Et > Br, with the H2O2 selectivity trend being Br > F > H > Et. Among these, CoPorF/CNT could provide the highest turnover frequency (TOF) of (3.51 ± 0.06) s-1 at an overpotential of 200 mV, and a high H2O2 production rate of up to 10.76 mol·gcat-1·h-1 (H2O2 selectivity > 94.3%). Sun et al[71] modified the derivatives of nickel phthalocyanine (NiPc) with organic groups of varying conjugation degrees (phenyl, naphthyl, and pyrene functional groups), demonstrating over 85% H2O2 selectivity under full pH conditions. Theoretical calculations further revealed that an increase in conjugation degree could adjust the HOMO-LUMO orbitals of the phthalocyanine derivatives' nickel active centers, lowering the d-band center of the nickel metal, thus optimizing the binding energy with *OOH intermediates and the theoretical limiting potential, enhancing the selectivity and activity for H2O2.
It is noteworthy that some supramolecular catalysts also need to be loaded on hierarchical porous carbon materials, carbon black, graphene, and other carriers through chemical adsorption, weak interactions, etc., for actual electrocatalytic processes. Therefore, modifying the loading carriers of supramolecular catalysts can also affect the coordination situation of metal active centers. Lee et al.[65] revealed that in a titanium phthalocyanine cobalt structure with carbon nanotubes as the carrier (CoPc/CNT), introducing oxygen functional groups (such as COOH, OH, C—O—C, and C=O) on the surface of carbon nanotubes could strengthen the π-π interaction between CoPc and the carbon surface by generating dipole moments. This could further enhance the adsorption of CoPc on the carrier, while shifting the d-band center of Co in CoPc downwards and producing electron-deficient Co centers. These effects can modulate the adsorption energy of key ORR intermediates, bringing them closer to the top of the volcano plot. Ultimately, it demonstrated a TOF exceeding 55 s-1 and a low overpotential of 280 mV, while being able to operate continuously for 100 h at 300 mA·cm-2 with H2O2 selectivity above 90%.

7 Conclusions and Prospects

The in-situ preparation of H2O2 through the oxygen reduction reaction (ORR) is a green, low-cost, and environmentally friendly electrosynthesis technology that has the potential for low-carbon production and the utilization of surplus electricity. However, the current synthesis of H2O2 via ORR still suffers from issues such as low selectivity and slow reaction kinetics, which have become key factors limiting the further large-scale application of this technology. Although existing research has developed strategies to modulate the intrinsic activity of catalysts, including chemical doping, defect construction, size/porosity/surface morphology control, surface functionalization by chemical modification, and atomic-level active site design, there remains a certain gap between current catalyst design strategies and practical application needs. Therefore, the following areas of research need to be given special attention:
(1) Under the current background of "carbon peak" and "carbon neutrality" goals, developing green, low-cost, and low-carbon emission efficient electrocatalysts remains a key to promoting development in the fields of green chemistry and sustainable energy. At present, noble metal-based electrocatalysts have become the main research direction in ORR reactions due to their excellent catalytic activity. However, non-noble metal materials (transition metals, carbon-based materials, etc.) have also gradually attracted attention from researchers in recent years due to their abundant resources, low cost, and higher ORR selectivity. In particular, M–N–C single-atom catalyst structures, with carbon-based materials as carriers and transition metals as active centers, not only reduce the cost of catalysts but also possess high selectivity and activity, making them ideal alternatives to noble metals. Additionally, the clear active sites of single-atom catalysts facilitate precise identification, providing flexible room for optimizing the electronic structure and adsorption characteristics of electrocatalysts. Combined with artificial neural networks and machine learning, it is possible to quickly screen metal center active sites, ligand structures, and carrier structures to guide the synthesis of electrocatalysts, which is expected to accelerate the development of high-performance electrocatalysts and achieve precise regulation of reaction pathways.
(2) In the actual electrocatalytic process, factors such as potential application and interfacial microenvironment may lead to dynamic reconstruction of the valence state and surface topological structure (roughness, porosity, crystallinity, and crystal facets) of the electrocatalyst, thereby causing the migration of active centers and adjustment of reaction pathways. Therefore, combining in-situ Raman, in-situ infrared, and in-situ X-ray absorption spectroscopy and other in-situ spectroscopic techniques, to strengthen the real-time identification of active sites under reaction conditions and the dynamic tracking of electrocatalysts/morphology/electronic structure, is conducive to further enhancing a deeper understanding and exploration of the electrocatalytic mechanism under real reaction environments. In addition, developing catalyst design strategies based on pre-regulation or in-situ regulation of active sites, such as constructing anion pre-oxidation-induced restructured structures, or using the plasmon resonance effect and the in-situ activation effect of pulsed electrocatalysis to alter the energy distribution at the electrode interface, can further induce favorable reconstruction of the electrocatalyst to enhance its catalytic performance.
(3) The ORR electrocatalytic reaction, as a typical gas-liquid-solid three-phase interfacial reaction, in addition to the design and construction of active sites at the atomic scale, also needs to consider the influence and coupling rules from the molecular scale to the particle scale (discrete units such as bubbles, droplets, etc.) and then to the reactor scale. Although methods including ab initio molecular dynamics simulations, microkinetic simulations, and Monte Carlo simulations have been developed to intuitively reflect the concentration gradient of the electrocatalytic process and the dynamic adsorption process of reactants and intermediates, it is still necessary to establish mesoscale catalytic models as a communication bridge between various scales, to deeply reveal the impact mechanisms of the interfacial microenvironment (electrolyte, pH, temperature, etc.) and the interfacial energy and mass transfer processes on the kinetics of electrochemical reactions, providing theoretical basis and guidance for cross-scale optimization of electrocatalytic performance and improvement of energy conversion efficiency.
(4) To meet the demands of industrial-level electrocatalysis, a balance must be sought between the precise controllable synthesis of electrocatalysts and practical production efficiency. It is necessary to deeply investigate the performance degradation mechanisms of catalysts under high current density and long-term operation, and to explore effective methods for catalyst regeneration or repair, in order to improve the durability and reliability of electrocatalysts and electrolytic devices in practical applications. In addition, since H2O2 produced through traditional liquid-phase electrocatalytic systems usually contains a certain concentration of electrolyte salt solutions, additional separation and purification steps are required in practical applications, which increases the preparation cost. Therefore, utilizing systems such as porous solid electrolytes and gas diffusion electrodes to directly synthesize high-purity H2O2 solutions without electrolytes could be a promising direction for future development.

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