Role and advances in nonmetal doping of electrocatalysts for the hydrogen evolution reaction: a review

Abstract

Hydrogen produced by water electrolysis is considered an ideal, safe, and clean energy source. However, the high cost of noble metal-based electrocatalysts and the high overpotential of non-noble metals for the hydrogen evolution reaction (HER) are key bottlenecks limiting the practical application of water splitting technology. To tackle this challenge, extensive efforts have been made to explore strategies for improving the HER activity. This paper systematically reviews the critical roles of nonmetallic doping in enhancing HER catalyst performance, including electronic structure adjustment, active site regulation, and the generation of synergistic effects. Subsequently, we categorized the reported nonmetal-atom-doped HER catalysts by composition and introduced the mechanisms behind their high performance and long-term stability. In addition, this review also provides a comprehensive summary of the synthesis strategies and characterization techniques associated with non-metallic doping, aiming to offer practical guidance for future research and catalyst development in this field. Finally, this review discusses the development of non-metal-doped electrocatalysts, the key challenges in industrial water electrolysis, and the promising application of artificial intelligence (AI) in accelerating catalyst design and system optimization.

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1. Introduction

Although fossil fuels have brought tremendous benefits to the development of human society, their associated environmental costs are becoming increasingly prominent.¹ This requires us to seek a balance between economic development and environmental sustainability.^2,3 In the long term, society needs to gradually reduce its dependence on fossil fuels and explore alternative energy forms and carriers.^4–6

Hydrogen, as a type of renewable energy, with high calorific value (1.42 × 10⁸ J kg⁻¹) and zero carbon emission, has attracted widespread attention from researchers.^7–9 There are various methods to obtain hydrogen, among which water electrolysis is considered to be promising technology with the potential for near-zero net CO₂ emissions.^9–11 Water electrolysis involves the hydrogen evolution reaction (HER) occurring at the cathode and the oxygen evolution reaction (OER) taking place at the anode.^12–14 Although the HER is considered to be kinetically favorable, its catalytic performance can still be limited under certain conditions, particularly at high current densities or in highly concentrated electrolytes, due to issues such as active site saturation, hydrogen bubble accumulation, and inefficient electron transport. Therefore, further optimization of the HER catalytic efficiency remains necessary.^15–17 To address this challenge, the use of effective electrocatalysts is necessary to accelerate the HER kinetics and enhance the efficiency of the hydrogen production process. The development of advanced electrocatalysts with optimized properties has become a central focus in water electrolysis research and development.^18–20

It is necessary to clarify that the key factors influencing the performance of electrocatalytic materials are the number of active sites on the electrocatalyst surface and the intrinsic activity of each active site.^21,22 Since the surface structure of electrocatalysts can be deliberately tuned and engineered, achieving high-performance catalysts requires extensive efforts in structural optimization and material design.^23–25 In recent years, a variety of advanced design strategies have been proposed to optimize the surface structure of catalysts and achieve excellent electrocatalytic performance, including atomic doping, interface engineering, defect engineering, strain engineering, and others.^26–28 Among these strategies, heteroatom doping is widely regarded as a highly promising method due to its numerous advantages.^29–31 Heteroatom doping refers to the introduction of different metal or metalloid atoms into the main structure of the catalyst, enabling the electrocatalytic material to undergo performance-related changes in conductivity, morphology, and band gap energy. Depending on the type of heteroatom, doping can be categorized as metal atom doping, nonmetal atom doping, and metal–nonmetal co-doping.^32–34 Among them, nonmetallic atom doping offers advantages such as low cost, excellent stability, uniform distribution, and minimal aggregation. Its impact on catalysts is primarily reflected in the following aspects (Fig. 1):

(1) Modulation of the electronic structure: nonmetal doping to modulate the electronic structure of catalysts is an effective strategy for enhancing the water-splitting performance by adjusting the electronic states, energy band structure, and adsorption characteristics.^35,36

(2) Regulating active sites: nonmetal doping can enhance the catalytic reaction kinetics by increasing the density of active sites and enhancing the intrinsic activity of catalytic centers.^37,38

(3) Enhancing electrical conductivity: the enhancement of electrical conductivity can accelerate the rate of electron transport in the electrocatalysis process, thereby improving the intrinsic activity of the catalyst.^39,40

(4) Synergistic effects: the synergistic effect between the doping atoms and the catalyst backbone can effectively regulate the electronic structure of the active center and increase the concentration of active sites, thereby significantly improving the intrinsic activity of the electrocatalytic process.^41,42

Fig. 1 Schematic representation of the role of nonmetal doping in the HER.^43–46 Reprinted with permission from ref. 43. Copyright 2021, American Chemical Society. Reprinted with permission from ref. 44. Copyright 2024, Elsevier. Reprinted with permission from ref. 45. Copyright 2022, Elsevier. Reprinted with permission from ref. 46. Copyright 2023, Elsevier.

The application of nonmetal-doped catalysts in the HER has witnessed significant progress in recent years (Fig. 2). This review systematically summarizes recent advances in nonmetallic doping strategies for enhancing the electrocatalytic performance of HER catalysts. First, six mainstream doping methods, including hydrothermal synthesis, calcination, and electrochemical deposition, are introduced, with a comparative analysis of their respective advantages and limitations. Next, four primary mechanisms through which nonmetallic doping improves the catalytic performance are discussed: regulation of the electronic structure, optimization of active sites, enhancement of electrical conductivity, and the construction of synergistic interfacial effects. The review also highlights typical characterization techniques applicable to nonmetal-doped systems, such as HRTEM, EDS, Cs-corrected TEM, XRD, XPS, XANES, and EXAFS. Subsequently, the catalytic behavior and regulatory features of representative nonmetallic dopants, including phosphorus (P), sulfur (S), nitrogen (N), boron (B), and halogens, are analyzed in detail through a survey of specific research studies. Finally, the review discusses existing challenges in the field and proposes future directions, including the application of artificial intelligence (AI) to accelerate catalyst design and optimization, the promotion of industrial-scale green hydrogen production, and the use of abundant seawater as a sustainable electrolyte. It is hoped that this work will serve as a valuable reference and inspiration for the design and application of nonmetal-doped electrocatalysts.

Fig. 2 Research progress for nonmetal-doped materials as HER electrocatalysts.^47–53 Reprinted with permission from ref. 47. Copyright 2018, Wiley. Reprinted with permission from ref. 48. Copyright 2019, Elsevier. Reprinted with permission from ref. 49. Copyright 2020, Wiley. Reprinted with permission from ref. 50. Copyright 2021, Wiley. Reprinted with permission from ref. 51. Copyright 2022, Wiley. Reprinted with permission from ref. 52. Copyright 2023, Elsevier. Reprinted with permission from ref. 53. Copyright 2024, American Chemical Society.

2. Mechanisms of water splitting

In the water electrolysis system for hydrogen production, the electrolyser is composed of the cathode, anode, and electrolyte. The overall electrochemical water splitting process comprises two half-reactions: the HER occurring at the cathode and the OER taking place at the anode. The specific chemical reaction pathways for water electrolysis are influenced by the prevailing reaction conditions:

Overall water splitting reaction:

H₂O → H₂ + 1/2O₂ (1)

In acidic electrolyte:

Anode OER:

H₂O − 2e⁻ → 1/2O₂ + 2H⁺ (2)

Cathodic HER:

2H⁺ + 2e⁻ → H₂ (3)

In alkaline electrolyte:

Anode OER:

2OH⁻ − 2e⁻ → 1/2O₂ + H₂O (4)

Cathodic HER:

2H₂O + 2e⁻ → H₂ + 2OH⁻ (5)

2.1 HER

The HER occurs at the cathode of the electrolytic cell, where water is reduced to produce hydrogen.⁵⁴ Generally, the HER is divided into two types:

In acidic media, the reduction reaction of H⁺/H₂O that occurs on the electrode surface goes through three consecutive steps, and the specific reactions are as follows.

H⁺ + e⁻ + * → H* (Volmer step)

H⁺ + e⁻ + H* → H₂ + * (Heyrovsky step)

H* + H* → H₂ + 2* (Tafel step)

In an acidic medium, the reactions occurring on the catalyst surface involve two prominent mechanistic pathways: the Volmer–Tafel mechanism and the Volmer–Heyrovsky mechanism. In the Volmer step, H⁺ is first firmly adsorbed on the surface of the cathode, generating adsorbed hydrogen species (H*). Then, when the concentration of H⁺ is relatively low, H⁺, H*, and electrons (e⁻) can combine to form molecular hydrogen (H₂) through the Heyrovsky step. Alternatively, when the coverage of adsorbed H* on the cathode surface is sufficiently high, the combination of two adjacent H* species can also produce H₂ via the Tafel step.

The reaction equation in an alkaline electrolyte is as follows:

H₂O + e⁻ → H* + OH⁻ (Volmer step)

H₂O + e⁻ + H* → H₂ + OH⁻ + * (Heyrovsky step)

H* + H* → H₂ + 2* (Tafel step)

In alkaline and neutral media, the Volmer and Heyrovsky processes involved in the HER differ from those under acidic conditions. In the absence of H⁺ ions, the Volmer step involves the generation of adsorbed H* and OH⁻ on the cathode surface. Subsequently, a chemical reaction occurs among H*, H₂O, and e⁻ to produce H₂ and OH⁻, which corresponds to the Heyrovsky step. Additionally, the Tafel step, involving the combination of two adsorbed H* species, is similar to the mechanism in acidic media.

Regardless of whether the conditions are alkaline, neutral, or acidic, the HER proceeds through either the two-step Volmer–Heyrovsky mechanism or the Volmer–Tafel mechanism.

3. Advanced methods for nonmetallic doping

At present, the methods for introducing nonmetallic atom doping are mainly: hydrothermal, solvothermal, calcination, ion exchange, plasma treatment, and electrodeposition. These methods are shown in Fig. 3.

Fig. 3 Common methods of introducing nonmetallic doping.^55–60 Reprinted with permission from ref. 55. Copyright 2022, Elsevier. Reprinted with permission from ref. 56. Copyright 2022, Wiley. Reprinted with permission from ref. 57. Copyright 2021, American Chemical Society. Reprinted with permission from ref. 58. Copyright 2023, Royal Society of Chemistry. Reprinted with permission from ref. 59. Copyright 2024, MDPI. Reprinted with permission from ref. 60. Copyright 2021, Wiley.

3.1. Hydrothermal method

The hydrothermal method is a synthetic approach in which water is employed as both the medium and reactant, with the process taking place in a sealed container under appropriate temperature and pressure conditions.⁶¹ The hydrothermal method offers several advantages, including simple operation, suitability for large-scale production, and excellent tunability in terms of morphology, crystallinity, and structural characteristics.⁶² The hydrothermal process can be classified into three categories based on the reaction temperature: low-temperature hydrothermal method (<100 °C), medium-temperature hydrothermal method (100–300 °C), and high-temperature hydrothermal method (>300 °C). In the medium-temperature heating process, a tetrafluoroethylene (Teflon) lining is typically used in the reactor, which provides low internal pressure, good sealing, and low synthesis cost.⁶³ The key to obtaining high-performance catalysts via the hydrothermal method is the selection of suitable precursors and appropriate reaction conditions (such as temperature, concentration, duration, and pH value).⁶⁴ With this method, Zhao et al.⁶⁵ directly grew oxygen-doped MoS₂ nanosheets on carbon paper substrates. Then, through a short H₂O₂ etching process, they obtained MoS₂ with dual defects of O doping and S vacancies (O–MoS_2−x). The O-doping increased the active site density and also favored the formation of S vacancies. Moreover, the synergistic effect of O-doping and S vacancies enhanced the HER activity by accelerating the reaction kinetics and improving charge transport.

3.2. Solvothermal method

The solvothermal method is developed based on the hydrothermal approach, which is a synthetic technique for reacting a desired precursor mixture in a closed system under appropriate temperature and pressure conditions, utilizing organic compounds or non-aqueous solvents as the reaction medium.^66,67 The key distinction from the hydrothermal method is that the solvent used in the solvothermal process is an organic solvent rather than water. This method is not only easy to operate but also enables rapid reactions. However, it is necessary to ensure that the operating temperature is higher than the critical temperature of the solvent, and the required pressure is also relatively high. Common organic solvents include EtOH, EG, IPA, PEG, t-BuOH, and DMF.⁶⁸ It is well-established that the morphology, crystallinity, and structure of catalysts can be tuned by adjusting parameters such as pH, temperature, and the choice of solvent.⁶² In recent years, the solvothermal method has been widely employed to prepare various nonmetal-doped electrocatalysts with outstanding catalytic performance. For example, Li et al.⁵⁶ synthesized a composite material (N–MoS₂/N-CNTs) composed of N-doped MoS₂ (N–MoS₂) nanosheets anchored on N-doped carbon nanotubes (N-CNTs) via a one-step solvothermal method. The incorporation of N atoms can expand the interlayer spacing of MoS₂ and replace S atoms to form more structural defects, exposing more catalytic active sites. Meanwhile, N doping also increases the defects of CNTs, which benefits the interface coupling between MoS₂ and CNTs, thereby greatly improving the HER catalytic performance.

3.3. Calcination method

Under an inert gas atmosphere, certain upstream compounds (e.g., NaH₂PO₂, (NH₄)₂CO₃, H₂S) that rapidly decompose into reactive gaseous species can be utilized, as these active gases will fully react with downstream precursors.⁶⁹ This process is primarily dependent on the heating temperature and duration, allowing the crystal structure of the material to be regulated by controlling these two key parameters. Based on the above considerations, Yin et al.⁷⁰ introduced P dopant into the PtZn alloy by calcination, which induced lattice tensile strain in Pt and electronic interactions between P and Zn, thereby optimizing the HER activity. Additionally, their study revealed that as the calcination temperature increased, the P content in the samples also increased. The sample with optimal P content exhibited the highest catalytic performance for the HER.

3.4. Ion-exchange method

The ion exchange method facilitates the replacement of ions in a liquid phase and in solid materials. Particularly effective for creating hollow transition metal oxide nanostructures, this approach offers significant advantages, such as a large specific surface area and abundant electron transfer channels, which together provide ideal structural conditions for electrochemical reactions. However, the ion exchange method has certain limitations, including slow reaction kinetics and specific requirements for the solubility product constant (K_sp).^14,62 Despite these challenges, recent research has actively leveraged ion exchange to develop doped materials with enhanced properties. For instance, Tian et al.⁴⁵ employed ion exchange to construct a core–shell heterostructure of S–Co₂P@Ni₂P, achieving interfacial S doping by carefully controlling the reaction time. This S doping promotes electron transfer from Co₂P to Ni₂P, resulting in electron-rich nickel and electron-deficient cobalt at the interface, thereby enhancing the catalytic activity.

3.5. Plasma treatment method

Plasma treatment typically employs nonmetallic gases such as N₂ and NH₃ to generate reactive nonmetallic species. These species are subsequently activated and dissociated into highly reactive forms, which then interact with metallic substrates to achieve effective nonmetal doping. Plasma treatment is a nonmetallic doping method characterized by high efficiency and environmental friendliness. This method not only significantly reduces the reaction time, sometimes to just a few minutes, but can also be performed at room temperature.^71,72 Chen and his colleagues⁷³ prepared N-doped yttrium–nickel–cobalt phosphide (N–YNiCoP/PNCF) by radio frequency plasma phosphorization under a nitrogen atmosphere. The YNiCo layered double hydroxide (LDH) precursor was transformed into the N-doped YNiCo phosphide (N–YNiCoP) structure. The incorporation of N dopants and the creation of multiphase heterointerfaces can expose a large number of active sites for catalysis. Additionally, the hydrophilic nature of these interfaces facilitates the transfer and diffusion of electrolytes and reaction product bubbles at the electrode–electrolyte interface.

3.6. Electrochemical deposition method

The electrochemical deposition method refers to technology for forming a coating through the migration of positively and negatively charged ions in an electrolyte solution under the influence of an external electric field, coupled with redox reactions involving the gain and loss of electrons at the electrode surface. Electrochemical deposition stands out as a simple, efficient, and cost-effective strategy for synthesizing doped materials for hydrogen production via water electrolysis. Compared to other electrocatalyst synthesis techniques, it offers several advantages, including mild reaction conditions, minimal equipment requirements, and straightforward operation, making it highly suitable for being scalable. It is worth noting that the electrochemical deposition process can also be carried out under ambient temperature and pressure conditions. This versatility provides the prerequisites for the large-scale industrial application of this method. Most importantly, the electrochemical deposition approach is a binder-free method, which enables the direct fabrication of electrodes with inherently high electrocatalytic activity.^74,75 For instance, Pham and co-workers⁷⁶ synthesized S-doped Ni–P nanospheres at different deposition frequencies. The optimal doping of S elements was found to enhance the intrinsic electrocatalytic activity of Ni–P, endowing the catalyst with bifunctional activity for water electrolysis.

To facilitate a more direct comparison of the advantages and disadvantages of various methods, relevant studies have been reviewed and are summarized in Table 1.

Table 1 Advantages and disadvantages of nonmetal doping methods

Synthesis methods	Advantages	Weaknesses
Hydrothermal	Simple operation, suitable for large-scale production	Long reaction time, high temperature and high pressure conditions
	The morphology, crystallinity, and structure are adjustable
Solvothermal	Simple technique	High temperature and high pressure conditions
	Fast reaction kinetics	High risk factor
	The morphology, crystallinity, and structure are adjustable
Calcination	Low cost	Prone to carbonization
	Simple technique	High deposition temperature
	Wide application range
	Controllable product structure
Ion-exchange	Moderate reaction conditions	Sluggish reaction kinetics stringent solubility product requirements
	Excellent controllability
	Versatile applicability
Plasma treatment	Rapid and efficient	High equipment investment
	Moderate reaction conditions	High energy consumption
	Environmentally friendly
Electrochemical deposition	Simple and efficient	Limited in depositing certain materials
	Equipment requirements are relatively simple	Challenging to achieve uniform film thickness
	Moderate reaction conditions

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4. The effect of nonmetal doping on HER electrocatalysts

The mechanisms of nonmetallic doping modification in HER catalysts can be broadly classified into four categories: (1) doping introduces impurity states and alters the electronic band structure, which can optimize the adsorption/desorption of reaction intermediates; (2) doping can modulate the activity and density of active sites, thereby optimizing the reaction kinetics; (3) doping can improve conductivity, thus enhancing the intrinsic catalytic activity; (4) the interplay between doped species and the host matrix can generate cooperative effects, such as charge transfer and electronic sensitization, to boost the overall catalytic performance.

These four regulatory mechanisms can often work in concert, where one or more aspects cooperate to synergistically enhance the intrinsic activity of the HER catalyst. The delicate interplay between these factors underpins the rational design of highly efficient nonmetallic doped HER electrocatalysts.

4.1. Adjust the electronic structure

Nonmetal doping plays a crucial role in modifying the electronic structure of catalysts by introducing new energy levels or altering the energy band structure.⁷⁷ This leads to a redistribution and adjustment of electrons within the catalyst. For instance, the incorporation of nonmetal atoms like N, S, or P creates additional energy levels, allowing for the fine-tuning of the catalyst's Fermi level position and electron density. Consequently, this manipulation of the electronic structure influences the charge transfer kinetics and the interaction between the active center and reactant molecules. By modulating the electronic structure, nonmetal doping can effectively enhance the catalyst's adsorption capacity and reduce the reaction activation energy, ultimately improving the catalyst's reaction activity.^50,78 Inspired by this concept, researchers have directed their attention towards the development of electrocatalysts with superior electrocatalytic properties by incorporating nonmetal dopants. For instance, Wang et al.⁴³ enhanced the catalytic activity of MoS₂ by doping it with phosphorus. The low electronegativity of P atoms induced electron redistribution among nearby Mo and S atoms, resulting in a more uniform charge gradient. In this study, the hydrogen adsorption free energy of medium-concentration P-doped MoS₂ (MP-MoS₂) was found to be optimal, approximately 0.16 eV, exhibiting a volcano-like trend that effectively balanced strong adsorption and rapid desorption. Furthermore, P doping increased the interlayer spacing of MoS₂ from 0.64 nm to 0.80 nm, which enlarged the ion transport channels and facilitated ion adsorption. Notably, after 30 h of continuous operation, the performance of doped MoS₂ remained virtually unchanged, demonstrating exceptional stability.

4.2. Regulate active sites

Active sites refer to specific locations on the catalyst surface that interact with reactants to facilitate catalytic reactions. Incorporating nonmetallic elements into electrocatalysts can influence these active sites in two core ways: first, increasing the number of active sites raises their overall density, thereby enhancing the material's electrocatalytic activity.⁷⁹ Second, introducing nonmetallic dopants allows for precise modification of these sites, improving catalytic properties and increasing efficiency in electrochemical reactions.⁸⁰ For example, Tong et al.⁸¹ introduced Co–S and Fe–S bonds into the oxygen vacancies within the FeCo₂O₄ spinel structure through S doping. This modification enabled the catalyst to expose more active sites for reactions, as S doping not only filled the vacancies but also provided a better chemical environment at these sites, enhancing their ability to adsorb reactants and improving the efficiency of the catalytic reaction. As a basic concept in catalysis research, the Sabatier principle suggests that the adsorption strength of catalytic active sites should be balanced—strong enough to facilitate effective adsorption of reactants but weak enough to enable timely desorption. In this field, noble metal-based catalysts with excellent hydrogen evolution activity often suffer from excessively strong adsorption strength. To address this, Kang and colleagues⁴⁴ introduced N doping into Ir-based catalysts to create two distinct types of active sites: Ir sites near N atoms exhibit weaker hydrogen adsorption energy, while those farther from N retain stronger adsorption energy. This “strong–weak” synergy enhances hydrogen's efficient adsorption and rapid desorption, boosting the catalytic performance.

4.3. Enhance the electrical conductivity

Conductivity refers to the ability of electrons to migrate within a material, which directly impacts the efficiency of electron transfer from the electrode to the active sites during electrocatalytic processes.⁸² Nonmetal doping plays a crucial role in optimizing electron transfer pathways by altering the material's band structure, Fermi level, and electron density of states (DOS), thereby enhancing its conductivity. Recently, Li's team⁸³ demonstrated a significant improvement in catalyst conductivity using a molten salt chemical-assisted strategy combined with N doping, leading to enhanced HER performance. In their study, N doped into the carbon substrate was categorized into various types, with graphitic N contributing electrons to the carbon layer and pyridinic N enhancing the electron-attracting ability of neighboring carbon atoms. Similarly, Shi et al.⁸⁴ developed a three-dimensional conductive network by integrating N-doped carbon substrates with carbon nanotubes (CNTs), which markedly improved the catalyst's electron transfer efficiency. By optimizing electron transfer pathways, these strategies effectively reduce electron migration impedance during the HER process, resulting in enhanced overall catalytic performance.

4.4. Generate synergistic effects

The synergy between nonmetallic doping and other modification strategies, such as vacancy engineering and heterostructure construction, has demonstrated significant potential for enhancing the electrocatalytic performance. Notably, nonmetallic doping can induce the formation of vacancies, particularly anion vacancies, by modulating the local bonding environment or introducing lattice strain. These vacancies contribute to charge redistribution and the exposure of additional active sites.^85,86 For example, Wang et al.⁸⁷ systematically investigated the mechanism by which nitrogen doping enhanced the catalytic performance of CoS. Due to the high electronegativity of nitrogen, its introduction reduces the electron density of neighboring Co²⁺, thereby facilitating the formation of Co³⁺ species and cobalt vacancies. Under the synergistic effect of nitrogen doping and Co vacancies, the originally inert sulfur atoms experience an upward shift in their 3p orbital energy level, leading to stronger orbital overlap with the 1s orbital of H. This significantly lowers ΔG_H*, enabling the S atoms adjacent to Co vacancies to serve as new active sites for the HER, and thus markedly enhances the HER catalytic activity of N-doped Co_xS. When coupled with heterostructures, the introduction of doping further facilitates interfacial charge transfer and promotes favorable electronic interactions, thereby synergistically optimizing the intrinsic activity, electrical conductivity, and structural stability of the catalyst.^88,89 Sun et al.⁹⁰ proposed a synergistic mechanism combining S-doping-induced interfacial coupling and built-in electric field effects. The electron-donating nature of the doped carbon support drives spontaneous electron transfer toward the Pt nanoparticles, generating electron-rich Pt active sites. This electronic reconstruction significantly optimizes the adsorption/desorption energy barriers of H*, thereby fundamentally enhancing the kinetics of the HER.

5. Characterization methods for nonmetal-doped HER catalysts

5.1 Morphology characterization methods

5.1.1. High-resolution transmission electron microscopy. High-resolution transmission electron microscopy (HRTEM) is an essential technique in materials science for microstructural characterization. Although nonmetallic dopant atoms are difficult to observe directly, HRTEM can indirectly reveal the crystal structure modifications induced by such doping through the analysis of microstructural features, including lattice fringe distortions, changes in interplanar spacing, and the presence of defect regions.^91,92 In practice, HRTEM is frequently employed in combination with other characterization techniques such as XRD and X-ray photoelectron spectroscopy (XPS) to comprehensively analyze the doping sites of nonmetallic atoms and their effects on the material's surface structure. For instance, Ma et al.⁹³ demonstrated that S doping induced significant lattice compression strain and structural distortion in the MoSe₂ lattice, as evidenced by a rightward shift in the XRD peak positions, a reduction in the (002) interlayer spacing, and interruptions and distortions in the lattice fringes observed via HRTEM. Further comparison with HRTEM data from undoped MoSe₂ confirmed that the substitution of Se by S was the primary cause of the observed changes in the crystal stress field.

5.1.2. Energy-dispersive X-ray spectroscopy. Energy-dispersive X-ray spectroscopy (EDS) is commonly used to confirm the successful incorporation of nonmetallic dopants and to analyze their spatial distribution on the material's surface. This technique provides qualitative and semi-quantitative information regarding elemental composition. When elemental mapping reveals a uniform distribution of dopant elements (such as P, N, or S), it suggests that the doping process was effectively achieved.⁹⁴ Although EDS does not provide information on the valence states of elements, it is often used in combination with other techniques such as XPS to comprehensively characterize nonmetallic doping behavior. Using this approach, Zhang et al.⁹⁵ demonstrated that Mo, C, N, and B were uniformly distributed within the sample, indicating the successful doping of N and B atoms. Subsequently, XPS was employed to investigate electronic and chemical structure modulation induced by the co-doping of Mo₂C with B and N.

5.1.3. Spherical aberration-corrected transmission electron microscopy. Spherical aberration-corrected transmission electron microscopy (Cs-corrected TEM) has emerged as a powerful technique for investigating the doping behavior of nonmetallic light elements, owing to its sub-angstrom spatial resolution in combination with high-angle annular dark-field (HAADF) and annular bright-field (ABF) imaging modes. When further integrated with electron energy loss spectroscopy (EELS), this technique enables spatial localization of nonmetal atoms, analysis of their coordination environments, and monitoring of associated electronic structure changes, thereby offering atomic-scale evidence for elucidation of the regulatory mechanisms of nonmetallic doping.⁹⁶

5.2 Structural characterization methods

5.2.1. X-ray diffraction. X-ray diffraction (XRD) is one of the most widely employed structural characterization techniques in materials chemistry. It can also be used to investigate the effects of nonmetallic element doping on the crystal structure. Although the doping concentration of nonmetallic elements is typically low and they are usually light elements, making direct detection via XRD challenging, doping often induces changes in lattice parameters and introduces lattice defects. These structural variations are indirectly reflected in the XRD patterns as shifts in diffraction peak positions, broadening of peak widths, and variations in diffraction intensity. Specifically, when nonmetallic doping leads to lattice expansion (e.g., a dopant with a larger atomic radius entering the lattice), the diffraction peak tends to shift toward lower angles; in contrast, lattice contraction causes a peak shift toward higher angles. Additionally, weakened or broadened peaks typically indicate reduced crystallinity or increased structural disorder. These changes provide indirect evidence for the incorporation of nonmetallic dopants into the crystal lattice and the resulting local distortion, offering important insights into their role in tuning the catalytic performance.⁹⁷ For instance, by comparing XRD patterns with varying P doping levels, Yin et al.⁷⁰ demonstrated that the interstitial doping of the Pt lattice with the light-element P led to a reduction in the lattice parameter. Moreover, the extent of lattice tensile strain was found to increase proportionally with the P doping content.

5.2.2. X-ray photoelectron spectroscopy. XPS is one of the most widely used surface analysis techniques for investigating nonmetallic doping. It not only confirms the presence of doped nonmetallic elements but also provides insights into their chemical states and associated electronic structure modifications. By analyzing and deconvoluting the XPS spectra of relevant elements, the valence states, electron transfer directions, and bonding environments (such as P–O, P–M) of the dopants can be elucidated, offering essential structural information to support our understanding of the catalytic performance.^98,99 On this basis, Cao et al.¹⁰⁰ verified the successful incorporation of P via XPS and revealed that electrons were transferred from Ni and S to P, thereby facilitating the formation of Ni–P and P–O bonds.

5.2.3. Synchrotron radiation-based X-ray absorption spectroscopy. Synchrotron radiation-based X-ray absorption spectroscopy (XAS) offers detailed insights into the structural regulation of nonmetal-doped materials. Specifically, X-ray absorption near-edge structure (XANES) analysis enables the investigation of valence state variations and electronic structure modulation of metal centers before and after doping, while extended X-ray absorption fine structure (EXAFS) provides information on local coordination changes, such as bond length contraction and coordination number (CN) reduction. Furthermore, wavelet transform (WT) analysis of the EXAFS signal allows for the clear differentiation of contributions from various coordinated atoms in systems with multiple atomic environments, offering a structural foundation for understanding how nonmetallic doping affects the catalytic performance.¹⁰¹ Recently, Zhao et al.¹⁰² demonstrated via XANES that S doping partially substituted lattice oxygen with S atoms, thereby inducing the formation of oxygen vacancies. The introduction of oxygen vacancies alters the coordination environment of Co, leading to electronic structure reconstruction and a partial reduction in the oxidation state of Co from Co³⁺ to Co²⁺. Complementary EXAFS and WT analyses revealed a significant decrease in the Co–O coordination number while maintaining the overall coordination geometry. These results suggest that S doping primarily leads to an adjustment of the coordination environment, manifested as reduced CN, rather than a complete rearrangement of the coordination structure.

To provide a clearer overview of the functions and distinguishing features of the characterization methods, the relevant content is systematically summarized in Table 2.

Table 2 Summary of characterization techniques used for nonmetal-based catalyst analysis

Methods	Available information	Feature description
HRTEM	Lattice fringes	Observation of structural changes
	Structural distortion
EDS	Elemental distribution	No valence state information Often combined with TEM
	Semi-quantitative analysis
Cs-corrected TEM	Location of doped nonmetallic elements	ABF is suitable for light elements
		HAADF provides Z-contrast
XRD	Lattice parameter changes	Suitable for detecting whether atoms cause structural changes
XPS	Presence, valence, and bonding state of doping elements	Surface sensitive
XANES	Electronic states of dopant atoms	Depth analysis of local structure and valence state
EXAFS/WT	Coordination environment

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6. Nonmetal-doped HER electrocatalysts

The development of highly active HER electrocatalysts is essential for achieving efficient hydrogen production through water electrolysis.¹⁰³ Among cathode HER catalysts, platinum group metals (PGMs) like Pt, Ru, and Ir are the most effective due to their moderate d-orbital electron density, which overlaps effectively with the 1s orbital of hydrogen atoms—an interaction that is highly favorable for the HER process.¹⁰⁴ However, these noble metals are scarce and costly, posing challenges for the large-scale production of hydrogen.^105,106 To mitigate costs, attention has increasingly focused on high-performance, low-loading noble metal catalysts and non-noble metal electrocatalysts.^24,107

Nonmetallic atoms typically exhibit higher electronegativity than metallic atoms. Consequently, doping with nonmetallic elements can induce more pronounced modifications in the electronic structure and chemical properties of the host materials. Such doping can adjust the catalyst's electronic configuration by altering its surface chemical state, thereby increasing the exposure of active sites, enhancing conductivity, and optimizing hydrogen adsorption energy—factors that may significantly boost the electrocatalyst's catalytic activity.^108,109 Furthermore, the introduction of nonmetallic doping is relatively straightforward. Based on these observations, we have summarized recent advancements in the synthesis of efficient water-splitting catalysts via nonmetallic doping methods, such as P doping, S doping, N doping, B doping, and halogen doping (Fig. 4).

Fig. 4 Types of non-metal doping discussed in this article.^55,110–123 Reprinted with permission from ref. 110. Copyright 2022, Wiley. Reprinted with permission from ref. 111. Copyright 2022, Elsevier. Reprinted with permission from ref. 112. Copyright 2025, Wiley. Reprinted with permission from ref. 113. Copyright 2024, Wiley. Reprinted with permission from ref. 114. Copyright 2021, Springer. Reprinted with permission from ref. 55. Copyright 2022, Elsevier. Reprinted with permission from ref. 115. Copyright 2021, Wiley. Reprinted with permission from ref. 116. Copyright 2022, Wiley. Reprinted with permission from ref. 117. Copyright 2023, Wiley. Reprinted with permission from ref. 118. Copyright 2022, American Chemical Society. Reprinted with permission from ref. 119. Copyright 2020, Elsevier. Reprinted with permission from ref. 120. Copyright 2022, Wiley. Reprinted with permission from ref. 121. Copyright 2022, Wiley. Reprinted with permission from ref. 122. Copyright 2021, Elsevier. Reprinted with permission from ref. 123. Copyright 2020, Wiley.

6.1. P-doped electrocatalysts for the HER

The large atomic size and excellent electron-donating capability of P atoms make them appealing as dopants, effectively enhancing the electrocatalytic activity by altering the electrocatalyst's electronic properties. To enhance intrinsic catalytic activity and accelerate reaction kinetics, P doping is employed to modulate the local charge density, optimize surface charge distribution, and supply a significant number of active surface sites, thereby boosting the efficiency of water electrolysis.^124,125 Additionally, compared to transition metal doping, P doping offers a more advantageous approach for enhancing the catalytic performance by leveraging the in situ adjustment of the electronic structure through P atoms while preserving the host material's intrinsic properties. Therefore, P doped into electrocatalytic materials usually endows the electrocatalyst with low overpotential and high conductivity.¹²⁶

In recent years, P doping has become a widely adopted strategy to enhance the performance of new high-efficiency electrocatalytic materials, significantly advancing the development of low-loaded PGM catalysts. Furthermore, the unique atomicity and electronegativity of PGM and P create synergistic effects that further boost catalytic activity. This synergy increases the density of active sites and enhances electron transfer efficiency within the catalyst, ultimately improving its catalytic performance. Based on this, our team¹²⁷ synthesized a new nanocatalyst, Ru/P–MoB, through a microwave quasi-solid-state method. In this work, strong interactions between Ru nanoclusters and the support are established through Ru–Mo and Ru–P bonds, providing abundant active sites to accelerate reaction kinetics. P doping lowers the work function and facilitates electron transfer from the bulk phase to the surface (Fig. 5(a)), creating an electron-rich state for ruthenium at the interface (Fig. 5(b)). This electron-rich interface proved beneficial for water molecule adsorption/dissociation and optimized the adsorption free energy of intermediate H species (Fig. 5(c)). As a result, Ru/P–MoB demonstrated not only efficient H₂ production with a low overpotential in alkaline pure water, alkaline seawater, and acidic media, but also exhibited excellent activity and performance in anion exchange membrane electrolyzed water, showcasing remarkable stability. In addition, Cho et al.¹²⁸ synthesized a catalyst, Ru/P–TiO₂, in which Ru clusters were supported on a P-doped, defective TiO₂ substrate. The P doping process partially replaces Ti⁴⁺ sites in TiO₂, leading to the formation of Ti–O–P bonds and the creation of abundant oxygen vacancies on the surface. Notably, the mass activity of the Ru/P–TiO₂ catalyst synthesized in this study was 34.3 times higher than that of Pt/C and 18.7 times higher than Ru/TiO₂ (Fig. 5(d)). This enhancement is attributed to the Ru/P–(R)TiO₂-V_O surface, where the alkaline HER involves water dissociation, strong adsorption of H on Ru sites, hydrogen spillover from Ru sites to P sites, and efficient hydrogen desorption on P sites (Fig. 5(e)). The synergistic effect of P doping and oxygen vacancies enhances water adsorption and lowers the energy barrier for water decomposition (Fig. 5(f)), significantly enhancing hydrogen production as it transfers from Ru sites to surface P sites, thereby markedly improving HER activity. Similarly, Li's team¹²⁹ developed a novel P-doped Pt₃Co electrocatalyst. Upon P doping, negatively charged P sites are generated, enabling effective capture and storage of protons (H⁺) (Fig. 5(g)). These P sites function as “proton reservoirs” during catalysis, providing a steady supply of protons for the HER. Through a short-path hydrogen spillover effect, P sites facilitate rapid proton transfer to catalytic Pt sites, thereby reducing energy loss along the reaction pathway (Fig. 5(h)). Additionally, the incorporation of P lowers the electronic density of states of Pt near the Fermi level, which promotes the electrochemical desorption of hydrogen (Fig. 5(i)). Consequently, the efficiency of hydrogen generation is significantly enhanced, as demonstrated by the P–Pt₃Co/NC catalyst, which delivers an overpotential of just 219 mV at 1.5 A cm⁻² under acidic conditions, while exhibiting a HER activity that is 4.7 times greater than that of Pt₃Co. In the study conducted by Luo et al.,¹³⁰ the impact of controlling the doping amount of P on ruthenium-based catalysts was explored. It was found that the P doping content should be carefully controlled, as both too low and too high doping amounts had undesirable effects. However, an appropriate P doping content can effectively regulate the surface electronic structure of the catalyst, leading to a more significant enhancement in the catalytic activity of hydrogen oxidation and reactions in alkaline electrolytes (Fig. 5(j)).

Fig. 5 (a) Working function of P–MoB and comparison samples. (b) Plane-averaged and charge density difference (inset) of Ru/P–MoB. (c) Water dissociation kinetic barriers of Ru/P–MoB and a control group. Reprinted with permission from ref. 127. Copyright 2023, Wiley. (d) Mass activities of Ru/P–TiO₂, etc. (e) Free energy diagrams for hydrogen spillover on Ru/P–(R)TiO₂-V_O (black for Ru, red for P atoms). (f) Kinetic barriers for water dissociation at different active sites. Reprinted with permission from ref. 128. Copyright 2022, Wiley. (g) Hydrogen adsorption energies on P–Pt₃Co and Pt₃Co. (h) Schematic of hydrogen spillover on P–Pt₃Co. (i) PDOS plots of P–Pt₃Co and Pt₃Co. Reprinted with permission from ref. 129. Copyright 2023, American Chemical Society. (j) Performance comparison of samples with different P doping contents. Reprinted with permission from ref. 130. Copyright 2020, American Chemical Society.

The development of non-noble metal catalysts as cost-effective alternatives has attracted substantial attention; however, matching the catalytic performance of noble metal systems continues to pose a major challenge. Among the various strategies employed, P doping has emerged as a popular and effective approach. Recent advancements in this field have been significant. For instance, Xu et al.¹³¹ conducted a study where they incorporated P into the cobalt molybdate lattice, resulting in the formation of a Co(OH)₂–CoMoO₄/P–CoMoO₄ structure on the surface of cobalt molybdate (Fig. 6(a)). The introduction of P through doping was employed to modulate the electronic structure and surface active sites of the material, leading to enhanced catalyst activity and stability. The study found that as the phosphating temperature increased, the degree of amorphization increased. The optimized catalyst exhibited the lowest overpotential of 44 mV for the HER to achieve a current density of 10 mA cm⁻² (Fig. 6(b)). Similarly, Waterhouse et al.¹³² synthesized a novel Fe–P-CMO catalyst with a sea urchin-like structure. P doping introduced numerous oxygen vacancies (O_V) and formed a Fe₂O₃/P–CoMoO₄ heterojunction on the material's surface, enhancing electronic conductivity and increasing the density of reactive active sites (Fig. 6(c)). During the HER process, the P-doped CoMoO₄ in Fe–P-CMO undergoes surface reconstruction, resulting in the in situ generation of Co(OH)₂ on its surface, forming a Co(OH)₂/P–CoMoO₄ heterostructure, as shown in Fig. 6(d) and (e). This reconstruction exposes a greater number of active sites. As a result, the activity of CoMoO₄ was significantly improved, facilitating the efficient reduction of H⁺. Remarkably, Fe–P-CMO exhibited outstanding performance in 1.0 M KOH electrolyte, requiring low overpotentials of only 68 and 151 mV to achieve current densities of 100 and 300 mA cm⁻², respectively, surpassing the performance of the benchmark Pt/C catalyst (Fig. 6(f)). Recently, Zhou's team¹³³ successfully demonstrated the effectiveness of P-doped W₂C nanoparticles anchored on graphene (P@W₂C–C) as efficient electrocatalysts for the HER, utilizing a wind-driven triboelectric nanogenerator (TENG). The P@W₂C–C catalyst exhibited an impressively low overpotential of only 179 mV and a Tafel slope of 87.8 mV dec⁻¹ (Fig. 6(g)), which correlated well with the low power output of the TENG. In this study, P doping altered the electronic structure of W₂C, shifting its d-band center closer to the ideal position (Fig. 6(h) and (i)) and optimizing the hydrogen adsorption free energy (ΔG_H*). This optimization not only balances hydrogen adsorption and desorption but also lowers the reaction energy barrier (Fig. 6(j)), further accelerating the HER and significantly enhancing the catalytic activity.

Fig. 6 (a) Illustration of the synthesis process for P-CMO/NF. (b) HER polarization curves of P-CMO/NF-400 and comparison samples. Reprinted with permission from ref. 131. Copyright 2022, Elsevier. XRD patterns of (c) Fe–P-CMO and P-CMO; and (d) Fe–P-CMO after reaction. (e) Raman analysis of Fe–P-CMO prior to and following the reaction. (f) HER polarization curves of different catalysts in 1.0 M KOH. Reprinted with permission from ref. 132. Copyright 2024, Elsevier. (g) HER performance comparison of P@W₂C–C and a control. (h and i) DOS for P@W₂C–C and W₂C–C. (j) ΔG diagrams for H₂ generation from H₂O on P@W₂C–C, Si@W₂C–C, and W₂C–C (inset: preferred configurations of intermediates adsorbed on P@W₂C–C). Reprinted with permission from ref. 133. Copyright 2024, Elsevier.

As a representative 2D transition metal dichalcogenide (TMDs), the modification of MoS₂ by heteroatom doping has been attracting much attention. Compared with pristine MoS₂, the introduction of P atoms can enhance the electronic conductivity and HER catalytic activity of MoS₂. Based on their findings, Ye and colleagues¹³⁴ successfully synthesized P-doped MoS₂ (P–MoS₂) using a one-step air calcination method. They observed that P doping significantly improved the HER performance of MoS₂ nanosheets, achieving a remarkably low onset potential of approximately 30 mV and a Tafel slope of 48 mV dec⁻¹ (Fig. 7(a) and (b)). Interestingly, they discovered that the edge sites of P–MoS₂ exhibited higher activity compared to the basal plane. Through in situ transport measurements, they determined that the enhanced HER performance was primarily attributed to an increase in intrinsic catalytic activity rather than a modulation of charge transport (Fig. 7(c) and (d)). Subsequent DFT calculations confirmed that the edge sites of P–MoS₂ were energetically more favorable for the hydrogen reaction (Fig. 7(e) and (f)). Similarly, our team¹³⁵ successfully synthesized MoS₂ nanoflowers with P doping, S vacancies, and a crystalline–amorphous heterostructure using a P-assisted rapid calcination method. The synergistic effect of these multiple strategies resulted in MoS₂–P₂ with outstanding stability at both low and high current densities throughout the entire pH range (Fig. 7(g) and (h)). This significant finding provides valuable insights for the rational design of edge-dominant P–MoS₂ catalysts. In addition, Kang et al.¹³⁶ successfully synthesized P-doped biphasic MoS₂ (P-BMS) nanosheets as electrocatalysts through a one-step hydrothermal method. The doping of P atoms can modify the electronic structure of MoS₂ (Fig. 7(i) and (j)), optimize its electrocatalytic reaction kinetics (Fig. 7(k)), and enhance its conductivity and structural stability. As a result, the optimized P-BMS electrocatalyst exhibited low overpotentials of 60 and 72 mV, respectively, at 10 mA cm⁻² in 0.5 M H₂SO₄ and 1.0 M KOH electrolytes, and smaller Tafel slopes of 76 and 80 mV dec⁻¹, respectively.

Fig. 7 (a) LSV and (b) Tafel plots for plane and edge sites of MoS₂ and P–MoS₂ in 0.5 M H₂SO₄. Electrochemical (orange) and electronic (violet) signals of a MoS₂ nanosheet before (c) and after (d) P doping for the HER. Minimum energy paths and critical HER structures on (e) basal plane and (f) armchair edge of P–MoS₂. Reprinted with permission from ref. 134. Copyright 2023, American Chemical Society. Durability of MoS₂–P₂ in different electrolytes, with HER polarization curves pre- and post-cycling in (g) 1.0 M KOH and (h) 0.5 M H₂SO₄. Reprinted with permission from ref. 135. Copyright 2022, Springer. XPS spectra of (i) Mo 3d and (j) S 2p. (k) Adsorption energy diagram for MoS₂ and P-BMS at different sites. Reprinted with permission from ref. 136. Copyright 2023, Wiley.

At present, while scientific researchers have successfully synthesized some high-performance catalysts through various approaches (Table 3), the understanding of P-doped catalysts still requires further improvement. The P-doped surface is susceptible to oxidation, which can potentially affect the nature of the active sites, reduce electrical conductivity, diminish catalytic activity, and lead to poor air stability.

Table 3 Comparison of the performance of recently reported P-doped electrocatalysts

Electrocatalyst	Electrolyte	η10 (mV)	TOF (s^−1)	η (mV)	Stability	Ref.
P–Pt1Ni2NH/NGA	1.0 M KOH	15	13.5	100	20 h	137
P0.3–CuMn2O4	1.0 M KOH	103	1.3	260	3000 cycles	138
P–Os	1.0 M KOH	32	0.28	100	100 h	139
PtP NDs	0.5 M H2SO4	13.3	0.3	50	5000 cycles	110
P,Mo–Ru@PC	1.0 M KOH	21			10 h	140
P–CoNS@CuPD	1.0 M KOH	166			10 h	141
P–PtNi networks	1.0 M KOH + 0.5 M NaCl	12	24.5	200	120 h	142
	1.0 M KOH	23	16.4	200
P/Cr60–NiMoO4	1.0 M KOH	35.7			120 h	143
P–Co3O4	1.0 M KOH	80			60 h	144
P/FeCo-NC	0.5 M H2SO4	38	0.6	50	10 h	112
P@MNTC	1.0 M KOH	120			25 h	145
P–CoS2/MoS2/MoO2	1.0 M KOH	85	0.87	200		146
P-doped MoS2/CFC	0.5 M H2SO4	162			24 h	147

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6.2. S-doped electrocatalysts for the HER

S is a representative chalcogen element, which can typically be introduced as a dopant into the crystal lattice of the primary catalyst through a sulfurization reaction. Studies have demonstrated that S doping can tune the electronic structure, engineer lattice defects, and increase the active surface area of the catalytic materials.¹⁴⁸ These adjustments enable the catalysts to achieve superior electrical conductivity, outstanding electrochemical activity, enhanced durability, and greater chemical stability.

MoX_n exhibits several advantages such as low cost, high stability, and abundant edge positions, which serve as active sites for the HER. S doping effectively adjusts the electronic structure of MoX_n and optimizes ΔG_H*. Building upon this, Zhang and his colleagues¹⁴⁹ achieved ultrathin S-doped molybdenum compound nanosheets by employing an in situ topological conversion method on MoS₂ ultrathin nanosheets grown on carbon cloth. Examples include S–Mo₂C, S–MoP, S–MoN, and S–MoO₃ (Fig. 8(a)). Notably, during the high-temperature phase transformation, MoS₂ nanosheets served as both the Mo source and S supplier, with the released S species being in situ doped into the synthesized Mo-based composites. The precisely engineered porous nanosheet structure offered abundant exposed active sites, improved electronic conductivity, and facilitated efficient mass and charge transfer, thus boosting interfacial electrocatalytic activity. Crucially, experimental and theoretical calculations demonstrated the synergistic effect between the S dopant and MoX_n, inducing an optimized electronic state in S–MoX_n (Fig. 8(b)). This resulted in favorable energy for proton adsorption and hydrogen desorption on the S–MoX_n surface. Consequently, 2D S–MoX_n/CC exhibited enhanced HER activity compared to MoX_n/CC. Notably, the optimized S–MoP/CC electrode displayed low overpotentials of 75, 75, and 127 mV at 10 mA cm⁻² in acidic, alkaline, and neutral media, respectively, while maintaining stable performance for 30 h during continuous operation. Furthermore, in the study conducted by An et al.,¹¹⁴ a chemical conversion method was employed to transform 1T phase MoS₂ into S-doped 2H-MoTe₂ (Fig. 8(c)). The remarkable research findings revealed that S-doped 2H-MoTe₂, with tellurium vacancies, exhibited exceptional electrocatalytic performance for H₂ production. Specifically, it reached an impressive current density of 100 mA cm⁻² at an overpotential of 217 mV, with negligible decay after 20 000 cycles (Fig. 8(d)). This improvement is attributed to the combined effects of S doping and Te vacancies, which increase the negative charge at active sites, making them more favorable for H* adsorption and facilitating the HER process, thereby boosting catalytic efficiency (Fig. 8(e)). Similarly, in their study, Zhao et al.¹⁵⁰ achieved the successful synthesis of the S–Mo₅N₆ catalyst by carefully controlling the ammoniation process of MoS₂. Through a combination of experiments and DFT simulations, it was found that during hydrogen adsorption, electrons transfer from N atoms to S atoms (Fig. 8(f)). This electron transfer reduces the adsorption strength of S atoms on protons, thereby optimizing the ΔG_H* value (Fig. 8(g)). As a result, when tested in 0.5 M H₂SO₄, this S-doped Mo₅N₆ catalyst exhibited exceptional performance in the HER. It achieved a current density of 10 mA cm⁻² with a low overpotential of 56 mV and a Tafel slope of 37.9 mV dec⁻¹ (Fig. 8(h) and (i)). These findings highlight the significant contribution of S doping in enhancing the electrocatalytic activity of Mo₅N₆ for an efficient HER.

Fig. 8 (a) Preparation schematic and (b) differential charge density of S–MoX_n/CC and its comparison samples. Reprinted with permission from ref. 149. Copyright 2023, Elsevier. (c) Synthesis path diagram of S–VTe-2H MoTe₂/NCC. (d) Polarization curves of S–VTe-2H MoTe₂/NCC before and after 20 000 cycles. (e) Relationship between HER free energy change and Bader charge. Reprinted with permission from ref. 114. Copyright 2021, Springer. S–Mo₅N₆ and comparison samples: (f) differential charge density, (g) free energy diagram for H adsorption. (h) Overpotentials corresponding to different current densities. (i) Tafel plots. Reprinted with permission from ref. 150. Copyright 2022, Elsevier.

Iron-based oxide electrocatalysts have garnered significant attention in recent research due to their affordability, non-polluting nature, and excellent durability. However, enhancing their catalytic activity has proved to be a challenge due to their poor conductivity. In a recent study, Liu et al.¹⁵¹ successfully developed a novel catalyst, S–Fe₂O₃ nanosheet array supported on iron foam (S–Fe₂O₃/IF), through a simple and convenient two-step synthesis process. The catalyst exhibited improved performance in alkaline media for the HER. It demonstrated a low overpotential of 134 mV, enabling a current density of 10 mA cm⁻², and displayed a small Tafel slope of 76 mV dec⁻¹ (Fig. 9(a) and (b)). Furthermore, DFT calculations reveal that S doping significantly reduces the adsorption free energy of water molecules (ΔG_H2O*), indicating that S–Fe₂O₃/IF more effectively promotes water adsorption and the Volmer reaction (Fig. 9(c)). Additionally, S doping optimizes ΔG_H*, with a near-zero ΔG_H* value suggesting that S–Fe₂O₃ is particularly well-suited for catalyzing the HER (Fig. 9(d)). These results offer novel insights for the development of iron oxide electrocatalysts with enhanced performance. Additionally, S doping has been utilized to enhance the efficiency of precious metal catalysts. A notable example is the work by Xia and his team,¹⁵² who developed a novel method for synthesizing network-like aerogels composed of nanowires in an aqueous solution, leading to the successful production of S-doped gold–lead–cobalt (S–AuPbPt) alloy aerogels. Within this alloy aerogel, S doping modifies the electronic structure of Pt, specifically lowering the d-band center (Fig. 9(e)). This shift reduces the adsorption strength of hydrogen intermediates (H*) on the Pt surface, allowing adsorbed H* to more readily combine to form H₂ and rapidly release it into the reaction system. As a result, the S–AuPbPt alloy nanowire network catalyst exhibited an intrinsic HER activity approximately 15.4 times higher than that of Pt/C, demonstrating its outstanding performance (Fig. 9(f)).

Fig. 9 Electrochemical properties of S–Fe₂O₃ and its comparative sample: (a) LSV and (b) Tafel plot of HER. (c) ΔG_H2O* and (d) ΔG_H* plots of Fe₂O₃ and S–Fe₂O₃. Reprinted with permission from ref. 151. Copyright 2022, American Chemical Society. (e) TDOS of Pt₄ on S–AuPbPt and AuPbPt surfaces. (f) TOF value at an overpotential of 0.05 V (vs. RHE) of S–AuPbPt (red) and Pt/C (black). Reprinted with permission from ref. 152. Copyright 2020, Royal Society of Chemistry.

Although researchers from various countries have achieved significant progress in the research and development of S-doped catalytic materials (Table 4), the underlying mechanisms of these S-doped catalysts still require further exploration. At present, most transition metal sulfides can exhibit excellent HER activity under acidic or alkaline conditions. However, there are relatively few studies on their HER performance under neutral pH conditions. A possible reason for this is the difficulty in adsorbing water molecules to produce adsorbed hydrogen intermediates under neutral conditions. Nonetheless, hydrogen production under neutral conditions is particularly important for industrial applications.

Table 4 Comparison of the performance of recently reported S-doped electrocatalysts

Electrocatalyst	Electrolyte	J (mA cm^−2)	η (mV)	Stability	Ref.
Pt/S–TiN NTs	0.5 M H2SO4	10	12	25 h	153
	1.0 M KOH	10	25	25 h
	1.0 M PBS	10	39	25 h
S–PtCo@MC	0.5 M H2SO4	10	23	30 h	154
Pt@S–NiFe LDH	1.0 M KOH	300	132	200h	155
S–Co2P@Ni2P	1.0 M KOH	100	103	20 h	45
S–WP2-5	0.5 M H2SO4	10	115	24 h	156
M–CoSe1.28S0.72	0.5 M H2SO4	10	67	1000 h	157
NFS-6	1.0 M KOH	10	171	20 h	158
S–CoMoO-12.4	1.0 M KOH	10	105	40 h	159
S–MoO2-2	0.5 M H2SO4	10	176	10 h	160
MoSe1.44S0.56	0.5 M H2SO4	10	167	24 h	55

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6.3. N-doped electrocatalysts for the HER

There are 3 single electrons and 1 lone pair of electrons in the valence electron shell of the N atom, which means the N atom has multiple bonding types. N atoms have a small atomic radius, and they can be embedded into the interstitial positions of the catalyst easily without affecting the overall conductivity. In addition, the high electronegativity of the N atom causes the surrounding atoms to be polarized easily. The vacant orbitals of the N atom facilitate electron transfer during the Tafel and Volmer steps, thereby enhancing the catalytic activity for hydrogen production.¹⁴⁸

N doping can adjust the electron density and d-band center of the active site, thereby enhancing the HER activity.¹⁶¹ Generally, we prepare electrocatalysts by doping N into the host lattice. Two common methods are as follows: (a) plasma treatment, also known as ammonification directly—it is a method of high-temperature treatment of precursors under NH₃ conditions; (b) the ion exchange method involves reacting the precursor with N-containing reagents (such as urea) in the solid phase. N atom doping can not only play a part in improving the HER performance of metal compounds, but also improve the HER performance of nonmetals such as carbon.

For metal compounds, N doping not only maintains the metallic character of the host catalyst, but it also improves the electrical conductivity and enhances charge transfer in some cases. Additionally, N doping can alter the surface crystal structure to create more reactive sites and lower the hydrogen adsorption energy. Inspired by this concept, researchers have directed their attention towards the development of electrocatalysts with superior electrocatalytic properties by incorporating N dopants. For instance, Huang et al.¹⁶² synthesized a N-doped Ni₅P₄@CoP/CFP nanocatalyst, which exhibited a remarkable HER performance. In acidic, alkaline, and neutral media, the N–Ni₅P₄@CoP/CFP catalyst achieved HER overpotentials as low as 55, 56, and 59 mV, respectively, at a current density of 10 mA cm⁻² (Fig. 10(a)–(c)). The DFT calculation results indicated that the integration of N atoms into Ni₅P₄ modified the coordination environment of Ni, thereby adjusting the distribution of the interface electronic field (Fig. 10(d)). This adjustment enhances charge transfer from CoP to N–Ni₅P₄, boosts the catalyst's electron transfer efficiency, and creates nucleophilic P sites in N–Ni₅P₄ with balanced hydrogen adsorption and desorption, thereby accelerating the HER (Fig. 10(e)). Furthermore, Sun et al.¹¹⁶ conducted a study on the rational design and preparation of a self-supporting N-doped Ni₂P (N–Ni₂P/NF) catalyst on NF using a simple NH₄H₂PO₂-assisted gas–solid reaction process. The catalyst exhibited remarkable catalytic activity and stability for the HER, demonstrating a low overpotential of 50 mV at 10 mA cm⁻², a small Tafel slope of 45 mV dec⁻¹, and long-term stability. DFT calculations were employed to investigate the catalyst. The incorporation of N causes electrons to transfer from nickel and P to N, forming new positive and negative charge centers, while significantly reducing the energy barriers for water adsorption and decomposition, accelerating the first step of the Volmer process of the HER (Fig. 10(f)). An optimized ΔG_H* achieves a favorable balance between hydrogen adsorption and desorption, thereby facilitating the HER process (Fig. 10(g)). In addition, N doping significantly increases the specific surface area of the Ni₂P/NF catalyst, facilitating the full exposure of active sites (Fig. 10(h) and (i)). Similarly, Zhang and his colleagues¹⁶³ successfully synthesized N–Co₉S₈/Ni₃S₂/NF using an electrodeposition method, showcasing remarkable activity for the HER (Fig. 10(j)). After N doping, the d-band center of the Co₉S₈/Ni₃S₂ catalyst shifts from −1.547 eV to −0.811 eV, indicating enhanced electron interactions with adsorbates and improved HER performance (Fig. 10(k)). Specifically, the water adsorption free energy (ΔG_H2O) at both cobalt and nickel sites is significantly lowered, facilitating water adsorption and dissociation processes (Fig. 10(l)). Additionally, the ΔG_H* value at the cobalt site near N approaches zero after doping, demonstrating that N doping markedly enhances the catalyst's intrinsic HER activity.

Fig. 10 LSV curves of N–Ni₅P₄@CoP/CFP and reference catalysts measured in (a) 0.5 M H₂SO₄, (b) 1.0 M KOH, and (c) 1.0 M PBS. (d) Charge density difference and (e) Gibbs free energy diagram of ΔG_H* of N–Ni₅P₄@CoP. Reprinted with permission from ref. 162. Copyright 2022, Elsevier. (f) The energy barriers associated with the kinetic process of water dissociation on N–Ni₂P/NF and Ni₂P/NF. (g) ΔG_H* on various samples at equilibrium potential. N₂ adsorption/desorption isotherms of (h) Ni₂P/NF and (i) N–Ni₂P/NF. Reprinted with permission from ref. 116. Copyright 2022, Wiley. (j) LSV curves of N–Co₉S₈–Ni₃S₂ and contrasting catalysts. (k) The d-band of the DOS for Co₉S₈/Ni₃S₂ and N–Co₉S₈–Ni₃S₂. (l) Water adsorption energy diagram for Co₉S₈/Ni₃S₂ and N–Co₉S₈/Ni₃S₂. Reprinted with permission from ref. 163. Copyright 2023, Wiley.

The widespread use of carbon-based materials in HER research can be attributed to their low cost, tunable structures, and robust stability over a wide pH range. Incorporating heteroatoms into carbon materials has proved effective at activating their intrinsically inert surfaces and enhancing catalytic performance.¹⁶⁴ Building on this strategy, Xi et al.¹⁶⁵ synthesized PtMo-NC, where the N-doped carbon with a high surface area and abundant nitrogen sites effectively immobilized metal atoms and prevented aggregation during high-temperature calcination, resulting in uniformly dispersed Pt and enhanced atomic utilization (Fig. 11(a) and (b)). Similarly, Sun et al.¹⁶⁶ synthesized Ir@NC using a simple impregnation followed by a calcination–reduction method. This process formed strong covalent Ir–N bonds, which effectively inhibited Ir corrosion and aggregation in acidic environments. In Ir@NC, the N atoms doped in the carbon support regulated the electronic structure of iridium, resulting in a lower valence state for Ir compared to that of Ir@C (Fig. 11(c)), which contributed to the superior HER performance of Ir@NC over Ir@C. High entropy alloys (HEAs) possess unique electronic structures and exceptional stability, making them promising candidates for HER catalysis. Using a low-temperature thermal reduction method, Hu's team¹⁶⁷ achieved the uniform dispersion of ultrasmall (sub-2 nm) PtRuCoNiCu HEA nanoparticles on hierarchical N-doped carbon nanocages (hNCNCs). This configuration benefits from electron transfer from pyridinic N in the hNCNC support to the metals (Fig. 11(d)), which strengthens the metal–support interactions and significantly enhances the catalyst's stability during prolonged HER operation. Recently, significant progress has been made in the research of non-precious metal HEAs. Yin et al.¹⁶⁸ successfully synthesized HEA catalysts coated with a N-doped graphene layer of controllable thickness (CuNiFeCoCrTi@NC NPs). The alloy core exhibits strong hydrogen adsorption, which impacts the H* desorption step, while the N-doped graphene layer has weaker hydrogen adsorption, affecting the adsorption step. Consequently, although the alloy and graphene layer individually show suboptimal catalytic performance, the ΔG_H* value is effectively modulated when the alloy is coated with the N-doped graphene layer, enhancing the overall catalytic efficiency (Fig. 11(e)). Furthermore, Xiao et al.¹⁶⁹ synthesized a highly efficient HER catalyst featuring an urchin-like N-doped carbon structure encapsulating a MoO₂/Mo₃P/Mo₂C triple-interface heterojunction. The catalyst exhibited exceptional performance, with an overpotential of merely 69 mV at a current density of 10 mA cm⁻² and demonstrated remarkable long-term stability (Fig. 11(f) and (g)). This outstanding performance can be attributed to the synergistic effect between the unique heterostructure and the encapsulated N-doped carbon, highlighting the importance of their combined influence for enhancing the catalytic activity.

Fig. 11 TEM images of (a) Pt-NC and (b) Pt–C after Ar/H₂ pyrolysis. Reprinted with permission from ref. 165. Copyright 2023, Wiley. (c) Ir 4f spectra of Ir@NC and comparison samples. Reprinted with permission from ref. 166. Copyright 2024, Elsevier. (d) N 1s, Pt 4f, Co 2p, and Cu 2p spectra of hNCNCs and control samples. Reprinted with permission from ref. 167. Copyright 2024, Springer. (e) ΔG_H* diagram of CuNiFeCoCrTi@NC and comparison samples. Reprinted with permission from ref. 168. Copyright 2024, Elsevier. HER performance evaluated in 1.0 M KOH. (f) Polarization curves; (g) the stability evaluation of MoOPC@NC-200 over an extended period. Reprinted with permission from ref. 169. Copyright 2023, Wiley.

In addition, several researchers have achieved significant results through N doping, see Table 5 for details.

Table 5 Comparison of the performance of recently reported N-doped electrocatalysts

Electrocatalyst	Electrolyte	J (mA cm^−2)	η (mV)	Stability	Ref.
LaCeOx@NGr/Ru1	1.0 M KOH	10	22	30 h	170
N-fcc-Ir-NSs	0.5 M H2SO4	10	19	72 h	44
N–MoO2/Cu	1.0 M KOH	1000	363	50 h	171
N–FeS2	1.0 M KOH	10	126	20 h	172
MoCoSex@NC	0.5 M H2SO4	10	60	12 h	173
4%Mo-Co3O4/NC	1.0 M KOH	10	91	40 000 s	174
MoP@NC-250	1.0 M KOH	10	96	35 h	161
MoSe2@NC	1.0 M KOH	10	175	12 h	175
	0.5 M H2SO4	10	183	12 h
Mo2C@NC@Pt	0.5 M H2SO4	10	27	10 h	176
	1.0 M KOH	10	47	10 h
	1.0 M PBS	10	25	10 h
Mo2C@NC-160	1.0 M KOH	10	90	12 h	177
MoC@NC	0.5 M H2SO4	10	171	70 h	178

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6.4. B-doped electrocatalysts for the HER

Boron (B), with its electron-deficient nature and three valence electrons, exhibits distinct advantages as a doping element. Compared to P, B has a lower electronegativity of 2.04, which allows it to interact more readily with a single electron from oxygen in water. This interaction facilitates the rapid release of a proton during the HER process. By effectively participating in the HER, B enhances the catalytic activity and efficiency of the system, contributing to improved performance in various electrochemical applications. Recently, our team⁵² developed an effective B-doped strategy, leading to the successful synthesis of a novel B-doped Ni₄Mo nanoarray (B_x–Ni₄Mo/NF, X = B doping level). By introducing electron-deficient B atoms, they effectively tuned the electronic structure of Ni₄Mo, optimized the H* adsorption affinity, and enhanced H₂O dissociation (Fig. 12(a)–(d)). The presence of B atoms also facilitated electronic interactions with molybdenum, resulting in improved stabilization of Mo atoms. Furthermore, the formation of B–metal bonds reduced the oxygen affinity of Mo, weakening OH⁻ adsorption and preventing Mo dissolution during the alkaline HER (Fig. 12(e) and (f)). The B_4.7–Ni₄Mo/NF catalyst exhibited exceptional catalytic performance in the alkaline HER, displaying a low overpotential of only 135.2 mV at 1.0 A cm⁻² and a low Tafel slope of 37.2 mV dec⁻¹ (Fig. 12(g) and (h)). Additionally, B_4.7–Ni₄Mo/NF achieved a current density of 1.0 A cm⁻² at 1.73 V in a 60 °C electrolyser, demonstrating its high efficiency. Notably, the catalyst maintained stable operation at this current density for an impressive duration of 800 h (Fig. 12(i)). Similarly, B doping can be employed to enhance the electrocatalytic performance of noble metal-based catalysts. A notable example is our group's study,¹⁷⁹ which applied the B doping method to strengthen the SMSI effect in Rh@NC nanoparticles. Through a combination of experimental and theoretical investigations, it has been demonstrated that B doping optimizes the electronic structure and water adsorption energy, effectively activating the SMSI between Rh and the NC support (Fig. 12(j)–(m)). Consequently, the B–Rh@NC catalyst synthesized exhibited remarkable electrocatalytic activity for the HER over a broad pH range, while maintaining impressive durability (Fig. 12(n)–(p)). In addition to the aforementioned studies, several other research works involving B doping are summarized in Table 6.

Fig. 12 XPS spectra of (a) Mo 3d and (b) Ni 2p, (c) free energy diagram of H₂O activation, (d) ΔG_H*, (e) Mo dissolution energy, (f) Mo dissolved in the electrolyte after the electrode worked for 48 h for B_4.7–Ni₄Mo/NF and comparison samples. HER performance of B_4.7–Ni₄Mo/NF and control samples in 1.0 M KOH: (g) HER polarization curve; (h) Tafel slope. (i) Stability test results for B_4.7–Ni₄Mo/NF//IrO₂/NF in an AEM electrolyser. Reprinted with permission from ref. 52. Copyright 2023, Elsevier. (j) Bader charge and (k) charge density difference analysis of B–Rh@CN. (l) ELF of B–Rh@CN. (m) Water dissociation free energy of B–Rh@CN and comparison samples. The HER performance test of B–Rh@CN and its comparison sample in (n) 0.5 M H₂SO₄, (o) 1.0 M PBS and (p) 1.0 M KOH. Reprinted with permission from ref. 179. Copyright 2023, Elsevier.

Table 6 Comparison of the performance of recently reported B-doped electrocatalysts

Electrocatalyst	Electrolyte	J (mA cm^−2)	η (mV)	Stability	Ref.
B–Ru@CNT	1.0 M KOH	10	17	10 000 cycles	180
	0.5 M H2SO4	10	62	5000 cycles
B–Ir NSs/NF	1.0 M KOH	10	18	24 h	181
M–B–Ni2P	1.0 M KOH	100	217	100 h	182
B-ZIS@TiO2	1.0 M KOH	10	237	18 h	122
6B-Fe7S8/FeS2	1.0 M KOH	10	113	30 h	121
Ni3Fe@BC-500 °C	1.0 M KOH	10	330	20 h	183
B–CoP	1.0 M KOH	100	112	20 h	184
NiCo@BC (500 °C)	1.0 M KOH	10	209	200 h	185

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6.5. Halogen element-doped electrocatalysts for the HER

Doping elements used to improve HER catalysts should possess certain advantages, such as the ability to adjust the electronic structure, increase active sites, inhibit surface oxidation, adjust acid–base properties, and improve catalyst stability. In addition to the commonly used elements mentioned earlier, such as P, N, S, and B, researchers have also been exploring other types of elements.

Halogen ions have been extensively studied for their ability to improve the selectivity and activity of the carbon dioxide reduction reaction (CO₂RR) by forming unique nanostructures and promoting adsorption/desorption of intermediates.¹⁸⁶ Recently, researchers have started applying halogen doping in the development of HER catalysts. For instance, our team¹⁸⁷ synthesized a halogen (X = Cl, Br, I) modified Ru/RuP₂ heterojunction structure using the microwave phosphide plasma method in just 1 min. The introduction of halogen elements increased the electrochemically active surface area. Experimental and DFT calculations demonstrated that the presence of halogen elements facilitated the dissociation of water molecules and optimized the hydrogen binding energy, thereby improving the electrocatalytic kinetics. Compared to I and Cl doping, the Br-doped catalyst demonstrates optimal electronegativity (Fig. 13(a)). Consequently, the Br–Ru/RuP₂ catalyst significantly reduced the energy barrier for breaking H–OH bonds, leading to accelerated water dissociation (Fig. 13(b)). Moreover, when H is adsorbed in the Br–Ru/RuP₂ system, the center of Ru's d band is appropriately located, resulting in a moderate strength of the Ru–H bond (Fig. 13(c)). According to Sabatier's principle, achieving the “just right” adsorption energy is crucial for optimizing all chemical reactions. Therefore, Br–Ru/RuP₂ exhibited optimal electrocatalytic performance, demonstrating a low overpotential of 34 mV to achieve a current density of 10 mA cm⁻² and a small Tafel slope of 27 mV dec⁻¹ in alkaline electrolytes (Fig. 13(d) and (e)). These indicated the high efficiency and effectiveness of Br doping at enhancing the HER catalytic activity. In the study conducted by our team,¹⁸⁸ we employed F doping to effectively narrow the work function difference between the Ru metal and the F–FeCoOOH support (Fig. 13(f)). This strategic adjustment facilitated the creation of a favorable interface between the metal and the support, enabling the efficient spillover of H species from Ru to F–FeCoOOH (Fig. 13(g)). By coordinating the hydrogen-rich and hydrogen-depleted components within the catalyst, the HER performance of Ru-based catalysts in alkaline seawater was significantly enhanced (Fig. 13(h)). Recent studies on halogen doping have proliferated, and a summary of these works is provided in Table 7.

Fig. 13 (a) Bader charge of Ru; (b) Gibbs free energy diagrams for HER via hydrogen spillover; (c) PDOS of Ru for Ru/Br–RuP₂ and comparison sample; (d) LSV and (e) Tafel curves of X (Cl, Br, I)–Ru/RuP₂ and Pt/C in 1.0 M KOH at 5 mV s⁻¹. Reprinted with permission from ref. 187. Copyright 2023, Wiley. (f) Work functions; (g) C_φ vs. η; (h) HER polarization curves for Ru/F–FeCoOOH and comparison sample. Reprinted with permission from ref. 188. Copyright 2023, Elsevier.

Table 7 Comparison of the performance of recently reported halogen elements-doped electrocatalysts

Electrocatalyst	Electrolyte	J (mA cm^−2)	η (mV)	Stability	Ref.
3F-FeP	1.0 M KOH	1000	369	100 h	189
F–Pt NCs	1.0 M KOH	500	274	24 h	190
Ru@F–Ni3N	1.0 M KOH	100	115	30 h	118
F–CoP–Vp-2	1.0 M PBS	10	108	20 h	191
Co–I–N/G	0.5 M H2SO4	10	52	5000 cycles	57
I-G-20	0.5 M H2SO4	10	245	30 h	192
ISA-CoPn/CoP-2	0.5 M H2SO4	10	44	50 h	193
Ni12P5−xBrx	1.0 M KOH	10	18	125 h	194

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7. Discussion

Although substantial progress has been achieved in this field, further improvements are still required to meet the standards for large-scale industrial applications (Fig. 14).

Fig. 14 Schematic illustration of ideal nonmetal-doped HER electrocatalysts and future perspectives.

The authors believe that for the future development of hydrogen production by electrolysis of water, we should focus on the following aspects:

1. In-depth understanding of the mechanism: nonmetal atom doping can enhance catalyst activity and stability by modifying the electronic structure, adjusting active sites, and inducing synergistic effects within the catalytic material. However, the precise mechanisms underlying these enhancements require further investigation and systematic study. In situ characterization techniques and first-principles calculations provide critical insights into how doping enhances catalyst performance and the key elements of the reaction mechanism, offering essential support for a deeper understanding and optimization of the reaction process.

Specifically, regulation of the electronic structure can be visualized using in situ XPS and synchrotron radiation, allowing direct comparison of surface electronic structure changes before and after doping. In situ infrared and Raman spectroscopy enable real-time detection of chemical bond changes, monitoring of active site surface states, and tracking of reaction intermediates. By observing reaction rates at varying temperatures, these methods also allow for the derivation of activation energies, providing a comprehensive view of the catalytic process. DFT calculations can simulate and model the catalyst's microstructure, while experimental characterization results provide a means to validate and explore the catalyst's reaction mechanism further. By combining theoretical calculations with experimental data, an effective structure–activity relationship (SAR) framework can be established to predict the catalytic activity of various materials and structural configurations in water electrolysis for hydrogen production. This approach holds significant potential in the rapidly advancing field of AI. If machine learning (ML) can process large-scale datasets derived from both DFT calculations and experimental data, while integrating and learning from past experiments, it could predict the catalytic activity and reaction mechanisms of new catalysts, thus greatly accelerating the catalyst design process.^195,196 The integration of machine learning with established computational methods and experiments can streamline the screening and development of new catalysts, improving overall R&D efficiency and aiding the discovery of cost-effective alternative materials to reduce or replace precious metal catalysts, thereby substantially lowering costs. This is crucial for advancing the practical implementation and commercialization of water electrolysis technology for hydrogen production. Moreover, detailed mechanistic studies have clarified the impact of various operational conditions on catalyst performance, enabling the design of more adaptable catalysts tailored to the specific demands of different applications.

2. Exploring prospects for industrial application: to promote the industrial application of water electrolysis for hydrogen production, the development of catalyst synthesis strategies that are both highly efficient and scalable remains a critical challenge. This endeavor requires the optimization of synthesis parameters to meet the demands of large-scale production while maintaining high catalytic activity. Furthermore, achieving a comprehensive balance between catalytic performance, long-term stability, economic feasibility, and environmental sustainability—particularly in terms of reducing pollutant emissions and greenhouse gas generation—is essential. Among various approaches, nonmetallic doping has emerged as a promising strategy due to its compatibility with existing industrial processes. Nevertheless, several nonmetal doping synthesis routes still encounter significant limitations when applied to large-scale manufacturing, and further research is needed to establish robust and scalable methods suitable for practical deployment.

To accelerate the technological transformation of water electrolysis, it is essential to establish an evaluation framework that closely aligns with industrial scenarios. Currently, electrolysers are widely employed in laboratory settings to simulate industrial production environments for assessing the potential application of electrocatalysts. However, most existing studies remain focused on activity and stability tests, with limited exploration of underlying reaction mechanisms, thereby constraining their ability to inform large-scale process design. With the rapid advancement of computational technologies, digital twin (DT) frameworks offer a promising approach to bridge this gap.¹⁹⁷ By integrating AI with multi-physics simulations, such as current distribution, heat transfer, and gas bubble dynamics within electrolysers, DT enables precise optimization of both electrolyser design (e.g., plate and flow channel architecture) and operational parameters (e.g., temperature gradient control). This approach not only reduces energy loss but also allows for predictive modeling of performance changes under scale-up conditions. Looking forward, the industrialization of water electrolysis-based hydrogen production will require the dismantling of disciplinary boundaries and the advancement of cross-disciplinary collaboration. In particular, the integration of materials science, fluid mechanics, and data science will be critical for addressing technical bottlenecks across the entire value chain, from catalyst synthesis to reactor design and system-level optimization.

3. Shift the focus towards seawater electrolysis: we are aware that the majority of the Earth's water resources are in the form of seawater, and many regions face scarcity of freshwater supplies. Developing direct seawater electrolysis for hydrogen production can effectively circumvent these limitations. However, it should be noted that seawater has a much more complex composition than fresh water, making it challenging to develop electrocatalysts that can operate stably in marine environments.^198,199 In cathodic reactions, the high concentration of calcium and magnesium ions in seawater tends to form insoluble precipitates on the cathode surface, leading to blockage and deactivation of active sites. Currently, research on the preparation of catalysts for direct seawater electrolysis using nonmetal atom doping is still limited. Future studies should focus more on whether nonmetal atom doping can improve the catalyst's surface microenvironment to prevent the deposition of calcium and magnesium ions from seawater on the cathode surface, thereby developing catalysts that can stably operate in natural seawater.

8. Summary

Designing efficient, low-cost, stable, and environmentally friendly electrocatalysts for hydrogen production through water electrolysis is of great significance for addressing the global energy crisis and advancing the development of a green hydrogen energy system. This review systematically summarizes nonmetal atom doping strategies from both theoretical and practical perspectives, aiming to provide a comprehensive reference for further fundamental research and engineering applications in this field.

This article first summarizes six mainstream nonmetallic doping methods, namely, the hydrothermal method, the solvothermal method, the calcination method, the ion exchange method, plasma treatment, and electrochemical deposition. For each approach, a brief introduction is provided, along with representative research examples and a discussion of its advantages and limitations, offering a clear and intuitive basis for the rational selection of synthesis strategies in future research.

To gain a deeper understanding of how nonmetallic doping influences electrocatalytic performance, this article outlines four key mechanisms: regulation of the electronic structure, optimization of surface active sites, enhancement of overall electrical conductivity, and induction of interfacial synergistic effects between metal centers and supports. These mechanisms are interrelated and collectively contribute to improved reaction kinetics and long-term stability in the HER. In parallel, this review summarizes commonly used experimental techniques for characterizing nonmetallic doping systems, including HRTEM, XPS, and XAS, which provide crucial insights into the structural and electronic effects underpinning the doping process.

From the perspective of practical applications, this review systematically compares the regulatory characteristics and representative advances of various nonmetallic dopants, including P, S, N, B, and halogens, in HER catalyst systems in recent years. For instance, P doping enhances electronic coupling between metal centers and supports, while optimizing the hydrogen adsorption free energy. S doping facilitates modulation of the d-band center of transition metals and improves overall conductivity. N doping effectively alters the electron distribution and tailors the structure of active sites by introducing lone-pair electrons. In contrast, B and halogen dopants demonstrate promising capabilities in regulating water dissociation pathways and tuning surface adsorption selectivity, respectively. This comprehensive summary and comparative analysis not only elucidates the mechanisms by which nonmetallic dopants influence catalytic performance but also provides a clearer theoretical framework and methodological reference for researchers working in related fields.

Looking ahead, considering the current limitations of nonmetallic doping strategies, such as insufficient mechanistic understanding, a lack of scalable synthesis methods, and limited integration with industrial hydrogen production, we propose the incorporation of AI-assisted data analysis and predictive modeling to accelerate the inverse design of structure–performance relationships and facilitate high-throughput catalyst screening. This approach is expected to enhance catalyst design efficiency and support the transition toward industrial-scale production. Moreover, in line with the principles of green development, this review advocates for the use of seawater as an alternative to pure water in electrolysis processes. This strategy not only addresses the global scarcity of freshwater resources but also holds promise for maintaining a high HER catalytic performance, offering a viable pathway toward the establishment of a sustainable clean energy system.

Data availability

The data are available upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (52072197, 52174283, and 22301156), the Natural Science Foundation of Shandong Province (ZR2021QE165), the Qingdao Natural Science Foundation (24-4-4-zrjj-16-jch), and the Shandong Province “Double-Hundred Talent Plan” (WST2020003).

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