1. Introduction

Sustainable clean energy technologies have the potential to address the pressing issues of severe environmental pollution and energy shortages faced today. Electrocatalysis, as the fundamental science underpinning these technologies, has taken a central role in their development. Electrocatalysts, being the core components of electrocatalysis, have attracted increasing attention in recent years. Currently, state-of-the-art electrocatalysts are noble metals such as platinum for the hydrogen evolution reaction (HER) and ruthenium dioxide (RuO₂) and iridium dioxide for the oxygen evolution reaction (OER)^[1-3]. However, their widespread industrial application is greatly limited by high cost, scarcity, and limited availability. Therefore, it is of paramount importance to develop low-cost and highly efficient electrocatalysts to enable the commercial viability of these promising technologies.

Consequently, significant efforts have been devoted to developing non-noble metal-based electrocatalysts, including transition metal compounds and metal-free materials^[4-6]. Among these, transition metal phosphides (TMPs) have emerged as promising electrocatalysts due to their unique physicochemical properties, such as high electronic conductivity, tunable structure and composition, and multifunctional active sites^[7,8]. Unlike many other non-noble electrocatalysts (e.g., metal oxides and sulfides), TMPs exhibit metallic or semi-metallic conductivity, which significantly enhances electron transfer rates during simultaneous HER and OER catalysis. Variations in the stoichiometric ratio of metal to phosphorus result in diverse structures and physicochemical characteristics of TMPs. Reports indicate that metal-rich TMPs possess superior electronic conductivity and chemical stability compared to phosphorus-rich TMPs^[9]. TMPs have been extensively studied as efficient catalysts for the HER, where negatively charged phosphorus sites and positively charged transition metal (TM) sites act as acceptors for protons and hydrides during the reaction^[10]. Additionally, the excellent OER activity of TMPs has been investigated, with several studies suggesting that the true OER active sites are gradually formed metal oxide/hydroxide species on the TMP surface^[11-13].

As is well known, both HER and OER are surface-sensitive reactions that primarily occur at the solid/liquid/gas three-phase interface^[14-16]. The reaction efficiency is closely related to the surface structure, morphology, composition, and interfacial properties of electrocatalysts. Coupling TMPs with other materials, such as metals or compounds, to form heterostructures can integrate the advantages of each component, thereby significantly enhancing electrocatalytic performance. TMP-based heterostructures include metal/TMP, TMP/TMP, transition metal (TM) oxides/TMP, TM hydroxides/TMP, TM chalcogenides/TMP, TM nitrides/TMP, TM carbides/TMP, TM phosphates/TMP, and carbon/TMP^[17-21]. Strong interfacial interactions between these components promote improved electronic conductivity and optimized adsorption energies, thus accelerating reaction kinetics^[22]. The enhanced activity and durability of heterostructured materials are attributed to the synergistic interactions occurring at the interfaces of their constituent phases. Establishing such interfacial coupling effects is a promising approach for improving both the activity and stability of heterostructures. Interfacial engineering, as a pivotal strategy in the construction of TMP-based heterostructures, allows precise control over charge transfer pathways, electronic structure modulation, and structural stability. By tailoring interfaces through compositional and morphological optimization, the synergistic interplay between components can be maximized, thereby enhancing catalytic performance. Several reviews have previously addressed TMPs and their electrocatalytic applications^[23-25]. However, most prior works focus on synthetic strategies or broad applications of TMPs, whereas this review uniquely emphasizes heterostructure engineering strategies to establish explicit structure-activity relationships. Moreover, comprehensive investigations and summaries regarding the origin of catalytic activity and the underlying mechanisms of TMP-based heterostructure electrocatalysts remain scarce and are urgently needed.

Herein, this review summarizes recent advances in TMP-based heterostructure electrocatalysts for water splitting, with a focus on the Mott-Schottky effect, support effect, and synergistic effect (Figure 1). The discussion encompasses the design of electrocatalyst structures and compositions and their corresponding electrochemical performance. Particular emphasis is placed on interfacial engineering and the synergistic interactions among different components. Finally, the existing challenges and future prospects of TMP-based heterostructure electrocatalysts for water splitting are presented.

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Figure 1. Illustration of TMP-based heterostructures in electrocatalytic water splitting. TMP: transition metal phosphide.

2. The Effects of TMP-Based Heterostructures

The design of TMP-based heterostructures harnesses three key effects to improve electrocatalytic performance in water splitting: the Mott-Schottky effect, the support effect, and the synergistic effect. Collectively, these effects enhance both catalytic activity and stability for the HER and OER.

2.1 Mott-Schottky effect

A Mott-Schottky heterojunction forms between a metal (or metallic compound) and a semiconductor (TMP), resulting in a space-charge region and a built-in electric field (BIEF) due to the difference in work function. This difference drives spontaneous electron transfer at the interface, inducing charge redistribution and creating an internal electric field that enhances active site accessibility and accelerates charge transfer rates^[26-29]. Similarly, p-n junctions leverage band alignment to promote charge separation. Furthermore, the surface chemisorption properties toward reaction intermediates can be finely tuned, which favors accelerated reaction kinetics. This adjustment optimizes the binding energies of intermediate species and reduces energy barriers, thereby improving catalytic activity. Overall, the formation of a Mott-Schottky heterojunction allows the catalyst to integrate the intrinsic advantages of its constituent materials while generating interfacial synergy that significantly enhances catalytic performance.

2.2 Support effect

Constructing TMP/defective carbon heterostructures by combining TMPs with defective carbon materials has attracted considerable attention. Various defective carbons, such as graphene quantum dots, carbon nanotubes, graphene, and porous carbons, have been widely reported^[30-33]. Strong interfacial interactions between TMPs and defective carbons facilitate the precise construction of heterostructures. Defective carbon materials typically exhibit high electrical conductivity and excellent resistance to acidic and alkaline environments. Therefore, coupling TMPs with defective carbon supports not only enhances electron transfer but also mitigates electrolyte-induced corrosion, thereby improving both the catalytic activity and stability of TMPs. The catalytic performance is strongly influenced by the properties of the carbon material. Compared to individual TMPs, incorporating defective carbon can modulate the electronic configuration and fine-tune the adsorption-desorption energetics of intermediates. Consequently, integrating TMPs with well-designed carbon matrices has become an effective strategy to achieve enhanced catalytic efficiency and long-term durability.

2.3 Synergistic effect

It is widely recognized that the rational design of multicomponent heterostructures can integrate the advantages of individual components to enhance specific properties, such as electronic conductivity and charge distribution. These heterostructures typically exhibit strong interfacial interactions, which result in improved electron transfer rates, optimized adsorption energies, and accelerated reaction kinetics^[34-36]. The deliberate construction of heterojunctions thus facilitates more favorable catalytic pathways. The pronounced synergistic effect between different components at the heterointerface plays a crucial role in boosting catalytic activity. Therefore, designing abundant multilevel interfaces and multifunctional interfacial sites is essential. Computational studies have revealed that interfacial synergy enables precise electronic structure engineering and optimizes the adsorption energetics of key intermediates, thereby significantly accelerating electrocatalytic reaction rates.

3. Activity Regulation Strategies of TMP-Based Heterostructures Towards Electrocatalytic Water Splitting

Common synthesis routes for TMP-based heterostructures include solvothermal/hydrothermal methods, electrodeposition, and phosphidation, among others. The catalytic performance of TMP-based heterostructures in water splitting can be systematically optimized by strategically regulating their electronic structure, interfacial interactions, phase composition, and support architecture. These approaches work synergistically to enhance charge transfer kinetics, fine-tune the adsorption energies of reaction intermediates, and improve structural stability, thereby delivering superior electrocatalytic activity and durability.

3.1 Electronic structure optimization

Heterojunctions formed between TMPs and secondary components generate a BIEF due to differences in work function. This intrinsic field drives spontaneous electron transfer, redistributes charges at the interface, and reduces energy barriers for water dissociation and intermediate adsorption^[37,38]. For example, Lin et al. constructed a Ni₂P/CoP heterojunction anchored on nickel foam (Ni₂P/CoP-NF) via a two-step phosphorization process, creating a robust p-n heterojunction that establishes a stable BIEF^[39]. This design promotes continuous electron transfer from CoP to Ni₂P, thereby optimizing the local electronic states of active catalytic centers during electrocatalysis. As a result, the catalyst demonstrates outstanding bifunctional performance, achieving low overpotentials of 238 mV for OER and 101 mV for HER at 50 mA·cm^-2, a cell voltage of 1.91 V for overall water splitting at 500 mA·cm^-2, and stability over 300 hours. In situ Raman spectroscopy and density functional theory (DFT) studies further confirm that the BIEF accelerates the formation of active CoOOH and Co(OH)₂ intermediates, reducing energy barriers for *OH deprotonation (OER) and H₂O dissociation (HER). Similarly, Yan et al. designed a NiCoP-Co/MXene Mott-Schottky heterostructure, where the work function difference (ΔΦ = 0.43 eV) between Co and NiCoP drives spontaneous electron transfer, creating an asymmetric charge distribution at the interface^[40]. The incorporation of MXene not only enhances electrical conductivity but also prevents nanosheet aggregation, resulting in excellent catalytic performance: a low cell voltage of 1.52 V at 10 10 mA·cm^-2 in 1 M KOH, stable operation for 50 hours, and remarkable overpotentials of 55 mV for HER and 236 mV for OER at 10 mA·cm^-2, outperforming Pt/C and RuO₂ benchmarks. These studies collectively highlight the critical role of BIEF in tuning adsorption energies of key intermediates and accelerating reaction kinetics for efficient water splitting.

The electronic structure of TMP-based heterostructures can also be optimized through d-band center regulation^[41]. A widely adopted strategy is heteroatom doping, as demonstrated by Br-induced Co_1.085P@NF, where the incorporation of bromine caused a negative shift in the Co d-band center. This shift weakened the adsorption of oxygen intermediates and reduced energy barriers, thereby enhancing catalytic efficiency^[42]. Combined with ultra-hydrophilicity, this design achieved remarkable performance, with overpotentials of 90 mV (HER) and 197 mV (OER) at 50 mA cm^-2, and a low cell voltage of 1.59 V for overall water splitting. Similarly, the construction of Co₄S₃/CoP₃ heterostructures facilitated interfacial charge redistribution, inducing a downward shift in the d-band center to optimize the adsorption/desorption balance of intermediates^[43]. This heterostructure exhibited outstanding OER activity, delivering overpotentials of 190 mV and 272 mV at 20 and 50 mA cm^-2, respectively, surpassing both monophase counterparts and RuO₂ benchmarks.

3.2 Interfacial engineering

Interfacial interactions between TMPs and other components (e.g., oxides, carbides, or carbon supports) play a critical role in determining catalytic efficiency. Introducing defects or lattice strain at these interfaces creates unsaturated coordination sites and enhances electron delocalization. For instance, compressive strain in Ni₂P/Co₂P heterostructures, achieved by “similar stacking” of hexagonal Co₂P on Ni₂P, optimized the hydrogen adsorption free energy (ΔG_H* = 0.23 eV) and delivered a HER current density of 1 A cm^-2 at 388 mV, outperforming Pt/C^[44]. Similarly, oxygen vacancies (O_v) in Co₂P/WO_3-x heterojunctions enhanced charge transfer kinetics, resulting in a low OER overpotential of 254 mV at 10 mA cm^-2[45]. Another effective strategy involves vacancy engineering. For example, Mg-doped FeP subjected to acid leaching generated Fe-vacancy-rich FeP (Vc-FeP)^[46], which exhibited excellent HER activity in both alkaline (1 M KOH) and acidic (0.5 M H₂SO₄) electrolytes, with overpotentials of 108 mV and 65 mV at 10 mA·cm^-2, respectively. The Tafel slopes were 33 and 49 mV·dec^-1, and nearly 100% Faradaic efficiency was maintained. Mechanistic studies revealed that Fe vacancies optimized the electronic structure, reducing hydrogen adsorption free energy and improving proton trapping. Similarly, Ni₂P with Fe vacancies (V_Fe-Ni₂P), prepared via electrodeposition of Fe-doped Ni₂P followed by acid etching, demonstrated outstanding bifunctional performance in 1 M KOH, requiring only 52 mV (HER) and 154 mV (OER) to achieve 10 mA·cm^-2. The corresponding Tafel slopes were 49.7 mV·dec^-1 (HER) and 82.57 mV·dec^-1 (OER), with negligible degradation after 1,000 CV cycles. The presence of cationic vacancies reduced charge-transfer resistance (Rct ≈ 1.88 Ω) and generated a phosphorus-rich environment conducive to proton adsorption and intermediate desorption^[47].

Dynamic surface reconstruction during operation is also crucial for activating catalytic phases. For instance, Wei et al. introduced phosphorus vacancies (P_v) into CoFeP via Ar plasma treatment, inducing electron redistribution, lowering the energy barrier for *H₂O dissociation, and achieving an HER overpotential of 367 mV at 1 A cm^-2 under industrial conditions (6 M KOH, 80 °C)^[48]. In situ Raman analysis revealed that P vacancies accelerated phase reconstruction into CoFeOOH during OER, reducing the overpotential to 382 mV at 1 A cm^-2. Similarly, a Ce-doped Fe₂P/NiCoP hybrid pre-catalyst was designed to enhance OER kinetics for anion exchange membrane water electrolysis (AEMWE)^[49]. The optimized Ce_0.1-Fe₂P/NiCoP exhibited an extremely low OER overpotential of 280 mV at 0.5 A cm^-2 and a small Tafel slope of 55.3 mV dec^-1 in 1.0 M KOH, outperforming RuO₂ benchmarks. When integrated into an AEMWE electrolyzer, it delivered a cell voltage of 1.812 V at 1.0 A cm^-2 and maintains stable operation for over 500 hours at 60 °C. In situ characterizations and DFT calculations revealed that Ce doping promoted dynamic surface reconstruction into Ce-FeOOH/NiCoP, optimizing the electronic structure, lowering energy barriers for intermediate formation, and enhancing charge transfer. The synergistic combination of Ce doping, bimetallic hybridization, and retained metallic NiCoP phases significantly improved both catalytic activity and long-term durability.

3.3 Phase regulation

Crystalline phases, characterized by long-range atomic order, provide excellent electrical conductivity and structural stability, enabling rapid electron transport. In contrast, amorphous phases, with their disordered atomic arrangement, expose abundant unsaturated coordination sites and defects, thereby increasing the density of active sites and modulating adsorption energy barriers for key intermediates. The synergistic integration of crystalline and amorphous phases through heterointerface engineering promotes interfacial charge redistribution, tunes electronic structures, and accelerates reaction kinetics. Thus, rational design of crystalline/amorphous heterojunctions represents a promising approach to enhance electrocatalytic performance, offering efficient, durable, and scalable solutions for hydrogen production via water electrolysis.

For example, CoP@WP₂/NF core-shell nanowires (featuring a crystalline CoP core and an amorphous WP₂ shell) deliver ultralow overpotentials of 13 mV (HER, acidic) and 254 mV (OER, alkaline) at 10 mA cm^-2, outperforming Pt/C and RuO₂ benchmarks^[50]. The amorphous WP₂ shell increases active site density and corrosion resistance, while interfacial charge coupling between CoP and WP₂ reduces kinetic barriers. Similarly, CoP/FeCoP_X hierarchical nanoarrays employ Fe doping and phosphorus vacancies to achieve HER and OER overpotentials of 74 mV and 237 mV in alkaline media, respectively, with a full-cell voltage of 1.53 V at 10 mA cm^-2, underscoring the role of electronic engineering in optimizing intermediate adsorption^[51]. The NiCoP/Co₂P@NF core-shell structure achieves an ultralow HER overpotential of 26 mV at 10 mA cm^-2, attributed to electron-rich Co sites at the amorphous-crystalline interface, where the amorphous NiCoP shell not only protects the crystalline Co₂P core but also provides a large electrochemical surface area^[52].

Amorphous components further facilitate dynamic surface reconstruction during catalysis. In reduced graphene oxide (RGO)-supported heterostructures comprising crystalline nickel phosphide and amorphous iron phosphate phases (Ni₂P/FePO_X/RGO), the amorphous FePO_X promotes the in situ formation of active oxyhydroxides under OER conditions, enabling outstanding performance at industrial current densities (421 mV at 1000 mA cm^-2)^[53]. Similarly, a medium-entropy NiCoFeP@NiCoFe-layered double hydroxide (LDH) architecture, which combines crystalline phosphide nanowires with amorphous LDH nanosheets, achieves HER/OER overpotentials of 46 mV and 148 mV at 10 mA cm^-2 and sustaining 1,000 mA cm^-2 for 600 hours^[54].

3.4 Support engineering

Defective carbon substrates, characterized by heteroatom doping, hierarchical porosity, and tailored nanostructures, play a crucial role in enhancing catalytic activity by improving electrical conductivity, exposing abundant active sites, and facilitating rapid electron/ion transport.

For example, nitrogen-doped porous carbon (NC) in NiFeP/NC hybrids, synthesized through KOH activation and ammonia annealing, provides a three-dimensional hierarchical structure that promotes lattice oxygen activation^[55]. N-doping redistributes electron density, lowers the energy barrier for oxygen evolution via the lattice oxygen oxidation mechanism, and achieves an OER overpotential of 241 mV at 10 mA cm^-2. The porous framework also prevents nanoparticle aggregation and improves mass transport. Similarly, nickel-nitrogen co-doped carbon nanofibers (Ni-NCNF) in NiFeP@Ni-NCNF leverage Ni-N co-doping to enhance electrical conductivity and create a superhydrophilic/superaerophobic surface, accelerating bubble detachment and electrolyte penetration^[56]. Coupled with vertically aligned NiFeP nanosheet arrays, this design achieves low bifunctional overpotentials (HER: 131 mV, OER: 204 mV) and an overall cell voltage of 1.61 V, attributed to maximized active site exposure and efficient charge transfer. Additionally, S-doped carbon nanobelts (SC) in Co₂P@SC modulate electron distribution via sulfur doping, strengthening proton adsorption for HER (78 mV) and optimizing intermediate binding for OER (242 mV)^[57]. The interconnected nanobelt network prevents Co₂P agglomeration and facilitates efficient mass and charge transport, resulting in a cell voltage of 1.57 V.

Nanostructure-dependent effects also play a pivotal role. For instance, ultrafine N-doped carbon nanotubes (U-NCNTs) in CoP-Ni₂P@U-NCNTs/NF, derived from Co-ZIF-L through phosphorylation, confine heterojunctions within 50 nm nanotube walls^[58]. The ultrafine diameter increases edge defect density and exposes abundant active sites, while N-doping synergistically interacts with CoP-Ni₂P to lower hydrogen adsorption energy (HER overpotential: 67 mV) and accelerate O-O coupling dynamics (OER overpotential: 203 mV). DFT calculations confirm that spatial confinement stabilizes the heterostructure, achieving a low cell voltage of 1.52 V.

Overall, the catalytic performance of TMP-based heterostructures in water splitting can be significantly improved through strategic regulation of electronic structures, interfacial interactions, phase compositions, and substrate engineering. Tailoring electronic configurations via heterojunction formation or heteroatom doping optimizes the d-band center, balancing adsorption/desorption energies of intermediates and reducing kinetic barriers. Interfacial engineering, through defects, lattice strain, and dynamic surface reconstruction, enhances charge transfer and promotes the exposure of metastable active phases. Synergistic integration of crystalline and amorphous phases ensures structural stability while increasing active site density to accelerate reaction kinetics. Finally, incorporating defective carbon supports improves conductivity, mitigates nanoparticle aggregation, and tunes electronic states, collectively enabling high activity and industrial-level durability.

4. Applications of TMP-Based Heterostructures in Electrocatalytic Water Splitting

TMP-based heterostructures demonstrate broad applicability in electrocatalytic water splitting, primarily enabled by precise interfacial engineering. These architectures deliver outstanding performance metrics, including ultralow overpotentials, high current densities, and long-term operational stability under industrial conditions. Such characteristics underscore their potential as scalable and cost-effective catalysts for next-generation clean energy technologies.

4.1 TMP/metal heterostructure

A heterostructured Co-Co₂P catalyst embedded in a three-dimensional N, P dual-doped carbon matrix further modified with carbon nanotubes (denoted as Co-Co₂P@CNT//CM) was synthesized using sodium hypophosphite (NaH₂PO₂)-modified Zeolitic Imidazolate Framework-67 (ZIF-67) as the precursor^[59]. Work function analysis revealed a higher value for Co₂P (4.80 eV) compared to Co (4.73 eV) (Figure 2a and Figure 2b). This difference drives electron transfer from Co to Co₂P at the interface, forming a Mott-Schottky junction. The resulting charge redistribution significantly accelerates electrocatalytic reaction kinetics. Computational studies further indicated that the Co-Co₂P heterojunction exhibits an optimized hydrogen adsorption energy (H*) and superior intrinsic activity for the HER relative to individual Co or Co₂P constituents. Consequently, Co-Co₂P@CNT//CM demonstrated excellent HER activity, achieving overpotentials of 205, 142, and 262 mV at 10 mA cm^-2 in 1.0 M KOH, 0.5 M H₂SO₄, and 1.0 M phosphate buffer saline, respectively. The work function difference (ΔΦ) between metals and supports also plays a critical role in hydrogen spillover kinetics. CoP was synthesized via hydrothermal growth of Co(OH)₂ followed by phosphidation, while PtM/CoP catalysts (M = Rh, Pd, Ag, Ir, Au) were obtained through in situ chemical reduction of metal precursors on CoP using NaBH₄^[61]. Electrochemical testing in acidic media revealed that minimizing ΔΦ reduces interfacial charge accumulation, weakens proton trapping, and lowers the energy barrier for hydrogen spillover. The optimized Pt₂Ir₁/CoP catalyst (ΔΦ = 0.02 eV) exhibited outstanding HER performance, achieving an ultralow overpotential of 7 mV at 20 mA·cm^-2, a Tafel slope of 25.2 mV·dec^-1, and exceptional stability over 500 hours. Its mass activity reached 110 A·mg_ptir^-1 at -50 mV vs. RHE, surpassing commercial Pt/C by 78-fold. Mechanistic insights from DFT and operando studies confirmed that a small ΔΦ promotes electron redistribution, alleviates interfacial charge accumulation, and enables efficient hydrogen transfer from PtIr to CoP. Control experiments excluded dominant contributions from isolated PtIr or CoP sites, confirming hydrogen spillover as the primary pathway. These findings establish ΔΦ as a universal design parameter for spillover-based HER electrocatalysts.

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Figure 2. Energy band diagrams illustrate Co and Co₂P (a) pre- and (b) post-Schottky junction formation. Republished with permission from^[59]; (c) Calculated d-band density of state for CoP, CoP/Co-N-C and Co-N-C. Republished with permission from^[60]. E_VAC: vacuum energy; E_Fm: Fermi level of metal; E_Fs: Fermi level of semiconductors; φ_m: vacuum electrostatic potential of metal; φ_s: vacuum electrostatic potential of semiconductors; E_C: conduction band; E_V: valence band.

Additionally, DFT simulations revealed that Mott-Schottky heterostructures can tune the d-band center to balance oxygen intermediate adsorption and desorption^[60]. A CoP/Co-N-C heterojunction was synthesized via controlled phosphorization of Co-decorated Co-N-C^[62]. The internal electric field at the Mott-Schottky interface facilitated charge transfer during oxygen electrocatalysis. As shown in Figure 2c, a Schottky barrier-induced d-band center shift modulated oxygenate intermediate adsorption/desorption, enhancing bifunctional catalytic activity. CoP/Co-N-C achieved high OER activity with an overpotential of 320 mV in 0.1 M KOH. Subsequently, bimetallic and trimetallic Mott-Schottky catalysts were also explored to further improve catalytic performance^[63].

4.2 TMP/defective carbon heterostructure

Incorporating heteroatoms such as nitrogen (N) and phosphorus (P) into carbon supports effectively tunes the electronic properties of electrocatalysts and generates additional active sites, thereby enhancing catalytic efficiency. For example, a three-dimensional cobalt phosphide nanoparticle embedded into N, P co-doped carbon nanotubes knitted hollow nanowall arrays on carbon textiles (CoPʘNPCNTs/CTs) was synthesized using a cobalt metal-organic framework (Co-MOF) as the precursor (Figure 3a)^[64]. The catalytic synergy between the CoP phase and the NPCNT support renders the composite highly active for both HER and OER. The CoPʘNPCNTs/CTs exhibit overpotentials of 101.9 mV in 1.0 M KOH and 53.1 mV in 0.5 M H₂SO₄ for HER at 10 mA cm^-2. When used as both the anode and cathode in a water electrolyzer, the system requires a cell voltage of 1.66 V to reach 200 mA cm^-2 (Figure 3b). Combined experimental and computational studies confirm that both the NPCNTs and the nitrogen, phosphorus-coating contribute to lowering the apparent activation energy barrier for HER (Figure 3c).

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Figure 3. (a) HRTEM images of hollow CoPʘNPCNTs nanowall; (b) Polarization curves depicting the HER/OER activity of CoPʘNPCNTs/CTs initially and following a 200-hour stability test. The inset chronopotentiometry curve demonstrates catalyst stability under a fixed potential at 100 mA cm^-2 for 200 h; (c) Calculated free energy diagrams illustrating H* adsorption intermediates on catalyst surfaces during the HER. Republished with permission from^[64]; (d) TEM image of borophene nanosheets. The inset presents the inverse FFT pattern of the selected region of the HR-TEM image indicating the interlayer distance; (e) TEM image of borophene_Ni₂P. (f) LSV plot of the OER for catalysts in 1 M KOH^[65]. HR-TEM: high-resolution transmission electron microscopy; CoP: cobalt phosphide; NPCNTs: nitrogen-doped porous carbon nanotubes; HER: hydrogen evolution reaction; OER: oxygen evolution reaction; CTs: carbon textiles; FFT: Fast Fourier Transform; HR-TEM: high-resolution transmission electron microscopy; LSV: linear sweep voltammetry; CNTs: carbon nanotubes.

Defect-rich carbons have also been reported to optimize the band structure of TMPs. For instance, CoP nanoparticles confined within a defect-rich carbon shell (CoP/DCS) were synthesized via self-assembly of modified polycyclic aromatic molecules^[66]. This method enables precise control over crystalline defects in the carbon matrix through strategic grafting and subsequent removal of C-N bonds from the precursor molecules. DFT calculations demonstrate that defects with unpaired electrons in the carbon shell significantly tailor the electronic band structure of encapsulated CoP nanoparticles. As a result, CoP/DCS exhibits excellent catalytic performance with overpotentials of 88 mV (HER) and 251 mV (OER) at 10 mA·cm^-2. When applied as both cathode and anode in a water electrolyzer, the system delivers a cell voltage of 1.49 V at 10 mA·cm^-2 and maintains good stability over 24 hours.

Beyond defective carbon supports, borophene, a monolayer boron nanostructure, has been utilized as a support (Figure 3d). Borophene features exceptional carrier mobility, mechanical robustness, and superconductivity. Nickel phosphide nanoparticles supported on borophene (borophene Ni₂P, 1:1) were synthesized and employed as an OER electrocatalyst (Figure 3e)^[65]. This catalyst demonstrated high activity with an overpotential of 299 mV and excellent durability for OER in 1 M KOH solution (Figure 3f). The presence of borophene prevents Ni₂P nanoparticle agglomeration and enhances charge transfer, thereby boosting OER performance.

4.3 TMP/TMP heterostructure

Research indicates that CoP, characterized by abundant Co-P bonds, effectively provides additional proton-acceptor sites for the HER, while Co₂P, featuring numerous Co-Co bonds, accelerates the formation of the OER intermediate OOH*^[67]. Accordingly, Fe-doped Co₂P/CoP integrated with N, P co-doped porous carbons were synthesized via ion exchange followed by pyrolysis in a hydrogen atmosphere^[68]. It was found that robust interfacial interactions between the CoP and Co₂P phases within these heterostructures facilitate accelerated electron transfer, consequently enhancing catalytic activity (Figure 4a and Figure 4b). Furthermore, Fe doping boosts intrinsic catalytic activity and modulates electron density distribution near the catalyst interface. As a result, Fe-doped Co₂P/CoP demonstrated excellent bifunctional performance, achieving overpotentials of 286 mV (OER) and 174 mV (HER) at 10 mA cm^-2, respectively. When Fe-doped Co₂P/CoP was used as both anode and cathode in a water splitting electrolyzer, the cell voltage was 1.711 V at 10 mA cm^-2 (Figure 4c).

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Figure 4. (a) The Co K-edge XAFS spectra; (b) k²-weighted χ(k)-function of Co-XAFS spectra of Fe-Co₂P/CoP, Co₂P/CoP, CoP, Co₂P, CoO and Co foil; (c) LSV curves of Fe-Co₂P/CoP||Fe-Co₂P/CoP and Co₂P/CoP||Co₂P/CoP toward overall water splitting in two-electrode configuration. Republished with permission from^[68]; (d) Band alignment diagrams illustrating Fe₂P and NiCoP before and after interfacial contact; (e) Schematic diagram of the electron transfer in Fe₂P/NiCoP. Republished with permission from^[69]. XAFS: X-ray absorption fine structure; LSV: linear sweep voltammetry.

Compared with single-metal phosphides, multi-metal phosphides exhibit synergistic effects between metal species, which favorably regulate microstructure, accelerate charge transfer, and increase conductivity and active surface area^[70-72]. For example, the CoP/CoMoP₂ heterojunction was synthesized via a CoMo-layered double hydroxide precursor^[73]. The interfacial interaction between CoP and CoMoP₂ leads to charge redistribution, enhancing catalyst conductivity and modulating the adsorption of H₂O and H*. The CoP/CoMoP₂ exhibited good HER activity with an overpotential of 93.6 mV at 10 mA cm^-2 in 1 M KOH.

Furthermore, a BIEF can be generated at the heterojunction interface. The Fe₂P/NiCoP heterostructure was synthesized via a solvothermal method^[69]. As illustrated in Figure 4d, upon intimate contact within the heterojunction, electrons transfer from NiCoP to Fe₂P, equilibrating their Fermi levels through downward and upward shifts in NiCoP and Fe₂P, respectively. This electron redistribution establishes a strong BIEF at the interface. The resulting positive charge accumulation on the NiCoP surface facilitates the recombination of electrons, supplied by OH^- ions, with holes located in the NiCoP valence band. Operando Raman analysis reveals that the BIEF promotes surface reconstruction of NiCoP, converting it into catalytically active NiCoOOH species (Figure 4e). This structural transformation modulates the binding energies of intermediates during the OER reactions, leading to enhanced catalytic performance. Consequently, the catalyst exhibits excellent OER activity, with overpotentials of 247 mV and 255 mV required to achieve 10 mA cm^-2 in alkaline freshwater and simulated seawater, respectively.

Trimetallic phosphides encapsulated within N-doped carbon nanofibers (CoNiP/CoNiFeP@NCNFs) were synthesized using a CoNi coordination polymer and CoNiFe layered double hydroxide as precursors^[74]. The CoNiP/CoNiFeP@NCNFs exhibited excellent catalytic performance for both OER and HER, achieving overpotentials of 260 mV and 177 mV, respectively, in 1 M KOH. When used as both the anode and cathode in a water electrolyzer, the system required a cell voltage of 1.55V to reach 10 mA cm^-2. The enhanced catalytic performance originates from the synergistic interactions among CoP, FeP, and Ni₂P, which facilitate interfacial electron transport. At the same time, Fe doping modulates the valence states of Co and Ni, thereby optimizing the adsorption energetics of oxygen-containing intermediates. Additionally, a simple one-pot pyrolysis strategy was employed to anchor composition-tuned Mn₂P-MnP heterojunction nanoparticles onto a P/N-doped porous carbon network^[34]. This design achieved ultralow overpotentials of 63 mV (acidic) and 98 mV (alkaline) at 10 mA cm^-2, with Tafel slopes of 41 mV dec^-1 and 48 mV dec^-1, respectively, outperforming individual Mn₂P or MnP phases. DFT calculations revealed that the Mn₂P-MnP interface optimized ΔG_H* to 0.057 eV, close to the thermoneutral value, while the carbon matrix enhances stability and charge transfer.

4.4 TMP/ metal oxide or hydroxide heterostructure

Interfacial engineering through in situ self-reconstruction dynamically generates active metal hydroxide/oxide layers, where the core-shell structure stabilizes the interface and modulates electronic states. It has been demonstrated that some catalysts, such as metal phosphides, undergo in situ self-reconstruction during oxidation reactions, transforming into corresponding metal oxides/hydroxides, which are considered the actual active sites. Accordingly, various metal oxides including CrO_x, CoO, NiO, Fe₃O₄, CeO₂ and CoNiO_x have been reported to composite with TMP to construct heterostructures^[75-80]. The synergistic effect between TMPs and metal oxides or hydroxides promotes charge transfer and modifies electronic structures to increase the number of active sites. For example, phosphidated cobalt oxide with a nanoscale heterointerface (PCO-nHI) was fabricated by a sol-gel reaction combined with a hot injection method, followed by phosphidation^[81]. DFT calculations demonstrated that CoO promotes water dissociation while CoP facilitates OH* removal. The presence of CoP helps maintain the stability of CoO and prevents its reduction to metallic cobalt. Consequently, PCO nHI exhibited high alkaline HER and OER activity with overpotentials of 168 mV and 396.4 mV at 100 mA cm^-2, respectively, along with superior stability over 100 hours.

Multi-component metal oxide TMP nanointerfaces have also been reported to outperform single-metal systems. Crystalline Ni₂P clusters anchored on amorphous NiMoO_x supported by nickel foam (Ni₂P-NiMoO_x/NF) were fabricated via sequential hydrothermal growth and phosphorization^[82]. When deployed in water electrolysis, the system required 1.66 V to reach 100 mA cm^-2 and demonstrated 100-hour durability under industrial operation (65 °C, 30% KOH) (Figure 5a). Both DFT calculations and characterizations indicated that interfacial coupling between Ni₂P and NiMoO_X downshifts d-band centers toward the Fermi level, facilitating water dissociation and enhancing overall electrocatalytic performance. (Figure 5b and Figure 5c).

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Figure 5. (a) LSV polarization curves of the Ni₂P-NiMoO_x/NF-catalyzed electrolyzer at various temperatures; (b) ΔG_H* diagram of Ni₂P, NiMoO₄, Ni₂P-NiMoP, and Ni₂P-NiMoO₄; (c) The tailoring relationship between ΔG_H* and the d-band centers of ε_d. Republished with permission from^[82]; (d) TEM image of Ni-FeOH@Ni-FeP powders with the inset of HR-TEM; The corresponding free energy diagrams for OER (e) and HER (f) on Ni₂P, Ni-FeP, NiFe(OH)₂, Ni-FeOH@Ni-FeP (both Ni site and Fe site) at U = 0 V. Republished with permission from^[83]. LSV: linear sweep voltammetry; HR-TEM: high-resolution transmission electron microscopy; OER: oxygen evolution reaction; HER: hydrogen evolution reaction.

The heterointerface between nickel phosphide and nickel hydroxide supported on nickel foam (Ni(OH)₂/Ni₂P/NF) was constructed via an electrodeposition phosphorization electrodeposition method^[84]. Introduction of Ni(OH)₂ triggers interfacial electron density rearrangement across the Ni₂P/Ni(OH)₂ heterojunctions, suppressing charge-transfer resistance and optimizing adsorption energetics of key intermediates. The synergistic effect between Ni(OH)₂ and Ni₂P contributes to bifunctional catalytic activity toward both hydrazine oxidation reaction and HER, achieving potentials of -14 mV and 72 mV at 10 mA cm^-2, respectively. When used as both electrodes in overall hydrazine splitting, cell voltages of 0.357 V and 0.513 V were required to deliver 100 and 200 mA cm^-2 in 1 M KOH with 0.5 M hydrazine, respectively. Furthermore, the construction of core shell heterojunction structures helps maintain structural stability. As shown in Figure 5d, Ni-FeOH@Ni-FeP needle arrays with a core shell heterojunction were synthesized through in situ hydroxide growth on the Ni-FeP surface^[83]. The catalyst exhibited superior intrinsic conductivity and optimized d-band center. During the OER process, the O₂ release step was identified as the rate-determining step on the Fe site of Ni-FeOH@Ni-FeP, with an energy barrier of 1.86 eV (Figure 5e). The catalyst possessed the lowest Gibbs free energy for hydrogen adsorption (ΔG_*H), yielding optimal catalytic activity (Figure 5f). The Ni-FeOH layer improves the structural stability of the Ni-FeP core and cooperatively interacts with Ni-FeP, modulating their electronic structures and surface microenvironments. Consequently, the Ni-FeOH@Ni-FeP-based alkaline water electrolyzer achieves a cell voltage of 2.14 V to reach 1 A cm^-2 and demonstrates long-term stability over 240 hours.

4.5 TMP/ metal chalcogenide heterostructure

Metal chalcogenides, including metal tellurides, sulfides, and selenides, have shown significant potential in electrocatalysis owing to their tunable physical, chemical, and electronic properties^[85-87]. Constructing TMP/metal chalcogenide heterostructures enables modulation of charge distribution, optimization of electronic structures, and acceleration of reaction kinetics^[88-92].

The synergistic interaction between Ni₂P and NiS₂ can effectively enhance electrocatalytic performance. A nickel phosphide/sulfide (Ni₂P/NiS₂) microsphere electrocatalyst was synthesized via a bubble-template electrodeposition process followed by phosphorization and sulfuration (Figure 6a)^[93]. NiS₂ modification induces interfacial charge redistribution and strengthens Ni-P bond covalency at heterojunctions, facilitating optimized adsorption of H* intermediates and water molecules. As a result, the catalyst exhibited high activity toward both OER and HER, requiring overpotentials of 320 mV and 169 mV at 100 mA cm^-2, respectively, in 1 M KOH. In alkaline seawater, the Ni₂P/NiS₂ electrocatalyst achieved overpotentials of 344 mV and 188 mV for OER and HER, respectively, at 100 mA cm^-2 (Figure 6b).Furthermore, the formation of polyanionic species derived from NiS₂ generates a protective surface film that inhibits chloride ion (Cl^-) penetration (Figure 6c). This barrier improves corrosion resistance, suppresses corrosive processes, and ensures long-term durability for alkaline seawater electrolysis.

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Figure 6. (a) SEM image of Ni₂P/NiS₂; (b) HER/OER polarization curves of Ni₂P/NiS₂ in 1.0 M KOH, 1.0 M KOH + 0.5 M NaCl and 1.0 M KOH + natural seawater; (c) Stability enhancement schematic for Ni₂P/NiS₂ anodes in seawater splitting. Republished with permission from^[93]; (d) ΔG_H* diagram and (e) reaction energy diagram for H₂O dissociation; (f) Temperature-dependent water splitting polarization curves of Ni₂P-NiSe₂/MoO_x/NF in 30% KOH electrolyte. Republished with permission from^[94]. SEM: scanning electron microscopy; HER: hydrogen evolution reaction; OER: oxygen evolution reaction.

In addition, multi-component synergistic effects can further enhance electrocatalytic performance through strong coupling interactions among multiple active phases. For instance, Ni₂P-NiSe₂ were decorated on amorphous MoO_x nanorods supported on nickel foam (Ni₂P-NiSe₂/MoO_x/NF)^[94]. The interfacial coupling between Ni₂P, NiSe₂ and MoO_x optimizes charge distribution and electronic structure, accelerating the initial steps of alkaline HER (Figure 6d and Figure 6e). The resulting catalyst displayed outstanding performance, requiring overpotentials of only 241 mV for OER and 23 mV for HER at 10 mA cm^-2. When applied in a water electrolyzer, the system required a cell voltage of 1.63 V at 50 mA cm^-2 and maintained stability for 1,000 hours at 20 mA cm^-2 in 1 M KOH. Under industrial conditions (30% KOH at 65 °C), the electrolyzer achieved 1,000 mA cm^-2 at 2.02 V with remarkable durability of 200 hours at 200 mA cm^-2 (Figure 6f).

4.6 TMP/ metal phosphate heterostructure

Metal phosphates, a class of multifunctional materials, exhibit tunable acidity, excellent thermal stability, and high conductivity. They have been widely reported as efficient OER electrocatalysts in both neutral and alkaline media. Structural distortions within phosphate groups can stabilize the high oxidation states of metals, thereby promoting water adsorption and oxidation on the catalyst surface^[95-97]. Constructing TMP/metal phosphate heterostructures increases the number of active sites and accelerates charge transfer, leading to strong synergistic effects that enhance overall electrocatalytic performance^[98-102].

Crystalline materials typically provide superior charge transport efficiency and structural stability, whereas amorphous phases offer abundant structural defects and unsaturated coordination sites^[103]. Based on these complementary characteristics, crystalline/amorphous heterophase engineering of TMP/metal phosphate systems emerges as an effective strategy to improve catalytic activity. For example, crystalline Ni₅P₄/amorphous CePO₄ core/shell heterostructure arrays were fabricated on Ni foam via in situ deposition followed by low-temperature phosphating (Figure 7a)^[104]. Both experimental data and theoretical calculations revealed that amorphous CePO₄ surface modification enhances the electrochemically accessible surface area and facilitates charge transfer. Moreover, CePO₄ effectively modulates the d-band center of Ni₅P₄, optimizing intermediate adsorption and accelerating water dissociation kinetics (Figure 7b and Figure 7c). Consequently, the heterostructure delivered low overpotentials of 191 mV (OER) and 94 mV (HER) at 10 mA cm^-2 in 1 M KOH. When assembled into a water electrolyzer, it achieved 10 mA cm^-2 at a cell voltage of 1.535 V.

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Figure 7. (a) SEM image of Ni₅P₄/CePO₄ nanosheets heterostructure array; Electronic density of state profiles for (b) crystalline Ni₅P₄ and (c) crystalline Ni₅P₄/amorphous CePO₄ hybrid. Republished with permission from^[104]; (d) FE-SEM micrographs of Ni₂P/V-Pi/CC; (e) Long-term water splitting stability of Ni₂P/V-Pi/CC||Ni₂P/V-Pi/CC electrolyzer; (f) Operational demonstration of the electrolytic system powered by a 1.50 V AA battery. Republished with permission from^[105]. SEM: scanning electron microscopy; FE-SEM: field emission scanning electron microscopy; CC: carbon cloth; AA battery: standard 1.5 V alkaline battery; TDOS: total density of states.

Similarly, Ni₂P nanocrystals immobilized on amorphous vanadium phosphate (V-Pi) nanosheets supported on carbon cloth (Ni₂P/V-Pi/CC) were synthesized via oxidation and phosphorization of NiV-LDH/CC (Figure 7d)^[105]. The numerous crystalline/amorphous interfaces and strong interfacial electronic interactions between Ni₂P and V-Pi create abundant active sites, optimizing adsorption-desorption of reaction intermediates. Furthermore, the amorphous V-Pi phase provides excellent resistance to both acidic and alkaline environments, thereby improving long-term durability. As a result, the Ni₂P/V-Pi/CC exhibited outstanding bifunctional activity, requiring overpotentials of 80.8 mV (HER) and 263 mV (OER) at 10 mA cm^-2 in 1 M KOH. A water electrolyzer assembled with Ni₂P/V-Pi/CC electrodes achieved 10 mA cm^-2 at a low cell voltage of 1.44 V and demonstrated exceptional durability (Figure 7e). Notably, a 1.5 V AA battery could power the Ni₂P/V-Pi/CC||Ni₂P/V-Pi/CC device (Figure 7f).

4.7 TMP/ metal nitride or carbide heterostructure

Metal nitrides and carbides are interstitial compounds in which nitrogen or carbon atoms occupy interstitial positions within metallic lattices^[106-108]. These materials exhibit significant potential for electrocatalytic water splitting due to their unique physicochemical properties, including noble metal-like electronic structures, high electrical conductivity, and exceptional mechanical strength^[109-112]. Constructing heterostructures between TMPs and metal nitrides/carbides offers a promising approach to synergistically optimize hydrogen adsorption and water dissociation kinetics^[113-115].

A Co₄N/Co₂P Mott–Schottky heterojunction electrocatalyst was synthesized through a hydrothermal method followed by annealing^[116]. The metallic Co₄N possesses a work function of 5.35 eV, which exceeds that of semiconducting Co₂P (4.8 eV). When combined to form a heterointerface, electrons transfer from Co₂P to Co₄N until their Fermi levels equilibrate. This process establishes a BIEF, which facilitates electron transfer, lowers the energy barrier, and accelerates reaction kinetics. As a result, the Co₄N/Co₂P heterostructure exhibits pH-universal HER activity, achieving overpotentials of 53, 32, and 19 mV at 10 mA cm^-2 in alkaline, acidic, and neutral electrolytes, respectively.

Another example involves an Fe₂P/Co₂N hybrid catalyst, in which Fe₂P nanoparticles are in situ integrated onto a Co₂N scaffold^[117]. Theoretical calculations revealed that the strong interfacial interaction between Fe₂P and Co₂N optimizes the H* binding energy at Fe sites (Figure 8a). Experimentally, the heterostructure demonstrated bifunctional activity with overpotentials of 283 mV (OER) and 131 mV (HER) at 500 mA cm^-2 in 1 M KOH, respectively. Moreover, the superior overall water splitting activity was observed with voltages of 1.561 V and 1.663 V to attain 100 and 500 mA cm^-2 and good durability of 120 h in 1 M KOH, respectively (Figure 8b and Figure 8c).

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Figure 8. (a) Calculated HER Gibbs free energy landscapes at Fe₂P/Co₂N and CoP/Co₂N interfaces with optimized interfacial H* structures; (b) The overall water electrolysis polarization curves of the Fe₂P//Co₂N^(+/-) and IrO₂⁽⁺⁾//Pt^(-) systems in 1 M KOH; (c) Long-term durability at 100/500 mA cm^-2 over 120-hour continuous operation. Republished with permission from^[117]; (d) TEM images of CoP/Ti₃C₂T_x; (e) HER free energy diagrams of Ti₃C₂T_x, CoP, and CoP/Ti₃C₂T_x; (f) Calculated d-band center of CoP/Ti₃C₂T_x. Republished with permission from^[118]. Co: cyan; Fe: purple; N: blue; P: pink; H: white; HER: hydrogen evolution reaction; TEM: transmission electron microscopy.

To further enhance performance, multilevel interfaces and multifunctional interfacial sites have been engineered. For instance, a Ni₃N/Co₃N/CoP heterostructure was fabricated via a thermal nitridation–phosphorization route^[119]. In this configuration, Ni₃N/CoP acts as a water dissociation promoter, generating adsorbed H*, while Co₃N/CoP serves as a hydrogen acceptor, optimizing H* adsorption strength. This dual functionality improves HER performance across alkaline, acidic, neutral, and near-neutral seawater electrolytes. Specifically, overpotentials of 140 and 290 mV were recorded at 500 mA cm^-2 in 0.5 M H₂SO₄ and 1 M PBS, respectively, while a neutral seawater electrolyte yielded an overpotential of 331 mV at the same current density.

Interfaces between TMPs and metal carbides also introduce abundant accessible active sites and promote efficient charge redistribution^[120-122]. For example, the tungsten carbide (W₂C) and tungsten phosphide (WP) embedded in N-decorated carbon (W₂C/WP@NC) were synthesized via a solid-state reaction and subsequent calcination^[123]. Strong coupling between W₂C and WP effectively suppresses the coarsening of WP nanocrystals due to the robust interaction between the dual catalytic centers. Additionally, the N-doped carbon framework facilitates charge transfer and protects the catalyst from corrosion. The W₂C/WPinterface enables efficient charge redistribution and provides abundant active sites. As a result, the catalyst achieves overpotentials of 116.4 mV and 196.2 mV at 10 mA cm^-2 in 1 M KOH and 0.5 M H₂SO₄, respectively, along with stable performance for 12 h.

Notably, the emerging family of 2D layered MXenes, composed of transition metal nitrides, carbides, and carbonitrides, has attracted significant attention owing to their unique features, including large surface area, high electrical conductivity, and strong interfacial interactions with composite materials^[124-127]. MXenes can integrate with TMPs to provide additional active sites and mitigate the agglomeration of transition metal nanoparticles, owing to their surface functional groups (-O, -OH and -F)^[128].

As illustrated in Figure 8d, a CoP/Ti₃C₂T_x MXene heterostructure was synthesized via a coprecipitation-phosphidation method^[118]. This catalyst exhibited excellent bifunctional activity, requiring overpotentials of 103 mV for HER and 312 mV for OER at 10 mA cm^-2 in 1 M KOH, with stable performance maintained for 24 h. Theoretical studies revealed that the CoP/Ti₃C₂T_x interface enhances water adsorption on CoP, modulates the d-band center toward the Fermi level, and increases the total density of states, thereby improving conductivity and HER activity (Figure 8e and Figure 8f).

Due to the negatively charged MXene surface, electron transfer occurs from MXene to TMP at the interface, resulting in the formation of a BIEF. This BIEF modulates the electronic structure and optimizes the density of active sites, thereby improving interfacial charge transfer, accelerating electron migration, and enhancing catalytic performance^[40,129,130]. Furthermore, the interfacial electronic structure and d-band center of TMP/MXene heterostructures can be further tuned through heteroatom doping and vacancy engineering in TMPs^[131,132].

5. Conclusions

TMP-based heterostructures have emerged as highly promising electrocatalysts for water splitting, offering a cost-effective and efficient alternative to noble metal catalysts. This review provides a comprehensive overview of recent advancements in TMP-based heterostructures and their applications in electrocatalytic water splitting.

The Mott–Schottky effect, resulting from differences in work function between TMPs and metals or metallic compounds, drives spontaneous interfacial electron transfer. This process creates a charge redistribution and establishes a BIEF, which optimizes intermediate binding energies, lowers energy barriers, and enhances catalytic performance. The support effect highlights the critical role of conductive carbon-based materials (e.g., graphene, carbon nanotubes) in improving electronic conductivity, preventing corrosion, and tuning the electronic structure of TMPs. Meanwhile, the synergistic effect underscores interfacial coupling between TMPs and secondary components (e.g., metal oxides, chalcogenides), which enhances active site exposure, adjusts d-band centers, and facilitates reaction pathways.

Despite these significant advances, several challenges remain to be addressed before TMP-based heterostructures can achieve large-scale practical applications:

(1) Interfacial engineering as a design principle. Interfacial modulation including charge redistribution, electronic coupling, and dynamic surface reconstruction is the cornerstone for unlocking synergistic effects that integrate structural stability with catalytic activity. Precise control over interfacial structure and composition is essential for maximizing performance. However, the atomic-scale structure of interfaces and their quantitative relationship to catalytic activity remain unclear. Since multiple effects may coexist in TMP based heterostructures, isolating their individual contributions is highly challenging. Advanced in situ and operando characterization techniques (e.g., Raman spectroscopy, X-ray absorption spectroscopy(XAS)) are indispensable for decoupling these effects and establishing reliable structure-activity correlations.

(2) Long-term stability under harsh conditions. TMP based heterostructures, particularly those containing dual-metal active sites, often suffer from corrosion or phase transformations during extended operation, compromising durability. Strategies such as surface functionalization, encapsulation, and protective coatings are urgently needed to enhance stability under practical electrolysis conditions.

(3) Scalable and cost-effective synthesis. Industrial deployment demands synthesis routes that are simple, reproducible, and economically viable. However, many current methods involve complex, multi-step processes with limited scalability and low yield. Developing high-throughput, scalable synthesis strategies remains a priority for practical applications.

(4) Mechanistic understanding and computational guidance. A deeper mechanistic understanding is critical for rational catalyst design. Combining in situ experimental techniques with computational modeling will enable accurate predictions of catalytic activity and selectivity based on electronic structure and interfacial properties, thereby guiding the development of next-generation electrocatalysts.

In summary, continued research into TMP based heterostructure electrocatalysts is expected to deliver breakthroughs in electrocatalytic water splitting and other gas involving reactions, ultimately accelerating the adoption of sustainable, clean energy technologies.

Authors contribution

Zhao H: Data analysis, investigation, resources, validation, writing-original draft, writing-review & editing.

Yuan ZY: Conceptualization, project administration, supervision, validation, writing-review & editing.

All authors have given approval to the final version of the manuscript.

Conflicts of interest

Zhong-Yong Yuan is the Editor-in-Chief of Smart Materials and Devices. The other author declares no conflict of interest.

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Funding information

This work was supported by Shandong Provincial Natural Science Foundation (Grant No. ZR2023MB090).

Copyright

© The Author(s) 2025.