The interaction between metal particles and their supports has been recognized for nearly a century^[14]. With the development of surface science and advanced characterization techniques， the MSIs concept has been further refined into several subtypes， including CMSIs， OMSIs， EMSIs， and SMSIs interactions^[15]. These distinct types arise from different mechanisms such as electron redistribution， orbital hybridization， and structural reconstruction at the MSIs， all of which play crucial roles in determining catalytic activity and durability^[1]. Since the strength and characteristics of these interactions are closely related to the physical and chemical properties of the catalyst system， the following section discusses the key external factors that govern MSIs formation and modulation in supported catalysts^[16].

Most of MSIs hold extensive practical application value in designed catalytic systems， with their performance largely determined by MSIs^[17]. In electrocatalytic research， the type and strength of MSIs are significantly influenced by multiple factors， including the morphological structure of the support， metal loading， and reduction temperature^[18]. For instance， reduction temperature serves as a critical parameter inducing SMSIs phenomena， with elevated temperatures typically inducing subshell migration within the support， thereby forming encapsulating structures around metallic nanoparticles^[18]. As summarized in Table 1 below， numerous application cases have been documented for MSIs systems exhibiting diverse characteristics， which will be discussed in detail in subsequent sections.

Synchrotron radiation techniques， particularly XAFS spectroscopy， have emerged as powerful probes for resolving the local electronic structure and coordination environment within metal-ligand interactions^[51]. In studies of the CuSAs-N/TiO₂ system （Fig.4d）， the Ti K-edge XANES spectrum reveals that the absorption edge position of Cu（24）-N（1.0）/TiO₂ lies between that of TiN and TiO₂， indicating that nitrogen doping increases the electron density of titanium and slightly reduces its oxidation state. Its EXAFS spectrum further confirms alterations in Ti-O/Ti-N bond lengths， revealing successful N incorporation into the TiO₂ lattice to form Ti-N bonds， fundamentally altering the electronic properties of support. Concurrently， the Cu K-edge XANES （Fig.4e） indicates that the valence state of Cu single atoms lies between +1 and +2. The Cu-O coordination peak observed at ~1.53 Å in the EXAFS spectrum suggests that Cu atoms are atomically dispersed and bonded to support oxygen sites， forming a localized Cu-O₃ configuration. Collectively， these results demonstrate that N doping not only modulates the intrinsic electronic structure of supports but also enhances the electronic interaction between the Cu single atom and the N-TiO₂ support by strengthening the coupling between the p orbitals （O 2p， N 2p） and the metallic d orbitals （Ti 3d， Cu 3d） of support. This constitutes the core manifestation of MSIs.

In the Ru-Cu metal/metal catalyst system （Fig.4f~j）， XAFS further reveals the fine-tuned regulation of MSIs by hydroxyl coverage. Cu K-edge XANES （Fig.4g） shows the Cu absorption edge progressively shifting towards higher energies with increasing HO^- coverage， reflecting elevated oxidation states and indicating electron transfer from Cu to Ru regions across the interface. Meanwhile， the Ru K-edge XANES （Fig. 4f） exhibits an absorption edge position intermediate between Ru foil and RuO₂， further confirming HO^-’s role as an electron ‘magnet’ attracting electrons across the Ru-Cu interface and enhancing interfacial charge flow. Moreover， the Ru-O coordination peak intensity in the Ru K-edge EXAFS （Fig. 4h） intensifies with increasing HO^- coverage， while the Ru-Ru metallic bond signal broadens and weakens. This indicates that HO^- modification not only alters Ru’s local coordination environment but also induces a slight lattice stretching， thereby strengthening the electron coupling and structural correlation between the metal and the support. Collectively， these spectroscopic evidences reveal that modulating the coverage of surface modifiers （such as HO^-） can effectively enhance the inherently weak metal-metal MSIs， thereby achieving charge redistribution and optimized catalytic performance.

In summary， XAFS serves as a pivotal technique for characterizing MSIs. It not only precisely detects the valence state， coordination structure， and electronic rearrangement induced by support modification at the metal center but also unveils the microscopic mechanisms of interfacial charge transfer and orbital coupling^[54]. This provides indispensable experimental evidence for understanding and designing high-performance MSIs catalysts. Looking ahead， advancements in in situ XAS hold promise for real-time tracking of dynamic evolution under operational conditions. This will enable a more comprehensive elucidation of its true role and regulatory patterns within catalytic reactions.

In elucidating the fine structure of MSIs， particularly within metal/metal composite systems， aberration-corrected high-angle annular dark-field scanning transmission electron microscopy （AC-HAADF-STEM） has played a pivotal role. As demonstrated in Figure 5a， high-resolution imaging of the Ru-Cu catalyst enabled direct observation of the distinct heterojunction interface formed between Ru species and the Cu support. The selected area diffraction patterns in Fig. 5a and the high-resolution images in Fig.5a1 and 5a2 distinctly resolve the lattice fringes of Cu（111） and Ru（101）， visually illustrating the attachment state of Ru nanoclusters or nanocrystals onto the Cu support. These AC-HAADF-STEM images provide the most direct structural evidence for the intimate interfacial contact between Ru and Cu. This interface constitutes the physical foundation for charge exchange and SMSIs， robustly confirming the successful implementation of the MSIs effect within this metal/metal catalyst.

Furthermore， in studies of Pt-doped MoO₃， AC-HAADF-STEM imaging reveals the precise positioning of Pt single atoms within the MoO₃ lattice. Fig. 5g demonstrates that at low doping levels， isolated Pt atoms （marked by blue circles） uniformly occupy Mo lattice sites without significant lattice distortion， indicating Pt is stably anchored within the substrate via Pt-O coordination bonds. Conversely， at elevated Pt doping levels， Pt atoms aggregate into nanoclusters （Fig. 5h）. This contrast provides compelling evidence that moderate MSIs is crucial for achieving stable single-atom dispersion， excessive metal loading renders MSIs insufficient to overcome surface energies， thereby inducing aggregation. Consequently， aberration-corrected electron microscopy not only confirms the presence of metal in a single-atom state but also reveals MSIs intensity and its decisive role in determining catalyst structural stability by contrasting structural differences under varying preparation conditions.

Theoretical calculations provide an essential understanding of the MSIs and their role in governing catalytic activity during water electrolysis. Density functional theory （DFT） simulations reveal how the coordination environment and interfacial coupling between metal centers and supports influence adsorption energetics， reaction barriers， and structural stability under operating conditions^[56].

Theoretical calculations are systematically employed to elucidate the origin and regulation mechanisms of MSIs within metal-metal （M₁-M₂） heterostructures. Fig.6a~d clearly demonstrates， through a comparative analysis of bonding characteristics between metal-oxide and metal-metal support systems， that the relatively longer metal-metal bond distances inherently result in slower and insufficient charge transfer at the interface， consequently yielding WMSIs. Building upon this insight， the authors employ the Ru-Cu system as a model to further elucidate the physical essence of MSIs through theoretical calculations. As depicted in Fig.6b， when two metals with differing work functions （Φ） contact each other， electrons flow from the metal with the lower work function （Cu） to the metal with the higher work function （Ru）， generating a contact potential （eΔΦ） that establishes an internal electric field at the interface. However， as depicted in Fig.6c， the theoretical contact potential between Ru and Cu is merely 0.06 V， indicating an intrinsically weak MSIs incapable of effectively modulating catalytic performance.

To overcome this limitation， theoretical calculations further revealed the potential for regulating MSIs through hydroxyl （HO^-） modification. Fig.6d schematically illustrates how HO^-， acting as an electron modulator， not only induces interfacial charge migration but also optimizes the adsorption behaviour of reactive intermediates at active Ru sites. Theoretical modelling with varying HO^- coverage demonstrated that introducing HO^- significantly enhances charge accumulation at the Ru-Cu interface， thereby strengthening the MSIs. This theoretical prediction provides clear guidance for subsequent experimental design： controllable enhancement of metal-metal interface MSIs can be achieved by regulating HO^- coverage， thereby optimising the kinetics of non-acidic HER. Consequently， theoretical calculations in this study not only elucidate the electronic origin of MSIs but also offer quantitative control strategies， demonstrating that interfacial charge transfer is the core mechanism for MSIs enhancement.

In this work， Fig. 6e， 6f， and 6g reveal， through systematic theoretical calculations， the composition of the MSIs within MoS₂/NGr-Co SACs and its regulatory mechanism on catalytic performance. The figures present Gibbs free energy diagrams for the HER and OER reaction pathways on the Co@4 N-doped graphene/MoS₂ model， alongside corresponding stability plots. Specifically， Fig.6e indicates an adsorption free energy （ΔG_H） of 0.50 eV for H* during the HER process. Whilst this value is marginally above the ideal threshold， it nonetheless demonstrates appreciable hydrogen evolution activity. More critically， Fig.6f reveals the free energy changes of intermediates during the OER process， where the highest energy barrier is merely 0.46 eV-approaching the ideal value （0.4 eV）. This confirms the configuration exhibits outstanding thermodynamic activity for oxygen evolution. The stability diagram in Fig.6g further confirms the catalyst’s robust structural stability under OER conditions， with its oxidation resistance markedly enhanced as the coordination number of the Co atom increases from 1 N to 4 N.

Theoretical calculations played a pivotal role in this study， not only quantifying the electronic effects of MSIs but also elucidating the origin of its catalytic activity. By constructing Co single-atom models with varying nitrogen coordination environments （1 N， 3 N， 4 N）， calculations revealed that the Co-N₄ coordination structure constitutes the optimal active site. This configuration substantially reduces the energy barriers for key steps in the OER process， such as OH→O and O→OOH. Concurrently， Bader charge analysis revealed pronounced electron loss at Co and Mo atoms， alongside electron enrichment at N and S atoms， at the MoS₂/NGr-Co interface. This directional charge transfer behaviour constitutes the core manifestation of MSIs， optimising the adsorption energy of reaction intermediates and thereby enhancing overall water splitting performance. Thus， theoretical calculations not only confirm Co-N₄ as a key component of MSIs but also elucidate its microscopic mechanism for enhancing catalytic activity through regulating interfacial charge distribution.

Overall， theoretical simulations clarify that the MSIs fundamentally determines the catalytic efficiency and stability of these systems. The interfacial charge redistribution， coordination-dependent adsorption behavior， and band structure alignment together create a favorable environment for both HER and OER. These insights provide a solid theoretical foundation for rationally designing next-generation water-splitting catalysts through precise control of metal-support coupling at the atomic level.

In this work， EPR and XPS characterization were effectively employed to elucidate the specific composition and electronic structural alterations of the MSIs^[47]， thereby underpinning the high catalytic performance of PtRu/WO₃-O_v. EPR testing （Fig.6h） provides crucial supplementary evidence within this characterization framework， directly confirming the presence and increased concentration of oxygen vacancies （O_v） via signal intensity at g=2.003. The strongest EPR signal was observed in PtRu/WO₃-O_v， indicating that microwave treatment and alloying introduced oxygen vacancies. These vacancies not only enhanced WO₃ conductivity but also provided active sites for MSIs， facilitating electron transfer and the adsorption-desorption equilibrium of reaction intermediates. The XPS analysis in Fig. 6i，j clearly demonstrates the electron transfer phenomenon induced by MSIs. The binding energy of the W 4f orbitals in PtRu/WO₃-O_v relative to the WO₃-O_v support （from 35.76/37.86 eV to 36.10/38.20 eV） shows a positive shift， indicating electron flow from the WO₃-O_v support towards the PtRu alloy and reflecting electron donation from the support to the metal core manifestation of MSIs. Concurrently， the negative shift in the Pt 4f orbitals （e.g.， Pt 4f {7/2} decreasing from 71.25 eV） in Figure 6i further confirms electron accumulation at the Pt surface. This electron-rich state optimizes the occupation of the d-band orbitals of Pt， facilitating the desorption of hydrogen intermediates and thereby enhancing catalytic activity. Moreover， the O 1s spectrum in Fig.6j reveals a slight positive shift in the binding energies of lattice oxygen and O_v on PtRu/WO₃-O_v. This indicates that electrons do not preferentially migrate to oxygen sites but instead redistribute via MSIs， reinforcing the synergistic effects between the metal and supports.

Consequently， XPS and EPR collectively establish a comprehensive chain of evidence： XPS reveals MSIs-induced electronic rearrangement and metal-support bonding， while EPR validates the enhancement of support defects （such as O_v）. These defects further amplify the MSIs effect， ultimately enabling the catalyst to exhibit outstanding HER activity across a broad pH range.

Beyond core characterization techniques such as XAFS， XPS， and aberration-corrected electron microscopy， a suite of auxiliary methods-particularly fundamental electrochemical approaches-provides crucial complementary evidence for elucidating the MSIs effect and its influence on catalytic behavior^[59]. For instance， cyclic voltammetry （CV） can be employed to detect characteristic redox peaks of the metal center. When strong MSIs forms， the d-band electronic structure of the metal undergoes alteration， leading to a systematic shift in its redox peak positions^[60]. This directly reflects the electronic effects induced by MSIs. The Tafel slope， meanwhile， provides corroboration from a reaction kinetics perspective： MSIs modifies the rate-determining step by optimizing the adsorption energy of reaction intermediates at active sites， thereby inducing significant changes in the apparent Tafel slope. Furthermore， electrochemical impedance spectroscopy （EIS） quantitatively assesses interfacial charge transfer resistance^[61]. Strong MSIs typically enhances electron transport at the metal-support interface， manifesting as reduced charge transfer resistance. This provides direct electrochemical evidence for MSIs enhancement of charge migration efficiency.

Concurrently， multiple characterization techniques provide robust corroboration for the presence and intensity of MSIs. STEM coupled with elemental mapping visually delineates the spatial distribution and dispersion of metallic species on the support^[62]. Raman spectroscopy， sensitive to local lattice vibrations， effectively detects defects （e.g.， oxygen vacancies） or lattice strain induced by MSIs in the support^[63]. XRD， examining long-range ordered structures， reveals that if metallic species are highly dispersed to the single-atom level， corresponding metallic crystal diffraction peaks will not appear in the XRD pattern^[64]. Conversely， if MSIs induces minor alterations in the support lattice parameters， this may cause shifts in diffraction peaks. Furthermore， infrared spectroscopy employing carbon monoxide as a probe molecule （CO-DRIFTS） serves as a highly sensitive characterization technique. Electron transfer between metal sites and supports alters the electron density of metal， causing a systematic shift in the stretching vibration frequency of adsorbed CO molecules （e.g.， blue shift for increased electron density， red shift for decreased density）. This constitutes a classical method for demonstrating interfacial electronic effects.

Mastery of multiple MSIs characterization techniques must be accompanied by a deep understanding of the physical essence， information dimensions， and inherent limitations of each method. Future research hinges on the organic integration and correlative analysis of these techniques， enabling cross-scale， multidimensional precision characterization of catalysts-from macroscopic morphology and crystal structure to microscopic electronic structure and coordination environment^[65]. Only through this ‘multidimensional perspective’ strategy can a clear ‘structure-performance’ relationship be established， propelling MSIs research beyond phenomenological observation towards a new phase of mechanism-driven regulation and rational design.

The physicochemical properties of the support material play a decisive role in the formation and strength of MSIs. Parameters such as support composition， electronic structure， surface defects， and oxygen vacancies govern how charge redistribution occurs at the metal-support interface^[66]. Defect engineering， particularly through the introduction of oxygen vacancies， has proven to be an effective strategy to enhance interfacial coupling and improve catalytic performance.

For example， Chen et al. constructed Ru cluster-modified NiFe layered double hydroxide （NiFe-LDH） catalysts by introducing oxygen vacancies （Vo^••） into the support lattice. As illustrated in Fig.7a， these defects alter the local coordination environment and induce metastabilization of interfacial Ru sites， thereby strengthening the MSIs effect. The modulation of the work function between Ru clusters and defect-rich NiFe-LDH （Fig.7b） confirms the enhanced charge transfer capability， while the partial density of states （PDOS） analyses （Fig.7d） reveal a clear shift in the d-band centers of both Ru and Ni atoms， indicative of electronic coupling across the interface. Structural and vibrational characterizations， such as FT-IR spectra （Fig.7c）， further demonstrate that the introduction of vacancies modifies the lattice microstructure and interfacial bonding， ultimately improving the catalyst’s electrical conductivity and durability in seawater electrolysis.

This example highlights that tailoring the electronic structure of the support through defect engineering provides an efficient approach to strengthening MSIs and optimizing the interfacial charge dynamics in electrocatalytic systems.

The strength and characteristics of MSIs depend not only on the intrinsic properties of the support but also on the type， valence state， and particle size of the loaded metal. Different metals possess distinct electronic configurations and bonding affinities， which influence charge transfer and orbital coupling at the interface^[67]. The geometric scale of the metal species， ranging from nanoparticles and clusters to single atoms， further determines the extent of electronic overlap and interfacial area， thereby affecting the MSIs strength and catalytic behavior.

Meng et al report Pt single atoms anchored on NiCrO₃ （Pt SACs-NiCrO₃/NF）， where electron transfer from Ni and Cr to Pt optimizes the local electronic environment and facilitates both water dissociation and hydrogen adsorption in the hydrogen evolution reaction （Fig.7g，h）. Although involving different metals and reaction systems， both studies demonstrate that the type and dispersion of the loaded metal strongly influence MSIs formation and catalytic performance. Xiao investigates Cu-loaded nitrogen-doped TiO₂ （CuSAs-N/TiO₂） to reveal how the MSIs varies with metal type and local coordination. Nitrogen incorporation into the TiO₂ lattice introduces localized electronic states and enhances the hybridization between Cu 3d and Ti 3d/O 2p orbitals， promoting interfacial electron delocalization and improving the hydrogen adsorption energetics. Cu species are atomically dispersed on the N-doped TiO₂ surface without aggregation （Fig.8b~d）. The synergistic interaction between Cu atoms and the nitrogen-doped TiO₂ lattice strengthens the MSIs and significantly improves photocatalytic hydrogen evolution activity^[52].

Reaction temperature plays an essential role in determining the formation and strength of MSIs. Thermal treatment affects the crystallization， surface energy， and defect structure of the support， as well as the electronic state and dispersion of the loaded metal^[68]. Increasing the calcination temperature usually enhances interfacial contact and facilitates phase reconstruction， thereby strengthening the MSIs and improving catalytic stability.

Ren prepared a series of Ni/MAO catalysts by calcining Mg-Al layered double hydroxide （MgAl-LDH） precursors at different temperatures to regulate the MSIs strength between Ni and the oxide support. The formation pathway from MgAl-LDH to （Mg， Al）O_x （MAO） supports， and Ni/MAO catalysts shows that higher calcination temperature promotes the transformation of MgAl-LDH into a well-crystallized MgAl₂O₄ spinel structure （Fig.8e）. This structure provides a stable framework for Ni anchoring and enhances the interfacial electronic coupling between Ni and the support. Ni L-edge XAS spectra show that the L₂ and L₃ peaks gradually shift to lower energies as the calcination temperature increases （Fig.8f）， indicating an increase in electron density around Ni and stronger electron transfer between Ni and the oxide support. As a result， Ni/MAO1000 exhibits the highest CO₂ conversion rate during methanation， reaching 1821 mmol CO₂·mol/Ni·min at 400 ℃， which surpasses the catalysts prepared at lower calcination temperatures. In-situ DRIFTS spectra provide additional insights into the reaction pathway. The spectra of CO₂ adsorption identify carbonate （CO₃^2-）， bicarbonate （HCO₃^-）， carboxylate （COOH）， and formate （^*HCOO） intermediates， while the methanation spectra show increasing intensities of ^*HCOO and CH₄ peaks with temperature （Fig.8g，h）. These observations confirm that the formate pathway dominates CO₂ methanation on Ni/MAO1000. The stronger MSIs formed at high temperature facilitates the adsorption and transformation of these intermediates， thereby enhancing catalytic activity and stability.

Overall， temperature adjustment provides an effective strategy to modulate MSIs through phase transitions and interfacial electronic rearrangements. High-temperature treatment improves the crystallization and electronic coupling between the metal and support， stabilizes small Ni particles， and strengthens the adsorption and conversion of intermediates， leading to superior performance in CO₂ methanation.

In noble metal systems， a representative example is the NO₂-Pt-Cl₂ structure， which achieves high intrinsic activity through strong coordination between the Pt centers and functionalized carbon supports. The catalyst preparation involves introducing nitro groups onto nitrogen-doped carbon， followed by the adsorption of Pt precursors to form a stable NO₂-Pt-Cl₂ configuration （Fig.9a）. The absence of Pt-Pt bonds and the clear atomic dispersion of Pt species indicate a strong interaction between Pt and the support， which prevents aggregation under electrochemical conditions （Fig.9b，c）. This interaction optimizes the local electronic environment of Pt， enhancing proton adsorption and facilitating charge transfer during HER. As a result， the catalyst exhibits a low overpotential of 25 mV at 10 mA/cm² and maintains almost unchanged performance after extensive cycling （Fig.9d，e）. The strong MSIs provides both high activity and remarkable structural stability under acidic conditions.

In non-noble metal systems， heterostructure engineering offers another effective way to modulate MSIs. A typical study constructs Co-based catalysts supported on nitrogen-doped graphene coupled with MoS₂ nanosheets， forming an integrated MoS₂-NGr-Co interface （Fig.9f~h）. The electronic coupling between the Co centers， MoS₂， and NGr enables efficient charge redistribution at the interface， which strengthens the adsorption of key intermediates and lowers reaction barriers for both HER and OER. The catalyst displays a low OER overpotential of 310 mV and high HER activity with a small overpotential of 200 mV at 10 mA/cm² （Fig.9i，j）. When used as both electrodes for overall water splitting， it achieves a cell voltage of only 1.49 V to deliver 10 mA/cm²， outperforming many reported non-noble metal systems （Fig.9k，9m）. These results confirm that the optimized MSIs within the heterostructure effectively promotes the dual catalytic reactions and ensures long-term stability.

In this paper， Fig. 9n presents the OER polarisation curves of IrO_x sub-2 nm clusters in 0.5 mol/L H₂SO₄. By comparing the electrochemical performance of IrO_x catalysts supported on different supports （TiO₂， V_O-TiO₂， and V_Ti-TiO₂）， it visually demonstrates the regulatory effect of MSIs on OER activity. Results indicate that IrO_x/V_O-TiO₂ exhibits the lowest overpotential （250 mV） at a current density of 10 mA/cm²， significantly outperforming IrO_x/V_Ti-TiO₂ （273 mV） and IrO_x/TiO₂ （303 mV）. This performance disparity is directly attributable to the adsorption-type MSIs induced by the V_O-TiO₂ support. It’s a relatively weaker interaction that optimizes the electronic structure of IrO_x， shifting the d-band centre downward. This weakens the binding strength with oxygen intermediates， thereby enhancing OER reaction kinetics. Conversely， the stronger embedded MSIs induced by V_Ti-TiO₂ did not yield higher activity. Instead， its excessive electronic regulation restricted the desorption of reaction intermediates， resulting in slightly inferior performance.

Fig.9o further extends the MSIs effect to practical application， presenting the polarisation curves at 80 ℃ for an electrolyzer assembled with an IrO_x/V_O-TiO₂ anode and a commercial Pt/C cathode within a proton exchange membrane （PEM） water electrolysis cell. Results indicate that at 1.6 V， the IrO_x/V_O-TiO₂‖Pt/C system achieves a current density of 240 mA/cm²， double that of the commercial IrO₂‖Pt/C system. This significant performance enhancement not only validates the superiority of absorptive MSIs at the laboratory scale but also demonstrates its feasibility and efficiency in practical PEM electrolyzers. A comprehensive analysis of Fig. 9n and 9o reveals that stronger MSIs is not necessarily superior. Instead， moderate electron transfer must be achieved by regulating the type of carrier defects （such as oxygen vacancies）， thereby maximizing catalytic activity while maintaining stability. This discovery provides clear theoretical guidance and experimental evidence for designing highly efficient and stable acidic OER catalysts.

In summary， both systems demonstrate that rational design of MSIs， whether through coordination engineering in noble metal catalysts or electronic coupling in non-noble metal heterostructures， provides a powerful strategy to enhance catalytic performance for water electrolysis. The optimization of MSIs regulates the local electronic structure， stabilizes active centers， and accelerates charge transport， which collectively enable efficient and sustained hydrogen production， providing crucial guidance for the design of high-performance water-splitting catalysts.

This paper systematically reviews the pivotal role of MSIs in electrocatalytic water splitting. It constructs a comprehensive cognitive framework spanning from microscopic mechanisms to macroscopic performance， encompassing theoretical foundations， formation mechanisms， characterization techniques， and regulation strategies， alongside their specific applications in the HER， OER， and overall water splitting. Research indicates that by rationally designing support properties， metal morphology， and preparation conditions， the intensity and pattern of MSIs can be effectively modulated. This optimizes the electronic structure of active sites， enhances interfacial charge transfer， and improves catalyst stability. Particularly， advances in defect engineering， interfacial structure design， and dynamic regulation offer novel approaches to intensifying MSIs effects.

Despite significant advances in MSIs research and its immense potential for enhancing water-splitting catalysts， numerous challenges persist. Current research has demonstrated the ability to preliminarily modulate the intensity and patterns of MSIs through support defect engineering （such as regulating oxygen vacancies and titanium vacancies）. Future work should integrate atomic-scale synthesis techniques （e.g.， confined growth， atomic layer deposition） with high-accuracy theoretical calculations （e.g.， first-principles-based machine learning potential function， microkinetic simulations） to achieve quantitative design and precise construction of the local coordination environment， electron transfer number， and bonding strength at MSIs interface active sites. Establishing quantitative structure-property relationships between MSIs intensity/type （e.g.， adsorption-type and insertion-type） and key catalytic descriptors （e.g.， d-band centers， reaction intermediate adsorption energies， rate-limiting step energy barriers） will transcend current qualitative correlations， enabling rational catalyst design tailored to specific reactions. Existing characterization methods （e.g.， XPS， HRTEM， EPR） are predominantly performed pre-reaction or under static conditions， making it challenging to capture the dynamic evolution of MSIs under actual electrochemical conditions （applied potential， reaction intermediate adsorption， local pH/electric field variations）. There is an urgent need to develop and integrate multiple in situ/operational characterization techniques， such as： electrochemical in situ X-ray absorption spectroscopy to track real-time changes in the valence state and coordination structure of metal active sites， electrochemical scanning tunneling microscopy to observe potential-driven interfacial structural rearrangement at the atomic scale， and online electrochemical mass spectrometry to correlate MSIs states with the generation/consumption kinetics of key reaction intermediates. This will reveal the stability and reconstruction mechanisms of MSIs within actual catalytic cycles， providing a basis for designing robust interfaces resistant to dynamic deactivation. Existing research has primarily focused on the role of strong interactions （SMSIs， CMSIs） in enhancing activity and stability， while the potential regulatory function of WMSIs in multi-electron， multi-step reactions （such as OER） remains underappreciated. Future work should systematically investigate the subtle effects of WMSIs on reaction intermediate adsorption energies， interfacial proton-electron transfer rates， and catalyst surface reconstruction kinetics within specific model catalytic systems， integrating surface-sensitive in situ spectroscopy with theoretical calculations. Particularly in non-precious metal or low-loading precious metal catalysts， WMSIs may provide essential electronic modulation and stabilization without inducing excessive structural encapsulation， warranting reevaluation of its value.

Advancing MSIs catalysts from laboratory to practical electrolysers requires bridging the substantial gap between academic stability demonstrations （typically tens to hundreds of hours） and industrial requirements （thousands of hours of continuous operation）. A critical yet underexplored challenge lies in catalyst durability under intermittent power loads， which are inherent to renewable energy coupling （e.g.， solar and wind）. These fluctuating conditions exacerbate key failure modes such as metal leaching and Ostwald ripening， leading to rapid performance degradation. MSIs offer a promising pathway to mitigate these issues by strengthening metal-support bonding， anchoring active sites against dissolution， and suppressing atomic migration under potential cycling. Future MSIs’ research should therefore prioritize the design of interfaces that maintain structural integrity and electronic coupling under dynamic on-off cycling and variable load conditions. This includes systematic investigation of how MSIs influence leaching thermodynamics and ripening kinetics， as well as the development of accelerated stress tests that mimic real-world renewable profiles to validate long-term durability.

In summary， MSIs， as a cross-scale core regulatory strategy， will continue to drive the design and development of highly efficient and stable water-splitting catalysts. Through the deep integration of multidisciplinary approaches and precise cross-scale characterization， future MSIs research is poised to transition from phenomenological description to mechanism-driven rational design， providing a robust scientific foundation for the large-scale application of green hydrogen energy.