Rational design of local microenvironment for electrocatalytic water splitting

Abstract

Electrocatalytic water splitting reaction kinetics are usually determined by the intrinsic activity of electrocatalysts and the microenvironment at the electrode–electrolyte interface. Compared to electronic structure regulation based on intrinsic activity, manipulating the local microenvironment is more effective at accelerating internal reactions and transfer processes, so herein we concentrate on the latest progress in local microenvironment design for electrocatalytic water splitting. Methods for the development and characterization of the electric double layer structure model closely related to the microenvironment are first introduced. Next, the influence of electrode surface wettability, local pH, interfacial water structure, and electrolyte composition on the composition of interface reactants, key intermediate adsorption, and reaction kinetics are discussed in detail. Local microenvironment design strategies based on bubble engineering, local electric fields, surface modification, interfacial water orientation/hydrogen bonding networks, and electrolyte anion and cation distributions are subsequently proposed. Finally, we outline the existing challenges and provide possible solutions to drive the future development of this emerging field.

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1. Introduction

The intensifying energy crisis and environmental pollution are driving the green and low-carbon transformation of energy systems.¹ As an important carrier for replacing traditional fossil fuels in the future, hydrogen energy with a high energy density is widely employed in ammonia synthesis, refining, metallurgy and the fuel cell industry.^2,3 Traditional methane reforming processes and the industrial by-product hydrogen have intensive energy demands and result in severe carbon dioxide emissions, which do not meet environmental protection concepts and sustainable development requirements.^4,5 Renewable energy-driven electrocatalytic water splitting has become an ideal solution for green hydrogen production due to its zero-carbon footprint, high efficiency and flexible controllability.⁶ In order to improve the energy conversion efficiency of this process, a large amount of work has focused on gradually or gently adjusting the electronic structure of electrocatalysts and the adsorption of key intermediates such as heterostructure construction, defect engineering, heteroatom doping and lattice strain.^7–12 However, these strategies cannot escape the pH-dependent kinetics of the hydrogen evolution reaction (HER). It is worth noting that the water splitting reaction occurs in the nanoscale confined space at the gas–liquid–solid three-phase interface, and the catalytic activity is not only limited by the composition and structure of the catalyst, but is also closely related to the microenvironment of the electrode–electrolyte interface.^13,14 The reactants, intermediates, and products in the electrocatalytic process diffuse to the catalyst surface or the bulk electrolyte through the interface microenvironment.¹⁵ Adjusting the reaction environment near electrochemically active sites is an effective way to accelerate the kinetics and thermodynamics of catalytic reactions.^16,17

The interface microenvironment is usually directly determined by the electrolyte properties and catalyst structure. Adjusting the surface wettability of electrodes, local pH, interfacial water structure, and electrolyte composition such as cations/anions are effective ways to achieve favorable reaction conditions.^18–22 The main insights into these strategies include: (1) The design of oriented micro-/nanostructures based on bubble wettability engineering and the introduction of field effects can increase the accessibility of reactive substrates at active sites and accelerate bubble convection transport, thereby optimizing three-phase interactions and mass transfer processes.^23,24 (2) The regulation of local pH can affect the proton-coupled electron transfer (PCET) rate, proton donor/acceptor composition, and adsorption intermediate binding strength, thereby determining the concentration of local reactants, which provides ideas for overcoming slow kinetics in non-acidic media.^25,26 (3) Adjusting the arrangement of interfacial water and hydrogen bonding interactions can effectively promote the activity of water splitting. The suspended O–H free water formed at the catalyst/electrolyte interface is proven to have a smaller activation energy for hydrolysis, which is beneficial for the dissociation of water molecules and provides more active intermediates.^27,28 A hydrogen bonding network of interfacial water molecules can mediate the diffusion of protons and hydroxides. Constructing intermolecular hydrogen bonds by modifying organic groups with specific adsorption properties can provide channels for the transport of active intermediates.^29,30 (4) Adjusting the ion distribution in the electrolyte will alter the interfacial water structure, directly affecting the interfacial reaction process. For instance, the orientation of interfacial water is influenced by the competitive arrangement of cation hydration, and larger solvated ions have been shown to disrupt the connectivity of hydrogen bonding networks.^27,31 In addition, electrolyte ions can also regulate the adsorption behavior of key intermediates and the structural composition of catalysts. Although relevant summaries of local microenvironments have been obtained, these works have usually only focused on the relationship between certain microenvironmental factors and electrocatalytic performance under specific reaction conditions or in the field of overall electrocatalysis. For example, Wang et al. emphasized the role of local environment in a mechanistic study of alkaline hydrogen evolution.²² Chen et al. briefly reviewed optimization strategies for interface reaction conditions in different electrocatalytic reactions.¹⁷ A comprehensive review has not yet been conducted for microenvironment regulation in electrocatalytic water splitting systems to promote its development.

In this review, we summarize the latest progress in local microenvironment regulation for electrocatalytic water splitting. Firstly, the establishment and development of the theory model for the double layer, as well as related technologies for characterizing the composition of the microenvironment are introduced to reveal the specific interactions and dynamic response patterns of electrolyte components at the interface. Subsequently, we focus on discussing the impact of environmental factors on the electrocatalytic performance and kinetics, and corresponding control strategies are proposed. Finally, some challenges and future development prospects in this field are proposed, aiming to provide guidance for the design of efficient catalysts and the scalable application of microenvironments.

2. Electric double layer (EDL)

2.1. The development of the EDL structure

The electric double layer of an electrode–electrolyte interface is an important concept in modern electrochemistry and surface science, playing a significant role in energy storage and conversion systems such as batteries and supercapacitors.³² Usually, the electron density, ion species, and solvent molecules in the EDL region are highly uneven in spatial distribution.³³ Due to the influence of the material's structure and electrolyte properties (such as pH value, ion type, and electrode potential), local polarization with spatiotemporal fluctuations will occur, mainly reflected in the distribution of electron density, ion arrangement, and solvent molecule orientation, leading to differences in reaction kinetics.³⁴ Understanding the double layer structure and specific interactions between electrolyte components at the interface provides assistance in constructing an adaptive microenvironment.

In 1853, Helmholtz first proposed the compact double layer model, which consisted of a polarized electrode surface and a tightly adhered counterion layer on the surface.³⁵ It can be defined as a simple double plate capacitor: the accumulation of counterions on the electrode surface is accompanied by the consumption of co-ions, and the electromotive force formed is linearly related to the thickness of the double layer (Fig. 1a).³⁶ The Helmholtz model laid the foundation for our early understanding of electrochemical interfaces. However, the model only considers the electrostatic repulsion experienced by counterions, without considering the mixed diffusion of ions in solution caused by thermal motion and the interaction between the solvent and electrode, and the differential capacitance predicted by the model depends on the dielectric constant of the solvent electrolyte and the thickness of the double layer, independent of charge density, which contradicts the minimum value of differential capacitance observed in dilute solutions.¹⁷ Based on the original model and the Poison–Boltzmann (PB) equation, Gouy and Chapman considered the thermal motion of ions themselves in the dielectric continuum and introduced the concept of the diffusion layer.^37,38 The resulting double layer model consists of charged surfaces in contact with the liquid region, which has an enhanced counterion concentration.³⁹ The distribution of excess ion charges is defined as a function of the distance from the electrode surface, and the potential decreases exponentially until the bulk solution is formed (Fig. 1b). This model theoretically derives the relationship between concentration and differential capacitance, but ignores the molecular properties of ions and solvents, as well as the presence of internal Helmholtz layers, resulting in significant discrepancies between the calculated and experimental results. On the basis of the Helmholtz model and the Gouy–Chapman model, Stern proposed the widely used model to describe the double layer structure, dividing the double layer into an internal compact layer with solvated ion radius limitation and a diffusion layer slightly away from the electrode surface.^40,41 The Gouy–Chapman–Stern model suggests that the electrostatic potential of the system varies linearly in the compact layer and then exponentially in the diffusion layer (Fig. 1c), which is closer to the actual situation. Considering the limited size of ions and the spatial variation of the dielectric constant, Graham, Bockris, Devanathan, and Müller proposed many improved models.⁴² At present, the improved double layer region is divided into an inner Helmholtz plane (IHP), an outer Helmholtz plane (OHP), and a diffusion layer.^43,44 The inner Helmholtz plane is composed of specifically adsorbed ions and molecules, and the adsorbed substances are influenced by electrostatic and chemical interactions.^45,46 The outer Helmholtz layer is considered to be an electrical center plane composed of non-specific ions, in which solvated ions are only involved in long-range electrostatic interactions with the electrode surface.⁴⁷ The substance in the diffusion layer extending from the outer Helmholtz plane to the bulk electrolyte is subjected to thermal motion and electrostatic forces. The successful description of the EDL model for aqueous solvents is mainly attributed to the fact that electrolytes in water can disrupt interfacial ordering.³⁹ With the development of ionic liquids, more and more experiments have shown that the EDL model has poor effectiveness and cannot accurately describe the surface potential.⁴⁸ This can be theoretically anticipated: the space charge density of ionic liquids is much higher than that of aqueous electrolytes.⁴⁹ In such a high-concentration environment, the repulsive effect between anions and cations will be much more pronounced than in dilute solutions; the premise of the Debye Hückel approximation cannot be satisfied. There is still room for improvement in the EDL model.

Fig. 1 The development of electric double layer models. (a) Helmholtz model, (b) Gouy-Chapman model and (c) Gouy-Chapman-Stern model.

2.2. Identification of the EDL microenvironment

Due to interference from the bulk electrolyte and external electric fields, the dynamic stability of the interface structure and the specific interactions between solvent molecules and reactants or intermediates are very complex, which increases the difficulty of local microenvironment construction.^50,51 Identifying the molecular scale structure of the electrocatalyst–electrolyte interface microenvironment and its dynamic response law to external electric fields are key to achieving a matched microenvironment design.¹⁵ Traditional interface experiments can only provide ex situ information and cannot accurately analyze electrode reaction processes.^52,53 Fortunately, theoretical calculations such as ab initio molecular dynamics (AIMD) simulations and in situ imaging/spectroscopy techniques enable the dynamic monitoring of local structure/properties, which will provide effective assistance for the visualization of the interface microenvironment.^54,55

2.2.1 AIMD simulations. With the development of computer technology and first principles algorithms, ab initio molecular dynamics simulations based on time evolution can achieve quantum chemical analysis of complex electrocatalytic interfaces by tracking the movement trajectory and energy changes of molecules and have been widely applied in the in-depth study of interface EDL properties such as the potential of zero charge (PZC), surface charge and differential capacitance,^56–58 in which surface charge density directly determines the interface electric field intensity and interfacial water structure, while PZC can eliminate the polarization effect of the surface and nearby molecules caused by electrode–electrolyte contact, which is an important parameter for quantifying surface charge density.^59,60 When the metal electrode is located at the PZC, there is no net charge at the interface, and water orientation at the interface has almost no contribution. When deviating from U_PZC, the interface begins to charge and forms an electric double layer.^61,62 Xu et al. estimated the U_PZC value at the Pt–water interface by constructing the H-down bilayer model, with integrated implicit and explicit solvation, which was in good agreement with the quantitative derivation value generated by electric field-induced second-harmonic generation, indicating the accuracy and feasibility of AIMD calculations (Fig. 2a).⁵⁹ AIMD simulations can also analyze the influence of complex experimental conditions such as adsorption configuration on electrode surfaces and liquid electrolyte composition, which cannot be achieved by classical surface science and technology.^63,64 Cai et al. employed AIMD simulations to reveal the effect of atomic localized electric fields on the reorientation of interface water, including the adsorption configuration evolution and changes in M–H distance (Fig. 2b).⁶⁵ Combining systematic experiments and AIMD analysis, Shah et al. revealed the indirect effect of cations on Pt during the alkaline HER: smaller cations lead to higher OH_ad coverage on the Pt surface in the HER potential window, thereby promoting water dissociation in alkaline media and Volmer step kinetics (Fig. 2c and d).⁶⁶ The calculated vibrational spectra of interface adsorbates obtained based on dynamic AIMD simulations can also verify in situ spectroscopic analysis. Li et al. reported the calculated spectra of water molecules on the Pt surface by AIMD-simulated EDLs, which showed consistency with in situ surface-enhanced infrared adsorption spectra, clearly confirming the presence of hydrogen bond network gaps at the alkaline interface (Fig. 2e).⁶⁷

Fig. 2 (a) U_PZC estimation and water configuration arrangement under different surface charge densities on the Pt surface. Reproduced with permission from ref. 59. Copyright 2023 Springer Nature. (b) Interface water orientation simulated by AIMD and statistical distance of H and O distribution in the Z direction. Reproduced with permission from ref. 65. Copyright 2023 Wiley-VCH. (c) Schematic diagram of surface OH_ad promoting the Pt alkaline Volmer step. (d) The optimized geometric shapes of H₂O and Li(H₂O)⁴⁺ in separate and solvated states. Reproduced with permission from ref. 66. Copyright 2022 Springer Nature. (e) The calculated VDOS spectrum of interfacial water molecules at the alkaline interface. Reproduced with permission from ref. 67. Copyright 2022 Springer Nature.

2.2.2 In situ imaging/spectroscopy technologies. At present, significant progress has been made in the static observation of electrocatalytic reactions, but it is impossible to directly measure the surface structure and surrounding environment during the actual reaction process, or to provide spatial heterogeneity information on the potential, reaction intermediates, and product distribution in the electric double layer.^68,69 Space-resolved in situ characterization techniques are needed to visually observe the dynamic evolution of the interface microenvironment of nanoscale catalysts under electrolytic conditions.^52,70 In recent years, some advanced surface-sensitive technologies have been developed for the in situ chemical characterization of charged interfaces at atomic resolution. According to working principles, these technologies can be divided into in situ microscopy and in situ spectroscopy. By using in situ microscopy techniques such as scanning tunneling microscopy (STM), scanning electrochemical microscopy (SECM), and atomic force microscopy (AFM), nanoscale imaging of interface structures and component changes can be obtained.^71–73 For instance, Gao et al. employed SECM to demonstrate non-destructive imaging of hydrogen evolution at the interface of individual Pt nanoparticles and the structure–activity relationship between current density and membrane thickness, revealing the complex dependence of H⁺ concentration, water content, and polymer structure on Nafion thickness (Fig. 3a).⁷⁴ Kosmala et al. monitored the insertion, adsorption and subsequent conversion of H⁺ at the graphene–iron interface using atomic resolution electrochemical STM (Fig. 3b).⁷⁵ Combining AFM and SECM, Nie et al. measured the local distribution of molecules and catalytic product molecules that could transfer electrons, achieving in situ imaging of electron transfer processes and electrocatalytic reaction processes (Fig. 3c).⁷⁶In situ spectroscopy technology can provide useful information on the composition and structural evolution of charged interface adsorbates, which is particularly important for identifying the local microenvironment.^77,78 Wang et al. found the generation of H₃O⁺ intermediates during hydrolysis and dissociation processes through in situ Raman measurements, leading to a unique acidic microenvironment on the catalytic surface of Pt/C nanomaterials; this explains the abnormally high activity of nanomaterials under high pH conditions (Fig. 3d and e).⁷⁹ Wang et al. used in situ surface-enhanced Raman spectroscopy to monitor the configuration and dynamic changes of water molecules at the Pd surface interface during electrocatalytic hydrogen evolution in real-time. Direct spectroscopy confirms that the structure of interface water undergoes a dynamic change from random distribution to an ordered structure under applied bias, which promotes efficient electron transfer at the interface and thus improves the HER performance in 0.1 M NaClO₄ solution (Fig. 3f).⁸⁰ Velasco-Velez et al. employed X-ray absorption spectroscopy to similarly study the interfacial water structure in actual electrochemical working environments and found that negative bias tended to change the orientation of water molecules, directing hydrogen atoms towards the gold surface, thereby increasing the number of dangling hydrogen bonds (Fig. 3g and h).⁸¹ Besides, in situ spectroscopy such as in situ XRD can also monitor the dynamic evolution/reconstruction processes of electrocatalysts, which helps us to understand the impact of crystal structure changes on the local microenvironment.

Fig. 3 (a) SECCM measurement of a single electrocatalytic particle at the anode of a simulated fuel cell. Reproduced with permission from ref. 74. Copyright 2020 American Chemical Society. (b) Potentiodynamic EC-STM images of Gr/Pt(111). Reproduced with permission from ref. 75. Copyright 2021 Springer Nature. (c) SECM image of Au nanoparticles operated in feedback mode. Reproduced with permission from ref. 76. Copyright 2021 American Chemical Society. In situ Raman spectroscopy of Pt/C (d) and bulk Pt (e) during the HER process in 0.1 M KOH. Reproduced with permission from ref. 79. Copyright 2019 Springer Nature. (f) In situ Raman spectra of interfacial water on a Pd(111) electrode. Reproduced with permission from ref. 80. Copyright 2021 Springer Nature. (g) Schematic of the electrochemical cell for XAS measurements. (h) The O K-edge XAS of water collected at the Au electrode under different potentials. Reproduced with permission from ref. 81. Copyright 2014 American Association for the Advancement of Science.

3. Engineering the surface microenvironment of electrocatalysts

3.1. Wettability

The electrocatalytic process usually involves liquid electrolytes and gaseous substances, which bring inherent complexity to the local environment.⁸² As one of the most fundamental surface characteristics of catalysts, wettability directly determines the distribution and diffusion of liquid and gaseous substances around the catalyst, thereby affecting mass transfer and reaction kinetics.⁸³ For water splitting reactions, a hydrophilic microenvironment is conducive to the contact between catalyst active sites and water electrolytes to promote gas evolution;⁸⁴ the adhesion of bubbles will form a shielding layer, increase the ohmic resistance of the electrode–electrolyte interface, and hinder the mass transfer and diffusion of the electrolyte to the surface active sites. Microconvection caused by bubble growth and departure will result in the concentration overpotential, which is also essential for overpotential loss of the entire system.^85,86 Especially for industrial-grade electrolytic water reactions, this phenomenon is even more severe. Adjusting electrode wettability to enhance surface affinity and reduce bubble adhesion can establish and maintain a balance between liquid electrolytes and gaseous products, optimize the interaction between the three phases, and achieve efficient electrocatalysis.^83,87

3.1.1 Fundamental concepts. The concept of superhydrophilicity/superaerophobicity based on micro-/nanostructures has been widely recognized. The superhydrophilic surface has a lower liquid contact angle (LCA < 10°). Superaerophobic surfaces are defined as underwater surfaces with high bubble contact angles (BCA > 150°).⁸⁸ Due to the competition between the water medium and the surface of bubbles attempting to adhere, the contact angle of bubbles on the underwater surface depends on the contact angle of droplets similar to those adhering to the surface in air.⁸⁹ Young's equation, which is used to quantify the wettability of droplets on ideal smooth surfaces, can also be used for the analysis of aerophilicity and aerophobicity, where the surface tension between different phases determines the movement of bubbles in water.⁹⁰ Considering the heterogeneity and rough composition of the real electrode surface, Wenzel and Cassie–Baxter equations established the relationship between surface structure and the apparent contact angle of bubbles on underwater electrodes. On aerophilic rough surfaces, bubbles that are nailed to the solid surface generate significant adhesion and are not easily detached. The apparent CA is directly proportional to the plane intrinsic CA and the surface roughness factor (the ratio of the true surface area to its horizontal projection) (Fig. 4a).⁹¹ The inherent aerophobicity of electrode materials can be improved by adjusting surface roughness. When the intrinsic CA becomes sufficiently large (Fig. 4b), the electrode surface becomes aerophobic, and bubbles cannot fill the grooves on the rough surface, leaving an air gap. In this case, the Cassie–Baxter model provides a relationship between the apparent contact angle and the actual contact area (Fig. 4c). The variation law of electrode surface wettability is consistent with the Wenzel model.⁹² Due to the discontinuous three-phase contact lines (TPCL) provided by rough surfaces, liquid is filled between isolated standing units, minimizing bubble adhesion and facilitating gas separation.⁹³

Fig. 4 (a–c) Different wettability states of a bubble. (d) Bubble nucleation, (e) growth and (f) detachment processes.

3.1.2 Bubble engineering. Generally, bubbles undergo a process of nucleation, growth, and detachment on the electrode surface (Fig. 4d–f).^94,95 The nucleation process occurs after the supersaturation diffusion of dissolved hydrogen and oxygen molecules.^88,96 According to classical nucleation theory, the Gibbs free energy of bubble nucleation is determined by the free energy of both forming a new interface and dissolving gas in the bubble phase.⁹⁷ By changing the composition of the liquid electrolyte, surface tension can be altered, thereby affecting the energy barrier that forms the gas–liquid interface. For example, the presence of surfactants is beneficial for reducing the surface tension at the gas–liquid interface and the degree of supersaturation of dissolved gas required for bubble nucleation.⁹⁸ Park et al. revealed that the solute Marangoni force caused by the concentration gradient of anions in platinum microelectrodes in acidic electrolytes, also known as the force generated by the surface tension gradient, played an important role in the detachment of H₂ bubbles within the studied potential range, providing effective insights into the regulation of bubble dynamics.⁹⁹ In addition, compared with planar electrodes, due to the reduction of the volume free energy formed by bubbles, rough defect surfaces are more prone to nucleation. Designing the surface structural composition of electrocatalysts has also become an effective strategy for controlling the nucleation rate of bubbles. After nucleation, bubbles continue to grow and absorb more dissolved gas molecules under a force higher than the internal Laplace force. They undergo initial inertial growth controlled by momentum interactions with the surrounding fluid, diffusion growth controlled by mass transfer from dissolved gas into the bubbles, and electrochemical growth controlled by the electrochemical reaction rate. The growth rate depends on the rate of hydrogen molecule formation and the coalescence behavior between bubbles.^96,100 Based on the simplified force analysis of a single bubble on the electrode surface, the upward buoyancy and downward adhesion control the separation of bubbles. When the buoyancy and adhesion are balanced, the bubbles reach a threshold driven from the electrode surface.^93,101 According to the Fritz formula and its derived form, the theoretical bubble detachment size is closely related to the sine function of surface tension and bubble contact angle (Fig. 4e).¹⁰² By changing the microstructure of the electrode surface, the adhesion force and peel size of bubbles can be effectively reduced.¹⁰³ The ordered superhydrophobic nanostructured electrode exhibits a good bubble transport ability due to the discontinuous region of interface division. The construction of nanoarrays has been widely applied in the design of efficient electrocatalysts for water electrolysis.¹⁰⁴ Yu et al. constructed a Ni₂P nanoarray on a nickel foam substrate, which showed excellent hydrogen evolution activity and stability in 1.0 M KOH. Due to the unique superhydrophobic property of the interface, it can withstand internal and external forces, and release the in situ H₂ bubbles over time under a high current density (Fig. 5a).¹⁰⁵ Compared to disordered nanoarrays, long-range ordered nanostructures are more effective at bubble management.¹⁰⁶ The ordered array ensures the stable flow of liquids and bubbles in microchannels, and suppresses the harmful generation of typical flow-blocking bubbles in randomly oriented nanoarrays. Kang et al. used microfluidic finite element simulations to systematically analyze the mass transfer and oxygen bubble evolution processes of parallel nanosheet arrays (PNAs) and traditional random nanosheet arrays (NAs). Traditional random NAs have a concave surface structure formed by oxygen bubbles. The generated bubbles block the channels, seriously hindering fluid flow. In contrast, the highly accessible microchannels of PNAs can promote mass transfer even in the presence of oxygen bubbles, leaving overall flow almost unaffected (Fig. 5b).¹⁰⁷ Inspired by the special composition of tissues or body surfaces in nature, biomimetic structures provide rich inspiration for the design of superaerophilic/superaerophobic electrodes by controlling the behavior of liquids/gases that are required for survival through adapting to complex and ever-changing environments.^87,108 Wang et al. successfully prepared the “axis” and “feather” layered fern alloy aerogel (LFA) self-supporting electrode by magnetic field induction design; this had a unique dynamic adaptive exhaust performance and could effectively avoid stress concentration caused by bubble aggregation, greatly reducing the size of released bubbles and promoting rapid gas evolution in 1.0 M KOH (Fig. 5c and d).¹⁰⁹ Xiao et al. developed a biomimetic two-dimensional structure of pitcher plants by etching asymmetric hydrophilic arrays on superhydrophobic surfaces (Fig. 5e). Under water, a stable and continuous gas film can form on the superhydrophobic surface, and the gas film can firmly adhere to bubbles; the etched area strongly adheres to water droplets and can form a series of “water fences” underwater. The patterned surface of this air film/watermark can generate asymmetric resistance, achieving directional and long-distance continuous spreading of bubbles on the biomimetic two-dimensional surface of Nepenthes pitcher (Fig. 5f).¹¹⁰ In addition to wettability control based on micro-/nanostructures, active strategies using external forces such as flowing electrolytes, magnetic forces, and ultrasound can also guide bubble detachment.¹¹¹ Under actual dynamic conditions, the shear force caused by electrolyte flow will detach bubbles from the electrode surface at a very small size (Fig. 5g). Through phase field simulations of smooth surface electrodes and arrayed surface electrodes under static and dynamic conditions, He et al. found that when the flow field acted on the surface electrodes, the critical bubble size significantly decreased, and compared with smooth surface electrodes, the flow on arrayed surface electrodes was less affected by bubbles (Fig. 5h).¹¹² Under the influence of a magnetic field, magnetohydrodynamic convection caused by Lorentz forces can promote electrolyte circulation and accelerate bubble separation.¹¹³ Ben et al. combined superhydrophobic wettable surfaces with magnetic responsive microfibril structures to achieve a new strategy for efficient, non-destructive, and reversible manipulation of bubbles. Under the action of an external magnetic field, flexible cilia will move together with the magnet and drive the bubbles to move within the small concave surface. The driving force for surface manipulation of bubbles comes from reversible changes in surface morphology. In addition to non-destructive transport of discrete bubbles, the transport speed of bubbles can also be regulated by adjusting the magnetic field velocity. This also provides innovative ideas for magnetic field-assisted bubble separation (Fig. 5i).¹¹⁴ The effect of ultrasound causes bubbles to become unstable and detach early through vibration.¹¹⁵ These all provide effective approaches for bubble engineering of water splitting electrodes. Briefly, adjusting electrolyte composition, constructing ordered nanoarrays/biomimetic structures, and applying external fields can achieve a good microenvironment for bubble transport.

Fig. 5 (a) Digital images of H₂ bubbles on blank NF and Ni₂P/NF. Reproduced with permission from ref. 105. Copyright 2019 American Chemical Society. (b) Finite element simulations of mass transfer and bubble evolution in PNAs and random NAs. Reproduced with permission from ref. 107. Copyright 2023 American Chemical Society. (c) Schematic illustration of the bubble transport behavior for O₂ bubbles in the fern-like structure and (d) the synthesis of LFA with a uniform magnetic field. Reproduced with permission from ref. 109. Copyright 2024 Wiley-VCH. (e) A surface composed of periodic duckbill microcavities with arched edges inspired by the microstructure of the periodontal surface of pitcher plants with tails. (f) Schematic illustration of the NATS electrode for continuous H₂ generation. Reproduced with permission from ref. 110. Copyright 2021 American Chemical Society. (g) Force analysis and (h) critical bubble size of bubbles on surface-arranged electrodes under static and dynamic conditions. Reproduced with permission from ref. 112. Copyright 2023 Wiley-VCH. (i) Bubble manipulation with a magnetic field applied on the surface of horizontally superhydrophobic FMMA. Reproduced with permission from ref. 114. Copyright 2022 Wiley-VCH.

3.2. Local pH

According to Koper's HER hemolysis and heterolysis models, the local concentration or composition of the solution components near the catalytic site is an important descriptor that determines the reaction kinetics.¹¹⁶ Due to the influence of mass transfer and electrode potential, there is a significant difference in the local reagent concentration in the reaction plane compared to the bulk electrolyte, which leads to changes in the concentration of proton donors and hydroxides at the interface.¹¹⁷ Regulating the local pH microenvironment is crucial for the distribution of reactive species. Single crystal/polycrystalline materials typically exhibit electrolyte pH-dependent catalytic activity, with even the most advanced precious metal Pt catalysts exhibiting HER rates two to three orders of magnitude lower under alkaline conditions than under acidic conditions.^118,119 This pH effect may be attributed to the variation of proton donors or acceptors with electrolyte pH. For the HER, the proton donor changes from H₃O⁺ in acidic solution to H₂O in alkaline solution, and the PCET energy barrier in the basic step significantly increases. For the oxygen evolution reaction (OER), as the pH increases, the oxidant changes from H₂O to OH⁻, and the oxidation ability is enhanced. Therefore, designing customized local acidic and alkaline environments for the HER and OER is an effective strategy for optimizing reaction kinetics. Lamoureux et al. used DFT activation energy and mean field micromechanics models to calculate the polarization curves of the HER/hydrogen oxidation reaction (HOR) for Pt(111), confirming the transition of proton donors from hydrated hydrogen ions to water with increasing pH (Fig. 6a).¹²⁰ Zhong et al. observed that the pK_a of hydrated hydrogen ions on the Pt surface was closely related to pH-dependent HER kinetics. At lower pH, the protonated surface state favors rapid Tafel kinetics, while at higher pH, the deprotonated surface state follows Volmer step confinement kinetics (Fig. 6b and c).¹²¹ The interaction between the interface electric field generated by surface charges and the adsorbent provides other explanations for the pH effect. The change in pH value can alter the surface charge state of the catalyst, thereby affecting the substrate adsorption capacity and reaction rate. The electric field can also strongly interact with adsorbents with dipole moments and polarizability, leading to changes in their adsorption energy.²⁶ Zhu et al. directly observed H and H₂O on Pt using surface-enhanced infrared spectroscopy, and inferred the relationship between their adsorption strength and pH through changes in their vibrational behavior. They all monotonically weakened with increasing solution pH (Fig. 6d).¹²² The change in pH value can also affect the rate constant of PCET steps. Lewis et al. studied the pH-dependent interface PCET kinetics using a graphite-conjugated diaminobenzoic acid electrode as a model system, the I-PCET equilibrium rate constants at pH values from 0 to 14 were measured by variable scan rate voltammetry, and it was found that the logarithm of the observed rate constants showed a V-shaped dependence on pH. This model calls for the donor/acceptor contributions of hydrated hydrogen ions/water reactions that dominate at low pH and water/hydroxide reactions that dominate at high pH (Fig. 6e).¹²³ Jung et al. found that due to the coverage of intermediates (metal-H), the reaction sequence of H₃O⁺ was fractional throughout the pH range, and proposed that the efficiency loss was caused by the slow proton-coupled electron transfer between H₂O and H₃O⁺ (Fig. 6f).¹²⁴ These studies emphasize the role of pH in electrocatalytic kinetics and stimulate strategies to optimize the reaction microenvironment through local pH regulation.

Fig. 6 (a) Computed HER/HOR polarization curves. Reproduced with permission from ref. 120. Copyright 2019 American Chemical Society. (b) The overpotential of HER vs. log current density. (c) Tafel slopes and electrical transport spectroscopy (ETS) signal from pH = 0 to pH = 7. Reproduced with permission from ref. 121. Copyright 2022 National Academy of Sciences. (d) Plot of the normalized hydrogen binding energy (HBE_measured) as a function of pH. Reproduced with permission from ref. 122. Copyright 2020 American Chemical Society. (e) The k_appversus pH contribution of the acid and base reactions. Reproduced with permission from ref. 123. Copyright 2024 Springer Nature. (f) The pH dependence of HER kinetics on Pt. Reproduced with permission from ref. 124. Copyright 2022 Elsevier.

3.2.1 Local electric fields. The construction of a local electric field based on the electrostatic polarization effect can effectively regulate the distribution of key H and OH⁻ intermediates at catalytic sites, thereby affecting local pH and reaction kinetics.¹⁷ The morphology and structure of catalysts are key factors determining the intensity of the local electric field. It has been determined that high curvature structures such as nanoneedles, nanocones, nanodendrites and nanosheets are conducive to creating an ideal microenvironment for overall catalytic reactions.^104,125 When an electric potential is applied, the charge will accumulate at the high curvature position of the catalyst, leading to an enhancement of the local electric field, thereby inducing mass transfer and achieving local pH control.¹²⁶ Liu et al. used finite element simulations to investigate the effect of the tip curvature radius on the OER reaction process. The simulation results show that the smaller the tip curvature radius, the higher the tip charge density, and the stronger the electric field. Additional research using the Gouy–Chapman–Stern model found that the stronger the electric field, the higher the density of OH⁻ adsorbed on the Helmholtz layer surface of the electrode surface's EDL, and the higher the OER current density (Fig. 7a).¹²⁷ Similarly, Nairan et al. found that for the same Pt–Ni and pure Ni catalysts, samples with surface-pointed cones exhibited better alkaline HER activity than those with smooth surfaces. Based on finite element calculations, it has been confirmed that the concentration of positively charged H protons and potassium ions on the electrode surface significantly increases as the cathode electric field increases in the pointed cone model, and the effect of increasing hydrogen ion concentration is more significant than that of potassium ions (Fig. 7b and c).¹²⁸ In addition to the tip effect, special sites such as corners and edges are also proven to have enhanced catalytic activity. Liu et al. used highly curved carbon carriers to anchor single atoms to simulate metal active sites at corners and edges. The DFT simulation of energy changes during the hydrogen evolution reaction shows that there is a strong local electric field near the Pt site, with the highest intensity at maximum curvature (Fig. 7d). Based on the double layer model, the enrichment of protons on the Pt surface can be observed intuitively (Fig. 7e).¹²⁹ Tan et al. demonstrated the edge effect of the local electric field on nanosheets. The platinum nanoparticles loaded on the edge of the Ni₂P/CoP nanosheets exhibit the strongest local electric field, which facilitates the rapid diffusion of H⁺ and induces a local “pseudo-acidic” environment in alkaline media (Fig. 7f).¹³⁰ The construction of heterojunctions can achieve directed migration and aggregation of charges, thereby promoting the concentration of reactants at the local microenvironment of the reaction interface.¹³¹ For example, Zhang et al. achieved targeted construction of positively and negatively charged surface regions through spontaneous polarization caused by the difference in work function between Co₆(CO₃)₂(OH)₈·H₂O (CoCH) nanowire arrays and Ni₂P nanoparticle heterointerfaces. The negative charge enriched region of Ni₂P is responsible for optimizing the adsorption of H*, while OH⁻ is captured by positively charged CoCH for catalyzing the OER, effectively improving the overall water splitting efficiency (Fig. 7g).¹³² The multiple physical and chemical interactions between substrates, metal active sites, and reaction intermediates provide another pathway for regulating the local reaction environment. Wang et al. improved the local reaction environment of Ru single atoms by precisely switching the crystallinity of the carrier, thereby significantly enhancing the HER activity. The Ru single atom catalyst anchored on low crystalline nickel hydroxide reconstructed the distribution equilibrium of interface ions due to the activation of metal dangling bonds on the support. Benefiting from the orbital coupling effect, single-center Ru with a low oxidation state can induce the aggregation of H₃O⁺, leading to the formation of a local acidic microenvironment in alkaline media (Fig. 7h).¹³³ Tan et al. constructed a local acid-like reaction environment on V_O–MgO loaded with Pt and achieved an efficient HER in alkaline media. MgO rich in oxygen vacancies is beneficial for the dissociation of water. In addition, due to the oxygen vacancies in MgO being occupied by unpaired electrons, when anchoring Pt, electrons transfer to the metal to form electron-rich centers, which can generate electrostatic attraction with positively charged H₃O⁺ species (Fig. 7i).¹³⁴

Fig. 7 (a) Positive charge density and OH⁻ density distribution on different electrode tip surfaces. Reproduced with permission from ref. 127. Copyright 2021 Wiley-VCH. (b) The iso-surface of NWs decorated with NTs. (c) The calculated hydrogen ion concentration at different distances to the electrode surface. Reproduced with permission from ref. 128. Copyright 2021 Royal Society of Chemistry. (d) Local electric field distribution of H₂Pt₁/OLC. (e) Enrichment of high-concentration protons around Pt sites. Reproduced with permission from ref. 129. Copyright 2019 Springer Nature. (f) Schematic illustration of the growth of Pt nanocrystals on the edge of TMP nanosheets. Reproduced with permission from ref. 130. Copyright 2023 Wiley-VCH. (g) Schematic diagram of charge transfer from CoCH to Ni₂P. Reproduced with permission from ref. 132. Copyright 2023 Wiley-VCH. (h) In situ Raman spectra of Ru–LC–Ni(OH)₂ during the HER process. Reproduced with permission from ref. 133. Copyright 2024 Wiley-VCH. (i) Schematic representation of water dissociation, formation of H₃O⁺ intermediates, formation of H₂ and OH⁻ desorption from the Pt/MgO surface. Reproduced with permission from ref. 134. Copyright 2022 Springer Nature.

3.2.2 Surface modification. In addition to regulating the local electric field, surface modification is an effective strategy for regulating the local pH microenvironment. Utilizing organic molecules, amorphous and other layer modifications can provide active sites and intermediate substrate receptor/donor transport channels, thereby promoting mass transfer. Gao et al. demonstrated a surface modification strategy using amino acid molecules to non-covalently connect CoP. The electron-donating functional groups on the side chains of amino acid molecules can regulate the valence state of the Co center, while the proton donor/acceptor functional groups can act as proton accumulators through hydrogen bonding networks, inducing a local proton-rich environment on the electron-rich Co center (Fig. 8a).¹³⁵ Zheng et al. proposed a neutral HER lattice hydrogen-mediated interface hydrogen evolution pathway catalyzed by Ir–H_xWO₃ hybridization. The H_xWO₃ carrier acts as a proton sponge and spontaneously injects protons into WO₃, forming a local acidic microenvironment around the Ir metal site. By utilizing the thermodynamically favorable Volmer Tafel step, the activated lattice hydrogen located at the interface is coupled with the H_ad atom on the surface of the Ir metal, thereby achieving rapid kinetics (Fig. 8b and c).¹³⁶ According to Lewis acid–base theory, the Lewis acid layer can utilize the lone pair electrons of the base to achieve a stable electron layer structure of its own atoms, providing an intrinsic driving force for substrate transport. Guo et al. achieved dynamic splitting of water molecules and capture of hydroxyl anions by introducing Lewis acid layers (such as Cr₂O₃) on transition metal oxide catalysts. The local alkalinity generated in situ is beneficial for the kinetics of the two electrode reactions, avoiding chloride ion attacks and precipitation formation on the electrode (Fig. 8d).¹³⁷ Zhu et al. found significant differences in OER performance in an alkaline environment between Fe–Co/WP and Co–Fe/WP electrocatalysts with heteroatom pairs of opposite configurations. Lewis acidic Fe³⁺ at the distal position in Co–Fe/WP has a significant impact on the redox potential of Co and the deprotonation of coordinated water, leading to more significant OER activity (Fig. 8e and f).¹³⁸ The modification of an amorphous phase with a large number of dangling bonds and unsaturated surface atoms can provide adsorption sites for reactants, which are also beneficial for the enrichment of reaction intermediates.¹³⁹ Wan et al. demonstrated that delocalized protons generated at the amorphous NiO_xH_y shell could easily cascade inward to the interface Pt sites through hydrogen bonding networks, improving the effective proton supply rate on the Pt surface for later Tafel steps (Fig. 8g–i).¹⁴⁰

Fig. 8 (a) Schematic illustration of the electrocatalytic HER over amino acid-modified CoP. Reproduced with permission from ref. 135. Copyright 2023 American Chemical Society. (b) Free energy calculations and (c) proposed reaction mechanism for Ir₁₀–H_xWO₃. Reproduced with permission from ref. 136. Copyright 2023 Springer Nature. (d) Schematic diagram of local alkaline microenvironment generation on a Lewis acid-modified anode. Reproduced with permission from ref. 137. Copyright 2023 Springer Nature. Schematic diagram of the individual effects of Lewis acidic Fe³⁺ at the distal end (e) and active Co⁴⁺ at the distal end (f). Reproduced with permission from ref. 138. Copyright 2022 Wiley-VCH. (g) The regulatory effect of amorphous Ni(OH)₂-layer modification on the local chemical environment of the Pt surface. (h) Schematic of the reaction pathway of Pt@NiO_xH_y. (i) HER activity colour map. Reproduced with permission from ref. 140. Copyright 2023 Springer Nature.

3.3. Interfacial water structure

Water molecules are the main component of the electrode/electrolyte interface of the EDL. In the inner Helmholtz plane, interfacial water molecules dissociate into intermediate H_ads and OH_ads adsorbed on the catalyst, directly participating in the dissociation of water.¹⁴¹ Therefore, the interfacial water structure and dynamic characteristics are key factors determining catalytic performance.¹⁴² Under external bias, the electrostatic interaction between the electrode and solution induces a reorientation of polarized interfacial water. The competition between the electric field dipole interaction and hydrogen bonding of water molecules leads to dynamic rearrangement of the hydrogen bonding network. Moreover, the presence of specific adsorbed cations makes interface interactions more complex, which can greatly disrupt the local structure of hydrogen bonding networks and thus affect proton transport.^143,144 Understanding the interface water orientation, hydrogen bonding network, and structural transformation at the atomic scale is crucial for tuning the solvent environment.

3.3.1 Water arrangement. Based on subtle differences in molecular vibration characteristics, spectroscopic characterization techniques can provide overall information on the composition and dynamic evolution of interfacial water structures.^145–147 By Gaussian fitting the wide Raman spectra of the O–H stretching mode, the configuration of interfacial water at metal interfaces can be divided into 4-coordinated hydrogen bonded water (4-HB·H₂O), 2-coordinated hydrogen bonded water (2-HB·H₂O), and water molecules bound to cations (Fig. 9a).¹⁴⁸ Under external bias, interface water typically exhibits potential-dependent redirection. Wang et al. directly observed that the structure of water molecules transitioned from a parallel configuration to a structure with one H atom facing down, and then to a structure with two H atoms facing down by dynamically changing the value of electric potential (Fig. 9b).¹⁴⁹ Compared with hydrogen bonded water, cation-coordinated water molecules are located closer to the electrode surface and are more likely to transform into ordered H-down structures. This can shorten the distance between the active site and H, enhance the M–H binding energy, and significantly improve the dissociation efficiency of interfacial water molecules.¹⁵⁰ According to the vibrational Stark effect, Wang et al. found that Na⁺ ion hydrated water (Na·H₂O) was more sensitive to local electric fields on the Pd(111) surface in 0.1 M NaClO₄ (Fig. 9c). At all potentials, the vibrational dipole moment (O–H bond direction) of Na·H₂O is more parallel to the electric field direction, indicating that Na·H₂O is more likely to convert into a double H-down structure. The AIMD simulation results further indicate the shorter Pd–H distance of Na·H₂O than that of non-Na·H₂O, which strengthens the Pd–H bond interaction (Fig. 9d).⁸⁰ It is precisely the synergistic effect of electrode potential and local hydrated cations that disordered bulk H₂O can effectively arrange into ordered interfacial H₂O. The recombination behavior of interfacial water is considered an important reason for the differences in acid–base HER kinetics.¹⁵¹ In alkaline media, there is a stronger interaction between dipoles with negatively charged surfaces and O–H bonds, resulting in a decrease in the number of adsorbed water molecules entering the inner Helmholtz layer. The formation of solvated species between H-down interface water and adsorbed hydrogen is not conducive to hydrogen desorption. In acidic media, H_ads are surrounded by more water in the “H-up” configuration, forming a smooth proton transfer network (Fig. 9e).¹⁴⁴ Adjusting the orientation of the first layer interface of H₂O to promote its dissociation is a promising method for improving the HER activity.

Fig. 9 (a) In situ Raman spectra of Ru during the HER process. Reproduced with permission from ref. 148. Copyright 2023 Springer Nature. (b) Plot of intensity changes to the OH stretching band. Reproduced with permission from ref. 149. Copyright 2023 Elsevier. (c) Frequency changes in the O–H stretching modes. (d) Interfacial water dissociation from the Pd surface. Reproduced with permission from ref. 80. Copyright 2021 Springer Nature. (e) Solvent and work function (WF) dependent HOR mechanism. Reproduced with permission from ref. 144. Copyright 2022 American Chemical Society.

The introduction of a local electric field can effectively regulate the adsorption configuration and orientation of H₂O. Cai et al. constructed a localized electric field with atomic asymmetry by introducing IrRu double single atom sites (IrRu DSACs), which resulted in dense charge redistribution and an asymmetric H-down adsorption of the H₂O configuration (Fig. 10a). The distance from H to the surface is significantly reduced, which is positively correlated with the dissociation ability of H₂O (Fig. 10b).⁶⁵ Deng et al. found that metal oxides could cause local electric field enhancement and induce interface water enrichment and redirection. On the surface of Pt, most interfacial water molecules tend to interact with Pt through oxygen atoms rather than H atoms, presenting an H-up configuration, which is not conducive to the continuous dissociation of H–O bonds to form adsorbed *H. In contrast, the arrangement pattern of interfacial water molecules on TiO₂ is significantly different. The binding between interfacial water molecules and Ti is closer, which is more conducive to the continuous activation of H₂O to form adsorbed *H (Fig. 10c).¹⁵² Similarly, interface water on amorphous TiO₂-modified CoP was demonstrated to tend to interact with TiO₂ through H atoms exhibiting an H-down configuration (Fig. 10d and e); interfacial water with an H-down configuration is more beneficial for successive water activation on catalysts, thus possessing a lower dissociation energy barrier (Fig. 10f).¹⁵³ In addition, coupling with metals also has a positive impact on the reorientation of interfacial water molecules. Shen et al. believed that Ni-induced high activity on the Pt surface was driven by the structure of interfacial water. As the Ni content increases, the area ratio of peaks related to suspended O–H bonds and tetrahedrally coordinated water increases (Fig. 10g and h), which means that the interface water perpendicular to the metal surface of the H atom increases, the water–metal interaction decreases, and Ni plays an active role in the dissociation of adsorbed interface water molecules.¹⁵⁴

Fig. 10 (a) Charge density distribution of CoP and IrRu DSACs. (b) Schematic of interface H₂O reorientation induced by an atomic electric field. Reproduced with permission from ref. 65. Copyright 2023 Wiley-VCH. (c) AIMD simulation snapshot of Pt(111) and TiO₂–Pt(111) in KOH solution. Reproduced with permission from ref. 152. Copyright 2022 American Chemical Society. (d) AIMD simulation snapshot of TiO₂–CoP in KOH solution. (e) The radial distribution functions of (H₂O) H and (H₂O) O atoms. (f) The dissociation energy of H₂O for CoP and TiO₂. Reproduced with permission from ref. 153. Copyright 2022 Elsevier. (g) In situ Raman spectra of the PtNi_1.5, acid–PtNi_1.5, and Pt surfaces in 0.1 M NaOH. (h) The relationship between peak area ratio and current density. Reproduced with permission from ref. 154. Copyright 2020 Wiley-VCH.

3.3.2 Hydrogen bonding interactions. Intermolecular hydrogen bonds can promote the simultaneous cleavage and formation of covalent O–H bonds between adjacent H₂O molecules, playing a crucial role in water splitting.¹⁵⁵ Yang et al. found that compared to isomorphic KMnPO₄·H₂O, NH₄MnPO₄·H₂O with hydrogen bond networks exhibited enhanced OER activity in 0.05 M PBS solution. The difference in proton transfer kinetics indicates that in NH₄MnPO₄·H₂O, the proton jump transfer process is fast and the activation energy is low (Fig. 11a and b).¹⁵⁶ Sun et al. proposed that the HER/HOR kinetics of platinum in acids and bases was controlled by the Grotthus diffusion of protons and hydroxides through the hydrogen bonding network of interfacial water. The electric field at the electrified Pt–water interface disrupts the interfacial hydrogen bonding network by reorienting interfacial water, thereby hindering the diffusion of protons and hydroxides at the interface (Fig. 11c and d).¹⁵⁷ In alkaline media, the interface water network strongly interacts with a strong interface electric field, making it more rigid and difficult to recombine during charge transfer through the electric double layer. This leads to slow hydrogen steps in alkaline media. Ledezma-Yanez et al. attributed the influence of PZC on the activation barrier of hydrogen adsorption to the energy penalty associated with interfacial water recombination, in order to adapt to the movement of charges through the bilayer. In acidic media, PZC approaches the hydrogen region—the recombination energy of protons moving through the bilayer by interfacial water is relatively small—emphasizing the promotion of the hydrogen evolution reaction by reducing the energy barrier required for interfacial water network recombination.¹⁵¹ In addition, Li et al. believe that the connectivity of the hydrogen bond network in the double layer is a potential reason for the dynamic pH effect on hydrogen electrocatalysis. There are obvious water gaps at the alkaline interface, which greatly reduce the connectivity of the hydrogen bond network and increase the hydrogen transfer barrier in the interface region (Fig. 11e–h).⁶⁷ The regulation of hydrogen bonding networks is an effective way to break pH-dependent kinetics.

Fig. 11 (a) Schematic illustration of proton hopping in NH₄MnPO₄·H₂O. (b) Key OER steps with proton hopping between adjacent Mn sites in NH₄MnPO₄·H₂O and proton transfer to a liberated form in KMnPO₄·H₂O. Reproduced with permission from ref. 156. Copyright 2023 Wiley-VCH. Atomic configuration of (c) H₂O–Pt(100) interface and (d) H₂O–Me–N₁C₂–Pt(100) interface. Reproduced with permission from ref. 157. Copyright 2023 Springer Nature. EDL structure at HER potentials on the Pt(111) electrode surface for (e) acid and (f) alkaline systems. (g) The concentration distribution map of O atoms in water and (h) statistical distribution of the number of hydrogen bonds along the surface normal direction. Reproduced with permission from ref. 67. Copyright 2022 Springer Nature.

Due to the presence of hydrogen bond acceptors and donors, the modified organic groups can usually participate in the construction of hydrogen bond networks on the electrode surface. Li et al. utilized the convenience of modifying various substituents with carbazole ligands and successfully characterized that the modification of crown ether groups could attract water molecules through hydrogen bonding and promote the formation of hydrogen bonded water network structures. At the same time, it was clarified that the hydrogen bonded water network structure could significantly promote the HER by assisting proton transfer (Fig. 12a).¹⁵⁸ Zhang et al. found that Ni(II)–porphyrins (o-NiTP_yP, m-NiTP_yP, and p-NiTP_yP, corresponding to pyridine N atoms located in the ortho, meta, or para positions) exhibited different HER and OER performances in 1.0 M KOH. The N atoms of pyridine substituents at different positions can form different strengths of H bond interactions with hydrogen donating intermediates (*OOH or H–OH), thereby regulating subtle changes in the microenvironment around the active site (Fig. 12b).¹⁵⁹ The rigidity of the hydrogen bonding water layer at the interface is not conducive to the transport of reaction intermediates, and it is necessary to weaken the water–water interaction at the catalytic site. Intermolecular hydrogen bonding also facilitates the formation of highly disordered and unstable water networks. Wen et al. confirmed that surface OH groups could induce strong non-covalent hydrogen bonds, thereby dragging 4-coordinated hydrogen bonded H₂O molecules through IHP to become free H₂O, continuously providing reactant water molecules for catalytic sites and participating in the HER (Fig. 12c).¹¹⁶ Ng et al. applied self-assembled functional molecular monolayers on the surface of a nanoporous gold bowl (NPGB) to directly and effectively manipulate the hydrogen bond network at the catalytic point; this is expected to improve the accessibility for free water at the electrocatalytic surface (Fig. 12d).¹⁶⁰ The introduction of metal compounds can also disrupt the interfacial water network. For example, RuSe_x can accelerate the transportation of H₂O*/OH* by disordering the hydrogen bonding network in the water interface region, increasing the amount of available H₂O* near RuNC and thereby enhancing the neutral HER activity (Fig. 12e).¹⁶¹ Ding et al. confirmed that introducing CeO₂ could optimize the interfacial water structure and significantly increase the content of free water on Ni₃N in 1.0 M KOH (Fig. 12f).¹⁶² Based on Li's viewpoint, forming a continuous hydrogen bonding network is beneficial for reducing the proton transfer barrier. Zhu et al. proposed reconstructing the connected hydrogen bond network by coupling hydrophilic nitrogen-doped carbon layer (CN) units. The hydrogen bond interaction between CN units and water molecules not only enriches the Mn–Co₃O₄ surface with free water, but also promotes the dehydrogenation process for the acidic OER (Fig. 12g).¹⁶³

Fig. 12 (a) The water network constructed by crown ether groups and its role in assisting proton transfer. Reproduced with permission from ref. 158. Copyright 2022 Wiley-VCH. (b) Optimized structures of *OOH and *H adsorbed on o-NiTP_yP, m-NiTP_yP, and p-NiTP_yP. Reproduced with permission from ref. 159. Copyright 2023 Wiley-VCH. (c) Surface hydroxyl groups drag 4-HB·H₂O molecules through IHP to form free H₂O. Reproduced with permission from ref. 116. Copyright 2022 Wiley-VCH. (d) A scheme to promote or inhibit the HER by combining dissociated/co-induced surface chemicals with NPGB. Reproduced with permission from ref. 160. Copyright 2024 Wiley-VCH. (e) Neutral HER mechanisms for RuNC and RuSe_x–RuNC. Reproduced with permission from ref. 161. Copyright 2022 Springer Nature. (f) In situ ATR-SEIRAS of Ni₃N/NF (left) and Ni₃N–CeO₂/NF (right). Reproduced with permission from ref. 162. Copyright 2023 Wiley-VCH. (g) ELF evaluations for H₂O adsorption on the Co site of Mn–Co₃O₄ (left) and CN site of Mn–Co₃O₄@CN (right). Reproduced with permission from ref. 163. Copyright 2023 Wiley-VCH.

3.4. Electrolytes

Electrolytes, as carriers for ion migration and soluble reactant mass transfer, directly participate in controlling the double-layer microenvironment.¹⁶⁴ Adjusting the composition of electrolytes such as cations, anions, and additives has been proved to play an important role in the regulation of active centers, changes in interfacial water structure, and adsorption of specific reaction intermediates.^165–167 Systematic analysis of the mechanism of electrolyte composition provides assistance in achieving efficient water splitting through optimization of electrolyte design.

3.4.1 Cations. The kinetic dependence of the electrolyte cation effect has been widely confirmed. Xu et al. reported the effect of cations on the interfacial water structure using second harmonic generation. In the presence of Ba²⁺, the second-order water (χ_H2O⁽²⁾) and third-order water (χ_H2O⁽³⁾) optical polarization on Pt is less than that on K⁺, indicating that cations can affect the orientation of interfacial water. The competition between cation hydration and interfacial water arrangement determines the orientation of water, which effectively confirms the hydrogen electrocatalytic mechanism mediated by cation hydration (Fig. 13a).¹⁶⁸ You et al. constructed Au@Pd core–shell nanostructures to enhance Raman spectroscopy to study the behavior of water at different cationic electrolyte interfaces on palladium surfaces and found that the order of increase in interface water peak frequency at a given potential was: Li⁺ < Na⁺ < K⁺ < Ca²⁺ < Sr²⁺ (Fig. 13b).¹⁶⁹ Li et al. proved that larger solvated shells or higher concentrations of ions could disrupt the hydrogen bonding network connectivity of water molecules in the acidic solutions, reduce the probability of long water chains, and prevent protons from rapidly transferring along the water chain like in bulk water, ultimately leading to a decrease in the diffusion coefficient of protons in concentrated salt electrolytes (Fig. 13c).³¹ The regulation of radius, valence state, and concentration of cations can achieve interfacial water optimization. Another important effect of cations is to regulate the adsorption behavior of reaction intermediates. Shah et al. revealed the indirect role of alkali metal cations in altering HER kinetics, with smaller cations exhibiting less instability in the OH_ad species within the HER potential window, which was beneficial for higher coverage of OH_ad on Pt surfaces (Fig. 13d). OH_ad can act as a proton acceptor and donor for nearby water molecules, thereby promoting Volmer step kinetics and HER activity in alkaline media.⁶⁶ Liu et al. found that hydrated alkali metal cations interacted with OH⁻ species adsorbed at the electrolyte and Pt/C interfaces in the form of OH_ad–(H₂O)_x–AM⁺via non-covalent bonds, which facilitated the removal/transportation of OH_ad in the body and was proportional to the hydration energy of metal ions (Fig. 13e).¹⁷⁰ Usually, the smallest cation with the highest hydration energy has the strongest interaction with the electrode surface. For example, Hou et al. found that the increase in the OER electrocatalytic activity of surface-mounted metal–organic framework-derived electrocatalysts from Li⁺ to Cs⁺ was closely related to the decrease in hydration energy (Fig. 13f and g).¹⁷¹ Goyal et al. suggested adjusting the kinetic energy barrier of the HER by favorably interacting with the transition state of rate-determining (*H–OH^δ−–Cat⁺) steps and cations: weakly hydrated cations had a higher near-surface concentration than strongly hydrated cations, making them more conducive to stabilizing the transition state of the hydrolysis dissociation step.¹⁷² In addition to these non-covalent interactions, the changes in bulk structure caused by alkali metal intercalation cannot be ignored. Jia et al. observed through in situ XAFS that cation insertion induced an increase in interlayer spacing of the original CoOOH in the order of Cs⁺ > K⁺ > Na⁺ > Li⁺, resulting in CoOOH–Cs⁺ as the highest Co valence state and the longest Co–O bond, which was beneficial for promoting the formation of active Co(IV) species and improving the OER activity (Fig. 13h and i).¹⁷³ Similarly, Garcia et al. indicated that larger cations (Cs⁺) better stabilized NiOO–M⁺ species.¹⁷⁴

Fig. 13 (a) Fitted values of |χ_S⁽²⁾| and |χ_S⁽³⁾|. Reproduced with permission from ref. 168. Copyright 2024 American Chemical Society. (b) DFT calculated models of interfacial water in different cation solutions on the Pd(111) surface. Reproduced with permission from ref. 169. Copyright 2023 Springer. (c) Hydrogen bonding network connectivity extracted for acidic solutions without and with K⁺. Reproduced with permission from ref. 31. Copyright 2023 Wiley-VCH. (d) Electron density difference of the Pt(111)–OH_ad–water interface after introducing Li⁺, Na⁺ and K⁺. Reproduced with permission from ref. 66. Copyright 2022 Springer Nature. (e) Schematic illustration of the catalytic roles of OH_ad–(H₂O)_x–AM⁺. Reproduced with permission from ref. 170. Copyright 2019 American Chemical Society. (f and g) The correlation between electrocatalytic activity and Raman peaks. Reproduced with permission from ref. 171. Copyright 2022 Wiley-VCH. (h) The variation of overpotential with the oxidation state of Co and Co–O bond length. (i) Schematic illustration of alkali cation intercalation and elongated Co–O bonds. Reproduced with permission from ref. 173. Copyright 2023 Wiley-VCH.

3.4.2 Anions. Anions, as an important component of electrolytes, usually adsorb on the Helmholtz layer and participate in various acid–base equilibrium reactions at the interface, thereby affecting the local microenvironment of the EDL.¹⁷⁵ Some specific adsorbed anions have been shown to promote reaction kinetics under certain conditions. This promoting effect is usually attributed to changes in the properties of the active center, adsorption of key reaction intermediates, and proton transfer processes of the adsorbate. For example, Tang et al. found that adding oxygen-containing anions (SO₄²⁻, CO₃²⁻, NO₃⁻) to electrolytes could effectively regulate the solid–liquid interface, significantly accelerate the surface reconstruction process of LaNiO_3−δ, and enhance the OER performance in 1.0 M KOH electrolytes. The anions adsorbed specifically can greatly affect the distribution of OH⁻ within the IHP layer, thereby breaking the dynamic balance between surrounding OH⁻ ions and their release, releasing more OH⁻ ions (Fig. 14a).¹⁷⁶ Considering the maintenance of catalytic performance and stability by electrode reconstruction and readsorption, Zhou et al. found that adding single or mixed MoO₄²⁻ and SeO₃²⁻ to the alkaline electrolyte was beneficial for improving the hydrogen adsorption free energy, thereby effectively enhancing the HER activity of MoSe₂; this has a universal effect on transition metal chalcogenides (Fig. 14b).¹⁷⁷ Jackson et al. found that the rate of the hydrogen evolution reaction on Au could serve as a function of the concentration of exogenous phosphate and borate proton donors. Among them, phosphate can outperform water as a proton donor for interface CPET (Fig. 14c).¹⁷⁸ Surendranath et al. reported the anti-first-order dependence of the OER rate on proton activity and the zero-order dependence on phosphate. In the absence of phosphate buffer, the Tafel slope significantly increases and the overall activity of the cobalt–phosphate catalyst significantly decreases at neutral pH. Phosphates can participate in reactions as proton acceptors.¹⁷⁹ The positive effect of phosphate ions can be extended to seawater electrolysis systems. Yu et al. achieved highly stable alkaline seawater decomposition at the industrial level of current density by adding phosphate ions. The phosphate ions adsorbed on the surface of NiFe-LDH can repel Cl ions, prevent transition metal dissolution, and act as a local pH buffer to compensate for rapid OH consumption under high current electrolysis without significantly blocking OH diffusion (Fig. 14d).¹⁸⁰ The synergistic effect of anions also proved to effectively activate water. Liu et al. revealed a strong synergistic effect between borate anions with acid–base reaction characteristics and fluoride anions with hydrogen bonding characteristics, which could promote electrocatalytic water oxidation under neutral conditions. At the anode potential, borate anions tend to reside in the inner Helmholtz layer closer to the Co(OH)₂ surface, promoting local PCET and water oxidation, while fluoride anions are located further outside to maintain a higher current density by increasing the exchange current (Fig. 14e and f).⁶⁴ This also provides ideas for designing electrolytes by adding anions.

Fig. 14 (a) Mechanism of surface reconstruction promoted by oxygen-containing anions. Reproduced with permission from ref. 176. Copyright 2023 Wiley-VCH. (b) Dynamic leaching and readsorption behavior of Mo and Se on the MoSe₂ surface. Reproduced with permission from ref. 177. Copyright 2022 Wiley-VCH. (c) Phosphate ions act as proton donors to participate in the HER process. Reproduced with permission from ref. 178. Copyright 2019 American Chemical Society. (d) The role of phosphate ions in seawater electrolysis. Reproduced with permission from ref. 180. Copyright 2022 Elsevier. (e and f) Distribution of borate anions and fluoride anions in the double layer. Reproduced with permission from ref. 64. Copyright 2022 Elsevier.

4. Summary and outlook

The local microenvironment is considered a key factor restricting the kinetics of electrocatalytic reactions. This article reviews the latest progress in utilizing local microenvironment regulation to achieve efficient water splitting. The focus was on the influence of wettability, local pH, interfacial water structure, and electrolyte on catalytic performance, and effective strategies were proposed to regulate these environmental factors. By switching the wettability of bubbles, such as the design of nano-/biomimetic structures and the introduction of external field effects, a balance between liquid electrolytes and gaseous products can be established and maintained, thereby accelerating the mass transport process; constructing a local electric field and modifying the proton/hydroxide transport layer can achieve the enrichment of reaction intermediates and the regulation of local pH; adjustment of the interface water arrangement and hydrogen bonding network can accelerate the efficiency of charge and proton transfer. Finally, the influence of electrolyte components such as cations/anions on the interfacial water structure, changes in the properties of active centers, and the adsorption of key reaction intermediates were emphasized. Although there has been some progress in the identification and careful design of microenvironments, there are still some issues worth paying attention to:

(1) Compatibility of local microenvironments. The design of electrocatalysts usually focuses only on optimizing a single environmental factor and lacks consideration for other environmental factors. For example, control based on morphology effects can simultaneously achieve effective bubble mass transfer, induce the formation of local electric fields, and achieve local pH regulation, interfacial water structure transformation, etc. This also provides ideas for the synergistic regulation of multiple environmental factors. In addition, due to the complex solvation effect of the interface, a reasonable design of the local microenvironment requires consideration for incompatibility.

(2) The stability of the microenvironment is a key indicator for evaluating its practical application. However, due to external bias and interference from the electrolyte, the interface microenvironment undergoes localized polarization with spatiotemporal fluctuations, which seriously affect its stability. In addition, due to the influence of material properties, the catalyst structure transformation and dissolution behavior will inevitably lead to changes in interface composition. Special packaging layers such as amorphous and organic ligands can be modified to accurately customize highly stable interface microenvironments, showing promising prospects.

(3) More efforts are needed to explore the electric double layer microenvironment. The establishment of theoretical models helps to understand the properties of the double layer and the composition of the local microenvironment of electrocatalysts, but there is still a lack of direct experimental evidence. It is necessary to use electrochemical testing and in situ characterization to reveal the relationship between the double layer structure and the performance of electrocatalyst. In addition, more strategies based on local microenvironment design of electrocatalysts need to be developed to drive their development.

(4) Besides electrocatalysts and electrolytes, designing appropriate reaction systems is equally important. An electrolyzer is feasible technology for achieving commercial-scale water electrolysis. An electrolyzer structure with flexible diffusion channels can ensure continuous transportation of gas products and electrolytes, as well as efficient electrolysis of water reactions. Membrane electrode assembly such as membranes and ionomers with mechanical and chemical stability can efficiently suppress gas and electron crossing while achieving ion transport. These all provide ideas for changing the local microenvironment under real operating conditions.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 22162025 and 22068037), the Youth Innovation Team of Shaanxi Universities, the Open and Innovation Fund of Hubei Three Gorges Laboratory (SK232001), the Regional Innovation Capability Leading Program of Shaanxi (2022QFY07-03, 2022QFY07-06), and the Shaanxi Province Training Program of Innovation and Entrepreneurship for Undergraduates (S202210719078).

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