1 Introduction
Condensed-phase chemistry is a new academic discipline that studies the composition, structure, properties, and functions of condensed-phase materials, as well as the laws governing chemical reactions, construction, and creation, along with their underlying theories. Its primary research subjects encompass various material states—solid, mesoscopic/nanoscale, molten, liquid, and gaseous—that are closely associated with chemical science
［1］. Developing advanced characterization techniques for condensed-phase chemical reactions and gaining a deeper understanding of the multi-level structures of condensed-phase materials and their evolution processes and patterns under reaction conditions are currently among the most important directions in the field of condensed-phase chemistry.
Energy storage and conversion technologies centered on electrochemical reactions have become an important approach to alleviating energy and environmental challenges
［2-3］. The electrochemical interface, as a critical region for electrochemical reactions, significantly influences the reaction processes and mechanisms through its chemical composition, surface structure, and other characteristics, thereby regulating the electrochemical performance of the system. Developing advanced characterization techniques for the dynamic processes at the electrochemical interface, gaining a deep understanding of the reaction mechanisms at the microscopic level, and establishing structure–property relationships in electrochemical systems are essential foundations for the design and fabrication of high-performance materials and devices.
As a complementary and closely related discipline to electrochemistry, the development of surface and interface science and characterization techniques can bring new ideas and methodologies to electrochemical interface research. For example, scanning probe microscopy techniques
［4-5］and surface/interface spectroscopic techniques
［6-8］have been widely applied in the study of electrochemical interfacial processes. With the continuous development of novel materials, increasingly complex interfacial systems have emerged, such as the coupled interface between multiphase catalysts and electrolytes
［9］, the local environment of single-atom catalysts embedded in carbon supports
［10］, and the gas–liquid–solid three-phase reaction interface
［11］. Meanwhile, electrochemical processes often need to operate under extreme conditions, such as vigorous gas evolution at high current densities
［12］, stability challenges under high voltages or wide potential windows
［13］, and operation in highly concentrated saline or strongly acidic/alkaline electrolytes
［14］. These conditions pose new challenges for characterization technologies. A systematic summary of electrochemical interfacial processes and characterization methods, together with an in-depth understanding of interfacial regulation mechanisms and the establishment of correlations between microscopic interfacial behavior and macroscopic catalytic performance, will provide important insights for the development of relevant fields in both electrochemistry and surface and interface science.
Understanding interfacial electrochemical processes from the perspective of condensed-phase chemistry can provide new insights for optimizing electrochemical system architectures and advancing characterization techniques. This paper reviews interfacial electrochemical processes and characterization methods within the framework of condensed-phase chemistry, systematically introducing the fundamental components of electrochemical systems, including electrodes, electrolytes, and dispersing media. From a condensed-phase chemistry standpoint, it analyzes the intrinsic characteristics of electrochemical interfaces. The paper summarizes various characterization approaches for electrochemical interface structures and processes and discusses interfacial electrochemical processes with distinct features. This work will contribute to the development of the scientific frameworks of "condensed-phase structural chemistry" and "condensed-phase reaction chemistry," while also fostering interdisciplinary integration and mutual complementarity between condensed-phase chemistry and electrochemistry.
2 Condensed Matter Systems in Electrochemistry
Electrochemical systems typically consist of electrodes, electrolytes, and their dispersing media, which contain condensed matter with diverse hierarchies and properties
［15］. The fundamental characteristics of condensed matter—such as its constituents, phases, structure, and aggregation forms—significantly influence the performance of electrochemical systems. Different types of condensed matter can be combined through various interactions to form electrode and electrolyte materials with tunable structures and properties. By controlling the components, interaction modes, and structures of composite material systems, it is possible to effectively expand the application fields and performance limits of electrochemical systems.
2.1 Electrodes
Electrodes are the sites where electron transfer occurs in electrochemical reactions and can be classified into cathodes and anodes based on the direction of electron flow. Electrodes are typically composed of metals or semiconductor materials with good electrical conductivity, and they are applied to different electrochemical systems according to their structure and function. Studies have shown that metals with different crystallographic facets exhibit distinct atomic arrangements and functional characteristics at the microscopic level, resulting in variations in their electrochemical properties
［16-18］. Tian et al.
［19］synthesized Pt nanoparticles bounded by various high-index crystal facets (e.g., {730}, {210}, {520}, etc.) in solution using square-wave potentiometry (
Figure 1a). These nanoparticles possess a large number of active atoms and dangling bonds on their surfaces, demonstrating outstanding electrocatalytic activity toward formic acid and ethanol oxidation reactions.
In addition to single-component electrodes, electrodes formed by the composite of various condensed-phase materials have also been extensively studied. Organic molecules
［20-21］and polymers
［22］, among other functional systems, can form composite electrodes through surface modification on substrates such as metals, thereby enabling a uniform dispersion of electrochemical active sites. By appropriately leveraging substrate effects, the properties of the active centers can be further optimized.
Thus, electrode performance can be enhanced. For example, porphyrin-based compounds
［23-24］,perylene diimide derivatives
［25］,and bipyridine derivatives
［26-27］are organic functional molecules that can self-assemble on surfaces via intermolecular interactions and molecule-substrate adsorption, forming structurally diverse assembly layers. Different adsorption configurations, molecular orientations, and substrates can effectively modulate the frontier orbitals and energy levels of the molecules. Furthermore, polymers can also adsorb onto substrate surfaces to form modified electrodes
［28-30］.The rational design of chain structures and substituents in polymers can regulate the properties of the electrical double layer and the microenvironment at the electrode interface, making polymer-based modifications one of the important approaches for constructing composite electrodes
［31］.
2.2 Electrolytes
Electrolytes generally refer to substances that can conduct ions under specific dispersion conditions, mostly existing in condensed states formed by ionic bonds or polar covalent bonds. Ionic compounds can form freely moving ions in solvents or in the molten state; the conductivity of some solid electrolytes originates from ion migration within their crystal lattices.
For electrolytes dispersed in a solvent, the ionic composition significantly influences electrochemical processes. Certain cations or anions can bind with electrochemical reaction intermediates, altering the energy of these intermediate species and thereby regulating the reaction pathway
［32-33］. Furthermore, differences in ion size, hydration degree, and differential distribution within the electrical double layer can effectively modulate the microenvironment at the electrode/electrolyte interface (e.g., pH value
［34］, hydrogen bonding
［35］, and counterions
［36］), thus changing the electrochemical performance of the system. Huang et al.
［37］found that K
+in the electrolyte can promote the activation of reactants in the carbon dioxide reduction reaction (CO
2RR), suppress the competitive hydrogen evolution process, and enhance the Faraday efficiency of the product (
Figure 1b).
2.3 Dispersed Medium
Water is the most common dispersing medium in electrochemical systems
［38］. In addition, organic solvents such as acetonitrile, ionic liquids, and certain solid-phase materials are also frequently used to disperse electrolytes. The water molecules exhibit a rich hydrogen-bonding network; variations in the structure and strength of this network under different experimental conditions can effectively regulate the elementary steps of electrochemical reactions. Wang et al.
［39］used organic additives to modulate the hydrogen-bond strength at the Au electrode surface. Their experiments revealed that strong hydrogen bonding could enhance proton tunneling kinetics by a factor of 103, thereby boosting the catalytic activity for the oxygen reduction reaction (ORR) (
Figure 1c). Li et al.
［40］combined theoretical simulations with experimental studies and proposed that differences in the connectivity of the hydrogen-bonding network within the electrical double layer constitute one of the key factors underlying the kinetic pH effect in hydrogen electrocatalysis. Furthermore, researchers have also developed various strategies to modulate organic electrolytes in order to improve electrochemical reaction efficiency. Tian et al.
［41］proposed a solvent-regulation strategy that precisely controls the water content in acetonitrile to achieve efficient CO production over metal catalysts. Their experiments demonstrated that CO exhibits weaker binding to metal centers in acetonitrile, which facilitates its dissociation. At a water concentration of 0.19 mol/L, Cu catalysts can attain nearly 100% CO conversion efficiency.
3 Understanding Electrochemical Interfaces from the Perspective of Condensed-Phase Chemistry
Electrochemical interfacial processes involve rich structural evolution, species transformation, and phase changes, which are closely related to condensed-phase chemistry. Understanding electrochemical interfacial processes from the perspective of condensed-phase chemistry helps to better comprehend the interfacial reaction mechanisms and establish structure–property relationships. This section will analyze the fundamental characteristics of electrochemical interfaces from the perspective of condensed-phase chemistry.
3.1 Holism
Electrochemical interfacial processes and properties are often not only related to single components such as electrolytes or electrodes; the interactions between them also have a significant impact on the interfacial behavior of the system
［42］. Therefore, it is particularly important to focus on the holistic nature of electrochemical interfacial systems and to gain a deeper understanding of the regulatory mechanisms by which electrolytes control electrode interfacial processes.
In lithium-ion battery systems, the solid electrolyte interphase (SEI) is a passivation film on the surface of the negative electrode, primarily composed of reduction decomposition products of the electrolyte
［43］. The SEI film can prevent further decomposition of the electrolyte while allowing selective ion transport. By reducing side reactions and promoting the Faraday process at the electrode, it significantly influences battery performance
［44］. Its formation is closely related to the interactions among multiple components, including the electrode and electrolyte. Understanding the formation process and underlying mechanisms from a holistic perspective of the battery interface system is crucial for optimizing battery performance. Hou et al.
［45］developed an in-situ characterization technique combining transmission electron microscopy (TEM) with scanning tunneling microscopy (STM), enabling simultaneous study of the SEI film formation process and its electrical properties (
Figure 2a–d). Liu et al.
［46］used atomic force microscopy (AFM) and electrochemical differential mass spectrometry to conduct in-situ investigations into the growth, morphology, and compositional evolution of the SEI film, thereby elucidating its formation mechanism.
In electrocatalytic reaction systems, interactions between different condensed phases under non-equilibrium potentials can induce reversible or irreversible structural changes at the interface. These structural alterations often lead to variations in electrochemical performance. Understanding the overall synergistic regulation mechanism of the electrolyte–catalytic electrode system is a crucial foundation for establishing structure–performance relationships in electrochemical systems. Wang et al.
［47］investigated the regulatory principles governing electrode interface roughening during the CO
2reduction reaction. By conducting experiments in electrolytes containing different alkali metal cations, they found that the Au(111) surface exhibited the highest degree of roughening and the most outstanding catalytic activity in solutions containing Cs
+, thereby directly revealing the microscopic mechanism by which cationic modulation of the interfacial structure optimizes catalytic performance (
Figure 2e).
3.2 Dynamics
The electrochemical interface is the transition region between the two phases of an electrode and an electrolyte, serving as the core site where electrochemical reactions occur. Dynamism is a fundamental characteristic of electrochemical interface processes, and electrochemical reactions often involve multiple dynamic interfacial processes at different levels. Condensed-phase chemistry regards dynamic processes as evolutionary behaviors under many-body interactions, thereby providing a new perspective for the study of dynamic interfaces.
The development of in-situ characterization techniques has enabled researchers to better understand the dynamic processes of interfacial species transformation. In-situ electrochemical STM can provide atomic-resolution information on reactant adsorption and electrocatalytic processes. For example, Gu et al.
［48］used in-situ electrochemical STM technology to observe the ORR process catalyzed by iron phthalocyanine (FePc) at the single-molecule level (
Figure 3a). In the STM images, highly contrasted complexes formed by O
2adsorption onto FePc could be clearly observed. Further in-situ imaging revealed the transformation of the FePc–O
2complex into a FePc molecule during the reaction. Quantitative analysis of the molecular dynamics at the interface helps researchers gain insights into the kinetics of electrochemical reactions. For instance, in the CO
2reduction reaction catalyzed by cobalt phthalocyanine (CoPc), after observing CO
2adsorption, they further analyzed the correlation between the molecular adsorption process and the reduction of Co sites
［49］. By statistically analyzing the temporal variation in the surface coverage of adsorbed molecular species, it was determined that the rate-determining step in the initial stage of the reaction is the adsorption of CO
2onto the active site.
The video-STM technique, capable of rapid scanning imaging with millisecond-level time resolution, has enabled the study of fast atomic dynamic processes such as interfacial diffusion. Rahn et al.
［50］used video-STM to observe the diffusion process of S atoms on halogen-atom-modified Cu(100) and Ag(100) surfaces (
Figure 3b). The experiments revealed that S atoms undergo exchange diffusion on the Cu surface, whereas diffusion on other surfaces generally occurs via a rotational mechanism. Further studies demonstrated that co-adsorbed halogen atoms significantly influence diffusion kinetics (surface mobility).
3.3 Multiscale
In electrochemical reactions, there are often interfacial processes occurring at different scales. These processes are interconnected and collectively influence the electrochemical performance of the system. For example, in energy storage and catalytic systems, atomic-scale interfacial changes gradually accumulate to the macroscopic (micrometer) scale as the reaction proceeds. Characterizing and understanding these electrochemical interfacial processes across multiple scales is of great significance for establishing the mechanisms of electrochemical reactions
［51］。
Electrochemical interfaces are characterized by dynamic processes at the atomic scale, such as interfacial deposition
［52］,diffusion
［53］,and microstructural evolution
［54］,among others. These atomic-scale transformations often exert a certain influence on catalytic performance. Taking the electrooxidation of CO as an example, Inukai et al.
［55］found that the atomic step structure on the Pt(111) surface undergoes significant changes during the reaction: the originally curved steps gradually become straighter, while the catalytic activity correspondingly decreases (
Figure 4a). This phenomenon is likely closely related to the reduction in the number of low-coordination, highly active metal sites located at the curved steps. High-resolution surface characterization techniques constitute an essential foundation for studying interfacial processes at the atomic scale. Soriaga et al.
［56］used in situ STM to observe the transformation of Cu crystal facets during the electrocatalytic reduction of CO
2. Under the reaction potential, the polycrystalline copper surface first transforms into the Cu(111) facet and subsequently into the Cu(100) facet. The resulting changes in the Cu–Cu interatomic spacing directly affect carbon coupling in the reaction, thereby regulating product selectivity.
At the mesoscopic scale, research on electrochemical interfaces primarily focuses on the dynamic behavior of surface adsorption layers and nanoscale domains, whose evolution is often closely related to interfacial reaction processes. Kim et al.
［57］used in situ STM to investigate the underpotential deposition of Cu atoms on the Au(111) surface. The experiments revealed an ordered (1 × 1) adsorption layer and Moiré fringes formed by Cu atom deposition. It was found that the addition of Cl
-to the solution could influence the structure of the deposited layer, likely due to the co-adsorption of Cl
-on the surface. Hernandez et al.
［58］employed electrochemical STM to observe the underpotential deposition of Pd on the Au(111) surface. At scales ranging from the atomic level to hundreds of nanometers, they were able to visualize the formation of monolayer island structures, step-edge extensions, and subsequent growth until complete coverage of the substrate, providing direct experimental evidence for understanding the underpotential deposition behavior of metals.
The accumulation of micro- and meso-scale processes can lead to changes in the macroscopic morphology of interfaces, thereby influencing electrochemical behavior. Li et al.
［59］performed *in situ* observations of the lithiation process of SnO
2nanowires during the charging of lithium-ion batteries. The experiments revealed that the reaction front propagates along the nanowires, inducing expansion and elongation at the nanoscale to micrometer scale, which subsequently results in battery performance degradation (
Figure 4b). These multi-scale features can be elucidated through appropriate characterization techniques. Such a multi-scale research approach provides a systematic and comprehensive perspective for a deeper understanding of electrochemical interfacial processes.
In summary, integrality, dynamics, and multi-scale characteristics, as important features of electrochemical interface systems, have provided new perspectives for understanding electrochemical interface processes from a condensed-phase chemistry standpoint, demonstrating the unique value and development potential of condensed-phase chemistry in electrochemical research.
4 Electrochemical Interface Characterization Techniques
Research on the relationship between electrochemical interfacial processes and performance is crucial for understanding electrochemical reaction mechanisms and subsequently optimizing electrochemical systems. In 2024, the Chinese Electrochemical Society listed “How to detect or simulate, at the microscopic level, the dynamic structural changes of complex electrochemical interfaces under in situ/operando conditions, and establish their correlation with macroscopic electrochemical performance” as one of the top ten scientific challenges in electrochemistry
［60］. From the perspective of condensed matter chemistry, the holistic, dynamic, and multiscale characteristics of electrochemical interfacial processes pose significant challenges to characterization methods. The development of in situ, real-time, and high spatiotemporal resolution characterization techniques can facilitate a precise understanding of interfacial dynamic evolution, species transformation, and microenvironmental changes, thereby establishing a comprehensive understanding that bridges microscopic mechanisms and macroscopic performance, which in turn provides essential support for elucidating electrochemical reaction mechanisms.
Currently, the in-situ characterization techniques for electrochemical interfacial processes have become relatively sophisticated. These techniques mainly include microscopy imaging and spectroscopic analysis methods, such as scanning probe microscopes (atomic force microscopy [61–62], scanning tunneling microscopy [4–5, 63]), transmission electron microscopy [5, 64–65], energy-dispersive spectroscopy techniques (X-ray photoelectron spectroscopy, X-ray absorption fine structure spectroscopy [66–67]), molecular vibrational spectroscopy (infrared/Raman spectroscopy [6–8, 68]), and electrochemical mass spectrometry coupling techniques [69–70]. This chapter will primarily focus on the characterization of condensed-phase systems at the electrochemical interface and introduce application examples of these techniques in electrochemical interfacial research.
4.1 Imaging Characterization of Electrochemical Interface Structure
Microscopic imaging techniques generally possess high spatial resolution, providing an intuitive research approach for studying the dynamic processes, structural evolution, and reaction mechanisms at condensed-phase interfaces. Electrochemical scanning probe microscopy (EC-SPM), by integrating a scanning probe microscope with an electrochemical testing system, enables in situ acquisition of surface morphology, electronic states, and mechanical properties under electrochemical conditions
［71］, thereby becoming a powerful tool for exploring the electrochemical behavior at condensed-phase interfaces. Depending on the differences in detection principles, EC-SPM primarily includes electrochemical atomic force microscopy (EC-AFM) and electrochemical scanning tunneling microscopy (EC-STM).
4.1.1 Electrochemical Scanning Probe Microscopy
AFM achieves high-resolution imaging of sample surface topography by detecting the displacement changes of the laser beam reflected from the back of the probe cantilever
［72］. AFM is applicable to various working environments and exhibits excellent compatibility with liquid-phase conditions. In EC-AFM experiments, the sample under test serves as the working electrode, forming a three-electrode system together with an external reference electrode and a counter electrode, which enables in-situ observation under potential control. Currently, EC-AFM has developed into an important tool for studying electrochemical interfacial behavior, widely used for dynamic characterization of electrochemical processes, such as SEI film formation and evolution, structural transformations of electrode materials including particle expansion/strain/crack propagation, as well as precise measurement of mechanical and electrical properties of electrode/electrolyte materials
［71］. In lithium-ion batteries, an unstable SEI can be disrupted by the deposition/stripping of metallic lithium on the negative electrode surface, leading to side reactions and Li dendrite growth
［59］. Zhang et al.
［61］designed a polymer SEI layer based on lithium polyacrylate (LiPAA) and demonstrated through in-situ EC-AFM that LiPAA can accommodate Li deformation and suppress dendritic structures, thereby yielding a smoother anode morphology after plating/stripping.
The principle of STM technology is based on the quantum tunneling effect, which utilizes the exponential relationship between the tunneling current and the distance between the tip and the sample to obtain atomic-resolution topographic or electronic-state information of the sample surface. EC-STM integrates electrochemical control into the STM framework; by applying a potential through a three-electrode system, it enables in situ observation of surface structures and interfacial dynamic processes at the atomic and molecular levels under electrochemical reaction conditions
［73］(
Figure 5a). EC-STM can probe the distribution of active sites, the adsorption and transformation of reactants
［74］, as well as the structural evolution of catalysts during electrocatalytic reactions. These atomic-scale dynamic insights provide direct experimental evidence for understanding electrochemical processes such as charge transfer and material transformation at condensed-phase interfaces. Auer et al.
［75］employed in situ EC-STM to monitor local morphological and structural changes during the electrocatalytic oxidation of CO-saturated electrolytes, discovering that the presence of CO promotes the structural evolution of the Cu(111) surface, leading to increased surface roughening and the formation of linear nanostructures and island-like structures (
Figure 5b), thereby enhancing electrochemical activity. Furthermore, EC-STM can be used to evaluate the adsorption strength of active species. Experiments have shown that O
2adsorption on molecular surfaces such as FePc, CoPc, and octaethyl cobalt porphyrin (CoOEP) can form high-contrast species, whose coverage can reflect the binding strength of M–O
2. Wang et al.
［26］revealed through EC-STM that the O
2binding strength at Co-N
4sites is positively correlated with ORR activity.
EC-SPM has enabled in situ visualization of electrochemical interfaces at the molecular scale, providing a unique research approach for elucidating dynamic interfacial processes. It is worth noting that real material systems exhibit complex structures and heterogeneous reaction sites. The design and precise construction of structurally controllable model systems can facilitate a clear understanding of electrochemical interfacial processes and mechanisms, which holds significant value for establishing structure–property relationships within these systems.
4.1.2 In-situ Transmission Electron Microscopy
The transmission electron microscope, which uses an electron beam as a “probe,” generates various signals through the interaction between electrons and the sample. These signals are focused and magnified by an electromagnetic lens system to produce high-resolution images or diffraction patterns. TEM can be combined with analytical techniques such as energy-dispersive X-ray spectroscopy (EDS) and electron energy-loss spectroscopy (EELS) to simultaneously study material structural evolution and compositional changes
［76］.
The high temporal resolution of TEM makes it a crucial tool in studying the microstructure and dynamic processes of materials. To conduct *in situ* TEM experiments in liquid environments, researchers have developed the windowed cell approach, which encapsulates the sample within a liquid layer, thereby isolating it from the ultra-high vacuum environment
［77］(
Figure 6). The thickness of the liquid layer can be controlled between tens and hundreds of nanometers and is sealed by two electron-transparent membranes. To meet experimental requirements, these sealing membranes must possess two key characteristics: first, they should exhibit minimal electron scattering to ensure image quality; second, they must have sufficient mechanical strength to withstand the substantial pressure difference across the membrane. In recent years, the application of novel materials such as graphene has opened up new possibilities for nanoscale imaging in liquid environments.
Gu et al.
［64］ Utilized this type of technique to successfully achieve in-situ observation of the structural changes in Si nanowire electrodes during lithium deintercalation. The experiment revealed that a core–shell structure can be formed within Cu–Si nanowires during the lithiation process of Si. Sun et al.
［5］ Employed in-situ TEM to investigate the sodiation mechanism of black phosphorus, and the TEM images clearly demonstrated a two-step sodiation process involving both intercalation and alloying.
4.2 Spectroscopic Characterization of Species
Spectroscopic techniques provide crucial molecular-level information for electrochemical interface studies through the analysis of characteristic peaks. By developing in-situ characterization methods, it has become possible to monitor in real time the dynamic changes of adsorbed species on electrode surfaces, the electronic structure of active sites, and the local chemical environment during electrochemical reactions, thereby deepening the understanding of complex multi-scale behaviors in condensed-phase systems.
4.2.1 In situ vibrational spectroscopy
Vibrational spectroscopy is one of the important characterization techniques for elucidating electrochemical interface mechanisms. Probing molecular vibrational features can provide significant support for understanding electrochemical interfacial processes. This technique utilizes the interaction between ultraviolet/visible/infrared photons and molecular systems. By detecting the characteristic vibrational modes and their dynamic changes in solvent molecules, electrode surfaces, and reactants/intermediates/products, it obtains information on structural evolution and species transformations during electrochemical reactions
［78］. With a specialized electrochemical cell setup, applying laser or infrared light to the working electrode enables spectroscopic measurements under potential-controlled conditions
［79］. Infrared spectroscopy is widely used to monitor adsorbates on metal electrode surfaces
［80］; Raman spectroscopy is more sensitive to most organic substances and less susceptible to interference from water molecule vibrational signals, demonstrating certain advantages in electrochemical system studies
［68］. Infrared and Raman spectroscopies are complementary to each other and help resolve different issues in complex multiphase interfaces.
To enhance the surface sensitivity of spectroscopic measurements, researchers have developed several techniques based on localized surface plasmon resonance, including surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS), and surface-enhanced infrared absorption spectroscopy (SEIRA) that utilizes enhanced electromagnetic fields at metal interfaces. Both SERS and TERS can identify vibrational fingerprints of molecules and other chemical species. Li et al.
［81］further developed shell-isolated nanoparticle-enhanced Raman spectroscopy by coating gold nanoparticles with an ultrathin silica shell, thereby achieving highly sensitive detection of molecules on various substrate surfaces. This technique has provided a powerful tool for studying molecular adsorption on single-crystal surfaces and interfacial processes. Using in situ electrochemical SERS, Dong et al.
［82］detected intermediates during the ORR process on Pt(hkl) electrodes, elucidating the catalytic mechanism of ORR on Pt(hkl) surfaces at the microscopic level. Bodappa et al.
［83］employed in situ shell-isolated nanoparticle-enhanced Raman spectroscopy to investigate the early stages of electrochemical oxidation of Cu(111) and polycrystalline Cu materials in electrolytes with varying pH levels. The in situ spectra revealed the presence of surface OH species, and on the Cu(111) surface, a transformation from OH to O species was observed (
Figure 7), providing crucial experimental evidence for the involvement of hydroxyl/oxide surface species in interfacial electrochemical reactions.
Attenuated total reflection infrared spectroscopy (ATR-IR) can minimize the influence of the electrolyte on the measurements, making it a commonly used experimental approach in electrochemical interface studies. Among various vibrational modes, the C—O bond exhibits high SEIRAS sensitivity
［84］; therefore, in situ ATR-SEIRAS has been widely applied to investigate interfacial processes involving carbon-containing species transformations, such as CO
2reduction reactions. Based on this technique, *CO was identified as a common intermediate during the CO
2reduction process
［85］. Furthermore, SEIRAS has also played an important role in elucidating the ionic effects in electrocatalytic reactions. Xu et al.
［36］combined SEIRAS with molecular dynamics simulations and found that cations significantly affect both the enthalpy and entropy of CO adsorption on copper surfaces, thereby explaining the regulatory mechanisms of different cations on electrochemical interfacial processes.
4.2.2 X-ray Absorption Spectroscopy
X-ray absorption spectroscopy is a technique used to determine the local geometric and electronic structures surrounding atoms. The XAS spectrum primarily comprises two regions: the X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS)
［86］. XANES is defined as the region within several tens of electronvolts before and after the absorption edge. The shape of the absorption edge, as well as the characteristic features preceding and following it, are determined by the density of unoccupied states of the inner-shell excited electrons. Furthermore, local electronic states can also influence XANES, leading to pronounced pre-edge features. Consequently, XANES can reflect the atomic density of states, oxidation state, and geometric symmetry. The region extending beyond XANES up to thousands of electronvolts is referred to as EXAFS. In this region, inner-shell electrons are excited into free states and propagate outward as photoelectrons. During their propagation, these photoelectrons are scattered by neighboring atoms, with a portion returning to the absorbing atom. Due to phase differences, the scattered photoelectrons undergo interference, thereby providing information on bond lengths and coordination numbers
［87］. XAS technology can precisely resolve the local atomic configuration and electronic structure of active sites within a system.
XAS technology is currently primarily based on synchrotron radiation sources with high brightness and a broad energy range. With the advancement of synchrotron radiation sources, in situ XAS has been increasingly applied to the study of electrochemical interfaces
［88］, thereby enabling real-time monitoring of material evolution and revealing the dynamic relationship between structure and performance. For example, in situ EXAFS characterization has been used to elucidate direct electronic modulation between Co centers and covalent organic framework materials
［66］, allowing partial reduction of Co
2+ to Co
+. Lee et al.
［89］ employed in situ XANES in combination with cyclic voltammetry (CV) scans to reveal the activation and degradation mechanisms of electrodeposited Ni(OH)
2 catalysts during alkaline OER (
Figure 8). The α-Ni(OH)
2 formed via electrodeposition was transformed into γ-NiOOH during the first anodic scan, becoming the active phase for OER. However, during subsequent cathodic scans, only a portion of the γ-NiOOH was reduced back to β-Ni(OH)
2, while the remaining γ-NiOOH was irreversibly separated out. This irreversible phase transformation led to a significant decline in OER catalytic activity. Furthermore, combining EXAFS with theoretical simulations can resolve single-atom coordination structures
［90-91］.
5 Electrochemical Interface Processes and Performance
The electrochemical interface is the site where electrochemical reactions occur, characterized by complex species, diverse structures, and dynamic processes. Electrochemical interfacial processes typically involve multi-faceted dynamic changes, including electrode structure evolution
［47］, active species transformation
［83］, and interfacial charge transfer, among others. These processes are closely related to the performance of electrochemical systems. Investigating and understanding the dynamic processes and regulation mechanisms at the electrochemical interface across different scales is of great significance for establishing the structure–performance relationship in electrochemical systems.
5.1 Interface Structure Evolution and Characterization
The evolution of condensed-phase structures under reaction conditions is one of the key scientific issues in condensed-phase chemistry. In electrochemical reactions, the structural evolution at the electrode/electrolyte interface is widespread. Under non-equilibrium potentials, influenced by charge transfer
［92］,species interactions
［93］,and solvent effects
［47］,the interface undergoes reversible or irreversible structural changes. These structural alterations often lead to differences in the electrochemical performance of the system. Understanding the synergistic relationships between structure and performance is an essential foundation for establishing structure–property correlations in electrochemical systems. Taking electrocatalytic systems as an example, single-crystal copper is one of the commonly used highly efficient catalysts for CO
2reduction reactions. Experimental studies have shown that, under open-circuit potential, a Cu
2O(111) oxide layer forms epitaxially on the Cu(100) surface
［94］.Under reaction potentials, the interfacial structure becomes rougher, and a reconstruction induced by the adsorption of species such as CO occurs. This process is accompanied by an increase in low-coordinated Cu atoms at step edges, leading to enhanced catalytic activity.
Magnussen et al.
［95］ utilized a variety of in-situ characterization techniques to further investigate the surface reconstruction of Cu(100) during the CO
2 reduction reaction (
Figure 9, a–d). The experiments revealed the formation of surface cluster structures, namely roughening reconstruction, which is reversible within a certain potential range. In-situ surface X-ray diffraction (SXRD) analysis of crystal truncation rods further confirmed the surface roughening during the reconstruction process. By analyzing the CTR data, the authors proposed a structural model for the reconstructed surface. They argued that the reconstruction of the Cu(100) surface primarily occurs in the initial stage of the reaction and is closely related to the reaction product CO, while the formation of the reconstructed structure significantly influences the catalytic activity of the electrode.
5.2 Interface Process Regulation
Developing strategies to regulate electrochemical interfacial processes and optimize system performance is one of the important research directions in this field. The commonly used regulation methods include intrinsic control over electrode materials and the structure and electronic properties of active sites
［96-97］, as well as microenvironmental regulation targeting electrolyte composition
［98-99］ and acidity/alkalinity
［40］. At their core, most of these strategies can be attributed to regulating fundamental processes such as interfacial mass transport, electron transfer, and chemical reactions. Therefore, developing characterization techniques for interfacial electrochemical processes and gaining a deeper understanding of the interfacial processes and mechanisms are crucial foundations for achieving efficient regulation.
Lithium dendrite deposition can lead to battery capacity fade and poses significant safety risks. Based on an understanding of its formation mechanism, developing strategies to suppress lithium dendrites is crucial for performance enhancement
［100］. Ning et al.
［101］employed a variety of in-situ characterization techniques to investigate the dendrite growth process in all-solid-state batteries (
Figure 9e) and established a theoretical model for dendrite nucleation and growth. They further proposed that enhancing the local strength of the electrolyte, reducing defect sizes within the electrolyte, and minimizing isolated defects are effective strategies for suppressing dendrite formation.
6 Conclusion and Outlook
In-depth research into the interfacial dynamic processes in electrochemical reactions is of great significance for understanding reaction mechanisms and designing high-performance material systems. The development of advanced *in situ* characterization techniques serves as an important foundation for elucidating interfacial electrochemical mechanisms. This article presents a systematic review focusing on electrochemical interfacial processes and characterization from the perspective of condensed-phase chemistry. It introduces the basic components of electrochemical systems, such as electrodes and electrolytes, and summarizes the characteristics of electrochemical interfacial systems based on condensed-phase chemical theory. The article also systematically discusses various methods and techniques for characterizing electrochemical interfaces and briefly outlines the relationship between interfacial processes and system performance. This work contributes to a multi-faceted understanding of electrochemical interfacial processes, characterization, and regulation principles, providing valuable insights for the establishment and advancement of condensed-phase chemical systems.
With the establishment of various electrochemical models and theories, the advancement of sophisticated characterization techniques, and the innovation of control strategies, electrochemical surface/interface research has made significant progress. The proposal and development of the “condensed-phase chemistry” framework have provided a new perspective for in-depth understanding of electrochemical surface/interface processes and mechanisms. As two highly theoretical and interdisciplinary fields, electrochemistry and interface science present the following issues that warrant further attention. 1) Multiscale studies on the theory and mechanisms of electrochemical interfacial reactions under nonequilibrium conditions and extreme operating environments. Efficient and stable electrochemical reactions are fundamental to the practical application of electrochemical devices; however, they also pose challenges such as high current densities, prolonged service life, and complex operational scenarios. Traditional electrochemical interfacial theories and mechanistic investigations are predominantly based on equilibrium or near-equilibrium conditions. Electrochemical interfacial processes far from equilibrium are characterized by rapid reaction rates, vigorous reactivity, and intricate pathways, which may differ significantly from those under near-equilibrium conditions. Moreover, the extreme operating conditions of electrochemical devices—such as high/low temperatures and high external pressures—have imposed new requirements on the development of interfacial research and characterization technologies. Therefore, it is crucial to develop high spatiotemporal resolution in situ characterization techniques capable of operating under extreme conditions, to elucidate the evolution mechanisms of condensed-phase interfaces at multiple scales, and to refine and establish nonequilibrium electrochemical interfacial reaction theories, which will be of great significance for the design and construction of high-performance material systems. 2) Investigating the microscopic mechanisms of electrochemical interface modulation from a holistic condensed-phase chemical perspective. Modulating the chemical environment of electrochemical active centers, the microenvironment within the electrical double layer, and introducing external fields during electrochemical reactions have become important strategies for optimizing electrochemical performance. A thorough understanding of the differences in interfacial properties, species, and processes across diverse environments is essential for advancing interface modulation strategies. Although researchers have conducted in-depth mechanistic studies on electrochemical interface modulation using theoretical simulations and other computational approaches, experimental evidence at the molecular level remains limited. Furthermore, for more complex interfacial systems—such as those involving multiple active sites or coupled regulatory parameters—their complexity may exceed the applicability of conventional theoretical modeling methods. In such cases, it becomes particularly important to experimentally resolve the reaction pathways and mechanisms at the interface. Additionally, changes in system parameters within electrochemical systems can induce synergistic alterations in factors such as the properties of the reaction center and the characteristics of the electrical double layer. Therefore, it is imperative to adopt a holistic, systems-oriented approach rooted in condensed-phase chemical principles, leveraging multiscale, multimodal, and multifunctional interfacial characterization techniques to disentangle the fundamental interfacial processes from individual parameters. Such innovative research paradigms will facilitate the precise establishment of relationships between interfacial modulation and electrochemical performance at the microscopic level.
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