Design Strategies and in situ Infrared, Raman, and X‐ray Absorption Spectroscopy Techniques Insight into the Electrocatalysts of Hydrogen Energy System

The challenging sluggish reaction kinetics of hydrogen energy‐related electrocatalysis can be overcome via exploring electrocatalysts with high‐efficient activity and long‐term durability. However, the deficiency of comprehensive and in‐depth understanding of the evolution of the electrocatalysts, nature of active centers, intermediate species absorbed in the electrocatalysts, and the reaction pathway during the electrocatalytic processes seriously limits the elucidation of the composition/structure–activity relationship of electrocatalysts. To this end, plenty of powerful in situ techniques that can provide atomic/molecular information are employed to bridge the understanding of fundamental mechanisms to the practical development of electrocatalysts. This review summarizes design strategies based on composition regulation and morphology design for tuning the electronic/geometric structures of electrocatalytic materials with improved activity and stability. Moreover, the recent application of in situ infrared, Raman, and X‐ray absorption spectroscopy is elaborated with emphases on tracking the dynamic evolution during electrocatalysis and building a link between the composition/structure and activity of electrocatalysts. Finally, the current challenges and future perspectives for in situ monitoring techniques to gain an understanding more deeply and comprehensively in the hydrogen energy‐related electrocatalysis are proposed. This review provides insights into the rational optimization of electrocatalysts and inspire the unraveling mechanism of the enhanced electrocatalytic performance in future research.


Introduction
Currently, not only social and economic development but also the promotion of industrial civilization is closely related to energy. [1] The overexploitation and consumption of conventional fossil fuels such as oil, coal, natural gas, and so on make them close to exhaustion. [2] To a certain extent, it has triggered energy competition between countries in the world. Meanwhile, the environmental pollution caused by the use of fossil fuels has also seriously damaged human living conditions and even health. [3] Therefore, it is vital to build clean and renewable energy systems to promote the sustainable development of the social economy and the harmonious development of human and nature. Due to the advantages of sustainability, renewability, pollution-free combustion products, and high combustion calorific value, hydrogen energy occupies an important position in the world energy industry today and even in the future, [4,5] and is considered a major energy source that is expected to replace fossil fuels to alleviate environmental issues and energy crisis.
The challenging sluggish reaction kinetics of hydrogen energy-related electrocatalysis can be overcome via exploring electrocatalysts with highefficient activity and long-term durability. However, the deficiency of comprehensive and in-depth understanding of the evolution of the electrocatalysts, nature of active centers, intermediate species absorbed in the electrocatalysts, and the reaction pathway during the electrocatalytic processes seriously limits the elucidation of the composition/structure-activity relationship of electrocatalysts. To this end, plenty of powerful in situ techniques that can provide atomic/molecular information are employed to bridge the understanding of fundamental mechanisms to the practical development of electrocatalysts. This review summarizes design strategies based on composition regulation and morphology design for tuning the electronic/geometric structures of electrocatalytic materials with improved activity and stability. Moreover, the recent application of in situ infrared, Raman, and X-ray absorption spectroscopy is elaborated with emphases on tracking the dynamic evolution during electrocatalysis and building a link between the composition/structure and activity of electrocatalysts. Finally, the current challenges and future perspectives for in situ monitoring techniques to gain an understanding more deeply and comprehensively in the hydrogen energy-related electrocatalysis are proposed. This review provides insights into the rational optimization of electrocatalysts and inspire the unraveling mechanism of the enhanced electrocatalytic performance in future research.
Hydrogen oxygen fuel cells and water electrolysis are two important systems to realize hydrogen energy recycling. [6] Specifically, a fuel cell is composed of two half-reactions: oxygen reduction reaction (ORR: 1/2O 2 þ 2 H þ þ 2e À ! H 2 O) at the cathode and hydrogen oxidation reaction (HOR: H 2 ! 2 H þ þ 2e À ) at the anode while electrocatalytic water splitting consists of the hydrogen evolution reaction (HER: 2 H þ þ 2e À ! H 2 (acid); 2H 2 O þ 2e À ! H 2 þ 2OH À (alkaline)) at the cathode and oxygen evolution reaction (OER: H 2 O ! 1/2O 2 þ 2 H þ þ 2e À (acid); 2OH À ! 1/2O 2 þ H 2 O þ 2e À (alkaline)) at the anode. [7][8][9] In a word, hydrogen produced by HER can provide the source for HOR, so as to realize the hydrogen energy cycle (Figure 1). However, the inevitable overpotential in the reaction process greatly limits the electrolysis efficiency. Especially, the sluggish kinetics of the multi-electron transfer ORR and OER process place restrictions on the overall energy conversion efficiency of the corresponding energy devices. [10][11][12][13] Noble-metal-based materials such as Pt-based compounds are the advanced electrocatalysts to date for fuel cells and water electrolysis, but their large-scale practical commercial application is still severely restricted by the high cost and low reserve abundance. [14,15] Therefore, it is rather urgent and crucial to develop higher activity and longer lifetime electrocatalysts with low noble metal usage via rational design to improve the reaction efficiency and reduce energy consumption.
Strenuous effort has been continuously invested in the design and development of electrocatalysts to improve the electrocatalytic reactions in recent years. Controllable fabrication of materials with different compositions and morphology is a considerable method to adjust the electrocatalysis performance. To be specific, the former aims to improve the intrinsic reactivity of each active site which can be called composition regulation, and the latter aims to increase the number and density of active sites in electrocatalyst which can be called the morphology design. [16] Unfortunately, the rational design and development of efficient and durable electrocatalysts are restricted by the inconclusive catalytic reaction mechanism and the ambiguous composition/ structure-activity relationship. Thus, it is necessary to understand the changeable information of electrocatalysts, interface, and intermediate species during the dynamic electrocatalytic reaction process via various in situ techniques.
To obtain such information to provide clear and targeted theoretical guidance for the development of efficient and stable electrocatalysts, two types of in situ characterization methods are widely utilized. One is mainly used to reveal the dynamic changes of the electrocatalyst itself such as phase transition and structure evolution process, for example, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICP-MS), transmission electron microscope (TEM), etc. [17][18][19] The other is spectroscopic techniques, such as sum-frequency generation (SFG), second harmonic generation (SHG), infrared spectroscopy (IR), Raman spectroscopy, X-ray absorption and spectroscopy (XAS), which can provide vibrational spectral information about surface adsorbed species and intermediate interactions to illustrate the reaction mechanism in atomic/molecular level. [20][21][22][23] The number of relevant works on the application of these in situ techniques in electrochemical reactions has increased exponentially in the past few years (Figure 2a).
Combined with the research background and experience of our group in in situ surface-enhanced Raman spectroscopy, we mainly summarize the second kind of technique mentioned earlier. Herein, we mainly summarized recent progress in the strategies for electrocatalysts design to improve the performance. Enormous focus is put on how advanced in situ characterization techniques reveal the reaction mechanism and establish the composition/structure-activity relationship between electrocatalysts and performance. We begin with a concise introduction encompassing the significance of hydrogen economy, the strategies for electrocatalyst performance optimization, and the importance of in situ characterization approaches for electrocatalytic reaction. Next, we discussed the basic principle, instrumentation, and electrochemical cell for in situ techniques. Then, we review recent advances in the application of in situ characterization methods on energy conversion systems. We described the various approaches of composition regulation and morphology  design for performance optimization, the advantages of each type, and the in situ characterizations on them during the electrochemical reaction process to reveal the mechanism for the enhanced activity in Section 3. Last but not least is our summary on the development of in situ characterization techniques on energy conversion systems and the perspectives/prospects, that is, it is expected to elucidate the reaction mechanism even in higher spatial resolution/higher temporal resolution/extreme environments at the atomic/molecular level and establish a composition/structure-activity relationship to provide guidelines for developing electrocatalysts with superior performance.

In situ IR, Raman, and XAS Characterization Techniques
To understand the correlation between composition/structure and performance of electrocatalysts, enormous works with emphases on the pre-/post-reaction states and the resulting electrocatalytic performance have been carried out using ex situ characterization. The results of unilateral and static analysis modes are not convincing because there are short-lived and dynamic-changeable intermediates during the electrocatalytic reaction process as well as the preparation of the electrocatalysts after the electrochemical test may make an impact on the results. Fortunately, in situ techniques that can avoid these problems and provide authentic real-time information about the key intermediate species, the electrocatalysts surface, and the material structure at the interface during the reaction process have been developed. The results of in situ techniques are conducive to investigate the in-depth reaction mechanisms and establish the composition/structure-activity relationship, as well as hold the key to rational design and development of electrocatalysts.
To reflect the information of catalysts at the reaction conditions in practical devices, it is crucial to design and develop electrochemical in situ cells or special devices to mimic the real working conditions of the catalysts for in situ measurement. Generally speaking, the electrochemical in situ cells are home-made Reproduced with permission. [20] Copyright 2022, Wiley-VCH GmbH. b,c) Schematic illustrations of electrochemical in situ cells for infrared (IR). Reproduced with permission. [24] Copyright 2020, Elsevier. Reproduced with permission. [25] Copyright 1990, Federation of European Biochemical Societies. d) Schematic diagrams of electrochemical in situ cells for surface-enhanced Raman spectroscopy. Reproduced with permission. [26] Copyright 2021, Annual Reviews. e) A schematic diagram of the custom-designed in situ X-ray absorption spectroscopy (XAS) cell. f ) Pictures of the photo-electrochemical flow-cell developed. Reproduced with permission. [27] Copyright 2017, Elsevier.
depending on different reactions, sample types, or techniques. Thus, principles, features, and electrochemical in situ cells of IR, Raman, and XAS are introduced in the following section.

Brief Introduction of IR Spectroscopy
IR spectrum, also named molecular vibrational spectrum or vibrational rotation spectrum, is a technique using the absorption of IR radiation by molecules. Specifically, molecules selectively absorb a photon in the near-or mid-infrared spectrum, causing the transition of vibrational and rotational energy levels. [28] It is connected with the changes in the dipole moment of the lattice or molecule. [29,30] Benefiting from improved sensitivity, the multiplex measurement, and the high throughput of Fourier transform infrared spectrometers (FTIR), the application of infrared spectroscopy in electrocatalysis become more widely. In situ electrochemical infrared spectroscopy (EIRS) which was first reported by Bewick and coworkers in the 1980s, [31] which not only can achieve the identification and analysis of composition, groups, and molecular structure of the electrode materials that are involved in the electrocatalytic reaction, but also can acquire the information of molecular, adsorbate, and key intermediate species during the dynamic process under various reaction temperatures, pressures, and atmospheres with FTIR spectrometer, providing an authentic and objective experimental foundation to understand the electrocatalytic reaction mechanism and the nature of the active center at the molecular level. The sensitivity to water makes it difficult to obtain the information of the intermediates below 1000 cm À1 by in situ IR technique. But it is more metal-universal than Raman which shows the surface enhancement effect on Pt, Pd, Ni, and so on. Additionally, the signal-to-ratios and temporal resolutions of IR are also better than Raman. [32]

Electrochemical Cells for in situ IR Spectroscopy
The design for the electrical contact between the electrochemical platform and the working electrode as well as the mass transport space between the electrolyte solution and the working electrode is critical for coupling electrocatalysis experiments and IR measurements. For this purpose, as shown in Figure 2b, the catalyst is usually composed of an ultrathin Au foil or a silver epoxy conductive paste coated with an insulating epoxy finish to connect to the potentiostat. [24] And the reference electrode is made of a narrow diameter capillary that is similar to a Luggin capillary, which is placed in the same chamber or in a separate compartment outside the cell. A schematic diagram with more detailed structural information about the electrochemical cell is shown in Figure 2c. [25] 2.2. In situ Raman Spectroscopy

Brief Introduction of Raman Spectroscopy
Different from IR spectroscopy which is based on the direct absorption of photons by vibrations, Raman originates inelastic scattering of photons caused by the vibrations of molecules or lattices. Unfortunately, limited by its low sensitivity, Raman spectroscopy could only be applied for the characterization of bulk electrocatalytic materials until the discovery of surface-enhanced Raman spectroscopy (SERS) in the 1970s. Thanks to the ultrasensitive and nondestructive characterization, SERS, as a powerful technique for molecular fingerprint recognition, has a better application prospect in in situ studying catalytic reaction process, avoiding that IR spectrum is affected by water which is not suitable for obtaining information of the key oxygen-related intermediates in the low wavenumber region. [33] Nevertheless, a strong SERS effect was only observed on certain rough-surfaced metals such as Au, Ag, and Cu, which seriously restricts the research and application of the transition metal with high electrocatalytic activity. To scale out the application of this technology in the electrocatalytic field, core-shell nanoparticle-enhanced Raman spectroscopy, such as "borrowing strategy"(Au/Ag/ Cu@TM, TM¼transition metal, such as Pt, Pd, Co, and Ni), [34] shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) [35] as well as SHINERS-satellites [36] were developed by our group to enlarge the weak Raman signal of absorbed intermediates on transition metal shell, single crystal surface, or practical nanocatalyst surface. These strategies provide a whole string of information about the compositional and structural alteration of intermediate species and/or electrocatalysts during the realtime electrochemical process to elucidate the underlying reaction mechanism.
Au/Ag/Cu@TM nanoparticles were synthesized by Au/Ag/ Cu nanoparticles coated with ultra-thin shells of various transition metals. [37] In such a nanoparticle, Au/Ag/Cu act as cores that can generate strong electromagnetic fields around them to amplify the Raman signal (10 4 -10 5 ) and the transition metal act as the shells that show the catalytic activity. And the enhancement effect of nanoparticles attenuates exponentially as the shell thickness increases. The thinner the shells, the stronger the SERS, optimally less than 3 nm. Despite all this, it cannot be denied there is an inevitable strain effect and electronic effect between the Au core and the TM shell to a certain degree, which may influence the performance of the TM shell. SHINERS, which consists of the Au core and the pinhole-free silica shell (Au@SiO 2 nanoparticles), is a promising technology that can well solve the problem of material universality in SERS. [38,39] Notably, in this strategy, the Au internal cores serve as signal amplifiers to enhance weak Raman signal and the pinhole-free silica shells can effectively isolate the contact between the Au core and the outside environment to eliminate the interference. Besides, the SHINERS-satellite technique, that is, the shell-isolated nanoparticles surrounded by the practical nanocatalysts, which not only insulates the influence of the internal Au core, but also exhibits a strong Raman signal enhancement for the catalytic nanoparticles. [40] Generally speaking, the Raman peak intensity can be affected by the polarizability of the phonons or the content of certain structures; the peak width would indicate the degrees of structural disorder; peak shifts are often related to the changeable bond strength or the variable vibration phonons.

Electrochemical Cells for in situ Raman Spectroscopy
To realize the real-time in situ Raman detection of catalytic materials during dynamic electrocatalytic reactions, we designed special electrochemical cells to meet the requirements of different reaction tests. As shown in Figure 2d, it is our most commonly used electrochemical cell, which is suitable for the reaction of a three-electrode system. [26] Pt wire is usually used as the counter electrode. Silver/silver chloride electrode, saturated calomel electrode, mercuric oxide electrode, etc. are the reference electrodes. The glassy carbon or metal electrode involving Au, Ag, and Pt with the catalysts spread is the working electrode.

Brief Introduction of XAS
XAS measurement is an inner-shell spectroscopy that can provide element-specific information on the coordination environment and electronic structures of electrocatalytic materials by collecting interactive information between the incoming X-rays and the core electrons within atoms. The resulting XAS spectra can be divided into X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The spectral region of the former is about 20-50 eV around the absorption edge and it is the closest region to the absorption edge with a characteristic structure of the electronic structure and local symmetry of the absorbing atom. [41] It can effectively reflect elementspecific information on the electronic structure, valence state, bonding geometry, and the density of unoccupied states on the target element. The EXAFS with the region up to 1000 eV after the absorption edge originates from the oscillation in the absorbance induced by the interference between the scattered wave of the photoelectron and the backscattered wave of the nearest neighbor atoms. [42] It can identify the local structural information at the atomic scale, for example, coordination number and bond distances of the absorbing atom. Benefiting from these advantages, in situ XAS technique has been extensively utilized to monitor the dynamic reaction process, helping us to identify the real active sites in the electrode materials and reveal the reaction mechanism at the molecular/atomic level.

Electrochemical Cells for in situ XAS
The development of a special photoelectrochemical cell is vital to solving the challenges for in situ XAS applied to electrocatalysis. Figure 2e shows a cartoon illustration of a typical in situ XAS cell, which provides us with an intuitive impression including the position of working electrode, electrolyte, and light incidence. Furthermore, a physical diagram of the electrolytic cell suitable for the three-electrode system was illustrated in Figure 2f. [27] The positions of the working electrode, reference electrode, and counter electrode are clearly visible.

Composition Regulation on and in situ Characterization of Electrocatalysts
To obtain superior intrinsic electrocatalytic activity, it is an efficient strategy to regulate the composition of electrocatalysts. Currently, heteroatom doping, vacancy defect engineering, multi-metal, and heterostructure design are popular strategies for composition modulation, leading to improved electrocatalytic performance through adjusting the internal electronic structure of the electrocatalyst, optimizing the intermediate adsorption/ desorption, and reducing the energy barrier.

Heteroatom Doping
Heteroatom doping can be divided into two categories: metallic atom doping and nonmetallic atom doping. The integration of metal elements allows to increase the number of active sites, optimize the binding energy of reaction intermediate species, and enhance the intrinsic catalytic activity. To this end, our group [43] constructed Au@PtNi core-shell nanoparticles (NPs) via a "borrowing" strategy and detected the ORR process by utilizing in situ enhanced Raman spectroscopy (Figure 3a). It is found that the ORR performance of the Au@Pt NPs can be optimized by introducing and adjusting the amount of Ni. We unexpectedly observed an OOH* signal (728 cm À1 ) on Au@PtNi NPs and the *OOH frequency exhibits a red-shift compared with Au@Pt NPs (731 cm À1 ) ( Figure 3b). Furthermore, as the plot of ΔE 1/2 and Raman peak shifts for different Ni loading content shown in Figure 3c, it is discovered that the ΔE 1/2 of these four samples positively shifts along with an obvious increase of Raman shift. Thus, we bridge the gap between the component and ORR activity through the direct intermediates evidence, that is, the improved Ni concentration causes more red-shift of the Raman frequency of OOH*, and the lower frequency represents the more easily broken O─O, which can accelerate the subsequent reaction steps and result in an advanced ORR activity. Zhang et al. [7] constructed Fe doping oxide/sulfide heterojunction (Fe─NiO/NiS 2 ) and NiO/NiS 2 as the OER electrocatalysts to reveal the effect of Fe doping via in situ enhanced Raman and in situ attenuated total reflection infrared (ATR-IR). As the results shown in Figure 3d,f, the peak of NiS 2 (278 cm À1 ) decreased at 1.16 V in NiO/NiS 2 while 1.36 V in Fe─NiO/ NiS 2 , suggesting that the corrosion of S was alleviated after the doping of Fe. And the peaks observed at 1.36 V in NiO/ NiS 2 are assigned to Ni─O vibrations in NiOOH which is considered the active phase for the OER. Therefore, it can be concluded that the doping Fe not only stabilizes M─S bonds against self-oxidation but also promotes the accumulation of OH on the electrocatalyst surface to form high active NiOOH at the earlier applied potential. Besides, in situ ATR-IR spectra find the surface-adsorbed OOH intermediates (Figure 3e,g, 1212 cm À1 is assigned to the O─O stretching mode in OOH). And the OOH ad species emerged at the open-circuit voltage (OCV) for Fe─NiO/NiS 2 while it appeared at 1.32 V for NiO/NiS 2 . These further illustrated that Fe doping is advantageous to the formation of active specie for OER and the enhanced activity of electrocatalyst.
Shao's group [10] prepared a Ru-modified Pt electrode by electro-assisted deposition of Pt rotating disk electrode in RuCl 3 solution. They find the Pt─H and Ru─H directly by utilizing in situ surface-enhanced infrared absorption spectroscopy with the attenuated total reflection (ATR-SEIRAS), which can hardly be obtained via other ex situ technologies. As shown in Figure 4a, the increase of the Ru coverage on the Pt surface gradually improves the intensity of the Ru─H and boosts the electrocatalytic activity. Furthermore, based on the density functional theory (DFT) calculations, such enhanced performance of Ru-modified Pt can be attributed to the strain effect and electronic effect of Pt which strengthen the adsorption of *H Figure 3. a) Schematic illustration of in situ electrochemical surface-enhanced Raman spectroscopy (SERS) study of Au@PtNi during the oxygen reduction reaction (ORR) process. b) In situ enhanced Raman spectra of Au@Pt and Au@PtNi nanoparticles (NPs). c) Plot of Raman peak shifts and ΔE 1/2 with different Ni loading contents. Reproduced with permission. [43] Copyright 2021, American Chemical Society. In situ enhanced Raman spectra of: d) Fe─NiO/NiS 2 and f ) NiO/NiS 2 . In situ attenuated total reflection infrared (ATR-IR) spectra of: e) Fe─NiO/NiS 2 and g) NiO/NiS 2 . Reproduced with permission. [7] Copyright 2022, Wiley-VCH GmbH.
www.advancedsciencenews.com www.small-structures.com and *OH, resulting in a lower energy barrier of the water dissociation which is the rate-determining Volmer step (Figure 4b).
Hung and coworkers [44] used in situ high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) to explore the mechanism of the improved OER performance for iron-doped spinel. As the results revealed, Co ions are the real active centers, while Fe ions play key roles in stabilizing the Co with a higher valent state which allows a stable hydroxide intermediate of reactant and enhanced activity. Benefiting from the greater electronegativity of anions than that of metals, the introduction of anions (such as P, N, and S) into the host materials can be more conducive to regulating the electronic structure, resulting in an optimal catalytic activity. [45][46][47] For instance, N─Ni 3 S 2 /NF, P─Fe 3 O 4 and N─Ni all displayed much more superior electrocatalytic performance than pristine materials. [48][49][50] Among these, it is established that P incorporation plays an important role in boosting catalytic activity due to the higher electron-donating ability and the larger atomic size. As reported by Lu et al., [51] they introduced P into the commercial platinum-carbon surface (Figure 4c) and found a sevenfold increase in its mass activity, reaching 1.00 mA μg Pt À1 . They carried out in situ electrochemical FTIR studies using adsorbed CO as a molecular probe to reveal the chemical properties of the electrocatalyst surface. The peak at 2065 cm À1 observed in Figure 4d is attributed to the linearly adsorbed CO on Pt, and an obvious blue-shift of the *CO frequency was identified after the introduction of P. The higher frequency represents the strong C─O, leading to a weaker CO chemisorption energy and enhanced ORR activity. Combined with the results of DFT calculations, the structure-activity relationship was established, that is, the P doping brings the distortion of the Pt lattice which reduces the d-band of Pt and results in a reduced chemisorption energy of CO with an optimal binding energy of OH, leading to an enhanced ORR activity.

Vacancy Defect
Similar to the effect of the introduction of heteroatoms, the vacancy defects in the electrocatalytic materials can also tune the surface electronic structure with charge and spin densities as well as bring about structural distortion, thereby enhancing the electrocatalytic activity. [52] Cai and coworkers introduced massive vacancy defects in ultrathin Co 3 O 4 nanosheets via a mild reduction condition. [53] And the as-prepared electrode material displayed remarkably enhanced OER performance. Yang's team [54] recently reported Co 3 O 4Àx /N-doped graphene (Co 3 O 4Àx /NG) with oxygen vacancy which effectively induced a modified charge density and optimize the intermediates adsorption energies of Co sites, significantly improving the intrinsic OER and ORR activity. Nevertheless, their specific role in modulating the surface reconstruction of electrocatalysts during the electrocatalytic reaction process remains ambiguous. To this end, Wu and coworkers [55] reported NiFe-LDH nanosheets with cationic vacancy defects, and shed light on the surface remodeling behaviors of the catalyst caused by defects via in situ Raman ( Figure 5). As the voltage increases, the various cation defects in NiFe-LDH are instrumental in converting local crystalline Ni(OH) x species to their defective status and eventually form the local NiOOH species which correspond to the formation of the vacancy defect V MOH-H . They believed that the evolution of cationic defects (V M ! V MOH ! V MOH-H ) is accompanied by . Reproduced with permission. [10] Copyright 2021, Nature. c) Schematic illustration of Fe─NiO/NiS 2 synthesis. d) In situ FTIR spectra of CO adsorbed on Pt/C and PNS-Pt/C at 0.10 V(0.1 M HClO 4 ). Reproduced with permission. [51] Copyright 2021, American Chemical Society. an essential motif of the surface restructuration process (crystalline Ni(OH) x ! disordered Ni(OH) x ! NiOOH).

Multi-Metal
Taking ORR and OER as examples, the construction of multimetal electrocatalysts can effectively speed up their reaction kinetics and improve the electrocatalytic activity. As for ORR, Pt is still considered the most efficient electrocatalyst currently, but its practical application is limited by its high-cost and low reserve. [56][57][58] One of the most effective ways to promote ORR activity with low Pt usage is to introduce other TMs into Pt which brings the strain effect and electronic effect, effectively optimizing the adsorption energy of intermediate species at active sites and then enhancing the electrocatalytic activity. It is crucial for developing such multi-metal electrocatalysts to elucidate the specific reaction mechanism and uncover the role of the second component via in situ techniques. To this end, our group detected the key intermediate species of ORR via the "borrowing" strategy and "SHINERS-states" strategy by utilizing in situ SERS. We successfully constructed layercontrolled Au@Pd@Pt core-shell nanoparticles which show superior ORR activity and durability than Au@Pt NPs. [59] In the results of XPS (Figure 6a,b), the binding energy of Pt 4f and Pd 3d in Au@Pd@Pt NPs presents upshift and downshift, respectively, which explicit that the electrons migrate from Pt to Pd and the electron-scarce Pt will be easier to adsorb O, resulting in an enhanced Pt─OOH. As displayed in Figure 6c, the *OOH intermediate (723 cm À1 ) was observed and its frequency exhibits a different degree red-shift via controllably adjusting the ratio of Pd/Pt (Figure 6d), which leads to a weaker O─O in *OOH. Based on the aforementioned results, we elucidated the composition/ structure-performance relationship at the atomic/molecular level, that is, such enhanced ORR performance and stability can be ascribed to the strengthened Pt─O and weakened O─O in Pt─OOH resulted from the strain and electronic effect after introducing Pd. We used the "SHINERS-satellite" strategy to study the ORR process of bimetallic Pt 3 Co catalysts. [60] We successfully captured direct spectroscopic evidence of *OOH on the surface of Pt 3 Co in both acidic and basic media, an important intermediate during the ORR process, revealing the association mechanism involves *OOH. Combined with DFT calculations, we speculated that the strain effect in Pt 3 Co results in weakened adsorption of surface *O and improved the catalytic activity of ORR.
For OER, it is usually agreed that the coexistence of Fe, Co, or Ni is required to obtain high catalytic activity. However, the identification of the active center and the structural change of the active phase are still debated, and it is essential to focus on the energetics of OER intermediates. As reported by Friebel et al., [61] to find out whether Ni or Fe is the real active site in Ni 1Àx Fe x OOH, they employed in situ XAS to observe the OER process on Ni 1Àx Fe x OOH. It is established that Fe 3þ in Ni 1Àx Fe x OOH occupies octahedral sites with unusually short Fe─O bond distances because of the edge-share with surrounding [NiO 6 ] octahedra, which leads to near-optimal adsorption energies between Fe sites and OER intermediates. While the Ni sites in Ni 1Àx Fe x OOH are not active sites for the oxidation of water. Xie's group [62] prepared bifunctional nanoparticles consisting of a plasmonic Au core and high OER performance Ni 3 FeO x shell and the role of the NiFe dual-catalytic center  Figure 7c,d, the two peaks of O─O À can be also observed in the former while not obtained in the latter, suggesting that Fe atoms rather than Ni atoms are the real active sites for adsorbing the O─O À intermediate. Thus, it is reasonable to conclude that Fe atoms are the authentic catalytic sites for the oxidation of initial OH À to O─O À while the Ni III converting sites play the role of electron acceptor to elevate the formation of O 2 . Apart from the in situ XAS and in situ Raman techniques, in situ IR techniques is popular to investigate the OER process on electrocatalysts. As reported by Cheng et al., [63] they employed in situ synchrotron radiation Fourier transform infrared (SR-FTIR) to reveal the improved ORR and OER activity caused by the strain effect in NiFe metal-organic frameworks (NiFe MOFs). As the potential decreasing (increasing), an absorption band at 1048 cm À1 was observed which is ascribed to the superoxide *OOH species on the high-valent Ni 4þ sites (Figure 7e), suggesting a 4e À pathway for ORR (OER).
Moreover, more and more literatures have reported that the application of multi-metal electrocatalysts in HOR achieved superior activity following a bifunctional mechanism. Generally, Pt is considered a good site for the adsorption of the reaction hydrogen intermediates (H ad ) due to the optimal interaction of H ad (near-zero free energy of adsorption). While its performance is limited by the inefficient adsorption of OH ad in alkaline media. [64,65] Therefore, it is necessary to construct multi-metal electrocatalysts via inducing oxyphilic metals, providing not only *H but also *OH adsorption sites which are promising to lay the foundation for improving HOR kinetics. For example, Markovic's group proposed that the introduction of Ru atoms as the oxyphilic sites in PtRu facilitates the adsorption of OH ad , significantly improving the HOR activity. [66] A similar conclusion was also obtained by our team. We explored the HOR process on the Au@PtRu by in situ SERS and obtained direct spectroscopic evidence for the presence of OH ad intermediate on the catalyst surface. [67] As per the SERS results of Au@PtRu shown in Figure 8a,b, the peak at 712 cm À1 shifts to 667 cm À1 in the deuterium isotope experiment is assigned to the OH ad . We further found that the frequency of OH ad displayed a blue-shift (from 709 to 717 cm À1 ) with the increase of Ru content which well matches the electrochemical results (the increase of Ru content effectively improves the HOR activity) (Figure 8c,d). Combined with theoretical calculations and other experimental characterizations, a conversion process from H 2 to H 2 O on the PtRu surface was illustrated in Figure 8e. We believed that the addition of ruthenium promoted the adsorption  HClO 4 ). d) Plot of Raman peak shifts and mass activity (0.8 V) of Au@Pd(x nm)@Pt NPs. Reproduced with permission. [59] Copyright 2022, American Chemical Society.
www.advancedsciencenews.com www.small-structures.com of hydroxyl groups, which is responsible for the increase in HOR activity. The evidence of OH ad intermediate on the Pt─Ru/C surface during the HOR process was also obtained via in situ XAS which is reported by Jia's group. [68] As seen in Figure 8f,g, the XANES of Ru K-edge for both PtRu/C and Ru/C displays an upshift with the increase of potential from 0.05 to 0.24 V. And the Δμ derived from the XANES spectra correspond well to the theoretic results by replacing the H ad by OH ad on the Ru 6 cluster model in Figure 8h, which confirms the dynamic existence of OH ad on Ru sites. All these findings present mechanistic references and guidelines for the rational selection and construction of HOR electrocatalysts. Recently, various multi-metal HOR catalysts have developed based on the bifunctional mechanism, such as Ru─Ni, [69] PtNi, [70] D-PtCu 3 /C, [71] and so on.

Heterostructure
Enormous effort is focused on promoting the electrocatalytic performance by introducing a hetero-junction or heterointerface into the electrocatalytic materials, which can change the electronic structure to obtain optimal surface properties (adsorption/desorption free energy). [72,73] In particular, heterostructure design is a very widespread and effective surface engineering strategy to optimize the HER performance due to the synergistic effect between different individual components that can hasten water dissociation and improve the HER kinetics.
To design an efficient electrocatalyst with superior HER performance in alkaline media, both mass transfer and water splitting must be considered. For instance, it has been reported by abundant literature that MoS 2 exhibits a high HER activity caused by a moderate hydrogen absorption energy at its edges in acidic media. It can be confirmed by the related in situ Raman works from our group. [74] We designed a tunable-size single-layer MoS 2 -coated polyhedral Ag core-shell heterostructure (Ag@MoS 2 ) for HER via a wet-chemical synthetic strategy. As the illustration shown in Figure 9a, in situ electrochemical-SERS was employed to monitor the HER process on Ag@MoS 2 . We draw a conclusion that the real electrocatalytic active site is the S atom which was revealed by in situ SERS spectroscopic evidence, that is, the S-H bonding was observed on the MoS 2 surface during the HER process (Figure 9b). Deng et al. [75] also achieved the direct spectroscopic evidence of S─H in MoS x (2530 cm À1 ) through in situ Raman technique and prove that the real active site in MoS x is the S atom (Figure 9c,d). MoS 2 shows excellent HER performance in acidic media due to the abundant S─H, but low activity in alkaline media which is restricted by the sluggish kinetics of the water dissociation step. To enhance the activity of MoS 2 in alkaline media, it is an efficient strategy to introduce a component that is instrumental in breaking the O─H bond and accelerating water dissociation. In response to this view, Luo and coworkers [76] designed a catalyst consisting of Co(OH) 2 nanoparticles confined in few-layer MoS 2 nanosheets. Profiting from the synergistic effect between the Co(OH) 2 nanoparticles and MoS 2 nanosheets (the former speed up water dissociation and the latter promote hydrogen generation), the as-prepared electrode exhibits excellent HER activity.

Surface Reconfiguration
The enhanced performance of heterostructures reported in many works is attributed to the electronic modulation and interfacial synergistic effect simply. However, at present, more and more Reproduced with permission. [62] Copyright 2015, American Chemical Society. e) In situ synchrotron radiation Fourier transform infrared (SR-FTIR) spectroscopy measurements for the NiFe metal-organic frameworks (MOFs). Reproduced with permission. [63] Copyright 2019, Nature. works reported that the state of the electrocatalyst surface after the electrocatalytic reaction is different from that before the reaction which may be the key to the improved performance. Gao's group [77] confirmed this conjecture by investigating the surface reconfiguration of Mo 2 C─MoO x on carbon cloth (Mo 2 C─MoO x / CC) during the HER process in 0.1 M HClO 4 using in situ Raman and DFT calculation. The peak intensity of MoO 3 (237, 820, 996 cm À1 ) begins to decline and even disappears and new peaks associated with MoO 2 (192, 340, 495, and 745 cm À1 ) appear with increasing the cycles of linear sweep voltammetry (Figure 10a), which demonstrates that Mo VI in Mo 2 C─MoO x /CC is in situ reduced to Mo IV during HER process. Combined with the results of electrochemical tests in Figure 10b, it is obvious that in situ surface reconfiguration of MoO x on Mo 2 C─MoO x /CC heterostructures is responsible for the great promotion of performance. Furthermore, the results of the DFT calculation revealed that the reduced surface with terminal Mo¼O moieties can effectively promote the HER kinetics limited by H* desorption because the ΔG H* on bare Mo 2 C can be adjusted close to a thermodynamic neutral value (Figure 10c). In the study of Yano's group, [78] they used in situ XAS to investigate the HER process on amorphous molybdenum sulfide (MoS x ) surfaces. The evidence for the direct involvement of disulfide units in MoS x was observed which suggest the surface reconfiguration from MoS 2 to MoS x . Such component structure reconfiguration is also widespread during the OER process. It is vital to conduct in-depth research on this using various in situ characterization techniques to identify the real active center of the catalyst. Fan et al. [79] performed in situ FTIR to deeply explore the transition process of CoS x during OER measurement. Combined with in situ TEM and other characterizations, they identified that CoOOH after the reconfiguration is the active specie for the electrocatalytic reaction (Figure 10d,e).

Morphology Design and in situ Characterization of Electrocatalysts
Morphology structural engineering is an important means to effectively modulate the electrocatalytic performance. The rational design of electrocatalyst morphology can not only facilitate the electrocatalytic activity by improving the sluggish reaction The inset is an enlarged area near the 0 V region. e) Schematic diagram of the HOR processes on PtRus. Reproduced with permission. [67] Copyright 2022, American Chemical Society. Ru K-edge XANES spectra of: f ) Ru/C and g) PtRu/C. h) Experimental Δμ signals derived from the XANES spectra displayed in f ) and g), and the theoretical Δμ with the model clusters given in the inset. Reproduced with permission. [68] Copyright 2017 Wiley-VCH GmbH.
www.advancedsciencenews.com www.small-structures.com kinetics, but also elevate the density of accessible active sites, enhance the robust stability and accelerate the mass transport. [80] In the following chapters, we will focus on some design principles of electrocatalyst morphology and the recent application of in situ characterization techniques in elucidating structure-activity relationships. . Reproduced with permission. [74] Copyright 2020, American Chemical Society. c) Enhanced Raman spectra before and during HER on MoS x . d) In situ enhanced Raman spectra of HER on MoS x . Reproduced with permission. [75] Copyright 2016, American Chemical Society. Reproduced with permission. [77] Copyright 2019, Wiley-VCH GmbH. d), e) Selected area electron diffraction (SAED) patterns and FTIR for the evolution of CoS x during the OER test. Reproduced with permission. [79] Copyright 2018, American Chemical Society.

Single Crystal
Research on electrocatalytic reaction processes at single crystal owning well-defined and atomically flat surfaces is in a role that plays the bridge between theoretical simulations and experimental observations, as well as provides guiding principles for the design and development of practical electrocatalysts. Single crystal with different coordination environments causes strong or weaken interactions with reaction intermediate species, resulting in a diverse electrocatalytic activity. [81][82][83] Based on this, our group has carried out some related work. It is found that the ORR performance order of different low-index single crystal surfaces of Pt is: (111) > (110) > (100). [84] To understand the mechanism of the diversity in ORR performance, we utilize electrochemical in situ SHINERS to investigate the ORR process on Pt(hkl) surface. No peak but at 732 cm À1 was observed in Pt(111) which is attributed to the *OOH species (Figure 11a), while there is just *OH species that occurred in Pt(100) (Figure 11b). It is revealed that the ORR process on Pt(111) is involving *OOH species and the accumulation of *OH species on Pt(100) is unfavorable to the reaction process (Figure 11c). It can even be speculated that the exposure of (100) in Pt-based catalysts is not conducive to promoting the electrocatalytic reaction. Besides, we explored the difference between the low-index and the high-index Pt(hkl). [85] We found out the ORR performance of Pt(211) is superior to the Pt(311). Based on the detailed analysis of in situ Raman spectroscopy and DFT calculations, it can be found that the adsorption free energy of *OOH intermediate specie at the Pt(311) surface is higher than at Pt(211) (Figure 11d,e), which has a significant impact on the results of ORR performance.

Single Atom
Owing to their promising activity and selectivity, single-atom electrocatalysts set off an intense upsurge of research. [86,87] There are kinds of types of single-atom materials that have been reported to date, such as Fe-based, Co-based, Ni-based, and Mn-based single-atom electrocatalysts. [88,89] And it is necessary and important to understand its dynamics during the catalytic process so as to provide guidance for the preparation of highefficiency electrocatalysts. XAS is not only a key technology for characterizing the single-atom catalytic materials, but also plays a crucial role in in situ studies for the electrocatalytic reactions of single-atom electrocatalysts. Fang et al. [90] recently reported single-atom Pt electrocatalyst for HER which possesses an exceptionally superior activity with only 19 and 46 mV overpotential at 10 mA cm À2 in acid and alkaline media, respectively (Figure 12a). They identified the general evolution of Pt single atom under the realistic condition at the atomic level by using in situ XAFS technique. As shown in Figure 12b, the intensities of white lines at ex situ, open circuit, and þ0.5 V (RHE) remain unchanged which manifests that Pt single atom is structurally stable. And the intensity of white lines decreases when the potential drops to þ0.15 V and even À0.07 V, suggesting less charge transfer and a weaker interaction between the N─C substrate and Pt. The intensities of the oscillation hump also show a decay with the down-shift of potential which implies the dynamically disordered structures around the Pt single atom. Combined with the results of theoretical simulations, they uncovered that the Pt single atom tends to be in a near-free state, which facilitates the adsorption of H 2 O in alkaline media and optimizes the adsorption free energy of H, responsible for the excellent Figure 11. In situ enhanced Raman spectra of the ORR system at: a) a Pt(111) and b) a Pt(100) electrode surface (O 2 -saturated 0.1 M HClO 4 ). c) The illustration of the ORR process on Pt(hkl) surfaces. Reproduced with permission. [84] Copyright 2019, Nature. d) Schematic of in situ electrochemical SHINERS study of the ORR reaction pathway on Pt(311) surface. e) Enhanced Raman spectra (normalized with the peak of 933 cm À1 ) of different Pt(hkl) surfaces (O 2 -saturated 0.1 M HClO 4 , at 0.8 V). Reproduced with permission. [85] Copyright 2020, American Chemical Society.  [91] used in situ XAS to identify the dynamic structure of Co single-atom catalysts (Co was immobilized in a phosphide carbonitride framework) under alkaline HER at the atomic level. They uncovered the formation process of HO─Co 1 ─N 2 and the preferred water adsorbate intermediate H 2 O─(HO─Co 1 ─N 2 ), as well as revealed that the high-valence Co sites are favorable to the improved HER activity. As the reaction mechanism proposed in Figure 12c, H 2 O adsorbed on the Co surface firstly, subsequently dissociated into adsorbed *H and *OH on the nearby N and Co, respectively. And then, the adsorbed *H reacted with the proton from another H 2 O to generate H 2 . Furthermore, Qiao's group [92] reported an atomically dispersed single-atom nickel iodide (SANi-I) electrocatalyst, and proclaimed the mechanism of the enhanced HER performance via in situ Raman spectroscopy. They obtained direct evidence of I-H ads intermediate and believed that this is responsible for the remarkable activity (Figure 12d).

Hollow Structures
The controllable construction of hollow structures is also a popular strategy for designing catalysts. Benefiting from the abundant active sites, well-defined void space, shorter diffusion pathways and efficient electronic regulation, hollow nanostructures were regarded as promising electrocatalyst candidates. [52] Inspired by the advantages of hollow nanostructures, enormous efforts have been made to design and construct high-efficiency electrocatalysts with different hollow structures. For example, as reported by Hu and coworkers, [93] hierarchical Ni─Co─P hollow nanobricks (HNBs) were successfully fabricated via using a template-engaged strategy followed by acid-alkaline sequential etching ( Figure 13a). The specific surface area of Ni─Co─P HNBs is 37.6 m 2 g À1 which is higher than that of Ni─Co─P nanosheets (29.4 m 2 g À1 ). Thanks to the unique hollow nanobrick architectures which allow abundant mass diffusion pathways and large active surface, Ni─Co─P HNBs possess much more excellent performance than nanosheets in Figure 13b. Similarly, Lou's group [94] reported that the hierarchical NiFe LDHs hollow nanoprisms perform an exceptionally enhanced OER activity due to the large surface areas in comparison with its bulk materials (Figure 13c,d).
Gong et al. [95] have employed in situ Raman characterization to verify the structural stability of the hollow structure over a wide pH range. The primary peaks of MoS 2 (387, 412 cm À1 ), MoO 2 (280, 816, 990 cm À1 ), and carbon (1347, 1572 cm À1 ) remained and their intensity of them showed negligible change during HER process at various potentials, suggesting the endurability in all-pH media (Figure 14a-c). Lima's group [96] synthesized Pt hollow nanostructure shows superior ORR Figure 12. a) LSV curves of the Pt 1 /N─C, Pt/C, and N─C framework. b) In situ XANES at different applied potentials during HER process, and the XANES data of Pt foil and PtO 2 . Inset: magnified white-line peak and post-edge XANES region. Reproduced with permission. [90] Copyright 2020, Nature. c) HER reaction pathway on HO─Co 1 /PCN in alkaline media. Reproduced with permission. [91] Copyright 2018, Nature. d) In situ enhanced Raman spectra of SANi-I. Reproduced with permission. [92] Copyright 2019, Wiley-VCH GmbH.
www.advancedsciencenews.com www.small-structures.com activity, which can be attributed to the lattice contraction induced by a hollow from the multilayer Pt shells as well as the strain effect and the ligand effect after the introduction of Ni, even than that of the state-of-the-art Pt/C. It can be seen in Figure 14d that the magnified white lines at 0.4 V for hollow@NiPt/C are higher than those for Pt/C, proving the higher density of the available states of the Pt atoms in hollow@NiPt/C. In contrast, the increase in the magnified white lines at 0.9 V compared to 0.4 V for hollow@NiPt/C are lower than for Pt/C which is related to the amount of Pt atoms and oxide formation on Pt atoms. Figure 13. a) Schematic illustration of the construction of hierarchical Ni─Co─P hollow nanobricks (HNBs). b) LSV curves of the Ni─Co─P HNBs and Ni─Co─P nanosheets for water electrocatalysis. Reproduced with permission. [93] Copyright 2018, Royal Society of Chemistry. c) Schematic diagram of the formation of Ni─Fe LDH hollow nanoprisms. d) Overpotential at 10 mA cm À2 for the Ni─Fe LDH hollow prisms and the reference samples. Reproduced with permission. [94] Copyright 2017, Wiley-VCH GmbH. Figure 14. a-c) In situ Raman spectra of catalysts in acidic, alkaline, and neutral media. Reproduced with permission. [95] Copyright 2021, Wiley-VCH GmbH. d) In situ XANES at the PtL 3 edge for the Hollow@NiPt/C and Pt/C. Reproduced with permission. [96] Copyright 2013, Elsevier.
www.advancedsciencenews.com www.small-structures.com The hollow structure and electronic modification result in a lower d-band center of Pt which would attenuate the adsorption strength of O/OH species, leading to enhanced performance.

Self-Supporting
Electrocatalysts with hollow structures guarantee lower loading, but their activity and stability will be affected by using binders to some degree. The direct growth of electrocatalytic materials based on conductive substrates, for instance, Ni foam, Cu foam, carbon paper, and carbon cloth can effectively avoid these problems, leading to a better uniformity of size, strong mechanical stability, and optimal activity. [97,98] Self-supporting is a class of promising strategies for regulating the performance of catalysts in water electrolysis. For example, Yin and coworkers [99] designed and constructed hierarchical network of NiCo 2 S 4 nanoflakes on nickel foam (NiCo 2 S 4 -NF) (Figure 15a,b). Benefiting from the binder-free electrode, the electrocatalytic activity and durability of NiCo 2 S 4 -NF are superior to those of NiCo 2 S 4 . Similarly, Wang's group [100] presented a simple one-step solvothermal method to synthesize FeCoNi hybrid nanotube arrays (FeCoNi-HNTAs) by using the ternary Fe, Co, Ni-based layered double hydroxide nanowire arrays (FeCoNi-LDH-NWAs) as template and ammonium thiomolybdate as reactants (Figure 15c).
Profiting from the advantages of multi-composition and hollow structure, that is, the synergistic effects among Fe, Co, and Ni, superb conductivity of 1 T' MoS 2 , abundant active sites as well as fast mass transfer, the derived FeCoNi-HNTAs exhibit dramatically enhanced HER performance and durability (Figure 15d). Wu et al. [101] synthesized self-supported Ni 3 S 2 /FeNi 2 S 4 nanosheets (Ni─Fe─S) on NiFe foam and adopted in situ Raman technique to real-time investigate the phase conversion during the OER process. They draw a conclusion that the formation of the Ni─OH, Ni─OOH, and Fe─OOH intermediates speeds up the sluggish kinetics and results in an exceptional catalytic activity of N─Fe─S.

Conclusion and Perspective
In the clean-energy economy, the hydrogen fuel cell and the water electrolysis are recognized as two important systems to realize hydrogen recycling without any subsidiary pollution. The development of electrocatalysts with high-efficiency and long-stability holds the key to the practical application of these two devises in energy conversion. Herein, we have systematically summarized the recent optimization principle of the electrocatalytic performance, with the emphasis on design strategies for Figure 15. a) Schematic diagram of the construction of NiCo 2 S 4 -NF. b) Electrochemical properties of NiCo 2 S 4 and NiCo 2 S 4 -NF for HER and OER in 1.0 M NaOH. Reproduced with permission. [99] Copyright 2017, Elsevier. c) Schematic diagram of the construction of FeCoNi-HNTAs. d) Polarization plots for HER processes in 1.0 M KOH media. Reproduced with permission. [100] Copyright 2018, Nature. composition and morphology of the electrocatalysts, which aim to improve the intrinsic activity of each active site and increase the density of active sites in electrocatalyst, respectively. Furthermore, we have reviewed the notable applications of in situ IR, Raman, and XAS techniques in fuel cells and water electrolysis. The application of in situ characterization techniques for detecting these reaction processes has contributed to bridging the gap between the theory and practice as well as establishing the composition/structure-activity relationship between electrocatalysts and electrocatalytic performance at the atomic/ molecular level, mainly through: 1) identifying the real active site in electrocatalysts; 2) uncovering the conversion of the electrocatalysts themselves under realistic condition; 3) revealing the reaction pathways occurring on the surface of various electrocatalysts. Currently, impressive research progresses in in situ techniques have been achieved in the field of electrocatalytic reactions. Looking toward the future for in situ characterization techniques in clean-energy conversion systems, we propose here a few following considerations worth investigating. 1) Combined applications of multiple in situ techniques. To a certain extent, the individual in situ technique can only provide limited but different aspect information about the electrocatalytic reaction. Integrating the complementary characterization techniques closely is expected to realize more comprehensive surface science studies. Take the combined application of Raman and XPS/XRD as the instance, it would provide information including the evolution of the electrocatalysts themselves and the pathway of intermediate species during the reaction process at the same time. Additionally, tip-enhanced Raman spectroscopy (TERS), which consists of Raman spectroscopy and scanning tunneling microscope/atomic force microscope, can furnish both chemical and physical information about the catalysts simultaneously. 2) Improving the sensitivity and resolution of in situ techniques. In fact, the capture of the dynamic changes during the electrocatalytic reaction process is still facing challenges. For example, the lifetime of intermediate species under realistic conditions is at the picosecond level and the content of intermediate species is extremely small, so it is difficult to capture all of them. It is vital to develop in situ techniques with higher time-resolution, spatialresolution, and surface-sensitive to study electrocatalytic systems with small variations involving angstroms-scale spatial resolution or single-atom reactions. As for Raman, the spatial resolution of TERS is much higher than SHINERS, which possesses the potential in the application of imaging. Furthermore, developing integrated techniques, for instance, combining the SHINERS with the microfluidic technique, is an efficient way to improve the temporal resolution. 3) Bridging the gap between real working devices and in situ electrochemical cells. It cannot be denied that in situ testing condition in the experiment is not completely consistent with the realistic industrial condition. And the complicated surface reaction situation due to the possible damage of electrocatalysts caused by the strong electron beams or X-rays also cannot be neglected. Thus, it is essential to optimize the real electrochemical cells to narrow the gap between realistic working conditions and in situ testing as well as minimize the damage of electrocatalysts from the operando system. For instance, developing a soft XAS with low-energy incident light can effectively offset the shortcoming of XAS (the high-energy incident light may result in a transformation or damage of catalytic material). 4) Designing and optimizing in situ techniques used under extreme reaction conditions. It will be an important step to expand the universality of the applied environment of in situ technologies for uncovering the mechanism of various reactions. As is known to all, some catalytic reactions require extremely high/low temperature or pressure or are carried out in strong acid/alkaline media. It is still challenging to investigate the reaction process under such harsh conditions. Take the SHINERS mentioned earlier as an example, the SiO 2 shell dissolves easily in the alkaline media (pH > 12), exposing the Au core which will affect the experimental results. Therefore, developing more tolerant shell materials such as MnO 2 which owns great stability in alkaline media is crucial to solving this challenge.