English

• 1. Introduction

  The increasing demand in energy for the future development stimulated a great growth in research on pursuing renewable and carbon-neutral energy supplies [1–3]. Nowadays, many efficient conversion technologies, including fuel cells, water splitting, and electrocatalytic CO₂ reduction, play a crucial role and appeal extensive interest as potential approaches to facilitate the energy conversion and supply highly valuable products [4–6]. As well known, catalysts play a crucial role in realizing efficient electrochemical reactions by significantly accelerating reaction kinetics [7–15]. Accordingly, the development of cost-effective and high-performance electrocatalysts is crucial for enhancing the energy conversion process. In essence, the catalysts used for electrochemical reactions should be highly efficient, stable, and economical [16–18]. Despite significant progress in the rational design and fabrication of advanced electrocatalysts, a fundamental question remains unanswered, "what is the nature of active sites under working conditions?" To this end, precisely analyzing the evolution process and comprehensively studying the mechanism are significant for the design and exploration of expected electrocatalysts [19,20].

  A deep understanding on the reaction steps, reaction mechanisms, and adsorption energy of intermediates is still lacking, which require more endeavors devoted. In recent years, some advanced in situ characterization techniques have been developed to investigate the morphology, structure, composition, and chemical valences of the catalyst [21–23]. Many advanced in situ technologies have been well developed for identifying the true reconstruction process of the catalysts, such as the in situ Fourier transform infrared spectroscopy [24,25]. Previous works have demonstrated that in situ characterization techniques play a pivotal role in deeply understanding the dynamic evolution of reaction sites and the specific reaction mechanisms [26,27]. As expected, in situ characterizations have emerged as promising techniques for researchers due to their significant achievements in investigating energy-related reactions [28].

  Aiming to describe the recent achievements in this interesting field, we have organized a comprehensive review to summarize the significant advancements in situ techniques for studying the reconstruction in the electrochemical reactions (Scheme 1). First, we manifest the fundamental principles of various in situ characterization technologies, including XRD, XAS, Raman, FTIR, and TEM. Then, a section on the applications of these in situ techniques used for investigating the dynamic evolution process and the nature of active sites of catalysts during electrochemical operation is also presented. At the end of this review, the future development and application of in situ technologies in this fascinating field are systematically discussed.

  Scheme 1

  

  Scheme 1.  Scheme of the in situ technologies and their applications in energy conversion.

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  2. In situ/operando characterization techniques

  In situ characterization is a technique used to study the properties of materials and surfaces by observing and measuring the structure, morphology, composition and electrochemical properties of materials under in situ conditions [29].

  The basic principle is to place the material to be studied in an experimental device, and then observe and measure it by various means. These means may include optical microscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, Raman spectrometer, X-ray diffractometer, electrochemical workstation, etc. Through these techniques, the changes in structure, morphology, composition and electrochemical properties of materials can be observed and measured in real time [30].

  In situ characterization technology has been widely used in materials science, energy storage and conversion, catalyst research, biomedicine and other fields. For example, in the field of materials science, in situ characterization technology can be used to study the process of material phase transition, crystal growth, interface reaction, etc. In the field of energy-related application, it can be used to study the changes in the performance of energy storage and conversion devices such as batteries and fuel cells. In the research of catalyst, it can be used to study the activity and stability of catalyst. In the biomedical field, it can be used to study properties such as biocompatibility and drug release of biological materials. In short, in situ characterization technology provides an important avenue and method for the research of material science and related fields by observing and measuring the property changes of materials under experimental conditions.

  2.1 In situ electron microscopy

  Electron microscopy, especially for TEM, is one of the most effective strategies for studying the microstructure and morphology of the materials [31,32]. The rapid development of TEM analysis techniques has boosted the explosion of nanoscience and nanotechnology. Particularly, in situ TEM is now attracting increasing interest as potential characterization technique for revealing the chemical and physical process dynamics at atomic resolutions [33]. For instance, Ma et al. [34] demonstrated the syntheses of free-supporting Pt and Pt-Ni nanowire-network electrocatalysts, and the formation mechanism was also investigated by an in situ TEM study. According to the in situ TEM images (Figs. 1a-c), it was observed that the Pt nanowires were formed through the oriented attachment of individual nanoparticles. Besides in situ TEM, a combination of other in situ electron microscopy is also demonstrated to be conducive to the investigation of the dynamic structural and morphological evolution during electrocatalysis.

  Figure 1

  

  Figure 1.  (a-c) In situ TEM observation of Pt nanowire growth at different stages. Reproduced with permission [34]. Copyright 2017, Wiley-VCH.

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  2.2 In situ XRD

  XRD shows diffraction patterns for different crystal phases, which can thus be a fingerprint for the analytical task. As well known, the majority of XRD investigations of materials can be used to identify the crystal phases. Therefore, the collection of an XRD pattern can be a potential strategy to study the crystal structures by altering the scattering vector, and XRD is also appearing as a promising technique for analyzing the crystal structure of materials [35] More recently, in situ XRD techniques are also developed and used for investigating the crystal plane and structure of the catalysts during electrochemical operation, providing a facile method for monitoring the structure evolution of catalyst and identifying the nature of catalytic active site [36–38]. As an illustration, Yuan et al. [39] have studied the structural reconstruction of Bi(OH)₃ during electrocatalytic CO₂ reduction reaction (CO₂RR) through a combination of ex situ XRD and in situ Raman (Figs. 2a-e). Specifically, the sample underwent XRD characterization each 5 mins during the electrochemical activation process in CO₂-saturated 0.1 mol/L KHCO₃ at −1.0 V_RHE. According to the ex situ XRD pattern, it is clearly observed that the peaks at 47.0^°, 42.4^°, and 32.9^° can be indexed to the Bi(OH)₃, while the other peak located at 48.7^°, 45.9^°, 44.6^°, 39.6^° and 37.9^° are associated with the metallic Bi (Figs. 2f and g). The ex situ XRD pattern suggested that the transformation of Bi(OH)₃ to metallic Bi during electrocatalytic CO₂RR.

  Figure 2

  

  Figure 2.  (a) Illustration of the fabrication of Bi(OH)₃ and the reconstruction to pits-Bi nanosheets. (b) TEM image, (c, d) STEM image and (e) AC-STEM image of the pits-Bi nanosheets. (f) Ex situ XRD and (g) Raman spectra for the in situ reconstruction process. Reproduced with permission [39]. Copyright 2022, Wiley-VCH.

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  2.3 In situ Raman

  The principle of in situ Raman spectroscopy is to use the scattering phenomenon that the incident light frequency is obviously changed by material surface molecules [40–43]. Operando Raman spectroscopy is also conducted to establish the evolution of electrocatalysts during reactions process and emerging as a potential approach to explore the structure-performance relationship of catalysts. For example, Wu et al. [44] have performed the in situ surface-enhanced Raman analysis to clarify the role of cation defect of the catalyst during electrochemical reaction. Specifically, they have synthesized the defective NiFe-LDH nanosheets via a facile aprotic-solvent-solvation-induced strategy. According to the Raman spectra, it is clearly observed that the as-generated cationic vacancy defects tend to exist as V_M (M = Ni/Fe) (Figs. 3a and b). As the voltage increases, the crystalline Ni(OH)_x on the surface of NiFe-LDH gradually transforms into a disordered state. When oxygen bubbles begin to evolve under sufficiently high voltage, local NiOOH species appear as residual products (Figs. 3c and d). Therefore, in situ Raman spectra are beneficial for understanding the reconstruction behaviors of the NiFe-LDH. Besides better understanding the surface reconstruction process, in situ Raman technology is also advantageous for providing insights into the adsorption and transfer of the reaction intermediates. Previous studies have reported that in situ Raman technology can be utilized to identify the electron transfer and hydrogen spillover effects, which are critical for comprehending the reaction mechanisms.

  Figure 3

  

  Figure 3.  In situ Raman spectroscopy tests for studying the transformation of (a) the pristine NiFe-LDH and (b) d-NiFe-LDH during OER. (c, d) The evolution of I₅₂₈/I₄₅₇ of NiFe-LDH and d-NiFe-LDH. Reproduced with permission [44]. Copyright 2021, Wiley-VCH.

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  2.4 In situ FTIR

  In situ FTIR, pioneered by Bewick et al. [24] has been utilized to acquire the molecular information regarding ionic and neutral adsorbates at the electrode as well as solution species during electrochemical reactions [45,46]. In situ FTIR is employed to characterize the adsorbates, molecules, and the intermediated species. Many researches employed in situ FTIR to study the electrocatalytic processes, including the study of the reaction mechanisms of organic molecules oxidation, CO₂RR, and the corresponding dynamic process [47,48]. Sun et al. have used this technique to investigate the reaction mechanism and intermediated species of many electrocatalytic processes [49]. For instance, they have investigated the reaction mechanisms of the propene electrooxidation on Pd/C and PdO/C via in situ FTIR [50]. Based on the in situ attenuated total reflection FTIR (Figs. 4a and b), a novel reaction pathway was distinctly observed compared with unconventional thermocatalysis (Figs. 4c-f). This work highlights the significant role of in situ FTIR in elucidating the electrochemical reaction mechanisms involved in propene oxidation.

  Figure 4

  

  Figure 4.  In situ ATR-FTIR spectra of C₃H₆ oxidation on (a) PdO/C and (b) Pd/C at different potentials in a C₃H₆-saturated 0.1 mol/L HClO₄ solution. Different propene adsorbed configurations (c) on PdO below 0.80 V, (d) above 0.8 V, (e) on Pd below 0.80 V, and (f) above 0.80 V. Reproduced with permission [50]. Copyright 2021, American Chemical Society.

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  2.5 Operando XAS

  XAS is extensively employed to investigate the local electronic and atomic structure of catalysts [51–53]. Compared with the XRD, XAS is sensitive to the electronic structure of materials, which can thus be utilized as advanced characterizations to measure the electronic properties of the catalysts [54]. Recently, XAS characterization techniques have been widely utilized in the electronic structure analysis of nanocatalysts and single-atom catalysts [55], which have gained widespread utilization and extensive investigation [56,57]. For example, Yang and coworkers explored the structural and chemical transformation of the AgCu nanocatalysts during the electrochemical CO₂RR [58]. According to a systematic investigation, it has been revealed that the two types of catalysts undergo similar structural evolution under CO₂RR conditions (Figs. 5a-d). In the case of intermixed AgCu catalysts, Cu may undergo leaching and subsequently become enriched on the surface of particles or recrystallize elsewhere as new particles (Fig. 5e).

  Figure 5

  

  Figure 5.  (a) Operando XANES of Cu K-edge of the I-AgCu as a function of the reaction time at −1.0 V vs. RHE in CO₂-saturated 0.1 mol/L KHCO₃. (b) XANES pre-edges of I-AgCu magnified from the dashed box region in (a) and the comparison with Cu and Cu₂O references (dashed lines). (c, d) Operando XANES and EXAFS spectra of I-AgCu for CO₂RR. (e) The in situ evolution of intermixed and phase-separated AgCu catalysts in electrochemical CO₂RR. Reproduced with permission [58]. Copyright 2023, American Chemical Society.

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  3. Applications of in situ/operando characterization techniques for electrocatalytic reactions

  3.1 ORR

  Fuel cells offer a green and high-efficiency technology for the conversion of chemical energy into electrical energy [59,60]. The development of high-performance ORR catalysts largely relies on a comprehensive understanding on the redox behavior of the active sites [61–67]. Therefore, the in situ/operando measurement techniques are highly imperative [68–71]. Recent works have confirmed the significant role of in situ techniques in enhancing our understanding on the redox behavior of metal sites during electrochemical ORR. For instance, Huang and colleagues reported the in situ synthesis of Ru-doped Pt₃Co octahedral nanoparticles grown on carbon (Ru–Pt₃Co/C) [72], which exhibited remarkable ORR activity up to 1.05 A/mg_Pt. Ru doping effectively mitigates Co corrosion, substantially improving the morphological stability of Ru-Pt₃Co/C. In situ XAFS analysis revealed that Ru promoted the Pt atom inversion (Figs. 6a-d), thereby limiting over-reduction of Pt sites to enhance the stability of Ru-Pt₃Co/C. Additionally, DFT calculations demonstrated that adding Ru atom accelerated the oxygen intermediate breach and desorption (Fig. 6e).

  Figure 6

  

  Figure 6.  In situ XAS tests. (a) Scheme of the electrochemical setup for the in situ XAS experiment. Pt L₃-edge XANES spectra of (b) Ru-Pt₃Co/C and (c) Pt₃Co/C during the ORR. (d) Peak positions of Pt 4f_7/2 XPS spectra for Ru-Pt₃Co/C, Pt₃Co/C, and commercial Pt/C. (e) The corresponding ORR mechanism of Ru-Pt₃Co/C. Schematic illustration of the (f) SHINERS study of the ORR at Pt (311) surface and the (g) ORR mechanism. (a-e) Reproduced with permission [72]. Copyright 2021, American Chemical Society. (f, g) Reproduced with permission [74]. Copyright 2020, American Chemical Society.

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  In situ characterization techniques can be employed not only to investigate the nature of active sites, but also to examine the intermediates during electrochemical ORR [73]. Dong et al. employed the shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) to study the ORR mechanism at Pt (211) and Pt (311) surfaces [74]. Through control and isotope substitution experiments, clear evidence of OH and OOH intermediates at Pt (311) and Pt (211) surfaces can be observed. It was concluded that the difference in OOH adsorption on high-index surfaces posed a significant impact on the electrocatalytic ORR performance (Figs. 6f and g). The research has enhanced the comprehension of ORR mechanism on high-index Pt (hkl) surface and highlighted the crucial significance of in situ techniques for a thorough understanding of the ORR mechanism.

  3.2 Small molecule oxidation

  The electrooxidation of hydrogen, CO, or small organic molecules is also a crucial electrode reaction of fuel cells [75,76]. Appraising the reaction intermediates is of crucial importance to unveil the pathways and mechanisms. Taking electrocatalytic hydrogen oxidation reaction (HOR) as an example, the HOR may follow the Heyrovsky-Volmer or Tafel-Volmer mechanism in acidic solution, where the H⁺ can be generated on the catalyst surface from adsorbed hydrogen and then convert to a proton by releasing an electron [77,78]. The EOR process typically involves a dual-pathway mechanism, namely the C1 and C2 pathways. The former can generate adsorbed carbon monoxide (CO_ad) and carbonate radical (CO₃²⁻) intermediates [79–82]. In contrast, the C2 pathway is involved in the generation of acetate radical (CH₃COO⁻). Therefore, measuring and analyzing the intermediated species will be significant for deeply understanding on the reaction pathways and mechanisms. Huang and coworkers investigated the EOR mechanism on the Pd-Sb hexagonal nanoplate (Figs. 7a-d) [83]. According to the in situ ATR-SEIRAS spectra, it is clearly found that there are two forms of CO adsorption on Pd₈Sb₃ HPs/C surface (bridge-bonded CO (CO_B) and multiply-bonded CO (CO_M)), while Pd/C has three forms of CO adsorption (linear-bonded CO (CO_L), CO_B and CO_M) (Figs. 7e and f). The negligible CO_L on Pd₈Sb₃ HPs/C reveals that the linear CO adsorption capacity of Pd₈Sb₃ HPs/C is weakened due to the introduction of Sb, and the stronger CO_M and CO_B on Pd₈Sb₃ HPs/C suggest a more positive CO, indicating that Pd₈Sb₃ HPs/C can greatly facilitate the C₂ pathway of the EOR.

  Figure 7

  

  Figure 7.  (a, b) HAADF-STEM images and the (c) elemental mapping images of the Pd₈Sb₃ HPs. (d) CV curves of the Pd₈Sb₃ HPs/C, Pd/C, and Pt/C for EOR. In situ ATR-SEIRAS spectra of (e) Pd₈Sb₃ HPs/C and (f) Pd/C during CV cycling. (g) Atomic-resolution HAADF-STEM image of the 22% YO_x/MoO_x-Pt nanowires. (h) Pt mass-normalized MOR, (i) comparison on the specific activities and mass activities of 22% YO_x/MoO_x-Pt nanowires and other references. (j) In situ FTIR spectra recorded from 0.15 V to 1.20 V and (k) magnified in situ FTIR spectra recorded from 0.15 V to 0.60 V versus RHE during MOR on 22% YO_x/MoO_x-Pt. (l, m) In situ FTIR spectra during MOR. (a-f) Reproduced with permission [83]. Copyright 2022, Wiley-VCH. (g-m) Reproduced with permission [84]. Copyright 2021, Wiley-VCH.

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  Guo et al. employed the in situ FTIR test to obtain in-depth understanding on the improvement MOF performance on the YO_x/MoO_x-Pt catalyst (Figs. 7g-i) [84]. In situ FTIR spectra showed several bands between 2500 cm⁻¹ and 1000 cm⁻¹, where the downward bands at 2341 cm⁻¹ was assigned to the asymmetric stretching vibration of CO₂, implying the complete oxidation of methanol (Figs. 7j-m). It was also demonstrated that MoO_x and YO_x contributed to the enhanced anti-CO poisoning capability of Pt nanowires. Moreover, DFT calculations further revealed that surface Y and Mo atoms, with enabled oxidation states, facilitated the binding of COOH* to the surface via both C and O atoms. This effectively reduced the free energy barriers for CO* oxidation into COOH*. As a result, a mass activity of 2.10 A/mg_Pt and a high specific activity of 3.35 mA/cm² were achieved for the optimized 22% YO_x/MoO_x-Pt.

  3.3 OER

  OER plays a critical role in electrochemical water splitting as it involves a four-electron transfer process, which directly determines the energy barrier and reaction efficiency of the overall reaction [85–90]. Therefore, significant efforts have been dedicated to the development and production of more efficient electrocatalysts to enhance the reaction kinetics of OER [90–92]. It is demonstrated that the in situ characterizations play a key role in identifying the nature of true active sites of catalysts during OER [93–95]. For instance, Zheng et al. [96] endeavored to explore the electrochemical reconstruction of MOFs and elucidate the function of in situ generated species during electrochemical OER (Figs. 8a and b). Dramatic morphological changes that expose electron-accessible Co sites are clearly observed (Figs. 8c-e). Subsequent OER experiments suggest that the dominating active sites are CoOOH species produced from α/β-Co(OH)₂.

  Figure 8

  

  Figure 8.  (a) In situ UV–vis and (b) Raman devices. (c) In situ UV–vis spectroelectrochemical study between 0.925 V and 1.705 V. (d) In situ Raman spectroelectrochemical study between 0.925 V and 1.585 V. (e) Scheme for the transformation of ZIF-67 to α-Co(OH)₂ and β-Co(OH)₂. Reproduced with permission [96]. Copyright 2019, American Chemical Society.

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  3.4 HER

  Electrocatalytic HER offers a promising strategy to provide clean energy carriers to alleviate the energy crisis [97–101]. According to the Sabatier's principle and volcano plot of hydrogen intermediate adsorption (∆G_H), the best HER catalyst should possess the close-to-zero ∆G_H [102,103]. To this end, designing the synthesizing the advanced HER electrocatalysts with close-to-zero ∆G_H is highly favorable for promoting the hydrogen production [104]. However, it is also demonstrated that the hydrogen spillover effect and surface reconstruction of catalyst during electrochemical reactions are also crucial for affecting the HER performance [105–108]. Operating in situ characterization tests is imperative to understand the reaction mechanisms and reconstruction process of catalyst, which will provide guidance for future rational design and fabrication of more excellent HER catalysts. Using Wei's work as an example, they have reported the preparation of Pt/TiO₂ catalyst with abundant deficient oxygen vacancies (V_O-deficient Pt/TiO₂) and rich oxygen vacancies (V_O-rich Pt/TiO₂) (Figs. 9a-c) [109]. Remarkably, it is measured that the V_O-rich Pt/TiO₂ has a mass activity of 45.28 A/mg_Pt at −0.1 V vs. RHE, which is 58.8 and 16.7 times higher than those of commercial Pt/C and V_O-deficient Pt/TiO₂, respectively (Figs. 9d and e). In order to uncover the outstanding electrocatalytic HER performance, they have operated the in situ Raman tests and DFT calculations. Based on the in situ Raman spectra, it is obviously observed the H* transfer from Pt to TiO₂ support. In contrast, no obvious peak related with E_g(1) are observed during the catalytic process due to the absence of H* (Fig. 9f). DFT calculations have demonstrated that TiO₂ with abundant oxygen vacancies can facilitate both reversed charge transfer and hydrogen spillover from Pt to the TiO₂ support, thereby contributing to the improved electrocatalytic performance for HER (Figs. 9g-k).

  Figure 9

  

  Figure 9.  (a) Illustration of the synthesis procedures of the V_O-rich Pt/TiO₂ and V_O-deficient Pt/TiO₂ catalysts. (b) ESR spectra and (c) XPS profiles. (d) HER polarization curves and (e) corresponding mass activity at the overpotential of 100 mV. (f) DFT calculation of the free energy. (g) In situ Raman spectra of V_O-rich Pt/TiO₂ at −0.1 V vs. RHE with different electrolysis durations. (h) Magnified view of the E_g(1) mode. (i–k) In situ Raman spectra of V_O-rich Pt/TiO₂ (i), V_O-deficient Pt/TiO₂ (j), and TiO₂ (k) at a potential of −0.2 V vs. RHE. Reproduced with permission [109]. Copyright 2021, Wiley-VCH.

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  3.5 CO₂RR

  Electrocatalytic CO₂RR is a promising renewable technology for the conversion of greenhouse gas into value-added liquid fuels and chemicals [110,111]. Cu, Pd, Pb, and Bi-based catalysts are established as the predominant candidate for selective CO₂ electroreduction to multicarbon products [111–113]. However, the comprehension of the nature of active sites during reaction conditions remains limited, particularly for some electrocatalysts with complicated components. To this end, in situ characterization technologies are emerging as promising candidates for addressing these issues. Yang and colleagues emphasized the importance of understanding and regulating the dynamic characteristics of nanocatalysts to attain optimized CO₂ electrolysis [114]. Specifically, they have systematically investigated the dynamic aspects of both the active site and its surrounding reaction microenvironment by analyzing the structural transformation via in situ/operando characterizations [115]. More recently, they also identified the nature of reaction sites of the high-activity Cu nanocatalysts through time-resolved operando techniques (Figs. 10a and b). After a systematic investigation, it is clearly found that an ensemble of monodisperse Cu nanoparticles would undergo a structural transformation process. According to the operando EC-STEM image, 4D-STEM image and XANES spectra, it is uncovered that the Cu nanograins underwent a typical structural reconstruction (Figs. 10c-m). As a result, a 7 nm Cu nanoparticle ensemble, with a unity fraction of active Cu nanograins, exhibits sixfold higher C₂₊ selectivity than the 18 nm counterpart with one-third of active Cu nanograins (Figs. 10n-p).

  Figure 10

  

  Figure 10.  (a) Operando/in situ methods used to reveal the dynamic changes during electrocatalytic CO₂RR. (b) Scheme of operando EC-STEM and 4D-STEM under CO₂RR-relevant conditions. (c-m) EC-STEM images of the initial growth of Cu nanocatalysts after a single negative-direction LSV scan, from 0.4 V to 0 V. (n) Quantitative analysis of relative fraction of metallic Cu. Operando XANES (o) and corresponding quantitative analysis (p) of 18 nm nanoparticles at −0.8 V. Reproduced with permission [115]. Copyright 2023, Nature Publishing Group.

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  4. Conclusion and perspectives

  In summary, we have focused on the in situ characterizations utilized for identifying the true active sites of catalyst and better understanding the reaction mechanism of electrocatalytic reactions. The aim of this review is to clarify the great significance of the in situ characterization techniques on various electrocatalytic reactions. By systematically discussing the fundamentals of various in situ technologies, such as in situ electron microscopy, in situ XRD, in situ FTIR, in situ Raman, in situ XAS, we hope to provide readers with insightful ideas about the development of in situ technologies. Subsequently, the applications of various in situ technologies used for identifying the nature of reaction sites and the surface adsorption of intermediated species are also discussed in detail, which will benefit for the development of advanced electrocatalysts. Despite the significant progress achieved in this interesting field, a comprehensive visualization is still far from being attained. More efforts need to be devoted and some challenges must also be overcome.

  Notably, many factors can govern the electrocatalytic performance of the catalysts, including the oxidation state, local electronic structure. Refining the monitoring of electrocatalytic processes through a single in situ technique and interpretation of mechanisms may result in an incomplete assessment. Therefore, it is of utmost importance to integrate multiple in situ analytical techniques to provide a comprehensive insight into the underlying mechanisms of electrochemical reactions.

  In addition, it should be noted that a significant disparity exists between the in situ employed devices and actual energy devices. Currently, most researches are focused on the half-reaction of energy conversion devices, such as detecting the intermediates of small molecules in fuel cells. To this end, optimizing the electrochemical performance of in situ cells related to real energy devices should be the future direction.

  Another important challenge that should be overcome is to establish a reconstruction-performance relationship. Previous works have demonstrated that the in situ reconstruction of some catalysts can yield highly active species to further enhance the electrocatalytic performance, while some are hindering the surface reconstruction to achieve high electrocatalytic performance. For example, the surface reconstruction of Ni(OH)₂ can yield highly active NiOOH to boost electrocatalytic OER, while some works are hampering the surface reconstruction of Ni(OH)₂ by incorporating the Fe³⁺. It is still unclear whether surface reconstruction can promote the electrocatalytic performance or not.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was financed by the National Natural Science Foundation of China (No. 22202169), Xuzhou Science and Technology Plan Project of China (No. KC21294).

  
  
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  Scheme 1  Scheme of the in situ technologies and their applications in energy conversion.

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  Figure 1  (a-c) In situ TEM observation of Pt nanowire growth at different stages. Reproduced with permission [34]. Copyright 2017, Wiley-VCH.

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  Figure 2  (a) Illustration of the fabrication of Bi(OH)₃ and the reconstruction to pits-Bi nanosheets. (b) TEM image, (c, d) STEM image and (e) AC-STEM image of the pits-Bi nanosheets. (f) Ex situ XRD and (g) Raman spectra for the in situ reconstruction process. Reproduced with permission [39]. Copyright 2022, Wiley-VCH.

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  Figure 3  In situ Raman spectroscopy tests for studying the transformation of (a) the pristine NiFe-LDH and (b) d-NiFe-LDH during OER. (c, d) The evolution of I₅₂₈/I₄₅₇ of NiFe-LDH and d-NiFe-LDH. Reproduced with permission [44]. Copyright 2021, Wiley-VCH.

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  Figure 4  In situ ATR-FTIR spectra of C₃H₆ oxidation on (a) PdO/C and (b) Pd/C at different potentials in a C₃H₆-saturated 0.1 mol/L HClO₄ solution. Different propene adsorbed configurations (c) on PdO below 0.80 V, (d) above 0.8 V, (e) on Pd below 0.80 V, and (f) above 0.80 V. Reproduced with permission [50]. Copyright 2021, American Chemical Society.

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  Figure 5  (a) Operando XANES of Cu K-edge of the I-AgCu as a function of the reaction time at −1.0 V vs. RHE in CO₂-saturated 0.1 mol/L KHCO₃. (b) XANES pre-edges of I-AgCu magnified from the dashed box region in (a) and the comparison with Cu and Cu₂O references (dashed lines). (c, d) Operando XANES and EXAFS spectra of I-AgCu for CO₂RR. (e) The in situ evolution of intermixed and phase-separated AgCu catalysts in electrochemical CO₂RR. Reproduced with permission [58]. Copyright 2023, American Chemical Society.

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  Figure 6  In situ XAS tests. (a) Scheme of the electrochemical setup for the in situ XAS experiment. Pt L₃-edge XANES spectra of (b) Ru-Pt₃Co/C and (c) Pt₃Co/C during the ORR. (d) Peak positions of Pt 4f_7/2 XPS spectra for Ru-Pt₃Co/C, Pt₃Co/C, and commercial Pt/C. (e) The corresponding ORR mechanism of Ru-Pt₃Co/C. Schematic illustration of the (f) SHINERS study of the ORR at Pt (311) surface and the (g) ORR mechanism. (a-e) Reproduced with permission [72]. Copyright 2021, American Chemical Society. (f, g) Reproduced with permission [74]. Copyright 2020, American Chemical Society.

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  Figure 7  (a, b) HAADF-STEM images and the (c) elemental mapping images of the Pd₈Sb₃ HPs. (d) CV curves of the Pd₈Sb₃ HPs/C, Pd/C, and Pt/C for EOR. In situ ATR-SEIRAS spectra of (e) Pd₈Sb₃ HPs/C and (f) Pd/C during CV cycling. (g) Atomic-resolution HAADF-STEM image of the 22% YO_x/MoO_x-Pt nanowires. (h) Pt mass-normalized MOR, (i) comparison on the specific activities and mass activities of 22% YO_x/MoO_x-Pt nanowires and other references. (j) In situ FTIR spectra recorded from 0.15 V to 1.20 V and (k) magnified in situ FTIR spectra recorded from 0.15 V to 0.60 V versus RHE during MOR on 22% YO_x/MoO_x-Pt. (l, m) In situ FTIR spectra during MOR. (a-f) Reproduced with permission [83]. Copyright 2022, Wiley-VCH. (g-m) Reproduced with permission [84]. Copyright 2021, Wiley-VCH.

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  Figure 8  (a) In situ UV–vis and (b) Raman devices. (c) In situ UV–vis spectroelectrochemical study between 0.925 V and 1.705 V. (d) In situ Raman spectroelectrochemical study between 0.925 V and 1.585 V. (e) Scheme for the transformation of ZIF-67 to α-Co(OH)₂ and β-Co(OH)₂. Reproduced with permission [96]. Copyright 2019, American Chemical Society.

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  Figure 9  (a) Illustration of the synthesis procedures of the V_O-rich Pt/TiO₂ and V_O-deficient Pt/TiO₂ catalysts. (b) ESR spectra and (c) XPS profiles. (d) HER polarization curves and (e) corresponding mass activity at the overpotential of 100 mV. (f) DFT calculation of the free energy. (g) In situ Raman spectra of V_O-rich Pt/TiO₂ at −0.1 V vs. RHE with different electrolysis durations. (h) Magnified view of the E_g(1) mode. (i–k) In situ Raman spectra of V_O-rich Pt/TiO₂ (i), V_O-deficient Pt/TiO₂ (j), and TiO₂ (k) at a potential of −0.2 V vs. RHE. Reproduced with permission [109]. Copyright 2021, Wiley-VCH.

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  Figure 10  (a) Operando/in situ methods used to reveal the dynamic changes during electrocatalytic CO₂RR. (b) Scheme of operando EC-STEM and 4D-STEM under CO₂RR-relevant conditions. (c-m) EC-STEM images of the initial growth of Cu nanocatalysts after a single negative-direction LSV scan, from 0.4 V to 0 V. (n) Quantitative analysis of relative fraction of metallic Cu. Operando XANES (o) and corresponding quantitative analysis (p) of 18 nm nanoparticles at −0.8 V. Reproduced with permission [115]. Copyright 2023, Nature Publishing Group.

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