In situ characterization techniques: main tools for revealing OER/ORR catalytic mechanism and reaction dynamics

Abstract

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are some of the most important reactions in electrochemical energy technologies such as fuel cells and metal–air cells. However, the lack of in-depth understanding of reaction mechanisms and clear identification of catalytic active sites hinders the development of high-performance and durable electrocatalysts for the OER and ORR. In situ characterization techniques enable on-site monitoring of the surface oxidation state and local atomic structural transitions, facilitating the identification of active sites and enhancing our fundamental understanding of reaction mechanisms in these systems. This article reviews the main progress in the OER/ORR process using various in situ techniques. By utilizing in situ characterization techniques, the factors affecting catalyst catalytic performance are revealed from a direct or indirect perspective. Finally, the main challenges and prospects of in situ characterization techniques in ORR/OER research are discussed.

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1. Introduction

Currently, the world is facing serious energy storage and utilization problems.¹ Therefore, developing clean and sustainable energy sources such as wind energy, solar energy, and hydropower, as well as related power storage and conversion technologies,^2,3 has become an inevitable course of action to guarantee safety and secure sustainable development prospects for the future. However, renewable energy is constrained by seasonal and regional variations, necessitating the development of novel energy conversion and storage technologies.^4,5 Among these technologies, the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) are two critical electrochemical processes integral to a multitude of energy storage devices and renewable energy conversion systems,^6,7 including renewable fuel cells, water electrolysis technology, or rechargeable metal–air batteries. In the realm of batteries, the sluggish kinetics significantly limit the widespread adoption of this energy storage technology.⁸ Hence, there is an urgent need for the development of efficient, low-cost, and abundant novel electrocatalysts to overcome this challenge.

Although there has been a concerted effort to cultivate efficient electrocatalysts, achieving a profound grasp of the structural attributes of these catalysts, the spatial distribution of charges, and the nuanced transformations of reaction intermediates throughout the catalytic cycle remains exceedingly difficult.⁹ Consequently, a comprehensive understanding of these processes is vital for the judicious design of novel electrocatalysts, which in turn can significantly bolster their catalytic efficacy and facilitate their optimized application in energy storage and conversion technologies. At present, traditional characterization techniques can analyze the microstructure, morphological composition, crystal phase structure, lattice parameters, chemical bond composition, mass changes, and information on intermediate substances of materials. For instance, techniques such as Raman or UV spectroscopy are instrumental in probing the molecular vibrational and rotational modes of materials, facilitating the determination of crystal structures and phase transitions on their surfaces.¹⁰ Additionally, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) can be used to investigate the alterations occurring before and after a catalytic reaction, thereby enabling the identification of potential active sites within the reaction mechanism.¹¹ Although conventional characterization methods can reflect some insights into catalysts, they are highly susceptible to environmental influences. For instance, when materials are exposed to air over an extended period, they face the risk of contamination, leading to inaccurate test results and affecting the final data.^12,13 Furthermore, comparing spectroscopic data obtained from non-in situ methods cannot ensure a meaningful trend or correlation between the chemical transformations and the electrochemical activity of catalysts.^14,15 Consequently, the development of in situ characterization techniques is imperative to elucidate the precise dynamics of catalyst changes throughout the reaction sequence.^16,17

This article reviews the main progress in characterizing transition metal-based catalysts in OER/ORR processes using various in situ techniques (Fig. 1). The underlying ORR/OER mechanisms are dissected in depth, with a comprehensive enumeration of the potential intermediates and products implicated in these reactions. A brief introduction is given to the working principles of various in situ technologies, and a detailed characterization of the structural evolution of catalysts during the ORR/OER process is carried out. The dynamic evolution information on intermediates and products is highlighted, while a discussion on monitoring the morphology of catalysts during the synthesis process is also partially covered, and presents prospects for the future development of in situ characterization techniques.

Fig. 1 Introduction of common in situ characterization techniques.^18,19

2. Brief overview of the ORR and OER mechanism and intermediates

The development of efficient and low-cost novel OER/ORR electrocatalysts demands a profound comprehension of their intrinsic mechanisms and complex oxygen intermediate processes.^20–22 This section delves into the intermediate processes, offering theoretical insights to guide the design of high-performance catalysts for the ORR/OER.

2.1. ORR: reaction pathway and mechanism

In general, for metal–air batteries or fuel cells, the anode undergoes electrochemical oxidation to release electrons, while oxygen molecules at the cathode receive electrons from an external circuit to be reduced to water.^23,24 The mechanism of the ORR process is complex, including multiple reaction pathways and intermediate products. It can be broadly categorized into a four-electron (4e⁻) pathway and a two-electron (2e⁻) pathway,²⁵ with the specific reaction pathway being influenced by the acidity and alkalinity of the electrolyte solution (Fig. 2a).

Fig. 2 Mechanism explanation diagram of (a) OER and (b) ORR in different solutions.

Acidic media:

4e⁻ ORR: O₂ + 4H⁺ + 4e⁻ → 2H₂O E° = 1.23 V (1)

2e⁻ ORR: O₂ + 2H⁺ + 2e⁻ → 2H₂O₂ E° = 0.70 V (2)

H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O E° = 1.76 V (3)

Alkaline media:

4e⁻ ORR: O₂ + 2H₂O + 4e⁻ → 4OH⁻ E° = 0.40 V (4)

2e⁻ ORR: O₂ + 2H₂O + 2e⁻ → HO₂⁻ + OH⁻ E° = −0.06 V (5)

HO₂⁻ + 2H₂O + 2e⁻ → 3OH⁻ E° = 0.86 V (6)

Monitoring the catalytic process and capturing intermediate information is essential for elucidating the mechanism of the ORR.^26,27 Typically, oxygen molecules adsorb to the catalyst surface in three distinct modes: the Griffiths adsorption, the Bridge adsorption, and the Pauling adsorption.²⁸ In the Pauling adsorption mode, only a single oxygen atom within the oxygen molecule is activated, typically going through a 2e⁻ process. Conversely, in the Griffiths and Bridge adsorption modes, both oxygen atoms within the molecule can be activated, typically going through a 4e⁻ process.

For applications of fuel cells or rechargeable metal–air batteries, the 4e⁻ pathway significantly enhances the reaction rate, whereas the 2e⁻ pathway can be exploited to produce hydrogen peroxide (H₂O₂).²⁹ Oxygen molecules initially diffuse to the surface of the catalyst, where they are adsorbed to form adsorbed oxygen molecules (*O₂). These adsorbed molecules can then be further processed along two distinct pathways, which are determined by the sequence in which the O–O bonds are broken: the dissociation mechanism and the association mechanism.^30–32 The dissociation mechanism mainly involves the direct cleavage of O–O bonds to form *O intermediates, which are further reduced to H₂O. The association mechanism reduces *O₂ to *OOH, followed by further reduction of *OOH to *O and the intermediate of *OH to H₂O. Based on this, it is necessary to ensure O–O fracture for the 4e⁻ pathway, which is conducive to the adsorption of *O. At the same time, *O will form double bonds with the catalyst surface, so catalysts with high oxygen binding energy will be more conducive to the adsorption of *O and occurrence of 4e⁻ reactions, while 2e⁻ is the opposite. Nørskov conducted theoretical calculations on the density functional theory (DFT) of commonly used metal oxygen reduction activity and obtained the famous “volcano curve”. The precious metals Pt and Pd are located at the top of the volcano curve, with the optimal adsorption and desorption ability. Metal elements such as Ni, Ru, Cu, and Co on the left of the curve have too strong an adsorption for *O and *OH, resulting in slow kinetic processes of activating the proton transfer step. On the right of the volcano curve, Ag and Au have a large energy barrier in the dissociation process due to their weak oxygen binding energy. Therefore, Pt-based catalysts are commonly used in ORRs, but their scarcity and high cost limit their practical applications. Meanwhile, DFT calculations indicate that the introduction of heteroatoms affects the distribution of charge density and spin density.^33–36 In comparison with single heteroatom-doped systems, the incorporation of double heteroatom doping in carbon leads to a reduced work function. This reduction in energy barriers facilitates more efficient electron transfer from the catalyst surface to the adsorbed oxygen molecules, thereby accelerating the formation of intermediate species.

2.2. OER: reaction pathway and mechanism

The OER involves multiple electron transfer processes and exhibits slower reaction kinetics, and as a reverse reaction of the ORR, OER in alkaline solutions also involves multiple reaction intermediates, as shown below^32,37,38 (Fig. 2b):

* + OH⁻ (aq) = HO* + e⁻ (7)

HO* + OH⁻ (aq) = O* + H₂O (l) + e⁻ (8)

O* + OH⁻ (aq) = HOO* + e⁻ (9)

HOO* + OH⁻ (aq) = * + O₂ (g) + H₂O (l) + e⁻ (10)

Overall: 4OH⁻ (aq) = O₂ (g) + H₂O (l) + 4e⁻ (11)

Simultaneously, both the ORR and OER involve the strength of the binding ability of oxygen-containing intermediates to the catalyst surface during the reaction process. According to the Sabatier principle, an excess or insufficient adsorption strength heightens the challenge in resolving *OOH or *OH species. The alterations in Gibbs free energy throughout the reaction process are depicted as follows:^22,39,40

ΔG₁ = ΔG_*OH (12)

ΔG₂ = ΔG_*O − ΔG_*OH (13)

ΔG₃ = ΔG_*OOH − ΔG_*O (14)

ΔG₄ = ΔG_O2 − ΔG_*OOH (15)

In the OER process, the intermediates demonstrate a linear correlation, characterized by the binding of *OH and *OOH to the catalyst surface via single bonds. The linear relationship between ΔG_*OOH and ΔG_*OH results in a minimum theoretical starting potential of 0.37 eV, and the difference between ΔG_*O and ΔG_*OH can serve as a predictive factor for OER activity; in other words, the catalytic activity can be regulated by optimizing the difference between ΔG_*O and ΔG_*OH.

The prevalent theories regarding the OER process are the adsorbate evolution mechanism (AEM) and the lattice oxygen mediation (LOM).^41–43 In the AEM, the 4e⁻ pathway occurs in both acidic and alkaline solutions. Under acidic conditions, water molecules participate in proton–electron transfer, yielding *OH as a product. This intermediate then reacts to form *O, which subsequently combines with another water molecule to produce *OOH. The *OOH intermediate is then protonated, leading to the evolution of oxygen. Throughout this process, a proton is released into the electrolyte, ultimately combining with the transferred electrons at the cathode. In alkaline media, the OH⁻ ions diffuse from the electrolyte solution and adsorb onto the electrocatalyst to form *OH. This species then reacts to produce *O, which further reacts with another OH⁻ to form *OOH. The *OOH intermediate is then deprotonated, resulting in the formation of O₂. In contrast to the AEM mechanism,^44,45 the LOM mechanism involves a dual site process. In this process, two *OH radicals deprotonate at the metal active site, forming two metal–oxygen groups. These groups then couple to form O–O bonds, ultimately resulting in the generation of O₂. The oxygen vacancies created are subsequently filled by OH species, and the *OOH intermediate is not involved in the LOM mechanism. Furthermore, beyond the AEM and LOM mechanisms, there have been some reports in the literature on the oxide pathway mechanism (OPM), which primarily facilitates oxygen evolution through O–O coupling on adjacent metal sites.^46–48 The sequential interaction of two adjacent active sites leads to the deprotonation of OH⁻ and triggers the coupling of *O radicals to produce O₂.

Similar to the ORR, the activity and selectivity of OER catalysts can also be predicted using volcano plots, with optimization of the linear relationship between ΔG_O and ΔG_OH to adjust OER activities.³⁵ Unlike the ORR, OERs involve surface/body reconstruction and phase transitions, which complicates the accurate identification of the true active species. To gain a deeper understanding of the catalyst's internal changes, further analysis using in situ characterization techniques is required.

3. Succinct overview of various in situ characterization techniques

As is commonly understood, the OER/ORR performance of catalysts is influenced by a series of intricate reaction processes. A profound comprehension of active sites and structural information is indispensable for elucidating the underlying reaction mechanism of catalysts.^49,50 Regrettably, the majority of contemporary research into these mechanisms is grounded in theoretical simulations, such as DFT, and there remains a notable gap in the exploration of catalytic active sites.⁵¹ The sluggish kinetic rates and the multifaceted electron transfer processes concurrently impede a thorough understanding of the intrinsic reaction mechanism. Non-invasive techniques like XPS and XAS are frequently employed to investigate the active phases of electrocatalysts, but they are inadequate for probing the internal structural evolution of catalysts.^52,53 To enhance the detection of structural alterations within catalysts and the alterations of key reaction intermediates, in situ characterization techniques, such as Raman, Fourier transform infrared spectroscopy (FTIR), XAS, transmission electron microscopy (TEM), and X-ray diffraction (XRD), have gradually come to the fore. Fig. 3 succinctly summarizes some of the conventional in situ characterization methods, concurrently elucidating their respective strengths and limitations.

Fig. 3 Structural schematic diagrams and advantages and disadvantages of different in situ characterizations.

This article presents a succinct overview of the operational principles and benefits of numerous advanced characterization techniques, offering cutting-edge insights into the structural and mechanistic aspects of catalysts. A summary has been compiled regarding the utilization of diverse in situ characterization techniques in the study of OER and ORR mechanisms over the past few years (Fig. 4).

Fig. 4 Application of in situ characterization in OER and ORR in recent years.^54–65

3.1. In situ Raman spectroscopy

The Raman effect arises from the inelastic scattering that occurs during the interaction between monochromatic probe light and materials. During testing, Raman spectroscopy can be integrated with electrochemical electrolytic cells (EC cells) to detect surface active species at the electrode–electrolyte interface under reaction conditions.^66,67 In situ Raman measurement is particularly well-suited for studying the structure and chemical effects of electrode surfaces during intercalation processes.^68–70 Compared with Fourier transform infrared (FTIR), Raman spectroscopy holds a distinct advantage due to the minimal scattering effect of water molecules and the absence of interference from water. However, the low probability of Raman scattering often results in a weak spectral signal for Raman analysis. To augment the surface sensitivity and selectivity of the system, an array of in situ Raman characterization techniques has been meticulously developed. These include surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS), and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), which are designed to provide deeper insights into the surface properties with enhanced precision and detail.^71,72

These in situ Raman techniques harness the phenomena of localized surface plasmon resonance (LSPR) to elicit reaction signals that furnish fingerprint-like information on key reaction intermediates, aiding in the identification of rate-determining steps (RDS).^70,73 Through in situ Raman spectroscopy, Ai and colleagues have shown that the transformation of the active site within NiSe₂ into NiOOH and SeO_x²⁻ is triggered by the structural collapse of NiSe₂, which is accompanied by the concurrent in situ formation of NiOOH.⁷⁴ Concurrently, Li et al. have established that with the change of time, the characteristic peaks of CoMoO₄/Co₃O₄@CC associated with Co–O–Mo, O–Mo–O, and Mo=O bonds dissipate, while CoOOH forms, signifying the emergence of the active species for OER and the occurrence of structural alterations.⁷⁵ In situ Raman spectroscopy is a prevalent technique for the analysis of amorphous or weakly crystalline materials, particularly those containing carbon, oxygen, hydrogen, and polysulfides. Nonetheless, its reliance on the vibrational and rotational energy levels of molecular/crystal bonds, along with its sensitivity to non-polar bonds, precludes the direct detection of metal signals.

Recently, Gan and his research team at Tsinghua University have employed in situ isotope-labeled Raman spectroscopy to explore the ORR intermediates, RDS and dynamic active sites of Fe–N–C catalysts under varying pH conditions (Fig. 5(a)).⁷⁶ In acidic media (Fig. 5(b)), FePc/C exhibits numerous Raman diffraction peaks, with the positions at 593, 685 and 754 cm⁻¹ primarily associated with the displacement of C/N atoms near the Fe center. Currently, as the applied voltage decreases, the peak intensity at 593 cm⁻¹ progressively diminishes. When the voltage fluctuates from 0.05 V to 1 V, the attenuation is reversible. However, it becomes irreversible when the voltage remains at 0.05 V for 15 min and then shifts to 1 V. This irreversible attenuation is attributed to the non-planar structure induced by the low potential adsorption of the ORR intermediate on Fe. In contrast to the acidic medium (Fig. 5(c)), the peak strength at 593 cm⁻¹ gradually increases with the voltage reduction, and the peak intensity does not exhibit an irreversible change with voltage variation. This further indicates that the stability of the Fe–N–C structure in an alkaline medium exceeds that in an acidic medium. In contrast in acidic media, a novel Raman signal appears emerges between 1100–1200 cm⁻¹ when the voltage dips below 0.2 V, indicative of oxygen-containing intermediates in the ORR process. To rigorously characterize these intermediates in alkaline conditions, in situ isotope substitution Raman spectroscopy is employed. The absence of any chemical shift resulting from the substitution confirmed that the adsorbed oxygen-containing intermediate does not incorporate hydrogen. Instead, this intermediate is attributed to the O–O tensile vibration (Fig. 5(d)). Drawing on these findings, we infer that the protonation of *O₂⁻ to form OOH (*O₂⁻ + H⁺ → *OOH) represents the RDS of the ORR on FePc under alkaline conditions. This implies a non-concerted proton–electron transfer process, which may well account for the pH-dependent ORR activity observed on the RHE scale. Apart from the conventional 4e⁻ pathway, the 2e⁻ pathway has also garnered significant attention. Recently, Chen and his co-workers have elucidated the oxygen-containing intermediates in the 2e⁻ pathway using in situ Raman spectroscopy.⁷⁷ Moreover, they have synthesized NiSe₂₋Vse with Se vacancies, which exhibited remarkable electrocatalytic activity for the 2e⁻ pathway, achieving H₂O₂ yields exceeding 90% within the voltage range of 0.25–0.55 V (Fig. 5(e)). As shown in Fig. 5(f), in the in situ Raman spectroscopy, a single peak at 206 cm⁻¹ at the open circuit potential (OCP) is observed, which corresponds to the Se–Se bond. As the reaction progress, Ni–O bands appear at 478 and 553 cm⁻¹ with their intensities gradually increasing, these peaks indicate that the Ni site is the real active site for the adsorption and desorption of the crucial intermediate (*OOH), and both peaks disappear when the potential returns to the OCP, which further indicates that the Ni atom has a high reversibility in this catalyst. Importantly, the Se–Se bond retained stable throughout the reaction, indicating the phase stability of the catalyst.

Fig. 5 (a) Diagram of the in situ Raman spectroscopy device for exploring the internal catalytic process of ORR electrocatalysis with FePc as a model catalyst. In situ Raman spectroscopy test results in (b) 0.1 M HClO₄ and (c) 0.1 M KOH. (d) Employing the isotope labeling method to investigate intermediate transformations.⁷⁶ Copyright (2022) American Chemical Society. (e) Selectivity of H₂O₂. (f) In situ Raman spectra (the red lines) for NiSe₂-V_Se electrocatalyst in O₂-saturated 0.1 M KOH.⁷⁷ Copyright (2022) Wiley. (g) In situ Raman test in 0.1 M KOH solution.⁷⁸ Copyright (2020) Wiley.

In addition, Yang and his colleagues analyzed the oxygen reduction intermediates on rough Au electrodes by in situ enhanced Raman scattering spectroscopy and proposed different reaction pathways and mechanism for ORR in dielectrics with different pH values.⁷⁸ Under alkaline conditions, the two distinct peaks mainly appear at 855 and 1130 cm⁻¹ under a 1.05–0.55 V potential scan (Fig. 5(g)). The peak at 855 cm⁻² is mainly caused by the stretching of the O–O bond in H₂O₂, while the peak at 1130 cm⁻¹ can be attributed to the stretching of the O–O bond in the superoxide ion, but the stretching of the O–O bond in the superoxide ion is significantly wider than that in H₂O₂, mainly due to the stretching of the O–O bond in H₂O₂. The occurrence of oxygen-containing intermediates is explored with the help of the negative scan, and unlike the forward scan, the weak peak at 1130 cm⁻¹ start to appear at about 0.9 V and then intensifies continuously. However, no oxygenated intermediates are detected at 0.9 V for either the forward or negative scan. Electrochemical tests reveal that no Raman signal or cathodic current is detected in the range of 1.05–0.9 V, meaning that no ORR occurs. When the voltage is around 0.85 V, the CV current is significantly enhanced and the O–O bond in the weak superoxide ion is expanded, indicating that this reaction may occur:

O₂ + * → O₂* (16)

O₂* + e⁻ → O₂⁻* (17)

Then H₂O₂ intermediates begin to appear, which may occur as:

O₂⁻* + 2H₂O + e⁻ → H₂O₂ + 2OH⁻ (18)

When the potential is more negative, the CV current decreases slowly and tends to be stable. When the potential sweeps back, n (H₂O₂) disappears at the same potential of negative sweep, which may be because H₂O₂ is further reduced in alkaline electrolyte, that is, this reaction may occur:

H₂O₂* + e⁻ → OH* + OH⁻ (19)

OH* + e⁻ → OH⁻ (20)

In order to increase the selectivity of H₂O₂, Wang and his associates coupled B and N co-doping with surface oxygen functionalization to create a carboxylated hexagonal boron nitride/graphene (h-BN/G) heterojunction on commercial activated carbon.⁷⁹ High 2e⁻ ORR selectivity was demonstrated by the catalyst. The active material co-doped with B and N effectively promotes the adsorption of O₂, stabilizes *OOH and *HOOH intermediates, inhibits the cleavage of O–O bonds, and promotes the formation of H₂O₂, according to in situ Raman spectroscopy performed in a 0.1 M Na₂SO₄ solution. Catalyst performance can also be greatly impacted by the size effect. Contrary to the widely held belief that smaller diameters result in higher catalytic performance, Chen and his colleagues produced zinc oxide nanosheet catalysts (L–ZnO, M–ZnO, S–ZnO) in three distinct thicknesses.⁸⁰ In neutral electrolytes, thick zinc oxide plates demonstrated superior 2e⁻ ORR selectivity. The crucial function of the aberrant size effect in oxygen electroreduction under elevated current density was investigated using in situ Raman spectroscopy. In addition to activating O₂, theoretical studies show that shifting the d-band center towards the Fermi level can stabilize important chemical intermediates. Pan and his collaborators proposed an effective controllable synthesis method to construct atomically dispersed N,S-codoped carbon-anchored single-atom Fe catalysts (Fe SAs/NSC-vd).⁸¹ It was shown that the I_D/I_G in the catalyst stayed at its highest value through in situ Raman testing of a 0.5 M H₂SO₄ solution, indicating the existence of numerous flaws. Certain peak positions in the catalyst displayed a red shift when compared with Fe SAs/NSC, suggesting a weaker bond energy between the Fe center and *O₂⁻/*OOH. This is more favorable for the desorption of compounds containing oxygen and the quick occurrence of ORR.

Meanwhile, Chi and his collaborators have demonstrated that F-FeCoP_v@IF surface reconstruction occurs through in situ Raman characterization,⁸² with the resulting hydroxide (FeCoOOH) serving as the true active site of the OER process. In comparison with samples without F doping, the earlier peak time observed for F-FeCoP_v@IF indicates that F doping is beneficial for the faster conversion of phosphides to hydroxyl oxides, which in turn is conducive to enhancing the OER activity. Liu and his collaborators have synthesized an Mn–FeP_v catalyst with superior OER activity by utilizing doping and vacancy engineering based on nickel foam.⁸³ In situ Raman spectroscopy has revealed that Mn doping and P vacancy can collectively accelerate the phase transition and facilitate the formation of FeOOH. Xu and his collaborators developed a novel catalyst with a heterogeneous structure through a two-step hydrothermal etching process, utilizing self-supporting materials as substrates (FeOOH/Ni₃S₂/NF).⁸⁴ In situ Raman spectroscopy results indicate that during the OER process, Ni₃S₂ in the catalyst rapidly transforms into NiOOH, accompanied by a phase transition from α-FeOOH to β-FeOOH. Through a detailed analysis of the Ni–O bond, it is demonstrated that the phase transition of FeOOH can modulate the lattice disorder of NiOOH, thereby enhancing the catalytic activity. This offers a new perspective for understanding the synergistic mechanisms of multiphase electrocatalysts in the OER process. In contrast to Xu's team, Zou and his collaborators discovered that vacancies can further induce remodeling and accelerate the OER process, although the underlying mechanism remains unclear.⁸⁵ The team synthesized three different samples: Co₃O₄, Co₃O₄-V_O, and Co₃O₄-V_Co. Through in situ Raman spectroscopy, they observed that E_g signals appeared at 290 cm⁻¹ in all samples. When a potential of up to 1.6 V was applied to the Co₃O₄-V_O, the E_g peak did not completely disappear; however, when a potential of 1.5 V was applied to Co₃O₄-V_Co, the E_g peak vanished. This phenomenon is attributed to the accelerated deprotonation process in Co₃O₄-V_Co. Notably, the E_g peak reappeared during the second cycle of the OER test, indicating that the catalyst surface can return to its original state after the first cycle. This observation suggests that Co₃O₄-V_Co maintains a high degree of integrity throughout the OER process, rather than becoming trapped in the first or second step, which would lead to the accumulation and irreversible transformation of intermediate products. The rapid deprotonation and high completeness of the OER in Co₃O₄-V_Co contribute to a reduced reconstruction rate and accelerated reaction kinetics during the OER.

Apart from doing in situ spectroscopic testing in alkaline settings, there has been a growing amount of research conducted in acidic or neutral environments. Ru-doped VO₂ catalysts have recently been effectively produced by Cao and his associates.⁸⁶ In VO₂ substrates, the self-sacrificial mechanism of V can prevent doped Ru from dissolving, shield Ru nanoparticles from oxidation, and greatly increase the stability of acidic OER. Ru–O is detected by in situ Raman spectroscopy in acidic environments, suggesting that Ru NPs were not oxidized to Ru^δ+ (δ > 0) during the OER process. Simultaneously, Zhang and associates presented a stable and reasonably priced manganese oxybromide catalyst that demonstrated superior OER activity in acidic electrolytes (Mn_7.5O₁₀Br₃).⁸⁷ For Raman testing, the team immersed the catalyst in two distinct solutions of 0.5 M H₂SO₄ + H₂¹⁶O and 0.5 M H₂SO₄ + H₂¹⁸O to remove the interference of by-products throughout the reaction process. Because the adsorbed species of Mn–¹⁸O–¹⁸OH were created by the isotope exchange of two ¹⁶O atoms, the peak location of MnOOH was slightly displaced. This spectrum was then used to investigate the stability and catalytic activity of various manganese-based materials (γ-MnO₂ and Mn–O–X, X = Cl, Br). It was determined through computation that the presence of halide ions gives the catalyst a greater capacity for electron transport, therefore improving OER activity.

Both the catalyst and the composition of the electrolyte have an impact on the electrocatalytic activity. Xu and his associates investigated the impact of electrolyte on the OER performance of Co (OH)₂ catalyst in neutral circumstances using KBi/KF as a mixed electrolyte.⁸⁸ It was shown that there were no appreciable structural changes in the catalysts with varying KF concentrations by comparing the in situ Raman spectra of two different KF concentrations. The peak positions in the two solutions were approximately the same, demonstrating that a crucial fluoroborate species produced in mixed KBi/KF to increase the OER rate at close to neutral pH is the BF₂(OH)₂⁻ anion. Meanwhile, Gong and his colleagues proposed an electrode/electrolyte synergistic strategy to achieve efficient neutral water oxidation at the interface of proton and electron transfer.⁸⁹ Due to the electrode/electrolyte synergy, Ir–O and Ir–OO⁻ intermediates could be directly detected by in situ Raman spectroscopy, and the rate-limiting step of Ir–O oxidation was determined.

3.2. In situ FTIR spectroscopy

Raman spectroscopy is predicated on the phenomenon of inelastic scattering, which arises from the vibrations of molecules or lattice structures. In contrast, infrared spectroscopy is derived from the absorption of photons by molecular vibrations.^90,91 The frequency corresponding to the infrared spectrum can be employed to discern specific functional groups, and moreover, the intensity of these infrared peaks is directly proportional to the number of species in the sample.⁹² The FTIR spectrum is particularly sensitive to the presence of water molecules. Typically, the band at 1640 cm⁻¹ corresponds to the H–O–H bending mode, while the –OH stretching mode is manifested at 3100 and 3700 cm⁻¹. In situ FTIR spectroscopy is a sophisticated technique that integrates electrochemical measurement methods with infrared spectroscopy techniques. It is instrumental in elucidating the mechanism of electrocatalytic reactions, modulating the electronic structure of catalysts, and validating new surface bonding configurations. When compared with other in situ technologies, in situ FTIR spectroscopy stands out by prioritizing the detection of catalytic reaction intermediates. It boasts advantages such as high sensitivity and rapid characterization speed, which collectively contribute to a more profound understanding of the intricacies of the catalytic mechanism.^52,93

There are two primary types of in situ infrared spectroscopy configuration, namely internal reflection and external reflection. The internal reflection configuration encompasses techniques such as attenuated total reflection FTIR (ATR-FTIR) and surface enhanced infrared absorption spectroscopy (SEIRAS). Dey et al. employed in situ infrared spectroscopy to elucidate that the peaks observed at 1080 and 1456 cm⁻¹ for Fe,Co-HPNC correspond to the stretching patterns of superoxide anions (O₂⁻) and O₂ molecules adsorbed on the catalyst,⁹⁴ respectively, proving that the ORR process is a 4e⁻ transfer process. Additionally, for the OER process, as the potential increases from 1.3 V to a higher value, new absorption bands also appear in the FTIR spectrum around 1048 cm⁻¹.

Liu and his collaborators developed a novel electrocatalyst featuring a NiZn core through a one-pot wet chemical synthesis method.⁹⁵ To elucidate the underlying factors responsible for the exceptional H₂O₂ selectivity exhibited by NiZn MOF catalysts, in situ SR-FTIR measurements are conducted under authentic catalytic conditions. Specifically, when the vibration range at 700–1300 cm⁻¹ is examined at 0.8 V versus RHE (Fig. 6(a)), no distinct absorption peaks are observed. Conversely, at lower applied potentials (≤0.7 V vs. RHE), an absorption band related to the IR absorption of *OOH at 1050 cm⁻¹ is observed, which corresponds to *OOH intermediate in the 2e⁻ ORR process. The intensification of this peak as the potential decreases suggests that a greater number of intermediates are actively participating in the catalytic reaction process. A similar transformation is noted at the 920 cm⁻¹, which is attributed to the Ni–O bond, and this observation suggests that an increased number of oxygen-containing species coordinate with the metal center with a decrease in voltage. At the same time, a distinct absorption band is detected at 3480 cm⁻¹, which is attributed to the stretching vibration of O–H bonds originating from adsorbed water molecules. And the enhancement in the peak intensity is also attributed to the buildup of adsorbed O–H groups at the metal surface sites. To elucidate the correlation between the formation of critical *OOH intermediates and the accumulation of O–H species, Fig. 6(b) illustrates the peak intensity of the absorption bands at 1050 and 3480 cm⁻¹ as a function of the applied potential. The intensity of the absorption bands centered at 1050 and 3480 cm⁻¹ exhibits a marked increase as the applied potential decreases. Notably, at potentials weaker than 0.5 V, they display a parallel enhancement trajectory and converge towards a state of near saturation. This synergistic effect implies that the adsorbed water molecules are most likely to be the predominant source of hydrogen atoms that facilitate the formation of crucial *OOH intermediates (Fig. 6(c)). Yuan and his collaborators have described a strategy for the development of a three-functional electrocatalyst featuring a core–shell structure (Fe–Co₂P@Fe–N–C).⁹⁶ The integration of transition metals with NC contributes to the formation of a novel M–N–C shell characterized by a M–N₄ coordination site (Fig. 6(d)). In situ infrared spectroscopy testing reveals that the hydroxyl stretching vibration at approximately 3450 cm⁻¹ is scarcely detectable within the voltage range of 1.10 to 0.95 V (Fig. 6(e)). However, a distinct absorption peak corresponding to this vibration becomes increasingly prominent at a voltage of 0.9 V and further develops at 0.8 V. The –OH bond stretching reveals that the Fe–N–C shell on the Fe–Co₂P@Fe–N–C surface facilitates the electrochemical reduction of O₂ to produce water. Concurrently, the –HOH bond is also detected in the voltage range of 0.9–0.8 V, and it begins to become stronger at the voltage of 0.8 V. When comparing the in situ infrared spectra of the Fe–Co₂P@NC samples, it becomes evident that there is no distinct stretching of the –OH and –HOH bonds in the reference sample. This observation indicates that the Fe–N–C reaction shell serves as the primary active site of ORR electrocatalysis.

Fig. 6 (a) The in situ SR-FTIR spectra of the NiZn MOF catalyst within the wavenumber range of 700–1300 cm⁻¹ and (b) 2900–3850 cm⁻¹ at various potentials. (c) Infrared signals of NiZn MOF at 1050 and 3480 cm⁻¹ versus the potential.⁹⁵ Copyright (2022) Wiley. (d) Schematic diagram of the ORR process of two different constructed electrocatalysts (from Co₂P@NC and M-Co₂P@M–N–C). (e) In situ FTIR spectra of Fe-Co₂P@Fe–N–C catalyst during the ORR process.⁹⁶ Copyright (2021) Wiley. (f) In situ FTIR spectra for Pt_0.2Pd_1.8Ge during the ORR CA at 0.4 V vs. RHE in 0.1 M KOH. (g) Enlargement of the peak near 1400 cm⁻¹. (h) Comparison of the peak maxima position shifting with time.⁹⁷ Copyright (2022) American Chemical Society.

Research on the use of in-situ infrared technology in ORR was also carried out by Peter and his associates.⁹⁷ Diverging from the approach of Liu and Yuan, the research team explored the oxygen-containing intermediates by varying the time under constant voltage. Germanium is found to alter the electronic structure and oxygen binding affinity of the intermetallic compound catalyst. Peter et al. studied the ordered intermetallic compound Pd₂Ge, which demonstrated excellent ORR performance. In order to explore enhance *OOH adsorption and H₂O production, further characterization is carried out by in situ infrared spectroscopy (Fig. 6(f)). It can be seen from the figure that the peaks at 1429 and 1080 cm⁻¹ correspond to the O–O stretching mode of adsorbed O₂ molecules and O₂⁻, respectively. Additionally, the peak at 1270 cm⁻¹ emerges gradually over time. The existence of this peak indicates that the *OOH species adsorb on the catalyst surface and exhibit bending mode vibration. At the same time, a H–O–H bending vibration is observed near 1580 cm⁻¹, indicating that water is produced during the reaction. Due to the O–H stretching frequency, a wide hump appeared at 3000 cm⁻¹, which strongly confirms the formation of water. As depicted in the Fig. 6(g), the shift in the peak position at 1429 cm⁻¹ can be observed over time, indicating that the strength of the O–O bond changed during the ORR process. At 200 seconds (Fig. 6(h)), the decrease in wavenumber is attributed to the weakening of the O–O bond, resulting from the electron transfer from the Pd catalyst to the active site of molecular oxygen. This weakening facilitates more facile cleavage of the O–O bond, favoring the 4e⁻ transfer pathway typically associated with the ORR.

To improve ORR in acidic conditions, Zhu et al. suggested a mixed ORR catalyst made up of ultrafine PdNC and Fe single atoms with favorable spin states (Fe–N–C/PdNC).⁹⁸ The strong electronic interaction between PdNC and Fe single atoms causes a spin state transition of Fe sites from low spin (LS) to medium spin (MS), which is responsible for the enhancement of PdNC. In situ FTIR revealed that the two peaks of HOOH_ad and OOH_ad were considerably weaker than those of Fe–N–C, firmly verifying the spin Fe(II) state shift from LS to MS and blocking the generation of H₂O₂ to increase stability and selectivity. In order to improve the acidic ORR performance and comprehend the connection between local atomic strain and reaction kinetics, Yin and associates simultaneously used the synergistic effect of diatoms to modify the atomic strain environment.⁹⁹ FeRu–N–C shows OOH and *O₂ instead of *HOOH, which increases the activity and stability of ORR by facilitating the hydrogenation dissociation of *OOH. The charge transfer between adsorbed oxygen and O₂ causes a noticeable red shift in *O₂ at the same moment. The ORR performance is further enhanced by the negative shift of the d-band center of FeRu–N–C, which is advantageous for the hydrogenation desorption of (oxygen) hydroxyl groups, according to the results of the electronic structure study. Using a straightforward one-step pyrolysis technique, Ye and his colleagues created ZnO–NG heterojunctions with a large number of oxygen defects and Zn–N chemical interactions at the interface in ZnO.¹⁰⁰ ZnO–NG shows more distinguishable *OOH bands than NG and ZnO, according to in situ FTIR spectroscopy of 1.0 M Na₂SO₄. Inhibiting the 4e⁻ ORR route and improving the 2e⁻ selectivity, the combined effect of metal and non-metal sites dramatically lowers the energy barrier for *OOH intermediate production.

Jiang and his associates used the cation exchange approach to create crystalline Ru electrocatalysts (Ru/MnO₂) supported by α-MnO₂ nanofibers.¹⁰¹ In addition to initiating the reconstruction of tiny Ru clusters into large Ru atomic arrays, the cation exchange process prevented metal leaching-induced catalyst deactivation. Additionally, this led to a shorter distance (2.9 Å) between Ru atoms than in RuO₂ (3.1 Å), which facilitates the coupling of O–O radicals and enhances the catalyst's OER performance. The catalyst's adherence to the OPM mechanism is further supported by in situ infrared spectroscopy, which further validates the presence of *O–O* in the catalyst.

3.3. In situ/operando X-ray-based techniques

The intricate interplay between X-rays and substances gives rise to distinct phenomena, including absorption, scattering, and transmission, which are harnessed for both qualitative and quantitative material analysis.^102,103 This powerful technique serves as a non-destructive probe to decipher the transformations in the phase, crystallographic structure, and chemical composition of materials across a wide range of spatial and temporal scales.^104,105 However, it is noteworthy that the intensity of conventional X-rays is limited in their ability to penetrate substantial thicknesses of matter, restricting their depth penetration capabilities. In the context of real-time investigations of advanced electrochemical systems, X-ray technology demands adequate penetration capabilities to traverse the equipment's outer layers. Based on this, a translucent window, typically made from materials like beryllium, Kapton tape, or polymer films, is strategically incorporated to allow X-ray transmission. While these materials are essential for practical implementation, they inadvertently compromise the resolution and precision of the analysis. However, with the advent of advanced in situ characterization methodologies, X-ray utilization has expanded, often operating across a broader spectral spectrum, enhancing the overall analytical capabilities.

X-ray light sources can be divided into ordinary X-ray and in situ X-ray. The intensity of the in situ X-ray source directly correlated with their enhanced penetration power, with the higher the intensity, the shorter the measurement duration, thus proving advantageous for real-time analysis,^106,107 which is indisputable. In situ X-ray technology boasts remarkable attributes, such as minimal sample disruption, high brightness and energy adjustability. It enables the acquisition of precise structural information about catalytic materials without compromising the integrity of the sample in the field. During the reaction process, it is capable of continuously monitoring (electrochemically) the reaction processes and quickly capturing dynamic information on catalysts. The spatial resolution ranging from nanoscale to microscale provides the potential for direct visualization and 3D reconstruction. In recent years, in situ X-ray characterization techniques, such as XRD, X-ray absorption spectroscopy (XAS) and XPS, have gain significant popularity in the study of electrocatalysis.

3.3.1. In situ XAS spectroscopy. XAS is a form of inelastic scattering that encompasses X-ray absorption near-edge structures (XANES) and extended X-ray absorption fine structures (EXAFS) (Fig. 7(a)).¹⁰⁸ XANES, which pertains to the region up to 25 eV above the edge, is used to uncover information about oxidation states and electronic configurations.¹⁰⁹ Specifically, atoms with higher oxidation states exhibit higher effective nuclear charges. On the other hand, EXAFS, which lies in the higher energy region, offers insights into interatomic spacing or coordination numbers.¹¹⁰ To gain a comprehensive understanding of the solid–liquid interface dynamics, it is imperative to employ a suite of advanced techniques.^104,111 In situ XAS spectroscopy stands out for its high sensitivity to local structural details. This technique is undeniably recognized as a robust instrument for investigating the structural transformations of catalysts as they undergo oxygen electrocatalytic processes.^11,111

Fig. 7 (a) Schematic for correlative FTIR and XAFS.¹⁰⁸ Copyright (2020) American Chemical Society. (b) ORR performance in 0.1 KOH. (c) The Pt L₃-edge and (d) Fe K-edge FT-EXAFS spectra and the corresponding fitting curves under different potentials. (e) The fitting results of the coordination number.¹¹⁵ Copyright (2022) Nature.

In situ XAS typically determines active sites by evaluating the disparity between the near-edge energy shift and implied atomic distance, reflecting alternations in the catalyst's oxidation state during electrochemical reactions. XANES analysis often proves time-consuming and less responsive to high temperatures, which actually enhances its suitability for in situ investigations conducted at high thermal conditions.^112,113 For OER or ORR, most in situ characterization studies use Raman or infrared spectroscopy, and there remains a scarcity of reports that specifically leverage in situ XAS for such analyses. Notably, Mukerjee and colleagues reported two alloy catalysts (D-PtCo/HSC and D-PtCo₃/HSC NPs),¹¹⁴ which exhibited excellent catalytic activity and durability for ORR compared with commercial Pt/C NPs. Through meticulous in situ XAS analysis, it is demonstrated that these alloy catalysts possess a strikingly similar coordination environment to Pt/C, thereby revealing the structural foundation that underpins the enhanced ORR efficiency at its core. Meanwhile, Abruña et al. employed in situ XAS to investigate the synergistic interactions within Co Mn oxide catalysts.¹¹² According to the XANES spectrum near the K-edge of Mn, as the applied potential decreases, the peak intensity exhibited a gradual rise, accompanied by a subtle shift towards lower energy. This observation suggests that the Mn valence state becomes more reduced at more negative potentials. Overall, the average valence of Mn has decreased from 3.15 to 2.91. The systematic valence transition from Mn (III, IV) to Mn (II, III) implies that various Mn species can act as active sites for catalyzing ORR.

Liu and his colleagues pioneered the development of a novel N-bridge Pt=N₂=Fe atomic bimetallic electrocatalyst, engineered with customized geometry.¹¹⁵ Pt=N₂=Fe exhibits excellent ORR performance under alkaline conditions (Fig. 7(b)). To gain a deeper understanding of the structural evolution of bimetallic assembled active sites, in situ XAFS analysis of the Pt L₃-edge and Fe K-edge was conducted. As depicted for the Pt L₃-edge (Fig. 7(c)), under 0.1 M KOH electrolyte and at an applied voltage of 1.05 V, the intensity of the principal peak at 1.55 Å increased significantly, which can be attributed to the coordination within the first shell of Pt–N/O interactions. Furthermore, at the Fe K-edge (Fig. 7(d)) under an applied voltage of 0.95 V, the intensity of the dominant peak at 1.42 Å increased, suggesting that both the Pt and Fe metal sites participated in the reaction. This observation indicates that these sites underwent a comparable coordination evolution throughout the ORR process. In order to quantify the local structural evolution of the Pt=N₂=Fe double site, the corresponding FT-EXAFS curve fitting was carried out. The comprehensive analysis of the fitting results for both sites shows that oxygen molecules can be adsorbed on Pt and Fe atoms at suitable distances, potentially facilitating the coupling of the intermetallic assembly sites of the dioxygen intermediate (O–O) through a two-site adsorption configuration (Pt–O–O–Fe) (Fig. 7(e)). This double-site adsorption arrangement can provide a robust driving force for the direct cleavage of the O–O bond, modulate the energy of multiple reaction intermediates, and enable the rapid 4e⁻ ORR process.

Huang and his collaborators have successfully prepared CdRu₂IrO_x nano-frameworks with twisted structures, specifically designed for acidic OER (Fig. 8(a)).¹¹⁶ The distorted model of the CdRu₂IrO_x structure is depicted in Fig. 8(b), leveraging atomic distance analysis based on AC-HAADF-STEM, and the synergistic effect between Ru and Ir in CdRu₂IrO_x is found to induce Ru–O, Ir–O and Ru–M (M = Ru) formation (Fig. 8(c) and (d)). This collaborative interaction significantly contributes to the enhanced OER performance. In situ XAS reveals that the increment in applied potential drives a positive shift in the Ir L₃ edge and the concurrent contraction of the Ir–O bond (Fig. 8(e) and (f)). These changes lead to a deformation in the octahedral arrangement of RuO_x and IrO_x, ultimately facilitating the stabilization of Ru⁵⁺ species during the OER process. The spectroscopic data thus demonstrate a clear correlation between the applied potential and the structural transformations that are crucial for enhanced OER kinetics (Fig. 8(g)).

Fig. 8 (a) Schematic diagram illustrating the synthesis of CdRu₂IrO_x. (b) A distorted model of the CdRu₂IrO_x structure. (c) AC-HAADF-STEM image of CdRu₂IrO_x and (d) measurement atomic distance map. In situ XANES spectra of CdRu₂IrO_x measured at different electrode potentials, focusing on the (e) Ir L₃-edge and (f) Ru K-edge during the OER. (g) Ir L₃-edge and Ru K-edge at different electrode potentials.¹¹⁶ Copyright (2023) Wiley.

In addition, Sun and colleagues prepared ORR electrocatalysts (Cu-HHTP) with strong π conjugated M–L covalent centers.¹¹⁷ The structural evolution of the metal Cu active site in Cu-HHTP is meticulously tracked using pertinent in situ synchrotron radiation spectroscopy techniques. These methods provided valuable insights into the structural changes that occur within the Cu-HHTP catalyst during the ORR process. As depicted in Fig. 9(a), Cu K-edge XANES data clearly exhibit a decline in the intensity of the white line peak, which is directly proportional to the decrease in applied potential. The XPS spectrum of Cu 2p shows a decrease in the oxidation state of Cu species, which is consistent with the XANES results (Fig. 9(b)). It is noteworthy that the bond length of the Cu–O bond exhibited a gradually decrease, from 1.94 Å to 1.92 Å at 0.66 V down to 1.91 Å at 0.46 V, signifying a contraction in the Cu–O coordination sphere within Cu-HHTP during the ORR. This observation is further substantiated in Fig. 9(c) and (d), which illustrates the progressive transformation of the Cu site in the catalyst as the reaction proceeds. At 0.66 or 0.46 V, a dynamic *OH species is formed in situ on the Cu site within the catalytic phase; subsequently, this dynamic coupling *OH effectively triggered the contraction of the Cu–O around the central Cu site, promoted the O–C elongation in the ligand and then induces self-polarization within the M–L Cu–O–C center. The electronic redistribution within the Cu–O–C center enhances the formation of the crucial intermediate *OOH at the C site, thereby facilitating the favorable 2e⁻ ORR pathway. Incorporation of transition metals through doping significantly enhances the catalytic performance of the catalyst. Huang and his collaborators proposed that Mo dopants can improve the ORR kinetics by changing the coordination environment of Pt atoms on the catalyst surface;¹¹⁸ through in situ XAFS and electrochemical investigations simultaneously, their research conclusively demonstrates the beneficial role of surface Mo dopants (Fig. 9(e) and (f)). At a potential of 0.54 V, compared with non-Mo-doped catalyst, the Pt L₃-edge XANES spectrum of the Mo-doped PtNi/C catalyst exhibits an intermediate characteristic between that of pure Pt/C and PtNi/C, substantiating its improved performance. The white line intensity of Pt L₃-XANES is due to the electron transition from the 2p orbit to vacancy 5d orbit, and it rises in tandem with an increase in d-band vacancies. The presence of additional electronic neighbors in Mo-PtNi/C leads to a lower d-band vacancy, consequently resulting in an anticipated lower white line intensity compared with PtNi/C. However, the Pt white line strength of Mo-PtNi/C is notably greater than that of PtNi/C, which indicates that electrons are transferred from Pt oxide to Mo oxide. This electron transfer direction is attributed to the intrinsic tendency of Pt, with its pair d electrons to donate electrons to oxygen in Mo cations or oxides, which possess relatively vacant d orbitals (Fig. 9(g)). Using in situ characterization techniques, Shi and his colleagues have ascertained the dynamic evolution of marginal monoatomic catalytic sites in Co–N–C materials at the atomic level.¹¹⁹ The distinctive edge-hosted design gives the atomic Co sites great structural flexibility and promotes effective ORR processes, as confirmed by in situ XAFS experiments.

Fig. 9 (a) In situ XANES spectra of Cu K edges in catalysts, (b) XPS spectrum of Cu 2p. (c) FT-XAFS fitting results in the catalyst. (d) Reaction diagram in the 2e⁻ pathway.¹¹⁷ Copyright (2022) Elsevier. In situ XANES spectra of (e) Pt L₃-edge and (f) Ni K-edge at 0.54 V in O₂-purged 0.1 M HClO₄. (g) Schematic diagram of the intermediate adsorption process on the catalyst surface and reaction of different catalysts under 0.9 V.¹¹⁸ Copyright (2018) American Chemical Society.

Using the co-precipitation approach, Zou and his colleagues created Ni Fe rich in ligand defects, which demonstrated outstanding OER performance in 0.1 M KOH solution.¹²⁰ The atomic and electronic structures of Fe hydroxide analogues in the α-NiFe catalyst were shown to have stayed nearly unaltered throughout the OER process through study of the Ni K-edge and Fe K-edge. The real catalytic phase of OER is the deprotonated and oxygen-deficient Ni⁴⁺ hydroxide produced in situ by the reversible conversion of Ni species between Ni hydroxide and Ni⁴⁺ hydroxide in the α-NiFe catalyst. By optimizing the electronic structure of the Ni species and preserving the structure of the hydroxide analogue, the Fe species in the α-NiFe catalyst increased the OER activity. Dong and his associates created a new catalyst by combining semiconductor CdS/CdSe-MoS₂ with NiFe layered double hydroxides using an interface engineering technique (TQ NiFe).¹²¹ According to in situ XAS data, photogenerated holes boost the oxidation of Ni²⁺ to active Ni⁴⁺, increase the OER catalytic activity, and encourage the structural transition of NiFe(OH)₂ to NiFeOOH.

CoS catalysts were synthesized by Song et al., who then thoroughly examined their OER performance in both neutral and alkaline environments.¹²² According to measurements using in situ XAFS and photoelectron spectroscopy, Co₉S₈-SWCNT experiences different levels of structural reconstruction in neutral and alkaline environments. In neutral environments, it transforms back into oxygen-containing cobalt sulfide (O-CoS-SWCNT), while in alkaline environments, it forms sulfur-containing hydroxylated cobalt oxide (S-CoOOH-SWCNT). This work offers comprehensive catalytic mechanisms to direct the logical design of advanced OER electrocatalysts and reveals the essence of the structural self-optimization process of OER catalysts. Wei and his associates created an Ir single atom catalyst that is heavily loaded and has a lot of d-band holes (h-HL Ir SAC).¹²³ It was shown by combining in situ XAFS that the intensity of the Ir–O peak stays nearly constant as the potential rises to 1.45 V, suggesting that a dynamic equilibrium between the adsorption and desorption of intermediates containing oxygen can be reached. The delayed OER kinetics process could be the cause of the LL Ir SAC's slow coordination number growth. High-loading iridium centers have been shown to rapidly aggregate important oxygen-containing intermediates on robust Ir–N₄ active centers at early voltages to form a nearly stable XO*–Ir–N₄ (X = H/OH) structural portion, which in turn softens the working potential to drive higher current densities and is the cause of greatly enhanced acidic OER activity and stability. This is demonstrated by combining SR-FTIR and Raman spectroscopy. Xing and colleagues suggested that the catalyst/carrier interaction plays a crucial role in catalysing the acidic OER process.¹²⁴ With I_x/Nb₂O_5−x as a model catalyst, in situ XAS results demonstrated that the Nb₂O_5−x substrate not only encourages Ir oxidation at low potentials but also inhibits its peroxidation at high potential biases, underscoring the critical role of the I_x/Nb₂O_5−x interface, in contrast to standard Ir NPs catalysts. Additionally, this offers a fresh theoretical framework for investigating the connection between catalytic performance and interface structure evolution.

3.3.2. In situ XRD spectroscopy. XRD is a highly effective technique for investigating the crystalline characteristics of materials. It involves the scattering of X-rays by a long-term ordered periodic array from a crystalline material,¹²⁵ and certain diffraction peaks at specific angles are obtained, which can provide accurate crystallographic information, including lattice parameters and preferred orientations.^126,127 The expression “2d sin θ = kλ” is derived from Bragg's Law, and this equation is crucial for understanding the behavior of monochromatic X-rays when they interact with a crystal.^126,128 When monochromatic X-rays are incident on a crystal, the distance between atoms is of the same order of magnitude as the wavelength of the incident X-rays. As these X-rays are scattered by different atoms within the material, they interfere with each other, resulting in distinct XRD patterns that are observable in specific directions.¹²⁹ Therefore, the analysis of strain, crystal plane characteristics, doping effect, and phase separation can be effectively conducted through XRD. The development of in situ characterization methods, which utilize small incident angles, has led to the emergence of in situ XRD as a potent instrument for real-time determination of the crystalline state at the catalyst surface.¹²⁸ In situ XRD enables the real-time detection of crystal phases during the ORR process, and the evolving diffraction patterns elucidate the phase transition dynamics of catalysts and intermediate species. Seo and his colleagues successfully synthesized choline chloride malonic acid under mild reaction conditions,¹³⁰ utilizing deep eutectic solvents (DES) as an auxiliary material for γ-CoV₂O₆. The incorporation of DES significantly reduces the energy barrier of the reaction, thereby facilitating the formation of a distinct octahedral morphology for the catalyst. In situ XRD analysis confirms the presence of α-CoV₂O₆ at high temperatures, and α-CoV₂O₆ is ultimately converted into γ-CoV₂O₆. The enhancement in the OER performance of the catalyst can be ascribed to its morphological benefits and the formation of related reaction intermediates.

Zhang and his research team employed electrochemical techniques to intercalate lithium into the lattice gaps of RuO₂, thereby achieving a tunable lithium concentration and enhancing the OER activity and durability of RuO₂ in acidic conditions.¹³¹ With a lithium concentration of 0.52 mA g⁻¹, the research team successfully synthesized a novel Li_0.52RuO₂ catalyst, which exhibited optimal catalytic performance. Through in situ XRD characterization analysis at a constant current density of 10 mA g⁻¹ (Fig. 10(a)), it is observed that a solid solution phase, Li_xRuO₂, emerged with a structure akin to the original RuO₂'s rutile framework during the initial stages of lithium intercalation. This phase preserved the rutile structure of the RuO₂ parent material, albeit with a subtle shift in the positions of its characteristic peaks towards more negative angles. Further lithiation triggered a first-order phase transition from the solid solution phase to the LiRuO₂ phase, precipitating a distinct set of diffraction peaks. However, upon completion of the electrochemical lithiation process, the LiRuO₂ phase becomes unstable. During the reaction process, the XRD peak intensity of the LiRuO₂ phase progressively diminishes, while the XRD peak intensity of the Li_xRuO₂ phase incrementally intensified, indicating a reversible transformation from the LiRuO₂ to Li_xRuO₂ phase. From this analysis, it can be inferred that the final structure of RuO₂ following lithium insertion is Li_xRuO₂, representing a solid solution phase. The comparison results of XRD and HAADF-STEM analyses conducted both before and after the reaction serve to further corroborate this observation (Fig. 10(b) and (c)). In parallel, Luo and his collaborators have observed that most current research concentrates on the noncovalent interactions between electrolyte cations and OER intermediates.¹³² However, there has been a notable absence of comprehensive studies that explore the dynamic relationship between the structure, adsorption, and catalytic performance of metal hydroxides in OER processes. Therefore, the research team employed in situ XRD and HRTEM to verify that electrolyte cations could be incorporated into the original CoOOH catalyst layer during OER (Fig. 10(d)). Simultaneously, in different electrolytes, the larger the cation size, the larger the interlayer spacing, and the higher the OER activity, with the order Cs⁺ > K⁺ > Na⁺ > Li⁺. At the same time, in situ Raman spectroscopy is employed to aid in substantiating the hypothesis that the augmentation in the size of electrolyte cations is directly correlated with the elongation of Co–O bonds within the phase structure (Fig. 10(e)). This lengthening facilitates the formation of OER-active Co(IV) species, leading to a shift of the Co 3d bond center towards higher binding energies, enhancing the adsorption of oxygen-containing intermediates, and accelerating the reaction kinetics of the OER (Fig. 10(f) and (g)). Chao and his collaborators conducted specific research on the crystal phase evolution of NiMoO₄, successfully achieving uniform structures that incorporate both α-NiMoO₄ and β-NiMoO₄ phases (Fig. 10(h)).¹³³ In situ thermal XRD analysis reveals the coexistence of α-NiMoO₄ and β-NiMoO₄ phases (Fig. 10(i)), suggesting that due to homogeneous boundary effects, the active Ni atoms exhibit more effective electron transfer capabilities at the α/β-NiMoO₄ interfaces. Li and his colleagues used in situ high-energy XRD to analyze the structural evolution in FeSA-N/TC.¹³⁴ ZIF-8 and ZnO gradually vanished from the catalyst as the temperature rose, and when the temperature reached 800 °C, the ZnO cores reacted with the carbon in the shell layer of Fe–N/C to form Zn metal, CO/CO₂ gas, and nanopores in Fe–N/C. A monoatomic crystalline phase was then obtained by evaporating Zn metal at the high temperature. DFT is used to show that the bent Fe–N₄ active sites can reduce the free energy barrier and enhance the performance of the ORR process by weakening the intermediate adsorption.

Fig. 10 (a) In situ XRD image of RuO₂ under constant current density. (b) Ectopic XRD patterns of RuO₂ and Li_xRuO₂. (c) HAADF-STEM images of RuO₂ (left) and Li_xRuO₂ (right).¹³¹ Copyright (2022) Nature. (d) Explanation diagram of the high-efficiency performance mechanism of CoOOH catalyst. (e) In situ Raman pattern of CoOOH in 1.0 M CsOH solution. (f) Changes in Co oxidation state and Co–O bond length of the catalyst at 10 mAcm⁻². (g) Illustration of molecular orbital energy from CoOOH-Li⁺ to CoOOH-Cs⁺.¹³² Copyright (2023) Wiley. (h) Schematic illustration of lattice matching in α-NiMoO₄ and β-NiMoO₄. (i) In situ thermal XRD analysis of the catalyst.¹³³ Copyright (2024) Wiley.

However, in situ XRD is not well-suited to the study of amorphous materials. Conversely, XRD data are intricate and necessitate meticulous analysis to prevent the potential omission of crucial information.

3.3.3. In situ XPS spectroscopy. XPS is a highly surface sensitive quantitative spectroscopic technique capable of assessing the elemental composition, as well as the chemical and electronic states of elements within samples.¹³⁵ XPS is also the method of choice for analyzing all chemical elements with atomic numbers 3 or greater (including Li), offering meticulous data on the atomic chemical configurations.¹³⁶

When X-rays come into contact with the surface of the catalyst, they excite electrons, thereby producing photoelectrons that are subsequently emitted into a vacuum.¹³⁷ By quantifying the binding energy of these emitted electrons, it becomes feasible to ascertain the elements and chemical states present on the catalyst surface. In contrast to in situ XRD, in situ XPS boasts a broader analytical scope, enabling the real-time observation of electron excitement, transfer, and bonding at the semiconductor interfaces.¹³⁸ This allows for the qualitative assessment of surface elements through the measurement of electron binding energies. Building upon the principles of conventional XPS spectroscopy, in situ XPS incorporates ultraviolet and visible light sources, necessitating high vacuum conditions for operation.¹³⁹

Zhou and his collaborators synthesized an amorphous FeOOH/Co(OH)₂ heterojunction microarray, which serves as an effective electrocatalyst for the OER, through the employment of mechanical stirring at room temperature.¹⁴⁰ The in situ XPS is used to study the changes in surface composition of the FeOOH/Co(OH)₂ catalyst during the OER process (Fig. 11(a)). Compared with the original sample, the surface valence of Co increased from +2 to +3 after applying a voltage of 1.124 V, indicating the formation of CoOOH in the sample. As the applied voltage is incrementally raised to 1.624 V, the peak position of the spectrum underwent a migration of 0.4 V, which is due to the formation of stable Co_1−xFe_xOOH caused by the addition of Fe to the sample. At the same time, the in situ XPS images of Fe show that as the potential gradually increases, the valence state of Fe does not change significantly. These results indicate that the surface FeOOH membrane can continue to penetrate into the bottom layer of CoOOH, forming a reconstructed Co_1−xFe_xOOH layer, which is also the reason why the catalyst has excellent OER activity (Fig. 11(b)). At the same time, Fu and his team discovered that the unique 4f orbitals in rare earth metals can couple with the 3d orbitals of transition metals (Fig. 11(c) and (d)).⁵⁷ Employing a gradient orbital coupling strategy, N-doped carbon loaded Eu₂O₃-Co was synthesized (Eu₂O₃-Co/NC). The catalyst exhibits excellent ORR performance under alkaline conditions, and the changes in the surface oxidation state of Eu₂O₃-Co/NC were investigated using quasi XPS technology (Fig. 11(e) and (f)). As the voltage decreases to 0.9 V, both Eu 3d and Eu 4d exhibit a shift towards higher binding energies, indicating that oxygen molecules such as O₂⁻ and OH⁻ undergo chemical adsorption, but Co 2p peaks remain largely unchanged. This further indicates that oxygen-containing substances are mainly adsorbed at Eu sites. Upon further voltage reduction to 0.6 V, an increase in the signal corresponding to Eu²⁺ is observed, which may be due to electrons receiving Eu³⁺ to generate Eu²⁺–Eu³⁺ pairs through surface gradient charges. At the same time, the main peak of Co 2p also exhibits a negative decrease (Fig. 11(g)). When the voltage drops to 0.4 V, both Eu 3d and Eu 4d undergo negative shifts, indicating that *OH in the catalyst becomes OH⁻. When the potential is further reduced to 0.2 V, there is a slight positive shift observed in both Eu 3d and Eu 4d peaks, denoting the gradual cover of oxygen species in alkaline electrolytes. The in situ characterization also indicates that the Eu²⁺–Eu³⁺ pairs formed by orbital gradient coupling accelerate electron transfer, optimize the binding strength of oxygen-containing intermediates in ORR, reduce the reaction energy barrier for *OOH to *O conversion, and accelerate the occurrence of the ORR process (Fig. 11(h)). Ma and his colleagues utilized Zn/Co MOF as a precursor, embedding Fe and S into the Co–N₄ structure through a high-temperature calcination process by employing a dual solvent method, thereby synthesizing a bimetallic S–Fe–Co–N₅ catalyst.¹⁴¹ In the OER process, M–O or M–OOH is commonly employed as the active substance, as shown in Fig. 11(i). By analyzing the XPS spectra of Co, Fe, and O at different voltages, it is observed that the peak intensity of M–O bonds progressively intensifies with the application of higher voltages (Fig. 11(j)). This also proves the formation of M–O axial ligands within the catalyst during the reaction process, with a shift in the binding energy of Co/Fe species towards higher values, indicating that Co and Fe elements jointly participate in the reconstruction process (Fig. 11(k) and (l)). The precise assembly of this O-axis ligand in the S–Fe–Co–N5–O structure significantly reduces the reaction energy from *O to *OOH, thereby accelerating the kinetic process of OER (Fig. 11(m) and (n)). Wang and his colleagues utilized simulated Na_xMn₃O₇ materials to devise an effective strategy for developing superior oxygen evolution electrocatalysts,¹⁴² and this is achieved by precisely tuning the reactivity of lattice oxygen and the structural dynamics through the manipulation of alkali metal ions. By utilizing in situ XPS spectroscopy, it has been elucidated that alkaline-metals are instrumental in modulating the reactivity of lattice oxygen and the associated structural interactions. Ling and his collaborators constructed the RuO₂/CoO_x interface, successfully overcoming the stability and activity constraints of RuO₂ in neutral and alkaline media.¹⁴³ In situ XPS analysis of the OER process under neutral conditions revealed that the alterations in the Ru 3d orbitals exhibit minimal variation with the escalation of potential. Quantitative analysis further revealed that Ru³⁺ and Ru⁴⁺ coexist in almost the same proportion, indicating that RuO₂ plays a crucial role in stabilizing the interface equilibrium. By using in situ UV spectroscopy and quantitative electron paramagnetic resonance (EPR) spectroscopy to analyze Co, it is observed that with the increase of potential, the valence state of Co progressively elevates, with no dissolution observed upon reaching the Co⁴⁺ state. This indicates that the carrier CoO_x selectively undergoes oxidation to safeguard RuO₂. Under neutral conditions, RuO₂/CoO_x composite demonstrates enhanced performance relative to pure RuO₂ and CoO_x. The excellent OER performance may be attributed to the unique electronic and geometric effects generated by the exposed Ru/Co diatomic sites in the vicinity of the interface. By combining Tb with Co species, Fu and his associates created a unique rare earth-based catalyst that performed better than Pt/C quasi-based catalysts (Co@Tb₂O₃/NC).¹⁴⁴ Quasi-in situ XPS spectroscopy was used to determine the binding energy and full width at half maximum of Co 2p and Tb 3d/4d, indicating that the ORR mechanism successfully covered and converted oxygen intermediates. Concurrently, the Co 3d band narrows with the addition of Tb, creating the perfect s/p bond combination between the Co sites and oxygen intermediates.

Fig. 11 (a) In situ XPS spectra of Co 2p and Fe 2p for FeOOH/Co(OH)₂. (b) Polarization curves of different catalysts in 1 M KOH solution.¹⁴⁰ Copyright (2024) Wiley. (c) Theoretical prediction for rational design of RE-based materials based on orbital coupling engineering. (d) The molecular orbital energy level diagram of O₂. In situ high-resolution XPS spectra of (e) Eu 3d spectra, (f) Eu 4d spectra and (g) Co 2p spectra. (h) LSV curves of catalysts in O₂-saturated 0.1 M KOH.⁵⁷ Copyright (2022) Wiley. (i) Schematic illustration of the preparation process of the S-Fe-Co-N₅. The quasi-in situ XPS spectra of (j) O, (k) Co, and (l) Fe for S–Fe–Co–N₅, respectively. (m) OER polarization curves of catalysts. (n) Corresponding Tafel plots of different catalysts.¹⁴¹ Copyright (2023) Wiley.

3.4. In situ Mössbauer spectroscopy

Mössbauer spectroscopy is a cutting-edge characterization technique that is considered a powerful tool for determining the physical phase of catalysts, and sites, and investigating the correlation between catalytic activity and the coordination structure of catalysts.¹⁴⁵ Furthermore, the technique is employed for the precise identification of distinct nuclear configurations within certain elements, including Fe, Sn, Ru, and Au. In situ Mössbauer spectroscopy enables the real-time monitoring of these elements’ nuclear states, encompassing their chemical environments, coordination geometries, spin orientations, and magnetic moments.¹⁴⁶ Isomeric displacement (IS) parameters are indicative of electronic structure and serve to elucidate chemical shift information. The quadrupole splitting (QS) sheds light on electronic symmetry and concurrently offers insights into the spin state. The magnetic Zeeman splitting (B) parameter is instrumental in providing magnetic structural information.¹⁴⁷

The investigation into the Fe activity contribution in porphyrin Fe-based catalysts by Kramm and colleagues utilized in situ Mössbauer spectroscopy.¹⁴⁶ The spectral test range was selected based on the LSV curve of ORR. Fig. 12(a)–(d) displays the Mössbauer fitted spectra at 0.9 V, 0.75 V, 0.6 V and 0.2 V. At 0.9 V, only two fitted and single line states, D₁ and D₂, are observed without any conformational changes. A new dual state D₃ is created when the voltage falls to 0.75 V. The D₃ condition endures when the voltage falls below 0.2 V. Reducing the potential to 0.6 V and further to 0.2 V, the D₃ state persists; however, a notable reduction in isomeric displacement and an increase in quadrupole splitting are observed (δ_iso = 0.91 mm s⁻¹, E_Q = 2.2 mm s⁻¹). Notably, upon returning the voltage to 0.9 V, the D₃ state disappears again, retaining only the two mimetic and single-line states D₁ and D₂, and this regression to the initial states demonstrates the reversibility of the electrochemical changes undergone by the catalyst. The in situ Mössbauer spectrum unequivocally illustrates that the electrochemical properties of the system are voltage-dependent. It reveals that the D₃ site is associated with the direct reduction of oxygen during the ORR, while the D₂ site is associated with the indirect reduction. Additionally, the spectrum suggests that both the D₂ and D₃ states are linked to the D₁ state, as indicated by the relative area of their peaks. The catalyst exhibits excellent ORR performance in acidic media, further confirmed by RRDE as a 4e pathway. Utilizing this spectrum, they meticulously analyzed the activity of these sites in the ORR (Fig. 12(e)), shedding light on the mechanism underlying the Fe sites’ behavior (Fig. 12(f)). Zhai and collaborators refined the conventional Fe–N–C catalyst by inducing a change in the spin polarization configuration of Fe–N–C through S doping, resulting in the formation of Fe₁-NC.¹⁴⁸ Analysis of the conventional Mössbauer spectrum revealed the existence of three different double states (D₁–D₃) within the sample (Fig. 12(g)). These corresponded to low spin (LS) Fe³⁺and high spin (HS) Fe²⁺, and the difference between D₂ and D₃ lies in their different coordination environments. At an applied potential of 0.85 V, the D₁ content decreases and the D₃ content increases, indicating that O₂ adsorbs on the D₁ site and generates oxygen intermediates. As the potential increases to 0.65 V, the proportion of D₁ decreases continuously, while the area associated with D₂ expands, signifying a transition in the spin state of the catalyst from LS Fe³⁺ to HS Fe²⁺. Further increasing the potential to 0.45 V results in a continued growth in the content of D₂, accompanied by a reduction in the areas of both D₁ and D₃. This behavior is attributed to the fact that both LS Fe³⁺ (D₁) and HS Fe²⁺ (D₃) can serve as active sites (Fig. 12(h)). The in situ Mössbauer spectrum indicates that the enhanced oxygen reduction activity is mainly attributed to the LS single Fe³⁺ atom in the C–FeN₄–S region. DFT calculations further demonstrate that S doping leads to a decrease in the spin state of Fe centers, which facilitates the desorption of OH* (Fig. 12(i)). The catalyst not only boasts exceptional ORR performance but is also noteworthy for surpassing the efficacy of the majority of materials that have been reported to date (Fig. 12(j) and (k)).

Fig. 12 In situ Mössbauer spectra obtained at potentials of (a) 0.9 V, (b) 0.75 V, (c) 0.6 V and (d) 0.2 V, (e) ORR LSV curves in 0.1 M H₂SO₄, and (f) H₂O₂ yield from RRDE.¹⁴⁶ Copyright (2021) Elsevier. (g) Operando ⁵⁷Fe Mössbauer spectrum at different potentials. (h) In situ ⁵⁷Mössbauer spectrum at various biases of different Fe moieties and reactive intermediates. (i) Free-energy diagram for different Fe-centered moieties. (j) ORR LSV curves for the catalysts in 0.1 M KOH solution. (k) The E_onset and E_1/2values for Fe₁-NS_1.3C and the catalysts in the literature.¹⁴⁸ Copyright (2021) Wiley.

Zhang and his collaborators utilized conductive nickel phosphide (NP) as a support to design and synthesize an Fe,V-based bimetallic composite nanosheet (FeVO_x/NP), and utilized in situ Mössbauer spectroscopy to probe the true active substances of Fe-based OER (Fig. 13(a)).⁵⁴ The in situ Mössbauer spectrum reveals a minor peak with application of increasing potential, which could correspond to Fe⁴⁺ (Fig. 13(b)–(e)). Upon the disappearance of the voltage, this peak ceases to exist as well. At the same time, as the applied voltage increases, the area of Fe⁴⁺ also increases. In situ XAS measurements provided additional confirmation for the formation of Fe⁴⁺ intermediate states (Fig. 13(f)). As the voltage gradually increased, the valence state of Fe gradually changed from +3 to +4. This finding strongly suggests that during the OER process, the high oxidation state of Fe serves as the true active species (Fig. 13(g)).

Fig. 13 (a) Schematic illustration on the fabrication process of FeVO_x/NP. In situ ⁵⁷Mössbauer spectra for FeVO_x/NP collected at (b) OCP, (c) 1.37 V, (d) post OER, and (e) 1.55 V. (f) In situ XANES spectra at the Fe K-edge for the FeVO_x/NP sample. (g) In situ ⁵⁷Mössbauer analysis under different Fe valence states.⁵⁴ Copyright (2023) American Chemical Society.

3.5. In situ electron microscopy (EM)

Electron microscopy technology, especially in situ electron microscopy characterization technology, represents an emerging and potent analytical method for observing nanoscale morphological details of individual particles.^149,150 In situ-TEM and scanning electron microscopy (SEM) are the two most common imaging techniques with high resolution, which can reveal the morphological evolution of electrodes, such as volume expansion and crack formation.^151,152 These advancements in imaging not only expand our observational capabilities but also facilitate a more profound understanding of the morphological dynamics and associated mechanisms of catalysts.^153,154

3.5.1. In situ SEM spectroscopy. In situ scanning electron microscopy involves the generation of an electron beam from an electron gun, which is then directed and concentrated into a focused point of light. This point, under the influence of an applied acceleration voltage, becomes a source of high-energy electrons. Subsequently, this high-energy electron beam is meticulously focused into an exceedingly small spot diameter, utilizing a pair of electromagnetic lenses.¹⁵⁵ The electron beam bombards the surface of the sample in a point-by-point fashion utilizing a grating scanning approach. It subsequently traverses the final stage involving an electromagnetic lens equipped with a scanning coil, thereby triggering the emission of electronic signals from various depths. At this time, the electrical signals are captured by the probes of various signal receivers and transmitted to the computer display through amplifiers, generating real-time imaging records. Moreover, due to the large depth of scanning electron microscopy, the resulting images exhibit a strong three-dimensionality characteristic. In the in situ SEM sample chamber, a variety of in situ experiments can be conducted to measure different phenomena, facilitated by the chamber's versatile capabilities.^156,157

3.5.2. In situ TEM spectroscopy. TEM relies on electron beams, which can be transmitted through thin samples to form ultra-high spatial resolution images and observe changes in catalysts before and after reactions. In situ TEM technology can provide real-time, comprehensive insights into the microstructure evolution and chemical composition changes of electrodes during electrochemical reactions, at high spatial resolution.^158,159 The underlying principle of in situ TEM involves directing accelerated and concentrated electron beams onto exceedingly thin samples. Electrons collide with atoms in the sample and change direction, resulting in stereo angular scattering. The size of the scattering angle is related to the density and thickness of the sample, which in turn creates images of varying levels of brightness and darkness.¹⁶⁰ These images are then visualized on imaging devices, such as fluorescent screens and photosensitive coupling components, allowing for subsequent magnification and focusing.^161,162

Strasser and his collaborators synthesized novel low PtNi alloy nanoparticles and characterized the structure and morphology of the catalyst during the reaction process using in situ heated TEM.¹⁶³ Throughout the annealing process, which spanned temperatures from 25 to 700 °C (Fig. 14(a)), the structural transformations of the catalyst were meticulously observed. When the temperature increased to 500 °C, the morphology of the catalyst remains unchanged relative to its initial state, and the presence of PtNi nanoparticles is confirmed. Upon a further increase to 600 °C, the PtNi nanoparticles begin to diminish in size, and as the temperature continued to rise, the nanoparticles experienced further changes. Over time, it is observed that certain PtNi nanoparticles underwent dissolution. The occurrence and subsequent disappearance of these PtNi nanoparticles are conclusively confirmed through the employment of this sophisticated characterization technique. Sui and his collaborators synthesized a novel ORR catalyst by high-temperature calcination using ZIF-67 as a precursor (Co@g-C) (Fig. 14(b)).⁶⁰ By using in situ TEM to investigate the origins of its superior performance (Fig. 14(c)), they discovered that ZIF-67 manages to retain its polyhedral structure up to 300 °C. As the temperature rises to 500 °C, the morphology of ZIF-67 remains largely unaltered. However, selected area electron diffraction (SAED) becomes amorphous and the original ordered microporous structure is disrupted. Upon reaching a temperature of 600 °C, the appearance of ultra-small nanodots becomes noticeable, and faint dispersion rings become detectable. As the temperature increases, the morphology has been disrupted, and the precipitated nanodots have become nanoparticles. At the same time, SAED has shown polycrystalline characteristics. At a temperature of 900 °C, ZIF gradually contracts while Co nanoparticles grow, which is consistent with the results of XRD. This further indicates that the excellent ORR performance is related to high-temperature pyrolysis, where the loss of nitrogen species and the reduction ability of the Co surface limit the ORR performance. Low-temperature pyrolysis results in a lower degree of graphitization of the catalyst, which consequently constrains its ORR performance. Huang and his collaborators synthesized a AgNbO₃ catalyst with isolated active sites using a perovskite as the raw material, effectively producing H₂O₂. In situ TEM was used to demonstrate that Ag serves as the active site for producing H₂O₂ in this catalyst.

Fig. 14 (a) In situ TEM analysis of octahedral PtNi particles.¹⁶³ Copyright (2018) American Chemical Society. (b) Schematic illustration on the fabrication process of Co@g-C. (c) In situ TEM and HRTEM images of Co@g-C.⁶⁰ Copyright (2021) Royal Society of Chemistry.

However, the transient electromagnetic method necessitates operation in a vacuum, which poses challenges for accomplishing the aforementioned tasks. Therefore, an in situ environmental TEM was constructed to provide a gas or liquid environment in the sample area. This microscope inherits the advantages of high spatial resolution and high energy resolution of traditional TEM, and introduces external fields such as light, heat, electricity, and magnetism into the electron microscope. Unfortunately, the SEM and TEM techniques employed in the literature are not in situ characterization methods. This is due to the nature of these technologies, which primarily involve the use of electron beams for imaging. Such beams are susceptible to interference during on-site data acquisition, thereby limiting their applicability in in situ settings.

4. Conclusions and perspectives

The development of efficient and low-cost new OER/ORR electrocatalysts is a key technology for achieving a low-carbon society in the future, and in situ characterization methods can further explore the internal changes of such catalysts. In this review, we compile a selection of in situ characterization techniques that have been employed in the study of OER/ORR over the past few years (Tables 1 and 2). These techniques have been used to demonstrate how local electronic structures can be detected, reaction processes can be monitored, and active sites as well as phases of transition metal-based electrocatalysts can be identified within the OER/ORR processes. The capabilities, advantages and disadvantages of various characterization techniques are discussed, which will help researchers choose appropriate characterization techniques based on their specific needs in future research. Despite the fact that analyzing electrocatalysts with in situ assurance technology has many advantages, as shown in Fig. 15 there are still many challenges before their widespread application.

Fig. 15 Challenges and prospects of in situ characterization.

Table 1 The relevant performance of OER catalysts mentioned in the article

Catalyst	Electrolyte	J, mA cm^−2	η, mV	Stability	Ref.
V-NiSe2/NF	1 M KOH	100	333.2	50 h	74
RCoMoO4/Co3O4@CC	1 M KOH	100	330	200 h	75
F-FeCoPv@IF	1.0 M KOH + 0.5 M NaCl	1000	360	20 h	82
Mn-FePV	1.0 m KOH seawater	1000	395	28 h	83
FeOOH/Ni3S2/NF	1 M KOH	100	268	200 h	84
Co3O4-VCo	1 M KOH	10	262	—	85
Ru-VO2	0.5 M H2SO4	10	228	125 h	86
Mn7.5O10Br3	0.5 M H2SO4	10	295	500 h	87
IrOx/Ni(OH)2	0.5 M KHCO3	20	421	—	89
12Ru/MnO2	0.1 M HClO4	10	161	200 h	101
CdRu2IrOx	0.5 M H2SO4	19	189	1500 h	116
Fe-Co2P@Fe–N–C	1 M KOH	10	300	24 h	96
Ni-Fc (25)	1 M KOH	10	228	570 h	120
TQ-NiFe	1 M KOH	450	470	100 h	121
Co9S8-SWCNT	1 M PBS	10	380	10 h	122
h-HL-Ir SAC	0.1 M HClO4	100	259	60 h	123
Ir/Nb2O5	0.5 M H2SO4	10	218	100 h	124
COV600	1 M KOH	10	324	24 h	130
Li0.52RuO2	0.5 M H2SO4	10	156	70 h	131
CoOOH	1 M CsOH	10	355	150 h	132
NMO-600	1 M KOH	10	270	21 h	133
FeOOH/Co(OH)2	1 M KOH	1000	304	150 h	140
NaMn3O7	1 M KOH	0.25 mA cm^−2ox	280	10 h	142
RuO2/CoOx	1.0 M PBS	10	240	20 h	63
FeVOx/NP	1 M KOH	500	290	1000 h	54

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Table 2 The relevant performance of ORR catalysts mentioned in the article

Catalyst	Electrolyte	E1/2 (V) (V vs. RHE)	Eonset (V vs. RHE)	H2O2 selectivity	Stability	Ref.
NiSe2-VSe	0.1 M KOH	0.603	0.72	90%	40 000 s	77
Au	0.1 M KOH	0.73	—	—	—	78
h-BN/G	0.1 M Na2SO4	—	0.67	95%	24 h	79
L-ZnO	0.6 M K2SO4	—	—	97.35%	20 h	80
FeSAs/NSC-vd	0.5 M H2SO4	0.76	1.06	—	50 000 s	81
Fe,Co-HPNC	0.1 M KOH	0.84	1.03	—	28 h	94
NiZn MOF	0.1 M KOH	—	0.73	90%	30 h	95
Fe-Co2P@Fe–N–C	0.1 M KOH	0.88	0.92	11.1%	32 000 s	96
Pt0.2Pd1.8Ge	0.1 M KOH	0.94	—	1.4%	—	97
Fe–N–C/PdNC	0.1 M HClO4	0.87	0.97	—	30 000 cycles	98
FeRu–N–C	0.1 M HClO4	0.860	0.975	4%	5000 cycles	99
ZnO-NG	0.1 M Na2SO4	—	—	85%	20 000 s	100
Pt=N2=Fe ABA	0.1 M KOH	0.95	1.05	1.5%	100 h	115
Cu-HHTP	0.1 M KOH	—	0.84	85.4%	24 h	117
CoNC	0.1 M HClO4	—	0.78	90%	6 h	119
FeSA-N/TC	0.1 M HClO4	0.825	—	3.2%	5000 cycles	134
Co@Tb2O3/NC	0.1 M KOH	0.85	1.02	4.5%	10 h	144
Eu2O3/NC	0.1 M KOH	0.887	1.007	2.2%	10 h	57
FeNC	0.1 M H2SO4	—	0.81	3.20%	—	146
Fe1-NS1.3C	0.1 M KOH	0.86	0.97	10%	25 000 s	148
ZIF550-7.5	0.1 M KOH	0.851	—	—	—	60

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First, in the presence of other interfering factors, selective observation of the active centers and intermediates of catalysts may become more complex, making it very difficult to identify the actual active centers. In addition, different characterization methods have different detection ranges and purposes. To date, there has been no unified, comprehensive overview of a technology capable of capturing valence state alterations, and morphological changes occurring under OER/ORR conditions. Driven by these challenges, there is an immediate and pressing need to conduct more extensive, comprehensive experiments coupled with the development of novel in situ technologies to accurately pinpoint the active sites involved in OER/ORR.

Second, additionally, there exists a wide array of electrocatalysts, ranging from powders to self-supporting forms. However, many current in situ characterization instruments lack compatibility with a diverse spectrum of electrocatalysts, which can prevent the direct in situ testing of certain materials. Therefore, it is essential for future advancements in in situ characterization technology to focus on enhancing the sample compatibility of these instruments with a broad range of electrocatalysts.

Third, at present, significant strides have been achieved by researchers in elucidating the catalytic mechanisms underlying ORR/OER materials. Nevertheless, how to successfully apply the obtained information to guide us in accurately designing and manufacturing electrocatalysts with the required structural features and practical large-scale applications remains an urgent challenge.

Last but not least, under high temperature and pressure, the structure of the catalyst is always damaged, and the atomic coordination environment changes, which may lead to a transformation of the active sites of the catalyst. Therefore, how to directly observe the structural evolution of catalysts under special conditions using in situ characterization techniques remains a challenge.

Author contributions

Siqi Wu: writing & original draft. Zexin Liang: formal analysis, conceptualization, investigation. Tianshi Wang: formal analysis, conceptualization, investigation. Xiaobin Liu: writing – review & editing. Shaobo Huang: writing – review & editing, funding acquisition, supervision, conceptualization.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Key R. & D., Promotion Project of Henan Province (No. 242102240084), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 21KJB480008)

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