1 Introduction

Classical electrochemical measurements typically rely on average behavior observed across large ensembles, failing to consider individual differences in the behavior of each catalyst particle during electrocatalysis. High-resolution single-particle electrochemical techniques are therefore of great interest as they allow for the study of intrinsic catalytic activity at the level of individual particles or even single sites [1,2,3,4]. Moreover, the heterogeneity of particle shape, size, and the surface structure can have a significant impact on the “structure-activity” relationship, yet this relationship remains difficult to understand through classical analysis [5,6,7,8,9,10,11,12,13,14,15,16]. Consequently, there is a compelling need to establish high-resolution electrochemical analysis with the goal of studying the structure-activity relationship at the nanoscale.

At present, various in-situ characterization techniques are widely used to investigate electrocatalytic processes [17,18,19,20]. These techniques, such as electron microscopy and in-situ/operando spectroscopy, permit to study the morphology, surface chemical properties, and intermediates during catalysis. However, they do not provide the analysis of catalytic activity in a quantitative manner [21,22,23]. To address this gap, high-resolution electrochemical measurements, such as scanning probe techniques and single-particle collision techniques, have been well developed over the past few decades, which offer in-situ investigation of catalytic activity with high spatial resolution, and for the ability to image and quantify the properties of individual particles. As a result, scanning probe techniques including scanning electrochemical microscopy (SECM) and scanning electrochemical cell microscopy (SECCM) have been widely used to study intrinsic electrocatalytic activity [11, 21, 22, 24,25,26,27,28,29,30]. Additionally, the single-particle collision technique with high temporal resolution can quantify the catalytic activity of isolated individual particles with known size, thus avoid the proximity effects between adjacent particles [31, 32].

The measurements at the single particle level are crucial for gaining a deeper understanding of electrocatalytic mechanisms and intrinsic activities. Recently, SECM, SECCM, and single-particle collision techniques have been proven to be powerful tools in investigating single-entity electrocatalysis [23, 33,34,35,36,37,38,39]. The research of single-entity electrochemistry is of great significance to the application of hydrogen energy and low carbon techniques. However, there has been no conclusive work on this important application of these high-resolution techniques, although many interesting reviews on these advanced techniques applied in electrocatalysis have been reported [30, 40,41,42]. Accordingly, this review outlines their fundamental principles in single-particle electrochemistry and recent research progress in these techniques, and aims to provide a comprehensive understanding of modern electrochemical measurement techniques, including their principles, advantages, limitations and applications. We anticipate that it could serve as a valuable reference for researchers interested in the latest trends and inspires further insights.

2 Instrumental and measurement principles for studying electrocatalysis

For scanning electrochemical microscopy (SECM), a bipotentiostat is used to measure the low currents (pA - nA level) and control potentials of both the tip and substrate with high accuracy. Additionally, a highly sensitive piezoelectric controller in SECM allows for the precise movement of the tip in a three-dimensional space (x, y, and z). The tip electrode used in SECM experiments is typically made of metal wires or carbon fibers with a small size ranging from 10 nm to 25 μm. Such ultramicroelectrode (UMEs) provides a high spatial resolution and very low background current to enable the investigation of electrocatalytic behavior of individual nanoparticles. The positive feedback mode of SECM is commonly performed to study the topography of nanoparticles (NPs) as shown in Fig. 1a. In this mode, a redox reaction occurs when the tip approaches toward the substrate. Surface imaging is achieved by monitoring the tip current (IT). The IT depends on the distance between the tip and the substrate (d) with the IT increases as d decreases. The redox competition mode has been developed for measuring catalytic activity associated with tip current reduction, in which the close SECM tip and the substrate compete for the same redox species (Fig. 1b). The generation/collection mode is also a powerful tool for studying single-entity electrochemistry, enabling the real-time detection of redox products as shown in Fig. 1c-d. Typically, this mode encompasses two distinct modalities, namely the substrate generation/tip collection (SG/TC) mode and the tip generation/substrate collection (TG/SC) mode, depending on whether the redox species originate from the substrate or the tip. In SG/TC mode, the redox species generated at the surface of the NPs diffuse across the gap between the tip and the substrate and subsequently collected by the tip. Conversely, in TG/SC mode, the redox species are generated at the tip and are then collected by the substrate. As a result, this approach has become a widely used and versatile means of measuring both the individual nanoparticle topography and the local electrocatalytic activity. In addition, tunneling mode, a new mode of the SECM operation based on electron tunneling between the tip and a NP immobilized on the insulating surface, has been developed by the Mirkin group in recent years (Fig. 1e). When the distances between the tip and the NP is less than about 3 nm, the electron tunneling occurs between the electrode and the NP, the NP can be regarded to act as part of the tip electrode at such a short separation distance. Using this new mode, high-resolution imaging of the NP topography and the heterogeneous kinetics measurement at a single NP could be achieved, which is particularly useful for studying the effects of NP size and geometry on electrocatalytic activity.

Fig. 1
figure 1

Principles of different measurements at a single NP. a The feedback modes of SECM. b The competition mode of SECM. c, d The generation-collection (G/C) modes of SECM. c TG-SC mode; d SG-TC mode. e The tunneling mode of SECM. f SECCM with single-barrel probe for studying individual particles. g Single-particle collision electrochemical measurement. h The experimental protocol for single-particle deposition and subsequent characterization by voltammetry

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In addition to the SECM, SECCM has also proven to be a powerful technique over the past decades for studying electrocatalytic behavior at the nanoscale. The SECCM experiment (Fig. 1f) is performed by scanning a single-barrel probe across the substrate surface with a piezoelectric controller while continuously measuring the localized voltammogram to probe the activity. Moreover, the “hopping” mode of SECCM is employed to image the electrochemical activity of a large number of isolated NPs. Single-particle collision analysis is a valuable tool for investigating the properties of individual nanoparticles and understanding how they contribute to ensembles. This technique provides high temporal resolution for the detection of tiny current signals from collisions, allowing for deeper insights into individual nanoparticle behavior. Figure 1g shows that at sufficiently low concentrations, particles randomly collide, and relevant electrochemical information about single nanoparticles can be obtained by observing their electrochemical responses. Bard group has developed a unique strategy for studying single atoms, molecules, and clusters. In this emerging approach on the basis of single-particle electrochemistry and electrocatalytic amplification (see Fig. 1h), single atoms, molecules, or clusters are obtained by electrodeposition on the surface of a nanosized ultramicroelectrode (UME) using a precursor solution with a femtomolar concentration, for a given period. Subsequently, the deposits are detected using an electrocatalytic amplification strategy that measures voltammograms in H+ or OH− with known concentrations. The size and kinetics information can be obtained by analyzing the steady-state catalytic current of the proton reduction or hydroxide oxidation reactions.

3 Recent progress in high-resolution electrochemical techniques for characterizing electrocatalysis

Understanding of the intrinsic catalytic activity of single NP or individual sites is not only of scientific interest but can also provide an effective method for catalyst design [43,44,45]. However, there is a great challenge to assess the fundamental behavior of individual nanoparticles by traditional electrochemical measurements. In contrast, SECM, as a scanning probe technique, can support both topographic and reactivity information of a single particle surface [23, 46,47,48,49]. To this end, using feedback and SG/TC mode of SECM, Sun et al. demonstrated a method that could map the electrochemical activity of individual Au NPs (Fig. 2) [34]. Figure 2a shows a low density of Au NPs attached to the carbon surface and the charge transfer process at a 20 nm Au NP was revealed with a 14 nm tip (Fig. 2b). Meanwhile, the current-distance curve obtained with the same tip positioned above the same NP (Fig. 2d) and polyphenylene film (Fig. 2e) fits the theory well. Importantly, the effective radius values are very similar and agree well with the theoretically calculated values (14 nm) as shown in Fig. 2d and e, indicating that SECM can quantitatively detect electrochemical reactions at a single 20 nm NP. Using a smaller tip (3 nm), similar measurements of a 10 nm Au NP were performed, and the results showed that the diameter of AuNP measured via this method was closer to the true value (10 nm) than that measured with atomic force microscope (AFM) (50 nm). Then, the HER kinetics at individual Au NPs was studied. The plots of the tip current versus the substrate potential (iT-ES) recorded at constant ET values at an individual 20 nm Au NP using SG/TC mode as shown in Fig. 2f. The linear portion of the Tafel plot for this process (Fig. 2f: Insert curve) obtained from curve 1 in Fig. 2f exhibits a slope of 116 mV/decade, which in good agreement with the reported value for the HER at a polycrystalline Au electrode. Overall, the developed methodology is of importance to the study of the effects of nanoparticle size, geometry on electrocatalytic activity.

Fig. 2
figure 2

a Non-contact mode topographic AFM image of 20 nm Au NPs; b, c Feedback mode SECM images of b 20 and c 10 nm Au NPs. d, e Experimental current-distance curves (symbol) and theoretical fitting curves (solid line) were obtained using the same tip as (b) approaching the same Au NP (d) and insulated substrate (e). f Tip/substrate voltammograms at a tip height of 60 above an Au NP. ET (in mV) was 500 (1), 400 (2), and 100 (3). Insert curve: Tafel plots for the HER at a single Au NP. Reproduced with permission from ref [34]. Copyright 2014 Wiley-VCH

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Similarly, the relationship between the structure and the reactivity of other NPs has been demonstrated as well [7]. For instance, Bard group developed the nanoscale SECM to investigate the hydrogen oxidation reaction (HOR) on individual Pt NPs with high spatial resolution [24]. The FE-SEM images of Pt NPs with a radius of 90 ± 30 nm present well-defined hemispherical or spherical geometry as shown in Fig. 3a. A focused ion beam (FIB)-milled Pt nanotip (radius 90 nm, Fig. 3b) was prepared for SECM imaging of single Pt NPs. The morphology (Fig. 3c-d) and catalytic activity (Fig. 3e) of individual NPs were studied with FcTMA+ and H+ as the redox mediators. Notably, an individual Pt NP presents a spherical shape as shown in Fig. 3c. Based on the SECM results, the size and distribution of NPs were analyzed by combining experimental data with numerical simulations. Moreover, the SECM imaging studied with H+ as the redox mediators were further quantitatively analyzed to estimate the electrocatalytic activity of each Pt NP for the HOR with a high effective rate constant (2 cm s−1).

Fig. 3
figure 3

a FE-SEM image of electrodeposited Pt NPs on HOPG. b SEM image of a FIB milled Pt nanoelectrode. c SECM image of an individual Pt NP. d SECM image of Pt NPs in 1 mM FcTMA+ at a tip height of 210 nm. e SECM image of HOR reaction at Pt NPs in 2 mM HClO4 at a tip height of 134 nm. Reproduced with permission from ref [24]. Copyright 2017 American Chemical Society

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In addition to single particles, SECM could also be used to study the local reactivity of other catalysts, for example, two-dimensional catalytic nanoflakes and other conductive nanostructures [50,51,52,53,54,55]. Sun et al. presented independent imaging morphologies and oxygen evolution reaction (OER) activity mapping of a semi-two-dimensional nickel oxide nanosheets at a spatial resolution of 15 nm using two complementary modes of the SECM (Fig. 4) [50]. By combining feedback mode and generation/collection mode, the OER catalytic activity of the NiO defect edge was found to be significantly higher than that of the basal plane. Most importantly, the mechanism of the enhanced NiO edge activity in this work was further investigated with the assistance of 3D electron microscopy and density functional theory calculations, which revealed that the (100) nanofacet was responsible for the enhancement of activity at the edge. This method of using multi-technology coupling collaborative analysis provides a viable strategy for revealing catalytic active sites on structurally complex electrocatalysts and promotes the development of future analysis technology.

Fig. 4
figure 4

a, b Schematic of SECM for imaging the OER activity of NiO nanosheets. Feedback mode (a) and SG-TC mode (b). c, d SECM images of HER activity on the edge region of NiO nanosheets. Under feedback mode (c) and SG-TC mode (d). e Line scans across the edge of a NiO nanosheet edge. Reproduced with permission from ref [50]. Copyright 2019 National Academy of Science

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Recently, tunneling mode of nano SECM has been developed to study the single-entity electrocatalysis, which enables measurements at a single NP without attaching it to the substrate surface [51, 55,56,57]. By means of the SECM tunneling mode, a sharp rectangular shape of a single Pd nanocube with extremely high lateral resolution (1 nm) (Fig. 5b) was observed, which is highly consistent with the shape in the TEM image (Fig. 5a) [56]. Mirkin and co-workers used this new mode to probe electrochemical processes at single Au NPs [55]. In such mode, the distances between the tip and NP are several nanometers as shown in Fig. 5c. Such a short distance allows electrons tunneling directly between the tip and the substrate. As a result, using this tunneling mode enables high resolution imaging of individual NPs. The steady-state voltammograms of HER obtained with a Pt tip in the bulk solution (curve 1) and within the tunneling distance from the Au NP (curve 2) as shown in Fig. 5d. The analysis of this curve exhibits a linear Tafel plot with a slope of 100 mV per decade for an individual Au NP (Fig. 5e insert), which matches well with literature data for the HER at polycrystalline gold. Besides, Mirkin group further demonstrated the possibility of applying this emerging mode on flat nanosheets and successfully investigated nanosheet samples with different sizes, geometries, conductivity and surface reactivity [51]. These results suggest the possibility of high-resolution SECM imaging through the tunneling mode. Importantly, this technique is expected to provide a new option for studying the effects of size and geometry on electrocatalytic activity at the single-particle level.

Fig. 5
figure 5

a TEM image of Pd nanocubes. b SECM images of a single Pd nanocube. Reproduced with permission from ref [56]. Copyright 2016 American Chemical Society. c Schematic of the tunneling model of SECM. d Voltammograms of HER at the same Au NP/Pt tip with (2)/without (1) the tunneling model. e Voltammograms of HER and Fc oxidation at the same Au NP/Pt tip. Inset: Tafel plot for the HER. Reproduced with permission from ref [55]. Copyright 2018 Wiley–VCH

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SECCM, first introduced by the Unwin group in 2010, has become a versatile technique for studying single-entity electrochemistry for the tip current can reflect the electrochemical properties of the substrate directly and locally, which offers high spatial resolution insight into the structure-activity relationship at nanoscale [25, 26, 58,59,60,61,62,63]. It has been used to investigate the effect of subtle changes in NP morphology on reactivity, such as mapping the reactivity of individual Pt NPs in an electrocatalytic ensemble [26]. This study suggested that a differences in reactivity among NPs with significantly similar size and highlighted the importance of other factors besides the NP size effect, such as morphology, on reactivity. In addition, Li et al. revealed the heterogeneity in OER activity of single hematite (α-Fe2O3) nanorods via SECCM, and the activity of the body part was always higher than that of the tip of the nanorods [64]. As for single ZIF-derived Co-N/C nanocomposite particles, the OER activity was relative to the number of particles in the wetted area of the droplet landing zone [65]. Based on the number of Co atoms in each particle, the turnover frequency (TOF) was obtained in alkaline media. Meanwhile, Quast et al. investigated the electrocatalytic OER activity (TOF) and structural changes (the formation of anoxy(hydroxide) surface layer during electrocatalysis) of individual Co3O4 nanocube particle by combining SECCM and the single-particle-at-the-tip technique [66]. Moreover, the OER activity of single NiFe2O4 nanocrystal superparticles were studied and demonstrated that the TOF is sensitive to size only when the superparticles are small enough (less than ~800 nm in diameter), and increases significantly (6-folds) with the incorporation of Au nanocrystals [67]. The catalytic activity for hydrazine oxidation at Au NP and the ORR reactivity at Pt nanoclusters was characterized by combining electrochemical and electron microscopy techniques at the single NP level [60, 68]. Recently, SECCM has been further used to investigate the differences in electro-oxidation of hydrazine rates on individual Au NPs surfaces [61]. Specifically, Au NPs electrodeposited on glassy carbon (GC) were used as electrodes and electrochemical reactivity and topographical imaging has been directly performed on it (Fig. 6a). Topographical and the electrochemical mapping at the same single Au NP were exhibited in Fig. 6b and c, respectively. In addition, the individual LSVs of four selected areas (marked in Fig. 6a) within a single Au NP were performed as shown in Fig. 6d. The study revealed a significant variation in electrocatalytic activity across the surface of individual AuNPs, indicating that the single entities cannot be assumed to be uniformly active.

Fig. 6
figure 6

a Schematic of the general procedure for voltammetric hopping mode SECCM with a single-channel nanopipet probe. b Topographical and c spatially resolved electrochemical maps of an Au NP. d Normalized LSVs collected at the individual pixels labeled in (a). Reproduced with permission from ref [61]. Copyright 2017 American Chemical Society. e, f SECCM images of Au NC (e) and Au OD (f) electrochemical response. Insert: SEM image of SECCM area. g Averaged CV response at individual Au NCs (n = 101) and Au ODs (n = 99). Inset: Same averaged CVs without standard deviation errors bars. Reproduced with permission from ref [25]. Copyright 2020 American Chemical Society. h SEM image of the Pt grains. i, j SECCM images of HER reactivity. Reproduced with permission from ref [69]. Copyright 2020 American Chemical Society

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In addition to spatially dependent activity, the facet-dependent electrocatalytic activity of individual nanocrystals has also been extensively studied. For example, using the scanning and hopping modes of SECCM, Mefford et al. investigated the OER reactivity of single single-crystalline β-Co(OH)2 nanostructures [62]. The finding suggested that the basal surface of the nano-catalysts was inactive after hydroxyl intercalation, while the edge surface of the particles still showed excellent electrochemical reactivity. Such activity difference was related to the local concentration of Co3+ at the edge sites. In addition, Choi et al. studied the HER activity of single-sided gold nanocubes (NCs) and nano-octahedral (ODs) as exhibited in Fig. 6e and f, respectively [25]. Voltammetry of single Au NCs and Au ODs was performed (Fig. 6g) and showed a similar trend to macroscale CV experiments. Evidently, compared to Au ODs, Au NCs exhibited higher HER activity and more particle-to-particle variation in catalytic activity, which was resulted from the distinct types of crystal planes exposed on their surfaces. In detail, Au NCs and Au ODs presented predominantly (100) and (111) crystal planes on the surface, respectively. Additionally, to investigate the crystal planes effect, Wang et al. demonstrated the HER activity on polycrystalline Pt [69]. The transfer coefficient (α) changes significantly between grain 1 and grain 2 as shown in Fig. 6i-j, while the standard rate constant (k0) remains almost constant. This proves that there is a different mechanism of the HER on different grains of Pt, which further revealed that the crystal planes have a great influence on the electrocatalytic reaction. Similarly, using the SECCM, Mariano et. al. investigated the CO2 electroreduction (CO2RR) on polycrystalline gold [63]. As a result, the grain-boundary surface terminations in the gold electrode showed higher activity for the electrochemical reduction of CO2 to CO than the grain surface, but this activity difference was not suitable for the competing HER. Recently, Jeong et al. further found that {110}-terminated gold rhombohedra displayed superior CO2-to-CO activity compared with {111}-terminated octahedra and high-index {310}-terminated truncated ditetragonal prisms [70]. Besides the facet-dependent electrocatalytic activity, the HER electrocatalytic activity of single Pd octahedra and icosahedra with the same surface bounded {111} crystal facet and similar sizes was studied by SECCM [71]. The results revealed that tensilely strained Pd icosahedra exhibited markedly superior HER activity.

Unlike scanning probe microscopy, the single-particle collision analysis could allow dynamic information of the size, motion, catalysis of single NPs [17, 70,71,72,73,74,75,76,77,78]. For example, the size-dependent ORR activity at a single Au NPs has been demonstrated by Zhang and coworkers, they found that higher catalytic activity could be obtained at larger Au nanoparticles [79]. Zhou et al. proposed an effective method to determine the number of Au atoms in polymerized sulfate particles through the current response generated by the collision of a single precursor [80]. The results matched well with the number of Au atoms estimated from dynamic light scattering (DLS). Furthermore, single-particle collision analysis was used to investigate the efficiency of NADH oxidation by collisions of modified Au NPs on UME [81]. By analyzing the peak charge and duration with and without NADH, a catalytic efficiency of 5000 NADHs per second was obtained for one catechol molecule. Recently, Long and coworkers demonstrated a well-defined 30 nm nanopore electrode (CNE; Fig. 7a) to directly observe the intrinsic feature of single nanoparticle collisions [82]. Using this electrode, the dynamic interactions of each single Ag NPs in mixtures of different particle sizes during collisions could be read directly with high current and time resolution (Fig. 7b). Notably, in this CNE system, the collision frequency was far higher than previously reported (two orders of magnitude). As a result, the hidden signals of nanoparticles with transient interaction time during the complex processes can be detected based on this strategy. Liu et al. further designed a gas-dissolved nanopore reactor for a detailed study of the single-molecule reaction kinetics under nanometer confinement [83]. The results showed that the single-molecule reaction kinetics inside a nanopore was relevant to the frequency of trapping a single reactant, as well as the ratio of effective collisions between a single reactant and the active site inside the nanopore confined space.

Fig. 7
figure 7

a Schematic of the confined nanopore electrode and SEM image. b Current-time curves (right) and the histograms (left) of 13 nm AuNPs 60 nm AuNPs, and the AuNP mixture. Reproduced with permission from ref [82]. Copyright 2018 Wiley-VCH. c Schematic of interaction-modulated dynamic electrochemical behaviors of single NPs. d Current-time curves (right) and representative current (left) of individual Ag NPs on two of UMEs in a different solution. e Durations and charges distribution histograms of individual Ag NPs. Reproduced with permission from ref [84]. Copyright 2020 Nature Publishing Group

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To the best of our knowledge, a typical collision event usually involves the diffusion of single particle and a random collision with a UME interface. Consequently, the dynamic electrochemical behavior of the oxidation of individual Ag NPs under the modulation of nanoparticle-electrode interaction was studied by Long group [84]. The established experimental framework in which NPs are located on two different UME surfaces with different adsorption properties as shown in Fig. 7c (calculated by DFT). The current response associate with the dissolution of individual Ag NPs was directly observed and shown in Fig. 7d. Evidently, individual Ag NPs with exhibit significantly different electrochemical oxidation characteristics in different adsorption systems was due to the interaction-mediated motion behaviors of the Ag NPs. This result was supported by measurements of the duration and integrated charge associated with each collision event (Fig. 7e). On this basis, the high-resolution measurement of Ag NPs mixtures with different particle sizes in liquid suspension was achieved by introducing a surface-confined nano-collision strategy, providing an effective method for particle size analysis in a complex environment. Furthermore, combined with theoretical simulation and high resolution photocurrent measurement, Peng et al. developed a new single nanoparticle photoelectrochemical system to quantify the electron transfer information of a single N719@TiO2 nanoparticle on a nanosphere TiO2@Au UME [85]. The electron diffusivity and the electron-collection efficiency for a typical collision of TiO2@Au UME were estimated to be (2.4 ± 0.9) x10–4 cm2 s−1 and 51 ± 4 per N719 molecule, respectively.

In fact, the current trace produced by a collision event usually involves complex electrochemical processes, thus it remains challenge to clearly understand the exact behavior of single particles in this way. With the desire to address this challenge, Bard group recently reported an atom-by-atom basis method for depositing isolated single Pt atoms or small clusters on a Bi UME from femtomolar H2PtCl6 solution and subsequently characterized their HER activity (Fig. 8a) [86]. The HER activity of the isolated NPs with varying sizes as shown in Fig. 8b, where the size effect on single particle electrocatalytic activity was significant. The reaction rate of HER was mapped by comparing the overpotential of clusters with different sizes at the same current density as illustrated in Fig. 8c. As a result, compared with platinum atoms, the clusters displayed the much smaller overpotential, suggesting that the clusters proceeded the HER with a faster rate while the reaction rate increased with the increase of cluster size. In addition, substrate effects on the HER were also studied by the same group by depositing isolated Pt on Bi and Pb UME [87]. There was a significant difference in the HER kinetic rate of isolated Pt deposits with the similar size on different UMEs, which was caused by the strong interaction between the Pt clusters and the substrate materials.

Fig. 8
figure 8

a Schematic diagram of the single platinum cluster deposition experimental protocol for studying the HER by voltammetry. b Voltammograms of the HER on Pt atoms and clusters. c Potential at given current density of platinum clusters with different atom numbers. Reproduced with permission from ref [86]. Copyright 2017 American Chemical Society. d Voltammograms of the OER on different size cobalt oxide clusters. e Potential at given current density of cobalt oxide clusters with different effective radii. Reproduced with permission from ref [35]. Copyright 2020 National Academy of Science

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Meanwhile, Jin et al. further extended this strategy to the electrosynthesis of compound or molecular catalysts, e.g., transition metal oxide (ConOy) on carbon UME, for studying the size-dependent activity of the OER [35]. The voltammograms of the OER on different size ConOy clusters is illustrated in Fig. 8d, where the radius was estimated from the diffusion-limited current of the hydroxide ion oxidation. The results showed that the smaller clusters proceeded the OER with a less positive potential (Fig. 8e), suggesting a faster reaction rate. It is noteworthy that this strategy could be performed on a single isolated atom/molecule level, enabling the quantitative analysis that overcomes the limitations associated with measuring ensembles.

Overall, SECM, as an important technique to study the structure-activity relationship at the single nanoparticle level, has made significant advances, including further application to the study of local activity of two-dimensional nanomaterials, and the development of emerging modes such as the tunneling mode. Notably, the SECM reflects the electrochemical activity on the substrate indirectly through the tip current, while SECCM current can directly obtain the activity information at the substrate. Therefore, SECCM is particularly well suited for imaging and quantifying the local variations in electrochemical behavior at nanoscale. However, the SECCM measurement of system that require strict drying is challenging due to its unique composition of the working electrode. Unlike scanning probe microscopy, the single-particle collision analysis could provide dynamic information about the size, motion, catalysis of single NPs, so it has been widely used in single-entity studies as a supplementary analysis method in recent years. In fact, the current trace produced by a collision event usually involves complex electrochemical processes, thus it remains challenge to clearly understand the exact behavior of single particles in this way. To this end, Bard group recently reported an atom-by-atom basis method for depositing isolated single metal atoms or small clusters on a UME and subsequently characterized their reactivity. In addition to these powerful techniques, advanced optical microscopies techniques with high sensitivity, excellent spatial and temporal resolution, have been widely reported for studying the single particle electrocatalysis [88,89,90]. Therefore, in practical application, the reasonable selection of methods and the combination of different techniques is significant for the analysis results. On the other hand, to obtain reliable and accurate results, the potential difference between the reaction environment of these advanced techniques and the solution environment of the bulk system is nonnegligible. Therefore, it is crucial to compare the electrochemical behavior observed at the nanoscale with that measured at the macroscopic level, as well as carefully control the experimental conditions to make the differences between the reaction environment and the solution environment are minimized.

4 Conclusion and outlook

In this review, three high-resolution electrochemical techniques for studying electrocatalysis at the single-particle level are discussed, which provide detailed information about the structure-activity relationships that remain tremendous difficulties in ensemble measurements. With these powerful tools, the characterization of electrode surface properties with nanoscale resolution, as well as the quantitative determination of the intrinsic activity of individual nanoparticles have been enabled. The SECM with high resolution that traditional voltammetry techniques cannot achieve is capable of imaging individual NP topography and local electrocatalytic activity, providing considerable insight into structure-activity relationships. Over the past decades, SECCM has become another versatile technique for studying electrocatalytic behavior at the nanoscale. Single-particle collision analysis with high temporal resolution is complementary to the scanning probe techniques (SECM, SECCM). An atom-by-atom basis strategy has been developed as an emerging approach based on single-particle collision. In addition, combining with other techniques, for instance, spectroscopy, electron microscopy and theoretical calculations, more accurate and complete information can be obtained, and its applications would be expanded. Future research on these advanced techniques would be focus on the integration of multiple technologies and multimodal analysis to enable comprehensive analysis of reactivity information involved in complex reaction processes. The reactions involved in these studies are mostly related to the energy conversion such as HER, OER, ORR and CO2RR, which have important implications for the efficient conversion and storage of clean energy, especially hydrogen energy. Therefore, the in-depth understanding of corresponding studies will further help accelerate the progress towards a greener and more efficient energy society.