Structural Self-Reconstruction of Catalysts in Electrocatalysis

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Structural Self-Reconstruction of Catalysts in Electrocatalysis Hongliang Jiang,† Qun He,† Youkui Zhang,†,‡ and Li Song*,† †

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National Synchrotron Radiation Laboratory, CAS Centre for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230029, P. R. China ‡ State Key Laboratory of Environment-friendly Energy Materials, School of National Defense Science & Technology, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China CONSPECTUS: Recent years have witnessed significant development of electrocatalysis for clean energy and related potential technologies. The precise identification toward active sites of catalysts and the monitoring of product information are highly desirable to understand how the materials catalyze a specific electrocatalytic reaction. For a long period, the identification of active sites and the cognition of corresponding catalytic mechanisms are generally based on various ex situ characterization methods which actually could not capture dynamic structure and intermediate information during electrocatalytic processes. With recent developments of in situ and operando characterization techniques, it has been extensively observed that most of the catalysts would undergo structural self-reconstruction as a result of electro-derived oxidation or reduction process of the catalysts at a given potential, often accompanied by the increase or decrease of catalytic activity as well as the change of catalytic selectivity. In fact, such structural self-change in the catalytic process does make it difficult to identify the true catalytically active sites efficiently, thus hindering the understanding of the real catalytic mechanism. Therefore, we believe that understanding the self-reconstruction by the combination of reliable characterization techniques and theoretical calculations holds the key to rational design of advanced catalysts. In this Account, we provide in-depth insights into recent progress regarding structural self-reconstruction of electrocatalysts in several typical electrochemical reactions with the emphasis on fundamental knowledge, structure−property relationships, structural evolution process, and modulation of self-reconstruction. To deliver a clear understanding, it has to be pointed out in advance that these catalysts with drastic structural and activity self-change in electrocatalytic processes are suggested to be called precatalysts under nonreaction conditions. The restructured active components in realistic reaction conditions are true catalysts. The structural self-reconstruction process bridges the precatalysts with true catalysts. To understand the self-reconstruction behavior, the following three critical aspects will be carefully disclosed and discussed in depth. First, fundamental origin of structural self-reconstruction of electrocatalysts is introduced. It is noteworthy that the atomic-level correlations between the self-reconstruction behavior and intrinsic structure of precatalysts are emphasized due to the fact that even if some precatalysts are congeneric, they often exhibit a diverse self-reconstruction phenomenon and catalytic performance. Second, the selfreconstruction process should be monitored by advanced characterization techniques, which is central to precisely unveil the self-reconstruction behavior. In situ or operando characterizations have been considered as judicious methods to track the selfreconstruction, capture dynamic structure and analyze real-time reaction products. Finally, based on the dynamic structure and product information together with comprehensive theory calculations, the enhancement or degradation mechanism of catalytic activities can be unambiguously clarified. With thoughtful studies toward the complete self-reconstruction process of electrocatalysts, some feasible methods to tune the self-reconstruction and improve the performance can be rationally proposed. Based on this progress, we hope to provide new insight into electrocatalysis, particularly the self-reconstruction and true active sites of electrocatalysts, and then to offer guidelines for rational design of advanced electrocatalysts.

1. INTRODUCTION With ever-increasing global concerns over energy and environment issues associated with the over-reliance on fossil fuels, fossil-free pathways to achieve energy storage and conversions have been extensively raised.1,2 In this regard, various prospective technologies such as fuel cells, metal-air batteries, water electrolysis, CO2 conversion, and N2 fixation attract considerable research interest from different basic and practical perspectives.3−6 It is well-known that electrochemical conversion processes toward molecules (e.g., hydrogen, oxygen, water, carbon dioxide, and nitrogen), typically hydrogen © XXXX American Chemical Society

evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR), are fundamental for the development of these technologies.4,7−10 To increase the catalytic efficiency and selectivity, electrocatalysts with high activities and stabilities are wellscreened in these electrochemical processes.11 Although significant developments in various electrocatalysts have been Received: September 6, 2018

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DOI: 10.1021/acs.accounts.8b00449 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. (a) Schematic for polarization curves of hydrogen- and oxygen-involving reactions. (b) Schematic illustration for the self-reconstruction of the AuNi heterodimers at corresponding potential windows. Reprinted with permission from ref 20. Copyright 2018 John Wiley and Sons.

sponding enhancement or degradation mechanism of catalytic activities in electrocatalysis. Accordingly, some strategies of inhibiting or promoting the self-reconstruction to achieve boosted performance are discussed. Finally, some perspectives on further studies of the self-reconstruction and rational design of advanced electrocatalysts in some emerging electrocatalysis are included.

achieved, rational and targeted designs based on insightful understanding of catalytic mechanisms are still unexplored mainly due to the lack of study of dynamic reaction processes. In fact, understanding catalytic processes and identifying active sites explicitly is pivotal to clarify the catalytic mechanism and facilitate the development of advanced electrocatalysts.12−14 In a specific electrocatalytic reaction, molecules at a given potential could be oxidized or reduced into target products at the active sites.11 Another fact that cannot be ignored is that most of the active sites of catalysts would undergo structural self-reconstruction owing to electrically driven structural oxidation or reduction processes.15,16 It is rational to believe that the self-reconstruction of active sites would induce the increase or decrease of catalytic activity as well as the change of selectivity.17,18 The generally employed ex situ characterizations are unconvincing for judiciously understanding the catalytic mechanism. By contrast, in situ or operando characterization techniques have garnered extensive interest and have been demonstrated as valid methods to track self-reconstruction, capture dynamic structure, and analyze real-time reaction product.16,18 However, from recent progress in electrocatalysis, more systematic understanding in terms of the self-reconstruction is urgently needed, and huge challenges are still existing: (1) What is the fundamental origin of the selfreconstruction of catalysts during electrocatalysis; what factors determine the self-reconstruction behavior. And, these factors should also be deeply correlated with the self-reconstruction and catalytic performance. (2) Tracking long-range and local structures as well as identifying the true active species under realistic reaction conditions should be further developed to better unveil the underlying mechanism.15,19−21 (3) Based on the dynamic atomic-level structure of catalysts as well as product information, a complete correlation among the structural feature of precatalysts, self-reconstruction behavior, and true catalytically active sites should be established, providing guidelines for the development of highly efficient and robust catalysts. In this Account, we spotlight typical cases in the structural self-reconstruction of electrocatalysts from the origin, the evolution of active sites and the establishment of structureperformance relationship, aiming at providing a systematic understanding for the self-reconstruction as well as corre-

2. STRUCTURAL SELF-RECONSTRUCTION IN ELECTROCATALYSIS 2.1. Fundamental Origin of Self-Reconstruction

To better illustrate structural self-reconstruction of electrocatalysts and the corresponding change of catalytic performance, first it is necessary to introduce the fundamental details of several typical electrocatalytic reactions, including HER, HOR, ORR, and OER. From the thermodynamics point of view, the equilibrium potentials (E0) of hydrogen-involving hydrogen oxidation reaction (HOR) and HER is 0 V versus reversible hydrogen electrode (RHE).5 The oxygen-involving ORR and OER is another one with the E0 of 1.23 V versus RHE.5 In practical electrocatalytic processes, experimentally much more negative or positive potential scales compared to the equilibrium potentials are often applied to obtain scalable molecule conversions due to nonideal reaction conditions and large kinetics barrier (Figure 1a).22,23 It has been extensively found that the applied potentials for driving the corresponding molecule conversion are to some extent overlapping with the redox potentials of the employed electrocatalysts, especially for those transition metal-based catalysts.24,25 The conversion of these molecules and structural self-change of the electrocatalysts would simultaneously occur at some potential regions.15,18,21 Electrocatalytic reactions as typical heterogeneous catalytic reactions occur on the surface of the applied electrocatalysts. The structural properties of the catalytic surface mainly determine catalytic behavior, including absorption, activation, and desorption.26 Therefore, it is rational to deduce that the self-change of catalysts would tremendously influence the catalytic performance.17 Herein, taking a recent cooperative study from our group and other researchers as an example,20 the structural self-reconstruction of catalysts at some given potential window is disclosed B


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Figure 2. (a) Schematic illustration for the synthesis of nickel vacancies-tunable nickel hydroxides. (b) Fourier transformed EXAFS results of the samples with different nickel vacancy (VNi) concentration. (c) Polarization curves of the samples. (d−g) Density of states of α-Ni(OH)2 with VNi of 0.0%, 3.7%, 7.4%, and 11.1%, respectively. (h) Charge density distribution at the Fermi level upon VNi incorporation. (i) Formation energies of high-valence Ni species from α-Ni(OH)2 of different VNi concentration. Adapted from ref 38. Copyright 2018 American Chemical Society.

with a direct 4e− transfer process relative to the existence of a 2e− transfer process with the gerneration of hydrogen peroxide for fresh AuNi HD. It can be seen that the applied activation potentials were much more positive than the electrode potential of Ni(OH)2 formation (E0 (Ni(OH)2/Ni) = 0.106 V versus RHE),30 leading to the enhanced oxidation. For the boosted ORR activity, DFT calculations indicated that the binding of O2 on the Ni(OH)2−Ni−Au interface is moderate relative to the strong binding at the pure metallic Ni surface.20 As to the OER catalysis of the activated AuNi HD, the formed Ni(OH)2 could be further transformed into high-valence Ni3+/4+ species as real active components due to the oxidation potential (centered at ∼1.4 V versus RHE) of Ni(OH)2 prior to the onset of OER.21,24 The Au part could promote the transformation process.31 Besides, the interfacial electron transfer from Au sites to Ni parts could optimize the absorption ability of intermediates, resulting the low overpotential in the RDS.32 On the other hand, it is readily acknowledged that reaction conditions, typically applied temperature and solvent (electrolyte pH and reaction gas), could alter the redox kinetics and shift the redox potentials of the electrocatalysts so that diverse catalytic performance could be displayed. Taking transition metal-based OER catalysts as examples, we illustrate how the electrolyte pH influences catalytic activity. Strasser and co-workers have revealed in operando spectra that high-pH electrolyte (pH > 13) could strongly shift the redox peak couple of Ni−Fe oxyhydroxides more cathodically and produce an evident enhancement of the catalytic activity.33 Such enhancement was ascribed to the more generation of high Ni oxidation states as actually active species, indicating that the high-pH electrolyte could facilitate the structural self-reconstruction. Therefore, preactivated strategy toward OER precatalysts in high-pH electrolyte has been developed and employed to promote the generation of active species for neutral OER catalysis.34,35 Additionally, the redox transitions of OER catalysts are also influenced by reaction gas that is generally used to maintain stable reaction

(Figure 1b). In this work, as-prepared AuNi heterodimers (HD) with NiO/Ni interfaces displayed initially comparable HER activity with a state-of-the-art Pt/C catalyst. However, the activity obviously deteriorated after constant HER catalysis at −0.27 V versus RHE for 1 h. Structural characterizations indicated that the increased surface oxidation and core−shell metal−metal oxide/hydroxide structure occurred, which may be attributed to the more negative standard electrode potential of E0 (Ni2+/Ni) compared to that of E0 (H+/H2).5,27 It should be noted that the details about the self-oxidation of metallic Ni surfaces in HER are not acknowledged to date, and further research is greatly needed.28 For understanding the activity decline induced by the surface oxidation, mechanism investigations toward alkaline HER catalysis is briefly introduced. Previous studies have revealed that nickel-based metal−metal oxide/hydroxide interfacial structures could increase the reaction kinetics of the water dissociation which is identified as the rate-determining step (RDS) of alkaline HER catalysis.29 At the interfacial sites, the water molecule is preferentially absorbed with H atoms interacting with metallic Ni sites and O atoms interacting with oxide/hydroxide sites. The absorbed H generated by water dissociation recombines to generate H2. Pure oxide/hydroxide or metallic Ni is unsatisfactory to catalyze alkaline HER, and thus, an optimized ratio of metal oxide/hydroxide−metal Ni should be considered. Therefore, it can be easily understood that the formed core−shell metal−metal oxide/hydroxide structure in AuNi heterodimers had decreased Ni/NiO interfacial sites and lower activity relative to the initial Ni/NiO−Au interfacial structure. For ORR and OER catalysis of the AuNi HD, first electrochemical activation by cyclic voltammogram cycles at 1.1 to 1.7 V versus RHE was performed to obtain activated AuNi HD with the ternary Ni(OH)2−Ni−Au interface.20 Electrochemical tests suggested that both the ORR and OER catalytic activities were boosted, along with more formation of Ni(OH)2 in high potential windows. Meanwhile, the ORR catalysis over the activated AuNi HD displayed high selectivity C


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Figure 3. (a) Illustration of the in situ sXAS experimental setup. (b) Ni L-edge spectra of NiCoFeP and relevant controls. (c−f) Normalized in situ XANES (c, e) and EXAFS (d, f) spectra of Mo K-edge for ternary Fe, Co, Ni-based hybrid nanotube arrays (FeCoNi-HNTAs) at potentiostatic HER and OER processes. Panels (a) and (b) are reprinted from ref 21. Copyright 2017 Nature Publishing Group. Panels (c)−(f) reproduced with permission from ref 19. Copyright 2018 Nature Publishing Group under a Creative Commons Attribution 4.0 International License: http:// creativecommons.org/licenses/by/4.0/.

conditions. For example, Shao-Horn and co-workers demonstrated that the oxidation of brownmillerite SrCoO3‑δ before OER catalysis could be promoted by the oxygen filling into SrCoO3‑δ oxygen vacancies in O2-saturated condition, thus leading to the enhanced activity.36 According to the above discussion, it can be briefly summarized that the applied potentials that are often selected by the experimental and theoretical results not only drive the conversion of the targeted molecules but also cover the redox potentials of the employed electrocatalysts. This results in the oxidation or reduction of electrocatalysts and alters the structure, thereby influencing the catalytic performance.

samples with gradually increased VNi were obtained by controlling hydrolysis rate (Figure 2a). Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra (Figure 2b) evidence the gradually increased VNi in these samples. In the combination of monitoring the structural change and analyzing electrocatalytic behavior (Figure 2c), it has been found that the sample with higher VNi exhibited more high-valence Ni species and larger current density during OER, revealing that VNi-assisted self-reconstruction leads to the boosted activities. Furthermore, we performed density functional theory (DFT) calculations to understand the role of VNi. The sample with more VNi displayed higher density of states (DOS) near the Fermi level (Figure 2d−h), which could promote the charge transfer to assist the reconstruction. Also, the calculated formation energies of high-valence Ni species (Ni3+) from the Ni(OH)2 were gradually decreased with increasing VNi (Figure 2i). Thus, we proposed in this work that structural self-reconstruction ability of OER precatalysts could be enhanced by elaborately tuning the local structures of the precatalysts. Similarly, the redox transitions can be tuned through the metal substitution without changing the longrange structure.24 For instance, Sargent’s group suggested in theoretical predictions that the formation energies of highvalence species for active OER catalysis could efficiently be lowered by the doping of Co, Fe, and P into nickel hydroxide (Ni(OH)2) precatalysts.21 Furthermore, inactive supports in heterostructural electrocatalysts also influence the self-reconstruction.31−33 Carbon materials are extensively applied as supports of active electrocatalysts. 33,39 A recent study suggested through experimental evidence that the redox potential of NiFe oxyhydroxides could be shifted to a more negative location after the introduction of carbon supports,33 which is in good

2.2. Correlation between Self-Reconstruction and Intrinsic Structure of Precatalysts

In addition to the aforementioned roles of reaction conditions on the self-reconstruction of electrocatalysts, understanding the correlation between the self-reconstruction and intrinsic structure of precatalysts is of great importance to approach the rational design of electrocatalysts, so we focus on it here. First it is easily understood that different congeneric metal compounds generally exhibit diverse redox potentials so that different self-reconstruction behaviors at the same reaction condition can be observed.24 However, it should also be noted that even if some precatalysts are congeneric with the same long-range structure, they still often exhibit diverse selfreconstruction phenomenon and catalytic performance, suggesting that local structure also plays a significant role on the self-reconstruction.24,37 Recently, our group demonstrated from the local structure of OER precatalysts that nickel vacancies (VNi) of Ni(OH)2 precatalysts could boost the selfreconstruction to form high-valence Ni species for efficient OER catalysis (Figure 2).38 In this work, the α-Ni(OH)2 D


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Figure 4. (a, b) Ir L3-edge XANES (a) and EXAFS (b) spectra of iridium incorporated cobalt-based hydroxide (CoIr)-derived components. (c, d) Co K-edge XANES (c) and EXAFS (d) spectra of CoIr-derived components. (e) Schematic illustration for the structural self-reconstruction of CoIr for OER. Reproduced with permission from ref 35. Copyright 2018 John Wiley and Sons.

reconstruction is highly important for identifying the detailed reconstruction processes in realistic reaction conditions.19−21 In particular, in situ and operando characterization techniques have been pushed to a central position for tracking the selfreconstruction.16,40 Herein, we emphasize X-ray absorption spectroscopy (XAS) due to its high sensitivity for local electronic and geometric structures which have above been discussed as important roles on the self-reconstruction and catalytic performance.16 Soft XAS (sXAS) is highly efficient to probe local electronic structures. Zheng et al. utilized in situ sXAS to investigate the oxidation-state changes of OER catalysts (Figure 3a).21 In the in situ sXAS (Figure 3b), the signal (located at 875.1 eV) of Ni4+ species was obviously observed with increasing the potential to above 1.6 V. In the combination with experimental results, the generation of highvalence metal sites (Ni4+) could achieve a high reactivity toward OER. Recently our cooperative investigations have successfully utilized in situ hard X-ray absorption fine structure (XAFS) spectra to probe the structural self-reconstruction of Fe, Co, Ni-based hybrid nanotube arrays (FeCoNi-HNTAs) catalysts in HER and OER.19 Through analyzing the X-ray absorption near edge structure (XANES, Figure 3c and e) and EXAFS (Figure 3d and f) results, it can be seen that the oxidation states and metal-O coordination increased when the

agreement with our investigations on the hybrid of cobalt phosphide (CoP) and carbon materials.39 In the hybrid, through the combination of XAS characterizations and density functional theory calculations, it was revealed that the electrondeficient CoP surface resulting from the electron transfer from the CoP sites to carbon sites would be transformed into cobalt oxyhydroxide more easily. Such promotion effect of inactive supports as a result of the interfacial charge polarization was also demonstrated through operando X-ray spectroscopic characterizations in the interface of La2O3 and CoFe nanoparticle precatalysts for OER.40 From the above discussion, it could be rationally concluded that the correlation between self-reconstruction and intrinsic structure of precatalysts is rooted in the different redox transitions resulting from the structural diversity and even the small disparity of local structures. 2.3. Tracking Structural Self-Reconstruction and Identifying True Active Sites

From the fundamental knowledge of the self-reconstruction, we have clearly realized that the location of the applied overpotentials, reaction conditions, as well as intrinsic structure of precatalysts could tune the redox transitions of electrocatalysts and determine the catalytic performance. On the other hand, it is widely accepted that tracking the selfE


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Figure 5. (a−d) X-ray diffraction (XRD) patterns (a), HRTEM image (b), X-ray photoelectron spectroscopy (XPS) (c), and Fe K-edge EXAFS spectra (d) of FeP4. (e) Schematic illustration of the structural self-reconstruction of Fe-based phosphides for OER. Reproduced from ref 42. Copyright 2018 American Chemical Society.

formed high-valence metal species display rich structural defects with low coordination relative to the corresponding bulks. The coordinatively unsaturated sites could enhance the interaction with intermediates of OER to achieve decreased overpotentials.41 Similarly, we also demonstrated that the FeP4 cubes synthesized by Prussian blue analogues would be transformed into FeOOH with abundant defects for OER (Figure 5), suggesting that the inefficient OER catalysis of single Fe-based materials could also be significantly activated by the defect engineering.42,43 This work further shows that the local structure of the reconstruction-derived components in OER mainly determines the performance. From the above cases, tracking the self-reconstruction from the angle of longrange structure is evidently inadequate. Achieving the atomiclevel capture of local structure in overall catalytic process is the doorknob to shed light on real active sites and catalytic mechanism.

potentials were more positive, which was obviously ascribed to the enhanced oxidation at more positive potentials. Such results are well consistent with the aforementioned AuNi cases,20 further verifying that the fundamental origin of the self-reconstruction is the potential window. The reconstruction initially occurs at the surface of electrocatalysts.16 Consequently, the reconstruction-derived components serve as actually active species for electrocatalysis.21 Therefore, identifying the structure of the derived components is imperative. Interestingly, for OER catalysts, considerable studies indicate that metal nitrides, phosphides, sulfides, selenides, and so forth would be transformed into apparently consistent metal (oxy)hydroxides for OER. However, these derived metal (oxy)hydroxides often show better catalytic activities than directly synthesized corresponding metal oxides/hydroxides.37 Thus, distinguishing structural difference among the derived components and corresponding bulk counterparts at atomic resolution holds the key to unveil the intrinsic mechanism. Recently, we synthesized defect-rich cobalt-based hydroxides with the incorporation of welldispersed iridium (CoIr) for efficient OER electrocatalysis in both alkaline and neutral electrolytes (Figure 4).35 We performed XANES and EXAFS measurements to identify the electronic properties and coordination structures of active components (Figure 4a−d). It has been revealed that the

2.4. Modulation of Self-Reconstruction

According to the aforementioned discussion and considerable reports, the self-reconstruction significantly influences catalytic performance. Thus, modulating the self-reconstruction would be a highly efficient strategy to improve catalytic performance. Obviously changing the reaction conditions, such as potentials, temperature, electrolytes, and so forth, are readily employed to tune the self-reconstruction. For example, the above F


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Figure 6. (a, b) Scanning transmission electron microscopy-atomic scale electron energy-loss spectroscopy (STEM-EELS) chemical maps for Ni (in blue), NiO (in green), and Cr2O3 (in red) before (a) and after (b) HER stability, showing structural change. (c) Chronoamperometry curves of NiO/Ni heterostructure with and without Cr2O3 blending with initial current densities of 20 mA cm−2. (d) Schematic illustration for the design of Ni/NiO@C. The red dotted circles illustrate carbon defect sites. (e) High-resolution transmission electron microscopy (HRTEM) image (inset: selected area electron diffraction) of Ni/NiO@C, indicating the ternary interfacial structures. (f, g) Polarization curves of Ni/NiO@C and control samples in 1 M KOH and 1 M PBS electrolytes. Panels (a)−(c) are reproduced with permission from ref 44. Copyright 2015 John Wiley and Sons. Panels (d)−(g) are reproduced with permission from ref 45. Copyright 2018 Elsevier Inc.

preactivated strategy toward OER precatalysts in high-pH electrolyte was proposed to promote the generation of active species for neutral OER catalysis.34,35 Herein, we would like to focus on the modulation of the self-reconstruction through the design of the precatalysts. In the HER electrocatalysis, the surface self-oxidation often occurs, such as the above AuNi and FeCoNi-HNTAs,19,20 leading to the degradation of catalytic activities. To suppress the self-oxidation and maintain stable interfacial structure in metallic Ni-based catalysts, Dai’s group demonstrated that the introduction of inactive Cr2O3 into the interfacial Ni/NiO could obtain stable HER catalysis (Figure 6a−c).44 More recently, we also proposed the design of a unique ternary interfacial superstructure comprising Ni, NiO clusters, and defective carbon layers by controllable thermal treatments toward Ni-based coordination compounds (Figure 6d).45 The ternary interfacial Ni/NiO/C sites were formed at the defective carbon sites (Figure 6e). In such ternary interfacial structure, the carbon layers around metallic Ni could enable fast electron transfer and efficiently inhibit the surface self-oxidation so that stable interfacial structures could be maintained during HER, thus high activities were delivered even after long-term catalytic processes (Figure 6f and g). Similarly, for OER catalysis, following the understanding for the role of defects on the reconstruction ability in the above VNi-tunable cases,38 we controlled the synthesis conditions in the CoIr OER precatalysts to achieve the modulation of defect level.35 It was predictably observed that the defect-rich samples displayed easier reconstruction into high-valence metal species for more efficient OER catalysis. Furthermore, we also designed transition metal phosphides with the containment of Na+ (Figure 7a).42 The Na+-contained metal phosphides during OER would be restructured into corresponding metal oxyhydroxides. It was experimentally found that the Na+

Figure 7. (a) Schematic illustration for the synthesis of Na-contained transition metal phosphide from Prussian blue analogues. (b) Schematic for the self-reconstruction of Sn-contained perovskite electrocatalysts during OER. Panel (a) is reproduced from ref 42. Copyright 2018 American Chemical Society. Panel (b) reproduced with permission from ref 46. Copyright 2017 Nature Publishing Group under Creative Commons Attribution 4.0 International License: http://creativecommons.org/licenses/by/4.0/.

continuously dissolved into the electrolyte during OER due to highly solubility of Na+ in KOH electrolyte. The Na+ G


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rational design of heterogeneous catalysts. We anticipate that this Account will stimulate more researchers to get into the structural identification and mechanistic studies of heterogeneous catalysts under realistic reaction conditions.

dissolution resulted in abundant structural defects to facilitate the self-oxidation of the metal phosphides. Likewise, Zhang’s group revealed that the superior OER activity of SnNiFe perovskite nanodots stemmed from Sn4+ dissolution from the solid phase into the electrolyte (Figure 7b).46 Overall, based on the fundamental understanding for the self-reconstruction, some efficient designs of precatalysts are indeed proposed to regulate the self-reconstruction.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

3. CONCLUSIONS AND OUTLOOK In this Account, we have highlighted recent achievements in understanding the structural self-reconstruction of the electrocatalysts in typical electrochemical reduction and oxidation reactions. The fundamental origin of the self-reconstruction is ascribed to the redox potentials of electrocatalysts which often locate at the applied potential windows. The self-reconstruction processes and the structure of reconstruction-derived species (namely, true catalysts) are mainly determined by the reaction conditions and structural features of the precatalysts. Combining the observed results with theoretical calculations, some valuable developments have been realized in understanding the structural self-reconstruction of electrocatalysts. Consequently, some efficient methods for the design of the precatalysts have also been proposed to modulate the selfreconstruction and improve the catalytic performance. From the above discussion, although some advances have been obtained, considerable challenges and new opportunities still exist. First, for some structure-sensitive materials in air or true active species only surviving in realistic reaction conditions, in situ and operando techniques are ideal and compelling selections to obtain the real-time structure information. In this regard, to obtain systematic understanding for the selfreconstruction, the coupling of various high spatial, time, and energy resolution characterizations should be further developed to capture dynamic long-range and local structures, such as in situ XRD, TEM, Raman, XPS, and XAS. Second, the understanding of the self-reconstruction for some emerging electrocatalytic reactions, such as electrochemical CO 2 reduction and N2 fixation, should be further researched.18,47−50 Taking the CO2RR catalysts as the example, tracking the change of oxidation states and coordination structure has been developed,18,47,48 but it is obviously inadequate. Achieving highly close coupling of the identifications toward both dynamic structures and product distribution is an attractive route due to the output of multiple products in electrochemical CO2 reduction. Third, extracting the atomic-level structural identification data and real-time product distribution under catalytic processes and developing theoretical calculation methods for systematically computing the entire selfreconstruction processes from the precatalyst to true catalyst is highly challenging yet urgent to facilitate the understanding of the self-reconstruction. Finally, it will come naturally that the development in the structural self-reconstruction of electrocatalysts for typical electrocatalytic reactions can be extended to other fields of heterogeneous catalysis, probably leading to completely new understanding for some traditional heterogeneous catalysis. All in all, the ultimate goal for the self-reconstruction is to establish a faithful correlation among the precatalyst, selfreconstruction process, and true catalyst through efficient tracking of self-reconstruction, and then unveiling the underlying mechanism and approaching the atomic-level

ORCID

Hongliang Jiang: 0000-0002-5243-3524 Li Song: 0000-0003-0585-8519 Notes

The authors declare no competing financial interest. Biographies Hongliang Jiang received his Ph.D. in East China University of Science and Technology in 2016. Currently, he is a Research Associate Professor in University of Science and Technology of China. His research interests focus on controlled synthesis, mechanism studies, and electrocatalytic applications of non-noble metal nanomaterials. Qun He is a Ph.D. candidate in University of Science and Technology of China. His research interests include the synthesis of lowdimensional materials and their applications in photocatalysis and electrocatalysis. Youkui Zhang is a Ph.D. candidate in University of Science and Technology of China. His research interests focus on the synthesis and electrocatalytic applications of two-dimensional nanomaterials. Li Song received his Ph.D. in 2006 from Institute of Physics, Chinese Academy of Science (advisor Sishen Xie). After 4 years as postdoctoral researcher at University of Munich, Germany and Rice University, he became an associate professor at Shinshu University in Japan. He was promoted to the position of a professor at University of Science and Technology of China in 2012. His current research interests are synchrotron radiation study of low dimensional nanostructures and energy related devices.

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ACKNOWLEDGMENTS This work is financially supported by MOST (2017YFA0303500), NSFC (U1532112, 11574280, 21706248), Innovative Research Groups of NSFC (11621063), CAS Interdisciplinary Innovation Team and CAS Key Research Program of Frontier Sciences (QYZDBSSW-SLH018), and Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2016FXCX003).

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REFERENCES

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