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Review

In Situ Surface Reconstruction of Catalysts for Enhanced Hydrogen Evolution

College of Electronic and Optical Engineering & College of Flexible Electronics (Future Technology), Nanjing University of Posts & Telecommunications (NJUPT), 9 Wenyuan, Nanjing 210023, China
*
Author to whom correspondence should be addressed.
These authors equally to this work.
Catalysts 2023, 13(1), 120; https://doi.org/10.3390/catal13010120
Submission received: 23 November 2022 / Revised: 22 December 2022 / Accepted: 28 December 2022 / Published: 5 January 2023

Abstract

:
The in situ surface reconstitution of a catalyst for hydrogen evolution refers to its structure evolution induced by strong interactions with reaction intermediates during the hydrogen evolution reaction (HER), which eventually leads to the self-optimization of active sites. In consideration of the superior performance that can be achieved by in situ surface reconstitution, more and more attention has been paid to the relationship between active site structure evolution and the self-optimization of HER activity. More and more in situ and/or operando techniques have been explored to track the dynamic structural evolution of HER catalysts in order to clarify the underlying mechanism. This review summarizes recent advances in various types of reconstruction such as the reconfiguration of crystallinity, morphological evolution, chemical composition evolution, phase transition refactoring, surface defects, and interface refactoring in the HER process. Finally, different perspectives and outlooks are offered to guide future investigations. This review is expected to provide some new clues for a deeper understanding of in situ surface reconfiguration in hydrogen evolution reactions and the targeted design of catalysts with desirable structures.

1. Introduction

In situ surface reconstruction is a phenomenon in which the structure of the active center of a catalyst spontaneously evolves during the catalytic process or evolves with a strong interaction between the catalyst and the reaction intermediates under the influence of certain catalytic environments due to the reactive mechanism of the catalysts involved in the reaction. The ultimate catalytic performance can be self-optimized by virtue of in situ reconstruction, which is an important method for designing highly active and efficient catalysts [1,2,3,4,5]. Various new structures can be derived from different types of in situ surface reconstruction, and active site structure evolution phenomena, such as morphology evolution [6,7,8], phase transition [9,10], and defect introduction [11,12], have attracted attention from both theoretical and experimental perspectives [13]. Therefore, it is essential to reveal the evolution of the active site structure at the molecular or even atomic level to deeply clarify the catalytic mechanism and further develop cost-effective catalysts with desirable catalytic performance [14]. In particular, in the field of HER catalysts, a large number of studies have focused on the characterization of catalyst structures before and after reactions while ignoring the possible dynamic evolution occurring during the reaction process. Due to temporal and spatial mismatch, it cannot be conclusively proved that the physical structure and chemical components of catalysts are exactly as the same during and after the reaction. Accordingly, it is hard to describe the real active site structure for the HER.
In recent years, with advancements in in situ and/or operando techniques [15,16,17], more and more researchers have highlighted the importance and prevalence of the in situ reconstitution of HER catalysts. In situ characterization has become an indispensable characterization method in catalytic reactions that not only helps us to further explore the catalytic mechanism but also allows for outstanding contributions to the further design of various effective catalysts. In situ infrared, in situ Raman, in situ X-ray photoelectron spectroscopy (XPS), and in situ synchrotron radiation have been widely used. For example, Qiu et al. reported that NiFe-layered double hydroxides were found to have improved crystallinity by means of in situ characterization during the electrolysis of water under alkaline conditions, and this resulted in a lower charge transfer resistance [18]. Similarly, Zou and colleagues found that ferrous sulfide nanosheet arrays prepared via the direct sulfidation of iron foam with thiourea could gradually generate Fe@FeOxSy nanoparticles, which acted as new active sites and thus enhanced their catalytic activity over time as the HER proceeded [19]. Homoplastically, our group reported and demonstrated the irreversible phase change of 1T-MoS2 to 1T’-MoS2 during the photocatalytic HER, confirming the occurrence of self-optimization in the HER process [20].
Understanding and applying the formation mechanism and impact of in situ surface reconstruction is a very important strategy to solve energy and resource problems [14]. This review summarizes the in situ reconstruction of catalysts during HERs, including the reconfiguration of crystallinity, morphology evolution, chemical composition evolution, phase transition, and introduced surface defects (as shown in Figure 1), as well as how they affect catalytic performance. Finally, this review provides some perspectives that are expected to be meaningful for future catalyst design and development.

2. Reconfiguration Types

In recent years, with the increasing interest in HER catalysts, non-noble metal-based HER catalysts, such as TMD (transition metal disulfide), TMN (transition metal nitride), and TMC (transition metal carbides) [21,22,23,24], have been developed at an unprecedented speed due to their abundant Earth abundance, low cost and superior performance. Although the catalytic performance of these catalysts has been reported to be comparable to that of Pt/C electrodes, there is still a lack of deep insight into the surface in situ reconstitution phenomena that occur during the catalytic process. Nowadays, in situ reconstitution has been reported to affect the catalytic activity and stability of catalysts [25,26,27], and catalysts for the HER have been shown to have a period of activation for enhancing catalytic performance. In other words, it is theoretically possible to achieve the self-optimization of HER catalyst performance through certain in situ reconfiguration designs. Here, we highlight five types of in situ reconfiguration phenomena, namely, crystallinity change, morphology evolution, chemical component change, phase reconfiguration, and defects, from the physical structure and chemical composition perspectives. Some of these phenomena are induced by strong interactions between catalyst surface atoms and reaction intermediates, while others are self-structural changes caused by specific catalytic conditions. In this section, we focus on latest processes of in situ surface reconstruction phenomena and establish a connection between changes in the structure of an active site center and the self-optimization of its performance, aiming to provide a solid foundation for the further design of excellent HER catalysts.

2.1. Reconfiguration of Crystallinity

The change in crystallinity during catalytic reactions has been shown to be one of the reasons for the improvement in catalyst performance. Qiu et al. discovered the phenomena of the in situ reconstitution of NiFe-layered double hydroxides (LDHs) during the electrolysis of water under alkaline conditions [18]. The performance of NiFe-layered double hydroxides was evaluated at 1.7 V for 100 h in a base electrolyte at room temperature. As shown in Figure 2A, the reacted sample (named H-NiFe LDH) required only −59 mV of overpotential to achieve a current density of 10 mA cm−2, which is more than twice the performance of NiFe LDH. This was confirmed by the relatively low Tafel slope shown in the figure, with a value of 62.30 mV dec−1 implying that H-NiFe LDH has a good conductivity and a low activity barrier. The authors further characterized H-NiFe LDHs after ruling out the possibility of changes in catalyst performance by nickel foam, inspired by previous reports on the increase in aging Ni(OH)2 activity [28,29]. The SAED (selected area electron diffraction) results (Figure 2B) revealed that the crystal plane spacing was 0.15 nm, corresponding to the nickel (113) and bicarbonate iron (113) planes, indicating that the catalyst crystal structure did not change over the course of the reaction. Notably, the sharpness of the ring pattern significantly increased from left to right, which indicated an increase in the crystallinity of the catalyst during the reaction. This facilitated the charge transport in the reaction by reducing the reaction resistance. Concomitant with the change in crystallinity was the valence increases. In the Ni XPS results shown in Figure 2C,D, the relative increase in peaks in the Ni p spectra and the accompanying broadening of the total peaks indicate the valence increase in Ni. For the O 1s spectra, the increase in peaks at higher energies (~531 eV) also implies an increase in the amount of O bound to higher valence elements. To further investigate the interfacial active phase of the catalyst, in situ Raman tests were performed on the catalyst samples (Figure 2E). Energy bands from 400 cm−1 to 600 cm−1 confirmed the presence of metal–oxygen vibrations for FeOOH (Fe3+) and Ni(OH)2. Additionally, combining XPS and Raman data revealed that the presence of Fe could inhibit the electrochemical self-reduction activity of Ni compounds, which consequently promoted the dissociation of water. That is, the formed substances (Had-NiO and OHad-FeO) were compatible with the water dissociation step (Volmer reaction), which could greatly improve the efficiency of the hydrolysis reaction (Figure 2G). It was the synergistic effect of better Had adsorption capacity and a good electron transfer rate that led to a significant improvement in the HER activity of the NiFe LDH. It appears that the reconfiguration of crystallinity in the HER process tends to result in a better charge transfer rate for the catalyst, which consequently further reduces reaction resistance and leads to the self-optimization of catalytic activity.

2.2. Morphological Evolution

With the development of in situ characterization techniques and the in-depth investigation of the dynamics of catalysts during hydrogen precipitation reactions, morphology evolution (a phenomenon that often occurs in catalysts during HERs) has gained much attention due to its ability to generate under-liganded active sites and thus enhance catalyst activity. Compared with more complex methods of optimizing the design of HER catalysts, morphological evolution offers advantages in terms of achieving higher catalytic activity with minimal additional processing. Recently, the morphological evolution of H-TaS2 during HERs was intensively investigated with microscopic analysis and EIS tests [30]. The AFM results (Figure 3A,B) showed that the flakes became smaller, thinner and more dispersed compared with the pre-cycling period. This was also confirmed by the results of the statistical thickness distribution before and after cycling (Figure 3C,D), which showed an increase in the number of flakes with thicknesses in range from 100 nm to 300 nm. This morphological evolution led to two favorable results for H-TaS2. First, the interlayer electron transfer pathway was shortened due to the thinning of the experimental sample. For weakly bound layered materials in general, the electron transfer efficiency in the stacking direction tended to be poor, and the test results of the effective capacitance, charge transfer resistance, and fragmentation cycle changes (Figure 3E) showed that the post-cycle morphology evolution decreased its charge transfer resistance. The increase in conductivity implied that the interlayer charge transfer became easier, suggesting that the reaction rate-determining step deviated from the initial electron transfer (Volmer reaction) process. Second, the increase in effective bilayer capacitance (as shown in Figure 3E) suggested that this morphological evolution improved the accessibility of water protons to the active basal sites, thereby increasing the effective active surface area. Thus, the increase in conductivity synergistically contributed to the overall catalyst performance with the accessible surface area. To facilitate understanding, a schematic diagram of the morphological evolution mechanism was presented (Figure 3F). It was the precipitation of hydrogen bubbles at the basal sites of the stacked layer that led to the perforation and exfoliation of the sample, which consequently led to an increasing catalyst performance as the reaction proceeded.
In addition, S. lqbal and colleagues discovered that the H2 bubbles generated in the photocatalytic HER process could exfoliate MoS2 layer by layer, and they applied this mechanism to the in situ preparation of multilayer MoS2/CdS van der Waals heterostructures for enhanced photocatalytic hydrogen precipitation [31]. Figure 3G shows that the experimentally obtained samples showed self-optimized performance, as demonstrated by the increase in the average H2 production rate from 27 mmol g(CdS)−1 h−1 in the first hour to 140 mmol g(CdS)−1 h−1 with AQY (apparent quantum yield) of 66% at 420 nm after two hours, which was higher than that of the recently reported photocatalytic hydrogen production system based on MoS2/CdS photocatalysts [32,33]. Based on these experimental results, a process based on the precipitation of H2 bubbles leading to the interlayer exfoliation of the sample and thus the preparation of highly active MoS2/CdS vdWHs was proposed, as shown in Figure 3H. Under the high H2 bubble growth rate, the Van der Waals forces between the MoS2 nanocrystal layers were weakened, and the layers were exfoliated into several layers. These experiments revealed the existence of morphological evolution during the HER process, i.e., the high growth rate of H2 bubbles during the reaction caused the perforation and even the interlayer exfoliation of the sample, as well as providing new ideas and methods to guide the preparation of self-optimized, highly active catalysts.

2.3. Chemical Composition Evolution

A catalyst works by participating in a unit step that changes the reaction rate by changing the activation energy of the reaction. During electrocatalytic hydrogen evolution, the reaction pathway of catalysts is highly sensitive to the characteristics of the active center and local reaction environment, which leads to the evolution of chemical components in some catalysts.

2.3.1. Complete Species Transition

Zou and colleagues analyzed the HER catalytic process of ferrous sulfide nanosheet arrays grown on iron foam prepared by the direct sulfurization of iron foam with thiourea in a solvothermal system, and they reported an ultra-high catalytic activity for the HER [19]. For the convenience of expression, the sample catalyzed by the HER was called Fe-H2cat. As shown in Figure 4A, both IF (iron foam) and FeS/IF were relatively inactive in the HER, while the current densities of Fe-H2cat reached 100 mA cm−2 and 1000 mA cm−2 (without rectification) at 300 mV and 510 mV of overpotential, respectively. Moreover, it was noteworthy that the catalytic activity of Fe-H2cat was much higher than that of Pt/C in the wide current density range above 300 mA cm−2. After correcting the original iR loss data (Figure 4B), Fe-H2cat only needed 243 mV and 336 mV of overpotential to achieve current densities of 100 mA cm−2 and 1000 mA cm−2, respectively. This indicated that the catalyst changed during the reaction, and the catalyst was further characterized following this idea. SEM images (Figure 4C) showed that the array structure of the FeS/IF nanosheets was almost intact in Fe-H2cat, and a large number of nanoparticles with sizes of 10–20 nm appeared on the surface of the FeS nanosheets. It was reported that the formed iron nanoparticles could be rapidly partially oxidized in an aerobic environment and that a thin crust of ferric oxide appeared around the nanoparticles [34]. This structure was found to have many advantages, and it was the integration of these advantages that led to the good catalytic performance of Fe-H2cat, as shown in Figure 4D. First, because FeS is inherently metallic (or conductive) and electrically connected to a metal collector (i.e., IF), FeS/IF could act as a 3D scaffold, ensuring that external electrons were readily available to all catalytically active phases (i.e., Fe@FeOxSy). Additionally, the use of 3D iron foam was able to increase the catalyst load per geometric area and improve the mass transfer performance. In addition, polymer adhesives were typically required for electrically driven catalysts such as powdered Pt/C catalysts, and the direct fixation of each Fe@FeOxSy nanoparticle to the conductive FeS nanosheet eliminated the need for polymer adhesives and reduced the interfacial resistance between them, thus improving the structure and catalytic stability of the electrode. Finally, this unique structure enabled Fe-H2cat to have an underwater superhydrophobic surface and thus excellent mass transfer performance. Next, an S/O-covered Fe (111) surface was constructed as a theoretical model to better understand the synergistic enhancement mechanism of the O and S binary nonmetals of Fe@FeOxSy (Figure 4E), and a model of O-covered Fe (111) surface was constructed for comparison (Figure 4F). The authors evaluated the free energy of voltametric H* (△GH*) on two surfaces of all possible adsorption locations (Figure 4G). Obviously, compared with the O-covered Fe surface and FeS, the S/O-covered Fe surface had a smaller △GH* value, that is, the S/O-covered Fe surface constituted a more efficient catalytic active center, which was consistent with the experimental results.

2.3.2. In Situ Surface Hydrogenation

Similarly, in situ hydrogenation, as an integral part of chemical composition evolution during in situ surface reconstruction, is considered a promising way to improve the visible light absorption and photocatalytic activity of semiconductor photocatalysts [35]. Zhu et al. prepared ZnIn2S4 nanosheets with a low-temperature reflux method and then peeled them off using a water-assisted surfactant/interlace-free stripping process [36]. Next, the ZIS nanosheets were processed in sealed quartz glass with Na2S/Na2SO3 as the sacrificial agent under UV–visible light. In the first four hours of the experiment, the H2 precipitation rate gradually increased with the increase in illumination time, and the color of ZIS changed from yellow to black within a few hours as the reaction progressed (Figure 5A). This indicated that the HER performance of the ZIS nanosheets was self-optimized within the first four hours. Then, the ZIS nanosheets (HxZIS-6 h) were measured with solid state nuclear magnetic resonance (NMR). The peaks at 3.35 and 4.0 PPM in the H-NMR spectrogram corresponded to S-H bonds (Figure 5B). The XPS spectrogram of the S 2P of HxZIS-6 h (Figure 5C) shows that there were two types of sulfur (original sulfur and hydrogenated sulfur) in the sample, which was consistent with the signals of two S-H bonds corresponding to the two peaks mentioned above. It was further confirmed that part of the sulfur in the ZIS structure was hydrogenated during the reaction. Diffuse UV–visible spectroscopy (UV–vis) was used to characterize the intrinsic optical properties of the original and illuminated samples (Figure 5D). It is worth noting that all samples showed a fairly high absorbance in the UV and blue regions (<400 nm). The color changes indicated the presence of appropriate modifications in the crystal structure and the electronic structure, which might have been related to the hydrogen adsorbed on the surface of ZIS in the hydrogen evolution reaction, and also indicated the absorption range of visible light following in situ hydrogenation. In addition, the structural changes of the ZIS nanosheets during hydrogenation were investigated with DFT calculations. Figure 5E,F, respectively, shows the atomic structure of the original ZIS sample and the HxZIS sample. The DOS (density of states) diagram of the original ZIS sample (Figure 5G) shows that the original ZIS had certain bandgap semiconductor characteristics. However, as shown in the atomic structure diagram and the DOS diagram of HxZIS sample (Figure 5F,H), H was adsorbed on the surface and inserted into the inner surface layer. H adsorption significantly changed the DOS distribution and Fermi energy level, which reflected the complete charge transfer of H adsorption. It also indicated that the HxZIS samples had zero band gap metal properties. In short, these results reflected the phenomena of the in situ surface reconstruction of ZIS nanosheets via in situ hydrogenation engineering during general UV–vis induced hydrogenation. These studies aid our understanding of the evolution of chemical components in in situ reconstruction phenomena and provide us with new inspiration for the design of new catalysts.

2.3.3. In Situ Surface Oxygen Doping

Contrary to the abovementioned ways of achieving chemical component changes, the adjustment of surface electron density by means of heteroatom doping is also considered an important strategy to achieve the self-optimization of properties in in situ surface reconstructions. In a recent report, our group proposed an effective photon-induced strategy for the in situ oxygen doping of MoS2 nanosheets in a low-oxygen environment and experimentally found that the intrinsically flexible MoS2 nanosheets exhibited self-optimization of properties under low-oxygen conditions [37]. The performance of the intrinsically flexible MoS2 nanosheets was found to be self-optimizing under the photocatalytic environment and moderate-oxygen conditions. The results of the real-time detection of the dynamic change of the H2 generation rate under the condition of 1.2 mL of O2 (Figure 6A) showed that the performance of MoS2 could be self-optimized under the stimulation of O2 for eight hours and finally reached its highest rate of 1.6 mmol h−1 g−1. In other words, under moderate-O2 conditions, MoS2 underwent in situ oxygen doping and the activity of MoS2 substrate was well-stimulated, showing efficient self-optimization performance. XPS spectra then confirmed the chemical composition change of the catalyst during the reaction. As shown in Figure 6B, the appearance of the O 1s peak at 530.6 eV, unlike the O 1s peak of pristine MoS2 (532.0 eV), confirmed the formation of Mo-O bonds in S-O-MoS2, i.e., the catalyst underwent in situ oxygen doping during the reaction. Raman spectroscopy tests were further used to reveal the type of bonding after O2 activation [38,39]. As seen in Figure 6C, the peak intensities measured from S-O-MoS2 were much weaker than those of pristine MoS2, indicating that the long-range ordering of the 2H structure of MoS2 was highly distorted due to oxygen doping, and the peaks appearing at 280.5 cm−1 and 337.5 cm−1 showed that the MoS2 lattice was contracted under the influence of oxygen doping. All these results fully demonstrated the successful doping of oxygen in the ultrathin MoS2 nanosheets. In fact, in experiments, oxygen doping inevitably strains ultrathin MoS2 nanosheets, and this strain also play an important role in enhancing the catalytic activity of the HER [40]. In order to more intuitively elucidate the effects of oxygen doping and strain on HER performance, the calculation of the free energy (ΔGH*) of atomic hydrogen adsorption (Figure 6D) was used to show that the ΔGH* of strained MoS2 with oxygen doping was only 0.3 eV, which was much lower than that of pristine MoS2. Based on the analysis of these experimental results, a schematic diagram of the formation mechanism of MoS2-xOx during the reaction was established. As shown in Figure 6E, the introduction of O2 induced the electron cloud rearrangement of the S 2p orbital, leading to the breakage of the Mo-S bond, the generation of volatile SO2 species, and the production of S vacancies. Figure 6F shows a complete actual reaction mechanism diagram. Excitation by the light of EY was found to produce the triplet excited state EY3* and the free radical EY through the reductive quenching of TEOA. Next the electrons were transferred between the highly reducible EY and O2, the generated O2 continued to react with MoS2, and oxygen doping was finally achieved. With the formation of MoS2-xOx, the catalyst had a locally higher electron density and better electrical conductivity. In other words, oxygen doping and the strain brought by oxygen doping could optimize the electronic structure and Gibbs free energy of the catalyst and realize the phenomenon of the self-optimization of catalyst performance. These studies aid our understanding of the evolution of chemical components in in situ reconstruction phenomena and also provide new insights into the design of new catalyst studies.

2.4. Phase Transition Refactoring

Phase change is the process of the transformation of a substance from one phase to another. Extensive studies have shown that some catalysts inevitably undergo phase transition due to the presence of cathodic potential and strong adsorption–surface interactions during the HER. The atomic density, coordination number, strain density, and electronic structure of the catalyst surface also change [41,42]. Our group reported that the irreversible phase transition of MoS2 during the photocatalytic HER resulted in a significant increase in catalytic activity [20]. The photocatalytic HER of TiO2@MoS2 was carried out under visible light irradiation using Eosin Y (EY) as the photosensitizer and triethanolamine (TEOA) as the sacrificial agent, and the hydrogen yields were separately examined. The results showed (Figure 7A) that the hydrothermally exfoliated TiO2@MoS2 exhibited better HER activity than the pre-exfoliated TiO2@MoS2, and the highest catalytic activity of the hydrothermally exfoliated TiO2@MoS2 was reached after 7 h from the start of the reaction (Figure 7B). The HRTEM test results clearly showed the reason for the self-optimization performance (Figure 7C,D). The catalyst showed a distortion of the 1T’ phase, as the 1T phase gradually relaxed into a highly distorted 1T’ phase with a superlattice, and its superlattice structure also aggregated from Mo atoms into a jagged chain structure. To further elucidate the mechanism by which this phase transition occurs, it was investigated by calculating ΔGH as a function of strain for different adsorption sites (Figure 7E,F) on different phases of MoS2 (Figure 7G,H). The calculated results showed that even though certain active sites were present on both 1T and 1T’ basal planes, the 1T’ phase confirmed its high hydrogen precipitation activity performance with ΔGH values of below 0.35 eV (regardless of the adsorption sites and the coverage of H), i.e., the self-optimization of the catalytic performance of MoS2 was induced by it undergoing a transition from the 1T to the 1T’ phase.
Unlike the direct observation of phase transition phenomena in the experiments mentioned above, our group innovatively proposed the in situ reconstruction of WS2 via local strain engineering and successfully transformed 2H-WS2 nanoribbons into 1T-WS2 nanospirals, and we found that it showed a high catalytic activity and good stability as a HER catalyst [43]. Due to the unique nanoribbon structure of 2H-WS2 with a strain gradient prepared by a simple hydrothermal synthesis method, coupled with the more flexible thinner nanoribbons, 2H-WS2 spontaneously twisted into a helix when subjected to anisotropic driving forces. In this process, the S-plane slipped due to the twisting of the atomic plane, but stable 1T-WS2 nanospirals were eventually formed due to the Van der Waals interactions between the layers and the self-stabilizing energy of the helix [44,45]. Based on this theory, three atomic models were constructed, as shown in Figure 7I, to illustrate that mechanically stretching WS2 nanoribbons into pleated sheets could be considered a typical way to induce shape anisotropy, thus making it possible to thermodynamically induce phase transitions by mechanical deformation [46,47,48,49]. Figure 7K clearly shows the formation energy difference Δγ between the 2H phase and the 1T phase (Figure 7J) as a function of strain. Clearly, the 1T phase was more stable than the 2H phase when the strain was close to 5%. This reinforced the feasibility of the theory. Remarkably, the next performance test on the 1T-WS2 nano helix in the HER showed good and efficient self-optimized catalytic activity, i.e., an overpotential of 170 mV at an electrocatalytic current density of 10 mA cm−2 (Figure 7L) and a Tafel slope of 40 mV dec−1 (Figure 7M), which implied the feasibility of the in situ reconstitution of WS2 by local strain engineering pairs. In the SEM image of WS2 that was subjected to 20,000 cycles (Figure 7N), it can be clearly seen that the WS2 nano helices were interconnected and formed an open porous framework, which not only exposed more active edges [50,51,52,53] but also provided the inner surface of the 1T-WS2 electrode with a larger contact area with the solution. Additionally, the results of the density generalization calculations of the atomic hydrogen adsorption free energy of deformed 1T-WS2 monolayers at different strains showed (Figure 7O) that the edges of 1T-WS2 had lower △GH values than 2H-WS2, indicating that both the electron transport efficiency and active sites were enhanced by the phase transition. Furthermore, Sun et al. studied the phase transition reconfiguration of multivalent nickel sulfides in the HER process and suggested that the phase reconfiguration process could be synergistically accelerated by moderate Fe doping to improve HER performance [54]. It was found that NiS2 doped with 20% Fe had better catalytic performance than NiS2 alone, and it was found that not only NiS2 but also multivalent nickel sulfides such as α-NiS, β-NiS, and Ni3S4 had similar phase transition processes, as shown in Figure 7P,Q. This not only indicated that the phase reconfiguration of NiS2 penetrated from the surface to the matrix over long catalytic durations and thus enhanced the catalytic activity of the catalyst but also illustrated the prevalence of the phase reconfiguration of multivalent nickel sulfide catalysts in the HER process. Overall, the phase transformation tended to bring benefits such as a lower charge transfer resistance, enhanced activity per site, and an increased number of active sites due to topological phase transformation of the catalysts, as well as providing a new approach for designing new catalysts with self-optimized activity via in situ reconfiguration.

2.5. Surface Defects

In recent years, defects have drawn a lot of attention and research for their ability to enhance intrinsic activity, enrich active sites, and improve electrical conductivity. Inspired by the fact that defects can change the charge distribution on a catalyst surface and thus directly generate active sites on the inert substrate, Charlie Tsai and colleagues proposed and verified that the application of a sufficient reduction potential could induce sulfur vacancies in monolayer and multilayer MoS2 to enhance its HER catalytic activity [55]. As shown in Figure 8A, the calculation of the surface free energy of a single atom showed that the surface on which sulfur vacancies were generated after the application of a potential (*) was more stable and had a higher capacity for hydrogen adsorption than the original substrate (*S). It also reflected that the generated sulfur vacancies were more inclined to bind to H2 than to the removed H2S gas. A further complete theory of sulfur vacancy generation was presented, as shown in Figure 8B, i.e., the desulfurization process at the applied potential was realized following the step of transferring two electrons and protons to the sulfur atoms in the basal plane. The free energy diagram of the corresponding steps below then proved the correctness of the proposed reaction process [56,57]. Next, the feasibility of the electrochemical activation strategy was verified with further experiments. The electrochemical processes of sulfur vacancy generation in monolayer and multilayer molybdenum disulfide under different electrode supports were tested, and optical and SEM images of the experimental samples were obtained (Figure 8C,D). The images showed that the samples exhibited significant morphological changes after the applied potential for both monolayer and multilayer MoS2, which implied the successful introduction of defects. The current density test results in Figure 8E show that the catalytic activity of both monolayer and multilayer MoS2 was enhanced after the electrochemical desulfurization process. That is, the sulfur vacancies created during the electrochemical reaction played a decisive role in the self-optimization of their HER activity. Finally, Figure 8F shows the TOF of each surface Mo atom of the sample (TOFMo) versus the applied potential and compares it with the TOF of other MoSx groups currently considered to be the most advanced. Various state-of-the-art MoSx catalysts from [58] are shown with non-bold labels. The light green area is the MoS2 monomolecular layer from [59] that underwent Ar desulfurization. The red solid curve is the monolayer MoS2 on the Au substrate after electrochemical desulfurization, and the blue solid curve is the multilayer MoS2 on carbon foam substrate after electrochemical desulfurization without iR correction. The blue dashed curve refers to the electrochemically desulfurized multilayer molybdenum disulfide on a carbon foam substrate with iR correction. The experimental samples exhibited higher TOF values. It was noteworthy that the experimentally prepared MoS2 samples containing S vacancies were not structurally optimized. Additionally, this could provide a feasible way to prepare more active and low-cost HER catalysts.

2.6. Interface Refactoring

Recently, the ability of a pre-catalyst designed based on interfacially induced self-assembly to achieve efficient performance self-optimization through interfacial reconfiguration during the HER was demonstrated by Wang and colleagues [60]. A CoC2O4@MXene pre-catalyst with hollow nanotubes was synthesized by completing the self-assembly of CoC2O4 nanorods and MXene nanosheets in an aqueous oxalic acid dihydrate solution at room temperature under the influence of interfacially induced effects. Next, sequential LSV scans were performed in 1 M KOH. The results showed (Figure 9A) that CoC2O4@MXene had a stable polarization profile after only five scanning cycles, and the overpotential sharply dropped by about 46 mV during this process, ending up with an overpotential of only 28 mV at 10 mA cm−2 (Figure 9B). This indicated that CoC2O4@MXene could complete a fast, efficient reconstitution in a short period of time, making its own performance substantially improved. Further HRTEM (high-resolution transmission electron microscopy) showed (Figure 9C) that CoC2O4@MXene generated new hexagonal nanosheets after cycling. The observed 0.276 nm lattice stripe length was consistent with the (100) plane of Co(OH)2, revealing that the formation of this sparse hexagonal nanosheet structure could be attributed to the generation of Co(OH)2. The interfacial reconfiguration mechanism of the conversion of CoC2O4@MXene to Co(OH)2 was proposed based on the experimental data, as shown in Figure 9D. Due to the ultrathin MXene and its unique nanotube structure, the high electron accessibility and abundant electrolyte diffusion channels introduced a highly disordered and sparse Co(OH)2 nanosheet structure for the catalyst as the reaction process proceeded, forming a Co(OH)2@MXene complex with low valence and rich defect levels. Finally, the catalytic mechanism of CoC2O4@MXene in the basic HER process after the interfacial reconfiguration is elucidated in the figure. As shown in Figure 9E,F, due to the low valence state and defect-rich nature of the complex formed by the rapid reconstruction, a large number of Co active sites were fully exposed, which promoted the adsorption of water molecules and optimized the binding energy of H* intermediates, thus accelerating the HER kinetics. Moreover, the large number of exposed low-valent Co sites also provided good orbitals for charge transfer, which consequently enhanced the electrical conductivity. In addition, the reconfigured Co(OH)2@MXene complex still had good hydrophilicity, excellent conductivity and abundant electrolyte diffusion channels due to the sparse structure and ultrathin MXene of the in situ reconfigured catalyst. Therefore, interface refactoring often causes changes in the surface structure, active site, and conductivity of catalysts (e.g., the surface structure becomes loose and amorphous, the number or types of active sites increases, and the conductivity is optimized) so as to realize the self-optimization of catalyst performance from different angles and directions.

3. Synergistic Effects

A synergistic effect, a common phenomenon in society and scientific research, refers to the addition or blending of two or more components together, resulting in an effect greater than the sum of the various components acting individually [61]. With the rapid development of in situ characterization techniques in recent years, it has been found that the in situ reconfiguration of the surface of a portion of HER catalysts is often accompanied by a series of more fantastic chain evolutions, which consequently act on the catalytic performance of the catalysts and result in a greater enhancement of their catalytic activity [62]. Therefore, appropriate or controlled synergistic effects can be used as a more efficient and cost-effective catalyst activation strategy to achieve the self-optimization of catalyst performance. As mentioned above in the work of Qiu and colleagues, the high catalytic activity of the H-NiFe LDH is influenced not only by its better electron transfer rate due to its increased crystallinity but also by the enhanced performance of the catalyst combined with the adsorbate that forms intermediates that accelerate the rate of hydrolysis [18]. The efficiency of the synergistic effect was also very well-demonstrated in the work of Zou and colleagues mentioned above. It was found that the newly generated Fe@FeOxSy nanoparticles were not in the free state but were directly immobilized on FeS nanosheets [19]. We know that polymer binders are necessary for powdered catalysts [63,64]. The direct immobilization of the nanoparticles on the nanosheets in this kind of experiment not only eliminates the need for polymer binders and thus reduces the experimental cost and operational difficulties but also reduces the interfacial resistance between them and thus improves the electrode structure and catalytic stability. From these two examples, we can draw certain conclusions. Firstly, the synergistic effect can not only improve the performance of catalysts but also save time and economic costs to a certain extent. Secondly, since the synergistic effect is often a series of chain effects caused by one evolution, and such subtle effects are not easily observed, we must treat each experimental phenomenon and all data with a rigorous and careful attitude in the experimental process, which is essential to promote the development of science and technology. Table 1 shows the catalytic performance of electrocatalysts modulated via in situ surface reconstruction. It can be seen that in situ surface reconstruction is an important strategy for regulating catalytic hydrogen evolution.

4. Conclusions and Future Prospects

Despite many practical achievements in transition metal catalyst design and performance enhancement, research on issues such as the understanding of in situ reconstructions and performance self-optimization is still in an immature stage. In this work, we reviewed the in situ surface reconstruction of catalysts during the HER process, focusing on six aspects of crystallinity change, morphology evolution, chemical component evolution, phase transition, surface defects, and interface refactoring, and we established the link between structure and performance. Overall, it is clear from our summary and analysis that in situ reconstructions have significant effects on the performance of catalysts in chemical reactions. Therefore, a more in-depth study on the in situ reconstruction of catalysts to reveal and summarize the potential mechanisms and principles is important to produce the next generation of catalysts with low cost, high activity, high selectivity, and long-term stability, as well as to further promote the commercialization of hydrogen energy production to achieve the goal of sustainable development [14,65]. However, the realistic feasibility of these in situ reconstructions remains to be explored in depth, so we suggest that future work should pay attention to the following ideas.
Firstly, in situ surface reconstruction is being continuously enriched, which can systematically reveal the predicted in situ surface reconstruction phenomena of HER catalysts. At present, the dynamic evolution of HER catalysts during the reaction process has not received sufficient attention and in-depth study, and more attention has been paid to CO2RR (carbon dioxide reduction reaction) [66,67,68,69,70], NRR (nitrogen reduction reaction) [71,72,73,74], and OER (oxygen evolution reaction) [75,76,77,78,79]. In addition, theoretical calculations, an indispensable tool for experimental characterization, have mostly been used to analyze the effects of these dynamics and have not been skillfully applied to analyze and predict changes in the catalysts themselves. Therefore, it is important to establish a mature knowledge system of the in situ surface reconstruction of HER catalysts and to analyze and predict the dynamic change process of catalysts with the help of theoretical calculations for the design of high-performance, high-stability and high-selectivity HER catalysts.
Secondly, a multifaceted and complementary in situ characterization technology system has been established in order to more comprehensively and deeply track the dynamic evolution of HER catalysts in the catalytic process and to identify and capture the true intrinsic active sites of catalysts in a timely and accurate manner. With the advancement of technology, more and more in situ characterization techniques, such as XPS, XRD, Raman spectroscopy and SECM, have been used as experimental characterization tools for HER catalysts [15,65]. However, it is worth noting that each of these technical tools has the characteristics of harsh application conditions and narrow application targets [38,80,81,82]. Therefore, it is difficult to achieve a comprehensive and profound understanding of the in situ surface reconstruction of HER catalysts with a single technical characterization tool [83,84,85,86,87,88,89]. Therefore, in future experimental studies, a combination of techniques with various temporal resolutions and probing regions (e.g., contact interfaces, solids, and liquids) should be adopted to characterize the in situ surface reconstruction phenomena in a comprehensive manner.
In addition, more groundbreaking methods should be developed, such as the integration of existing in situ reconstruction examples by means of the statistical analysis of big data and artificial intelligence algorithms, in order to build and analyze all possible in situ reconstruction models [81,90,91,92,93]. Big data and artificial intelligence technologies are both hot research fields nowadays, and they have seen widespread attention and rapid development due to their time-saving and efficient nature. With the involvement of artificial intelligence and machine learning, theoretical simulations may be able to provide a clear description of structural evolutions and reactive pathways [70,82], which will consequently guide researchers in deeper exploration and discovery.

Author Contributions

Writing—review and editing, Y.Z., J.P., G.G., R.S., Y.Y., M.L.; W.H., P.F. and L.Y., project administration, L.W.; All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Natural Science Foundation of China (51902101), the Youth Natural Science Foundation of Hunan Province (2021JJ540044), the Natural Science Foundation of Jiangsu Province (BK20201381), and the Science Foundation of Nanjing University of Posts and Telecommunications (NY219144).

Conflicts of Interest

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

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Figure 1. Schematic diagram of the in situ surface reconstruction of HER catalysts.
Figure 1. Schematic diagram of the in situ surface reconstruction of HER catalysts.
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Figure 2. (A) Polarization curves obtained for pristine NiFe and H-NiFe LDHs in a three-electrode structure with a 1 M KOH aqueous electrolyte at a scan rate of 5 mV s−1. The inset plots are Tafel plots with 100% iR compensation for both at the abovementioned conditions. (B) SAED plots of NiFe and H-NiFe LDHs. High-resolution XPS patterns of Ni 2p (C) and O 1s (D) in NiFe and H-NiFe LDHs. (E) In situ Raman spectroscopy test results of NiFe LDH in the large wave number region during the HER in the 1 M KOH environment at different overpotentials. (F) The amplification of the orange region in (E). (G) Diagram of the reaction mechanism of NiFe LDH in the HER process. Reproduced with permission from [18], ©The Royal Society of Chemistry 2014.
Figure 2. (A) Polarization curves obtained for pristine NiFe and H-NiFe LDHs in a three-electrode structure with a 1 M KOH aqueous electrolyte at a scan rate of 5 mV s−1. The inset plots are Tafel plots with 100% iR compensation for both at the abovementioned conditions. (B) SAED plots of NiFe and H-NiFe LDHs. High-resolution XPS patterns of Ni 2p (C) and O 1s (D) in NiFe and H-NiFe LDHs. (E) In situ Raman spectroscopy test results of NiFe LDH in the large wave number region during the HER in the 1 M KOH environment at different overpotentials. (F) The amplification of the orange region in (E). (G) Diagram of the reaction mechanism of NiFe LDH in the HER process. Reproduced with permission from [18], ©The Royal Society of Chemistry 2014.
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Figure 3. AFM images of H-TaS2 before (A) and after (B) the reaction. (C,D) Statistical thickness distributions of H-TaS2 before and after the reaction, respectively. (E) Variations of effective capacitance (Ceff/Ceff,0, where 0 is the initial value) and charge transfer resistance (Rct/Rct,0) with cycling. (F) Schematic diagram of the mechanism of the morphological evolution process. The flat plate represents the MX2 layer, and the sphere represents the H2 bubble generated during the HER. Reproduced with permission from [30], ©Springer Nature 2017. (G) Photocatalytic hydrogen production performance of vdWHs under high (red line) and low (blue line) pressure conditions. (H) Schematic diagram of the process principle based on bubble exfoliation to prepare highly active MoS2/CdS vdWHs. Reproduced with permission from [31], ©The Royal Society of Chemistry 2014.
Figure 3. AFM images of H-TaS2 before (A) and after (B) the reaction. (C,D) Statistical thickness distributions of H-TaS2 before and after the reaction, respectively. (E) Variations of effective capacitance (Ceff/Ceff,0, where 0 is the initial value) and charge transfer resistance (Rct/Rct,0) with cycling. (F) Schematic diagram of the mechanism of the morphological evolution process. The flat plate represents the MX2 layer, and the sphere represents the H2 bubble generated during the HER. Reproduced with permission from [30], ©Springer Nature 2017. (G) Photocatalytic hydrogen production performance of vdWHs under high (red line) and low (blue line) pressure conditions. (H) Schematic diagram of the process principle based on bubble exfoliation to prepare highly active MoS2/CdS vdWHs. Reproduced with permission from [31], ©The Royal Society of Chemistry 2014.
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Figure 4. (A) Linear sweep voltammetry (LSV) curves of the HER against IF, Pt/C (20 wt %), Fe-H2cat and FeS/IF in a 1 M KOH solution (without iR correction). (B) LSV curves of Fe-H2cat with Pt/C (20 wt %) corrected with 85% iR for the HER. (C) From left to right, SEM images of Fe-S/IF, low-magnification SEM images of Fe-H2cat, and high-magnification SEM images of Fe-H2cat. (D) Schematic of the principle of the HER activity enhancement of Fe-H2cat. (E) Possible H* adsorption locations on the O-covered Fe (111) surface and (F) O/S-covered Fe (111) surface. All top positions on the surface (indicated by elemental symbols) and top shallow bridge positions (indicated by “b”) are considered. (G) Calculated free energy diagrams for Pt (111), FeS (001), O-covered Fe (111), and O/S-covered Fe (111) surfaces at equilibrium potential for the HER. Reproduced with permission [19], Copyright 2018, Elsevier.
Figure 4. (A) Linear sweep voltammetry (LSV) curves of the HER against IF, Pt/C (20 wt %), Fe-H2cat and FeS/IF in a 1 M KOH solution (without iR correction). (B) LSV curves of Fe-H2cat with Pt/C (20 wt %) corrected with 85% iR for the HER. (C) From left to right, SEM images of Fe-S/IF, low-magnification SEM images of Fe-H2cat, and high-magnification SEM images of Fe-H2cat. (D) Schematic of the principle of the HER activity enhancement of Fe-H2cat. (E) Possible H* adsorption locations on the O-covered Fe (111) surface and (F) O/S-covered Fe (111) surface. All top positions on the surface (indicated by elemental symbols) and top shallow bridge positions (indicated by “b”) are considered. (G) Calculated free energy diagrams for Pt (111), FeS (001), O-covered Fe (111), and O/S-covered Fe (111) surfaces at equilibrium potential for the HER. Reproduced with permission [19], Copyright 2018, Elsevier.
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Figure 5. (A) Hydrogen content and corresponding color change of ZIS nanosheets at different time periods during the reaction. (B) H1 solid-state NMR spectra of HxZIS-6 h nanosheets. δ3 and δ4 are the signals of the S-H bonds; δ1 and δ2 are the signals of the adsorbed water molecules. (C) XPS spectrum of S 2p of HxZIS-6 h. (D) UV–vis DRS spectra and photos of ZIS nanosheets at different time periods during the reaction. Top (E) and side (F) views of relaxed pristine ZIS systems and HxZIS systems. (G,H) Electronic density of states of typical pristine ZIS systems and HxZIS systems (DOS). Reproduced with permission [36], Copyright 2019, Elsevier.
Figure 5. (A) Hydrogen content and corresponding color change of ZIS nanosheets at different time periods during the reaction. (B) H1 solid-state NMR spectra of HxZIS-6 h nanosheets. δ3 and δ4 are the signals of the S-H bonds; δ1 and δ2 are the signals of the adsorbed water molecules. (C) XPS spectrum of S 2p of HxZIS-6 h. (D) UV–vis DRS spectra and photos of ZIS nanosheets at different time periods during the reaction. Top (E) and side (F) views of relaxed pristine ZIS systems and HxZIS systems. (G,H) Electronic density of states of typical pristine ZIS systems and HxZIS systems (DOS). Reproduced with permission [36], Copyright 2019, Elsevier.
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Figure 6. (A) Plot of the results of hydrogen production per hour of MoS2 during the HER detected with an online gas chromatograph every 15 min. (B) O 1s high-resolution XPS spectra and (C) Raman spectra of pristine MoS2 and S-O-MoS2. (D) Hydrogen precipitation ΔGH* plots of pristine MoS2, strained MoS2 (S-MoS2, 6% strain), O-doped MoS2 (O-MoS2, 25 atom% O) and O-doped strained MoS2 (S-O-MoS2, 6% strain, 25 atom% O). (E) Schematic diagram of the S vacancy formation process. (F) Schematic diagram of the formation mechanism of MoS2-xOx during the actual reaction. Reproduced with permission [2021], Copyright 2021, Elsevier.
Figure 6. (A) Plot of the results of hydrogen production per hour of MoS2 during the HER detected with an online gas chromatograph every 15 min. (B) O 1s high-resolution XPS spectra and (C) Raman spectra of pristine MoS2 and S-O-MoS2. (D) Hydrogen precipitation ΔGH* plots of pristine MoS2, strained MoS2 (S-MoS2, 6% strain), O-doped MoS2 (O-MoS2, 25 atom% O) and O-doped strained MoS2 (S-O-MoS2, 6% strain, 25 atom% O). (E) Schematic diagram of the S vacancy formation process. (F) Schematic diagram of the formation mechanism of MoS2-xOx during the actual reaction. Reproduced with permission [2021], Copyright 2021, Elsevier.
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Figure 7. (A) H2 accumulation and (B) H2 production rate of TiO2@MoS2 over stripped TiO2 during each hour of the HER. All experiments were performed in 80 mL of a 15% (v/v) TEOA aqueous solution under visible light irradiation (γ > 420 nm). (C) HRTEM images of MoS2 in TiO2@MoS2 exfoliated after 7 h. (D) Magnified image of the square in (C) (arrows point to the serrated chain structure of 1T’). Supercell of H adsorbed on the surface of monolayer 1T-MoS2 (E) and 1T’-MoS2 (F). The yellow and blue balls indicate S and Mo atoms, respectively. The numbers indicate the possible sites of H binding. The Gibbs free energy for H adsorption on 1T-MoS2 (G) and 1T’-MoS2 (H) was calculated for strain (ε) in the range of 0.0% to 3.0%. (1) corresponds to 1/16 coverage of one H at any one site. (1,2), (1,3), (1,5) and (1,6) correspond to the coverage of two H atoms (one at 1 and the other at 2, 3, 5, and 6, respectively). 1T-MoS2: (1,2) = (1,4); (1,3) = (1,5) = (1,7); (1,6) = (1,8). 1T’-MoS2: (1,2) = (1,4); (1,3) = (1,5) = (1,7); (1,6) = (1,8). 1T’-MoS2: (1,2) = (1,4); (1,3) = (1,7); (1,6) = (1,8). Reproduced with permission [20], Copyright 2017, Wiley-VCH. (I) Strain-induced topological transition of WS2 nanoribbons to nano helices. The yellow and blue spheres represent S and W atoms, respectively. Side views of the terminated armchair and twisted armchair WS2 nanoribbons and helical WS2 nanowires are shown from top to bottom. (K) The calculated formation energy difference Δγ between the 2H and 1T (J) phases as a function of strain. (L) Electrochemical data of WS2 nanoribbons with a Pt reference catalyst in H2SO4 (0.5 M). Polarization curves are shown as a function of the number of potential cycles. (M) Corresponding Tafel plots of WS2 nanoribbons, Pt/C, and pristine WS2 nanoribbons after 10,000 cycles. (N) SEM image of WS2 nanospiral (after 20,000 cycles of WS2 nanoribbon). (O) ΔGH plots of different H adsorption sites and comparison of the effect of different sites and strains on HER properties in 1T and 2H-WS2 phases. The inset is a TEM image of a WS2 nano helix. Reproduced with permission [43], Copyright 2019, Elsevier. (P) Schematic diagram of the phase reconfiguration of multivalent nickel sulfide HER catalysts. (Q) XRD spectra of different catalysts after the instantaneous action of the HER at a constant current density of 10 mA cm−2. Reproduced with permission from [54], ©The Royal Society of Chemistry 2014.
Figure 7. (A) H2 accumulation and (B) H2 production rate of TiO2@MoS2 over stripped TiO2 during each hour of the HER. All experiments were performed in 80 mL of a 15% (v/v) TEOA aqueous solution under visible light irradiation (γ > 420 nm). (C) HRTEM images of MoS2 in TiO2@MoS2 exfoliated after 7 h. (D) Magnified image of the square in (C) (arrows point to the serrated chain structure of 1T’). Supercell of H adsorbed on the surface of monolayer 1T-MoS2 (E) and 1T’-MoS2 (F). The yellow and blue balls indicate S and Mo atoms, respectively. The numbers indicate the possible sites of H binding. The Gibbs free energy for H adsorption on 1T-MoS2 (G) and 1T’-MoS2 (H) was calculated for strain (ε) in the range of 0.0% to 3.0%. (1) corresponds to 1/16 coverage of one H at any one site. (1,2), (1,3), (1,5) and (1,6) correspond to the coverage of two H atoms (one at 1 and the other at 2, 3, 5, and 6, respectively). 1T-MoS2: (1,2) = (1,4); (1,3) = (1,5) = (1,7); (1,6) = (1,8). 1T’-MoS2: (1,2) = (1,4); (1,3) = (1,5) = (1,7); (1,6) = (1,8). 1T’-MoS2: (1,2) = (1,4); (1,3) = (1,7); (1,6) = (1,8). Reproduced with permission [20], Copyright 2017, Wiley-VCH. (I) Strain-induced topological transition of WS2 nanoribbons to nano helices. The yellow and blue spheres represent S and W atoms, respectively. Side views of the terminated armchair and twisted armchair WS2 nanoribbons and helical WS2 nanowires are shown from top to bottom. (K) The calculated formation energy difference Δγ between the 2H and 1T (J) phases as a function of strain. (L) Electrochemical data of WS2 nanoribbons with a Pt reference catalyst in H2SO4 (0.5 M). Polarization curves are shown as a function of the number of potential cycles. (M) Corresponding Tafel plots of WS2 nanoribbons, Pt/C, and pristine WS2 nanoribbons after 10,000 cycles. (N) SEM image of WS2 nanospiral (after 20,000 cycles of WS2 nanoribbon). (O) ΔGH plots of different H adsorption sites and comparison of the effect of different sites and strains on HER properties in 1T and 2H-WS2 phases. The inset is a TEM image of a WS2 nano helix. Reproduced with permission [43], Copyright 2019, Elsevier. (P) Schematic diagram of the phase reconfiguration of multivalent nickel sulfide HER catalysts. (Q) XRD spectra of different catalysts after the instantaneous action of the HER at a constant current density of 10 mA cm−2. Reproduced with permission from [54], ©The Royal Society of Chemistry 2014.
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Figure 8. (A) The unit cell surface energy of 2H-MoS2 at fixed sulfur vacancies as a function of the applied potential of 2H-MoS2 substrates with different adsorbent species (3.1%). (*) refers to the S vacancy, and the adsorbed species in the S vacancy is indicated by the prefix *. For example, (*S) refers to the sulfur adsorbed in the vacancy, i.e., the original basal plane without the S vacancy. (B) A schematic diagram of the EC desulfurization process is shown at the top of the figure. The lower part shows the free energy diagram for the protonation and removal of S. The green line indicates the free energy path at the applied potential required to make all paths functional. (C) Optical and SEM images of monolayer MoS2 before desulfurization (P-MoS2) and after desulfurization (V-MoS2). (D) Optical and SEM images of multilayer MoS2 films before (P-MoS2) and after (V-MoS2) desulfurization. (E) Statistics regarding the desulfurization effect of multilayer molybdenum disulfide on carbon rods and carbon foam substrates and the desulfurization effect of monolayer molybdenum disulfide on gold substrates on improving HER activity. The rods without MoS2 attachment show negligible effects of the electrochemical desulfurization process on the substrate itself. Error bars indicate the standard deviation between samples. (F) Comparison of the TOFMo flip frequency of Mo atoms per surface between experimental samples and state-of-the-art MoSx electrocatalysts. Reproduced with permission [55], Copyright 2017, Springer Nature.
Figure 8. (A) The unit cell surface energy of 2H-MoS2 at fixed sulfur vacancies as a function of the applied potential of 2H-MoS2 substrates with different adsorbent species (3.1%). (*) refers to the S vacancy, and the adsorbed species in the S vacancy is indicated by the prefix *. For example, (*S) refers to the sulfur adsorbed in the vacancy, i.e., the original basal plane without the S vacancy. (B) A schematic diagram of the EC desulfurization process is shown at the top of the figure. The lower part shows the free energy diagram for the protonation and removal of S. The green line indicates the free energy path at the applied potential required to make all paths functional. (C) Optical and SEM images of monolayer MoS2 before desulfurization (P-MoS2) and after desulfurization (V-MoS2). (D) Optical and SEM images of multilayer MoS2 films before (P-MoS2) and after (V-MoS2) desulfurization. (E) Statistics regarding the desulfurization effect of multilayer molybdenum disulfide on carbon rods and carbon foam substrates and the desulfurization effect of monolayer molybdenum disulfide on gold substrates on improving HER activity. The rods without MoS2 attachment show negligible effects of the electrochemical desulfurization process on the substrate itself. Error bars indicate the standard deviation between samples. (F) Comparison of the TOFMo flip frequency of Mo atoms per surface between experimental samples and state-of-the-art MoSx electrocatalysts. Reproduced with permission [55], Copyright 2017, Springer Nature.
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Figure 9. (A) Polarization curves of different LSV scans (IR correction) and (B) corresponding overpotentials of CoC2O4@MXene in a 1 M KOH solution. (C) HRTEM image of CoC2O4@MXene after the cycle. (D) Schematic diagram of the reconfiguration process of CoC2O4@MXene. (E,F) Schematic illustration of the catalytic mechanism of the alkaline HER on the CoC2O4@MXene after the cycle. Reproduced with permission from [60], ©Springer Nature 2017.
Figure 9. (A) Polarization curves of different LSV scans (IR correction) and (B) corresponding overpotentials of CoC2O4@MXene in a 1 M KOH solution. (C) HRTEM image of CoC2O4@MXene after the cycle. (D) Schematic diagram of the reconfiguration process of CoC2O4@MXene. (E,F) Schematic illustration of the catalytic mechanism of the alkaline HER on the CoC2O4@MXene after the cycle. Reproduced with permission from [60], ©Springer Nature 2017.
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Table 1. Catalytic performance of electrocatalysts modulated via in situ surface reconstruction.
Table 1. Catalytic performance of electrocatalysts modulated via in situ surface reconstruction.
CatalystReconstruction TypesElectrolytePerformance
H-NiFe LDH [18] Reconfiguration of crystallinity1.0 M KOHTafel slope of 62.30 mV dec−1
H-TaS2 [30]Morphological evolution0.5 M H2SO4Tafel slope of 37 mV dec−1
Fe@FeOxSy [19]Complete species transition1 M KOHTafel slope of 77 mV dec−1
HxZIS [36]In situ surface hydrogenation0.35 M/0.25 M Na2S-Na2SO390 μmol of H2 at each hour of the HER
S-O-MoS2 [37]In situ surface oxygen dopingWith O doping (20.0 mg) and 20 mg of EY dye in 100 mL of aqueous TEOA solution (15%, v/v) at pH 7The maximum rate of H2 production reached 1.6 mmol h−1 g−1
TiO2@MoS2 [20]Phase transition refactoring80 mL of 15% (v/v) aqueous TEOA solution under visible light
irradiation (l > 420 nm); catalysts: 20 mg; EY: 20 mg
The maximum rate of H2 production reached 10.5 mmol h−1 g−1
1T-WS2 [43]Phase transition refactoring0.5 M H2SO4Tafel slope of 40 mV dec−1
Multilayer MoS2 [55]Surface defects0.5 M H2SO4Tafel slope of 102 mV dec−1
Monolayer MoS2 [55]Surface defects0.5 M H2SO4TOF = 1.0 s−1 at −100 mV
Co(OH)2@MXene [60]Interface refactoring1 M KOHA low overpotential of 28 mV at 10 mA cm−2
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Zhang, Y.; Pan, J.; Gong, G.; Song, R.; Yuan, Y.; Li, M.; Hu, W.; Fan, P.; Yuan, L.; Wang, L. In Situ Surface Reconstruction of Catalysts for Enhanced Hydrogen Evolution. Catalysts 2023, 13, 120. https://doi.org/10.3390/catal13010120

AMA Style

Zhang Y, Pan J, Gong G, Song R, Yuan Y, Li M, Hu W, Fan P, Yuan L, Wang L. In Situ Surface Reconstruction of Catalysts for Enhanced Hydrogen Evolution. Catalysts. 2023; 13(1):120. https://doi.org/10.3390/catal13010120

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Zhang, Yingbo, Junan Pan, Gu Gong, Renxuan Song, Ye Yuan, Mengzhu Li, Weifeng Hu, Pengcheng Fan, Lexing Yuan, and Longlu Wang. 2023. "In Situ Surface Reconstruction of Catalysts for Enhanced Hydrogen Evolution" Catalysts 13, no. 1: 120. https://doi.org/10.3390/catal13010120

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