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Review

A Mini Review: Recent Advances in Asymmetrically Coordinated Atom Sites for High-Efficiency Hydrogen Evolution Reaction

by
Junyang Ding
1,2,
Wenxian Liu
3,*,
Shusheng Zhang
4,
Jun Luo
1,5 and
Xijun Liu
2,*
1
Institute for New Energy Materials and Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China
2
State Key Laboratory of Featured Metal Materials and Life-Cycle Safety for Composite Structures, School of Resource, Environments and Materials, Nanning 530004, China
3
College of Materials Science and Engineering, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou 310014, China
4
College of Chemistry, Zhengzhou University, Zhengzhou 450001, China
5
Shensi Laboratory, Shenzhen Institute for Advanced Study, University of Electronic Science and Technology of China, Longhua District, Shenzhen 518110, China
*
Authors to whom correspondence should be addressed.
Energies 2023, 16(6), 2664; https://doi.org/10.3390/en16062664
Submission received: 21 February 2023 / Revised: 10 March 2023 / Accepted: 10 March 2023 / Published: 12 March 2023
(This article belongs to the Special Issue Atomic Electrocatalyst of Efficient Energy System)
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Abstract

:
Energy is the substance foundation of human society. Single−atom catalysts (SACs) have emerged as promising electrode materials in the energy field owing to their unique characteristics. It was demonstrated that the hydrogen evolution reaction (HER) performance of SACs relies on the metal−centric species and the corresponding local coordination engineering. Herein, the recent progress relating to asymmetric atomic catalysts for the HER is reviewed, including low coordination, heteroatomic coordination, and bimetallic coordination. In addition, the connection between the coordination structures and the presented electrocatalytic performance was discussed. The main challenges that need to be addressed for the asymmetric atomic catalysts in the HER are summarized. Finally, some insights into the development of high−quality asymmetric atomic catalysts are included.

Graphical Abstract

1. Introduction

With the objectives of carbon neutrality being established across multiple countries and regions, the utilization of renewable energy sources, such as hydrogen, wind, solar, and tidal energy, is paramount for the sustainment of present day society [1,2,3,4,5,6,7]. High−performance catalysts are the key to constructing inexpensive, high−efficiency energy storage and conversion systems since they allow for altering the reaction path, reducing the activation energy barrier, and improving the reaction rate [8,9,10,11,12]. Typically, catalysts are categorized into two groups: homogeneous and heterogeneous [13,14,15,16,17]. Homogeneous catalysts are located within the reactants in one phase and maximizes the effectiveness of the interaction. However, their widespread practical adoption is hampered since it is challenging to isolate them from the other reaction components [18,19,20,21]. Heterogeneous catalysts reside in the distinct phases from the reactants during the reaction. They often comprise metal particles of uneven size and are still employed extensively in mass manufacturing due to their low cost and good recoverability. However, they perform poorly regarding their activity, selectivity, and atom consumption [22,23,24,25,26].
Among the various hydrogen evolution reaction (HER) catalysts involved in the electrolysis of the H2O molecule, single−atom catalysts (SACs) on diverse carriers (organic−, metal−, and carbon−based materials) have emerged in recent years that are capable of successfully incorporating the respective merits of the homogeneous and heterogeneous catalysts, such as well−defined electronic structures, defined geometric configurations, virtually 100% atomic availability, desirable catalytic efficiency, and recoverability [27,28,29,30,31,32]. Moreover, researchers now have gained a deeper insight into the atomic centers’ neighboring environment due to the continuous enhancement and advancement of characterization techniques such as the Fourier transform infrared (FTIR) spectra, X−ray photoelectron spectroscopy (XPS), X−ray absorption fine structure (XAFS), and transmission electron microscopy (TEM), etc. [33,34,35]. Therefore, contrary to the conventional symmetric atomic sites, researchers have attempted to construct diverse SACs, e.g., low coordination, lateral heteroatomic coordination, axial heteroatomic coordination, and dual−metal SACs, to obtain asymmetric atomic sites (Figure 1) [36,37,38]. The asymmetric coordination configuration can precisely modulate the active moieties’ electronic distribution and its adsorption–desorption behavior for the key reaction intermediate of H*, which could decrease the original overpotential, enhance the catalytic activity, and accelerate the electrocatalytic hydrogen production [39,40,41]. This demonstrates that apart from the metal catalytic center, the geometric coordination structures around the center considerably influence the reaction energy barriers and pathways [42,43,44].
In this mini review, the merits of the asymmetric atom catalysts in the HER have been systematically described. Three types of asymmetrically atomic catalysts with carbon−based supports were presented, namely, low−coordination structures, lateral/axial heteroatomic coordination structures, and dual−metal coordination structures. In addition, the organic/metallic support−loaded asymmetrical sites were briefly introduced. Finally, the difficulties and openings of the asymmetric atomic catalysts toward electrocatalytic hydrogen generation have been further prospected.

2. The Mechanistic Principles of HER

The HER, occurring at the cathode, is a half reaction in water splitting that undergoes a two−electron transfer process. The two reaction mechanisms, i.e., the Volmer–Heyrovsky and Volmer–Tafel, are as follows (* represents the active site) [45,46,47]: Volmer step
In acidic medium: H+ + e− → H*
In alkanline/neutral medium: H2O + e− → H* + OH−
Heyrovsky step
In acidic medium: H* + H+ + e− → H2
In alkanline/neutral medium: H* + H2O +e− → H2 + OH−
Tafel step
In acidic/alkaline/neutral medium: H* + H* → H2
The initial Volmer reaction can form absorbed H* from H+ ions or H2O molecules in all kinds of electrolytes. The selectivity of the next reaction step, i.e., the Heyrovsky or Tafel steps, depends on the H* coverage of the catalytic sites. Generally, the Heyrovsky step occurs when there is a low H* coverage, otherwise the Tafel step occurs [48,49,50]. The rate−determining steps can be judged by measuring the Tafel slope values, where 120, 40, and 30 mV dec−1 correspond to Volmer, Heyovsky, or Tafel reactions, respectively [51,52,53]. Moreover, the low H* adsorption free energy (|△GH*| close to zero) points toward a better inner activity in the density functional theory [54,55,56,57].

3. Asymmetric Atom Sites for HER

3.1. Low−Coordination Structure

Generally, the metal centers with low−coordination sites can be grouped into two species, precious metals and non−precious metals, and the corresponding carbon−based carriers are mainly graphene, nitrogen−doped graphene, graphdiyne, and C3N4, etc.
For the asymmetric MNx, Zang et al. designed atomic catalysts (Ni−SA/NC) with a Ni−N3−O2 coordination and low overpotentials in both freshwater (η10: 102 mV overpotential at 10 mA cm−2) and seawater (η10: 139 mV), demonstrating that low−coordination Ni−N3 was more beneficial for H2O dissociation and H* adsorption and was considered as an HER active site (Figure 2a) [58]. Liu et al. discovered that the edge models i.e., Co−2N−armchair, Co−2N−A, Co−2N−zigzag, and Co−2N−Z, were more reactive than the plane model of Co−4N−P for the HER. The edge−rich Co single atoms (E−Co SAs) (Figure 2b) were composed of Co−4N−P (65.49%), Co−2N−A (13.64%), and Co−2N−Z (20.86%), showing a significantly enhanced HER performance account for the edge effect of E−Co SAs (η10: ~59 mV) compared to CoN4 SACs (η10: ~111 mV) with an extraordinary durability of 200 h at 500 mA cm−2 (Figure 2c) [59]. For the asymmetric MCx, the defective sites on the carbonaceous supports, similar to the hexa−carbon ring−like hollow sites, were often used as anchor sites for the metal−based single atoms. Meanwhile, Zhang et al. adopted another graphene vacancy for capturing Ni atoms as the SACs, identifying the presence of the common double vacancy aNi@Di−vacancy (NiC4) and the Stone–Wales graphene defect aNi@5775 (NiC7), as shown in Figure 2d,e. Differing from the perfect hexagonal (NiC6) and divacancy Ni site, aNi@5775 demonstrates a higher density in the states close to the benchmarked Fermi level, pointing to stronger interactions between Ni and the key intermediates (e.g., H+ and OH−) than the divacancy ones, thus noting aNi@D5775 as more favorable for the HER (η10: 70 mV in 0.5 M H2SO4) and OER (Figure 2f,g) [60]. In other works, Ni single atoms were steadily coordinated with neighboring C atoms on nanoporous graphene (np−G), and the atomic resolution HAADF−STEM also clearly suggested this substituted configuration (Nisub, NiC3). Compared the absorbed and defective Ni sites, the substitutional Nisub exhibited a minimum ΔGH* and could work as a prominent active center for the HER (η10: ~50 mV in 0.5 M H2SO4) [61]. For an atomic−level identification of the active site properties, Fang et al. examined the structural evolvement of Pt in the nitrogen–carbon substrate (Pt1/N−C) and revealed a weak interaction between them, with the Pt atom tending to show a low coordination, especially when the coordination number decreased to 2 (Pt1C2−1 Pt1C2−2) when the applied potential was at −0.07 V (Figure 2h). This contributed to a better bonding strength between the Pt atom and the reactant molecule, i.e., a lower coordination number enhanced the activity of the Pt sites (η10: 19 mV in 0.5 M H2SO4; 46 mV in 1.0 M NaOH), as shown in Figure 2i [62]. Wei et al. found that there was a smaller negative ΔGH* compared to the Pt (111) when the Pt atoms were adsorbed on the defects (Pt_on SV, PtC3) or edges (Pt_on edge, PtC2) of the graphene of the mesoporous carbon, verifying the enhanced HER activities of the Pt SACs with low−coordination numbers (65 mV at 100 mA cm−2 in 0.5 M H2SO4) [63]. Aside from the graphene and N−doped carbon substrates, the C3N4 localized RuN2 sites with an η10 of 140 mV and a graphdiyne anchored four−coordinated C2−Pt−Cl2 of 23.64 A mg−1 at 100 mV and an overpotential of 0.5 M H2SO4, were also reported in the HER electrocatalysis [64,65].
Based on the above−mentioned reports, it can be noticed that the low−coordination atom sites were mainly located on the carbon defects/vacancies and edges compared to the perfect−coordination sites. The low−coordinated MNxCy was more favorable for H2O dissociation or H* adsorption, and thus was reasonably considered as the HER active site.

3.2. Lateral/Axial Heteroatom Coordination Structure

Heteroatomic coordination is usually composed of two element types (C/N, P/N, B/N, and I/N) or even coexists with multiple elements (C/N/O). Heteroatoms also appear in different coordination shell positions and spatial orientations, increasing the diversity of the modulating metal active centers.
Lu et al. demonstrated that Ru and N co−doped carbon nanowires (η10: 12 mV) outperformed the benchmark 20% platinum/carbon (η10: 49 mV) for the HER in a 1 M KOH alkaline media (Figure 3a). The control experiments and the first−principles principle calculations showed that the HER activity of Ru−NC−700 mainly came from atomically dispersed Ru that coordinated with N and C, while the contribution of the ruthenium NPs was almost negligible. Ru−C2N2 has the finest |△GH*| close to zero and was more active than Ru−C4 and Ru−N4, in which the Ru center and the C site served as the catalytic center, while the N site accelerated the hydrogen bonding steps (Figure 3b) [66]. Chen et al. successfully achieved a rational design of the W1N1C3 moiety local structure at the atomic level by anchoring the W atom to an N−doped carbon substrate from UiO−66−NH2 (Figure 3c). The absolute value of △GH* on the W1N1C3 site was less than other possible coordination structure of WN4, W−N2C2−a, W−N2C2−b, W−N3C1, which implied its remarkable alkaline HER performance (η10: 85 mV in 0.1 M KOH) [67]. To expand the multifunctional active materials scaled−up synthesis methods, Lai et al. constructed a bifunctional electrocatalyst with dual active sites (Ir@NC3 for HER, Ir@Co(Ir) for OER), i.e., Ir single atoms loaded onto Co−embedded N−doped carbon nanostructures that can simultaneously accelerate both the HER and the OER, as shown in Figure 3d [68]. Jin et al., constructing the Pt SACs comprising the Pt−N2C2 active sites using ball milling comprising a mixed precursor of nitrogen−doped carbon spherical NPs and K2PtCl4 powder [69]. Further, the Mo single atom was stabilized over porous nitrogen−doped mesopores carbon, constructing a Mo1N1C2 local structure and showing a superior activity compared to Mo2C and MoN [70].
Replacing the N/C atoms in MNxCy with other species of a weaker electronegativity, e.g., P and B, can also regulate the local environment of the metal atom centers. Wang et al. applied an in situ phosphatizing strategy—a direct pyrolysis treatment of triphenylphosphine encapsulated within ZIF−67—to obtain an atomic Co1−P1N3 interfacial structure that was denoted as Co−SA/P (Figure 3e) with a η10 of 98 mV in 0.5 M H2SO4. They discovered that a key factor for enhancing the activity was a bond length extending high−valent Co1−P1N3 interfacial configuration (Figure 3f,g) [71]. Meanwhile, according to the DFT calculation, the sp− and sp2−hybridized B species−doped Co@GY (Co@B2−GY) material was found to boast an exceptional activity for the HER as well [74]. The iodine (I) species, with 2.66 electronegativities, were distinct from B (2.04), S (2.58), and P (2.19), and thus were endowed with several distinguishing features. Liu et al. synthesized a cobalt–nitrogen–carbon (Co−N−C) sample, whose interior had an I−doped carbon matrix with low−coordinated CoN3 and IN3 sites. These sites needed to be anchored at the edge or defect within the graphene since the I was difficult to replace and dope onto the carbon lattice due to its high atomic radius (Figure 3h,i). The incorporation of the iodine heteroatoms not only enhanced the catalyst’s specific surface area but also lowered the chemical oxidation state of the Co sites, ultimately optimizing the hydrogen adsorption and enhancing its HER kinetics (η10: 85 mV in 0.5 M H2SO4) [72]. In an asymmetric axial coordination structure, Li et al. prepared a configuration of a N/C/O triple−coordinated Mo atom site with an additional axial O atom (Mo−O1N1C2). The coordination of the nonmetallic atoms, e.g., C, N, and O, dramatically modulated the Mo centers’ electronic reconfiguration at the atomic scale, and the rich local microenvironment imparted a beneficial H* adsorption energy to the formed Mo−O1N1C2, thus enhancing its inherent activity (Figure 3j). Note that for the reduction potential of the acidic media, the dangling O was readily hydrogenated to form hydroxyls, and the obtained MoC2N−OH local structure might have been a prospective catalytic center for the HER (η10: 66 mV in 0.5 M H2SO4) [73].
The incorporation of extra nonmetallic atoms in the carbon−based supports to boost the activity of the catalytic sites for the HER has become a versatile strategy. In particular, the doped species with a different electronegativity from C, mainly N, S, P, B, O, I, Cl, etc., can be doped onto different coordination shells or even in the axial space. As one of the highly electronegative atoms, N is often combined into carbon−based carriers together with other heteroatoms to form non−traditional M−Nx−O/S/P/B/Cl/I structure catalysts.

3.3. Dual−Metal Coordination Structure

Bimetallic heteroatomic coordination, i.e., diatomic catalysts, as an extension of the SACs, have higher metal loads and complex and flexible center site configurations, and afford more electrocatalytic possibilities, and thus have received much attention recently. They can be classified as direct bimetallic interbonds (M1−M2) or indirect bridging oxygen bonds (M1−O−M2) according to the difference in the bimetallic center connection.
Zhou et al. designed an Rh−Fe interbonding promoter that can disperse Fe NPs into the corresponding Fe atoms at low temperatures (FR−NCS). The theoretical calculations also revealed that the Fe cluster−Fe−N4 intermediate, which causes the neighboring carbon vacancies to trap the moving Rh atoms, as shown in Figure 4a, b, is the source of the atomic dispersion process. The Rh/Fe bond substantially aids in the thermodynamic stability at an atomic level and results in the FR−NCS exhibiting a higher HER activity (η10: 22 mV in 0.5 M H2SO4) than the commercial Pt/C, as shown in Figure 4c [75]. Zhang et al. used a more precise atomic layer deposition (ALD) technique to selectively form the diatomic sites by carefully controlling the deposition conditions, so that the next Ru could only attach to the primary Pt to construct high−quality Pt−Ru dimer structures on the nitrogen−doped carbon nanotubes. According to calculation results, the Pt atoms had a significant impact on the Ru sites’ electronic distribution and also modified the original inertness between H and Ru through the synergistic effects of the Pt−Ru dimer, which boosted the HER activity [76]. Further, Hu et al. reported a special bimetallic site that coexisted with the NPs, i.e., part of the single atom (FeN4, RuN4) and part of the nanoparticle (Ru NPs)−loaded Fe/N co−doped carbon support via a coupling of an Ru atom and Ru NPs (inset of Figure 4d). In the synthesis, the Fe−N4 moieties diffused larger Ru NPs into smaller Ru clusters, even the atomically dispersed Ru−N4. In the hydrogen evolution, the FeN4 groups effectively modulated the electron distribution of the smaller Ru clusters, and Ru−N4 partially promoted the splitting of the H2O molecules, intermediated their generation, and then recombined them into H2 molecules on the nearby modulated Ru cluster, as shown in Figure 4d,e. In other words, they synergistically improved the HER performance under alkaline conditions (η10: 9 mV in 1 M KOH), as shown in Figure 4f [77]. For the bridging oxygen bimetallic coordination, Yu et al. accurately bridged the oxygen molecules to graft a single−atom Pt onto another Fe atom site, forming Pt1@Fe−N−C catalysts with novel Pt1−O2−Fe1−N4 groups and obtaining a high Pt loading of 2.1wt% (Figure 4g). The DFT calculations confirmed that Pt1@Fe−N−C possesses better H* adsorption–desorption properties, accounting for the high active grafting bond of the Pt1−O2−group (Figure 4h) and exhibited an unexpected performance close to 20% Pt/C (η10: 60 mV in 0.5 M H2SO4) [78]. Moreover, Yang et al. designed a similar O−bridged W−Mo dual atom structure catalyst (W1Mo1−NG DAC), as shown in Figure 4i, through a self−assembling and a subsequent nitrogenation procedure. The synthesized O−bridged W−O−Mo−O−C local structure was anchored to the N−doped graphene vacancies, and the electron delocalization effect of this configuration provided the optimized H* adsorption strength. Thus, the synthesized W1Mo1−NG DAC achieved a Pt−like catalytic activity and an ultra−high durability for the HER (η10: 24 mV in 0.5 M H2SO4; η10: 67 mV in 1 M KOH) in the common electrolytes with a wide pH [79].
According to the aforementioned reports, it was noted that one metal species can encourage the dispersion of the other species and increase their thermodynamic durability. In the catalytic process, the bimetallic site catalysts have a unique advantage in their synergistic effect, which makes them superior to the SACs in modulating the adsorption state toward the hydrogen intermediates and reducing the reaction energy barrier of the reaction steps for the HER.

3.4. Asymmetric Atom Sites of Organic/Metal−based Supports for HER

Due to the original MOFs’ instability or poor conductivity under alkaline and acidic conditions, which limited them from acting as electrocatalysts for the HER, researchers have designed the structures of the MOFs or combined them with other substrates with better conductivity.
Huang et al. designed bimetallic site MOFs (M23(M13∙HAHATN)2) with extra M−N2 moieties by employing hexaiminohexaazatrinaphthalene (HAHATN) to serve as a conjugated ligand, as shown in Figure 5a,b. The optimal Ni3(Ni3∙HAHATN)2 nanosheets possessed a satisfactory η10 of 115 mV and a Tafel slope of 45.6 mV dec−1 in 0.1 M KOH. Both the theoretical and experimental data indicated that the metal atoms on the excess M−N2 group had a higher unsaturation in comparison to those on the M−N4 sites, for promoting the donation of electrons and enhancing the catalytic activity (Figure 5c) [80]. Meanwhile, Wu et al. reported two novel Co−MOFs (CTGU−5 and −6), which could be selectively crystallized to other new two−dimensional (2D) or three−dimensional (3D) nets by changing the anionic or neutral surfactants (Figure 5d,f). As shown in Figure 5e,g, the coordination model of the H2O molecules, which is a key structural distinction between CTGU−5 and CTGU−6, differed significantly. The H2O was coordinated to the Co site in CTGU−5, whereas in CTGU−6, the H2O was a lattice molecule of the framework through a hydrogen bond. This difference produced two distinct crystal structures, which in turn, influenced the electrocatalytic characteristics of the HER. The composite composed of CTGU−5(1:4) and acetylene black (AB) was determined to have the highest HER performance (η10: 388 mV in 0.5 M H2SO4) of all the MOFs after systematically altering the ratio of the two materials [81]. Moreover, Lin et al. created binderless FeNi(BDC)(DMF,F)/NF polymer electrodes by using a nickel foam surface to grow the bimetallic MOFs compounds in situ, which were fully modulated by alloying Ni and Fe, where each contained two molecularly well−combined metal nanoclusters and delivered 10 and 400 mA cm−2 current densities at the ultra−low applied potentials of 1.58 and 1.90 V, respectively. This superior activity might be owed to the intra−molecular synergism between the FeO6 and NiO6 clusters [82]. Another reported crystalline composition, i.e., a two novel polyoxometalates (POM)−encapsulated metal–organic nanotube (HUST−200, HUST−201), can serve as stable and highly effective HER electrocatalysts [83]. For the metal−based supports, in 2017, Chao et al. designed a Cu−Pt two−site alloy with Pd nanorings (Pd/Cu−Pt NRs), which underwent a two−step approach composed of sequentially reacting the Cu− and Pt−based solutions with the Pd nanorings solutions. The FT−EXAFS measurement confirmed the appearance of the Cu−Pt coordination with a 2.67 Å bond length (Figure 5h). The synthesized novel Pd/Cu−Pt NRs can greatly boost the HER (η10: 22.8 mV in 0.5 M H2SO4), as shown in Figure 5k, based on the theoretical calculation and experimental results, and the higher activity may originate from the Cu−Pt double atom sites (Figure 5i,j) [84].
MOF materials are often used as precursors to obtain catalysts with atomically dispersed products, while the direct use of the MOF as the HER electrocatalysis for the hydrogen evolution is less reported. The majority of the catalytic activities toward the MOF are hardly similar to those of the materials composed of carbon and metal. However, the well−defined metal nodes with a coordination environment and a customizable structure of the MOF can be an ideal platform to study the catalytic activity centers.

4. Summary and Outlook

The SACs catalyst with an asymmetric local coordination and a flexible adjustment of the coordination configuration was rapidly expended in the HER catalysis as compared to the traditional M−N4 sites, which resulted from their unique electronic and geometric structure (Table 1).
In order to compare the differences in the HER properties between the symmetric and asymmetric atomic sites, two elements−Co and Pt−were selected as the representatives of the non−precious and precious metal atomic catalysts for the investigation (Figure 6). In the Co−based SACs, it was discovered that a low−coordination CoN2 active site demonstrated a greater HER performance than the symmetric coordination CoN4 moieties in the 1 M KOH alkaline electrolytes. This finding remains consistent in the 0.5 M H2SO4 solutions, i.e., the heteroatom−coordination CoP1N3 center was superior to the traditional CoN4 sites. Nevertheless, the variations in the HER performance of the symmetric/asymmetric Pt sites were not considerable, and some prominent features should not be neglected for the asymmetric sites. Consequently, when designing and creating atomic catalysts, the origin of the catalytic active component (metal atom center and nearby coordination atoms) and the modulation of the activity (electronic structure of the central single atom and its interaction with the support) are the two wider aspects that must be considered [95,96,97,98,99]. Moreover, from reviewing and examining the employment of the asymmetric atom electrocatalysts in the HER, it is evident that some new discoveries and insights have been made in the structural design of the asymmetric atomic catalysts for the HER, which can be briefly summarized in the following, hoping to provide some instructions for the design of the HER catalyst at the atomic level [100,101,102,103].
(i) Among these low−coordination asymmetric sites, NiN3 and CoN2 are commonly used in SACs and possess a better HER performance, which may arise from the differences in the electrolyte environment and the material microstructure [104,105,106,107]. Both noble and non−precious metals are involved in the HER, and by comparing the reported experimental data, the carbon coordination seems to be better than N coordination. (ii) In the published lateral/axial coordination works for the HER, the species of the heteroatoms are numerous, consisting of C, N, O, P, B, and I, etc. [108,109,110,111,112,113]. The representative metal centers for the HER are Pt, Ru, Ir, Mo, Co, and W. As can be observed, compared to the coordination environment, the role of the metal−centered species is crucial. (iii) For the diatomic HER, except for Mo, W, and Fe, noble metals (Pt, Ru, Rh) are predominantly present. For the coordination mode, compared to the monotonous M2N6, there are more types of HER, e.g., the bridged M−O−M, which can offer some new ideas for the structural design of SACs [114,115,116,117,118,119]. (iv) The comparison of the above−mentioned data reveals that the asymmetric coordination of the organic supports seems to have poorer results than the carbon supports. Except for the variation in the coordination environment, the metal species include precious metals or transition metals, however the reports on the metal supports are few [120,121,122,123,124].
Hydrogen, a widespread element in nature, is perceived as being one of the most hopeful renewable energy substitutes. To minimize the barrier of H2O electrolysis, SACs are adopted as effective and inexpensive catalysts, whose catalytic features are often determined by the metal center sites and the local coordination atoms that come from the support, demonstrating various characteristics such as higher activity and more ideal selectivity. In the design of the atomic configuration, the asymmetric coordination structure exhibits a more flexible combination of manners, e.g., through spatial structure optimization and electron transfer, the bond strength of the reactants/intermediates on the catalytic site can be optimized to balance the energy barriers, thus accelerating the overall reaction rate [125,126,127,128,129,130]. Moreover, we present the remaining challenges and the expected directions of the asymmetric atomic catalysts for hydrogen production from water splitting.
(i) Advanced atomic structural characterization. The precise determination of the coordination structure over the atomic resolution is a significant matter for a comprehensive knowledge of the evolution of SACs. The interpretation of the coordination atoms/types on the central sites is still challenging, even with the most widely used characterization methods, such as FTIR spectra, XPS, XAFS, and HAADF−STEM, etc. [131,132,133,134]. For example, XAFS has difficulty distinguishing between the various coordination atoms with close atomic numbers (Z) and/or similar bond distances [135,136,137,138]. The HAADF−STEM also has difficulty in identifying the active atoms when their Z−values are close to those of the support [139,140,141,142]. Impressively, several in situ/operando characterization strategies enable us to unravel the evolution of the coordination environment and elucidate the catalytic mechanism during the reaction process.
(ii) Long−term stability. In comparison to conventional catalysts, SACs are more sensitive to the surrounding conditions and more prone to degradation during storage or application, especially when operating in highly corrosive electrolytes with large current densities. In terms of the HER, the numerous reports of SACs perform on par with or even better than commercial Pt, but their long−term durability poses a significant obstacle to their practical implementation in industrial hydrogen production, as they must generally retain their activity and stability for longer than 1000 h [143,144,145,146].
(iii) Scale−up production. Controlled and streamlined methods are needed to generate asymmetric SACs with high activity that meet the requirements of continuous and large−scale production. It should be noted that the presence of symmetric atoms, nanoclusters, and nanoparticles is difficult to avoid in the preparation of asymmetric atomic catalysts, and thus more attention should be paid to the quality of the finished catalyst. Furthermore, increasing the atomic loading is an efficacious strategy for further boosting the mass activity of the SACs. However, the present reported atom loading is limited to ~10 wt%, for which this still needs more effort [147,148,149,150].

Funding

This work was financially supported by the National Natural Science Foundation of China (22075211, 21601136, 51971157, and 22275166) and the Tianjin Science Fund for Distinguished Young Scholars (19JCJQJC61800).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Model illustration for the symmetric/asymmetric atom sites (yellow and green, metal atom; blue, N atom; gray, C atom; red, heteroatom, such as O, P, S etc.).
Figure 1. Model illustration for the symmetric/asymmetric atom sites (yellow and green, metal atom; blue, N atom; gray, C atom; red, heteroatom, such as O, P, S etc.).
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Figure 2. (a) X−ray adsorption near−edge structure (XANES) curves of Ni−SA/NC. Reprinted with permission from ref. [58]. Copyright (2020) Wiley−VCH GmbH. (b) Fourier transform extended X−ray absorption fine structure (FT−EXAFS) fitting curve for E−Co SAs in the R space. (c) Linear sweep voltammetric (LSV) curves in 1 M KOH. Reprinted with permission from ref. [59]. Copyright (2021) Wiley−VCH GmbH. (d) High−resolution high−angle annular dark field scanning transmission electron microscopy (HAADF−STEM) images of the defective area with atomic trapped Ni. (e) Coordination structure of a Ni single atom supported on D5775. Energy profiles for the HER (f) and the OER (g). Reprinted with permission from ref. [60]. Copyright (2017) Elsevier Inc. (h) Theory models of the Pt1−C3N1, two different Pt1−C2, and two different Pt1−C1N1 sties. (i) Adsorption free energies toward H2O and H*. Reprinted with permission from ref. [62]. Copyright (2020) Nature Publishing Group.
Figure 2. (a) X−ray adsorption near−edge structure (XANES) curves of Ni−SA/NC. Reprinted with permission from ref. [58]. Copyright (2020) Wiley−VCH GmbH. (b) Fourier transform extended X−ray absorption fine structure (FT−EXAFS) fitting curve for E−Co SAs in the R space. (c) Linear sweep voltammetric (LSV) curves in 1 M KOH. Reprinted with permission from ref. [59]. Copyright (2021) Wiley−VCH GmbH. (d) High−resolution high−angle annular dark field scanning transmission electron microscopy (HAADF−STEM) images of the defective area with atomic trapped Ni. (e) Coordination structure of a Ni single atom supported on D5775. Energy profiles for the HER (f) and the OER (g). Reprinted with permission from ref. [60]. Copyright (2017) Elsevier Inc. (h) Theory models of the Pt1−C3N1, two different Pt1−C2, and two different Pt1−C1N1 sties. (i) Adsorption free energies toward H2O and H*. Reprinted with permission from ref. [62]. Copyright (2020) Nature Publishing Group.
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Figure 3. (a) LSV curves. (b) Free energy diagram of the different RuCxNy structures. Reprinted with permission from ref. [66]. Copyright (2019) Nature Publishing Group. (c) k3−weight FT−EXAFS fitting curves of the W−SAC at W L3−edge. Reprinted with permission from ref. [67]. Copyright (2018) Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Schematic of the formation for M1@Co/NC. Reprinted with permission from ref. [68]. Copyright (2019) Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Scheme of the formation for Co−SA/P−in situ. Free energy diagrams of (f) the Volmer–Heyrovsky step and (g) the Volmer–Tafel step of the HER. Reprinted with permission from ref. [71]. Copyright (2020) American Chemical Society. (h) Atomic−scale HAADF−STEM images of Co−I−N/G. (i) Schematic illustration of the electronic construction. Reprinted with permission from ref. [72]. Copyright (2021) American Chemical Society. (j) Free energy profile of Mo−SACs and Pt (111) (inset, the hydrogenated MoC2N−OH). Reprinted with permission from ref. [73]. Copyright (2021) American Chemical Society. * represents the active site.
Figure 3. (a) LSV curves. (b) Free energy diagram of the different RuCxNy structures. Reprinted with permission from ref. [66]. Copyright (2019) Nature Publishing Group. (c) k3−weight FT−EXAFS fitting curves of the W−SAC at W L3−edge. Reprinted with permission from ref. [67]. Copyright (2018) Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Schematic of the formation for M1@Co/NC. Reprinted with permission from ref. [68]. Copyright (2019) Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Scheme of the formation for Co−SA/P−in situ. Free energy diagrams of (f) the Volmer–Heyrovsky step and (g) the Volmer–Tafel step of the HER. Reprinted with permission from ref. [71]. Copyright (2020) American Chemical Society. (h) Atomic−scale HAADF−STEM images of Co−I−N/G. (i) Schematic illustration of the electronic construction. Reprinted with permission from ref. [72]. Copyright (2021) American Chemical Society. (j) Free energy profile of Mo−SACs and Pt (111) (inset, the hydrogenated MoC2N−OH). Reprinted with permission from ref. [73]. Copyright (2021) American Chemical Society. * represents the active site.
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Figure 4. (a) FT−EXAFS data of Rh in the FR−NCS. (b) Possible sites to adsorb H. (c) ΔGH* of the different sites. Reprinted with permission from ref. [75]. Copyright (2020) Wiley−VCH GmbH. (d) ΔGH* of the possible sites toward the Ru/Fe−N−C system. (e) Comparison of the ΔGH* of the HER. (f) LSV curves. Reprinted with permission from ref. [77]. Copyright (2020) The Authors. Advanced Science published by Wiley−VCH GmbH. (g) Schematic diagram of Pt1−O2−Fe1−N4−C12 as the active moiety of Pt1@Fe−N−C. (h) Calculated ΔGH*. Reprinted with permission from ref. [78]. Copyright (2017) Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim. (i) Mo K−edge and W L3−edge EXAFS fitting curves for W1Mo1−NG at the R−space and k−space, respectively. Reprinted with permission from ref. [79]. Copyright (2020) The Authors, some rights reserved; exclusive licensee of the American Association for the Advancement of Science.
Figure 4. (a) FT−EXAFS data of Rh in the FR−NCS. (b) Possible sites to adsorb H. (c) ΔGH* of the different sites. Reprinted with permission from ref. [75]. Copyright (2020) Wiley−VCH GmbH. (d) ΔGH* of the possible sites toward the Ru/Fe−N−C system. (e) Comparison of the ΔGH* of the HER. (f) LSV curves. Reprinted with permission from ref. [77]. Copyright (2020) The Authors. Advanced Science published by Wiley−VCH GmbH. (g) Schematic diagram of Pt1−O2−Fe1−N4−C12 as the active moiety of Pt1@Fe−N−C. (h) Calculated ΔGH*. Reprinted with permission from ref. [78]. Copyright (2017) Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim. (i) Mo K−edge and W L3−edge EXAFS fitting curves for W1Mo1−NG at the R−space and k−space, respectively. Reprinted with permission from ref. [79]. Copyright (2020) The Authors, some rights reserved; exclusive licensee of the American Association for the Advancement of Science.
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Figure 5. (a) Experimental and simulated PXRD patterns of Ni3(Ni3∙HAHATN)2. (b) Hydrogen adsorption slabs. (c) Free energy profiles for the HER over the electrocatalytic sites. Reprinted with permission from ref. [80]. Copyright (2020) The Authors. Published by Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim. The crystal structure diagrams and the corresponding asymmetric unit for (d,e) CTGU−5 and (f,g) CTGU−6. Reprinted with permission from ref. [81]. Copyright (2017) Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim. (h) k3−weighted χ(k)−function of the EXAFS spectra for the Pt L3−edge. (i) The free energy diagram. (j) Models for the Pd/Cu−Pt NRs. (k) LSV curves. Reprinted with permission from ref. [84]. Copyright (2017) Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim.
Figure 5. (a) Experimental and simulated PXRD patterns of Ni3(Ni3∙HAHATN)2. (b) Hydrogen adsorption slabs. (c) Free energy profiles for the HER over the electrocatalytic sites. Reprinted with permission from ref. [80]. Copyright (2020) The Authors. Published by Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim. The crystal structure diagrams and the corresponding asymmetric unit for (d,e) CTGU−5 and (f,g) CTGU−6. Reprinted with permission from ref. [81]. Copyright (2017) Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim. (h) k3−weighted χ(k)−function of the EXAFS spectra for the Pt L3−edge. (i) The free energy diagram. (j) Models for the Pd/Cu−Pt NRs. (k) LSV curves. Reprinted with permission from ref. [84]. Copyright (2017) Wiley−VCH Verlag GmbH & Co. KGaA, Weinheim.
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Figure 6. The HER performance overviews of the symmetric/asymmetric coordination sites.
Figure 6. The HER performance overviews of the symmetric/asymmetric coordination sites.
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Table
Table 1. The comparison of the HER performance of the carbon−based SACs with the different active sites.
Table 1. The comparison of the HER performance of the carbon−based SACs with the different active sites.
CatalystActive SiteElectrolyteCurrent Density (mA cm−2),
Overpotential (mV)		StabilityRef.
E−Co SAsCoN21 M KOH10, ~59500 mA cm−2, 200 h[59]
CoN4CoN41 M KOH10, ~150−
Co1/PCNCoN41 M KOH10, 8910 mA cm−2, 24 h[85]
CoNx/CCoN41 M KOH10, 170−[86]
CoN40.5 M H2SO410, 133LSVs, 5000 cycles
Co−NG−MWCoN40.5 M H2SO410, 127LSVs, 1000 cycles[87]
Co−SA−without PCoN40.5 M H2SO410, 148−[71]
Co−SA/P−in situCoP1N30.5 M H2SO410, 98LSVs, 1000 cycles[71]
Pt@PCMPtN40.5 M H2SO410, 105~22.5 mA cm−2, 5 h[88]
Pt SACs/AGPtN40.5 M H2SO410, 12LSVs, 2000 cycles[89]
Pt1/NPCPtN40.5 M H2SO410, 25LSVs, 3000 cycles[90]
Pt1/MCPtC30.5 M H2SO410, ~27LSVs, 1000 cycles[63]
Pt@NC−BPtN2C20.5 M H2SO410, 3910 mA cm−2, 40 h[69]
Pt1/OLCPtO2C10.5 M H2SO410, ~3810 mA cm−2, 100 h[91]
Ru−NC−700RuC2N21 M KOH10, 12LSVs, 10,000 cycles[66]
Ru1/N−CRuN41 M KOH10, 173−[92]
RuN40.5 M H2SO410, 282−
Ru SAs@PNRuN40.5 M H2SO410, 24LSVs, 5000 cycles[93]
Ni−SA/NCNiN31 M KOH10, 102LSVs, 5000 cycles[58]
Ni−N−CNiN41 M KOH10, 307−[94]
Nisub/GNiC30.5 M H2SO410, ~50~9 mA cm−2, 120 h[61]
W−SAWN1C30.1 M KOH10, 85LSVs, 10,000 cycles[67]
Mo1N1C2MoN1C20.1 M KOH10, 132LSVs, 1000 cycles[70]
Mo@NMCNFsMoN1C2−O10.5 M H2SO410, 66LSVs, 3000 cycles[73]
FR−NCSRh−FeN40.5 M H2SO410, 22LSVs, 1000 cycles[75]
Pt1@Fe−N−CPt−O2−FeN40.5 M H2SO410, 605 mA cm−2, 5.5 h[78]
W1Mo1−NGW−O−Mo−O−C0.5 M H2SO410, 2410 mA cm−2, 100,000 s[79]
1 M KOH10, 6710 mA cm−2, 100,000 s
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Ding, J.; Liu, W.; Zhang, S.; Luo, J.; Liu, X. A Mini Review: Recent Advances in Asymmetrically Coordinated Atom Sites for High-Efficiency Hydrogen Evolution Reaction. Energies 2023, 16, 2664. https://doi.org/10.3390/en16062664

AMA Style

Ding J, Liu W, Zhang S, Luo J, Liu X. A Mini Review: Recent Advances in Asymmetrically Coordinated Atom Sites for High-Efficiency Hydrogen Evolution Reaction. Energies. 2023; 16(6):2664. https://doi.org/10.3390/en16062664

Chicago/Turabian Style

Ding, Junyang, Wenxian Liu, Shusheng Zhang, Jun Luo, and Xijun Liu. 2023. "A Mini Review: Recent Advances in Asymmetrically Coordinated Atom Sites for High-Efficiency Hydrogen Evolution Reaction" Energies 16, no. 6: 2664. https://doi.org/10.3390/en16062664

APA Style

Ding, J., Liu, W., Zhang, S., Luo, J., & Liu, X. (2023). A Mini Review: Recent Advances in Asymmetrically Coordinated Atom Sites for High-Efficiency Hydrogen Evolution Reaction. Energies, 16(6), 2664. https://doi.org/10.3390/en16062664

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