Single-Atom Catalysts for Electrochemical Water Splitting

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Perspective

Single-Atom Catalysts for Electrochemical Water Splitting Chengzhou Zhu, Qiurong Shi, Shuo Feng, Dan Du, and Yuehe Lin ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018

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ACS Energy Letters

Single-Atom Catalysts for Electrochemical Water Splitting Chengzhou Zhu*†,‡ Qiurong Shi,† Shuo Feng, † Dan Du, †,‡ and Yuehe Lin*,† †

School of Mechanical and Materials Engineering, Washington State University, Pullman, WA

99164, USA. ‡

College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C. Z.), [email protected] (Y.L.)

ABSTRACT: High-efficiency electrocatalysts with superior activity and stability are very crucial to their practical applications in water splitting, including the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Downsizing the conventional nanoparticle catalyst to single atoms and constructing single-atom catalysts (SACs) represent a rapidly emerging research focus. Because of the involvement of unique single-atom active moieties and the strong metal−support interactions arising from interfacial bonding, SACs as promising alternatives to noble metal-based nanoparticle catalysts exhibit profound power in the HER/OER. Here, we present a perspective on the exciting advances of SACs for HER/OER applications, with an emphasis on innovative synthetic strategies and an in-depth understanding of the

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structure-activity relationship through a combination of systematic characterization and theoretical studies. Finally, the challenges and some of the critical issues in this field are addressed.

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Increasing energy demands and environmental crisis have motivated the extensive investigation of alternative high-efficiency energy conversion and storage technologies. Electrochemical water splitting shows substantial promise for converting electricity from renewable sources into hydrogen, achieving the large-scale application of abundant yet intermittent renewable energy sources. Two half reactions, i.e., the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode, are involved in the overall electrochemical water splitting. Both of these reactions highly active electrocatalysts to expedite slow processes and lower the dynamic overpotentials. Currently, noble metal Pt and metal oxides (e.g., RuO2 and IrO2) are the widely used electrocatalysts for the HER and OER, respectively. However, the high cost and scarcity of these noble metals pose a formidable


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challenge to their widespread implementation. To this end, low or non-noble metal electrocatalysts have been extensively studied as advanced water splitting catalysts in recent years.1-4 Focusing on how to increase the number of active sites and how to enhance their intrinsic reactivity, several effective strategies, such as generating porous nanostructures5-6 as well as building bi/multimetallic systems7-8 and favorable hybrid nanomaterials,9-10 have been widely proposed, aiming to provide enhanced electrolyte permeability as well as fast mass transport and electron transfer, therefore accelerating the reaction kinetics of these gas-involved electrocatalysts. Aside from the extraordinary catalytic activity and stability, there have been long-lasting efforts to maximize metal atom use and minimize the cost of the heterogeneous catalysts. Downsizing a catalyst nanoparticle to a nanocluster or even a single atom has proven to be extremely

effective

in

achieving

these

implementation

goals

for

heterogeneous

electrocatalysts.11-13 Furthermore, the inhomogeneity of catalytic active sites and the complex interactions among them pose a serious challenge for fundamental studies. A typical example is that less stable metal oxides/sulfides/nitrides/phosphides under oxidizing potentials, which pose a formidable challenge to the verification of the true catalytically active species of the OER.14 An in-depth understanding of the nature of electrocatalysis and the structure-activity relationship has inspired the research community to accurately tune the catalytic active sites at the atomic level. Single-atom catalysts (SACs) only contain isolated metal atoms that are anchored to supports, which serve as active centers along with the neighboring atoms on the supports.15-16 The emergence of SACs, which bridge homogeneous and heterogeneous catalysts meets the urgent need due to the utmost use of the supported metals and the limit in material design (e.g., size, dimension, and composition). Specifically, electron microscopy (atomic-resolution aberration-


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corrected scanning transmission electron microscopy (STEM)) and X-ray absorption fine structure (XAFS) spectroscopy (X-ray absorption near-edge structure (XANES) and extended Xray absorption fine structure (EXAFS)) provide a wealth of information concerning the structure and properties of SACs including a verification of the single metal atom, the coordination environment and the chemical state of the metal center.17-18 Moreover, these advanced characterization techniques along with density functional theory (DFT) have brought unprecedented insight into the nature of electrocatalysis, enabling the precise design of targeted catalysts with tailored activity.19 Recent research efforts triggered an intensive investigation of SACs in the HER/OER and other gas-involved electrocatalytic applications.20-21 Due to their superior electrocatalytic activities and stabilities, SACs have shown substantial power in electrochemical water splitting. Several critical review/perspective articles on SACs and their applications in different fields have been published in recent years. However, the rational design and synthesis of novel SACs and the structure-activity of SACs in the HER/OER are rarely highlighted. In this Perspective, we discuss the exciting recent advances for SACs in electrochemical water splitting application. In addition to the innovative synthetic strategies, special emphasis will be placed on the strong metal–support interactions in SACs from the aspect of some key parameters including the effect of metal atom species, the active moiety and the support on HER/OER activity and stability. Specifically, by examining the common principles of the HER/OER, theoretical calculations coupled with advanced characterizations are highlighted to profoundly understand the catalytic nature and guide the development of new catalysts. Finally, we present both challenges and opportunities for future directions of SACs in water splitting, offering a promising opportunity to


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understand the structure−activity relationship at the atomic level and promote substantial breakthroughs in this field. Noble metal SACs for HER. The HER process involving H2 production consists of three possible reaction steps and may proceed through either the Volmer-Heyrovsky mechanism (H++ e−+ *→H*; H* + H+ + e−→H2 + *) or the Volmer-Tafel mechanism (H++ e−+ *→H*; 2 H*→ H2+2*), where one catalytic intermediate H* is always involved.22 Thus, the free energy of hydrogen adsorption (∆GH) is widely accepted as a descriptor of the HER process. With a nearly zero ∆GH, Pt sits very near the top of the hydrogen volcano plot and is considered the bestperforming catalyst for the HER. Downsizing catalyst nanoparticles to single atoms maximizes their efficiency by using nearly all Pt atoms. Engineering the Pt nanostructures and constructing Pt-based SACs via suitable synthetic strategies pave a new approach to high-performance HER catalysts that are comparable to commercial Pt catalysts. Using carbon as the supporting material to load a metal single atom for the synthesis of Ptbased SACs has aroused substantial attention due to the high electrical conductivity, stability in a harsh liquid-phase, structural diversity and tailored surface chemistry of these SACs.23-24 In particular, the doping of a heteroatom within carbon supports plays a critical role in stabilizing single metal atoms, modifying their electronic structures and metal–support interactions. For example, Sun et al. succeeded in loading isolated single Pt atoms on N-doped graphene (Pt/NGNs, 2.1 wt%) through atomic layer deposition (ALD).25 Pt sizes from a single atom to clusters and nanoparticles can be facilely controlled by the number of ALD cycles. The electrocatalytic performance showed that the obtained Pt/NGNs possessed an excellent HER activity and stability in 0.5 M H2SO4 in comparison to those of a Pt cluster on N-doped graphene


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and a commercial Pt catalyst. The electron transfer between the Pt single atom and N-doped grapheme, unambiguously confirmed via XANES and DFT analysis, was not only paramount for the stabilization of the Pt single atom but also benefited the distinct HER activity. Additionally, the Tafel mechanism, which is closely associated with high H coverage on single Pt atom catalysts, was clarified via experiments and theoretical calculations. Such a finding is of utmost importance to understanding the structure-activity relationship at the single atom level. Subsequently, Lou and coworkers proposed a new method for loading isolated Pt atoms in the Ndoped mesoporous carbon via loading the Pt precursor followed by pyrolysis (Figure 1A).26 The resultant SACs showed at 25-fold increase in mass activity for HER compared to a commercial Pt/C catalyst (Figure 1B). EXAFS and XANES analyses verified the evolution of Pt single atoms and the strong metal-support interactions. The introduction of a Pt single atom leads to an increased density of electronic states and a favorable charge density distribution (Figures 1C-F), which is believed to promote HER activity. A SCN- poisoning experiment uncovered the fact that single Pt sites and adjacent C and N atoms constitute HER active sites, which agrees well with theoretical calculations. The rate-determining step (RDS) for the obtained SACs follow the Heyrovsky step, which may be ascribed to the low Pt amount (0.53 wt. %), and the Volmer– Heyrovsky mechanism was determined. In another work, a simple electroplating deposition method is demonstrated to synthesize atomically disperse Pt on the sidewall of single-walled carbon nanotubes (SWNTs).27 As shown in Figure 1G, by accurately tuning the Pt anodic dissolution in sulfuric acid at a specific voltage range, an electroplating deposition of single Pt atoms or subnanoclusters in a low amount (0.190.75 at. %) can be facilely realized. As expected, the as-prepared Pt-based SACs exhibited superior HER activity, approaching that of a commercial Pt/C catalyst with a high loading


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ACS Energy Letters

amount (Figure 1H, I). Note that increasing the deposition cycles leads to the generation of subnanoclusters and an increase in HER activity, which can be attributed to the increased number of active sites despite the subnanoclusters involved. Interestingly, as with N-doped carbon, DFT calculations revealed that a sidewall of SWNT can immobilize Pt single atoms more efficiently than graphene, which accounts for the high physical and electrochemical stability of the Pt single atoms. Similarly, Liu et al. also adopted a universal electrochemical method and realized the dispersion of a Pt single atom on CoP-based nanotube arrays, which holds profound potential for fabricating large-area and unsupported Pt-based SAC electrodes.28 These unsupported SACs exhibited an extraordinary HER performance in neutral media, delivering 10 mA cm−2 at η=24 mV, which is close to that of commercial Pt/C (17 mV), and a better HER stability than Pt/C. Volmer–Tafel mechanism with a low Tafel slope of 30 mV dec-1 was involved. These works clearly imply that the use of a Pt counter electrode can enhance the HER activity of the catalysts and that the effect of the low Pt deposition on the HER measurement should receive more attention in future work. To enhance the intrinsic activity of the MoS2 nanosheets and enable the inert, in-plane atoms of 2D MoS2 to be active, Bao and coworkers succeeded in rationally tuning the MoS2 surface and thus altering the catalytic activity via single-atom Pt doping.13 By taking advantage of the modulated electronic properties, the obtained Pt–MoS2 exhibited significantly increased HER activity in comparison to pure 2D MoS2. DFT calculations indicated that S atoms neighboring the single Pt atom are the prior adsorption sites and that the ∆GH is approximately 0 eV, which is lower than that of the edge S of pure MoS2 (0.1 eV). The distinct HER enhancement was closely associated with the unique electronic states of Pt–MoS2, where Pt single atoms can effectively regulate the adsorption behavior of H atoms on the neighboring S sites and consequently


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improve their electrocatalytic activity. Accordingly, a volcano plot along the ∆GH was obtained using different single transition metal atom doping, broadening the understanding of the role of a single metal atom in electrocatalysis. In addition to the abovementioned non-precious supports, Li et al. used Pd nanorings as the support to immobilize atomically dispersed Cu–Pt dual sites (Pd/Cu-Pt NRs) containing 1.5 at.% Pt and investigated their HER performance.29 The resultant Pd/Cu-Pt NRs exhibited excellent HER activity in comparison to those of the Pd support, Pd/Cu with a Cu single site and commercial 20 wt.% Pt/C. Notably, in addition to the Tafel slope of 25 mV dec-1 that is lower than that of commercial Pt/C (28 mV dec-1), the obtained Pd/Cu-Pt NRs also possessed superior stability compared to Pt/C, making it one of the best HER catalysts reported. The theoretical study also indicated that the neighboring Cu atoms play a pivotal role in balancing the interaction between the single Pt atom-based active site and the H atom, thus enhancing HER activity. Non-noble metal SACs for HER. Earth-abundant catalysts, especially transition metalnitrogen-carbon (M−N/C, M=Fe, Co, Ni, Mn, etc.) have been widely studied as appealing nonprecious metal catalysts (NPMCs) for different applications.30-32 Recently, M−N/C nanomaterials as promising Pt alternatives also showed good HER activity.33-34 Notably, in addition to the active MNx species, metal oxide/carbide/nitride and carbon-encapsulated metal nanoparticles show electrocatalytic activity. However, the synergistic effects among these active species make it difficult to uncover the real active site and electrocatalytic mechanism. The M-N-C nanostructure featured in single-atom catalysis has received a surge of unprecedented interest as one of the most promising alternatives for circumventing the abovementioned roadblocks. Towards this end, Müllen et al. reported the synthesis of novel Co-N-C catalysts through pyrolysis and etching and identified the well-dispersed CoNx moiety on the carbon support as the


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ACS Energy Letters

robust HER active site.35 Significantly, the obtained catalysts exhibited unprecedented TOFs of 0.39 and 6.5 s-1 at an overpotential of 100 and 200 mV, respectively. Subsequently, Tour et al. synthesized atomically dispersed cobalt on N-doped graphene via pyrolysis of graphene oxide and cobalt salts in NH3 atmosphere and confirmed the presence of the Co single metal atoms via the HAADF and EXAFS techniques (Figures 2A,B).36 CoNx was believed to serve as the excellent active site, raising the level of understanding of single-atom catalysis in HER. Recently, a single Mo atom on N-doped carbon was also synthesized using the versatile template and pyrolysis methods.37 The direct observation of a single Mo atom was made using AC-STEM, and the atomic structure model of Mo1N1C2 was verified according to an EXAFS fitting (Figure 2C), shedding new light on electrocatalytic nature of the HER. Benefitting from their structural merits, the as-prepared Mo1N1C2 manifested excellent electrocatalytic HER activity superior to that of Mo2C and MoN and greater stability than that of commercial Pt/C (Figure 2D). DFT calculations were consistent with the experimental result and indicated that a low ∆GH value, as well as a large density of states, favored the outstanding electrocatalytic activity. As a new method for generation of SACs, an electrochemical potential was applied to Ni–C-based catalysts derived from carbonization of metal-organic frameworks to trigger the evolution of Ni-based SACs and enhance the HER performance.38 In another work, a single Ni atom anchored on nanoporous graphene without nitrogen doping was synthesized through chemical vapor deposition followed by acid etching (Figure 2E).39 The HER can be significantly enhanced for the resultant Ni-C SACs in comparison to those with a short Ni dissolution time, affording a low overpotential and a high stability with a low Tafel slope of 45 mV dec-1 in 0.5 M H2SO4 solution (Figure 2F). Experimental and theoretical studies suggest that charge transfer in the stable form of Ni-C bonding, in which substitutional dopants


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occupy C sites in the graphene lattice led to empty C–Ni hybrid orbitals and a small ∆GH, which benefits the excellent HER activities. Table 1 lists selected state-of-the-art SACs for the HER in 0.5 M H2SO4. Noble metal SACs for OER. The kinetically sluggish OER process has restricted the use of realistic water splitting. It is well accepted that four steps are involved in the OER mechanism:40 * + H2O → *OH + H+ + e-

(1)

*OH →*O + H+ + e-

(2)

*O + H2O →*OOH + H+ + e- (3) *OOH → O2 + H+ + e-

(4)

where * represents the active sites of the electrocatalyst. For OER catalysis in acid, IrO2 is a reasonably active metal oxide catalyst. Although some potential SACs have been predicted using DFT calculations,41-42 there are relatively few studies devoted to the construction of Ir/Ru-based SACs for OER applications. Rational design of these materials and investigation of their feasibility as OER catalysts is desirable for an intensive understanding of the more complex OER reaction mechanism. Song et al. reported the well-dispersed single-atom Ir and Ir clusters on Co(OH)2 nanosheets via a facile one-step NaBH4 reduction.43 The resultant Ir species incorporated into the defect-rich hydroxide nanosheets exhibited good OER activity with rapid catalytic kinetics (Figures 3A,B). The optimized sample showed an overpotential of 373 mV at 10 mA cm−2 in 1.0 M PBS, which is slower than that of a commercial IrO2 catalyst. Importantly, the detailed characterization including XRD and XAFS analysis further verified the evolution to


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high valence unique β-CoOOH and Ir species with a low-coordination structure under oxidation potential (Figure 3C), accounting for the superior OER performance. Very recently, Yu and coworkers conducted the modulation toward atomically dispersed Fe-N4 moieties and created “single-atom to single-atom” grafting of a Pt single atom onto the Fe center via an incipient wetness impregnation method.44 The new active Pt1-O2-Fe-N4 moiety was proposed based on the XAS analysis (Figure 3D). The newly designed SACs not only exhibited excellent ORR and HER performance in acidic media but also showed an outstanding OER activity in alkaline medium in comparison to that of RuO2 (Figure 3E). The Tafel plot of Pt1@Fe-N-C (62 mV dec−1) was close to that of RuO2 catalyst (60 mV dec−1) (Figure 3F). It can be determined that these novel active sites featuring unique geometric and electronic structures play a key role in enhancing OER performance based on the fact that neither F-N4 or Pt1@C can catalyze OER efficiently. This finding provides a good reason for studying the interaction between different single atoms and exploring unexpected electrochemical properties. Substantial efforts are required to deeply understand the structure-activity relationship and allow for more active OER catalysts. Non-noble metal SACs for OER. Compared with their noble counterparts, theoretical and experimental studies on non-noble based SACs for OER have sparked much research interest due to their distinctive OER activity (Table 2).45 Importantly, much effort has been devoted to synthesizing energetic M-N-C catalysts with SAC features and investigating their OER performance. In addition to the verification of the single metal atom and related active moieties, another important aspect of SAC research lies in the effect of the metal species on the evolution of the active moieties and the detailed reaction mechanism. However, the difference in the physicochemical properties of metal-dependent supports and the corresponding active sites


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makes the elucidation of the roles of different metals more difficult. To address this issue, considering that the profuse pyridine-N in C3N4 provides rich electron lone pairs to accommodate metal ions, Qiao et al. constructed a molecule-level Co−C3N4/CNT catalyst featuring a CoN3C2 ring through the direct grafting of a transition metal onto the surface of g‑C3N4, aiming to design a highly active ORR/OER catalysts.46 Aside from the good electrocatalytic activity of Co−C3N4 for the OER, the effect of the metal species on the OER activity was investigated and a volcano plot for the OER was obtained. On the basis of an extensive investigation of the interactions between C3N4 and metal ions, Song and coworkers synthesized atomically a dual-metal doped NiFe-C3N4/CNT catalyst and an OER performance superior to those of the single-metal counterparts was observed due to the synergistic effect between C3N4 and the metal ions.47 Recently, Duan et al. reported a general approach to various single transition metal atoms (Fe, Co and Ni) on N-doped holey graphene frameworks (M-NHGFs).48 On the one hand, the same MN4C4 moiety regardless of metal species was unambiguously confirmed by systematic XAS and STEM analyses. On the other hand, similar nitrogen (~5 at.%) and metal (~0.05 at.%) contents along with the same morphological characteristic make the comparison more trusted. As shown in Figure 4A, the catalytic activity was evaluated via DFT simulations and they found that a single-site mechanism applied to Fe(Co)-NHGFs in which all intermediates bind more strongly at the M site than the C site. In contrast, a dual-site mechanism was involved in the Ni-NHGFs catalyzed OER, in which O* and OH* prefer to reside at the C site, whereas the OOH* is favorably adsorbed on the M atom (Figure 4B). The calculated energy diagrams clearly show that the RDS of the Ni–NHGF catalyzed OER with a dual-site mechanism had the smallest barrier of 0.42 eV among these catalysts with an activity trend of Ni > Co > Fe (Figure 4C). The


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subsequent electrochemical experiment showed that Ni–NHGF possessed the best OER activity, delivering j=10 mA cm-2 at 331 mV with a low Tafel slope (Figure 4D). Note that the TOF of Ni–NHGF was calculated to be 0.72 s–1. This value was higher than those of OER catalysts reported previously and Ni–NHGF is considered the most active OER catalysts reported to date. The activity trend was consistent with that of the DFT calculations. In addition to the investigations of the metal species, some studies focus on the supports and aim to gain insights into the unique catalytic mechanism at the atomic level. Creating a high density of defects on graphene is claimed to accommodate more single metal atoms.48-49 Yao et al. used the defective graphene as a support to anchor single-atom Ni and the loading amount reached 1.24 wt.%, which is much higher than that of pristine graphene.49 The obtained SACs can serve as bifunctional electrocatalysts for HER and OER. It is also found that a single-atom Ni@defect with different configurations serves as active sites and the dominant active sites were verified for the HER and OER. Wang and coworkers succeeded in grafting molecular Co2+ on different heteroatom (S, N and O)-doped graphene (Figure 4E).50 Systematic characterizations using XAS and electron paramagnetic resonance indicated that heteroatom-containing functional 2+

groups in SNG combined with one acac− group can efficiently immobilize Co . The effect of heteroatom doping was investigated and it was observed that the activity was highest for SNGCo2+ and followed the trend SNG-Co2+ > SG-Co2+ > NG-Co2+> OG-Co2+ (Figure 4F). The active sites involving the coupling of C-S=O with Co2+ contributed more to the OER performance based on the spectra and electrochemical characterizations. Specifically, sequential oxidation of Co2+ with a single Co site mechanism was proposed for the OER process (Figure 4G), which brought new insight into the SAC mechanism study. As such, an atomically dispersed FeNx species anchored on the S, N-codoped carbon was reported and it was found that the OER


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performance was distinctively enhanced compared with that on N-doped carbon, paving a new route to tuning and optimizing supports and thus improving OER activity.51 In contrast to the carbon-supported abovementioned SACs, Wang and coworkers proposed a new route to creating coordinatively unsaturated metal sites created in conventional ZIF-67 through an efficient dielectric barrier discharge plasma treatment.52 For most MOF structures, the fully coordinated configuration and the inferior conductivity are a major concern that limits their electrocatalytic activity. After treatment, a decreased charge-transfer resistance and an enhanced OER activity were observed in comparison to that of pristine ZIF-67. DFT calculation revealed that the main active sites consist of unsaturated ZIF-67 with three ligands on Co sites. Challenges and Outlook. Due to remarkable characteristics such as maximum atom use efficiency as well as high activity and selectivity, the development of SACs has fueled the possibility of probing the nature of catalysis and of designing more efficient and low-cost electrocatalysts. SACs have shown profound potential in electrochemical water splitting. However, the HER/OER activities of most of the SACs reported previously remain lower than the benchmark commercial noble metal catalysts and the conventional nanoparticle catalysts. Given that investigations of SACs are still in their infancy, there are some opportunities and challenges that should be considered for future applications. (1) Increasing the number of active sites. Because single metal atoms are not stable because of the high surface energy, increasing the single metal atom loading content and thus increasing the number of the active sites are of profound importance for their practical applications. Intensive studies suggest that the enhancement of metal−support interactions might provide a viable route to stabilizing more single metal atoms, efficiently preventing the agglomeration of


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metal atoms and guaranteeing the accessibility of the numerous active sites. Much work to date has focused on the functionalization of the support such as through heteroatom doping and the introduction of defects to accommodate more metal atoms. Another effective strategy is to develop three-dimensional porous supports and maximize the number of available active sites and accelerate electron transfer and mass transportation. (2) Increasing the reactivity of the active site. General strategies in the construction of advanced SACs with increased reactivity of the active site are listed as follows. First, more attention should be paid to the role of the metal atom because the experimental and theoretical results verified that they not only affect the electrocatalytic activity of the SACs but also determine the reaction pathway. For example, a systematic framework combining experiment with the common principles of HER/OER catalysis has been established based on different transition metal atoms, realizing the optimization of metal atom species and the high function of SACs. Second, emphasis should be placed on the exploration of more functional supports to achieve synergy with a single metal atom. Aside from alloying, doping, the introduction of defects, the use of non-carbon supports, e.g., metal phosphides, having intrinsic catalytic activity towards HER/OER for anchoring a specific single metal atom, are favorable for increasing the reactivity of SACs. Third, some new systems including atomically dual-metal doping and dualheteroatom-doped supports are suggested to be effective for improving the intrinsic activity of SACs for water splitting. Last but not least, considering the advantages of the SACs, the bifunctional catalysts for the HER and OER should be addressed to further promote the commercialization of proton exchange membrane electrolyzers. Because the observed differences in catalytic activity are closely related to the active moieties, precise verification of the structure of the active moieties and a deep understanding of the


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contribution of the single metal atom and neighboring atoms to the binding of hydrogen/oxygen intermediates is required. The charge redistribution resulting from the single metal atoms and other adjacent atoms leads to the different binding sites and the role of the single metal atoms varies with different active sites. In contrast to conventional nanoparticle catalysts, SACs may suggest different reaction pathways due to the lack of metal-metal bonds. In other words, only a single metal site-involved reaction mechanism rather than a dual metal site mode was observed. However, little attention has been paid to the variation of the coordination environment, the binding mode and the oxidation states of the single atoms during the electrochemical process. In situ characterizations are largely needed to further probe the nature of the electrocatalysis in SAC systems. Furthermore, the pH-dependent activity due to the altered kinetic barriers will require that we quantitatively calculate electrochemical barriers for proton transfer reactions involved in the HER/OER. Searching for the advanced SACs and exploring their catalytic nature leads to a substantial challenge in innovative synthesis and systematic characterization as well as the underlying mechanism study. It is anticipated that gradually increasing the number of active sites and/or enhancing the reactivity of the active sites will be achieved in SACs through novel synthetic methods, resulting in the largest improvements in overall electrochemical performance. Specifically, cost-reduction and high efficiency synthetic strategies for maximizing the use of noble metal precursors are urgently needed. The coexistence of metal subnanoclusters was also observed in pseudo-SACs, and the role of the metal single atom in the catalytic mechanism should be investigated carefully, despite the dominant dispersion of single atoms. Meanwhile, coupling advanced characterization techniques with theoretical calculations, the study of the information-rich SACs in the HER/OER is beneficial for uncovering the governing principles


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that help to explain the electrochemical transformations at the single-atom level. The perspective presented here not only fuel the possibility of improving the reactivity of the OER/HER for water splitting but also promotes the development of other important gas-involving electrocatalytic reactions, further broadening the horizons of durable energy storage and conversion applications.

Figure 1. (A) Schematic illustration of the synthesis of Pt@PCM. (B) Linear sweep voltammetry curves of Pt@PCM and control samples in 0.5 M H2SO4. DFT calculated DOS (C), distribution of charge density (D) of Pt@PCM. C and N atoms of Pt@PCM with different coordination shell (E) and the corresponding ∆GH (F) (reprint from ref 26; Copyright 2018,


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American Association for the Advancement of Science). HAADF-STEM image (G) and related HER polarization curves (H) and corresponding Tafel plots (I) of Pt/ SWNTs (reprinted from ref 27).

Figure 2. HAADF-STEM images of Co-NG (A) (reprint from ref 36; Copyright 2015, Nature Publishing Group), Mo1N1C2 (C) (reprint from ref 37; Copyright 2017, John Wiley & Sons) and C–Ni (E) (reprint from ref 39; Copyright 2015, John Wiley & Sons). HER polarization curves of Co-NG (B), Mo1N1C2 (D) and C–Ni (F) in 0.5 M H2SO4, 0.1 M KOH and 0.5 M H2SO4, respectively.


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Figure 3. (A) OER polarization curves and (B) the corresponding Tafel plots of CoIr-x in 1.0 M PBS. (C) Atomic Ir-incorporated in Co(OH)2 and phase transformation after OER test (reprint from ref 43; Copyright 2018, John Wiley & Sons). (D) The proposed active moiety configurations of Pt1@Fe-N-C. HER polarization curves (E) and corresponding Tafel plots (F) of Pt1@Fe-N-C and control samples in 0.1 M KOH (reprint from ref 44; Copyright 2018, John Wiley & Sons).


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Figure 4. (A) The proposed reaction pathway for OER process via single-site (A) and dual-site mechanisms (B). (C) Free energy diagram for OER over Fe–NHGF, Co–NHGF and Ni–NHGF with different reaction mechanisms. (D) OER polarization curves of different catalysts in 1 M KOH (reprint from ref 48; Copyright 2018, Nature Publishing Group). (E) The scheme of the binding of Co2+ ions onto S-, N-, O-doped graphene. (F) OER polarization curves of SNG-Co2+, SG-Co2+, NG-Co2+, OG-Co2+ in 1 M KOH. (G) Sequential oxidation of Co2+ with single Co site mechanism (reprinted from ref 50). Table 1. Comparison of selected state-of-the-art SACs for HER 0.5 M H2SO4. Catalyst

onset overpotential (mV)

Overpotential@j=10 mA cm-2 (mV)

Tafel slope (mV/dec)

Reference

105

65.3

26

0

27

38

27

~0

-

29

25

Pt@PCM

0

(0.53 wt%) 400-SWNT/Pt (0.75 at%) ALD50Pt/NGNs (2.1 wt%)


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ACS Energy Letters

CoNC–Cl (1.45 at%)

-

130

41

53

Co-NG (0.57 at%)

30

147

82

36

_

34

41

50

20

133

57

35

50

~180

45

38

Mo1N1C2 (1.32 wt%)

48

154

86

48

A-Ni@DG (1.24wt%)

_

70

31

49

A-Ni-C (1.5 wt%) CoNx/C (0.14 at%) Ni-doped graphene


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Table 2. Comparison of selected state-of-the-art SACs for OER in 1 M KOH.

Catalyst

onset overpotential (mV)

Ni–NHGF ((~0.05 at%)

200

SNG-Co2+

-

Ni-CN-200 (2.4 at%)

Overpotential@j=10 mA cm-2 (mV)

Tafel slope (mV/dec)

Reference

63

48

370

62

50

310

-

60

54

270

380

68.4

46

~270

326

67

47

CUMSs-ZIF-67

-

320

53.7

52

A-Ni@DG (1.24 wt%)

-

~270

47

49

Co−C3N4/CNT (0.2 at%) NiFe@g-C3N4/CNT

331

Ni (0.84 at%), Fe (0.92 at%)

AUTHOR INFORMATION Notes The authors declare no competing financial interest. Biographies Dr. Chengzhou Zhu is currently a full professor at Central China Normal University. He received his Ph.D. degree at the Changchun Institute of Applied Chemistry. Since then, he worked as a Humboldt Research Fellow at Dresden University of Technology, and then joined Washington State University (2013–2014) as an assistant research professor (2014–2018). His scientific interests focus on nanomaterial-based electrochemical energy and analytical applications.


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Qiurong Shi received her B.S. and M.S. degrees from Shandong University in 2011 and 2014. Currently, she is pursuing her Ph.D. degree in the School of Mechanical and Materials Engineering at Washington State University under the supervision of Prof. Yuehe Lin. Her scientific interests focus on engineering nanostructured materials for fuel cells and water splitting. Mr. Shuo Feng is a Ph.D. student in Material Science and Engineering Department, Washington State University. He received his Master degree in 2016 from WSU and joined in Dr. Yuehe Lin’s group in 2017. His research topics mainly focus on Nanomaterials synthesis for electrochemical energy devices, including fuel cells, lithium-sulfur and lithium-ion batteries. Dr. Dan Du received her Ph.D. in Analytical Chemistry from Nanjing University in 2005. She joined Central China Normal University in 2005 and was promoted to Full Professor in 2011. Currently she is a Research Professor at Washington State University. Her research interests include functional nanomaterials for fuel cells and electrochemical biosensing. Dr. Yuehe Lin is a professor at Washington State University and a Laboratory Fellow at Pacific Northwest National Laboratory. He has been actively working in the nanotechnology area, particularly in the synthesis of functional nanomaterials for energy and environmental applications. His other research activities include development of new biosensors and bioelectronic devices and nanomaterials for biomedical diagnosis and drug delivery. Dr. Lin has published 430 papers, with 42,300 total citations and an h-index of 106, according to Google Scholar. He was listed as a highly cited researcher in chemistry on Thompson Reuters and Clarivate Analytics's lists in 2014, 2015, 2016, and 2017.


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ACKNOWLEDGMENT This work was supported by a start-up fund of Washington State University, USA.

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Quotes 1. Multiscale tuning of the sizes, shapes, compositions and structures of the catalysts provides a vast opportunity to rationally design advanced HER/OER catalysts that are comparable or superior to benchmark catalysts, while SAC has reached its limits at the atomic level. 2. Fully understanding of the strong metal−support interactions arising from interfacial bonding benefits the rapid development of the SAC in the HER and OER. The alteration of the electron structure as a result of the strong metal−support interaction plays a decisive role in their catalytic activity and stability. 3. With the rational design of SACs, the enhanced HER and OER performances along with the deep understanding of the underlying mechanism are greatly expected.


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Single-Atom Catalysts for Electrochemical Water Splitting

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