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Review

Bimetallic Single-Atom Catalysts for Electrocatalytic and Photocatalytic Hydrogen Production

by
Mengyang Zhang
1,
Keyu Xu
1,
Ning Sun
1,
Yanling Zhuang
1,
Longlu Wang
1,* and
Dafeng Yan
2,*
1
College of Electronic and Optical Engineering and College of Flexible Electronics (Future Technology), Nanjing University of Posts and Telecommunications (NJUPT), 9 Wenyuan Road, Nanjing 210023, China
2
College of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, China
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(11), 1409; https://doi.org/10.3390/catal13111409
Submission received: 8 October 2023 / Revised: 27 October 2023 / Accepted: 27 October 2023 / Published: 30 October 2023
(This article belongs to the Section Nanostructured Catalysts)
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Abstract

:
Electrocatalytic and photocatalytic hydrogen evolution reactions (HERs) provide a promising approach to clean energy generation. Bimetallic single-atom catalysts have been developed and explored to be advanced catalysts for HER. It is urgent to review and summarize the recent advances in developing bimetallic single-atom HER catalysts. Firstly, the fundamentals of bimetallic single-atom catalysts are presented, highlighting their unique configuration of two isolated metal atoms on their supports and resultant synergistic effects. Secondly, recent advances in bimetallic single-atom catalysts for electrocatalytic HER under acidic/alkaline conditions are then reviewed, including W-Mo, Ru-Bi, Ni-Fe, Co-Ag, and other dual-atom systems on graphene and transition metal dichalcogenides (TMDs) with enhanced HER activity versus monometallic analogs due to geometric and electronic synergies. Then, photocatalytic bimetallic single-atom catalysts on semiconducting carbon nitrides for solar H2 production are also discussed. Finally, an outlook is provided on opportunities and challenges in precisely controlling bimetallic single-atom catalyst synthesis and gaining in-depth mechanistic insights into bimetallic interactions. Further mechanistic and synthetic studies on bimetallic single-atom catalysts will be imperative for developing optimal systems for efficient and sustainable hydrogen production.

1. Introduction

Hydrogen is considered one of the most promising clean energy carriers for the future [1,2,3,4]. Electrocatalytic and photocatalytic water splitting provide sustainable approaches to hydrogen production using electricity from renewable resources [5]. The kinetics of the hydrogen evolution reaction (HER) depends on the active sites as well as synergistic interactions with the catalyst support [6,7]. For example, graphene, known for its inert properties, and transition metal dichalcogenides (TMDs), which typically exhibit catalytic promotion, have emerged as versatile supports for modulating the catalytic activity of anchored metal sites toward enhanced HER [8,9,10,11].
Single-atom catalysts, while innovative, face limitations including scarce active sites and susceptibility to aggregation or leaching [12,13]. Bimetallic single-atom catalysts address these drawbacks through proximity interactions between two distinct metal atoms that induce cooperative electronic effects and geometric synergies [14,15,16]. This leads to enhanced HER kinetics surpassing single-atom catalysts. The tailored environment in bimetallic single-atom catalysts allows for optimizing adsorption energies, reaction rates, conductivity, and stability [17,18,19,20,21]. The HER involves volcano plot-limited Volmer–Heyrovsky/Volmer–Tafel steps, where hydrogen binding and desorption influence kinetics [19]. Modulating the geometric and electronic structure in bimetallic single-atom catalysts helps optimize these rate-determining steps [20,21]. Furthermore, compared to traditional catalysts, bimetallic single-atom catalysts maximize the utilization of expensive metals and provide a platform for systematically modulating catalytic properties through proximity interactions [22,23]. This enables performance tuning beyond what is possible with single-metal SACs. Photocatalytic bimetallic single-atom catalysts also create dual sites for redox reactions, enhancing charge separation and surface proton reduction. For electrocatalysis, bimetallic single-atom catalysts lower kinetic barriers by optimizing hydrogen binding energies and catalytic site interactions [21,23,24]. This review provides a timely summary of recent advances in bimetallic single-atom catalysts for electrocatalytic and photocatalytic HER. We focus on synthetic strategies, characterization techniques, and mechanistic insights into key synergies that underpin performance gains versus single-atom catalysts. Elucidating fundamental principles for the rational design of bimetallic single-atom catalysts will accelerate the development of optimal systems for clean hydrogen production.
In summary, bimetallic single-atom catalysts supported on 2D materials are at the forefront of developing active, robust, and inexpensive systems for clean hydrogen generation [20,25]. Further mechanistic insights into bimetallic synergies [22] and synthetic strategies to incorporate different metal pairs will be imperative to guide the rational design of optimized catalysts for hydrogen production [26,26,27,28,29]. The choice of appropriate support materials and engineering optimal catalyst–support interfaces continue to be key focuses in this emerging field.

2. The Fundamentals of Bimetallic Single-Atom Catalysts for Electrocatalytic and Photocatalytic Hydrogen Production

Bimetallic single-atom catalysts have recently emerged as a promising class of electrocatalysts and photocatalysts for clean and sustainable hydrogen production as shown in Figure 1. Bimetallic single-atom catalysts are composed of two different metal species anchored in close proximity as isolated atoms on a support material. The choice of support plays a critical role in determining bimetallic single-atom catalysts’ performance. For electrocatalytic HER, graphene and TMDs have shown promise as versatile scaffolds. Defect engineering of these materials creates anchors to firmly immobilize the dual metal atoms [22,30,31]. For photocatalytic HER, semiconducting carbon nitrides like g-C3N4 provide excellent support to coordinate isolated metal atoms through their heteroatom-rich structure. This unique configuration allows bimetallic single-atom catalysts to harness synergistic effects between the two metals to optimize catalytic performance beyond their monometallic counterparts. The isolated dual metal atoms in bimetallic single-atom catalysts exhibit several advantageous features including high utilization efficiency, tunable adsorption energies, modulated electronic structure, enhanced stability, and cooperative catalysis [32,33,34]. With all metal atoms exposed as solitary active sites, bimetallic single-atom catalysts maximize the efficiency of the expensive noble metals [35,36,37]. The distinct adsorption capabilities of the two metals allow for optimizing hydrogen binding energies and intermediates activation [38,39,40]. The proximity interactions between the two metal atoms induce charge transfer and band structure tuning, facilitating electron transport and light absorption. Anchoring by one metal strengthens the binding of the second metal, improving thermal and chemical stability [41,42,43]. The two metals can selectively adsorb reactive intermediates and synergistically promote the multi-step catalytic process. These unique properties make bimetallic single-atom catalysts promising next-generation catalysts for electrocatalytic and photocatalytic HER. For photocatalytic HER, bimetallic single-atom catalysts create dual sites for redox reactions, promoting charge carrier separation and surface proton reduction. For electrocatalytic HER bimetallic single-atom catalysts lower the kinetic overpotentials and improve reaction rates for the critical Volmer–Heyrovsky/Volmer–Tafel steps. Overall, the tailored synergies in bimetallic single-atom catalysts stem from the proximity interactions between the two isolated metal species, which modulate the geometric and electronic environment. Further understanding of bimetallic cooperation mechanisms will be key to guiding the rational design of optimized bimetallic single-atom catalysts for efficient and durable hydrogen production across a wide pH range [22,44,45].
Meanwhile, the loading of metal atoms is typically very low in single-atom catalysts, which limits their catalytic efficiency. For example, the loading of Pt is usually below 0.5 wt% in Pt single-atom catalysts supported on carbon nitrides. Compared to the corresponding single-atom catalysts, bimetallic single-atom catalysts can achieve higher catalytic activity at low loadings through synergistic effects. A recently reported Co1Ag1-PCN dual-metal single-atom catalyst achieved a 3.5 times higher H2 production rate than the corresponding Co single-atom catalyst at a low Co loading of 0.1 wt%. This demonstrates that bimetallic single-atom systems can optimize the electronic structure and reactive sites to realize synergistic promotion at low loadings by modulating metal-metal interactions. Therefore, designing bimetallic single-atom catalysts provides an effective strategy to overcome the loading limitations of single-atom catalysts and improve catalytic efficiency.

3. Bimetallic Single-Atom Electrocatalytic Hydrogen Production

Bimetallic single-atom electrocatalysts have shown great promise for enhancing the HER across a wide pH range. The supports for anchoring dual metal atoms play a critical role in determining the catalytic performance. Graphene and TMDs have emerged as two predominant substrates owing to their unique properties [48,49].
Graphene offers an atomically smooth surface, excellent electrical conductivity, and high carrier mobility. However, defect engineering is required to create anchors for immobilizing isolated metal atoms. Oxygen functionalization and ion bombardment have enabled graphene to firmly stabilize bimetallic atoms and achieve synergy. For instance, W and Mo heteroatoms anchored on O-sites of nitrogen-doped graphene delivered optimized HER kinetics across acidic and alkaline conditions [4,50,51].
Meanwhile, TMDs like MoS2 provide abundant coordination sites and metallic character. Still, additional defects are needed to strengthen the interaction with single atoms. Sulfur vacancies and heteroatom doping allow TMDs to achieve dual-site anchoring. A combination of Ru and Ni atoms on defective MoS2 exhibited markedly enhanced alkaline HER performance compared to single metals. The Ni substitution modulated MoS2′s electronic structure to facilitate Ru incorporation and hydrogen adsorption [52,53].
In summary, graphene and TMDs have emerged as versatile scaffolds for bimetallic single-atom electrocatalysts. Defect engineering unlocks the potential of these materials to act as supports. Ongoing research to better understand metal–substrate interactions and synergies will pave the way for rational design of optimized dual-atom electrocatalysts.
Table 1 summarizes recent representative bimetallic single-atom electrocatalysts supported on graphene across different acidic and alkaline electrolytes. The W1Mo1 catalyst achieved ultralow overpotentials of 24 mV and 67 mV in 0.5 M H2SO4 and 1.0 M KOH, respectively, displaying excellent universal activity across the entire pH range. This was enabled by the modulated hydrogen binding energy from the synergistic W-Mo dual sites. Meanwhile, Rubi single-atom alloys (SAA) on oxidized graphene delivered a high mass activity of 65,000 mA/mgRu at 150 mV overpotential in 1.0 M KOH, which is over 70 times higher than Pt/C. This exceptional performance arose from the tuned adsorption energies and electronic structure from the Ru-Bi dual sites. Additionally, the Ru/Ni bimetallic single-atoms on MoS2 exhibited an ultralow overpotential of 32 mV and a Tafel slope of 41 mV/dec in alkaline media, outperforming monometallic Ru and Ni analogs. The Ni atoms facilitated Ru incorporation while creating S sites for hydrogen adsorption. Overall, the results highlight the capability of graphene-supported bimetallic single-atom electrocatalysts to achieve enhanced HER activity, universality, and stability compared to monometallic systems.

3.1. Bimetallic Single-Atom Catalysts Supported on Graphene

Graphene, with its hexagonal lattice structure and sp2 hybridized orbitals, presents an atomically smooth surface lacking anchors to immobilize metal atoms. To enable graphene as a support for single-atom catalysts, the surface requires modification to create anchoring sites. Oxidative treatment is a common approach, where oxygen-containing functional groups like hydroxyl and carboxyl form on graphene through partial oxidation. These polar groups can coordinate metal atoms. Alternatively, ion bombardment can be used to induce defects in the graphene lattice, generating vacancies that trap metal atoms [20,54,55].
Graphene provides various coordination sites to anchor isolated metal atoms. Oxygen-containing functional groups like hydroxyl and carboxyl formed through partial oxidation can coordinate with metal atoms. Ion bombardment creates defects and vacancies in the graphene lattice that can trap metal atoms. Modifying graphene with oxidative or ion beam treatments enables the fabrication of anchors to firmly immobilize bimetallic single atoms. This allows for leveraging the conductivity and stability of pristine graphene while harnessing dual-site cooperative interactions. Further research into optimizing anchor site generation and exploring diverse bimetallic pairs on graphene holds great promise for advancing bimetallic single-atom electrocatalysis.
This paper reports the development of O-coordinated W-Mo dual-atom catalysts (DACs), W1Mo1-NG, for electrocatalytic HER over a wide pH range. The W1Mo1-NG catalyst is synthesized through a three-step hydrothermal self-assembly and chemical vapor deposition (CVD) method. In the W1Mo1-NG material, O-bridged W-Mo heteroatoms are inserted into the vacancies of nitrogen-doped graphene (NG) via the self-assembly of tungstate and molybdate precursors (Figure 2A). As revealed via aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM), the distance between W-Mo atoms is below 3.6 Å (Figure 2B,C), markedly longer than Mo-Mo/W-Mo/W-W bonds, confirming that the W and Mo atoms are bridged by oxygen atoms. X-ray absorption fine structure (EXAFS) spectroscopy further demonstrates that the O-bridged W-Mo heteroatoms are stabilized on NG vacancy with a W-O-Mo-O-C configuration (Figure 2D). In electrocatalytic measurements, the as-prepared W1Mo1-NG catalyst delivers a current density of 10 mA/cm2 at an ultralow overpotential of 24 mV in 0.5 M H2SO4 (Figure 2E), outperforming Mo2-NG, W2-NG, nitrogen-doped graphene, and commercial Pt/C catalysts. Moreover, W1Mo1-NG exhibits an overpotential of 67 mV at 10 mA/cm2 in 1.0 M KOH (Figure 2F), displaying excellent catalytic activity over a wide pH range from 0 to 14. Density functional theory calculations reveal that the delocalized electrons in the distinctive W-O-Mo-O-C configuration provide near-optimal hydrogen binding energy, facilitating superior HER kinetics (Figure 2G). From an overarching perspective, it is imperative to place a strong emphasis on evaluating the stability of catalysts. This attribute holds utmost significance when considering the practical applications of these catalysts in industrial settings. After subjecting W1Mo1-NG to continuous electrolysis for a substantial duration of 80,000 s, operating at a current density of 10 mA/cm2, the remarkable observation is that it sustains an impressive 98% and 95% of its initial overpotential in both acidic and alkaline environments, respectively (Figure 2H).
This outstanding stability is a pivotal factor that underscores the catalyst’s potential for real-world usage, particularly in industries where sustained, long-term performance is of paramount importance. This in-depth evaluation is instrumental in assessing their suitability and viability for practical, industrial applications. The synergistic effect and electronic structure modulation of the W-Mo dual atoms in W1Mo1-NG is crucial for the high catalytic activity and stability over the entire pH range.
This paper reports an efficient alkaline HER electrocatalyst composed of RuBi single-atom alloy (SAA) decorated on Bi-single-atom-doped oxidized graphene (RuBi SAA/Bi@OG) (Figure 3A). This novel nanostructure was successfully synthesized via a facile pyrolysis approach. Morphological characterizations via transmission electron microscopy (TEM) (Figure 3B) demonstrate the uniform distribution of RuBi SAA nanoparticles on the OG substrate. The high-resolution TEM and selected area electron diffraction patterns confirm the dominant exposure of the Ru crystal facet in RuBi SAA. More importantly, as displayed in the element mapping results in Figure 3C, the Ru and Bi elements are homogeneously distributed in the RuBi SAA/Bi@OG sample, confirming the dual positioning of Bi and Ru atoms. X-ray absorption spectroscopy (XAS) provides critical structural insights. The Ru K-edge and Bi L3-edge XANES spectra verify the metallic nature of Ru and the lower oxidization state of Bi in Rubi SAA/Bi@OG, respectively (Figure 3D,E). No characteristic Bi-Bi peak is observed in FT-EXAFS (Figure 3E), demonstrating the isolated single-atom feature of Bi. These results strongly validate the proposed dual-positioning of Bi single atoms in the RuBi SAA and OG support. Owing to the modulated electronic structure, Rubi SAA/Bi@OG delivers substantially enhanced alkaline HER performance relative to Ru NP/OG and Pt/C (Figure 3F,G). At an overpotential of 150 mV, its exceptional mass activity reaches 65,000 mA mg−1, which is 72.2 and 3.6 times higher than commercial Pt/C and Ru NP/OG, respectively (Figure 3F). DFT calculations reveal the Rubi SAA/Bi@OG nanostructure possesses the optimal hydrogen adsorption free energy on Ru sites (−0.37 eV), benefiting from the double atomic tuning effect (Figure 3H).

3.2. Bimetallic Single-Atom Catalysts Supported on TMDs

TMDs like MoS2 have emerged as promising supports for bimetallic single-atom catalysts, owing to their metallic character, high conductivity, and abundance of unsaturated coordination sites [57,58,59]. However, pristine TMD surfaces lack sufficient anchoring sites to firmly immobilize isolated metal atoms. Similar to graphene, creating defects in the lattice structure of TMDs has proven an effective approach for engineering stable anchors. For example, sulfur vacancies in the MoS2 basal plane can coordinate single metal atoms through strong bonding with the exposed Mo atoms. Alternatively, heteroatom doping, such as partially replacing Mo with Ni or Co, can also modulate the TMD electronic structure and generate additional low-coordination sites [60,61,62,63,64]. In TMDs such as MoS2, sulfur vacancies present important coordination sites where exposed Mo atoms can coordinate with transition metal atoms. Additionally, heteroatom doping, such as partially replacing Mo with Ni or Co, can modulate the electronic structure of TMDs and generate extra low-coordination positions
In summary, creating sulfur vacancies and heteroatom doping enables TMDs to firmly stabilize isolated bimetallic atoms. Further research into defect control and TMD compositional tuning will open up new possibilities for designing optimized TMD-supported single-atom catalysts.
Recent advances in single-atom catalysts have shown great promise for electrochemical reactions like the HER. However, stability and activity limitations persist for single-metal systems. Ge et al. report a bimetallic single-atom catalyst composed of Ru and Ni anchored on MoS2 nanosheets (Ru/Ni-MoS2) (Figure 4A–C) that exhibits excellent HER performance and durability.
Using a combination of microscopy, spectroscopy, and computations, they confirm that Ru atoms are primarily bonded through Ni atoms substituting Mo sites in the MoS2 lattice (Figure 4D). The strong electronegativity of Ni facilitates electron transfer to the MoS2 support and stable incorporation of Ru. Electrochemical testing reveals the Ru/Ni-MoS2 catalyst has an ultralow overpotential of 32 mV and Tafel slope of 41 mV/dec (Figure 4E), significantly outperforming MoS2, Ru/MoS2, and Ni/MoS2. DFT calculations suggest a synergistic effect where Ni-bonded S sites adsorb H while Ru sites bind OH, together accelerating the water dissociation step crucial for alkaline HER (Figure 4F). DFT calculations also elucidate how Ni substitution enables Ru incorporation and modulates MoS2′s electronic structure (Figure 4G). Ni atoms replacing Mo are energetically favorable and lead to electron accumulation on neighboring S sites. This increases S affinity for hydrogen and promotes charge transfer from Ru atoms. The modulated electronic structure is central to achieving the high HER activity of Ru/Ni-MoS2. The alkaline HER mechanism involves water adsorption, dissociation, and hydrogen generation steps. Ge et al. constructed free energy diagrams to gain insights into how Ru/Ni-MoS2 promotes this process (Figure 4H). Compared to MoS2, Ru-MoS2, and Ni-MoS2, Ru/Ni-MoS2 has the lowest hydrogen adsorption free energy, facilitating hydrogen evolution. More importantly, the water dissociation barrier is significantly reduced on Ru/Ni-MoS2 due to the synergistic adsorption of hydrogen and hydroxide discussed earlier. Furthermore, the bimetallic single-atom structure enhances conductivity by narrowing the bandgap of MoS2 (Figure 4I), while the Ni anchoring minimizes Ru leaching to give remarkable 20 h stability.
Overall, this rational design of a bimetallic single-atom catalyst utilizing heteroatom electronegativity differences provides a promising strategy for developing highly active and stable HER electrocatalysts. The novel insights into controlling metal–support interactions and catalytic mechanisms will inform further advances in this emerging field.
The development of highly efficient and stable electrocatalysts for acidic overall water splitting is of great importance for sustainable hydrogen production but remains a huge challenge (Figure 5A). Recently, Jiang et al. proposed an ingenious approach to assemble diatomic species (DAs) of Ni and Fe into the interlayer of MoS2. This interlayer-confined NiFe@MoS2 structure was synthesized via laser molecular beam epitaxy and subsequent self-curving treatment (Figure 5B). Aberration-corrected scanning transmission electron microscopy (STEM) directly revealed the atomic configuration of Ni and Fe DAs anchored at sulfur vacancies within the MoS2 interlayer (Figure 5C). Compared to pristine MoS2, electron spin resonance spectra confirmed the anchoring of NiFe DAs at sulfur vacancies in planar NiFe@MoS2 (Figure 5D). Density functional theory calculations demonstrated the interlayer confinement effect optimized the hydrogen and oxygen adsorption capability and reduced the energy barriers, thereby facilitating acidic overall water splitting (Figure 5E–G). Impressively, it achieved ultra-efficient acidic overall water splitting with superb durability (Figure 5H,I). Experimentally, the interlayer-confined NiFe@MoS2 exhibited enhanced HER and oxygen evolution reaction (OER) activities.
This work embodies the significance of interlayer confinement in regulating the adsorption properties and stabilizing the highly active dual atoms even in harsh conditions. The atomic-level spatial restriction provides a protective “shelter” for the dual atoms to survive acidic electrolytes. Moreover, this general confinement approach based on 2D materials interlayer allows the incorporation of various active species, which opens up possibilities for designing advanced interlayer-confined electrocatalysts for diverse energy-related applications. In summary, the atomic-scale interlayer-confinement of multiple species represents a promising strategy to develop optimized and robust catalytic systems with both high efficiency and durability. Further exploitation of such confined configurations and the confinement effects may provide valuable insights into rational catalyst design.

4. Bimetallic Single-Atom Photocatalysts for Hydrogen Production

Semiconducting carbon nitrides, including phosphorus-doped carbon nitride (PCN) and two-dimensional polymerized carbon nitride (2D CN), have attracted great interest as metal-free photocatalysts for solar-driven hydrogen production. However, rapid recombination of photogenerated charge carriers remains a key limitation. Anchoring isolated dual metal atoms on the carbon nitride surface creates catalytic sites to facilitate charge separation and surface redox reactions [14,39,66,67,68]. The conjugate structure of carbon nitrides with delocalized π electrons enables strong coordination with metal atoms through the electron-rich nitrogen sites. Studies have shown that single atoms of Pt, Co, and Ag can be stably incorporated into the heptazine units of the carbon nitride lattice. The dual metal atoms act as electron trapping sites, spatially separating the photogenerated electrons and holes [69,70,71,72,73].
In summary, carbon nitride semiconductors provide versatile scaffolds for stabilizing bimetallic single-atom photocatalysts. The heteroatom-rich structure firmly anchors isolated metal atoms to create dual-site systems with tunable properties and synergistic effects. Further research into synthetic control and mechanistic elucidation will advance the development of efficient carbon nitride-based photocatalysts.
Table 2 summarizes selected bimetallic single-atom photocatalysts based on polymeric carbon nitrides for solar H2 production. The Co1Ag1 dual sites anchored on P-doped CN achieved an exceptional H2 evolution rate of 1190 μmol/g/h under simulated sunlight irradiation, which is over five times higher than Ag1-CN and Co1-CN. The proximity interactions between Co and Ag optimizes band structures and charge densities to enhance light absorption, carrier separation, and proton reduction. In another example, the Pt1-Co1 bimetallic single-atoms on 2D CN delivered high solar-to-hydrogen conversion efficiency exceeding 1% and rates up to 915.8 mmol/g/h, which is 19.8 times higher than Co1-CN. The incorporation of Pt modulates the Co coordination environment and electronic structure to facilitate charge transfer. Overall, the results showcase the capability of polymeric carbon nitride-supported bimetallic single-atom photocatalysts to achieve substantially increased solar H2 production through synergistic effects between the dual metal sites.

4.1. Bimetallic Single-Atom Catalysts Supported on Phosphorus-Doped Carbon Nitride

Photocatalytic water splitting for hydrogen production using solar energy has attracted tremendous attention. Single-atom catalysts (SACs) were developed to maximize atom utilization efficiency. However, the catalytic activity of mono-metallic SACs is still limited due to insufficient active sites. Constructing dual-atom catalysts (DACs) by incorporation of a second metal has proven an effective approach to achieve cooperative effects between dual metal sites. Recently, Liu et al. reported double-metal SACs with Co and Ag anchored on phosphorus-doped carbon nitride (PCN) sheets using supramolecular and solvothermal methods.
Figure 6A presents the proposed mechanism that the Co-Ag dual sites act as electron sinks for photogenerated electrons while the remaining holes oxidize water. This Co1Ag1-PCN DAC exhibited remarkably enhanced photocatalytic hydrogen evolution from water splitting under simulated sunlight, achieving an impressive H2 production rate of 1190 μmol g −1 h−1 (Figure 6B). In contrast, the incorporation of Ag or Co alone only led to moderate improvement over PCN. Figure 6C presents the high apparent quantum efficiency (AQY) of 1.49% at 365 nm, consistent with the enhanced light harvesting capability. Figure 6D highlights the impressive stability of Co1Ag1-PCN throughout numerous cycles, a quality of utmost importance for practical industrial use. The endurance of catalysts plays a pivotal role in ensuring sustained performance. Thorough assessments in real-world applications are crucial, as they provide insights into the catalyst’s potential usefulness in scenarios where durability and resilience are often the primary factors for success. Consequently, conducting comprehensive investigations and delivering detailed reports on catalyst stability are indispensable steps in evaluating their appropriateness for practical industrial applications. Figure 6E presents the Mott–Schottky plots suggesting the optimized band structure of Co1Ag1-PCN. The partial density of states calculated via DFT (Figure 6F,G) shows the band center shift closer to the Fermi level for the dual metal sites. Density functional theory calculations illustrate the modulated electronic structure and synergistic effects between the dual-metal centers (Figure 6H). The integration of Co redistributes the charge density surrounding Ag atoms, facilitating interfacial charge transfer.
Furthermore, (Figure 6I) shows the DFT calculated free energy diagram, suggesting the optimized Gibbs free energy of H* (0.15 eV) for H2 generation over Co1Ag1-PCN compared to the mono SACs. The cooperative interactions between Co and Ag lower the kinetic barrier and facilitate electron accumulation on Ag, thereby enhancing photocatalytic performance.
In summary, this rational design of double-metal SACs creates opportunities to optimize catalytic performance by engineering the atomic coordination environment. The cooperative effects between Co and Ag were successfully established via modulation of charge density and band structures. Further studies on the synthetic control and applications of multi-metallic SACs are expected.

4.2. Bimetallic Single-Atom Catalysts Supported on Two-Dimensional Polymerized Carbon Nitride

This paper reports the development of a bimetallic Pt1-Co1/CN single-atom catalyst for enhanced photocatalytic hydrogen production. The catalyst is synthesized by dispersing Co single atoms on two-dimensional polymeric carbon nitride (CN) sheets, followed by incorporation of Pt single atoms via a freezing-visible-light-deposition method.
Characterization via EXAFS (Figure 7A) reveals that the Co single atoms in Co1/CN exhibit a Co1N4 configuration, while in Pt1-Co1/CN the coordination number decreases to Co1N3, indicating modulation of the Co sites by Pt. XPS and EPR were used to verify the decrease in Co oxidation state and increase in unpaired electrons in Pt1-Co1/CN. Accordingly, Pt1-Co1/CN shows dramatically enhanced photocatalytic H2 production (Figure 7B), with rates 19.8 and 3.5 times higher than Co1/CN and Pt1/CN, respectively. Optical characterization (Figure 7C–F) confirms Pt1-Co1/CN has superior visible light response and charge separation. XPS (Figure 7G) verifies the modulation of Co sites by Pt single atoms. DFT calculations (Figure 7H,I) show the Co1N3 sites have unpaired 3d electrons crossing the Fermi level, narrowing the bandgap. This unique configuration improves visible light absorption, charge separation, and proton activation.
In summary, this work demonstrates a synergistic effect between Pt and Co single atoms in CN, where Pt modulates the geometric and electronic configuration of Co. This long-range electron synergy promotes photocatalytic performance by enhancing light absorption, carrier separation, and proton activation. The concept of utilizing bimetallic single atoms and their interactions to tune photocatalyst properties represents an exciting new direction for designing efficient catalysts. Key achievements include maximizing noble metal efficiency, elucidating the electron-level synergistic mechanism, and providing a general strategy to regulate single-atom sites via long-range electron effects. Overall, this work makes important fundamental advances in photocatalysis. The results are significant because they provide critical insights into engineering highly active photocatalysts through precise control and synergistic design of bimetallic single-atom sites. The knowledge gained can guide the continued development of efficient, low-cost photocatalytic systems for renewable energy applications. We find this work highly innovative in exploiting interactions between single atoms to tune catalytic properties.

5. Conclusions and Outlook

This review summarizes recent advances in bimetallic single-atom catalysts for enhancing hydrogen evolution across electrochemical and photocatalytic pathways. Bimetallic single-atom catalysts supported on 2D materials are at the forefront of developing active, robust, and inexpensive catalytic systems for clean hydrogen production. The proximity interactions and synergies between two different metal atoms in these catalysts create cooperative electronic effects and novel reactivity.
We first highlighted the fundamentals of bimetallic single-atom catalysts, including their unique configuration and resultant geometric and electronic modulation. Next, we reviewed recent progress in bimetallic single-atom electrocatalysts for HER under acidic and alkaline conditions. Graphene and TMDs acted as versatile scaffolds to coordinate isolated metal atoms and achieve synergistic improvements in activity and stability versus monometallic analogs. We then discussed photocatalytic bimetallic single-atom systems on polymeric carbon nitrides that harness dual-site cooperativity to enhance solar hydrogen production.
Several priorities can guide future efforts from fundamental studies to practical applications. Precision synthetic techniques are imperative to control bimetallic compositions and configurations at the atomic level, enabling the optimization of catalytic performance. Advanced spectroscopic and microscopic characterization techniques will provide critical insights into synergistic mechanisms. Exploring ternary and quaternary single-atom systems may uncover new cooperative phenomena toward enhanced HER kinetics. Surface functionalization and semiconductor coupling represent promising directions for solar-driven hydrogen generation. Translating laboratory-synthesized catalysts into industrial systems will require comparative scale-up studies.
In summary, this emerging field holds immense potential for enabling clean, efficient hydrogen production. Further insights into bimetallic single-atom interactions, metal–support anchoring, and characterization at the atomic scale will accelerate the rational design of optimal bimetallic single-atom catalysts. We hope this review provides a useful summary of recent progress and future opportunities in harnessing synergistic dual-metal single-atom systems for electrocatalytic and photocatalytic hydrogen evolution.

Author Contributions

Conceptualization, D.Y., L.W. and M.Z.; investigation, D.Y. and L.W.; data curation, M.Z., D.Y. and L.W.; writing—original draft preparation, M.Z.; writing—review and editing, M.Z., K.X., N.S., Y.Z., L.W. and D.Y.; funding acquisition, L.W. and D.Y. 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, 22202065), 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).

Data Availability Statement

All of the data analyzed in this review came from articles that mentioned studies.

Conflicts of Interest

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

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Figure 1. (A) Bimetallic single-atom catalysts for the photocatalytic generation of hydrogen. Adapted with permission from [46], Copyright 2023 Wiley—Vch Verlag GMBH & Co., KGAA; Wiley. (B) Bimetallic single-atom catalysts for electrocatalytic hydrogen production. Adapted with permission from [47], Copyright 2023 American Association for the Advancement of Science.
Figure 1. (A) Bimetallic single-atom catalysts for the photocatalytic generation of hydrogen. Adapted with permission from [46], Copyright 2023 Wiley—Vch Verlag GMBH & Co., KGAA; Wiley. (B) Bimetallic single-atom catalysts for electrocatalytic hydrogen production. Adapted with permission from [47], Copyright 2023 American Association for the Advancement of Science.
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Figure 2. (A) Schematic illustration of the synthetic procedure. (B) AC HAADF-STEM images. This image shows paired W-Mo atoms. (white circles). (C) The distance between statistical W-Mo diatomic in AC HAADF-STEM images. (D) The corresponding Mo K-edge and W L3-edge EXAFS fitting curves for W1Mo1-NG at R-space. (E,F) The polarization curves of Mo2-NG, W1Mo1-NG, W2-NG, NG, and Pt/C were measured in 0.5 M H2SO4 and 1.0 M KOH. (G) ΔGH diagrams of W1Mo1-NG, Mo2-NG, and W2-NG. The differential charge density maps of heteronuclear W1Mo1-NG system (H) Galvanostatic responses of W1Mo1-NG and Pt/C were recorded at 10 mA cm−2 under acidic and alkaline solutions. Adapted with permission from [47], Copyright 2023 American Association for the Advancement of Science.
Figure 2. (A) Schematic illustration of the synthetic procedure. (B) AC HAADF-STEM images. This image shows paired W-Mo atoms. (white circles). (C) The distance between statistical W-Mo diatomic in AC HAADF-STEM images. (D) The corresponding Mo K-edge and W L3-edge EXAFS fitting curves for W1Mo1-NG at R-space. (E,F) The polarization curves of Mo2-NG, W1Mo1-NG, W2-NG, NG, and Pt/C were measured in 0.5 M H2SO4 and 1.0 M KOH. (G) ΔGH diagrams of W1Mo1-NG, Mo2-NG, and W2-NG. The differential charge density maps of heteronuclear W1Mo1-NG system (H) Galvanostatic responses of W1Mo1-NG and Pt/C were recorded at 10 mA cm−2 under acidic and alkaline solutions. Adapted with permission from [47], Copyright 2023 American Association for the Advancement of Science.
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Figure 3. (A) Schematic illustration for preparation of RuBi SAA/Bi@OG (B) TEM. (C) The Ru and Bi elements in the RuBi SAA/Bi@OG were mapped using EDS. (D) The K3-weighted EXAFS signals of Ru foil, RuBi SAA/Bi@OG, and RuO2 were analyzed using wavelet transforms. (E) The wavelet transforms for the K3 -weighted EXAFS signals of Bi foil, RuBi SAA/Bi@OG, and Bi2O3. (F) The mass-specific activity at an overpotential of 50, 100, and 150 mV. (G) Comparison of TOF values of state-of-the-art Ru-based alloy HER electrocatalysts at an overpotential of 100 mV. (H) Calculated hydrogen-adsorption Gibbs free energy (∆GH*) of different adsorption sites in Ru13/O6G, Ru13/Bi1@O6G and Ru12Bi1/Bi1@O6G, respectively. Adapted with permission from [56], Copyright 2023 ANGEWANDTE CHEMIE.
Figure 3. (A) Schematic illustration for preparation of RuBi SAA/Bi@OG (B) TEM. (C) The Ru and Bi elements in the RuBi SAA/Bi@OG were mapped using EDS. (D) The K3-weighted EXAFS signals of Ru foil, RuBi SAA/Bi@OG, and RuO2 were analyzed using wavelet transforms. (E) The wavelet transforms for the K3 -weighted EXAFS signals of Bi foil, RuBi SAA/Bi@OG, and Bi2O3. (F) The mass-specific activity at an overpotential of 50, 100, and 150 mV. (G) Comparison of TOF values of state-of-the-art Ru-based alloy HER electrocatalysts at an overpotential of 100 mV. (H) Calculated hydrogen-adsorption Gibbs free energy (∆GH*) of different adsorption sites in Ru13/O6G, Ru13/Bi1@O6G and Ru12Bi1/Bi1@O6G, respectively. Adapted with permission from [56], Copyright 2023 ANGEWANDTE CHEMIE.
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Figure 4. (A) SEM image of Ru/Ni-MoS2/CFP. (B) The HAADF-STEM image and (C) corresponding enlarged image of Ru/Ni-MoS2. (D) EXAFS fitting curves of Ni K-edge in Ru/Ni-MoS2 (insets: atomic structure model of Ru/Ni-MoS2). (E) The LSV curves of CFP, MoS2/CFP, Ni-MoS2/CFP, Ru-MoS2/CFP, Ru/Ni-MoS2/CFP and 5 wt.% Pt/C without IR correction (inset: overpotential (ƞ10) of these samples). (F) DFT-calculated adsorption energies of H and OH at different positions on the surfaces of Ru/Ni-MoS2, respectively. (G) The illustration of the mechanism for the electrocatalytic HER under alkaline conditions. (H) Free energy diagrams on the surface of these catalysts in an alkaline solution. (I) The calculated DOS of pristine MoS2 and Ru/Ni-MoS2. Adapted with permission from [53], Copyright 2021 ELSEVIER BV.
Figure 4. (A) SEM image of Ru/Ni-MoS2/CFP. (B) The HAADF-STEM image and (C) corresponding enlarged image of Ru/Ni-MoS2. (D) EXAFS fitting curves of Ni K-edge in Ru/Ni-MoS2 (insets: atomic structure model of Ru/Ni-MoS2). (E) The LSV curves of CFP, MoS2/CFP, Ni-MoS2/CFP, Ru-MoS2/CFP, Ru/Ni-MoS2/CFP and 5 wt.% Pt/C without IR correction (inset: overpotential (ƞ10) of these samples). (F) DFT-calculated adsorption energies of H and OH at different positions on the surfaces of Ru/Ni-MoS2, respectively. (G) The illustration of the mechanism for the electrocatalytic HER under alkaline conditions. (H) Free energy diagrams on the surface of these catalysts in an alkaline solution. (I) The calculated DOS of pristine MoS2 and Ru/Ni-MoS2. Adapted with permission from [53], Copyright 2021 ELSEVIER BV.
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Figure 5. (A) This proposal aims to assemble DAs with atomic-scale precision in adjacent layers within the 2D limit. (B) HAADF-STEM of sidewalls of interlayer-confined NiFe@MoS2, inset is the corresponding low-magnification TEM image. (C) An atomic-resolution aberration-corrected HAADF-STEM image is shown, with Mo and S represented by blue and orange balls, respectively, in interlayer-confined NiFe@MoS2. (D) ESR spectra of pristine monolayer MoS2 and planar NiFe@MoS2. (E) Polarization curves and (F) Tafel plots of interlayer-confined NiFe@MoS2, planar NiFe@MoS2, interlayer-confined Sv@MoS2, bare glassy carbon electrodes and 20% Pt/C for HER. (G) Amperometric j–t tests of interlayer-confined NiFe@MoS2 and planar NiFe@MoS2. (H) Water dissociation barrier comparison. (I) Standard free energy diagram of the HER process. Adapted with permission from [65], Copyright 2023 Wiley-VCH.
Figure 5. (A) This proposal aims to assemble DAs with atomic-scale precision in adjacent layers within the 2D limit. (B) HAADF-STEM of sidewalls of interlayer-confined NiFe@MoS2, inset is the corresponding low-magnification TEM image. (C) An atomic-resolution aberration-corrected HAADF-STEM image is shown, with Mo and S represented by blue and orange balls, respectively, in interlayer-confined NiFe@MoS2. (D) ESR spectra of pristine monolayer MoS2 and planar NiFe@MoS2. (E) Polarization curves and (F) Tafel plots of interlayer-confined NiFe@MoS2, planar NiFe@MoS2, interlayer-confined Sv@MoS2, bare glassy carbon electrodes and 20% Pt/C for HER. (G) Amperometric j–t tests of interlayer-confined NiFe@MoS2 and planar NiFe@MoS2. (H) Water dissociation barrier comparison. (I) Standard free energy diagram of the HER process. Adapted with permission from [65], Copyright 2023 Wiley-VCH.
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Figure 6. (A) The charge separation and transfer in the Co1Ag1-PCN DAC system under solar light irradiation. (B) Time course of H2 production of CN, Ag1-CN, Co1-PCN, and 0.1Co1Ag1-PCN (C) Wavelength-dependent AQY for photocatalytic H2 evolution of 0.1Co1Ag1-PCN. (D) Recycled H2 evolution performance of 0.1Co1Ag1-PCN. (E) Mott–Schottky plots PDOS plots of (F) Ag 3d for Ag1-CN, 0.1Co1Ag1-PCN, and (G) Co 3d for Co1-PCN, 0.1Co1Ag1-PCN. (H) Differential charge of Co 3d and Ag 3d for 0.1Co1Ag1-PCN. (I) Gibbs free energy diagrams and the pathways of HER of Ag1-CN, Co1-PCN, and 0.1Co1Ag1-PCN. Adapted with permission from [46], Copyright 2023 WILEY—V C H VERLAG GMBH & CO. KGAA; Wiley.
Figure 6. (A) The charge separation and transfer in the Co1Ag1-PCN DAC system under solar light irradiation. (B) Time course of H2 production of CN, Ag1-CN, Co1-PCN, and 0.1Co1Ag1-PCN (C) Wavelength-dependent AQY for photocatalytic H2 evolution of 0.1Co1Ag1-PCN. (D) Recycled H2 evolution performance of 0.1Co1Ag1-PCN. (E) Mott–Schottky plots PDOS plots of (F) Ag 3d for Ag1-CN, 0.1Co1Ag1-PCN, and (G) Co 3d for Co1-PCN, 0.1Co1Ag1-PCN. (H) Differential charge of Co 3d and Ag 3d for 0.1Co1Ag1-PCN. (I) Gibbs free energy diagrams and the pathways of HER of Ag1-CN, Co1-PCN, and 0.1Co1Ag1-PCN. Adapted with permission from [46], Copyright 2023 WILEY—V C H VERLAG GMBH & CO. KGAA; Wiley.
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Figure 7. (A) Co K-edge R space EXAFS fitting results of Co1/CN and Pt1-Co1/CN samples. Insert are the schematic model of Co1N4 sites on Co1/CN and Co1N3 sites on Pt1-Co1CN obtained from EXAFS fitting results. (B) Photocatalytic hydrogen evolution activities of CN, Pt1/CN, Co1/CN, Pt1-Co1/CN and PtNC-Co1/CN samples (C) UV-vis absorption spectra. (D) Tauc plots. (E) Electrochemical impedance spectra (EIS). (F) Photocurrent response. (G) Co 2p XPS spectra of Co1/CN and Pt1-Co1/CN samples. (H) Projected density of states of CN and Co1N3/CN. (I) Work functions of CN and Co1N3/CN. Adapted with permission from [74], Copyright 2023 Elsevier B.V.; Elsevier.
Figure 7. (A) Co K-edge R space EXAFS fitting results of Co1/CN and Pt1-Co1/CN samples. Insert are the schematic model of Co1N4 sites on Co1/CN and Co1N3 sites on Pt1-Co1CN obtained from EXAFS fitting results. (B) Photocatalytic hydrogen evolution activities of CN, Pt1/CN, Co1/CN, Pt1-Co1/CN and PtNC-Co1/CN samples (C) UV-vis absorption spectra. (D) Tauc plots. (E) Electrochemical impedance spectra (EIS). (F) Photocurrent response. (G) Co 2p XPS spectra of Co1/CN and Pt1-Co1/CN samples. (H) Projected density of states of CN and Co1N3/CN. (I) Work functions of CN and Co1N3/CN. Adapted with permission from [74], Copyright 2023 Elsevier B.V.; Elsevier.
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Table 1. Summary of bimetallic single-atom catalysts supported on graphene for electrochemical HER.
Table 1. Summary of bimetallic single-atom catalysts supported on graphene for electrochemical HER.
Bimetallic CatalystMetal Loading SupportElectrolyteOverpotentialRemarks
W1Mo1N-doped graphenepH universality0.5 M H2SO4 24 mV
1.0 M KOH 67 mV		Excellent pH universality
RuBi SAA RuOxidized graphene1.0 M KOH150 mV (at 20 mA/cm2)High activity
Ru/NiMoS2alkaline32 mVStability
Table 2. Summary of bimetallic single-atom photocatalysts supported on polymeric carbon nitrides for solar H2 production.
Table 2. Summary of bimetallic single-atom photocatalysts supported on polymeric carbon nitrides for solar H2 production.
Bimetallic CatalystMetal Loading SupportH2 Production RateRemarksRef.
Co1Ag1P-doped CN1190 μmol/g/hHigh apparent quantum yield[46]
Pt1Co12D CN915.8 mmol·g−1Pt·h−1Stability[74]
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Zhang, M.; Xu, K.; Sun, N.; Zhuang, Y.; Wang, L.; Yan, D. Bimetallic Single-Atom Catalysts for Electrocatalytic and Photocatalytic Hydrogen Production. Catalysts 2023, 13, 1409. https://doi.org/10.3390/catal13111409

AMA Style

Zhang M, Xu K, Sun N, Zhuang Y, Wang L, Yan D. Bimetallic Single-Atom Catalysts for Electrocatalytic and Photocatalytic Hydrogen Production. Catalysts. 2023; 13(11):1409. https://doi.org/10.3390/catal13111409

Chicago/Turabian Style

Zhang, Mengyang, Keyu Xu, Ning Sun, Yanling Zhuang, Longlu Wang, and Dafeng Yan. 2023. "Bimetallic Single-Atom Catalysts for Electrocatalytic and Photocatalytic Hydrogen Production" Catalysts 13, no. 11: 1409. https://doi.org/10.3390/catal13111409

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