The recent progress of single-atom catalysts on amorphous substrates for electrocatalysis
0 Abstract
Single-atom catalysts (SACs) have emerged as a focal point in energy catalytic conversion due to their remarkable atomic efficiency and catalytic performance. The challenge lies in efficiently anchoring active sites on a specific substrate to prevent agglomeration, maximizing their effectiveness. Substrate characteristics play a pivotal role in shaping the catalytic performance of SACs, influencing the dispersion and stability of single atoms. In recent years, amorphous materials have gained attention as substrates due to their unique surface structure and abundance of unsaturated coordination sites, offering an ideal platform for capturing and anchoring single atoms effectively, thus enhancing catalytic activity. To clarify the interaction between single atoms and amorphous substrates, this review outlines amorphization methods, the mechanism of single-atom anchoring and the characterization methods of amorphous SACs. Subsequently, it summarizes the physical properties and electrocatalytic mechanisms of amorphous materials. Then, interactions between single atoms and amorphous substrates are categorized and summarized. Finally, the paper consolidates the research progress of amorphous SACs and outlines future development prospects. By exploring the synergistic relationship between single atoms and amorphous substrates, this review aims to deepen the understanding of their interaction mechanisms, thereby propelling advancements in SACs for energy catalytic conversion.
Keywords
INTRODUCTION
The increasing consumption of fossil fuels has highlighted environmental concerns, necessitating an immediate exploration of more sustainable and renewable energy sources[1]. Electrochemical conversion and storage devices, including water electrolyzers, rechargeable metal-air batteries and renewable fuel cells, currently form a vital component of the sustainability framework. They rely on a series of electrochemical redox reactions, such as hydrogen evolution (HER) at the cathode, oxygen evolution (OER) at the anode, oxygen reduction (ORR), and the electric reduction of carbon dioxide (CO2RR)[2-4]. Consequently, there is an urgent need to develop efficient electrocatalysts to enhance electrochemical performance[5,6].
Special supported metal catalysts, known as single-atom catalysts (SACs), are characterized by the presence of all metal components in the form of single atoms dispersed on the substrate[7]. As the size of the metal center increases from single atoms to nanoclusters and nanoparticles, the energy levels become more continuous and approach the metallic state[8]. Unlike nanoclusters and nanoparticles, single atoms maximize the exposure of active sites, resulting in high atomic efficiency for catalysis. In larger nanoclusters and nanoparticles, there is greater orbital overlap between metal atoms, and the internal contribution of particles is stronger. In contrast, small clusters and single atoms, with fully accessible orbital structures, have a lower efficiency of orbital overlap between the metal center and the substrate[9]. Single atoms, however, offer advantages in this regard. Their electronic structure is directly influenced by the coordination environment, making it easily adjustable and more conducive to enhancing the efficiency of electrocatalytic reactions. A key scientific challenge concerning SACs revolves around understanding the stabilization mechanism of single atoms on the substrate[10]. Strong interactions or charge transfer between the metal single atoms and coordination substrates are essential to prevent atom aggregation into particles[11-14]. This necessitates the selection and optimization of suitable substrates to stabilize and disperse the target atoms effectively[15]. Therefore, the correct choice and optimization of substrates can significantly synergize with single atoms to enhance their catalytic efficiency and activity.
Amorphous materials possess higher specific surface areas, more active sites, and greater surface defects compared to crystalline materials. These characteristics contribute to their unique structures and distinctive surface chemical states in the field of electrocatalysis, playing a significant role in the adsorption of intermediates and catalytic activity. Typically, defects present in substrate materials play a crucial role in regulating the electronic structure and surface properties of single atoms[10,16]. Metal atoms can be effectively immobilized onto the surface by capturing them through substrate defects[17-19]. While crystalline material defects have been extensively studied, amorphous materials exhibit irregular internal particle arrangement and short-range order due to their lack of a crystalline structure. The relatively weak bonding energy of amorphous surface atoms results in abundant unsaturated coordination sites and dangling bonds[20,21]. This facilitates the absorption or binding of substances at the atomic scale. Additionally, amorphous materials boast a significant surface area, providing an increased number of active sites for anchoring single atoms. This abundance of additional reservoirs and potential active reaction centers serves to enhance catalytic behavior and electrochemical storage. Anchoring single atoms onto the surface of amorphous materials allows for precise control and localization of the active site, effectively avoiding the deactivation and aggregation of active sites and thereby enhancing the catalyst's activity[22,23]. Consequently, stabilizing metal single atoms using amorphous substrates emerges as an effective approach for developing highly efficient catalysts.
Based on the above analysis, amorphous materials possess unique characteristics for activating and stabilizing catalysts, while single atoms offer the advantage of maximizing atomic utilization. Therefore, investigating the preparation and mechanisms of catalysts anchored with single atoms on amorphous materials is particularly crucial for future developments. This research can effectively support the subsequent development of high-performance and highly stable catalysts. In this review, as shown in Figure 1, we first provide an overview of general synthesis methods for amorphous materials and the mechanism of anchoring single atoms. Subsequently, the catalytic mechanism of amorphous materials is systematically explored. Finally, the anchoring mechanism of single atoms on amorphous substrates is discussed in detail, along with future prospects. This review presents a viable strategy for addressing sustainability challenges through the utilization of single-atom anchoring on amorphous materials.
Figure 1. Schematic diagram of single-atom anchoring sites, synthesis strategies of amorphous substrates and interactions in the field of electrocatalysis.
PREPARATION OF AMORPHOUS MATERIALS, SINGLE ATOM ANCHORING METHODS AND CHARACTERIZATION TECHNIQUES
Synthesis methods of amorphous substrates
Due to the disordered atomic arrangement inherent in amorphous systems, these materials typically exhibit higher entropy compared to their crystalline counterparts, thus showing a propensity for crystalline transformation under external conditions[24]. Consequently, preventing or delaying the transition from amorphous to crystalline states remains a significant challenge, crucial for achieving controlled synthesis and precisely modulating the local structure of amorphous materials to enhance their efficacy in specific applications. Although the synthesis mechanisms of amorphous nanomaterials are not fully clear, there are still some similarities in the methods of preparing nanomaterials. Generally, several primary techniques are employed in the synthesis of amorphous materials.
Wet chemical method
Wet chemistry synthesis utilizes simple setups and readily available materials, ensuring operational ease, lower energy consumption, and scalability. By adjusting reaction conditions and ligand ratios, precise control over catalyst structure, morphology, and surface properties is achieved, meeting diverse catalytic needs. However, optimal catalytic performance often requires iterative optimization, potentially increasing time and cost. Wet chemical synthesis methods generally encompass solvothermal, ion exchange, and template-assisted approaches.
Leaching of ions occurs during wet chemical processes, resulting in structural changes. The gradual leaching of ions leads to local defects or distortions in the lattice structure. These defects and distortions form amorphous regions on the catalyst surface, gradually expanding and leading to the disordering of the overall crystal structure. For instance, Chen et al. subjected bulk-phase LaNiO3 perovskite oxide to FeCl3 solution, resulting in corrosion-induced amorphization[25]. Continuous leaching of large-sized La3+ ions led to the gradual collapse of the crystalline perovskite structure[25]. Similarly, Zhu et al. treated NiMoO4·H2O pre-catalyst with an alkaline solution, resulting in the exfoliating from the bulk NiMoO4·H2O nanorods due to the dissolution of Mo species and crystalline water. Alkaline etching and multi-step dissolution formed the superthin and amorphous Ni(OH)2[26].
Heteroatom doping through wet chemical methods induces lattice distortion, resulting in partial or complete amorphization[27]. For instance, Kang et al. utilized dimethylamine borane and NaBH4 as dual reducing agents and boron sources to generate three-dimensional mesoporous Ni-B amorphous alloy spheres [Figure 2A][28]. The selected-area electron diffraction (SAED) pattern [Figure 2B] confirms that the mesoporous Ni-B spheres are amorphous. This method introduces a fraction of B into the Ni lattice, creating a disordered local structure around the Ni atoms. High-resolution transmission electron microscopy (HRTEM) [Figure 2C] indicates that there is no noticeable long-range atomic alignment because there are few clear domains with lattice fringes. Similarly, Cheng et al. achieved the transformation of Pd nanomaterials from a face-centered-cubic phase into an amorphous phase without destroying their integrity by simply mixing them with bismuth mercaptan I solution at room temperature[29]. Bismuth mercaptan I induced amorphization of outer Pd nanoparticle atoms, causing lattice expansion of inner atoms and eventual transformation into an amorphous phase. In another work, electrochemical lithiation of layered Pd3P2S8 crystals yielded amorphous lithium-incorporated palladium phosphosulfide nanodots with abundant vacancies[30]. The lithium treatment deformed the Pd-S shell and reduced structural uniformity, introducing sulfur vacancies and Li doping. These amorphous nanodots exhibited excellent HER properties in acidic solutions due to their unique structure and high electrical conductivity.
Figure 2. Various amorphous alloys prepared by different methods and structural characterization results of the amorphous alloys. (A) Mesoporous Ni-B nanospheres obtained from a dual chemical reduction. (B) SAED pattern of a single mesoporous Ni-B alloy sphere. (C) HRTEM images of 3D mesoporous Ni-B nanostructures. Reproduced with permission from Ref.[28] Copyright 2020, Wiley-VCH. (D) The synthesis process of P-NiMoO4 nanoarrays via hydrothermal reaction and phosphorization process. (E) TEM specimen prepared by FIB lift-out techniques from P-NiMoO4 nanorod. The inset shows the electron diffraction pattern of region A. (F) The magnified TEM image of region C in panel (E). Reproduced with permission from Ref.[35] Copyright 2024, American Chemical Society. (G) LITV method used to synthesize and direct-write mixed metal oxides from fluid precursors. (H) SEM images and SEM-EDS maps of single, binary, and ternary metal oxides and mixed metal oxides of Fe, Co, and Ni as prepared from solutions containing the ions listed at left. Reproduced with permission from Ref.[41] Copyright 2021, American Chemical Society.
Thermal annealing
Thermal annealing is a prevalent method for synthesizing amorphous materials, encompassing processes such as thermal reduction, oxidation, and phosphating. This approach is suitable for large-scale production and exhibits a degree of industrial feasibility. However, its implementation is costly due to the requirement for high-temperature equipment and precise atmosphere control. During appropriate thermal annealing, atomic diffusion and rearrangement lead to the formation of an amorphous phase.
In the thermal annealing process, parameters such as temperature, atmosphere, and annealing time significantly influence amorphous form formation. At low temperatures, partial phase transitions yield a composite of metal oxides and amorphous metals. Conversely, crystalline metals form at high temperatures, indicating a transition from an amorphous to a crystalline structure. For instance, in the work of Wu et al., amorphous nanosheets must be annealed between the melt points of the metal acetylacetonate and the alkali salt[31]. Annealing at temperatures above the melt point of the alkali salt results in crystalline material formation, while temperatures below the metal acetylacetone melt point fail to drive the reaction. In practical applications, thermal reduction techniques are often combined with inert gases and high temperatures. For example, amorphous Li3VO4 anodes are synthesized via high-temperature annealing in vacuum, leading to surface oxygen escape and reduction of V5+ to V4+[32]. This results in structural rearrangement, increasing lattice stress and decreasing oxygen vacancy, leading to gradual amorphization of the material. Additionally, the right reaction time is crucial for the manufacture of pure amorphous materials[33].
In thermal oxidation, a high-temperature oxygen atmosphere is employed to expose the substrate material to oxygen, facilitating a chemical reaction between oxygen molecules and the substrate surface. This process induces lattice deformation and distortion through oxidation reactions and atomic diffusion, thereby resulting in the amorphization of the material. Serrapede et al. demonstrated that amorphous TiO2 formed at low temperatures, while anatase TiO2 formed at high temperatures, suggesting that the oxidation temperature needs to be adjusted to obtain an ideal amorphous structural material[34].
Using a simple phosphating strategy, the interface between amorphous and crystalline phases and dislocation defects can be introduced into the catalyst. For instance, Zhang et al. synthesized a self-supporting electrocatalyst [Figure 2D], P-NiMoO4, featuring a dislocation-rich structure comprising an amorphous Ni2P/crystalline P-doped NiMoO4/amorphous P-doped NiMoO4 sandwich structure through thermal phosphidation strategies[35]. The phosphating process induces numerous edge and screw dislocations, prompting electron transfer from Ni and Mo ions to neighboring O and P atoms. Figure 2E gives a cross-sectional view of P-NiMoO4 nanorods prepared by a focused-ion-beam lift-out technique, indicating that the P-NiMoO4 nanorods are composed of the amorphous-crystalline-amorphous sandwiched structure. The HRTEM image [Figure 2F] confirms that the outer shell of the nanorod is also amorphous. The heterogeneous interface effect between crystalline and amorphous phases modulates the electronic structure of electrocatalysts, enhancing catalytic activity in alkaline water and seawater.
Laser ablation
Femtosecond lasers, characterized by ultrashort pulse durations and ultrahigh peak power density, possess unique attributes such as nonlinear, non-equilibrium, and non-thermal processing capabilities[36-38]. Laser synthesis offers a cost-effective, time-saving, contactless, maskless, and environmentally friendly approach for producing various nanomaterials. Laser ablation is well-suited for low-volume production due to the sophisticated nature of the laser processing equipment.
The characteristics of the laser can induce transient heating in processing solutions during the reaction, potentially introducing vacancies or generating defect sites, leading to material amorphization. In the work of Zhao et al., CoSx nanospheres anchored to carbon fiber cloths were found to provide more active sites due to the presence of sulfur vacancies caused by a high-pressure field induced by laser processing, enhancing HER and nitrogen reduction reaction (NRR) activity[39]. Upon cessation of laser irradiation, the large temperature gradient induced by local heating alters the kinetics of phase transitions, affecting the phase transition pathways and rates of crystalline structures. Rapid cooling of localized high-temperature regions often results in a delay in phase transitions of crystal structures. Structures fail to rearrange into stable crystalline phases within a sufficient timeframe. The inability of crystal structures to undergo complete rearrangement and relaxation in a timely manner leads to the formation and diffusion of lattice defects. These factors collectively contribute to the synthesis of non-crystalline phases via laser irradiation. Liu et al. stated that when the laser interacts with a solid target, plasma is generated in the gas phase[40]. At the interface of the cool liquid and laser-induced plasma, a chemical reaction occurs, leading to the condensation of atoms in the liquid and the formation of small amorphous nanoparticles[40]. Additionally, As shown in Figure 2G, McGee et al. synthesized metastable and mixed transition metal oxide materials directly from solution precursors using a simple laser-induced thermal voxel method[41]. The resultant materials deposited from the single, binary, and ternary combinations of precursors were analyzed with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy [Figure 2H]. This laser processing method enhanced catalytic activity by generating distorted active sites at the interface between crystal nanoparticles and amorphous substrates, thus reducing overpotential and improving OER performance[41].
Adjusting laser parameters can enhance hybrid microstrain and lattice strain, introducing structural irregularities and local defects[42]. Li et al. utilize instantaneously formed femtosecond laser double pulse sequences to maximize the proportion of MoV defective substances, thus improving catalytic activity[43]. Budiyanto et al. conducted molecular dynamics simulations of the Co3O4/water interface at different temperatures[44]. They revealed stronger interactions between the reconstructed Co3O4(001) surface and water/hydroxide molecules, resulting in more Co2+ pushing up from the surface into the tetrahedral coordination. The formation of amorphous cobalt hydroxide precursors potentially plays a crucial role in the formation of active sites during OER.
Others
In addition to the methods mentioned above, there are many ways to synthesize amorphous materials, such as electrochemical synthesis, electrospinning process[45], reactive magnetron sputtering deposition and physical approaches.
Reactive magnetron sputtering deposition allows for highly controllable catalyst morphology and composition by adjusting deposition parameters and target materials. However, this method requires specialized equipment and conditions, resulting in higher costs. At room temperature, reactive magnetron sputtering deposition increases the number of crystal defects and sulfur defects of MoSx, leading to the discovery of coordinated unsaturated Mo atoms at the edge of the S-Mo-S layer. These crystal defects and sulfur defects produce coordination-unsaturated atoms. A reaction mechanism using Mo atoms with overhanging bonds as the catalytic active site was proposed by Xi et al.[46].
In physical methods of synthesis, chemical reactions are not involved, which helps maintain the purity and stability of the catalyst. However, for certain complex catalyst structures, physical methods may be infeasible. For the physical strategy, the mechanism of amorphization is that the crystal lattice will be broken by the application of external stresses (e.g., mechanical force, irradiation, and heat). Therefore, the material will no longer have a long-range order, but a short-range order of amorphous. High pressures and mechanical processes cause the crystal structure to collapse to form an amorphous structure[47,48]. Low-temperature irradiation such as X-ray photooxidation and microwave-induced thermal agitation also produce amorphous processes[49-51].
Anchoring sites of single-atom
While various methods can be employed to anchor single atoms onto different amorphous substrates, a significant challenge persists: the high surface energy of loaded single atoms often leads to their agglomeration, forming nanoclusters and nanoparticles during catalysis. To address this issue and design stable single-atom electrocatalysts, a comprehensive understanding of the interaction between anchoring position and single atoms is imperative. Anchoring positions for single atoms primarily include heteroatoms, defects, and metal atoms within the substrate. Therefore, in the subsequent chapters, we will systematically outline the anchoring mechanisms of single atoms on various types of anchoring sites, including heteroatoms, defects, metal atoms, and others present in the substrate. Additionally, we will highlight some of the advantages offered by amorphous substrates in facilitating the anchoring of single atoms.
Doped heteroatoms
Heteroatoms, such as N, O, S, and P, are commonly employed for anchoring single atoms in catalysis[52]. Carbon-based materials such as graphene, featuring atomically dispersed TM-Nx sites, offer a promising alternative to noble metal oxygen reduction catalysts. These catalysts typically coordinate metal single-atom sites with heteroatoms through pyrolysis, providing a flexible and adjustable coordination environment. This approach enables fine-tuning of single-atom charge density, leading to significant advancements in single[53] and multi-heteroatom doping[54,55] and multi-metal active sites[56-58]. For instance, Zhao et al. employed a misplaced deposition strategy to anchor Co atoms with nitrogen[58], resulting in the formation of Dual-Metal Hetero-Single-Atoms [Figure 3A]. X-ray absorption fine structure (XAFS) analysis revealed that Co coordinated with four nitrogen atoms, forming CoN4 sites [Figure 3B and C]. This active site was incorporated into CuC4@HC, leading to a significantly polarized surface charge distribution [Figure 3D] and enhanced catalytic activity for HER.
Figure 3. Single atoms are anchored at doped heteroatoms(A-D), defect sites(E-G) and metal atoms(H-K): (A) anchoring diagram of CuC4/CoN4@HC. (B) FT k3-weighted EXAFS spectra for the Co K-edge of CuC4/CoN4@HC, Co foil, CoO, and Co3O4. (C) Model of CoN4 sites and fitting curves of CuC4/CoN4@HC. (D) Top view of the differential charge densities of CuC4/CoN4@HC. Reproduced with permission from Ref.[58] Copyright 2021, American Chemical Society. (E) The electron density differences in the C-MoS2, the structure model of C-M-Mo. (F and G) AC TEM images of C-Co-MoS2. Reproduced with permission from Ref.[63] Copyright 2023, Wiley-VCH. (H) The model structure of Fe-Rh interbonds. Reproduced with permission from Ref.[67] Copyright 2020, Wiley-VCH. (I) Plot of the H adsorption free energy versus η10. (J and K) The differential charge densities of Sn-MoSxOy (J), RhSn-MoSxOy (K). Reproduced with permission from Ref.[68] Copyright 2022, Wiley-VCH.
Defect sites
When employing defect engineering to design SACs, the defect structure on the substrate serves to capture single atoms and stabilize them through charge transfer between defects and single atoms. Therefore, under-coordinated anionic defects[59,60] and cationic defects[61,62] in the catalytic material can serve as anchor sites for single atoms. S and O vacancies are common anionic defects used in single-atom anchoring.
Compared to crystalline substrates, amorphous substrates offer significant advantages for anchoring single atoms due to their abundance of intrinsic defects. Zhang et al. demonstrated this by anchoring single Pt atoms and clusters in amorphous Fe5Ni4S8[65]. The presence of numerous defects in the substrate facilitated the capture of highly active single atoms. This resulted in efficient catalytic performance, with overpotentials of 30 mV under acidic conditions, 65 mV under alkaline conditions, and 98 mV under neutral conditions at a current density of 10 mA cm-2.
Metal atoms
Single atoms can be anchored by forming metal-metal (M-M) bonds with adjacent metal atoms. Metal atoms can serve as anchor sites for another single metal atom loaded onto the substrate. This approach has been reported in several studies and offers a unique strategy for anchoring single atoms on various substrates. Wang et al. demonstrated this method by introducing Pt atoms onto the surface of commercial CeO2, utilizing them to anchor inert Fe atoms nearby[66]. Their results revealed electron transfer between Fe and Pt, with the presence of adjacent Fe atoms reducing the electron density of Pt, thereby activating reverse water-gas shift reaction performance. Our group's related research also employed M-M bonds to anchor a series of single atoms. For instance, we converted Fe nanoparticles at low temperatures through Fe-Rh interbonds [Figure 3H], resulting in FeRh dual-atom catalysts supported on nitrogen-doped hollow carbon spheres[67]. This catalyst exhibited superior HER performance. Amorphous materials, characterized by short-range ordered structures, offer highly exposed surfaces conducive to metal atom proximity and active site exposure. Hence, we synthesized a Rh-Sn coordinated high-efficiency HER catalyst on an amorphous molybdenum oxysulfide substrate[68]. Calculation results revealed a synergistic effect between Rh and Sn, where Rh transferred the attack site of H to Rh through electron interaction with Sn and apical S, thereby altering hydrogen adsorption strength and achieving low overpotential HER [Figure 3I-K].
Anchoring single atoms can also involve utilizing a metal array. Evenly distributing single atoms on the metal array to create a single-atom alloy structure serves as an efficient anchoring method. This approach is widely adopted, and recent studies[69-73] have focused on synthesizing single-atom alloys through M-M bonds and applying them across various fields.
Others
In addition to the above anchoring mechanisms, single atoms can also be anchored by bonding with non-metallic atoms on the surface of the material[60,74,75] or by utilizing specific substrates to constrain their spatial arrangement[76]. For instance, Jiang et al. demonstrated the assembly of Ni and Fe single atoms within the interlayer of MoS2[77]. This interlayer constraint structure provides protection to single atoms while enhancing the overall water splitting activity. Moreover, when the confinement occurs at the interface between crystal and amorphous materials, it can promote the formation of heterogeneous interfaces and the introduction of single atoms simultaneously, further enhancing the catalytic performance of the catalyst. For example, Xi et al. synthesized Pd SACs dispersed at the interface of graphene oxide and amorphous carbon[78]. By loading single-atom Pd on the constrained interface, they achieved catalysts with exceptionally high activity and stability.
Characterization techniques of amorphous single-atom catalyst
The characterization techniques for crystalline SACs have been extensively reported[79-81]. However, the complex composition of amorphous substrates poses significant challenges for accurate characterization. Therefore, summarizing the characterization methods for amorphous SACs is essential.
X-ray diffraction (XRD) is a widely used characterization method. The spacing between crystals is generally close to the wavelength of X-rays, so when the material is irradiated by X-rays, diffraction occurs. The superposition of diffraction waves strengthens or weakens the rays in certain directions, allowing the analysis of the crystal structure based on diffraction results at different angles. Crystalline materials have a long-range order with atoms arranged periodically, resulting in clear diffraction peaks. In contrast, amorphous materials lack a periodic structure and exhibit a broad peak at a lower diffraction angle, which is a key indicator of an amorphous structure. Conventional XRD analysis cannot accurately characterize amorphous materials. However, it is feasible to analyze the short-range ordered structure using the Pair Distribution Function (PDF). The PDF can be obtained through the Fourier transform of the scattering pattern[82]. The intensity of the diffuse scattering may be related to the short-range structure of the material. For amorphous materials, the measured PDF typically represents the average of all variations. This information can be processed using modeling methods to determine the possible distribution of structural properties within the material.
Transmission electron microscopy (TEM) serves as a crucial tool in the field of electrocatalysis for examining the microstructure of materials. By irradiating the sample with an electron beam, TEM enables the acquisition of high-resolution images through the analysis of phase and interference information transmitted by electrons. TEM proves adept at identifying amorphous materials by analyzing diffraction patterns, density distributions, atomic alignment, and high-resolution microscope images. Due to the disordered atomic arrangement of amorphous materials, it is common for TEM diffraction patterns to exhibit continuous annular or diffuse spots instead of clear lattice diffraction points. These irregular diffraction spots reflect the disorder and short-range order inherent in amorphous materials. The density distribution observed in TEM images of amorphous samples typically exhibits homogeneity or irregularity. In contrast to crystals, where atoms exhibit significant periodicity, amorphous materials lack a well-defined periodic structure in their density distribution due to the absence of ordered atomic arrangements. Furthermore, in high-resolution TEM images, the atomic arrangement of amorphous samples tends to appear random or irregular. Although localized order may be present to some extent, the overall atomic arrangement remains disordered and defies description through lattice constants or identifiable lattice points.
In addition to the commonly used methods mentioned above, several techniques have been employed to describe the precise structure of amorphous substrates[21]. Raman spectroscopy, scanning TEM (STEM), nanobeam electron diffraction (NBED), and XAFS are utilized to reveal the true geometric structure of amorphous catalysts. These methods provide a foundation for the accurate characterization of amorphous SACs.
CHARACTERISTICS AND MECHANISM
The design of efficient electrocatalysts has become an important part of the exploration of sustainable energy technology. A comprehensive understanding of the structure-activity relationship and reaction mechanisms is crucial for designing rational and efficient catalysts. The structure-function relationship of amorphous materials and the significance of the structure of anchored single atoms on amorphous materials in specific catalysis are elucidated from the perspective of physical properties and catalytic mechanisms.
Physical characteristics
The distinct physical properties of amorphous materials profoundly influence electrocatalysis performance, underscoring the importance of comprehending the internal relationship between these properties and electrocatalytic reaction mechanisms. Amorphous materials typically exhibit a larger specific surface area compared to other materials, providing an expanded reaction space and more pore structure conducive to catalytic reactions. This loose and porous structure facilitates the diffusion of reactants and products between catalytically active sites, thereby enhancing mass transfer in the electrocatalytic process and improving reaction kinetics.
The inherent complexity of amorphous materials enables a more flexible and easily adjustable composition, allowing for the regulation of adsorption and desorption strengths of reactant molecules on the material surface. By manipulating specific components within the material, specific reaction pathways can be promoted, reaction intermediates can be stabilized, and catalytic activity can be enhanced. The abundant dangling bonds and coordination unsaturated atoms of amorphous materials are also related to their own flexible composition, so that the active sites can be better exposed from the bulk phase, rather than the crystal can only be exposed from the surface. Klingan et al. observed that the turnover frequency of Co active sites in the bulk phase of amorphous materials closely resembles that of surface activity, leading to their proposition that the OER activity of amorphous bulk materials is more aptly described by volume activity[83].
Moreover, the degree of disorder in materials often correlates with their electrical properties, where microscopic disordered structures can influence macroscopic electrical properties. Tian et al. synthesized two-dimensional (2D) amorphous carbon at varying temperatures to investigate the impact of structural disorder on material properties[84]. Utilizing a 2D system simplified complexity, allowing for a closer examination of structural issues. Atomic-resolution electron microscopy revealed the presence or absence of medium-range order and temperature-dependent densities of nanocrystallites, serving as proposed order parameters to comprehensively describe disorder and correlate microstructure with macroscopic electrical properties. Since electron conduction is integral to the electrocatalytic process, the influence of micro-disordered structure on the conductivity of amorphous materials can affect efficient electron conduction and subsequently influence catalytic processes. In 2018, Su et al. observed continuous improvement in the catalytic performance of Ni-Fe Prussian blue analogs, accompanied by the formation of an amorphous phase during the OER[85]. Theoretical calculations suggested that the formation of the amorphous phase increased the component of O 2p near the Fermi level and reduced the band gap, promoting improvements in kinetics.
Reaction mechanisms
Amorphous materials can adsorb multiple reactant molecules simultaneously, thereby facilitating catalytic reactions. The absence of long-range ordered structure leads to the presence of numerous defects and active sites on their surfaces, promoting the adsorption of reactant molecules. Duan et al. observed that amorphous NiFeMo oxides synthesized exhibited a more rapid surface self-rebuilding process compared to crystals[86]. This rapid evolution led to a significant loss of Mo cations, as depicted in Figure 4A and B, resulting in the generation of abundant oxygen vacancies. Further insights were obtained through DFT calculations aimed at elucidating the role of oxygen vacancies in facilitating the OER process. As shown in Figure 4C, the presence of oxygen vacancies in the structure decreased the ΔGH* value associated with the release of O2 by *OOH intermediates at the Fe site. These findings suggest that such vacancies promote weak *OOH adsorption, thereby enhancing the OER activity. Numerous experimental and theoretical studies have demonstrated that the surface reaction proceeds through the initial activation of the CO2 molecule, followed by the conversion of the intermediate *CO to subsequent hydrogenated species via proton-electron transfer. Figure 4D illustrates the CO2RR process facilitated by high-altitude InOx nanorods (H-InOx NRs), as prepared by Zhang et al. via a straightforward heat treatment method[87]. CO2 molecules undergo vigorous adsorption and activation upon interaction with H-InOx NRs, facilitated by the introduction of oxygen vacancies. Simultaneously, electrons generated by the oxygen vacancies facilitate charge transfer in subsequent reactions. Consequently, the combination of strong adsorption, effective activation, and efficient charge transfer mechanisms contributes to the favorable selectivity observed in
Figure 4. The impact of amorphous materials and synergistic monoatomic components on enhancing substance adsorption. STEM images of a-NiFeMo (A) and c-NiFeMo (B) after OER, respectively. Insets in (A and B) show corresponding EDX line scan profiles. (C)
When a single atom lacks surrounding atoms, its surface reactivity increases, facilitating effective adsorption of reactant molecules. This adsorption capacity enhances interaction with the catalyst surface, thereby boosting the reaction rate[88]. Zhang et al. discovered that C60-modified Pt single-atom sites enhance water adsorption and hydrogen desorption, representing the optimal active sites within Pt/C60[89]. Numerous Single-Atom Catalysts have demonstrated remarkable selectivity towards the reduction of CO2 over the competing HER. This preference is attributed to the favorable adsorption of carboxyl (*COOH) or formate (*OCHO) intermediates over hydrogen (*H) on these catalysts[90]. Cheng et al. noted that Pt single atoms supported on nitrogen-doped graphene nanosheets acquire a positive charge[91]. During the reaction, two hydrogen atoms bond to the Pt. Bader charge analysis reveals a significant overlap between Pt 5d and H 1s orbitals, generating numerous new states beyond the Fermi level. This indicates charge transfer from Pt to H during the pivotal proton reduction process.
Catalysts formed by anchoring single atoms onto amorphous materials offer several advantages over pure amorphous materials or SACs. Firstly, amorphous materials provide abundant active sites and surface defects, facilitating the adsorption of intermediate molecules and promoting catalytic reactions. Secondly, anchoring single atoms enhances the stability of active sites, reducing the possibility of uncontrolled deactivation and thus extending the catalyst's lifespan. Additionally, the structure of amorphous materials typically exhibits higher surface area and porosity, facilitating the diffusion rate of intermediate molecules and thereby enhancing the rate and efficiency of catalytic reactions. Through the rational design of the interaction between single atoms and amorphous materials, precise control over the adsorption and desorption processes of substances can be achieved, thereby enhancing the performance and efficiency of catalysts. Zhang et al. discovered that amorphous TiO2 nanofibers, as opposed to vacancy-engineered TiO2 nanocrystals, possess abundant intrinsic oxygen vacancies and dangling bonds in nature[92]. Utilizing the confinement effect of oxygen vacancies, well-isolated Ga single atoms were successfully synthesized. These Ga single atoms promote the selective adsorption of N2 on the catalyst surface, while oxygen vacancies facilitate the subsequent activation and reduction of N2 [Figure 4E]. Chen et al. exploited the synergistic effect of strong binding and stabilization of single Pt atoms along with strong metal-support interaction between the in situ anchored Pt atoms and Fe2O3 to reduce CO adsorption[93]. Furthermore, the presence of distorted amorphous Fe2O3 with oxygen vacancies facilitates O2 activation, thereby enhancing CO oxidation on nearby Pt sites or the interface between Pt and Fe2O3. This leads to an atomic catalyst with exceptionally high CO oxidation activity.
INTERACTIONS BETWEEN AMORPHOUS SUBSTRATES AND SINGLE ATOMS
The amorphous substrate presents numerous defects, unsaturated bonds, and dangling bonds, offering significant advantages in capturing single atoms to enhance active sites and prevent the agglomeration. Additionally, in contrast to crystals, amorphous substrates afford a more flexible coordination environment for single atoms, facilitating the synergistic activation of various sites and structures and the optimization of catalytic center electronic configurations. In order to further study the interaction between single atoms and amorphous substrates, the subsequent sections will primarily summarize and propose corresponding adjustment strategies from four perspectives: increasing active sites, preventing single-atom agglomeration, fostering multi-component collaboration, and optimizing electronic structure [Table 1]. It is important to note that while these classifications are not entirely mutually exclusive, detailed categorization is necessary for a better comprehension of the interaction between single atoms and amorphous substrates.
Summary table of interaction mechanism
| Electrocatalyst | Cooperative mode | Electrolyte | Application | Performance | Ref. |
| Ir1/CoOOH(F) | Increasing active sites | 1.0 M KOH | OER | η10 = 238 mV | [94] |
| In1/a-MoO3 | Increasing active sites | 0.5 M Na2SO4 | NORR | NH3 yield rate = 242.6 μmol h-1 cm-2 FE = 92.8% | [95] |
| Pt1/MoO3-x | Increasing active sites | 0.5 M H2SO4 | HER | η10 = 23.3 mV | [96] |
| Pt-Fe5Ni4S8 | Increasing active sites | 0.5 M H2SO4 | HER | η10 = 30 mV | [65] |
| 1.0 M KOH | η10 = 65 mV | ||||
| 1.0 M phosphate buffer solution (PBS) | η10 = 98 mV | ||||
| Ru SAs-MoO3-x/NF | Increasing active sites | 1.0 M KOH | OER | η10 = 209 mV | [98] |
| 1.0 M KOH | HER | η10 = 36 mV | |||
| 1 M KOH seawater | OER | η100 = 539 mV | |||
| h-Pt1-CuSx | Increasing active sites | O2-saturated 0.5 M HClO4 | ORR | H2O2 productivity = 546 ± 30 mol kgcat-1 h-1 FE = 92%-96% | [97] |
| PBN-300-Ir | Preventing agglomeration | 0.5 M H2SO4 | HER | η10 = 17 mV | [99] |
| SA-Pt/WO3 | Preventing agglomeration | 0.1 M KOH | ORR | / | [100] |
| Ru SAs@PN | Preventing agglomeration | 0.5 M H2SO4 | HER | η10 = 24 mV | [101] |
| Rh-SACs/HNCR | Preventing agglomeration | N2-saturated 0.5 M H2SO4 + 0.5 M HCOOH | FAOR | jp/jn = 2.10 | [102] |
| IrCoOx ANSs | Multi-component collaboration | 1.0 M KOH | OER | η10 = 152 ± 5.2 mV | [103] |
| NiFeIrx/Ni NW@NSs | Multi-component collaboration | 1.0 M KOH | OER | η10 = 200 mV η100 = 250 mV | [104] |
| Pt/NiRu-OH | Multi-component collaboration | 1.0 M KOH | HER | η10 = 38 mV | [105] |
| Fe/SAs@Mo-based-HNSs | Multi-component collaboration | 1.0 M KOH | HER | η10 = 38.5 mV | [106] |
| Fe1-N-NG/RGO | Multi-component collaboration | O2 saturated 0.1 M HClO4 | ORR | E1/2 = 0.84 V | [107] |
| Pt/TiBxOy | Optimizing electronic structure | H2-saturated 0.5 M H2SO4 | HER | Mass activity = 37.8 A mgPt-1 η = 50 mV | [109] |
| Pt-SA/a-MoOx | Optimizing electronic structure | 0.5 M H2SO4 | HER | η10 = 19 mV | [110] |
| Ru SAs/AC-FeCoNi | Optimizing electronic structure | O2-saturated 1 M KOH solutions | OER | η10 = 205 mV | [111] |
| Ru-a-CoNi | Optimizing electronic structure | 1.0 M KOH | HER | η10 = 15 mV | [20] |
| amorphous Fe-ZrP | Optimizing electronic structure | 1.0 M KOH | OER | η10 = 209 mV | [112] |
| Sb1/a-MoO3 | Optimizing electronic structure | NO saturated 0.5 M Na2SO4 | NORR | NH3 yield rate = 273.5 μmol h-1 cm-2 FE = 91.7% | [113] |
Increasing active sites
An increase in the density of single atoms anchored on suitable substrate materials can enhance the number of active sites for electrocatalytic reactions, thus leading to improved catalytic effects. However, achieving higher loading of single atoms necessitates the provision of a large number of anchoring sites by the substrate, which poses a significant challenge for crystalline materials.
Amorphous materials, lacking the long-range ordered structures found in crystalline materials, offer a large number of defects and unsaturated structures, facilitating the anchoring of single atoms with high content. For instance, during the co-precipitation process, the addition of F ions can promote the transformation of Co(OH)x to CoOOH, resulting in the formation of an amorphous structure with an edge surface[94]. HRTEM images demonstrate that the incorporation of F ions benefits the transformation of the structure, leading to the formation of a wider amorphous boundary [Figure 5A and B]. This wider boundary, rich in oxygen vacancies, provides numerous anchoring sites for Ir single atoms. HAADF-STEM images further confirm that the number of Ir single atoms anchored on the highly amorphous boundary is significantly larger
Figure 5. The amorphous substrates to increase active sites and prevent the agglomeration of single atoms: (A) HRTEM image of
Moreover, compared with crystalline materials, amorphous materials with more defects can also serve as additional active sites for synergistic single-atom catalysis. For example, Feng et al. combined Ru single atoms with amorphous MoO3 substrates to achieve low-energy water splitting[98]. The presence of numerous oxygen vacancies on the amorphous substrate enables the uniform dispersion and anchoring of Ru single atoms. Electrochemical tests reveal that the amorphous SAC exhibits a larger electrochemical active surface area than the crystal SAC, providing more active sites for the reaction. Additionally, Zhang et al. anchored a single Pt atom and cluster in Fe5Ni4S8, observing that as the ball milling time increased, the defects in
Preventing agglomeration
The defects in the amorphous substrate not only provide a large number of active sites, but also promote the stronger anchoring of single atoms, thereby preventing the occurrence of agglomeration. Inspired by the trapping of single atoms via nanometer defects through C-C bond reconstruction, Liu et al. synthesized a series of amorphous carbon-supported SACs. These catalysts feature a self-healing 2D conjugated carbon framework and C-M bond, augmenting the stability of single atoms[99]. Zhang et al. prepared Pt SAC
Moreover, amorphous substrates can utilize spatial constraints to achieve stronger anchoring of single atoms. For instance, Xi et al. initially employed theoretical calculations to predict that the graphene/amorphous carbon structure could form a stronger bond with Pd[78]. Subsequently, they synthesized Pd SACs dispersed at the interface between graphene oxide and amorphous carbon based on theoretical guidance. Hu et al. utilized the hollow structure and effective carbon confinement of carbon nanorods to constrain Rh single atoms[102]. Test results demonstrated that no CO oxidation current was detected in the CO stripping curves [Figure 5I], indicating the construction of a solid single-atom site and thereby circumventing the co-conformation dehydration pathway in the electrooxidation of formic acid [Figure 5J].
Multi-component synergy
The catalytic effect of an electrocatalyst can be adjusted by altering the chemical state of the electrode surface, the atomic arrangement structure, and other factors. Several studies have demonstrated that the amorphous substrate facilitates changes in the chemical state of the electrode surface and enables multi-site synergistic catalysis. For instance, amorphous CoOx nanosheets, with their large surface area and high chemical activity, are capable of adsorbing numerous oxygen atoms on the surface[103]. The distributed oxygen atoms enable the capture of Ir single atoms through strong bonds. The introduction of single Ir atoms induces a synergistic effect with Co atoms in the substrate. Due to the disparate electron affinities of Ir and Co, high-concentration electrons migrate to the Ir site, promoting the adsorption of intermediates on the Ir site, thereby altering the adsorption Gibbs energy difference of different intermediates and achieving ultra-high activity oxygen evolution. Luo et al. also demonstrated the synergistic effect between Ir single atoms and Ni elements in amorphous substrates on OER[104]. The synthesized three-dimensional NiFeIr ternary amorphous catalyst exhibits unique morphology and ternary composition, providing many advantages in catalytic activity. In addition to the catalytic activity of Ir single atoms, the Ni-based shell structure is also the OER active center, and the synergistic effect between different catalytic centers promotes the improvement of OER performance. Insights from Li et al. shed light on the understanding of multi-element synergistic catalysis[105]. They synthesized amorphous NiRu-LDH anchored Pt SAC, which benefits from the amorphous structure, featuring both NiRu-OH sites for hydrolysis dissociation and single Pt atom sites for hydrogen recombination. The synergy of multiple sites promotes efficient HER in alkaline media.
The characteristics of the amorphous substrate dictate the absence of long-range ordered structure, yet it exhibits short-range order. This short-range order extends only a few atomic distances and does not repeat periodically throughout the material. This flexible short-range ordered structure enables the regulation of catalytic activity by various structures. Lyu et al. fabricated 2D wrinkle-like iron-phosphomolybdic acid mineral hydrogel nanosheets with an amorphous layered structure[106]. Subsequently, they synthesized carbon-free single Fe-atom-dispersed heterostructured Mo-based nanosheets (Fe/SAs@Mo-based-HNSs) via phosphating. Fe/SAs@Mo-based-HNSs possess Fe single-atom sites and multiple heterostructures (MoP, MoP2, MoO2), prompting the authors to establish various models to calculate the reaction mechanism. The calculation results demonstrate that both single-phase and heterostructure interface models promote water decomposition (Volmer step). For the Heyrovsky step, except for MoP/MoP2, the free energy of hydrogen adsorption in the heterostructure interface and the single-atom dispersion model is superior to that in the single-phase model. Even the
Figure 6. Amorphous substrates-assisted multi-component synergy and electronic structure optimization: (A) Representative atomic configurations after H*adsorption at the surface sites of MoP/MoP2, MoP/MoO2, MoP2/MoO2, Fe@MoO2-1, and Fe@MoO2-2, with corresponding ΔGH*. Reproduced with permission from Ref.[106] Copyright 2022, Springer Nature. (B) Atomic ratio of O/Ti on Ir1O6 after different etching times. (C) Atomic ratio of O/Ti on Ir1O4 after different etching times. Reproduced with permission from
Optimizing electronic structure
Amorphous substrates exhibit remarkable potential in adjusting the electronic structure of catalysts and enhancing electronic metal-support interactions (EMSIs). Defects and special structures on amorphous substrates can readily induce changes in the electronic structure of single atoms. Inspired by the ligand charge donation-acquisition balance strategy, Cheng et al. reported a Pt SAC anchored on amorphous
In amorphous substrates, structural disorder leads to a wide distribution of electron density of states, promoting orbital coupling and adjusting the position of the d-band center. This affects the adsorption energy of the intermediate, thereby influencing catalytic activity. Liu et al. prepared Ru single atoms uniformly dispersed on amorphous cobalt/nickel (oxy) hydroxide, resulting in different properties compared to Ru single atoms dispersed on crystal substrates[20]. High amorphization of the substrate induces orbital coupling and promotes electron transfer from Ru to nearby Co/Ni/O centers, enhancing water dissociation ability and regulating electron distribution of the local configuration [Figure 6E]. This reduces the oxygen affinity of the metal site and the affinity of the O site to H*, greatly promoting HER kinetics. Wang et al. introduced a large number of Fe atoms on amorphous Zr(HPO4)2, where the Fe atoms dispersed on the surface of amorphous Zr(HPO4)2 have adjustable electron density[112]. Calculations show that the amorphous catalyst has a wider state density distribution and a lower d-band center, reducing the adsorption strength of oxygen-containing intermediates and improving OER performance. Chen et al. calculated the electronic structure of an amorphous MoO3 catalyst loaded with Sb single atoms, revealing a wider density of state distribution in amorphous MoO3[113]. This results in strong electronic interaction between Sb single atoms and amorphous MoO3, leading to a smaller band gap and stronger occupied electronic state in Sb1/a-MoO3 [Figure 6F], accelerating electron transfer in nitric oxide electroreduction to ammonia process.
In summary, the interaction between the amorphous substrate and single atoms enhances the electrocatalytic reaction. These interactions distinguish amorphous SACs from crystalline SACs. Due to the higher defect density and disordered structural composition of the amorphous substrate, amorphous SACs possess various active sites, including single atoms and defects, and a range of heterostructures. This diversity aids in the adsorption and desorption of different reaction intermediates, providing more opportunities for optimizing the adsorption energy regulation of electrocatalytic reaction intermediates. Furthermore, because single atoms are more likely to interact with unique structures in amorphous substrates, amorphous SACs have advantages in adjusting the charge density of single atoms, modifying the energy band structure, and promoting orbital coupling compared to crystalline SACs [Figure 7]. Based on these characteristics and advantages, amorphous SACs are expected to see broader applications in the field of electrocatalysis in the future.
Figure 7. Summary of interactions between amorphous substrates and single atoms.
CONCLUSION AND OUTLOOK
Electrocatalysis offers a promising avenue for converting intermittent green energy sources such as solar and wind power into sustainable energy, laying the groundwork for robust energy storage and conversion systems. Amorphous materials, characterized by intrinsic defects and excellent catalytic performance, have garnered significant attention in electrocatalysis research. However, there remains a dearth of studies on SACs supported on amorphous substrates within this field. This review begins by summarizing the preparation techniques for amorphous substrates and elucidates the anchoring mechanisms and sites for single atoms. Subsequently, it highlights the interactions between single atoms and amorphous substrates based on existing literature. Compared to crystalline substrates, amorphous substrates offer distinct advantages, providing more anchoring sites for single atoms, preventing single-atom agglomeration, and fully exploiting the multi-component collaboration and electronic effects between the substrate and single atoms. Consequently, SACs supported on amorphous substrates often exhibit superior electrocatalytic performance. Despite the notable progress in this area, several challenges persist:
(1) Large-scale synthesis methods: Existing synthesis approaches for amorphous SACs are often inefficient in preparing amorphous substrates and anchoring single atoms. Simplified and universally applicable preparation methods are essential to meet the catalyst demands in practical applications.
(2) Accurate structural characterization methods: The complex local structure of amorphous materials necessitates more precise and comprehensive characterization techniques to delve deeper into the binding modes between single atoms and amorphous substrates and understand the precise local geometric structures of amorphous SACs. In addition, high facet SACs have abundant dislocations and defects, similar to amorphous SACs, which prevent single-atom aggregation and adjust the electronic structure. These properties need more comprehensive study. In the future, we can leverage the easier characterization of high facet SACs to thoroughly investigate the interaction mechanisms between high defect content substrates and single atoms. This research will further reveal the interaction mechanisms between amorphous substrates and single atoms.
(3) In-situ structural transformation characterization: Amorphous SACs are susceptible to chemical structural transformations during reaction processes. In-situ techniques are required to monitor and elucidate the mechanisms of structural transformations.
(4) Improved calculation methods: The lack of long-range ordered structures in amorphous materials poses challenges in accurately modeling and calculating amorphous SACs. Hence, there is a pressing need for more accurate and rational calculation methods to analyze amorphous SACs and comprehend reaction mechanisms in detail.
Addressing these challenges will pave the way for further advancements in the development of amorphous SACs, leveraging their ultra-high atom utilization efficiency and the intrinsic activity of amorphous catalysts to create a range of catalyst families suitable for electrocatalysis and beyond.
DECLARATIONS
Authors’ contributions
Conceived the review and wrote the manuscript: Zhou Y
Wrote the manuscript: Liu C, Cui Y
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was financially supported by the “Teli Young Scholar” Program, Technology Innovation Program of Beijing Institute of Technology, “Xiaomi Scholar” Program, “Langyue” Program, and Beijing Municipal Natural Science Foundation (No. 2232023).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
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Liu C, Cui Y, Zhou Y. The recent progress of single-atom catalysts on amorphous substrates for electrocatalysis. Energy Mater 2025;5:500001. http://dx.doi.org/10.20517/energymater.2024.41
AMA Style
Liu C, Cui Y, Zhou Y. The recent progress of single-atom catalysts on amorphous substrates for electrocatalysis. Energy Materials. 2025; 5(1):500001. http://dx.doi.org/10.20517/energymater.2024.41
Chicago/Turabian Style
Liu, Cheng’ao, Yanping Cui, and Yao Zhou. 2025. "The recent progress of single-atom catalysts on amorphous substrates for electrocatalysis" Energy Materials. 5, no.1: 500001. http://dx.doi.org/10.20517/energymater.2024.41
ACS Style
Liu, C.; Cui Y.; Zhou Y. The recent progress of single-atom catalysts on amorphous substrates for electrocatalysis. Energy. Mater. 2025, 5, 500001. http://dx.doi.org/10.20517/energymater.2024.41
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