Rapid Joule heating synthesis of metal single-atom materials: theory, device construction, and functional applications

Abstract

Joule heating (JH) synthesis has been widely applied to disperse atomic metals onto supports, thereby enhancing metal utilization efficiency and enabling precise control over the electronic structure at the atomic level. This method holds considerable promise for metal single-atom (SA) synthesis. This review systematically investigates and summarizes recent advancements in JH synthesis of metal SA-based functional materials. It begins by clarifying the JH fundamental principles, including the methodology for calculating heating temperatures. After concluding an overview on the development of the JH technique, it details the necessary equipment and systematically compares plate-type and tube-type heating apparatuses, highlighting their differences and respective application fields. The JH synthesis and traditional processes for SA materials synthesis are also compared to highlight the advantages of the JH technique. Furthermore, we conclude and compare various types of metal SA along with their corresponding JH synthesis parameters and diverse functional applications in the fields of catalysis and electromagnetic wave adsorption. Finally, we provide a brief conclusion and outlook and discuss emerging trends and challenges that could shape future research on metal SA-based functional materials by rapid JH synthesis.

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Xinxing Shi

Xinxing Shi received his bachelor's and master's degrees from Jilin University. He is currently pursuing a Ph.D. degree in Materials Science and Engineering at Jiamusi University and serves as a faculty member at Taizhou University. His research focuses on the design of advanced electrocatalysts and devices for energy conversion.

Zhenjie Cheng

Dr Zhenjie Cheng earned his bachelor's degree from Tianjin Chengjian University and his master's degree from the University of Science and Technology of China, followed by his Ph.D. degree in Inorganic Chemistry from Shandong University. His research focuses on the development of advanced catalysts and devices for CO2 electroreduction.

Peng Du

Dr Peng Du is currently a researcher at Taizhou University, Zhejiang, China. He received his Ph.D. degree in Electronic Science and Technology from Beijing University of Posts and Telecommunications in 2024. His research focuses on the rapid Joule-heating synthesis of advanced electrocatalytic electrodes for hydrogen production through water splitting and related industrial device applications.

Jiacheng Wang

Dr Jiacheng Wang (FRSC) is currently a full professor at Taizhou University, Zhejiang, China, and head of the Zhejiang Key Laboratory for Island Green Energy and New Materials. He obtained his Ph.D. from the Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS) in 2007. From 2013 to 2023, he was a professor at SICCAS. His present research focuses on the rational design and preparation of advanced functional materials for energy transformation and photoluminescence.

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1. Introduction

Fast Joule heating (JH) synthesis occupies a critical position in the preparation of metal single-atoms (SAs) that are extensively employed in various catalytic processes and electromagnetic applications, such as hydrogen/oxygen evolution,^1–9 oxygen reduction,^10–14 hydrazine oxidation,¹⁵ carbon dioxide reduction,^16,17 methane combustion,¹⁸ water oxidation,¹⁹ CO oxidation,²⁰ and electromagnetic wave absorption.²¹ In the process of making metal SAs through JH, a high-energy-density electrical pulse is transmitted through solid precursors, rapidly raising the sample temperature to above 3500 K at a rate exceeding 10⁵ K s⁻¹, followed by rapid cooling at a rate greater than 10⁴ K s⁻¹,^22–25 as shown in Fig. 1. The core mechanism underlying the preparation of materials with metal SA distribution via the JH process relies on the non-equilibrium thermodynamic process, atomic diffusion kinetics, and strong metal–support interaction (SMSI). JH can instantaneously generate temperatures >3500 K. Metal salts (e.g., nitrates, chlorates, acetates, and oxalates) absorb heat within milliseconds, overcoming lattice and chemical bond energies to pyrolyze into metal atomic vapors (e.g., Pt²⁺ → Pt⁰). Under equilibrium conditions, metal atoms tend to aggregate to minimize surface energy, forming particles. However, JH creates a non-equilibrium state, bypassing nucleation-growth stages and maintaining atoms in their isolated state. Upon rapid cooling at rates >10⁴ K s⁻¹, the kinetic energy of metal atoms decreases sharply, reducing the diffusion coefficient (D) and minimizing atomic collisions. Consequently, cluster formation via diffusion and aggregation is inhibited, resulting in the formation of metastable single-atom (SA) distribution rather than the thermodynamically stable particulate state. SMSI is a key mechanism for stabilizing single atoms and modulating catalytic activity. If carriers (e.g., C, SiO₂, and MOFs) are present in the system, during rapid cooling, the insufficient kinetic energy of metal atoms prevents their detachment from the carrier surface. Instead, they are captured by active sites on the carrier surface (e.g., defects and functional groups), forming a SA anchored structure.^26–31 Most Group VIII transition metals (e.g., Pt, Pd, Fe, Co, and Ni) can form SAs from their precursors via the JH mechanism. Pt and Pd, with high melting points and chemical stability, resist agglomeration at high temperatures. Fe, Co, and Ni stabilize in the SA state through interactions with carriers or ligands due to their unfilled d orbitals. However, forming stable SAs from metals (e.g., Na, K, Ca) and high-melting-point refractory metals (e.g., W, Mo) is more challenging. Alkali metals are highly reactive and easily react with surrounding substances during heating, preventing their existence as SAs. Refractory metals have strong interatomic bonding energies, making them difficult to disperse under JH. Additionally, their weak interactions with common carriers or ligands hinder the stable loading of SAs.^{9,10,17,18,20,21,32}

Fig. 1 Joule heating (JH) synthesis of metal single-atom (SA) materials: synthesis parameters, properties, and advanced functional applications.

In the field of ultrafast SA synthesis, in addition to the JH method, He et al. also reviewed microwave heating, solid-phase laser irradiation, flame-assisted synthesis, and arc-discharge methods. Compared with the JH method, the equipment required for solid-phase laser irradiation and arc-discharge methods is more complex. The flame-assisted synthesis process lacks controllability, while microwave heating necessitates special substrates that must meet several criteria: (a) abundant surface functional groups; (b) adequate electrical conductivity; and (c) high thermal conductivity.³¹ In contrast, the JH method offers advantages such as a controllable process, easy equipment, and low manufacturing costs.

As is well established, metal SA materials constitute a distinctive class of individual metal atoms anchored onto the surface of a support via coordination bonds. Due to the spatial isolation of these metal atoms on the support, the metal centers achieve uniform atomic-level dispersion.³³ The unique electronic properties, quantum size effects, and unsaturated coordination environment of the metal centers in SAs result in strong chemical bonding with the support and facilitate charge transfer, thereby conferring excellent catalytic activity and selectivity.^34,35 Moreover, metal SAs can be immobilized on non-metallic or non-noble metal supports, which increases the surface area-to-volume ratio and improves the utilization efficiency.^36,37 There are several methods for synthesizing metal SAs, including high-temperature vapor transport,³⁸ mass-separated soft landing,³⁹ deposition of organometallic complexes,⁴⁰ and co-precipitation.⁴¹ These technologies depend on sophisticated equipment and rigorous synthetic environments, and they also encounter challenges related to processing efficiency. Compared to the traditional methods, JH technology, characterized by its ultra-high reaction temperatures, ultra-short reaction times, ultra-low energy consumption, and controllable manufacturing process, exhibits remarkable effectiveness in fabricating metal SAs and holds significant potential for large-scale production.¹²

To date, several reviews have examined the characteristics, synthesis, and applications of metal SA-based materials.^{29,31,42–48} However, no comprehensive review has addressed the preparation and applications of SAs using the JH method. To achieve a more profound and comprehensive understanding of the advantages of the JH method in synthesizing metal SAs, this review aims to provide a detailed and qualitative reference for future research endeavors in fast JH synthesis of metal SAs for functional applications (Fig. 1). This review explores the fundamental principles of JH synthesis, tracing its evolution from early concepts to advanced applications. A comparative analysis between JH synthesis and traditional methods highlights several key distinctions. Additionally, this review examines the diverse types of metal SAs that can be synthesized using JH techniques, along with their respective application fields. Furthermore, we discuss the future challenges associated with the implementation of JH synthesis and propose potential research directions aimed at enhancing the rational design of metal SAs through the utilization of JH techniques.

2. Introduction of Joule heating

2.1 Theory and advancement timeline of Joule heating

In 1841, the British physicist James Prescott Joule discovered that an electric current passing through a conductor can generate heat, a phenomenon now referred to as Joule heating. The unit of energy associated with this effect is the Joule (J). According to Joule's Law, the heat (Q) generated in a current-carrying conductor is proportional to the square of the current (I), the resistance (R) of the conductor, and the duration time (t) of the current flow. The mathematical expression for Joule's law is Q = I²Rt, where the units for Q, I, R and t are joules (J), amperes (A), ohms (Ω), and seconds (s), respectively.⁴⁹

Joule's Law can be used to quantify the rate of Joule heating. For example, when considering the heating of graphite paper, it is important to note that all generated heat is exclusively directed towards elevating the temperature of the material. The rate of heating can be estimated by considering the mass and specific heat capacity of the carbon material. This relationship is expressed in eqn (1), which can be derived from eqn (2)–(7).

P = U × I (2)

U = R × I (3)

R = ρ × l/(w × h) (4)

G = ρ_density × l × w × h (5)

dQ = P × dt (6)

dT = dQ/(C_p × G) (7)

where dT/dt represents the heating rate (K s⁻¹). P denotes the heat power (W), C_p is the specific heat capacity (J K⁻¹ g⁻¹), G is the mass (g), U is the Voltage (V), ρ is the resistivity (Ω mm), ρ_density is the conductor density (g cm⁻³), l is the conductor length (mm), w is the conductor width (mm), and h is the conductor thickness (mm).

According to eqn (1)–(7), the approximate heating temperature at a given time can be calculated. Here, a graphite paper for Joule heating was used as a calculation example. It has a length (l) of 100 mm, a width (w) of 25 mm, and a thickness (h) of 0.2 mm. Its resistivity (ρ) ranges from 0.008 to 0.013 Ω mm, with an average value of 0.01 Ω mm. The specific heat capacity C_p is 0.71 J K⁻¹ g⁻¹ at room temperature and 0.5 J K⁻¹ g⁻¹ at 1273 K, averaging 0.6 K⁻¹ g⁻¹. The graphite paper density ρ_density is 1.2 g cm⁻³. Assuming an input current I of 70 A and a heating time t of 0.5 s, the heating temperature can be calculated. By substituting these values into eqn (1)–(7), it is determined that within 0.5 s, the temperature T of the graphite paper rapidly increases to 1361.11 K. Given that the amount of support material is fixed, the temperature can be adjusted by varying the current magnitude and heating duration to meet the activation energy requirements of different metal SAs.

Following the groundbreaking discovery of JH in 1841,⁴⁹ Tsong et al. reported that Eigen and de Maeyer introduced the pioneering technique of JH in 1963.⁵⁰ Due to its characteristics of ultra-high reaction temperatures and ultra-short reaction times, JH technology has been widely employed for the synthesis and modification of functional materials including various metal SAs (Fig. 2). In 1970, Tsong et al. further documented that Harms and Thalmann utilized JH to explore synthetic membranes and biofilm bilayer structures in medical research.⁵⁰ The rapid annealing process via JH (T ≈ 800 K, t ≈ 10 s) was first employed by Yavari et al. in 1987 to relax Fe–Si–B metallic glass materials, achieving an equivalent annealing effect as conventional furnaces at 655 K for 2 h under inert gas protection or vacuum conditions.⁵¹ In 2017, Li et al. pioneered the use of a rapid JH and cooling process (T ≈ 2400 K, t < 0.1 s) to synthesize carbon-coated nickel nanoparticles on a reduced graphene oxide support as an efficient H₂O₂ fuel catalyst for fuel cell applications.⁵² In 2018, Yao et al. employed JH technology (T ≈ 2000 K, t < 55 ms) to synthesize high-entropy alloy nanoparticles on carbon carriers for ammonia oxidation.⁵³

Fig. 2 Concise timeline of JH synthesis of metal-based functional materials from nanoparticles to metal SA-based materials.^{9,10,18,20,21,26,49–53} Since 2019, the JH technology has been broadly applied to synthesize Pt-, Co-, Pd-, and Fe-based SA materials.

To date, researchers have successfully developed a wide range of noble and non-noble metal SA materials. A first synthesis example of metal SAs using the JH method was conducted by Yao et al. in 2019. They successfully synthesized mono-atomic catalysts of Pt, Ru, and Co on various supports, including C, C₃N₄, and TiO₂, under conditions of T = 1 500–2000 K, and t < 55 ms.²⁶ These metal SAs were used as active catalysts for ammonia conversion and CO oxidation. In 2021, Jiang et al. used thermal-shock (T = 1503 K, t ≈ 500 ms) to synthesize a Pt SA catalyst on a CeO₂ support for low-temperature CO oxidation.²⁰ In 2021, Xing et al. developed a graphene oxide-supported Co single-atom catalyst (SACs) by JH within 2 s, which demonstrated remarkable efficiency in the hydrogen evolution reaction (HER).⁹ In 2023, Wang et al. employed JH to synthesize a Co SA absorber on ZIF-67 at 873 K, achieving tunable impedance characteristics for enhanced electromagnetic wave absorption.²¹ Also in 2023, Tian et al. fabricated a Pd SACs supported on CeO₂ to significantly improve methane combustion.¹⁸ In 2024, Liu et al. successfully synthesized Fe SA anchored on porous carbon spheres, thereby enhancing the performance of the oxygen reduction reaction (ORR).¹⁰ To date, a wide range of both noble and non-noble metal SA materials have been successfully developed. It is believed that more systems of metal SAs, dual atoms, and clusters could be prepared as advanced functional materials by the JH technology in the near future.

2.2 Structures of Joule heating devices

The JH devices primarily comprise display, control, and reaction systems, and a sintering table (Fig. 3a(1)). Fig. 3a(2) illustrates a schematic diagram of a JH device, which mainly includes a power supply, electrodes, conductive materials, reactants, and a switch. The switch regulates different states: the on-state of the pulsed current provides sufficient activation energy to achieve thermodynamic stability, thereby ensuring atomic dispersion. Meanwhile, the off-state plays a critical role in maintaining the stability of atomic sites and the substrate. Repeated high-temperature shockwaves generated by the on/off cycles can transform nanoparticles into stably dispersed individual atoms.^19,54

Fig. 3 (a) (1) Picture of a JH device.¹⁰ Copyright 2025, Elsevier. (2) Schematic of a JH device. (b) (1) Schematic of electrothermal equipment that can increase the temperature within 1 s.⁷ (2) Thermal plate in a JH device.¹⁹ Copyright 2022, Springer Nature. Copyright 2023, Elsevier. (c) (1) Schematic of a single tubular JH device.⁵⁷ (2) Flash JH quartz tubes with different sizes and shapes that applied to synthesize FG.⁵⁷ Copyright 2020, Nature Publishing Group. (d) (1) Schematic of a tube-in-tube JH device.⁵⁸ (2) Current (denoted in blue) and temperature profile (denoted in black) of a 340 V FWF reaction.⁵⁸ Copyright 2024, Nature Publishing Group.

Based on the geometric configuration of the reaction platform, JH devices are primarily classified into open plate-type and semi-enclosed tube-like designs. Each design possesses distinct characteristics and applications that render it suitable for various scientific processes. The open plate-type configuration is especially advantageous in scenarios where maximizing surface area is critical, such as catalytic reactions or heat exchange processes. The semi-enclosed tube design exhibits superior heat transfer performance and reliable sealing properties, making it particularly suitable for applications that demand efficient heat exchange and the containment of volatile gases.

2.2.1. Open plate-type JH devices. The open plate-type reaction platform can be further classified into two distinct categories (Fig. 3b). The first category involves direct heating of the conductive substrate, whereas the second entails transferring Joule heat to non-conductive materials for heating applications.

Fig. 3b(1) illustrates the method of direct heating of the conductive substrate, where the support impregnated with the precursor is in direct contact with the electrodes.⁷ Electrical current flows directly through the precursor-impregnated support, generating high temperature, which is termed the electrothermal flash reaction process.⁵⁵ This process facilitates the loading of atoms from the precursor onto the support, forming stable coordination bonds such as metal/carbon or metal/doped species (e.g., N, P, etc.). The materials for the support should be conductive; for example, carbon paper, carbon black, reduced graphene oxide, and carbon nanotube (CNT) films.^{8,9,17,26,52,56} Abdelhafiz et al. utilized the electrothermal flash reaction process to synthesize transition metal catalysts.⁸ They initially employed a drop-casting method to deposit multi-metal chloride salts, dissolved in ethanol at a concentration of 50 mM, onto a gas diffusion layer (GDL) composed of carbon fiber paper. After overnight solvent evaporation at room temperature, the GDL coated with metal precursors underwent rapid JH by applying an electrical current of 15 A for three pulses (each lasting 500 ms). This procedure elevated the GDL temperature to approximately 1573–1973 K, resulting in the formation of highly active and durable non-noble transition metal electrocatalysts for the oxygen evolution reaction (OER) in an alkaline aqueous medium. Li et al. also employed this method to synthesize a Mo₂C/MoC/CNT composite film serving as a highly efficient hydrogen evolution reaction (HER) electrode.⁷ The heating process took approximately 256 ms to reach a temperature of 1770 K from room temperature, followed by cooling to 600 K within 330 ms.

Compared to Fig. 3b(1) and 3b(2) demonstrates the transfer of Joule heat to non-conductive materials for heating applications, where the reactant is positioned on the surface of conductive substrates such as carbon paper, graphite paper, or carbon cloth.^18–21 When an electric current passes through the conductive material it generates high temperatures to heat the reactant, which is referred to as the purely thermal flash reaction process.⁵⁵ The reactant can be in the form of powders or films. For instance, Li et al. prepared a NaH₂PO₂-treated Fe₂O₃ photoanode by positioning it face down between two layers of graphite papers (100 mm in length, 25 mm in width, and 0.2 mm in thickness).¹⁹ These graphite papers were connected to the electrical contacts of JH equipment to fabricate a P-doped hematite photoanode. Eddy et al. compared electrothermal and purely thermal JH methods for graphene fabrication, demonstrating that passing an electric current directly through the reactant significantly facilitates the phase transition process, resulting in energy requirements approximately half that of the purely thermal process.⁵⁵

2.2.2. Semi-enclosed tube-like JH devices. The tube reaction platform for JH synthesis can also be categorized into two distinct types (Fig. 3c and d). The first type comprises a single tube, whereas the second type incorporates an additional tube, enhancing its sealing properties. This makes the second type more suitable for the synthesis of compounds that produce volatile gases.

The first type is designed for conductive powders as reactants. Conductive powders, such as carbon, are gently compacted within a quartz or ceramic tube positioned between two electrodes made of copper, graphite, or other refractory materials with excellent conductivity. These electrodes are loosely inserted into the quartz tube to facilitate gas release during JH. Upon passage of a high-voltage electric discharge, the powder undergoes an electrothermal reaction, experiencing a rapid temperature increase to over 3000 K within less than 100 ms, as illustrated in Fig. 3c(1).⁵⁷ The flash JH process can be effectively scaled up by increasing the diameter of the quartz tube, as shown in Fig. 3c(2). Using quartz tubes with diameters of 4 mm, 8 mm, and 15 mm, it was possible to synthesize batches of fluorine-doped graphene (FG) weighing 30 mg, 120 mg, and 1 g, respectively.

The second type of tubular reaction platform for JH synthesis is based on a tube-in-tube structure design, which comprises an outer flash tube packed with metallurgical coke and an inner semi-closed tube containing volatile reagents. This configuration is specifically designed for reactions involving nonconductive reactants or volatile substances such as sulfur and selenium (Fig. 3d(1)). This apparatus, referred to as a flash-within-flash (FWF) system, was developed by Choi et al.⁵⁸ The electric current flowing through the conductive feedstock in the outer tube generated temperatures reaching approximately 2273 K (Fig. 3d(2)). The intense heat produced in the outer tube was transferred to the inner tube via thermal conduction, enabling rapid heating of the inner reactant. Complete conversion of the reactants was achieved through consecutive flashing cycles, typically ranging from 2 to 5 cycles. This device has potential applications in producing phase-selective and single-crystalline bulk powders.

To achieve better atomic dispersion and higher loading capacity, JH equipment and processes must be adapted to different carriers. High-melting-point oxide carriers (e.g., ZrO₂, CeO₂, TiO₂) have high lattice energy and low thermal expansion, allowing them to withstand JH temperatures without decomposition. The SMSI effect stabilizes single atoms via electron transfer (e.g., hybridization of metal d orbitals with carrier oxygen p orbitals), preventing agglomeration. However, at high temperatures, these oxides may crystallize, reducing surface defects and anchoring sites. Acid etching (e.g., HCl treatment of Al₂O₃) can increase surface defects and hydroxyl density. Carbon materials (e.g., graphene and carbon nanotubes) offer high electrical conductivity for rapid Joule heating, fixing metal atoms and preventing agglomeration. However, at high temperatures (>1073 K), functional groups (e.g., hydroxyl and carboxyl) on carbon surfaces are removed, weakening chemical adsorption and hindering single-atom loading. Pre-oxidation (e.g., HNO₃ treatment) or plasma treatment introduces more oxygen-containing groups. MOFs and derivatives provide porous structures and metal nodes for pre-positioning atoms. Most MOFs decompose below 573 K, so pulsed JH is preferred to reduce heat input and suppress excessive reactions.^20,21

During the laboratory stage, carriers are limited by equipment size (millimeter level) or volume (milligram level), hindering large-scale manufacturing. To address this, Du et al. and Shi et al. developed Roll-to-Roll JH devices. Du et al. used it to process meter-scale electrodes, while Shi et al. achieved high production rates (116.69 cm² s⁻¹, up to 7 m min⁻¹), offering a reference for the preparation of SA metals via JH.^59,60

2.3 Comparing JH with traditional methods for synthesizing metal SAs

Prior to the advent of JH technology, a wide range of methodologies was employed for the synthesis of metal SAs. These methodologies primarily included high-temperature vapor transport, atomic layer deposition, mass-separated soft landing, deposition of organometallic complexes, and co-precipitation. Over the years, these techniques have been thoroughly reviewed by researchers such as Mashkovsky, Kment, He, and Li, who have provided detailed insights into the definitions, characteristics, and limitations of traditional metal SAs synthesis methods.^{31,35,42–48} For a deeper understanding, Table 1 offers a comprehensive comparison of these conventional approaches, highlighting their advantages and limitations.^31,46,48 Physical methods include physical vapor deposition (PVD), atomic layer deposition (ALD), and mass-separated soft landing. The principle of PVD involves evaporating a metal source (e.g., Pt, Au) at high temperatures (>1273 K) to generate metal atomic vapor. These atoms diffuse in an inert gas (e.g., Ar) and deposit on a cooler carrier (e.g., oxides, carbon materials), where they are rapidly cooled and ‘frozen’ in a SA state. However, the SAs loading is typically less than 0.1 wt%, which is insufficient for industrial catalysis. ADL achieves atomic-level dispersion by pulsing metal precursors and reactive gases (e.g., O₂, H₂O) on the carrier surface through saturated chemical adsorption. Hydroxyl groups or defects react with the precursors, depositing one atomic layer per cycle. Metal loading is controlled by the number of cycles. However, the equipment is expensive, deposition time is long (>1 min per cycle), and large-scale production efficiency is low. Mass-separated soft landing produces metal ions (Fe²⁺) via thermal ionization, sputtering, or laser evaporation. These low-energy ions bind to surface defects or functional groups on a carrier, forming SAs. This technique requires expensive, high-vacuum systems, mass spectrometers, and precise ion manipulation devices. The low ion beams current (<1 nA) results in sample preparation times ranging from hours to days for a 1 cm² area, limiting scalability.^61–64

Table 1 Comparison of Joule heating and traditional methods for synthesizing typical metal SAs

Synthesis strategies	Synthesis methods	Metal SAs	Supports	Advantages	Disadvantages	Ref.
Electrical thermal	Joule heating	Pt, Pd, Co, Ni, Fe	carbon paper, carbon black, reduced graphene oxide, CNT film, porous carbon spheres	1. Fast reaction with high temperature	1. Easily contaminated by sample stages	9, 20, 26 and 31
				2. Controllable conditions	2. Use of the conductive supports
				3. High loading of SAs (0.2–3.0 wt%)
				4. Extremely high yield and scalable
Physical	Physical vapor Deposition	Pd, Pt, Rh	CeO2, Al2O3	Precisely controlled deposition parameters at the atomic level	1. High costs	38, 67 and 68
					2. Low yield
	Atomic-layer deposition	Pt, Pd, Ni	CeO2, SiO2, graphene	1. Precisely controlled deposition parameters at the atomic level	1. Low loading of SAs	69–74
				2. Uniformity and repeatability of deposition	2. Strict synthetic conditions
					3. High costs
	Mass-separated soft-landing	Pd	Any substrate without high-surface-area carriers, such as MgO	Precisely controlled deposition parameters at the atomic level	1. Ultrahigh vacuum deposition conditions	39
					2. Extremely low yield
					3. Easy aggregation
Wet chemical	Deposition of organometallic complexes	Au, Pd, Pt, Rh, Fe, Co, Cr, Mn, Cu, Ni, Zn, Ru	MOFs, zeolites, 1,10-phenanthroline hydrate, Al2O3, ZrO2, SiO2, TiO2, etc.	High loading of SAs	Finding or designing a suitable organic complex	40 and 75
	Co-precipitation	Pt, Pd, Ir, Au	CeO2, SiO2, etc.	Simple procedure	Use of a highly diluted solution of the target metals	41, 76–79

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On the other hand, wet chemical synthetic approaches, including the deposition of organometallic complexes and co-precipitation, have been widely explored due to their accessibility and adaptability to a variety of precursors. The deposition of organometallic complexes occurs through coordination or substitution reactions between the metal center and functional groups on the carrier surface, enabling targeted metal atom deposition. Low-temperature heat treatment (<573 K) decomposes organic ligands, leaving strongly interacting metal atoms. However, limited active sites on the carrier restrict loading to below 1 wt%, which does not meet industrial requirements. Precursors should be selected for easy decomposition and stable coordination. Co-precipitation involves mixing metal salts with carrier precursors, adjusting the pH (e.g., with NaOH) to precipitate metal ions and carrier precursors as hydroxides or carbonates. High-temperature calcination (>773 K) decomposes these precursors into metal oxides. The carrier lattice confines metal atoms, preventing agglomeration and achieving SA dispersion. However, metal atoms can migrate and aggregate during calcination, creating a trade-off between loading capacity and dispersion. Using highly diluted metal solutions helps address this issue.^62,65,66

In contrast, JH synthesis represents a transformative technology that effectively addresses numerous challenges associated with traditional methods. Specifically, it offers several advantages, including rapid reactions at elevated temperatures, high loading capacities, exceptionally high yields, scalability, and more efficient anchoring and interaction of atoms and carriers. Additionally, it enhances thermal stability and prevents agglomeration. The simplicity of operation and cost-effectiveness further underscore its appeal, making it a promising alternative for both research and industrial applications. Most importantly, JH synthesis achieves substantially higher yields compared to conventional methods, positioning it as a key enabler for large-scale production of metal SAs. It eliminates the need for high-temperature vacuum conditions or long processing times, enhancing practicality. Its high controllability also ensures broad applicability.

In summary, while traditional methods have laid the foundation for advancements in metal SAs, their limitations in scalability, cost, and operational complexity underscore the need for alternative approaches. JH synthesis stands out as a next-generation solution that bridges these gaps, offering unprecedented efficiency and practicality for producing metal SAs at scale. This transformative approach not only enhances the accessibility of these materials but also paves the way for their broader application in catalysis, energy storage, and other cutting-edge fields.

3. Progress of preparing metal SAs by Joule heating

In recent years, the JH method has garnered considerable attention in the field of materials science, especially for the manipulation and synthesis of SAs of both noble and non-noble metals. Since 2019, this state-of-the-art technique has been successfully implemented across a broad spectrum of metal SAs, including Pt, Pd, Co, Ni, Fe, and others, as demonstrated in Fig. 4.^{9,10,17,18,20,21,26,32}

Fig. 4 Synthesis parameters, including temperature (K), time (ms) and content (wt%) of various metal SAs by JH.^{9,10,17,18,20,21,26,32}

3.1 Noble metal SAs

Pt, Pd, Ru, and other noble metals, renowned for their exceptional stability and catalytic efficiency, play a pivotal role in various catalytic processes. These processes include the trimerization of acetylene to benzene, methanol oxidation, the water–gas shift reaction, and the hydrogen evolution reaction.^{39,67–70,76,77} However, the limited reserves and exorbitant costs (e.g., Pt/C: ∼9100 $ per m²) have significantly constrained their large-scale application.^4,80–83 To overcome these challenges, supported noble metals with atomic dispersion have shown considerable promise in achieving optimal atom efficiency, as well as enhanced activity and selectivity, in catalyzing an expanding range of thermally-, electrochemically-, and photo-driven chemical reactions.^{33,38,83–89} The JH synthesis technology with ultra-high reaction temperatures and ultra-short reaction times has been applied to the synthesis of noble metal SAs because it can prevent metal segregation and enhance metal dispersion.

The following outlines a detailed JH method for preparing Pt and Pd SAs, serving as a reference for the synthesis of related elements in future studies. Yao et al. utilized controllable JH to synthesize and stabilize SAs of Pt, Ru, and Co.²⁶ The schematic illustration of the synthesis of platinum high-temperature SAs on CO₂-activated carbon nanofibers (CA-CNFs) is depicted in Fig. 5a(1). The process involved loading ethanol-based H₂PtCl₆ precursors onto defective CA-CNFs at 0.01 μmol cm⁻² with good wetting. The CA-CNFs film loaded with precursors underwent electrical JH, rapidly reaching 1500 K in 55 ms, followed by quick cooling to 400 K. This heating and cooling cycle was repeated ten times over 6 s (Fig. 5a(2)-(3)). After a single heat shock, the CA-CNFs exhibited a high density of SAs along with some Pt clusters. However, after ten heat shocks, the substrate displayed a uniform distribution of SAs, with virtually no Pt–Pt bonds remaining. The CA-CNFs substrate had a surface area of ∼56 m² g⁻¹ and Pt loading of ∼0.24 wt%.

Fig. 5 (a) (1) Schematic of the HT-SA synthesis and dispersion process (grey, carbon atoms; cyan, metal precursor; and red, metallic atoms).²⁶ (2) Temperature evolution during the shockwave synthesis and a detailed heating/cooling pattern. Inset: light emitted from the material at a high temperature.²⁶ (3) Ten-pulse shock heating pattern demonstrates the uniform temperature in each cycle with a high-temperature “on” state and a low-temperature “off” state.²⁶ Copyright 2019, Springer Nature. (b) (1) Atom trapping (AT) and thermal shock (TS) synthesis of 1 wt% single-atom Pt₁/CeO₂ catalysts showing the symmetric (near-perfect) and asymmetric (distorted square-planar) Pt₁O₄ coordination in Pt₁/CeO₂_AT and Pt₁/CeO₂_TS, respectively.²⁰ (2) Schematic of the thermal-shock synthesis of 1 wt% SA Pt₁/CeO₂_TS catalyst.²⁰ (3) Temperature profile of thermal shock heating, which shows high temperature (∼1500 K) and short duration (∼500 ms).²⁰ Copyright 2021, Wiley-VCH. (c) (1) Schematic of the synthesis of Pd₁/CeO₂ SACs.¹⁸ (2) Histogram of Pd₁/CeO₂ catalytic activity.¹⁸ Copyright 2023, American Chemical Society.

The researchers studied the influence of different heating temperatures and durations on the dispersion of Pt atoms. Heating at 573 K for 1 h resulted in poor dispersion and stability. Heating at 1500 K for 10 min led to long-term overheating, causing graphitization of the carbon substrate and long-range atomic diffusion, with severe aggregation of Pt. Heating at 1500 K for 55 ms followed by cooling, resulted in the coexistence of Pt clusters and single atoms. Repeating the pulse of heating at 1500 K for 55 ms and cooling for 550 ms ten times enabled the uniform dispersion of Pt atoms. The experimental findings indicate that short-duration, multiple high-temperature heating cycles provide sufficient activation energy for diffusion and allow the energy barriers associated with bond formation to be overcome. This process enables single Pt atoms to detach from Pt-30 clusters, forming Pt–C bonds with higher binding energy and covalent characteristics.

Jiang et al. utilized a thermal-shock (TS) synthesis, also referred to as JH, to tailor the local environment of isolated Pt²⁺, resulting in a highly active and thermally stable Pt₁/CeO₂ catalyst.²⁰ As illustrated in Fig. 5b(1), tetraammineplatinum nitrate (TAPN) was introduced onto CeO₂ via incipient wetness impregnation (IWI). Subsequent TS treatments produced Pt₁/CeO₂_TS catalysts. The preparation of the 1 wt% Pt/CeO₂ precursor involved the following steps: crystalline CeO₂ support was synthesized by thermally decomposing Ce(NO₃)₃·6H₂O at 623 K in air for 2 h. An appropriate amount of TAPN (1 wt% CeO₂ base for Pt) was dissolved in 300 μL of deionized water to impregnate 1 g of CeO₂ powder (particle size 150 μm). The impregnated Pt/CeO₂ was dried overnight at 353 K in air, preparing it for catalyst synthesis. A thin layer of the dried Pt/CeO₂ precursor powder was evenly spread on a tungsten (W) plate and rapidly heated to a high temperature before being quenched using a carbon heater driven by electrified JH (Fig. 5b(2)). Controlled high-temperature (>1473 K) shockwaves, generated by periodic on–off heating cycles (500 ms on, 3000 ms off, repeated six times), restructured the CeO₂ surface, promoting Pt dispersion and strong Pt–O–Ce bonding (Fig. 5b(3)). In an inert atmosphere, ultrafast shockwaves exceeding 1473 K can reconfigure the CeO₂ surface, forming Pt SA in an unpaired Pt₁O₄ configuration. This adjusted the local coordination environment of isolated Pt, creating a highly active and thermally stable Pt₁/CeO₂ catalyst.

Tian et al. developed a highly active and thermally stable Pd₁/CeO₂ catalyst through thermal-shock synthesis.¹⁸ Initially, the Pd/CeO₂ precursor was synthesized via incipient wetness impregnation. The crystalline CeO₂ support was prepared by calcining Ce(NO₃)₃·6H₂O at 623 K in air for 2 h. Palladium(II) nitrate dihydrate (1 wt% on CeO₂) was dissolved in 350 μL of deionized water and used to impregnate 1 g of CeO₂ powder. The impregnated Pd/CeO₂ was dried overnight at 353 K in air. Subsequently, the dried Pd/CeO₂ precursor was placed on a carbon plate and rapidly heated to approximately 1273 K in 0.5 s, using JH equipment (40 V, 350 A). This process was repeated six times to ensure uniform heating and SA dispersion. The final catalyst was designated as Pd₁/CeO₂_TS (Fig. 5c(1)). The Pd₁/CeO₂_TS catalyst featured an active site characterized by unsaturated Pd–O* species, which significantly enhanced its catalytic activity (Fig. 5c(2)).

The study indicated that a thermal shock at 1273 K within 0.5 s induces an unbalanced PdO₃ configuration with a reduced coordination number. This configuration features Pd–O* species, which lowers the activation energy barrier for C–H bond activation and enhances catalytic performance.

3.2 Non-noble metal SAs

Transition metals, such as Ni, Co, Cu, Fe, Mo, and Zn, have emerged as promising alternatives to noble metals for catalytic applications due to their cost-effectiveness and superior atomic efficiency. Unlike noble metals, which are often scarce and expensive, transition metals are abundant and economically accessible, offering significant advantages for large-scale and industrial use. Furthermore, their exceptional atomic efficiency allows for effective utilization of active sites, ensuring optimal performance while reducing material waste. In addition to their economic and resource-related benefits, transition metals exhibit diverse catalytic properties that can be fine-tuned for specific reactions.^90–97

The subsequent section presents an in-depth JH method for synthesizing Ni, Co, and Fe SAs. Xi et al. synthesized carbon-supported Ni–N_x SACs via JH using a metal–ligand complex adsorbed on carbon black as the precursor.¹⁷ To prepare the Ni SAC, as illustrated in Fig. 6a(1), 8.3 mg of nickel(II) acetate tetrahydrate and 23.2 mg of 1,10-phenanthroline monohydrate were dispersed in 50 mL of ethanol at a molar ratio of 1 : 3.5 and stirred for 10 min. Subsequently, 100 mg of carbon black (Ni : C molar ratio of 1 : 250) was added, and the mixture was stirred and sonicated for 20 min. The resulting black suspension was placed between carbon layers (10 × 40 mm²) and subjected to JH at 1573 K for 0.5 s under an argon atmosphere, with this process repeated five times (Fig. 6a(2)). The final product contained 1 wt% nickel. It showed that JH at 1573 K for 0.5 s enabled up to 80% of the N-admixture to coordinate with the metal core. At high temperatures, metal-free N-species decompose and leave the matrix, reducing the negative impacts on catalytic performance. Besides Ni, metals like Ge, Co, Cu, and Zn can also bond with N to form isolated SAs.

Fig. 6 (a) (1) Schematic of the homemade JH reaction system.¹⁷ (2) Temperature, direct current, and voltage changes with the evolution of JH duration (left), and optical images before, during, and after heating of the samples on the reaction platform (right).¹⁷ Copyright 2021, Wiley-VCH. (b) (1) Schematic showing the preparation of self-supported CoAGO hydrogels.⁹ (2) Electrical JH process of CoAGO aerogel films: heating and quenching occur instantly with on/off switching.⁹ Copyright 2022, Springer Nature. (c) Schematic of sample preparation via the JH strategy and Co species formed on the carbon prepared at a heating temperature of 873 K.²¹ Copyright 2024, Wiley-VCH. (d) Schematic of the synthesis of Fe–N-DCSs.¹⁰ Copyright 2025, Elsevier. (e) Schematic of the synthesis process used to prepare P-MeN₄@CNTs catalysts (Me = Fe, Co, Ni, and Cu).³² Copyright 2024, Elsevier. (f) Schematic of RuMo@MoO_x-JH catalyst synthesis via rapid JH and a schematic of the JH device's operating conditions.⁵⁶ Copyright 2024, Springer Nature.

Xing et al. developed a JH-triggered transient heating–quenching strategy to synthesize three-dimensional porous monolithic CoNG-JH materials.⁹ Fig. 6b(1) illustrates the synthesis process used to generate the self-supported porous CoNG-JH film. Initially, graphene oxide (GO) underwent functionalization with aqueous ammonia to form amine-functionalized GO (AGO), which facilitates nitrogen doping. Subsequently, cobalt salts and ethanol were introduced into the AGO solution, leading to the formation of a Co²⁺ containing AGO (CoAGO) hydrogel via hydrothermal self-assembly. Ethanol served to lower the freezing point, thereby influencing ice crystal growth during the ice-templating process. The CoAGO hydrogel was then freeze-dried to produce a hierarchically porous CoAGO aerogel. This aerogel film subsequently underwent JH (1 A, 2 s) in an NH₃ atmosphere, where NH₃ acted as a secondary nitrogen source (Fig. 6b(2)). JH promoted metal atom dispersion, removed oxygen functional groups, and ejected carbon atoms, creating defects that anchored N and Co atoms to form Co–N–C sites within the graphene structure. As a result, the metal content of CoNG-JH was 0.81 wt%, with a mass loading of 1 mg cm⁻².

Heteroatoms (e.g., N, P, and S) doped into graphene can enhance the embedding of transition metals dispersed at the atomic level. Based on this principle, researchers utilized JH to supply the activation energy required for Co–N–C sites, thereby ensuring the uniform dispersion of metal atoms. Additionally, JH facilitated the removal of oxygen functional groups in graphene oxide (GO), the ejection of carbon atoms in the form of CO₂ or CO, and the formation of defects during this process.

Wang et al. demonstrated precise tuning of Co from SA to nanocrystals on carbon support using JH. Commercial ZIF-67, synthesized by bridging 2-methylimidazolate anions and cobalt cations, served as the MOF precursor.²¹ Fig. 6c shows that a direct current source was applied to the MOFs-loaded carbon cloth. JH enabled rapid heating and cooling, decomposing the MOFs precursor into metal/carbon composites. By adjusting the heating temperatures (873 K, 1173 K, and 1573 K) for 1 s, followed by 10 s of cooling over two pulses, different Co/C composites with varying Co particle sizes were prepared. At 873 K, 2-methylimidazole acts as an organic ligand, with its N atom deprotonating and coordinating with cobalt ions to form dispersed Co SA. At 1173 K, Co atoms start clustering into nanoparticles (average size ∼5 nm). At 1573 K, further clustering occurs, forming particles with an average size of ∼12 nm. These results showed that increasing temperature promotes the growth of cobalt particles in cobalt/carbon composites.

Liu et al. used JH to anchor Fe SAs on defect-rich porous carbon spheres (Fe–N-DCSs) within milliseconds;¹⁰ a schematic of Fe–N-DCSs is presented in Fig. 6d. First, defect-rich porous carbon spheres (DCS) were synthesized.⁹⁸ DCS retained its original nanosphere morphology and exhibited a highly porous structure. Next, N-doped porous carbon spheres (N-DCS) were fabricated using 1,10-phenanthroline as the nitrogen source. Following this, Fe³⁺ was impregnated into N-DCS to form Fe–N-DCS. Ultimately, Fe–N-DCSs with a metal content of 0.682 wt% was obtained via JH. This process involved placing 5 mg of the catalyst on carbon cloth, securing it in a fixture, and connecting it to a DC power supply on a sintering table. The reaction system was evacuated and filled with nitrogen. Heating was performed at 1273 K, 1473 K, and 1673 K under automatic temperature control (52 V, 60.5 A) for 1 or 2 s. The results showed that the Fe–N-DCSs catalyst performed best at 1473 K for 1 s (including heating and cooling). Qin et al. synthesized atomically dispersed Me–N–C catalysts through heteroatom engineering during an ultrafast JH process.³² The synthesis of FePc-CNT precursors involved dispersing 100 mg of multi-walled carbon nanotubes (MWCNTs) in 120 mL of DMF and 150 mg of FePc in 60 mL of DMF. Both suspensions were ultrasonicated for 0.5 h, then combined and ultrasonicated for an additional 1 h. After stirring for 24 h, the mixture was washed with ethanol and deionized water, followed by drying in a vacuum oven at 353 K for 8 h to produce FePc-CNTs. As illustrated in Fig. 6e, for the synthesis of P-FeN₄@CNTs using the FJH method, 100 mg of FePc-CNTs and 50 mg of NaH₂PO₂·H₂O were ground and placed in a 10.0 mm quartz tube. Conductive graphite plugs were inserted at both ends of the tube, which was then secured to the reaction frame with the sample's resistance set to 1.0–2.0 Ω. The setup was placed under vacuum and the flash voltage was set to 130 V. This method transformed the feedstocks into atomically-dispersed P–MeN_x moieties on CNTs within 200 ms. Reaction parameters, including peak temperature (3535 K) and current (333 A), were recorded in real-time. The product yield was approximately 60%, and gram-level production could be achieved by increasing the quartz tube diameter.

The study found that P doping suppresses nitrogen loss during ultrafast JH (3535 K in 200 ms), forming more active FeN₄ sites. DFT results show that P admixture disrupts FeN₄ planar symmetry, leading to uneven charge distribution and reducing the energy barrier for the rate-determining step, enhancing ORR performance.¹⁰

When synthesizing SACs using the JH method, the dispersion and loading of metal active sites are influenced by multiple factors, including temperature, heating/cooling duration, precursor type, and carrier characteristics. To avoid nanoparticle aggregation and Ostwald ripening, optimal heating conditions must be determined through rigorous simulation or experimentation. Typically, the precursor is a metal salt or a combination of metal salts doped with heteroatoms (e.g., N, P, S), which are uniformly dispersed on the carrier material. The carrier should possess abundant defects or incorporate heteroatoms to enhance atomic dispersion and improve loading efficiency. Consequently, the resulting SACs exhibit superior catalytic performance due to their unique coordination environments and strong interactions with the carrier.

Significant advancements have been achieved in utilizing JH to synthesize isolated single metal atoms. Nonetheless, there is limited literature reporting the successful synthesis of multiple metal SAs on a single support. The distinct properties of the metals hinder their simultaneous and uniform dispersion at the same temperature.¹¹ It is well established that bimetallic interactions exert a synergistic catalytic effect and modulate the adsorption and desorption free energies between intermediates and active sites through the incorporation of an appropriate second metal, thereby enhancing catalytic performance.⁹⁹ Zhao et al. have made significant contributions to the synthesis of dual-metal systems.⁵⁶ They reported a rapid JH method to synthesize RuMo nano-alloys (NAs) embedded in a MoOx matrix (denoted as RuMo@MoO_x-JH). A 1 cm × 1.5 cm × 1 mm piece of MoO₃ was heated to 973.1 K for 60 s in an Ar–H₂ (10%) flow, with a ramp-up rate of 84.4 K s⁻¹ and a cool-down rate of 361 K s⁻¹, resulting in MoOx-JH. This was then immersed in a ruthenium(III) chloride solution for 2 min, dried in an infrared desiccator, and heated to 1303.1 K with a ramp-up rate of 2010 K s⁻¹ to obtain RuMo@MoO_x-JH (Fig. 6f). The RuMo NAs have a size of ∼5 nm and a Ru concentration of ∼1.73 at%. The utilization of JH for the synthesis of multiple metal polyatomic species on a common carrier represents a promising direction for future research. The types of noble- and base-metal single-atom catalysts, along with their synthesis methods, are summarized in Table 2.

Table 2 JH synthesis parameters for noble and non-noble metal SA materials

Type	Metal SAs	Support	Content	Sample name	Functional applications	JH parameters	Ref.
Noble metals	Pt (Ru, Co)	CA-CNFS	0.24 wt%	Pt HTSAs	Direct methane conversion	10 pulse cycles, each heating to 1500 K in 55 ms and cooling for 550 ms	26
	Pt	Carbon paper & W plate	1.0 wt%	Pt1/CeO2	Low-temperature CO oxidation	6 pulse cycles, each with 1503 K rapid heating for 500 ms and 6 s cooling	20
	Pd	Carbon plate	1.0 wt%	Pd1/CeO2	Low-temperature methane combustion	6 pulse cycles at approximately 1273 K for 0.5 s	18
Non-noble metals	Ni (Fe, Co, Cu, Zn)	Carbon black	1.0 wt%	Ni–Nx SACs	CO2RR	5 pulse cycles, each with 1573 K rapid heating for 0.5 s	17
	Co	Porous graphene	0.81 wt%	CoNG-JH	HER	<2 s	9
	Co	Carbon cloth	—	Co/C-600	EMW	2 pulse cycles, heating to 873 K in 1 s, cooling for 10 s	21
	Fe	Porous carbon spheres	0.682 wt%	Fe–N-DCSs	ORR	Heating at 1473 K with automatic temperature control (52 V, 60.5 A)	10
	Fe (Co, Ni, Cu)	CNTs	1.487 wt%	P-FeN4@CNT	ORR	1 pulse, peak temperature is 3535 K, reaction time under 200 ms	32

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4. Advanced applications of metal SAs prepared by Joule heating

4.1 New energy field

In the rapidly evolving landscape of renewable energy, the development and application of advanced battery technologies play a pivotal role.¹⁰⁰ Metal–air batteries, with their high energy density (e.g., Zn–air battery (ZAB) theoretical 1086 Wh kg⁻¹) and cost-effective production, are particularly promising (Fig. 7a(1)).^101–103 They use a metal (zinc, lithium, or aluminum) as the anode and atmospheric oxygen as the cathode to generate electricity through the ORR. The ORR efficiency is key to the battery's performance.^104–106 Liu et al. developed a JH synthesis method to prepare Fe–N₄ sites anchored to a support of defect-rich porous carbon spheres referred to as Fe–N-DCSs.¹⁰ The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) images showed numerous bright dots representing Fe SAs, confirming the successful synthesis of SACs and atomically dispersed on Fe–N-DCSs (Fig. 7a(2)). The k²-weight Fourier transformed (FT) EXAFS spectrum for Fe–N-DCSs showed two main peaks at 1.5 Å and 2.5 Å, corresponding to the first and second coordination shells of Fe–N and Fe–C, respectively (Fig. 7a(3)). The first coordination shell of Fe–N had a bond length of 2.03 Å and a coordination number of 5.13, indicating the successful formation of Fe–N₄ sites with an axial O atom from adsorbed oxygen on the carbon support. The axial oxygen did not affect the planarity of FeN₄. The second coordination shell, Fe–C, had a bond length of 2.99 Å and a coordination number of 4.45.^107,108 The Tafel slopes in Fig. 7a(4) showed that the kinetics of Fe–N-DCSs (62.8 mV dec⁻¹) was similar to that of Pt/C (66.8 mV dec⁻¹) and better than that of N-DCSs (79.5 mV dec⁻¹), confirming its excellent ORR kinetics.¹⁰⁹ To clarify the excellent ORR performance of Fe–N-DCSs, DFT calculations using VASP were performed. The author constructed four configurations: A-Fe–(N–C₂)₄, A-Fe–(N–C₂)₄–N, Z-Fe–(N–C₂)₄, and Z-Fe–(N–C₂)₄–N, to investigate how different edge (armchair-type and zigzag-type) defects affect ORR performance. As shown in Fig. 7a(5), the ΔG_OH* (0.62 eV) for A-Fe–(N–C₂)₄ was similar to that of A-Fe–(N–C₂)₄–N (0.60 eV), but significantly higher than those of Z-Fe–(N–C₂)₄–N (0.57 eV) and Z-Fe–(N–C₂)₄ (0.54 eV). This indicated that A-Fe–(N–C₂)₄ with armchair-type edge defects had the highest OH* adsorption free energy, enhancing OH* desorption and improving ORR performance. This proved that armchair-type edge defects can promote the adsorption–desorption behaviors of ORR intermediates. Fig. 7a(6) shows that the ZAB built with Fe–N-DCSs has remarkable stability, maintaining an initial charge–discharge voltage gap of approximately 0.92 V, even after 1100 h of continuous cycling at 10 mA cm⁻².

Fig. 7 (a) (1) Schematic of the aqueous ZAB.¹⁰ (2) AC-HAADF-STEM images of Fe–N-DCSs synthesized by the JH process.¹⁰ (3) FT-EXAFS spectra of Fe–N-DCSs, FeO, FePc, and Fe Foil at the Fe K-edge.¹⁰ (4) Tafel plots of Fe–N-DCSs, N-DCSs, and Pt/C.¹⁰ (5) Free energy diagram of the OH* desorption step.¹⁰ (6) Rate performance of aqueous ZABs.¹⁰ Copyright 2025, Elsevier. (b) (1) Schematic of an electrolyzer for water splitting.¹¹⁴ (2) ADF-STEM images at various magnifications of CoNG-JH showing well-dispersed Co SAs in the carbon matrix. The inset highlights an individual Co atom, approximately 2 Å in size.⁹ (3) FT-IR spectra of CoNG-JH, CoG-JH, CoAGO and CoGO.⁹ (4) High-resolution XPS Co 2p spectra of CoNG-H before and after accelerated cycling.⁹ (5) Tafel slopes for CoNG-JH, CoG-JH, NG-JH and Pt/C.⁹ (6) Stability of CoNG-JH evaluated by the η–t curve at a current density of 10 mA cm⁻² for 48 h. The inset curve shows the polarization curves of CoNG-H before and after 10 000 CV cycles.⁹ Copyright 2022, Springer Nature.

In conclusion, Liu et al. achieved atomically dispersed Fe-N₄ sites on defective carbon, enhancing the ORR kinetics compared to Pt/C. The assembled Zn–air battery shows outstanding stability (1100 h cycling, ∼0.92 V gap). DFT reveals armchair defects optimize ORR intermediates, guiding efficient catalyst design and advancing renewable energy applications.¹⁰

4.2 Clean fuel field

In the context of ‘Carbon Peaking and Carbon Neutrality’, the hydrogen economy, driven by the high energy density (∼130 MJ kg⁻¹) of renewable hydrogen (H₂), emerges as a crucial technology to address the significant depletion of fossil fuel reserves and mitigate their adverse environmental impacts.^110–113 The production of H₂ through electrochemical water splitting has emerged as one of the most promising strategies (Fig. 7b(1)).^114–116 Xing et al. synthesized three-dimensional porous monolithic CoNG-JH materials that were used as electrodes for HER.⁹ The atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) images unequivocally demonstrated that the isolated cobalt atoms (bright spots) with a diameter of approximately 2 Å were uniformly dispersed within the CoNG-JH sample (Fig. 7b(2)). Fourier-transform infrared spectroscopy (FT-IR) spectra, as depicted in Fig. 7b(3), elucidated the structural modifications induced by JH. Notably, the quantity of oxygen-containing functional groups in the CoAGO and CoGO precursors markedly diminished upon transformation into CoNG-JH, underscoring the high efficacy of 2 s JH in reducing graphene oxide. Furthermore, the chemical state of cobalt in CoNG-JH was investigated through XPS Co 2p spectroscopy, which revealed two dominant peaks that were ascribed to the atomically dispersed CoN_x moieties (Fig. 7b(4)). Fig. 7b(5) revealed that CoNG-JH exhibited a low Tafel slope of 66 mV dec⁻¹. In addition to catalytic performance, electrocatalytic stability was essential for practical applications. As illustrated in Fig. 7b(6), the galvanostatic stability test of CoNG-JH at 10 mA cm⁻² indicated an increase in overpotential of only approximately 10 mV after 48 h, highlighting its exceptional stability. The cyclic stability test, conducted with up to 10 000 CV cycles, further corroborated the electrocatalytic stability of CoNG-JH (inset of Fig. 7b(6)).

Meanwhile, methane is one of the cleanest energy sources; therefore, enhancing its low-temperature combustion capability is crucial for minimizing the greenhouse effect.¹¹⁷ Tian et al. found that modifying the local environment around isolated Pd sites can create active and stable Pd single-atom catalysts for low-temperature methane combustion.¹⁸ They synthesized Pd₁/CeO₂_TS using TS, which showed a 20-fold improvement in catalytic activity compared to the Pd₁/CeO₂_AT catalyst.

The hydrogen economy leverages the high energy density of renewable hydrogen. Among its key strategies, electrochemical water splitting plays a pivotal role. Xing et al. used Co SA in CoNG-JH for HER, showing low Tafel slope (66 mV dec⁻¹) and stability. Tian et al. engineered Pd SA in Pd₁/CeO₂_TS, boosting low-temperature methane combustion activity 20×, highlighting the critical role of metal SAs in clean energy technologies.

4.3 Environment protection field

The extraction and utilization of natural resources result in the emission of carbon monoxide, carbon dioxide, sulfur oxides, and nitrogen oxides, leading to substantial environmental degradation.^45,118 Carbon monoxide, a colorless, odorless, and toxic gas, is generated through the incomplete combustion of various fuels, including coal, gasoline, and natural gas, as well as vehicle exhaust. From an environmental standpoint, the removal of CO from the atmosphere is essential due to its known detrimental effects on living organisms (Fig. 8a(1)).^79,119 Jiang et al. customized the local environment of isolated Pt²⁺ ions through TS synthesis, leading to the development of a highly active and thermally stable Pt₁/CeO₂ catalyst for low-temperature CO oxidation.²⁰ For comparative purposes, they also prepared the catalyst using the atom trapping (AT) method. The atomic dispersion of platinum (Pt) was confirmed through aberration-corrected scanning transmission electron microscopy (AC-STEM) utilizing high-angle annular dark-field (HAADF) imaging (Fig. 8a(2)). Given the absence of Pt–Pt scattering, as evidenced by extended X-ray absorption fine structure (EXAFS) analysis, it can be concluded that the as-prepared Pt₁/CeO₂_TS sample primarily consists of isolated Pt²⁺ cations (Fig. 8a(3)). Fig. 8a(4) illustrates that TS induced an asymmetric Pt₁O₄ coordination. During CO oxidation, the Pt²⁺ in Pt₁/CeO₂_TS dynamically adopted a partially reduced Pt₁O_4−x coordination. This reduction in electronic states significantly enhanced the activity of the evolved Pt₁^δ+ species, promoting CO oxidation at low temperatures. As shown in Fig. 8a(5), Pt₁/CeO₂_AT exhibited no activity below approximately 473 K and demonstrated superior activity over bare CeO₂ only at temperatures above approximately 513 K. In contrast, Pt₁/CeO₂_TS displays markedly enhanced low-temperature activity, as evidenced by a substantially lower T₅₀ value (the temperature required for 50% conversion of CO) of approximately 423 K, compared to the T₅₀ value of about 560 K for Pt₁/CeO₂_AT. Furthermore, it exhibited excellent cycling stability, showing no signs of deactivation in low-temperature CO oxidation after repeated light-off tests (Fig. 8a(5)). It can be reasonably inferred that, compared to the over-stabilized Pt²⁺ on CeO₂ via AT, the asymmetric Pt₁O₄ coordination induced by TS significantly enhanced CO activation, thereby accelerating low-temperature CO oxidation.

Fig. 8 (a) (1) Schematic of the CO oxidation. (2) HRTEM of isolated Pt atoms synthesized by the JH process.²⁰ (3) Fourier transform of k²-weighted EXAFS of as-synthesized Pt₁/CeO₂ catalysts and the Pt foil ref. 20. (4) Proposed dynamic evolution of the local environments of isolated Pt²⁺ in Pt₁/CeO₂_TS from asymmetric (distorted) Pt₁O₄ to partially reduced Pt₁O_4−x for greatly enhanced low-temperature CO oxidation.²⁰ (5) CO light-off curves collected over as-synthesized Pt₁/CeO₂_AT, Pt₁/CeO₂_TS, and bare CeO₂ calcined at 1073 K in air. Reaction conditions: 1% CO, 10% O₂, with N₂ balance, GHSV of 200 L/gh.²⁰ Copyright 2021, Wiley-VCH. (b) (1) Schematic of the EMW. (2) HRTEM of Co single atom/N–C.²¹ (3) Fourier-transform Co K-edge EXAFS spectra.²¹ (4) Dielectric loss tangent of various Co/C materials.²¹ (5) Co single atom/N–C with an isosurface of 2.8 × 10⁻³ e Å⁻³.²¹ (6) Calculated RL for Co/C-600, Co/C-900, and Co/C-1300 with filler loading of 30 wt% at the thickness of 2.48 mm.²¹ Copyright 2023, Wiley-VCH.

Volatile organic compounds (VOCs) from sources such as chemical industries, textiles, indoor products, and transportation contribute significantly to air pollution, threatening both the environment and health. To effectively eliminate VOCs, Du et al. developed atomically dispersed Pt/CeO₂ catalysts via the JH method. Ce(NO₃)₃·6H₂O and NaOH were mixed and reacted hydrothermally at 373 K for 24 h. The resulting product was centrifuged, washed, dried under vacuum, and calcined at 773 K for 4 h to produce CeO₂ nanorods. Next, 300 mg of CeO₂ nanorods were dissolved in 20 mL of ethanol, and 125 μL of Pt precursor solution (0.1 mol mL⁻¹) was added. After ball milling for 30 min, the Pt/CeO₂ catalyst was obtained via vacuum filtration. Finally, the Pt/CeO₂ solution was sprayed onto HCl-treated nickel foam (NF) to prepare Pt/CeO₂-NF. This advanced catalytic system demonstrated a 100% conversion rate for 1000 ppm toluene (flow rate of 33 mL min⁻¹) with an input power of only 6.5 W, which is 87% lower than that required by a conventional heating furnace. This technology offers a new approach for developing efficient, energy-saving VOCs catalytic removal equipment with broad application prospects.¹²⁰

Overall, the extraction and use of natural resources release pollutants such as CO and VOCs, causing environmental degradation. Metal SACs offer promising solutions. The Pt₁/CeO₂ catalyst prepared via TS synthesis enables efficient low-temperature CO oxidation with excellent cycling stability, outperforming traditional methods in activity (e.g., T₅₀ value of ∼423 K vs. ∼560 K for other Pt/CeO₂ catalysts). The atomically dispersed Pt/CeO₂ catalyst developed by the JH method achieves high-efficiency VOCs (e.g., toluene) removal at low energy consumption (6.5 W input power), reducing energy use by 87% compared to conventional heating furnaces. These advancements highlight innovative pathways for sustainable environmental governance using SACs.

4.4 Electronic information field

Prolonged exposure to electromagnetic waves (EMWs) poses substantial risks to human health, contributing to a range of health issues, including headaches, fatigue, and more severe conditions such as sleep disorders and elevated stress levels.^121,122 To address these concerns, the scientific community has intensified its efforts to develop efficient EMW absorbers (Fig. 8b(1)).^123,124 Wang et al. synthesized a series of Co/C composites with varying cobalt particle sizes by adjusting the JH temperatures to 873 K (Co/C-600), 1173 K (Co/C-900), and 1573 K (Co/C-1300).²¹ HAADF-STEM images of Co/C-600 showed isolated cobalt atoms distributed throughout the sample (Fig. 8b(2)). Co/C-600 exhibited a primary peak at 1.43 Å, attributed to the Co–N bond, indicating that cobalt atoms were mainly coordinated with nitrogen. In contrast, Co/C-900 and Co/C-1300 display a prominent peak at 2.2 Å, corresponding to the Co–Co bond, suggesting that higher temperatures promoted cobalt nanoparticle formation (Fig. 8b(3)). The Co/C-600 sample exhibited the highest dielectric loss tangent (tan δ_ε) among all the prepared samples (Fig. 8b(4)), indicating its superior dielectric loss performance. This was due to the large interface between Co and C atoms, facilitated by the Co single-atom structure and abundant defects in the carbon matrix, which enhanced interfacial polarization. To explore the underlying mechanism, they calculated the charge density differences of Co atoms, clusters, and particles on N-doped carbon using VASP with the projector augmented wave method. The introduction of Co to N-doped carbon caused charge redistribution, leading to symmetry breaking and the formation of electrical dipoles, which contribute to additional interfacial polarization loss (Fig. 8b(5)). The number of transferred electrons (Δq) for Co atoms, clusters, and particles are 0.33, 0.21, and 0.07 e/Co, respectively. Higher electron transfer at the Co atom/C–N interface resulted in increased polarization and relaxation loss. Consequently, mixing Co/C-600, Co/C-900, and Co/C-1300 samples at a thickness of 2.48 mm achieved an excellent absorber with a wide EMW absorption range, covering the C–X–Ku bands (Fig. 8b(6)).

In summary, prolonged EMW exposure risks drive efficient absorber development. Metal SA materials show unique application value. Wang et al. tuned temperatures to synthesize Co/C composites, and found that Co SA in Co/C-600 (873 K) showed superior dielectric loss via Co–N bonds and carbon defects. Mixing samples achieves broad EMW absorption (C–X–Ku bands), highlighting SA structures for high-performance absorbers.

5. Conclusion and outlook

In conclusion, this review presents a comprehensive exploration of JH synthesis, including its historical development, fundamental principles, equipment design, the fabrication of metal SAs, and their versatile applications. JH has emerged as a groundbreaking technique in the field of catalysis, particularly in the synthesis of metal SAs. Its unique characteristics and advantages position it as a vital tool for advancing catalyst development. One of the most remarkable features of JH synthesis is the ability to address the aggregation of isolated metal atoms—a persistent challenge in the fabrication of SAs. By employing a controlled electric current, JH generates localized, rapid heat that ensures metal atoms remain dispersed and stable on the support material, effectively preserving their atomic-scale catalytic properties. This stabilization is crucial for maintaining the distinct reactivity of SA-based catalysts, which often outperforms conventional nano-catalysts in terms of activity and selectivity. Additionally, JH provides an unprecedented level of precision in tailoring the local coordination environments and electronic structures of metal atoms. This fine control enhances the catalytic activity, selectivity, and durability of metal SA materials, rendering them highly effective for a range of chemical transformations.

Despite numerous achievements, the preparation of metal SAs using JH technology still faces significant challenges, such as large-scale production, the preparation of multiple metal types on a single carrier, and efficient rapid screening. To address the large-scale production of metal SAs using JH, researchers can draw inspiration from the successful methodologies employed in the large-scale synthesis of high-entropy alloys, oxides, and their heterostructures via the roll-to-roll process. By adapting these methodologies for the preparation of metal SAs, researchers aim to develop suitable carriers and precursors utilized in the process based on the mechanism of preparing SAs by JH. The synthesis of multiple SA metals on a single support via JH remains an aspirational goal. This concept, driven by the potential benefits of bimetallic or even multi-metallic interactions, offers promising pathways to further enhance catalytic performance. However, this objective presents substantial challenges. The intricate interplay between different metal atoms and their interactions with the support material requires precise control to achieve uniform atomic dispersion and long-term stability. To address the challenge, innovations in JH methodologies are imperative. Techniques such as pulsed current heating and localized heating via nanoscale electrodes are currently being explored to achieve uniform heating and mitigate the agglomeration of metal atoms during synthesis. Future research should focus on developing innovative support materials specifically tailored for multi-metallic SAs and refining JH strategies to facilitate controlled and scalable synthesis. Advanced computational modeling and in situ characterization techniques will also be crucial in elucidating the intricate dynamics of multi-metallic interactions and guiding the rational design of these materials. To achieve rapid screening, it is essential to integrate contemporary AI technologies by applying machine learning to assist in process optimization. A comprehensive database should be established, encompassing carrier types, pretreatment conditions, and heating parameters (temperature, time, rate). Through the use of a neural network model, the SA loading efficiency can be predicted with high accuracy. Furthermore, an automated system for screening the optimal process can be developed.

By tackling these challenges, JH synthesis can unlock new possibilities in catalysis, paving the way for the development of highly efficient and versatile materials for emerging energy and environmental applications.

Author contributions

Conceptualization: J. W. and X. L.; investigation: X. S., L. J., Z. C., J. Z., and J. W.; validation: J. W., X. S., L. J., P. D., X. L., and L. Z.; writing (original draft): X. S.; writing (review & editing): all authors. Visualization: J. W.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included, and no new data were generated or analysed as part of this review.

Acknowledgements

The authors are grateful for financial support from the National Natural Science Foundation of China (52472231, 52311530113, W2521017), the Science and Technology Commission of Shanghai Municipality (22DZ1205600), and the Central Guidance on Science and Technology Development Fund of Zhejiang Province (2024ZY01011).

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