Single-Atom Catalysts for Electrochemical Hydrogen Evolution Reaction: Recent Advances and Future Perspectives

Non-Noble-Metal-Based Electrocatalysts for Acidic Oxygen Evolution Reaction: Recent Progress, Challenges, and Perspectives

The oxygen evolution reaction (OER) plays a pivotal role in diverse renewable energy storage and conversion technologies, including water electrolysis, electrochemical CO2 reduction, nitrogen fixation, and metal-air batteries. Among various water electrolysis techniques, proton exchange membrane (PEM)-based water electrolysis devices offer numerous advantages, including high current densities, exceptional chemical stability, excellent proton conductivity, and high-purity H2. Nevertheless, the prohibitive cost associated with Ir/Ru-based OER electrocatalysts poses a significant barrier to the broad-scale application of PEM-based water splitting. Consequently, it is crucial to advance the development of non-noble metal OER catalysis substance with high acid-activity and stability, thereby fostering their widespread integration into PEM water electrolyzers (PEMWEs). In this review, a comprehensive analysis of the acidic OER mechanism, encompassing the adsorbate evolution mechanism (AEM), lattice oxygen mechanism (LOM) and oxide path mechanism (OPM) is offered. Subsequently, a systematic summary of recently reported noble-metal-free catalysts including transition metal-based, carbon-based and other types of catalysts is provided. Additionally, a comprehensive compilation of in situ/operando characterization techniques is provided, serving as invaluable tools for furnishing experimental evidence to comprehend the catalytic mechanism. Finally, the present challenges and future research directions concerning precious-metal-free acidic OER are comprehensively summarized and discussed in this review.

Hydrogen production catalysed by atomically precise metal clusters

Atomically precise metal clusters that possess the exact atom number, definitive composition, and tunable geometric and electronic structures have emerged as ideal model catalysts for many important chemical processes. Recently, metal clusters have been widely used as excellent catalysts for hydrogen production to explore the relationship between the structure and catalytic properties at the atomic level. In this review, we systematically summarize the significant developments concerning metal clusters as electrocatalysts and photocatalysts for hydrogen generation. This review also puts forward the challenges and perspectives of atomically precise metal clusters in electrocatalysis and photocatalysis in the hope of providing a valuable reference for the rational design of high-performance catalysts for hydrogen production.

Next-Generation Green Hydrogen: Progress and Perspective from Electricity, Catalyst to Electrolyte in Electrocatalytic Water Splitting

Green hydrogen from electrolysis of water has attracted widespread attention as a renewable power source. Among several hydrogen production methods, it has become the most promising technology. However, there is no large-scale renewable hydrogen production system currently that can compete with conventional fossil fuel hydrogen production. Renewable energy electrocatalytic water splitting is an ideal production technology with environmental cleanliness protection and good hydrogen purity, which meet the requirements of future development. This review summarizes and introduces the current status of hydrogen production by water splitting from three aspects: electricity, catalyst and electrolyte. In particular, the present situation and the latest progress of the key sources of power, catalytic materials and electrolyzers for electrocatalytic water splitting are introduced. Finally, the problems of hydrogen generation from electrolytic water splitting and directions of next-generation green hydrogen in the future are discussed and outlooked. It is expected that this review will have an important impact on the field of hydrogen production from water.

Enhancing Nitrogen Reduction Reaction through Formation of 2D/2D Hybrid Heterostructures of MoS2@rGO

Given the challenging task of constructing an efficient nitrogen reduction reaction (NRR) electrocatalyst with enhanced ambient condition performance, properties such as high specific surface area, fast electron transfer, and design of the catalyst surface constitute a group of key factors to be taken into consideration to guarantee outstanding catalytic performance and durability. Thereof, this work investigates the contribution of the 2D/2D heterojunction interface between MoS2 and reduced graphene oxide (rGO) on the electrocatalytic synthesis of NH3 in an alkaline media. The results revealed remarkable NRR performance on the MoS2@rGO 2D/2D hybrid electrocatalyst, characterized by a high NRR sensitivity (faradaic efficiency) of 34.7% with an NH3 yield rate of 3.98 ± 0.19 mg h-1 cm-2 at an overpotential of -0.3 V vs RHE in 0.1 M KOH solution. The hybrid electrocatalysts also exhibited selectivity for NH3 synthesis against the production of the hydrazine (N2H4) byproduct, hindrance of the competitive hydrogen evolution reaction (HER), and good durability over an operation period of 8 h. In hindsight, the study presented a low-cost and highly efficient catalyst design for achieving enhanced ammonia synthesis in alkaline media via the formation of defect-rich ultrathin MoS2@rGO nanostructures, consisting predominantly of an HER-hindering hexagonal 2H-MoS2 phase.

A nitrogen-doped carbon nanosheet composited platinum-cobalt single atom alloy catalyst for effective hydrogen evolution reaction

An electrocatalyst with ultra-small PtCo single atom alloy species evenly dispersed on nitrogen-doped ultra-thin carbon nanosheets (PtCo SAA/NC) was designed. The introduction of single-atom Pt not only maximizes the atomic utilization efficiency of Pt species, but also synergistically enhances the charge transfer characteristics of Co cluster surfaces, thereby increasing the migration and evolution rate of hydrogen ions. The PtCo SAA/NC catalyst exhibits a Tafel slope of 42 mV dec-1 and a low overpotential of 45 mV at 10 mA cm-2 in 0.5 M H2SO4 solution.

Strategies Toward High Selectivity, Activity, and Stability of Single-Atom Catalysts

Single-atom catalysts (SACs) hold immense promise in facilitating the rational use of metal resources and achieving atomic economy due to their exceptional atom-utilization efficiency and distinct characteristics. Despite the growing interest in SACs, only limited reviews have holistically summarized their advancements centering on performance metrics. In this review, first, a thorough overview on the research progress in SACs is presented from a performance perspective and the strategies, advancements, and intriguing approaches employed to enhance the critical attributes in SACs are discussed. Subsequently, a comprehensive summary and critical analysis of the electrochemical applications of SACs are provided, with a particular focus on their efficacy in the oxygen reduction reaction , oxygen evolution reaction, hydrogen evolution reaction , CO2 reduction reaction, and N2 reduction reaction . Finally, the outline future research directions on SACs by concentrating on performance-driven investigation, where potential areas for improvement are identified and promising avenues for further study are highlighted, addressing challenges to unlock the full potential of SACs as high-performance catalysts.

Metal Electrocatalysts for Hydrogen Production in Water Splitting

The rising demand for fossil fuels and the resulting pollution have raised environmental concerns about energy production. Undoubtedly, hydrogen is the best candidate for producing clean and sustainable energy now and in the future. Water splitting is a promising and efficient process for hydrogen production, where catalysts play a key role in the hydrogen evolution reaction (HER). HER electrocatalysis can be well performed by Pt with a low overpotential close to zero and a Tafel slope of about 30 mV dec-1. However, the main challenge in expanding the hydrogen production process is using efficient and inexpensive catalysts. Due to electrocatalytic activity and electrochemical stability, transition metal compounds are the best options for HER electrocatalysts. This study will focus on analyzing the current situation and recent advances in the design and development of nanostructured electrocatalysts for noble and non-noble metals in HER electrocatalysis. In general, strategies including doping, crystallization control, structural engineering, carbon nanomaterials, and increasing active sites by changing morphology are helpful to improve HER performance. Finally, the challenges and future perspectives in designing functional and stable electrocatalysts for HER in efficient hydrogen production from water-splitting electrolysis will be described.

Rare earth oxide based electrocatalysts: synthesis, properties and applications

As an important strategic resource, rare earths (REs) constitute 17 elements in the periodic table, namely 15 lanthanides (Ln) (La-Lu, atomic numbers from 57 to 71), scandium (Sc, atomic number 21) and yttrium (Y, atomic number 39). In the field of catalysis, the localization and incomplete filling of 4f electrons endow REs with unique physical and chemical properties, including rich electronic energy level structures, variable coordination numbers, etc., making them have great potential in electrocatalysis. Among various RE catalytic materials, rare earth oxide (REO)-based electrocatalysts exhibit excellent performances in electrocatalytic reactions due to their simple preparation process and strong structural variability. At the same time, the electronic orbital structure of REs exhibits excellent electron transfer ability, which can reduce the band gap and energy barrier values of rate-determining steps, further accelerating the electron transfer in the electrocatalytic reaction process; however, there is a lack of systematic review of recent advances in REO-based electrocatalysis. This review systematically summarizes the synthesis, properties and applications of REO-based nanocatalysts and discusses their applications in electrocatalysis in detail. It includes the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), methanol oxidation reaction (MOR), nitrogen reduction reaction (NRR) and other electrocatalytic reactions and further discusses the catalytic mechanism of REs in the above reactions. This review provides a timely and comprehensive summary of the current progress in the application of RE-based nanomaterials in electrocatalytic reactions and provides reasonable prospects for future electrocatalytic applications of REO-based materials.

Emerging Xene-Based Single-Atom Catalysts: Theory, Synthesis, and Catalytic Applications

In recent years, the emergence of novel 2D monoelemental materials (Xenes), e.g., graphdiyne, borophene, phosphorene, antimonene, bismuthene, and stanene, has exhibited unprecedented potentials for their versatile applications as well as addressing new discoveries in fundamental science. Owing to their unique physicochemical, optical, and electronic properties, emerging Xenes have been regarded as promising candidates in the community of single-atom catalysts (SACs) as single-atom active sites or support matrixes for significant improvement in intrinsic activity and selectivity. In order to comprehensively understand the relationships between the structure and property of Xene-based SACs, this review represents a comprehensive summary from theoretical predictions to experimental investigations. Firstly, theoretical calculations regarding both the anchoring of Xene-based single-atom active sites on versatile support matrixes and doping/substituting heteroatoms at Xene-based support matrixes are briefly summarized. Secondly, controlled synthesis and precise characterization are presented for Xene-based SACs. Finally, current challenges and future opportunities for the development of Xene-based SACs are highlighted.

Single-Atom Catalysts for Selective Oxygen Reduction: Transition Metals in Uniform Carbon Nanospheres with High Loadings

Transition metal single-atom catalysts (SACs) in uniform carbon nanospheres have gained tremendous interest as electrocatalysts owing to their low cost, high activity, and excellent selectivity. However, their preparation typically involves complicated multistep processes that are not practical for industrial use. Herein, we report a facile one-pot method to produce atomically isolated metal atoms with high loadings in uniform carbon nanospheres without any templates or postsynthesis modifications. Specifically, we use a chemical confinement strategy to suppress the formation of metal nanoparticles by introducing ethylenediaminetetraacetic acid (EDTA) as a molecular barrier to spatially isolate the metal atoms and thus generate SACs. To demonstrate the versatility of this synthetic method, we produced SACs from multiple transition metals, including Fe, Co, Cu, and Ni, with loadings as high as 3.87 wt %. Among these catalytic materials, the Fe-based SACs showed remarkable catalytic activity toward the oxygen reduction reaction (ORR), achieving an onset and half-wave potential of 1.00 and 0.831 VRHE, respectively, comparable to that of commercial 20 wt % Pt/C. Significantly, we were able to steer the ORR selectivity toward either energy generation or hydrogen peroxide production by simply changing the transition metal in the EDTA-based precursor.

Atomically Dispersed Ruthenium Catalysts with Open Hollow Structure for Lithium-Oxygen Batteries

Lithium-oxygen battery with ultra-high theoretical energy density is considered a highly competitive next-generation energy storage device, but its practical application is severely hindered by issues such as difficult decomposition of discharge products at present. Here, we have developed N-doped carbon anchored atomically dispersed Ru sites cathode catalyst with open hollow structure (h-RuNC) for Lithium-oxygen battery. On one hand, the abundance of atomically dispersed Ru sites can effectively catalyze the formation and decomposition of discharge products, thereby greatly enhancing the redox kinetics. On the other hand, the open hollow structure not only enhances the mass activity of atomically dispersed Ru sites but also improves the diffusion efficiency of catalytic molecules. Therefore, the excellent activity from atomically dispersed Ru sites and the enhanced diffusion from open hollow structure respectively improve the redox kinetics and cycling stability, ultimately achieving a high-performance lithium-oxygen battery.

Hierarchical Dual Single-Atom Catalysts with Coupled CoN4 and NiN4 Moieties for Industrial-Level CO2 Electroreduction to Syngas

Renewable-driven electrochemical CO2 reduction reaction (CO2RR) to syngas is an encouraging alternative strategy to traditional fossil fuel-based syngas production, and the development of industrial-level electrocatalysts is vital. Herein, based on theoretical optimization of metal species, hierarchical CoxNi1-x-N-C dual single-atom catalyst (DSAC) with individual NiN4 (CO preferential) and CoN4 (H2 preferential) moieties was constructed by a two-step pyrolysis route. The Co0.5Ni0.5-N-C exhibits a stable CO Faradaic efficiency of 50 ± 5% and an industrial-level current density of 101-365 mA cm-2 in an ultrawide potential window of -0.5 to -1.1 V. The CO/H2 ratio of syngas can be conveniently tuned by regulating the Co/Ni ratio. The coupled effect of NiN4 and CoN4 moieties under a local high-pH microenvironment is responsible for the regulation of the CO/H2 selectivity and yield for the CoxNi1-x-N-C catalyst, which is not present in the mixed Co-N-C and Ni-N-C catalyst. This study provides a promising DSAC strategy for achieving industrial-level syngas production via CO2RR.

Single-atom nanozymes: classification, regulation strategy, and safety concerns

Nanozymes, nanomaterials possessing enzymatic activity, have been studied extensively by researchers. However, their complex composition, low density of active sites, and inadequate substrate selectivity have hindered the maturation and widespread acceptance of nanozymes. Single-atom nanozymes (SAzymes) with atomically dispersed active sites are leading the field of catalysis due to their exceptional performance. The maximum utilization rate of atoms, low cost, well-defined coordination structure, and active sites are the most prominent advantages of SAzymes that researchers favor. This review systematically categorizes SAzymes based on their support type and describes their specific applications. Additionally, we discuss regulation strategies for SAzyme activity and provide a comprehensive summary of biosafety challenges associated with these enzymes.

Mechanistic insights on Bi-potentiodynamic control towards atomistic synthesis of electrocatalysts for hydrogen evolution reaction

Herein, electrochemically assisted dissolution-deposition (EADD) is utilized over a three-electrode assembly to prepare an electrocatalyst for hydrogen evolution reaction (HER). Cyclic voltammetry is performed to yield atomistic loading of platinum (Pt) over SnS2 nanostructures via Pt dissolution from the counter electrode (CE). Astonishingly, the working electrode (WE) swept at 50 mV/s is found to compel Pt CE to experience 1000-3000 mV/s. The effect of different potential scan rates at the WE have provided insight into the change in Pt dissolution and its deposition behaviour over SnS2 in three electrode assembly. However, uncontrolled overpotentials at CE in a three-electrode assembly made Pt dissolution-deposition behavior complex. Here, for the first time, we have demonstrated bi-potentiodynamic control for dissolution deposition of Pt in four-electrode assembly over Nickel (Ni) foam. The dual cyclic voltammetry is applied to achieve better control and efficiency of the EADD process, engendering it as a pragmatically versatile and scalable synthesis technique.

Recent progress of molybdenum carbide based electrocatalysts for electrocatalytic hydrogen evolution reaction

Electrocatalytic hydrogen evolution reaction (HER) is one of the most promising and clean strategies to prepare hydrogen on a large scale. Nevertheless, the efficiency of HER is greatly restricted by the large overpotential at the anode, and it is necessary to develop low cost electrocatalysts with excellent performance and stability. Molybdenum carbide has shown great potential in the field of HER due to its unique electronic structure and physical and chemical properties. In this paper, the current progress of molybdenum carbide-based catalysts for HER is summarized. The influence of phase structure, nanostructure, heterostructure and heteroatoms doping on its catalytic performance is discussed in detail. Especially, the catalytic mechanisms are analyzed according to structural characterization and theoretical calculation results. Finally, the challenges and prospects for the further development of molybdenum carbide-based catalysts for HER are put forward to guide the progress of this field.

Strain-Regulated Pt-NiO@Ni Sub-Micron Particles Achieving Bifunctional Electrocatalysis for Zinc-Air Battery

Highly active bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) have always been the key factors to affect the performance of zinc-air batteries. However, integrating the independent reaction sites of ORR and OER in a catalyst remains a major challenge. Herein, a collaborative strategy based on defect induction and doping is proposed to prepare the strain-regulated Pt-NiO@Ni sub-micron particles (Pt-NiO@Ni SP). Benefiting from the synergistic effect of tensile strain and Pt-doped, the metallic Ni-based sub-micron particles with tensile strain as the catalyst carriers can effectively optimize the electronic distribution of atomic structures in Pt and NiO on the surface of particles, leading to reduce the energy barrier of intermediates for ORR and OER. Consequently, the Pt-NiO@Ni SP exhibits outstanding bifunctional catalytic activity with the ΔE index of 0.65 V under a low Pt loading, outperforming that of the benchmark Pt/C+IrO2 catalysts (0.76 V). Impressively, the Pt-NiO@Ni SP-based liquid zinc-air battery develops a high open-circuit potential (1.47 V), excellent energy density (188.2 mW cm-2 ), and favorable cyclic charge-discharge cycling durability (200 h at 20 mA cm-2 ). This work provides an innovative avenue for the rational construction of highly active bifunctional electrocatalysts for practical applications.

Structure-Property Relationship of Defect-Trapped Pt Single-Site Electrocatalysts for the Hydrogen Evolution Reaction

Single-site catalysts (SSCs) have attracted significant research interest due to their high metal atom utilization. Platinum single sites trapped in the defects of carbon substrates (trapped Pt-SSCs) have been proposed as efficient and stable electrocatalysts for the hydrogen evolution reaction (HER). However, the correlation between Pt bonding environment, its evolution during operation, and catalytic activity is still unclear. Here, a trapped Pt-SSC is synthesized by pyrolysis of H₂PtCl₆ chemisorbed on a polyaniline substrate. In situ heated scanning transmission electron microscopy and temperature-dependent X-ray photoelectron spectroscopy clarify the thermally induced structural evolution of Pt during pyrolysis. The results show that the nitrogen in polyaniline coordinates with Pt ions and atomically disperses them before pyrolysis and traps Pt sites at pyridinic N defects generated during the substrate graphitization. Operando X-ray absorption spectroscopy confirms that the trapped Pt-SSC is stable at the HER working potentials but with inferior electrocatalytic activity compared with metallic Pt nanoparticles. First principle calculations suggest that the inferior activity of trapped Pt-SSCs is due to their unfavorable hydrogen chemisorption energy relative to metallic Pt(111) surfaces. These results further the understanding of the structure-property relationship in trapped Pt-SSCs and motivate a detailed techno-economic analysis to evaluate their commercial applicability.

Artificial Photosynthesis: Current Advancements and Future Prospects

Artificial photosynthesis is a technology with immense potential that aims to emulate the natural photosynthetic process. The process of natural photosynthesis involves the conversion of solar energy into chemical energy, which is stored in organic compounds. Catalysis is an essential aspect of artificial photosynthesis, as it facilitates the reactions that convert solar energy into chemical energy. In this review, we aim to provide an extensive overview of recent developments in the field of artificial photosynthesis by catalysis. We will discuss the various catalyst types used in artificial photosynthesis, including homogeneous catalysts, heterogeneous catalysts, and biocatalysts. Additionally, we will explore the different strategies employed to enhance the efficiency and selectivity of catalytic reactions, such as the utilization of nanomaterials, photoelectrochemical cells, and molecular engineering. Lastly, we will examine the challenges and opportunities of this technology as well as its potential applications in areas such as renewable energy, carbon capture and utilization, and sustainable agriculture. This review aims to provide a comprehensive and critical analysis of state-of-the-art methods in artificial photosynthesis by catalysis, as well as to identify key research directions for future advancements in this field.

Electrochemically Grown Ultrathin Platinum Nanosheet Electrodes with Ultralow Loadings for Energy-Saving and Industrial-Level Hydrogen Evolution

Nanostructured catalyst-integrated electrodes with remarkably reduced catalyst loadings, high catalyst utilization and facile fabrication are urgently needed to enable cost-effective, green hydrogen production via proton exchange membrane electrolyzer cells (PEMECs). Herein, benefitting from a thin seeding layer, bottom-up grown ultrathin Pt nanosheets (Pt-NSs) were first deposited on thin Ti substrates for PEMECs via a fast, template- and surfactant-free electrochemical growth process at room temperature, showing highly uniform Pt surface coverage with ultralow loadings and vertically well-aligned nanosheet morphologies. Combined with an anode-only Nafion 117 catalyst-coated membrane (CCM), the Pt-NS electrode with an ultralow loading of 0.015 mgPt cm-2 demonstrates superior cell performance to the commercial CCM (3.0 mgPt cm-2), achieving 99.5% catalyst savings and more than 237-fold higher catalyst utilization. The remarkable performance with high catalyst utilization is mainly due to the vertically well-aligned ultrathin nanosheets with good surface coverage exposing abundant active sites for the electrochemical reaction. Overall, this study not only paves a new way for optimizing the catalyst uniformity and surface coverage with ultralow loadings but also provides new insights into nanostructured electrode design and facile fabrication for highly efficient and low-cost PEMECs and other energy storage/conversion devices.

Double-Confined Ultrafine Cobalt Clusters for Efficient Peroxide Activation

The construction of highly active catalysts presents great prospects, while it is a challenge for peroxide activation in advanced oxidation processes (AOPs). Herein, we facilely developed ultrafine Co clusters confined in mesoporous silica nanospheres containing N-doped carbon (NC) dots (termed as Co/NC@mSiO₂) via a double-confinement strategy. Compared with the unconfined counterpart, Co/NC@mSiO₂ exhibited unprecedented catalytic activity and durability for removal of various organic pollutants even in extremely acidic and alkaline environments (pH from 2 to 11) with very low Co ion leaching. Experiments and density functional theory (DFT) calculations proved that Co/NC@mSiO₂ possessed strong peroxymonosulphate (PMS) adsorption and charge transfer capability, enabling the efficient O-O bond dissociation of PMS to HO^• and SO₄^•- radicals. The strong interaction between Co clusters and mSiO₂ containing NC dots contributed to excellent pollutant degradation performances by optimizing the electronic structures of Co clusters. This work represents a fundamental breakthrough in the design and understanding of the double-confined catalysts for peroxide activation.

Nanocomposite Electrocatalysts for Hydrogen Evolution Reactions (HERs) for Sustainable and Efficient Hydrogen Energy-Future Prospects

Hydrogen is considered a good clean and renewable energy substitute for fossil fuels. The major obstacle facing hydrogen energy is its efficacy in meeting its commercial-scale demand. One of the most promising pathways for efficient hydrogen production is through water-splitting electrolysis. This requires the development of active, stable, and low-cost catalysts or electrocatalysts to achieve optimized electrocatalytic hydrogen production from water splitting. The objective of this review is to survey the activity, stability, and efficiency of various electrocatalysts involved in water splitting. The status quo of noble-metal- and non-noble-metal-based nano-electrocatalysts has been specifically discussed. Various composites and nanocomposite electrocatalysts that have significantly impacted electrocatalytic HERs have been discussed. New strategies and insights in exploring nanocomposite-based electrocatalysts and utilizing other new age nanomaterial options that will profoundly enhance the electrocatalytic activity and stability of HERs have been highlighted. Recommendations on future directions and deliberations for extrapolating information have been projected.

Recent advances in the theoretical studies on the electrocatalytic CO2 reduction based on single and double atoms

Excess of carbon dioxide (CO2) in the atmosphere poses a significant threat to the global climate. Therefore, the electrocatalytic carbon dioxide reduction reaction (CO2RR) is important to reduce the burden on the environment and provide possibilities for developing new energy sources. However, highly active and selective catalysts are needed to effectively catalyze product synthesis with high adhesion value. Single-atom catalysts (SACs) and double-atom catalysts (DACs) have attracted much attention in the field of electrocatalysis due to their high activity, strong selectivity, and high atomic utilization. This review summarized the research progress of electrocatalytic CO2RR related to different types of SACs and DACs. The emphasis was laid on the catalytic reaction mechanism of SACs and DACs using the theoretical calculation method. Furthermore, the influences of solvation and electrode potential were studied to simulate the real electrochemical environment to bridge the gap between experiments and computations. Finally, the current challenges and future development prospects were summarized and prospected for CO2RR to lay the foundation for the theoretical research of SACs and DACs in other aspects.

Atomically dispersed Mn atoms coordinated with N and O within an N-doped porous carbon framework for boosted oxygen reduction catalysis

Developing efficient and robust catalysts to replace Pt group metals for the oxygen reduction reaction (ORR) is conducive to achieving highly efficient energy conversion. Here, we develop a general ion exchange strategy to construct highly efficient ORR catalysts consisting of various atomically dispersed metal atoms anchored on N-doped porous carbon (M-SAs/NC) to investigate the atomic structure-catalytic activity relationship. The structure characterization results demonstrated that the achieved atomic structure varied due to the presence of different metal centers. Mn-SAs/NC consists of MnN₃O₁ centers, in which the Mn single atoms are stabilized by 3 N and 1 O. In contrast, the center metals in Fe-/Co-/Cu single-atom catalysts are coordinated by merely N atoms. Mn-SAs/NC delivers superior performance for the ORR with a half-wave potential (E_1/2) of 0.91 V vs. RHE in 0.1 M KOH solution, outperforming that of the commercial Pt/C catalyst and the control Fe-/Co-/Cu single-atom catalysts. Furthermore, Mn-SAs/NC also shows excellent methanol tolerance and stability up to 5000 cycles. Density functional theory (DFT) calculations reveal that Mn single atom catalysts with MnN₃O₁ centers contributed to the superior ORR performance with lower energy barriers and optimized adsorption capacity of intermediates. These findings provide insights into the design and development of specific coordinated structures of atomically dispersed catalysts to facilitate the practical applications of energy conversion.

Bioinspired and Bioderived Aqueous Electrocatalysis

The development of efficient and sustainable electrochemical systems able to provide clean-energy fuels and chemicals is one of the main current challenges of materials science and engineering. Over the last decades, significant advances have been made in the development of robust electrocatalysts for different reactions, with fundamental insights from both computational and experimental work. Some of the most promising systems in the literature are based on expensive and scarce platinum-group metals; however, natural enzymes show the highest per-site catalytic activities, while their active sites are based exclusively on earth-abundant metals. Additionally, natural biomass provides a valuable feedstock for producing advanced carbonaceous materials with porous hierarchical structures. Utilizing resources and design inspiration from nature can help create more sustainable and cost-effective strategies for manufacturing cost-effective, sustainable, and robust electrochemical materials and devices. This review spans from materials to device engineering; we initially discuss the design of carbon-based materials with bioinspired features (such as enzyme active sites), the utilization of biomass resources to construct tailored carbon materials, and their activity in aqueous electrocatalysis for water splitting, oxygen reduction, and CO₂ reduction. We then delve in the applicability of bioinspired features in electrochemical devices, such as the engineering of bioinspired mass transport and electrode interfaces. Finally, we address remaining challenges, such as the stability of bioinspired active sites or the activity of metal-free carbon materials, and discuss new potential research directions that can open the gates to the implementation of bioinspired sustainable materials in electrochemical devices.

Interfacial Chemical Bond Modulation of Co3(PO4)2-MoO3-x Heterostructures for Alkaline Water/Seawater Splitting

The development of a high current density with high energy conversion efficiency electrocatalyst is vital for large-scale industrial application of alkaline water splitting, particularly seawater splitting. Herein, we design a self-supporting Co₃(PO₄)₂-MoO_3-x/CoMoO₄/NF superaerophobic electrode with a three-dimensional structure for high-performance hydrogen evolution reaction (HER) by a reasonable devise of possible "Co-O-Mo hybridization" on the interface. The "Co-O-Mo hybridization" interfaces induce charge transfer and generation of fresh oxygen vacancy active sites. Consequently, the unique heterostructures greatly facilitate the dissociation process of H₂O molecules and enable efficient hydrogen spillover, leading to excellent HER performance with ultralow overpotentials (76 and 130 mV at 100 and 500 mA cm^-2) and long-term durability of 100 h in an alkaline electrolyte. Theoretical calculations reveal that the Co₃(PO₄)₂-MoO_3-x/CoMoO₄/NF promotes the adsorption/dissociation process of H₂O molecules to play a crucial role in improving the stability and activity of HER. Our results exhibit that the HER activity of non-noble metal electrocatalysts can be greatly enhanced by rational interfacial chemical bonding to modulate the heterostructures.

Advances in the Electrocatalytic Hydrogen Evolution Reaction by Metal Nanoclusters-based Materials

With the development of renewable energy systems, clean hydrogen is burgeoning as an optimal alternative to fossil fuels, in which its application is promising to retarding the global energy and environmental crisis. The hydrogen evolution reaction (HER), capable of producing high-purity hydrogen rapidly in electrocatalytic water splitting, has received much attention. Abundant research about HER has been done, focusing on advanced electrocatalyst design with high efficiency and robust stability. As potential HER catalysts, metal nanoclusters (MNCs) have been studied extensively. They are composed of several to a hundred metal atoms, with sizes being comparable to the Fermi wavelength of electrons, that is, < 2.0 nm. Different from metal atoms/nanoparticles, they exhibit unique catalytic properties due to their quantum size effect and low-coordination environment. In this review, the activity-enhancing approaches of MNCs applied in HER electrocatalysis are mainly summarized. Furthermore, recent progress in MNCs classified with different stabilization strategies, that is, the freestanding MNCs, MNCs with organic, metal and carbon supports, are introduced. Finally, the current challenges and deficiencies of these MNCs for HER are prospected.

Electron-Deficient Zn-N6 Configuration Enabling Polymeric Carbon Nitride for Visible-Light Photocatalytic Overall Water Splitting

Despite of suitable band structures for harvesting solar light and driving water redox reactions, polymeric carbon nitride (PCN) has suffered from poor charge transfer ability and sluggish surface reaction kinetics, which limit its photocatalytic activity for water splitting. Herein, atomically dispersed Zn-coordinated three-dimensional (3D) sponge-like PCN (Zn-PCN) is synthesized through a novel intermediate coordination strategy. Advanced characterizations and theoretical calculations well evidence that Zn single atoms are coordinated and stabilized on PCN in the form of Zn-N₆ configuration featured with an electron-deficient state. Such an electronic configuration has been demonstrated contributive to promoted electron excitation, accelerated charge separation and transfer as well as reduced water redox barriers. Further benefited from the abundant surface active sites derived from the 3D porous structure, Zn-PCN realizes visible-light photocatalysis for overall water splitting with H₂ and O₂ simultaneously evolved at a stoichiometric ratio of 2:1. This work brings new insights into the design of novel single-atom photocatalysts by deepening the understanding of electronic configurations and reactive sites favorable to excellent photocatalysis for water splitting and related solar energy conversion reactions.

Competitive Coordination-Pairing between Ru Clusters and Single-Atoms for Efficient Hydrogen Evolution Reaction in Alkaline Seawater

The coordination environment of Ru centers determines their catalytic performance, however, much less attention is focused on cluster-induced charge transfer in a Ru single-atom system. Herein, by density functional theory (DFT) calculations, a competitive coordination-pairing between Ru clusters (RuRu bond) and single-atoms (RuO bond) is revealed leading to the charge redistribution between Ru and O atoms in ZnFe₂ O₄ units which share more free electrons to participate in the hydrogen desorption process, optimizing the proton adsorption and hydrogen desorption. Thus, a clicking confinement strategy for building a competitive coordination-pairing between Ru clusters and single-atoms anchored on ZnFe₂ O_x nanosheets over carbon via RuO ligand (Ru_{1, n} -ZnFe₂ O_x -C) is proposed. Benefiting from the optimized coordination effect and the electronic synergy between Ru clusters and single-atoms, such a catalyst demonstrates the excellent activity and excellent stability in alkaline and seawater media, which has exceptional hydrogen evolution reaction activity with overpotentials as low as 10.1 and 15.9 mV to reach the current density of 10 mA cm^-2 in alkaline and seawater media, respectively, higher than that of commercial Pt/C catalysts as a benchmark. Furthermore, it owns remarkably outstanding mass activity, approximately 2 and 8 times higher than that of Pt catalysts in alkaline and seawater media, respectively.

Synergizing Hydrogen Spillover and Deprotonation by the Internal Polarization Field in a MoS2 /NiPS3 Vertical Heterostructure for Boosted Water Electrolysis

Hydrogen spillover (HSo) has emerged to upgrade the hydrogen evolution reaction (HER) activity of Pt-support electrocatalysts, but it is not applicable to the deprotonated oxygen evolution reaction (OER). Non-precious catalysts that can perform well in both HSo and deprotonation (DeP) are extremely desirable for a sustainable hydrogen economy. Herein, an affordable MoS₂ /NiPS₃ vertical heterostructure catalyst is presented to synergize HSo and DeP for efficient water electrolysis. The internal polarization field (IPF) is clarified as the driving force of HSo in HER electrocatalysis. The HSo from the MoS₂ edge to NiPS₃ can activate the NiPS₃ basal plane to boost the HER activity of the MoS₂ /NiPS₃ heterostructure (112 mV vs reversible hydrogen electrode (RHE) at 10 mA cm^-2 ), while for OER, the IPF in the heterostructure can facilitate the hydroxyl diffusion and render MoS₂ -to-NiPS₃ /P-to-S dual-pathways for DeP. As a result, the stacking of OER-inactive MoS₂ on the NiPS₃ surface still brings intriguing OER enhancements. With them serving as electrode couples, the overall water splitting is attested stably with a cell voltage of 1.64 V at 10 mA cm^-2 . This research puts forward the IPF as the criterion in the rational design of HSo/DeP-unified non-precious catalysts for efficient water electrolysis.

Coupling Amorphous Ni Hydroxide Nanoparticles with Single-Atom Rh on Cu Nanowire Arrays for Highly Efficient Alkaline Seawater Electrolysis

Exploring efficient catalysts for alkaline seawater electrolysis is highly desired yet challenging. Herein, coupling single-atom rhodium with amorphous nickel hydroxide nanoparticles on copper nanowire arrays is designed as a new active catalyst for the highly efficient alkaline seawater electrolysis. We found that an amorphous Ni(OH)₂ nanoparticle is an effective catalyst to accelerate the water dissociation step. In contrast, the single-atom rhodium is an active site for adsorbed hydrogen recombination to generate H₂. The NiRh-Cu NA/CF catalyst shows superior electrocatalytic activity toward HER, surpassing a benchmark Pt@C. In detail, the NiRh-Cu NA/CF catalyst exhibits HER overpotentials as low as 12 and 21 mV with a current density of 10 mA cm^-2 in fresh water and seawater, respectively. At high current density, the NiRh-Cu NA/CF catalyst also exhibits an outstanding performance, where 300 mA cm^-2 can be obtained at an overpotential of 155 mV and shows a slight fluctuation in the current density over 30 h.

Single-Atom Catalysts (SACs) for Photocatalytic CO2 Reduction with H2 O: Activity, Product Selectivity, Stability, and Surface Chemistry

In recent years, single-atom catalysts (SACs) have attracted the interest of researchers owing to their suitability for various catalytic applications. For instance, their optoelectronic features, site-specific activity, and cost-effectiveness make SACs ideal for photocatalytic CO₂ reduction. The activity, product selectivity, and photostability of SACs depend on various factors such as the nature of the metal/support material, the interaction between the metal atoms and support, light-harvesting ability, charge separation behavior, CO₂ adsorption ability, active sites, and defects. Consequently, it is necessary to investigate these factors in depth to elucidate the working principle(s) of SACs for catalytic applications. Herein, the recent progress in the development of SACs for photocatalytic CO₂ reduction with H₂ O is reviewed. First, a brief overview of CO₂ photoreduction and SACs for CO₂ conversion is provided. Several synthesis strategies and useful techniques for characterizing SACs employed in heterogeneous catalysis are then described. Next, the challenges of SACs for photocatalytic CO₂ reduction and related optimization strategies, in terms of activity, product selectivity, and stability, are explored. The progress in the development of noble metal- and transition metal-based SACs and dual-SACs for photocatalytic CO₂ reduction is discussed. Finally, the prospects of SACs for CO₂ reduction are considered.

Nanoengineering of Catalysts for Enhanced Hydrogen Production

Hydrogen (H2) has emerged as a sustainable energy carrier capable of replacing/complementing the global carbon-based energy matrix. Although studies in this area have often focused on the fundamental understanding of catalytic processes and the demonstration of their activities towards different strategies, much effort is still needed to develop high-performance technologies and advanced materials to accomplish widespread utilization. The main goal of this review is to discuss the recent contributions in the H2 production field by employing nanomaterials with well-defined and controllable physicochemical features. Nanoengineering approaches at the sub-nano or atomic scale are especially interesting, as they allow us to unravel how activity varies as a function of these parameters (shape, size, composition, structure, electronic, and support interaction) and obtain insights into structure–performance relationships in the field of H2 production, allowing not only the optimization of performances but also enabling the rational design of nanocatalysts with desired activities and selectivity for H2 production. Herein, we start with a brief description of preparing such materials, emphasizing the importance of accomplishing the physicochemical control of nanostructures. The review finally culminates in the leading technologies for H2 production, identifying the promising applications of controlled nanomaterials.

Nickel sulfide nanorods decorated on graphene as advanced hydrogen evolution electrocatalysts in acidic and alkaline media

Nowadays, the fabrication of robust and earth-abundant hydrogen evolution electrocatalysts with noble-metal-like catalytic activities is still facing great challenges. In this report, nanorod (NR)-shaped nickel sulfide (NiS) is successfully decorated on graphene (Gr) by utilizing carbon cloth (CC) as a substrate (NiS-Gr-CC). Benefiting from the NR morphology and strong interfacial synergetic effect between NiS and Gr, the NiS-Gr-CC electrocatalyst shows good catalytic activity for hydrogen evolution reaction (HER). Specifically, the low Tafel slopes of 46 and 56 mV dec^-1 along with the small overpotentials of 66 and 71 mV at 10 mA cm^-2 are obtained in the acidic and alkaline electrolytes, respectively. Density functional theory results indicate that the combination of NiS and Gr can optimize the adsorption energy of H* during the HER process. The long-term durability measurement result reveals that our NiS-Gr-CC heterostructure has good electrocatalytic cycling stability (∼80 h) in both acidic and alkaline electrolytes. These results confirm that the NiS-Gr-CC heterostructure is a promising candidate for hydrogen evolution electrocatalyst with high catalytic activity.

Efficient Hydrogen Evolution Reaction with Bulk and Nanostructured Mitrofanovite Pt3Te4

Here, we discuss the key features of electrocatalysis with mitrofanovite (Pt₃Te₄), a recently discovered mineral with superb performances in hydrogen evolution reaction. Mitrofanovite is a layered topological metal with spin-polarized topological surface states with potential applications for spintronics. However, mitrofanovite is also an exceptional platform for electrocatalysis, with costs of the electrodes suppressed by 47% owing to the partial replacement of Pt with Te. Remarkably, the Tafel slope in nanostructured mitrofanovite is just 33 mV/dec, while reduced mitrofanovite has the same Tafel slope (36 mV/dec) as state-of-the-art electrodes of pure Pt. Mitrofanovite also affords surface stability and robustness to CO poisoning. Accordingly, these findings pave the way for the advent of mitrofanovite for large-scale hydrogen production.

Tuning Phase Structure of Nickel-Ruthenium Alloys via MOFs In Situ Hydrolysis toward Enhanced Hydrogen Evolution Performance in Alkaline

Metal organic frameworks (MOFs) and corresponding derivatives have attracted wide attention. As electrocatalysts, these derivatives (metal, metal compound, and associated composites) have a wide range of application in water-splitting devices, fuel cells, and other hydrogen-related technologies. However, with the exception of pyrolysis, limited studies have documented generated metal nanoparticles from MOFs hydrolysis reactions. Herein, NiRu dual-phase alloy nanoparticles are synthesized via in situ MOFs hydrolysis mediating solvothermal reduction reaction. The hcp-phase NiRu alloys can be rationally tuned by modulating experimental parameters of feeding metal ratio and reaction time. The volcanic link between hydrogen evolution reaction activity and the descriptor of d band center is investigated using experimentally determined valence bands. Furthermore, compared with fcc-phase NiRu alloys, it is theoretically revealed that hcp-phase NiRu alloys optimize d band structure and have a lower energy barrier. This finding broadens the range of application for MOFs hydrolysis reactions and highlights advantages of metal alloys manufactured from MOFs hydrolysis reactions.

Vanadium doped FeP nanoflower with optimized electronic structure for efficient hydrogen evolution

Reasonable design of hydrogen evolution reaction (HER) electrocatalyst from the perspective of electronic structure is a vital way to optimize the catalytic activity. Mono-metallic iron-based phosphates have been shown to be active toward HER, but their performance remains unsatisfactory despite their abundant reserves and low preparation cost. Here, guided by the d-band center and band structure theories, V-doped FeP nanoflower grown directly on iron foam are constructed. Combining the density functional theory (DFT) simulations with physical characterizations reveal that the enhanced HER activity is mainly attributed to the lowed d-band central position, increased water dissociation capacity, decreased hydrogen formation energy barrier and reduced charge transfer impedance. As a HER catalyst in 1 M KOH, the obtained V-FeP shows low overpotentials of ∼149, ∼246 and ∼290 mV to deliver the current densities of 100, 500 and 1000 mA cm^-2 with at least 24 h. When coupled with other highly active oxygen evolution reaction (OER) catalyst (NiFe-LDH/IF), the NiFe-LDH/IF⁽⁺⁾ || V-FeP/IF^(-) pair also performs a low cell voltage and over 100-h stability at high current density of 1000 mA cm^-2, which endows it a large potential in the practical electrolytic water industry. Our work may provide a reference for the enhancement of inert and low-cost HER-active iron phosphide.

PtAu Nanoparticles Supported by Reduced Graphene Oxide as a Highly Active Catalyst for Hydrogen Evolution

PtAu nanoparticles spontaneously deposited on graphene support, PtAu/rGO, have shown remarkably high catalytic activity for hydrogen evolution reaction (HER) in sulfuric acid solution. SEM images of the PtAu/rGO electrode surface showed that Pt nanoparticles that are non-uniform in size occupy both the edges of previously deposited uniform Au nanoparticles and the edges of graphene support. XPS analysis showed that the atomic percentages of Au and Pt in PtAu/rGO were 0.6% and 0.3%, respectively. The atomic percentage of Au alone on previously prepared Au/rGO was 0.7%. Outstanding HER activity was achieved for the PtAu/rGO electrode, showing the initial potential close to the equilibrium potential for HER and a low Tafel slope of −38 mV/dec. This was confirmed by electrochemical impedance spectroscopy. The chronoamperometric measurement performed for 40 min for hydrogen evolution at a constant potential indicated good stability and durability of the PtAu/rGO electrode.

Role of Dihydride and Dihydrogen Complexes in Hydrogen Evolution Reaction on Single-Atom Catalysts

The hydrogen evolution reaction (HER) has a key role in electrochemical water splitting. Recently a lot of attention has been dedicated to HER from single atom catalysts (SACs). The activity of SACs in HER is usually rationalized or predicted using the original model proposed by Nørskov where the free energy of a H atom adsorbed on an extended metal surface M (formation of an MH intermediate) is used to explain the trends in the exchange current for HER. However, SACs differ substantially from metal surfaces and can be considered analogues of coordination compounds. In coordination chemistry, at variance with metal surfaces, stable dihydride or dihydrogen complexes (HMH) can form. We show that the same can occur on SACs and that the formation of stable HMH intermediates, in addition to the MH one, may change the kinetics of the process. Extending the original kinetic model to the case of two intermediates (MH and HMH), one obtains a three-dimensional volcano plot for the HER on SACs. DFT numerical simulations on 55 models demonstrate that the new kinetic model may lead to completely different conclusions about the activity of SACs in HER. The results are validated against selected experimental cases. The work provides an example of the important analogies between the chemistry of SACs and that of coordination compounds.

What Is the Real State of Single-Atom Catalysts under Electrochemical Conditions—From Adsorption to Surface Pourbaix Plots?

The interest in single-atom catalysts (SACs) is increasing, as these materials have the ultimate level of catalyst utilization, while novel reactions where SACs are used are constantly being discovered. However, to properly understand SACs and to further improve these materials, it is necessary to consider the nature of active sites under operating conditions. This is particularly important when SACs are used as electrocatalysts due to harsh experimental conditions, including extreme pH values or high anodic and cathodic potential. In this contribution, density functional theory-based thermodynamic modelling is used to address the nature of metal centers in SACs formed by embedding single metal atoms (Ru, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au) into graphene monovacancy. Our results suggest that none of these SAC metal centers are clean at any potential or pH in the water thermodynamic stability region. Instead, metal centers are covered with Hads, OHads, or Oads, and in some cases, we observed the restructuring of the metal sites due to oxygen incorporation. Based on these findings, it is suggested that setting up theoretical models for SAC modelling and the interpretation of ex situ characterization results using ultra-high vacuum (UHV) techniques requires special care, as the nature of SAC active sites under operating conditions can significantly diverge from the basic models or the pictures set by the UHV measurements.

Design of Aligned Porous Carbon Films with Single-Atom Co-N-C Sites for High-Current-Density Hydrogen Generation

Metal- and nitrogen-doped carbon (M-N-C) materials as a unique class of single-atom catalysts (SACs) have increasingly attracted attention as the replacement of platinum for the hydrogen evolution reaction (HER); however, their employment as HER electrodes at high current densities of industrial level remains a grand challenge. Herein, an aligned porous carbon film embedded with single-atom Co-N-C sites of exceptional activity and stability at high current densities is designed. Within the film, the atomic CoN_x moieties exhibit high intrinsic activity, while the multiscale porosity of the carbon frameworks with vertically aligned microchannels afford facilitated mass transfer under the conditions of high production rate and ultrathick electrodes. Moreover, the superwetting properties of the film promote electrolyte wetting and ensure the timely removal of the evolving H₂ gas bubbles. The as-designed film can work as an efficient HER electrode to deliver 500 and 1000 mA cm^-2 in acid at overpotentials of 272 and 343 mV, respectively, and can operate uninterruptedly and stably at 1000 mA cm^-2 for at least 32 h under static conditions. These findings pave the road toward the rational design of SACs with improved activity and stability at high current densities in gas-evolving electrocatalytic processes.

Metal-Organic Frameworks-Derived Self-Supported Carbon-Based Composites for Electrocatalytic Water Splitting

Electrocatalytic water splitting has been considered as a promising strategy for the sustainable evolution of hydrogen energy and storage of intermittent electric energy. Efficient catalysts for electrocatalytic water splitting are urgently demanded to decrease the overpotentials and promote the sluggish reaction kinetics. Carbon-based composites, including heteroatom-doped carbon materials, metals/alloys@carbon composites, metal compounds@carbon composites, and atomically dispersed metal sites@carbon composites have been widely used as the catalysts due to their fascinating properties. However, these electrocatalysts are almost powdery form, and should be cast on the current collector by using the polymeric binder, which would result in the unsatisfied electrocatalytic performance. In comparison, a self-supported electrode architecture is highly attractive. Recently, self-supported metal-organic frameworks (MOFs) constructed by coordination of metal centers and organic ligands have been considered as suitable templates/precursors to construct free-standing carbon-based composites grown on conductive substrate. MOFs-derived carbon-based composites have various merits, such as the well-aligned array architecture and evenly distributed active sites, and easy functionalization with other species, which make them suitable alternatives to non-noble metal-included electrocatalysts. In this review, we intend to show the research progresses by employment of MOFs as precursors to prepare self-supported carbon-based composites. Focusing on these MOFs-derived carbon-based nanomaterials, the latest advances in their controllable synthesis, composition regulation, electrocatalytic performances in hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting (OWS) are presented. Finally, the challenges and perspectives are showed for the further developments of MOFs-derived self-supported carbon-based nanomaterials in electrocatalytic reactions.