Tailoring Bond Microenvironments and Reaction Pathways of Single-Atom Catalysts for Efficient Water Electrolysis

Synergistic effect of multi-metal site provided by Ni-N4, adjacent single metal atom, and Fe6 nanoparticle to boost CO2 activation and reduction

Single transition metal (TM) atom embedded in nitrogen-doped carbon materials with M-Nx-C configuration have emerged as a promising class of electrocatalysts for electrochemical CO2 reduction (CO2RR). However, at high TM atom densities, a comprehensive understanding of the active site structure and reaction mechanisms remains a significant challenge, yet it is crucial for enhancing CO2RR performance. In this work, we use first-principles calculations to investigate the electrocatalytic performance of Ni-N4 sites for CO2 reduction to CO, co-assisted by neighboring TM atoms and a Fe6 nanoparticle. Unlike many previously studied Ni-N4 catalysts that maintain a linear CO2 structure, the combination of adjacent TM atoms and Fe6 induces bending and activation of CO2 at the Ni site, enhancing its protonation to form key *COOH intermediate while maintaining efficient *CO desorption. The newly designed hybrid electrocatalyst demonstrates a synergistic effect of multi-metal sites in boosting CO2 reduction to CO. Specifically, the TM atom facilitates C-Ni bond formation between the Ni site and *CO2/*COOH species, while Fe6 forms an Fe…O coordination bond. Detailed analysis of reaction mechanisms and energetics show that Ni-N4, co-assisted by a single TM atom and Fe6 (especially TM = Ni, Cu, or Ag), exhibits enhanced catalytic activity for CO production with a low limiting potential of -0.5 V. This work presents an effective strategy for improving the catalytic activity of single-atom catalysts (SACs) at high metal content.

Recent advances in microenvironment regulation for electrocatalysis

High-efficiency electrocatalysis could serve as the bridge that connects renewable energy technologies, hydrogen economy and carbon capture/utilization, promising a sustainable future for humankind. It is therefore of paramount significance to explore feasible strategies to modulate the relevant electrocatalytic reactions and optimize device performances so as to promote their large-scale practical applications. Microenvironment regulation at the catalytic interface has been demonstrated to be capable of effectively enhancing the reaction rates and improving the selectivities for specific products. In this review we summarize the latest advances in microenvironment regulation in typical electrocatalytic processes (including water electrolysis, hydrogen-oxygen fuel cells, and carbon dioxide reduction) and the related in situ/operando characterization techniques and theoretical simulation methods. At the end of this article, we present an outlook on development trends and possible future directions.

Triboelectric Nanogenerator Drives Electrochemical Water Splitting for Hydrogen Production: Fundamentals, Progress, and Challenges

Currently, triboelectric nanogenerators (TENGs) are drawing significant attention owing to their potential in harvesting wave and wind energy from environment as well as their capability for driving electrochemical water splitting for hydrogen fuel production. This review aims to summarize the recent progress of ocean wave and wind energy harvesting TENGs and TENG-driven electrochemical water splitting processes for hydrogen evolution reaction. For better understanding, this review begins from the fundamentals of TENG and electrochemical water splitting. And then the working principle of TENGs and mechanism of electrochemical (EC) water splitting for hydrogen evolution reaction (HER) are introduced. Subsequently the progress of output performance enhancement in ocean wave and wind energy harvesting TENGs are systematically discussed including structure design, triboelectric material selection and power management all of which are important for output performance enhancement and the integration of TENGs with electrochemical water splitting cell. Although this review focus on the promotion strategies of TENG-driven electrochemical water splitting processes for HER, challenges for water splitting are also highlighted. While envision that this review provides a deep insight and direction to the design of TENG-driven electrochemical system for promoting the hydrogen fuel production in an active, economical manner.

A Metal-Sulfur-Carbon Catalyst Mimicking the Two-Component Architecture of Nitrogenase

The production of ammonia (NH3) from nitrogen sources involves competitive adsorption of different intermediates and multiple electron and proton transfers, presenting grand challenges in catalyst design. In nature nitrogenases reduce dinitrogen to NH3 using two component proteins, in which electrons and protons are delivered from Fe protein to the active site in MoFe protein for transfer to the bound N2. We draw inspiration from this structural enzymology, and design a two-component metal-sulfur-carbon (M-S-C) catalyst composed of sulfur-doped carbon-supported ruthenium (Ru) single atoms (SAs) and nanoparticles (NPs) for the electrochemical reduction of nitrate (NO3 -) to NH3. The catalyst demonstrates a remarkable NH3 yield rate of ~37 mg L-1 h-1 and a Faradaic efficiency of ~97 % for over 200 hours, outperforming those consisting solely of SAs or NPs, and even surpassing most reported electrocatalysts. Our experimental and theoretical investigations reveal the critical role of Ru SAs with the coordination of S in promoting the formation of the HONO intermediate and the subsequent reduction reaction over the NP-surface nearby. Such process results in a more energetically accessible pathway for NO3 - reduction on Ru NPs co-existing with SAs. This study proves a better understanding of how M-S-Cs act as a synthetic nitrogenase mimic during ammonia synthesis, and contributes to the future mechanism-based catalyst design.

Unveiling the Cation Dependence in Alkaline Hydrogen Evolution by Differently-Charged Ruthenium/Molybdenum Sulfide Hybrids

The sluggish kinetics of hydrogen evolution reaction (HER) via water reduction limits the efficiency of alkaline water electrolysis. The HER kinetics is not only intimately related to the catalyst surface structure but also relevant to the cation identity of the electrolyte. The cation dependence also relies on the surface electronic structure and applied potential, but this interrelated effect and its underlying mechanism awaits elucidation. Herein, differently-charged molybdenum sulfide (MoSx) cluster supports ([Mo3S13]2- and [Mo3S7]4+) are utilized to hybridize with the identical metallic Ru centers. The specific electrostatic interaction between MoSx clusters and Ru precursors induces different Ru valences of the hybrids, with a higher valence state for Ru/Mo3S13 endowing a higher activity. The Ru/Mo3S13 and Ru/Mo3S7 exhibited drastically-different cation dependence, in which the charged support determines the local accumulation of cations and resulting water structures. The more negatively-charged Mo3S13 support induces the facile accumulation of cations, especially for less-hydrated K+ cations. The water activation capability by Ru valences and cation accumulation from the support effect in-together determine the cation-dependent alkaline HER activity. This work not only enriches the understanding about the cation-dependent HER mechanism but also shines a light on the rational optimization strategy of electrode/electrolyte interfaces.

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

Ru/Ir-Based Electrocatalysts for Oxygen Evolution Reaction in Acidic Conditions: From Mechanisms, Optimizations to Challenges

The generation of green hydrogen by water splitting is identified as a key strategic energy technology, and proton exchange membrane water electrolysis (PEMWE) is one of the desirable technologies for converting renewable energy sources into hydrogen. However, the harsh anode environment of PEMWE and the oxygen evolution reaction (OER) involving four-electron transfer result in a large overpotential, which limits the overall efficiency of hydrogen production, and thus efficient electrocatalysts are needed to overcome the high overpotential and slow kinetic process. In recent years, noble metal-based electrocatalysts (e.g., Ru/Ir-based metal/oxide electrocatalysts) have received much attention due to their unique catalytic properties, and have already become the dominant electrocatalysts for the acidic OER process and are applied in commercial PEMWE devices. However, these noble metal-based electrocatalysts still face the thorny problem of conflicting performance and cost. In this review, first, noble metal Ru/Ir-based OER electrocatalysts are briefly classified according to their forms of existence, and the OER catalytic mechanisms are outlined. Then, the focus is on summarizing the improvement strategies of Ru/Ir-based OER electrocatalysts with respect to their activity and stability over recent years. Finally, the challenges and development prospects of noble metal-based OER electrocatalysts are discussed.

Multiscale Anion-Hybrid in Atomic Ni Sites for High-Rate Water Electrolysis: Insights into the Charge Accumulation Mechanism

Single-atom catalysts, characterized by transition metal-(N/O)4 units on nanocarbon (M-(N/O)4-C), have emerged as efficient performers in water electrolysis. However, there are few guiding principles for accurately controlling the ligand fields of single atoms to further stimulate the catalyst activities. Herein, using the Ni-(N/O)4-C unit as a model, we develop a further modification of the P anion on the outer shells to modulate the morphology of the ligand. The catalyst thus prepared possesses high activity and excellent long-term durability, surpassing commercial Pt/C, RuO2, and currently reported single-atom catalysts. Notably, mechanistic studies demonstrated that the pseudocapacitive feature of multiscale anion-hybrid nanocarbon is considerable at accumulating enough positive charge [Q], contributing to the high oxygen evolution reaction (OER) order (β) through the rate formula. DFT calculations also indicate that the catalytic activity is decided by the suitable barrier energy of the intermediates due to charge accumulation. This work reveals the activity origin of single atoms on multihybrid nanocarbon, providing a clear experiential formula for designing the electronic configuration of single-atom catalysts to boost electrocatalytic performance.

SnS/SnS2 Heterostructures Embedded in Hierarchical Porous Carbon as Polysulfides Immobilizer for High-Performance Lithium-Sulfur Batteries

Driven by the strong adsorptive and catalytic ability of metal sulfides for soluble polysulfides, it is considered as a potential mediator to resolve the problems of shuttle effect and slow reaction kinetics of polysulfides in lithium-sulfur (Li-S) batteries. However, their further development is limited by poor electrical conductivity and bad long-term durability. Herein, one type of new catalyst composed of SnS/SnS2 heterostructures on hierarchical porous carbon (denoted as SnS/SnS2-HPC) by a simple hydrothermal method is reported and used as an interlayer coating on the conventional separator for blocking polysulfides. The SnS/SnS2-HPC integrates the advantages of a porous conductive network for promoting the transport of electrons and an enhanced electrocatalyst for accelerating polysulfides conversion. As a result, such a cell coupled with a SnS/SnS2-HPC interlayer exhibits a long-term lifespan of 1200 cycles. This work provides a new cell configuration by using heterostructures with a built-in electric field formed from a p-n heterojunction to improve the performance of Li-S batteries.

Metal Alloys-Structured Electrocatalysts: Metal-Metal Interactions, Coordination Microenvironments, and Structural Property-Reactivity Relationships

Metal alloys-structured electrocatalysts (MAECs) have made essential contributions to accelerating the practical applications of electrocatalytic devices in renewable energy systems. However, due to the complex atomic structures, varied electronic states, and abundant supports, precisely decoding the metal-metal interactions and structure-activity relationships of MAECs still confronts great challenges, which is critical to direct the future engineering and optimization of MAECs. Here, this timely review comprehensively summarizes the latest advances in creating the MAECs, including the metal-metal interactions, coordination microenvironments, and structure-activity relationships. First, the fundamental classification, design, characterization, and structural reconstruction of MAECs are outlined. Then, the electrocatalytic merits and modulation strategies of recent breakthroughs for noble and non-noble metal-structured MAECs are thoroughly discussed, such as solid solution alloys, intermetallic alloys, and single-atom alloys. Particularly, unique insights into the bond interactions, theoretical understanding, and operando techniques for mechanism disclosure are given. Thereafter, the current states of diverse MAECs with a unique focus on structural property-reactivity relationships, reaction pathways, and performance comparisons are discussed. Finally, the future challenges and perspectives for MAECs are systematically discussed. It is believed that this comprehensive review can offer a substantial impact on stimulating the widespread utilization of metal alloys-structured materials in electrocatalysis.

Ultrafine Pt Nanoparticles Anchored on 2D Metal-Organic Frameworks as Multifunctional Electrocatalysts for Water Electrolysis and Zinc-Air Batteries

Multifunctional electrocatalysts are crucial to cost-effective electrochemical energy conversion and storage systems requiring mutual enhancement of disparate reactions. Embedding noble metal nanoparticles in 2D metal-organic frameworks (MOFs) are proposed as an effective strategy, however, the hybrids usually suffer from poor electrochemical performance and electrical conductivity in operating conditions. Herein, ultrafine Pt nanoparticles strongly anchored on thiophenedicarboxylate acid based 2D Fe-MOF nanobelt arrays (Pt@Fe-MOF) are fabricated, allowing sufficient exposure of active sites with superior trifunctional electrocatalytic activity for hydrogen evolution, oxygen evolution, and oxygen reduction reactions. The interfacial Fe─O─Pt bonds can induce the charge redistribution of metal centers, leading to the optimization of adsorption energy for reaction intermediates, while the dispersibility of ultrafine Pt nanoparticles contributes to the high mass activity. When Pt@Fe-MOF is used as bifunctional catalysts for water-splitting, a low voltage of 1.65 V is required at 100 mA cm-2 with long-term stability for 20 h at temperatures (65 °C) relevant for industrial applications, outperforming commercial benchmarks. Furthermore, liquid Zn-air batteries with Pt@Fe-MOF in cathodes deliver high open-circuit voltages (1.397 V) and decent cycling stability, which motivates the fabrication of flexible quasisolid-state rechargeable Zn-air batteries with remarkable performance.

Rational Modulation of Single Atom Coordination Microenvironments in a BCN Monolayer for Multifunctional Electrocatalysis

Single-atom (SA) catalysts (SACs) have demonstrated outstanding catalytic performances toward plenty of relevant electrochemical reactions. Nevertheless, controlling the coordination microenvironment of catalytically active SAs to further enhance their catalytic oerformences has remained elusive up to now. Herein, a systematic investigation of 20 transition metal atoms that are coordinated with 20 different microenvironments in a boroncarbon-nitride monolayer (BCN) is conducted using high-throughput density functional theory calculations. The experimentally synthesized ternary BCN monolayer contains carbon, nitrogen, and boron atoms in its 2D network, thus providing a lot of new coordination environments than those of the current Cx Ny nanoplatforms. By exploring the structural/electrochemical stability, catalytic activity, selectivity, and electronic properties of 400 (20 × 20) TM-BCN moieties, it is discovered that specific SA coordination environments can achieve superior stability and selectivity for different electrocatalytic reactions. Moreover, a universal descriptor to accelerate the experimental process toward the synthesis of BCN-SACs is reported. These findings not only provide useful guidance for the synthesis of efficient multifunctional BCN-SACs but also will immediately benefit researchers by levering up their understanding of the mechanistic effects of SA coordination microenvironments on electrocatalytic reactions.

Cascade and Ultrafast Artificial Antioxidases Alleviate Inflammation and Bone Resorption in Periodontitis

Periodontitis, one of the most common, challenging, and rapidly expanding oral diseases, is an oxidative stress-related disease caused by excessive reactive oxygen species (ROS) production. Developing ROS-scavenging materials to regulate the periodontium microenvironments is essential for treating periodontitis. Here, we report on creating cobalt oxide-supported Ir (CoO-Ir) as a cascade and ultrafast artificial antioxidase to alleviate local tissue inflammation and bone resorption in periodontitis. It is demonstrated that the Ir nanoclusters are uniformly supported on the CoO lattice, and there is stable chemical coupling and strong charge transfer from Co to Ir sites. Benefiting from its structural advantages, CoO-Ir presents cascade and ultrafast superoxide dismutase-catalase-like catalytic activities. Notably, it displays distinctly increased Vmax (76.249 mg L-1 min-1) and turnover number (2.736 s-1) when eliminating H2O2, which surpasses most of the by-far-reported artificial enzymes. Consequently, the CoO-Ir not only provides efficient cellular protection from ROS attack but also promotes osteogenetic differentiation in vitro. Furthermore, CoO-Ir can efficiently combat periodontitis by inhibiting inflammation-induced tissue destruction and promoting osteogenic regeneration. We believe that this report will shed meaningful light on creating cascade and ultrafast artificial antioxidases and offer an effective strategy to combat tissue inflammation and osteogenic resorption in oxidative stress-related diseases.

Metal-oxo Cluster Mediated Atomic Rh with High Accessibility for Efficient Hydrogen Evolution

Achieving single-atom catalysts (SACs) with high metal content and outstanding performance as well as robust stability is critically needed for clean and sustainable energy. However, most of the synthesized SACs are undesired on the loading content of the metal due to the anchored metals and the supports as well as the synthesizing methods. Herein, a Rh-SAC with high accessibility by loading it on the metal nodes of metal-porphyrin-based PCN MOFs (PCN-224) as supporting material is reported. Significantly, the PCN-Rh_15.9 /KB catalyst with a high Rh content of 15.9 wt% exhibits excellent hydrogen evolution activity with a low overpotential of 25 mV at a current density of 10 mA cm^-2 and a mass activity of 7.7 A mg^-1 _Rh at overpotential of 150 mV, which is much better than that of the commercial Rh/C. Various characterizations reveal the Rh species is stabilized by the metal nodes bearing -O/OH_x in MOFs, which is of importance for the high loading amount and the good activity. This work establishes an efficient approach to synthesize high content SACs on the nodes of MOFs for wide catalyst design.

Electrocatalytic Porphyrin/Phthalocyanine-Based Organic Frameworks: Building Blocks, Coordination Microenvironments, Structure-Performance Relationships

Metal-porphyrins or metal-phthalocyanines-based organic frameworks (POFs), an emerging family of metal-N-C materials, have attracted widespread interest for application in electrocatalysis due to their unique metal-N4 coordination structure, high conjugated π-electron system, tunable components, and chemical stability. The key challenges of POFs as high-performance electrocatalysts are the need for rational design for porphyrins/phthalocyanines building blocks and an in-depth understanding of structure-activity relationships. Herein, the synthesis methods, the catalytic activity modulation principles, and the electrocatalytic behaviors of 2D/3D POFs are summarized. Notably, detailed pathways are given for modulating the intrinsic activity of the M-N4 site by the microenvironments, including central metal ions, substituent groups, and heteroatom dopants. Meanwhile, the topology tuning and hybrid system, which affect the conjugation network or conductivity of POFs, are also considered. Furthermore, the representative electrocatalytic applications of structured POFs in efficient and environmental-friendly energy conversion areas, such as carbon dioxide reduction reaction, oxygen reduction reaction, and water splitting are briefly discussed. Overall, this comprehensive review focusing on the frontier will provide multidisciplinary and multi-perspective guidance for the subsequent experimental and theoretical progress of POFs and reveal their key challenges and application prospects in future electrocatalytic energy conversion systems.

Modulating Electronic Environment of Ru Nanoclusters via Local Charge Transfer for Accelerating Alkaline Water Electrolysis

Compared to platinum catalysts, ruthenium (Ru) is disclosed as a promising alternative for alkaline water electrolysis due to its similar hydrogen adsorption energy and relatively lower water dissociation barrier. However, in the challenging alkaline media, the dissatisfied Volmer step during water dissociation of Ru metal prohibits its practical applications. Here, a new pathway to modulate the electronic environment of Ru catalysts via a local charge transfer strategy for tuning the water dissociation kinetics and accelerating the alkaline water electrolysis is proposed. The obtained catalysts are engineered by assembling and subsequently pyrolyzing the layer-stacked and 2D porphyrin-based Ru-N coordination polymers on nanocarbon supports. Benefiting from the well-defined Ru nanocluster-N_x -coordination bonds (Ru_nc -N_x ), unique electronic environments, and local charge transfer properties, the catalysts exhibit the exceptional activity of 17 mV overpotential at 10 mA cm^-2 and robust stability in water, which is more efficient than state-of-the-art Ru catalysts. The theoretical calculation suggests that the Ru_nc -N_x sites enhance the nucleophilic attack of water and weaken the HOH bond. This study manifests that tailoring the bond environments of Ru clusters can significantly modulate their intrinsic catalytic activities and stabilities, which may open new avenues for developing high-active and durable catalysts for water electrolysis.

Multifaceted Catalytic ROS-Scavenging via Electronic Modulated Metal Oxides for Regulating Stem Cell Fate

Developing reactive oxygen species (ROS)-scavenging nanostructures to protect and regulate stem cells has emerged as an intriguing strategy for promoting tissue regeneration, especially in trauma microenvironments or refractory wounds. Here, an electronic modulated metal oxide is developed via Mn atom substitutions in Co₃ O₄ nanocrystalline (Mn-Co₃ O₄ ) for highly efficient and multifaceted catalytic ROS-scavenging to reverse the fates of mesenchymal stem cells (MSCs) in oxidative-stress microenvironments. Benefiting from the atomic Mn-substitution and charge transfer from Mn to Co, the Co site in Mn-Co₃ O₄ displays an increased ratio of Co²⁺ /Co³⁺ and improved redox properties, thus enhancing its intrinsic and broad-spectrum catalytic ROS-scavenging activities, which surpasses most of the currently reported metal oxides. Consequently, the Mn-Co₃ O₄ can efficiently protect the MSCs from ROS attack and rescue their functions, including adhesion, spreading, proliferation, and osteogenic differentiation. This work not only establishes an efficient material for catalytic ROS-scavenging in stem-cell-based therapeutics but also provides a new avenue to design biocatalytic metal oxides via modulation of electronic structure.