Theoretical Inspection of M1/PMA Single-Atom Electrocatalyst: Ultra-High Performance for Water Splitting (HER/OER) and Oxygen Reduction Reactions (OER)

Phosphorus-doped nickel cobalt oxide (NiCo2O4) wrapped in 3D hierarchical hollow N-doped carbon nanoflowers as highly efficient bifunctional electrocatalysts for overall water splitting

Properly design and fabricate capable electrocatalysts with 3D hierarchical hollow framework to realize cost-effective and efficacious overall water splitting (OWS) are particularly meaningful for the large-scale arrangement of pivotal energy technology. In this study, P-doped NiCo2O4 nanoparticles encapsulated in N-doped carbon hierarchical hollow nanoflowers (P-NiCo2O4@NCHHNFs) were constructed using the hydrothermal-pyrolysis-phosphorization approach. This fascinating architecture can not merely serve as a conductive pathway for electron transfer, but at the same time effectively inhibited the aggregation and corrosion of the NiCo2O4 nanoparticles. Additionally, the P doping not only regulates electronic structure configuration to boost the intrinsic activity of the catalyst, but also enhance electrochemical surface areas to reveal more accessible active sites. Attributing to these characteristics, the as-prepared P-NiCo2O4@NCHHNFs exhibit preeminent electrocatalytic performance with low overpotentials of 283 mV and 162 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) (at 10 mA cm-2), respectively. Specifically, by using the P-NiCo2O4@NCHHNFs as bifunctional catalysts, a low potential of 1.56 V (at 10 mA cm-2) is sufficient to drive overall water splitting with splendid durability. This study proposed an innovative strategy for the conceiving and fabricating high-performance catalysts via heteroatom-doping.

Designing core-shell heterostructure arrays based on snowflake NiCoFe-LTH shelled over W2N-WC nanowires as an advanced bi-functional electrocatalyst for boosting alkaline water/seawater electrolysis

The pursuit of efficient and sustainable hydrogen production through water splitting has led to intensive research in the field of electrocatalysis. However, the impediment posed by sluggish reaction kinetics has served as a significant barrier. This challenge has inspired the development of electrocatalysts characterized by high activity, abundance in earth's resources, and long-term stability. In addressing this obstacle, it is imperative to meticulously fine-tune the structure, morphology, and electronic state of electrocatalysts. By systematically manipulating these key parameters, the full potential of electrocatalysts can unleash, enhancing their catalytic activity and overall performance. Hence in this study, a novel heterostructure is designed, showcasing core-shell architectures achieved by covering W2N-WC nanowire arrays with tri-metallic Nickel-Cobalt-Iron layered triple hydroxide nanosheets on carbon felt support (NiCoFe-LTH/W2N-WC/CF). By integrating the different virtue such as binder free electrode design, synergistic effect between different components, core-shell structural advantages, high exposed active sites, high electrical conductivity and heterostructure design, NiCoFe-LTH/W2N-WC/CF demonstrates striking catalytic performances under alkaline conditions. The substantiation of all the mentioned advantages has been validated through electrochemical data in this study. According to these results NiCoFe-LTH/W2N-WC/CF achieves a current density of 10 mA cm-2 needs overpotential values of 101 mV for HER and 206 mV for OER, respectively. Moreover, as a bi-functional electrocatalyst for overall water splitting, a two-electrode device needs a voltage of 1.543 V and 1.569 V to reach a current density of 10 mA cm-2 for alkaline water and alkaline seawater electrolysis, respectively. Briefly, this research with attempting to combination of different factors try to present a promising stride towards advancing bi-functional catalytic activity with tailored architectures for practical green hydrogen production via electrochemical water splitting process.

Polyoxometalate Supported Single Transition Metal Atom as a Redox Mediator for Li-O2 Batteries

Keggin-type polyoxometalate (POM) supported single transition metal (TM) atom (TM1/POM) as an efficient soluble redox mediator for Li-O2 batteries is comprehensively investigated by first-principles calculations. Among the pristine POM and four kinds of TM1/POM (TM = Fe, Co, Ni, and Pt), Co1/POM not only maintains good structural and thermodynamic stability in oxidized and reduced states but also exhibits promising electro(chemical) catalytic performance for both oxygen reduction reaction and oxygen evolution reaction (OER) in Li-O2 batteries with the lowest Gibbs free energy barriers. Further investigations demonstrate that the moderate binding strength of Li2-xO2 (x = 0, 1, and 2) intermediates on Co1/POM guarantees favorable Li2O2 formation and decomposition. Electronic structure analyses indicate that the introduced Co single atom as an electron transfer bridge can not only efficiently improve the electronic conductivity of POM but also regulate the bonding/antibonding states around the Fermi level of [Co1/POM-Li2O2]ox. The solvent effect on the OER catalytic performance and the electronic properties of [Co1/POM-Li2O2]ox with and without dimethyl sulfoxide solvent are also investigated.

Single Transition-Metal Atom Anchored on a Rhenium Disulfide Monolayer: An Efficient Bifunctional Electrocatalyst for the Oxygen Evolution and Oxygen Reduction Reactions

Developing efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) bifunctional electrocatalysts is attractive for rechargeable metal-air batteries. Meanwhile, single metal atoms embedded in 2D layered transition metal chalcogenides (TMDs) have become a very promising catalyst. Recently, many attentions have been paid to the 2D ReS2 electrocatalyst due to its unique distorted octahedral 1T' crystal structure and thickness-independent electronic properties. Here, the catalytic activity of different transition metal (TM) atoms embedded in ReS2 using the density functional theory is investigated. The results indicate that TM@ReS2 exhibits outstanding thermal stability, good electrical conductivity, and electron transfer for electrochemical reactions. And the Ir@ReS2 and Pd@ReS2 can be used as OER/ORR bifunctional electrocatalysts with a lower overpotential for OER (ηOER) of 0.44 V and overpotentials for ORR (ηORR) of 0.26 V and 0.27 V, respectively. The excellent catalytic activity is attributed to the optimal adsorption strength for oxygen intermediates coming from the effective modulation of the electronic structure of ReS2 after Ir/Pd doping. The results can help to deeply understand the catalytic activity of TM@ReS2 and develop novel and highly efficient OER/ORR electrocatalysts.

Polyoxometalate-Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion

Polyoxometalates (POMs), a kind of molecular metal oxide cluster with unique physical-chemical properties, have made essential contributions to creating efficient and robust electrocatalysts in renewable energy systems. Due to the fundamental advantages of POMs, such as the diversity of molecular structures and large numbers of redox active sites, numerous efforts have been devoted to extending their application areas. Up to now, various strategies of assembling POM molecules into superstructures, supporting POMs on heterogeneous substrates, and POMs-derived metal compounds have been developed for synthesizing electrocatalysts. From a multidisciplinary perspective, the latest advances in creating POM-structured materials with a unique focus on their molecular fundamentals, electrocatalytic roles, and the recent breakthroughs of POMs and POM-derived electrocatalysts, are systematically summarized. Notably, this paper focuses on exposing the current states, essences, and mechanisms of how POM-structured materials influence their electrocatalytic activities and discloses the critical requirements for future developments. The future challenges, objectives, comparisons, and perspectives for creating POM-structured materials are also systematically discussed. It is anticipated that this review will offer a substantial impact on stimulating interdisciplinary efforts for the prosperities and widespread utilizations of POM-structured materials in electrocatalysis.

In Situ Growth of Interfacially Nanoengineered 2D-2D WS2/Ti3C2Tx MXene for the Enhanced Performance of Hydrogen Evolution Reactions

In line with current research goals involving water splitting for hydrogen production, this work aims to develop a noble-metal-free electrocatalyst for a superior hydrogen evolution reaction (HER). A single-step interfacial activation of Ti3C2Tx MXene layers was employed by uniformly growing embedded WS2 two-dimensional (2D) nanopetal-like sheets through a facile solvothermal method. We exploited the interactions between WS2 nanopetals and Ti3C2Tx nanolayers to enhance HER performance. A much safer method was adopted to synthesize the base material, Ti3C2Tx MXene, by etching its MAX phase through mild in situ HF formation. Consequently, WS2 nanopetals were grown between the MXene layers and on edges in a one-step solvothermal method, resulting in a 2D-2D nanocomposite with enhanced interactions between WS2 and Ti3C2Tx MXene. The resulting 2D-2D nanocomposite was thoroughly characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) analyses before being utilized as working electrodes for HER application. Among various loadings of WS2 into MXene, the 5% WS2-Ti3C2Tx MXene sample exhibited the best activity toward HER, with a low overpotential value of 66.0 mV at a current density of -10 mA cm-2 in a 1 M KOH electrolyte and a remarkable Tafel slope of 46.7 mV·dec-1. The intercalation of 2D WS2 nanopetals enhances active sites for hydrogen adsorption, promotes charge transfer, and helps attain an electrochemical stability of 50 h, boosting HER reduction potential. Furthermore, theoretical calculations confirmed that 2D-2D interactions between 1T/2H-WS2 and Ti3C2Tx MXene realign the active centers for HER, thereby reducing the overpotential barrier.

Dipole Effect on Oxygen Evolution Reaction of 2D Janus Single-Atom Catalysts: A Case of Rh Anchored on the P6m2-NP Configurations

Catalytic performance of single-atom catalysts (SACs) relies fundamentally on the electronic nature and local coordination environment of the active site. Here, based on a machine-learning (ML)-aided density functional theory (DFT) method, we reveal that the intrinsic dipole in Janus materials has a significant impact on the catalytic activity of SACs, using 2D γ-phosphorus carbide (γ-PC) as a model system. Specifically, a local dipole around the active site is a key degree to tune the catalytic activity and can be used as an important descriptor with a high feature importance of 17.1% in predicting the difference of adsorption free energy (ΔGO* - ΔGOH*) to assess the activity of the oxygen evolution reaction. As a result, the catalytic performance of SACs can be tuned by an intrinsic dipole, in stark contrast to those external stimuli strategies previously used. These results suggest that dipole engineering and the revolutionary DFT-ML hybrid scheme are novel approaches for designing high-performance catalysts.

Descriptor for C2N-Supported Single-Cluster Catalysts in Bifunctional Oxygen Evolution and Reduction Reactions

Developing highly active cluster catalysts for the bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is significant for future renewable energy technology. Here, we employ first-principles calculations combined with a genetic algorithm to explore the activity trends of transition metal clusters supported on C2N. Our results indicate that the supported clusters, as bifunctional catalysts for the OER and the ORR, may outperform single-atom catalysts. In particular, the C2N-supported Ag6 cluster exhibits outstanding bifunctional activity with low overpotentials. Mechanistic analysis indicates that the activity of the cluster is related to the number of atoms in the active site as well as the interaction between the intermediate and the cluster. Accordingly, we identify a descriptor that links the intrinsic properties of the clusters with the activity of both the OER and the ORR. This work provides guidelines and strategies for the rational design of highly efficient bifunctional cluster catalysts.

Three-dimensional ordered macroporous design of heterogeneous cobalt-iron phosphides as oxygen evolution electrocatalyst

Oxygen evolution reaction (OER) plays a key role in electrochemical conversion, which needs efficient and economical electrocatalyst to boost its kinetics for large-scale application. Herein, a bimetallic CoP/FeP2heterostructure with a three-dimensional ordered macroporous structure (3DOM-CoP/FeP2) was synthesized as an OER catalyst to demonstrate a heterogeneous engineering induction strategy. By adjusting the electron distribution and producing a lot of active sites, the heterogeneous interface enhances catalytic performance. High specific surface area is provided by the 3DOM structure. Additionally, at the solid-gas-electrolyte threephase interface, the electrocatalytic reaction exhibits good mass transfer.In situRaman spectroscopy characterization revealed that FeOOH and CoOOH reconstructed from CoP/FeP2were the true OER active sites. Consequently, the 3DOM-CoP/FeP2demonstrates superior OER activity with a low overpotentials of 300/420 mV at 10/100 mA cm-2and meritorious OER durability. It also reveals promising performance as the overall water splitting anode.

Computational Design of Ni6@Pt1M31 Clusters for Multifunctional Electrocatalysts

High-efficiency and low-cost multifunctional electrocatalysts for hydrogen evolution reaction (HERs), oxygen evolution reaction (OERs) and oxygen reduction reaction (ORRs) are important for the practical applications of regenerative fuel cells. The activity trends of core-shell Ni6@M32 and Ni6@Pt1M31 (M = Pt, Pd, Cu, Ag, Au) were investigated using the density functional theory (DFT). Rate constant calculations indicated that Ni6@Pt1Ag31 was an efficient HER catalyst. The Volmer-Tafel process was the kinetically favorable reaction pathway for Ni6@Pt1M31. The Volmer-Heyrovsky reaction mechanism was preferred for Ni6@M32. The Pt active site reduced the energy barrier and changed the reaction mechanism. The ORR and OER overpotentials of Ni6@Pt1Ag31 were calculated to be 0.12 and 0.33 V, indicating that Ni6@Pt1Ag31 could be a promising multifunctional electrocatalyst. Ni6@Pt1M31 core-shell clusters present abundant active sites with a moderate adsorption strength for *H, *O, *OH and *OOH. The present study shows that embedding a single Pt atom onto a Ni@M core-shell cluster is a rational strategy for designing an effective multifunctional electrocatalyst.

Photothermal-Magnetic Synergistic Effects in an Electrocatalyst for Efficient Water Splitting under Optical-Magnetic Fields

The slow oxygen evolution reaction (OER) limits water splitting, and external fields can help improve it. However, the effect of a single external field on the OER is limited and unsatisfactory. Furthermore, the mechanism by which external fields improve the OER is unclear, particularly in the presence of multiple fields. Herein, a strategy is proposed for enhancing the OER activity of a catalyst using the combined effect of an optical-magnetic field, and the mechanism of catalytic activity enhancement is studied. Under the optical-magnetic field, Co3 O4 reduces the resistance by increasing the catalyst temperature. Meanwhile, CoFe2 O4 further reduces the resistance via the negative magnetoresistance effect, thus decreasing the resistance from 16 to 7.0 Ω. Additionally, CoFe2 O4 acts as a spin polarizer, and electron polarization results in a parallel arrangement of oxygen atoms, which increases the kinetics of the OER under the magnetic field. Benefiting from the optical and magnetic response design, Co3 O4 /CoFe2 O4 @Ni foam requires an overpotential of 172.4 mV to reach a current density of 10 mA cm-2 under an optical-magnetic field, which is significantly higher than those of recently reported state-of-the-art transition-metal-based catalysts.

Fe-Regulated Amorphous-Crystal Ni(Fe)P2 Nanosheets Coupled with Ru Powerfully Drive Seawater Splitting at Large Current Density

Water electrolysis is an ideal method for industrial green hydrogen production. However, due to increasing scarcity of freshwater, it is inevitable to develop advanced catalysts for electrolyzing seawater especially at large current density. This work reports a unique Ru nanocrystal coupled amorphous-crystal Ni(Fe)P2 nanosheet bifunctional catalyst (Ru-Ni(Fe)P2 /NF), caused by partial substitution of Fe to Ni atoms in Ni(Fe)P2 , and explores its electrocatalytic mechanism by density functional theory (DFT) calculations. Owing to high electrical conductivity of crystalline phases, unsaturated coordination of amorphous phases, and couple of Ru species, Ru-Ni(Fe)P2 /NF only requires overpotentials of 375/295 and 520/361 mV to drive a large current density of 1 A cm-2 for oxygen/hydrogen evolution reaction (OER/HER) in alkaline water/seawater, respectively, significantly outperforming commercial Pt/C/NF and RuO2 /NF catalysts. In addition, it maintains stable performance at large current density of 1 A cm-2 and 600 mA cm-2 for 50 h in alkaline water and seawater, respectively. This work provides a new way for design of catalysts toward industrial-level seawater splitting.

Pencil-like Hollow Carbon Nanotubes Embedded CoP-V4P3 Heterostructures as a Bifunctional Catalyst for Electrocatalytic Overall Water Splitting

Electrocatalytic water splitting is one of the most efficient ways of producing green hydrogen energy. The design of stable, active, and efficient electrocatalysts plays a crucial role in water splitting for achieving efficient energy conversion from electrical to hydrogen energy, aimed at solving the lingering energy crisis. In this work, CNT composites modified with CoP-V₄P₃ composites (CoVO-10-CNT-450P) were formed by carbonising a pencil-like precursor (Co₃V₂O₈-H₂O) and growing carbon nanotubes in situ, followed by in situ phosphorylation on the carbon nanotubes. In the HER electrocatalytic process, an overpotential of only 124 mV was exhibited at a current density of 10 mA cm^-2. In addition, as an OER catalyst, a low overpotential of 280 mV was attained at a current density of 10 mA cm^-2. Moreover, there was no noticeable change in the performance of the catalyst over a 90 h test in a continuous total water splitting experiment. The unique electronic structure and hollow carbon nanotube structure of CoVO-10-CNT-450P effectively increased the catalytic active sites, while also significantly improving the electrocatalytic activity. This work provides theoretical guidance for the design and synthetic route of high-performance non-precious metal electrocatalysts, and actively promotes the commercial application of electrochemical water splitting.

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.

Challenges and Perspectives of Single-Atom-Based Catalysts for Electrochemical Reactions

Single-atom catalysts (SACs) are emerging as the most promising catalysts for various electrochemical reactions. The isolated dispersion of metal atoms enables high density of active sites, and the simplified structure makes them ideal model systems to study the structure-performance relationships. However, the activity of SACs is still insufficient, and the stability of SACs is usually inferior but has received little attention, hindering their practical applications in real devices. Moreover, the catalytic mechanism on a single metal site is unclear, leading the development of SACs to rely on trial-and-error experiments. How can one break the current bottleneck of active sites density? How can one further increase the activity/stability of metal sites? In this Perspective, we discuss the underlying reasons for the current challenges and identify precisely controlled synthesis involving designed precursors and innovative heat-treatment techniques as the key for the development of high-performance SACs. In addition, advanced operando characterizations and theoretical simulations are essential for uncovering the true structure and electrocatalytic mechanism of an active site. Finally, future directions that may arise breakthroughs are discussed.

Structure evolution and durability of Metal-Nitrogen-Carbon (M = Co, Ru, Rh, Pd, Ir) based oxygen evolution reaction electrocatalyst: A theoretical study

Developing low-cost, high activity and stability oxygen evolution reaction (OER) catalysts is significantly important but still challenging for water electrolyzers. In this work, we calculated the OER activity and stability of Metal-Nitrogen-Carbon (MNC, M = Co, Ru, Rh, Pd, Ir) based electrocatalyst with different structures (MN₄C₈, MN₄C₁₀, MN₄C₁₂) using density functional theory (DFT) method. These electrocatalysts were divided into three groups based on the value of ΔG_*OH, that is ΔG_*OH > 1.53 eV (PdN₄C₈, PdN₄C₁₀, PdN₄C₁₂), ΔG_*OH < 1.23 eV (RuN₄C₈, RuN₄C₁₀, RuN₄C₁₂, CoN₄C₈, CoN₄C₁₀) and 1.23 eV < ΔG_*OH < 1.53 eV (RhN₄C₈, RhN₄C₁₀, RhN₄C₁₂, IrN₄C₈, IrN₄C₁₀, IrN₄C₁₂, CoN₄C₁₂), and ΔG_*OH determine whether the structure evolution will appear. The results proved that MNC (M = Rh, Ir) with 1.23 eV < ΔG_*OH < 1.53 eV shows higher OER activity due to moderate binding energy between reaction intermediates and MNC. Furthermore, these catalysts could maintain MNC structure without further oxidation and structural evolution under working conditions (high temperature, dynamic condition, local electric field and strong specific adsorption), therefore show excellent stability. However, MNC electrocatalyst with ΔG_*OH > 1.53 eV or ΔG_*OH < 1.23 eV revealed less stability under working conditions, due to their low intrinsic stability or structural evolution under working conditions, respectively. In conclusion, we proposed a comprehensive evaluation method for MNC electrocatalysts by taking ΔG_*OH as the screening criterion for OER activity and stability, as well as ΔE_b under working condition as descriptor of stability. This is of great significance for the design and screening of ORR, OER and HER electrocatalysts under working conditions.

Morphology Modulation and Phase Transformation of Manganese-Cobalt Carbonate Hydroxide Caused by Fluoride Doping and Its Effect on Boosting the Overall Water Electrolysis

Increasing demands for pollution-free energy resources have stimulated intense research on the design and fabrication of highly efficient, inexpensive, and stable non-noble earth-abundant metal catalysts with remarkable catalytic activity for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Morphology control of the catalysts is widely implemented as an effective strategy to change the surface atomic coordination and increase the catalytic behavior of the catalysts. In this study, we have designed a series of Mn-Co catalyts with different morphologies on the graphite paper substrate to enhance OER and HER activities in alkaline media. The prepared catalysts with different morphologies were successfully obtained by adjusting the amount of ammonium fluoride (NH₄F) in the hydrothermal process. The electrochemical tests display that the cubic-like Mn-Co catalyst with pyramids on the faces at a concentration of 0.21 M NH₄F exhibits the best activity toward both OER and HER. The cubic-like Mn-Co catalyst with pyramids on the faces showed overpotentials of 240 and 82 mV at a current density of 10 mA cm^-2 for OER and HER, respectively. Also, the cubic-like Mn-Co catalyst with pyramids on the faces required overpotentials of 319 and 216 mV to reach the current density of 100 mA cm^-2 for OER and HER, respectively. The current density of this catalyst at η = 0.32 V was 701.05 mA cm^-2 for OER, and for HER, the current density of the catalyst was 422.89 mA cm^-2 at η = 0.23 V. The Tafel slopes of the Mn-Co catalyst with cubic-like structures with pyramids on the faces were 78 and 121 mV dec^-1 for OER and HER, respectively. A two-electrode overall water electrolysis system using this bifunctional Mn-Co catalyst exhibited low cell voltages of 1.60 in the alkaline electrolyte at the standard current density of 10 mA cm^-2 with appropriate stability. These electrochemical merits exhibit the considerable potential of the cubic-like Mn-Co catalyst with pyramids on the faces for bifunctional OER and HER applications.

Different Growth Behavior of MOF-on-MOF Heterostructures to Enhance Oxygen Evolution

The exploration of non-noble metal electrocatalysts with high catalytic activity and stability for oxygen evolution reaction (OER) has become particularly urgent. Here, FeNi-based Prussian blue analogues (PBAs) were obtained by adding different solvents, where PBA particles preferentially grew on the surface plane/edge of coordination polymer precursor (Ni-ABDC) with various polarities. This resulted in the formation of FeNi-PBA-plane/edge morphologies, respectively. Notably, on account of more exposed PBAs, FeNi nanoparticles were uniformly supported on the porous N-doped carbon nanomaterials. Among them, the calcined FeNi-NC-800 underwent an interesting pre-activation process and exhibited a low overpotential of 281 mV at 10 mA cm^-2 and a small Tafel slope of 82 mV dec^-1 in 1.0 m KOH. This bimetallic sample showed superior OER activity and stability in comparison with control materials, which could be attributed to its abundant FeNi nanoparticles, high nitrogen content, large specific surface area, and synergistic effects between Fe and Ni atoms. In addition, relevant theoretical calculation on the optimal catalyst, FeNi-NC-800, further demonstrated its efficient OER performance with effective Fe-doping in the Ni-based oxyhydroxides.

Tuning the Coordination Microenvironment of Central Fe Active Site to Boost Water Electrolysis and Oxygen Reduction Activity

In heterogeneous catalysis, single-atom catalysts are the frontier and important prototypes for many reactions, and revealing the intrinsic structure-activity relationship is presently a critical task, but remains challenging. In this work, water electrolysis and oxygen reduction performances of FeXY_i N_{3 -i} (X, Y = B, C, O, P and S; i = 0, 1) moiety in Fe-porphyrin are studied by the first-principles calculations, aiming at unraveling how and why tuning the coordination microenvironment of the active metal atom can improve the activity. It can be concluded that breaking the coordination shell symmetry breaks the well-accepted standard scaling relationship, adjusts *O adsorption behavior and thus optimizes the oxygen evolution reaction (OER) activity, for example, to an extremely low overpotential of 0.17 V. In combination with the Fe atom spin configuration and ligand field theory, the dramatically improved OER activity can be well explained. In the present work, the significance of the coordination microenvironment of central metal atom in studies of electrocatalysis is highlighted.

Unique amorphous/crystalline heterophase coupling for an efficient oxygen evolution reaction

Designing amorphous/crystalline heterophase catalysts is still in the initial stage, and the study of amorphous/crystalline heterophase and carbon-free catalysts has not yet been realized. Herein, we report a unique amorphous/crystalline heterophase catalyst consisting of NiFe alloy nanoparticles (NPs) supported on Ti₄O₇ (NiFe/Ti₄O₇) for the first time, which is achieved by a heterophase supporting strategy of dual heat treatment. Surprisingly, the amorphous/crystalline heterophase is flexibly composed of amorphous and crystalline phases of alloy NPs and Ti₄O₇. The heterophase coupling endows the catalyst with a low overpotential (256 mV at 10 mA cm^-2), a small Tafel slope (47 mV dec^-1) and excellent endurance stability (over 100 h) in 1 M KOH electrolyte, which already outperforms commercial RuO₂ (338 mV and 113 mV dec^-1) and exceeds most reported representative carbon-based and titanium-based non-precious metal catalysts. The density functional theory (DFT) calculations and experimental results reveal that the unique amorphous/crystalline heterophase coupling in NiFe/Ti₄O₇ results in electron transfer between the alloy NPs and Ti₄O₇, allowing more catalytically active sites and faster interfacial electron transfer dynamics. This work provides insights into the synthesis of amorphous/crystalline heterophase catalysts and can be generalized to the heterophase coupling of other transition metal-based electrocatalysts.

Work-function-induced Interfacial Built-in Electric Fields in Os-OsSe2 Heterostructures for Active Acidic and Alkaline Hydrogen Evolution

Theoretical calculations unveil that the formation of Os-OsSe₂ heterostructures with neutralized work function (WF) perfectly balances the electronic state between strong (Os) and weak (OsSe₂ ) adsorbents and bidirectionally optimizes the hydrogen evolution reaction (HER) activity of Os sites, significantly reducing thermodynamic energy barrier and accelerating kinetics process. Then, heterostructural Os-OsSe₂ is constructed for the first time by a molten salt method and confirmed by in-depth structural characterization. Impressively, due to highly active sites endowed by the charge balance effect, Os-OsSe₂ exhibits ultra-low overpotentials for HER in both acidic (26 mV @ 10 mA cm^-2 ) and alkaline (23 mV @ 10 mA cm^-2 ) media, surpassing commercial Pt catalysts. Moreover, the solar-to-hydrogen device assembled with Os-OsSe₂ further highlights its potential application prospects. Profoundly, this special heterostructure provides a new model for rational selection of heterocomponents.

Synergistic regulation of coordination sites of Co3O4 by selective doping for efficient water oxidation

Herein, we designed selectively doped Co₃O₄ to explore the effect of coordination sites, in which the octahedral and tetrahedral sites were modulated by simple doping with ions of different radiuses (Al and In). Compared to octahedral regulation, the synergistic regulation of both tetrahedral sites and octahedral sites can induce a higher intrinsic activity, which is because they possess improved electrical conductivity and facilitate the formation of highly active Co³⁺/Co⁴⁺ species during the oxygen evolution reaction (OER).

Doping of the Mn vacancy of Mn2B2 with a single different transition metal atom as the dual-function electrocatalyst

The design of efficient electrocatalysts is essential to enhance the performance of rechargeable metal-air cells, renewable fuel cells and overall water splitting. Based on this, how to improve the catalytic activity of oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) becomes self-evident. Currently, single atom catalysts (SACs) are widely used as structural design models for the OER, ORR and HER because of the single active site and maximum metal atom utilization, but significant challenges remain. Herein, the catalytic properties of the OER, ORR and HER with a single metal atom as the active site are discussed through first-principles calculations by introducing a single metal atom in the Mn vacancy of Mn₂B₂ (TM@Mn₂B₂, TM = Au, Ag, Co, Cd, Cu, Ir, Pd, Ni, Rh, Ru and Pt). The results show that Ni@Mn₂B₂ is suitable as a dual-function electrocatalyst for the OER/ORR with overpotentials of 0.38 V and 0.37 V, which are lower than those of the OER overpotential of RuO₂/IrO₂ (0.42 V/0.56 V) and the ORR overpotential of Pt (0.45 V). Meanwhile, Pt@Mn₂B₂ is available as an OER/HER dual-function electrocatalyst for overall water splitting with a lower overpotential of OER (0.45 V) and lower |ΔG_H| (-0.15eV) under 1/4 hydrogen coverage for the HER. This work proposes a practical strategy for developing single metal atom doped MBene as a dual-function electrocatalyst.

Investigation of the Stability and Hydrogen Evolution Activity of Dual-Atom Catalysts on Nitrogen-Doped Graphene

Single atom catalysts (SACs) have received a lot of attention in recent years for their high catalytic activity, selectivity, and atomic utilization rates. Two-dimensional N-doped graphene has been widely used to stabilize transition metal (TM) SACs in many reactions. However, the anchored SAC could lose its activity because of the too strong metal-N interaction. Alternatively, we studied the stability and activity of dual-atom catalysts (DACs) for 24 TMs on N-doped graphene, which kept the dispersion state but had different electronic structures from SACs. Our results show that seven DACs can be formed directly compared to the SACs. The others can form stably when the number of TMs is slightly larger than the number of vacancies. We further show that some of the DACs present better catalytic activities in hydrogen evolution reaction (HER) than the corresponding SACs, which can be attributed to the optimal charge transfer that is tuned by the additional atom. After the screening, the DAC of Re is identified as the most promising catalyst for HER. This study provides useful information for designing atomically-dispersed catalysts on N−doped graphene beyond SACs.

Synergetic Catalysis of Magnetic Single-Atom Catalysts Confined in Graphitic-C3N4/CeO2(111) Heterojunction for CO Oxidization

Magnetic single-atom catalysts (MSAC), due to the intrinsic spin degree of freedom, are of particular importance relative to other conventional SAC for applications in various catalytic processes, especially in those cases that involve spin-triplet O₂. However, the bottleneck issue in this field is the clustering of the SAC during the processes. Here using first-principles calculations we predict that Mn atoms can be readily confined in the interface of the porous g-C₃N₄/CeO₂(111) heterostructure, forming high-performance MSAC for O₂ activation via a delicate synergetic mechanism of charge transfer, mainly provided by the p-block g-C₃N₄ overlayer mediated by the d-block Mn active site, and spin selection, preserved mainly through active participation of the f-block Ce atoms and/or g-C₃N₄, which effectively promotes the CO oxidization. Such a recipe is also demonstrated to be valid for V- and Nb-MSACs, which may shed new light on the design of highly efficient MSACs for various important chemical processes wherein spin-selection matters.

Surface Design Strategy of Catalysts for Water Electrolysis

Hydrogen, a new energy carrier that can replace traditional fossil fuels, is seen as one of the most promising clean energy sources. The use of renewable electricity to drive hydrogen production has very broad prospects for addressing energy and environmental problems. Therefore, many researchers favor electrolytic water due to its green and low-cost advantages. The electrolytic water reaction comprises the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Understanding the OER and HER mechanisms in acidic and alkaline processes contributes to further studying the design of surface regulation of electrolytic water catalysts. The OER and HER catalysts are mainly reviewed for defects, doping, alloying, surface reconstruction, crystal surface structure, and heterostructures. Besides, recent catalysts for overall water splitting are also reviewed. Finally, this review paves the way to the rational design and synthesis of new materials for highly efficient electrocatalysis.

Unveiling the Role of Charge Transfer in Enhanced Electrochemical Nitrogen Fixation at Single-Atom Catalysts on BX Sheets (X = As, P, Sb)

To tune single-atom catalysts (SACs) for effective nitrogen reduction reaction (NRR), we investigate various transition metals implanted on boron-arsenide (BAs), boron-phosphide (BP), and boron-antimony (BSb) using density functional theory (DFT). Interestingly, W-BAs shows high catalytic activity and excellent selectivity with an insignificant barrier of only 0.05 eV along the distal pathway and a surmountable kinetic barrier of 0.34 eV. The W-BSb and Mo-BSb exhibit high performances with limiting potentials of -0.19 and -0.34 V. The Bader-charge descriptor reveals that the charge transfers from substrate to *NNH in the first protonation step and from *NH₃ to substrate in the last protonation step, circumventing a big hurdle in NRR by achieving negative free energy change of *NH₂ to *NH₃. Furthermore, machine learning (ML) descriptors are introduced to reduce computational cost. Our rational design meets the three critical prerequisites of chemisorbing N₂ molecules, stabilizing *NNH, and destabilizing *NH₂ adsorbates for high-efficiency NRR.

A Sulfur-Tolerant MOF-Based Single-Atom Fe Catalyst for Efficient Oxidation of NO and Hg0

Catalytic oxidation of NO and Hg⁰ is a crucial step to eliminate multiple pollutants from emissions from coal-fired power plants. However, traditional catalysts exhibit low catalytic activity and poor sulfur resistance due to low activation ability and poor adsorption selectivity. Herein, a single-atom Fe decorated N-doped carbon catalyst (Fe₁ -N₄ -C), with abundant Fe₁ -N₄ sites, based on a Fe-doped metal-organic framework is developed to oxidize NO and Hg⁰ . The results demonstrate that the Fe₁ -N₄ -C has ultrahigh catalytic activity for oxidizing NO and Hg⁰ at low and room temperature. More importantly, Fe₁ -N₄ -C exhibits robust sulfur resistance as it preferably adsorbs reactants over sulfur oxides, which has never been achieved before with traditional catalysts. Furthermore, SO₂ boosts the catalytic oxidation of NO over Fe₁ -N₄ -C through accelerating the circulation of active sites. Density functional theory calculations reveal that the Fe₁ -N₄ active sites result in a low energy barrier and high adsorption selectivity, providing detailed molecular-level understanding for its excellent catalytic performance. This is the first report on NO and Hg⁰ oxidation over single-atom catalysts with strong sulfur tolerance. The outcomes demonstrate that single-atom catalysts are promising candidates for catalytic oxidation of NO and Hg⁰ enabling cleaner coal-fired power plant operations.

NP monolayer supported transition-metal single atoms for electrochemical water splitting: a theoretical study

The development of cost-effective and highly efficient electrocatalysts for water splitting is highly desirable but remains an ongoing challenge. Numerous single-atom catalysts (SACs) have achieved satisfactory performances in this area; however, non-carbon metal-free substrates have been rarely explored. Herein, we report a series of single-metal atoms supported on a novel two-dimensional NP monolayer as promising electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) by theoretical calculations. Our results disclose that Ti@NP, V@NP and Ir@NP exhibit desirable catalytic activity for the HER with extremely low of -0.004, -0.051, and 0.017 eV, respectively. More importantly, the calculated activation barriers for the Tafel reactions of these SACs are much lower than those for the benchmark Pt catalysts. In addition, Pt@NP shows the lowest η^OER of 0.495 V, followed by Rh@NP (η^OER = 0.548 V), which are even superior to that of state-of-the-art IrO₂. This work highlights the potential application of metal-free supports in SACs, which also further enriches the application of a NP monolayer in other related electrochemical processes.

Hydrothermal synthesized delafossite CuGaO2 as an electrocatalyst for water oxidation

Hydrogen production from water splitting provides an effective method to alleviate the ever-growing global energy crisis. In this work, delafossite CuGaO₂ (CGO) crystal was synthesized through hydrothermal routes with Cu(NO₃)₂·3H₂O and Ga(NO₃)₃·xH₂O used as reactants. The addition of cetyltrimethylammonium bromide (CTAB) was found to play an important role in modifying the morphology of CuGaO₂ (CGO-CTAB). With the addition of CTAB, the morphology of CGO-CTAB samples changed from irregular flake to typical hexagonal sheet microstructure, with an average size of 1-2 μm and a thickness of around 100 nm. Furthermore, the electrocatalytic activity of CGO-CTAB crystals for oxygen evolution reaction (OER) was also studied and compared with that of CGO crystals. CGO-CTAB samples exhibited better activity than CGO. An overpotential of 391.5 mV was shown to be able to generate a current density of 10 mA/cm². The as-prepared samples also demonstrate good stability for water oxidation and relatively fast OER kinetics with a Tafel slope of 56.4 mV/dec. This work highlights the significant role of modification of CTAB surfactants in preparing CGO related crystals, and the introduction of CTAB was found to help to improve their electrocatalytic activity for OER.

Metal-organic framework derived bimetal oxide CuCoO2 as efficient electrocatalyst for the oxygen evolution reaction

Metal-organic framework (MOF) materials with tunable porous morphology, controlled crystalline structure, various compositions, and high specific surface area are widely used as precursors to synthesize electrocatalysts for water splitting, which is beneficial for improving their oxygen evolution reaction (OER) performance. Using ZIF-67 as a Co source and Cu-BTC as a Cu source, hexagonal MOF-derived CuCoO₂ (MOF-CCO) nanocrystals with the size of ∼288 nm were prepared through a one-step solvothermal method. The influence of the content of the precursor solvents (absolute ethanol and deionized water), reaction temperature, mass ratio of reactants, NaOH addition, and reactant concentration of precursors on the structure and morphology of the products was investigated. The optimal CuCoO₂ nanocrystals (MOF-CCO1) around 288 nm present the highest OER activity, such as a low overpotential of 364.7 mV at 10 mA cm^-2, a small Tafel slope of 64.1 mV dec^-1, and attractive durability in 1.0 M KOH solution. The XPS results showed that the higher catalytic efficiency of MOF-CCO1 nanocrystals could be due to the oxygen vacancies caused by lattice oxygen loss, the increase of OH^- content on the surface, and the synergistic effect of Cu²⁺/Cu⁺ and Co²⁺/Co³⁺ redox pairs. Finally, a possible OER mechanism for MOF-CCO nanocrystals of water splitting was proposed. This study provides a new approach for the preparation of delafossite nanomaterials and for the improvement of their OER performances.

A minireview on the synthesis of single atom catalysts

Single atom catalysis is a prosperous and rapidly growing research field, owing to the remarkable advantages of single atom catalysts (SACs), such as maximized atom utilization efficiency, tailorable catalytic activities as well as supremely high catalytic selectivity. Synthesis approaches play crucial roles in determining the properties and performance of SACs. Over the past few years, versatile methods have been adopted to synthesize SACs. Herein, we give a thorough and up-to-date review on the progress of approaches for the synthesis of SACs, outline the general principles and list the advantages and disadvantages of each synthesis approach, with the aim to give the readers a clear picture and inspire more studies to exploit novel approaches to synthesize SACs effectively.

Transition Metal and N Doping on AlP Monolayers for Bifunctional Oxygen Electrocatalysts: Density Functional Theory Study Assisted by Machine Learning Description

It is vital to search for highly efficient bifunctional oxygen evolution/reduction reaction (OER/ORR) electrocatalysts for sustainable and renewable clean energy. Herein, we propose a single transition-metal (TM)-based defective AlP system to validate bifunctional oxygen electrocatalysis by using the density functional theory (DFT) method. We found that the catalytic activity is enhanced by substituting two P atoms with two N atoms in the Al vacancy of the TM-anchored AlP monolayer. Specifically, the overpotential of OER(ORR) in Co- and Ni-based defective AlP systems is found to be 0.38 (0.25 V) and 0.23 V (0.39 V), respectively, showing excellent bifunctional catalytic performance. The results are further presented by establishing the volcano plots and contour maps according to the scaling relation of the Gibbs free-energy change of *OH, *O, and *OOH intermediates. The d-band center and the product of the number of d-orbital electrons and electronegativity of the TM atom are the ideal descriptors for this system. To investigate the activity origin of the OER/ORR process, we performed the machine learning (ML) algorithm. The result indicates that the number of TM-d electrons (N_e), the radius of TM atoms (r_d), and the charge transfer of TM atoms (Q_e) are the three primary descriptors characterizing the adsorption behavior. Our results can provide a theoretical guidance for designing highly efficient bifunctional electrocatalysts and pave a way for the DFT-ML hybrid method in catalysis research.