A pyrolysis-free Ni/Fe bimetallic electrocatalyst for overall water splitting

Built-in electric field control of electron redistribution (NiFe-based electrocatalyst) with efficient overall water splitting at industrial temperature

Nickel-iron layered double hydroxide (NiFe-LDH) is hindered in its further development in water splitting due to its slow kinetics of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). In this study, the synthesis of OER (FeO(OH)/NiFe-LDH) and HER (Fe7S8(NiS)/NiFe-LDH) catalysts endowed with inherent electric fields exhibited exceptional electrocatalytic properties. The presence of the built-in electric field modulated the redistribution of electrons within the catalyst, while the formation of a heterostructure preserved the intrinsic characteristics of the catalyst. Moreover, this electron redistribution optimized the catalyst's adsorption of reaction intermediates (O*, OH*, OOH*, and H*) during the catalytic process, thereby enhancing the performance of both OER and HER. The electrolytic cell, equipped with these catalysts, achieved the current density of 10 mA cm-2 at a remarkably low potential of 1.409 V under industrial temperature conditions and demonstrated an ultra-long-term stability of 200 h.

Rational Construction of a 3D Self-Supported Electrode Based on ZIF-67 and Amorphous NiCoP for an Enhanced Oxygen Evolution Reaction

The development of efficient and Earth-abundant electrocatalysts for the oxygen evolution reaction (OER) is an urgent requirement in the field of electrochemical water splitting. The electrocatalytic performance of the OER can be greatly enhanced by the synergistic combination of zeolite imidazolate frameworks (ZIFs) and transition-metal phosphides, both of which individually exhibit promising capabilities in this regard. In this study, a novel amorphous NiCoP deposited on ZIF-67 sheets supported on Ni foam (labeled as NiCoP/ZIF-67/NF) as an OER electrocatalytic material was successfully synthesized using a simple, secure, and time-efficient two-step strategy. The experimental results demonstrate that NiCoP/ZIF-67/NF possesses a large active surface area with abundant active sites. Also, the synergistic effect and interaction between NiCoP and ZIF-67, as well as between Ni and Co within NiCoP, effectively enhance its electrochemical performance under alkaline conditions. Consequently, NiCoP/ZIF-67/NF exhibits outstanding catalytic activity for OER with an overpotential (η) of 175 mV at a current density of 10 mA cm-2 and a long-term stability over 40 h at 20 mA cm-2 in a 1.0 M KOH electrolyte. The corresponding analyses suggest that the real active sites responsible for the OER are identified as NiOOH and CoOOH species within the structure of NiCoP/ZIF-67/NF. Additionally, the catalytic function and stability of ZIF-67 toward the OER under alkaline conditions were also briefly discussed. This work provides a novel catalytic material for the OER along with a facile strategy to fabricate superior, efficient, and noble metal-free catalysts suitable for energy-related applications.

Metal-organic framework derived heterostructured phosphide bifunctional electrocatalyst for efficient overall water splitting

Developing high active and stable cost-effective bifunctional electrocatalysts for overall water splitting to produce hydrogen is of vital significance in clean and sustainable energy development. This work has prepared a novel porous unreported MOF (Ni-DPT) as a precursor to successfully synthesize a non-noble bifunctional NiCoP/Ni12P5@NF electrocatalyst through doping strategy and interface engineering. This catalyst is constructed by layered self-supporting arrays with heterojunction interface and rich nitrogen-phosphorus doping. Structural characterizations and the density function theory (DFT) calculations confirm that the interface effect of NiCoP/Ni12P5 heterojunction can regulate the electronic structure of the catalyst to optimize the Gibbs free energy of hydrogen (ΔGH*); simultaneously, the defect-rich layered nanoarrays can expose more active sites, shorten mass transfer distance, and generate a self-supporting structure for in-situ reinforcing the structural stability. As a result, this NiCoP/Ni12P5@NF catalyst exhibits favorable electrocatalytic performance, which simply needs overpotentials of 100 mV for HER and 310 mV for OER, respectively, at a current density of 10 mA·cm-2. The anion exchange membrane electrolyzer assembled with this NiCoP/Ni12P5@NF as both anode and cathode catalysts can operate stably for 200 h at a current density of 100 mA·cm-2 with an insignificant voltage decrease. This work may provide some inspiration for the further rational design of inexpensive non-noble multifunctional electrocatalysts and electrode materials for water splitting to generate hydrogen.

Surface-Modified Ruthenium Nanorods for an Ampere-Level Bifunctional Hydrogen Evolution Reaction/Oxygen Evolution Reaction Electrocatalyst

The practical applications of bifunctional ruthenium-based electrocatalysts with two active sites of Ru nanoparticles covered with RuO2 skins are limited. One reason is the presence of multiple equally distributed facets, some of which are inactive. In contrast, ruthenium nanorods with a high aspect ratio have multiple unequally distributed facets containing the dominance of active faces for efficient electrocatalysis. However, the synthesis of ruthenium nanorods has not been achieved due to difficulties in controlling the growth. Additionally, it is known that the adsorption capacity of intermediates can be impacted by the surface of the catalyst. Inspired by these backgrounds, the surface-modified (SM) ruthenium nanorods having a dominant active facet of hcp (100) through chemisorbed oxygen and OH groups (SMRu-NRs@NF) are rationally synthesized through the surfactant coordination method. SMRu-NRs@NF exhibits excellent hydrogen evolution in acid and alkaline solutions with an ultralow overpotential of 215 and 185 mV reaching 1000 mA cm-2, respectively. Moreover, it has also shown brilliant oxygen evolution electrocatalysis in alkaline solution with a low potential of 1.58 V to reach 1000 mA cm-2. It also exhibits high durability over 143 h for the evolution of oxygen and hydrogen at 1000 mA cm-2. Density functional theory studies confirmed that surface modification of a ruthenium nanorod with chemisorbed oxygen and OH groups can optimize the reaction energy barriers of hydrogen and oxygen intermediates. The surface-modified ruthenium nanorod strategy paves a path to develop the practical water splitting electrocatalyst.

NiO/RuO2 p-n Heterojunction Nanofoam as a High-Performance Electrocatalyst for Desulfurization and Concurrent Hydrogen Evolution

The development of bifunctional electrocatalysts with excellent performance in both the hydrogen evolution reaction (HER) and sulfide oxidation reaction (SOR) remains a formidable challenge. Herein, we experimentally synthesize a NiO/RuO2 p-n heterojunction nanofoam that exhibits highly desirable electrocatalytic properties for both the HER and the SOR. We further design an electrolytic cell by pairing alkaline HER with SOR utilizing the NiO/RuO2 heterojunction nanofoam as both the anode and the cathode, which demands a low applied voltage of 0.846 V to achieve a current density of 10 mA cm-2. Density functional theory calculations confirm that the formation of the NiO/RuO2 p-n heterojunction nanofoam effectively regulates the electronic structure, thereby boosting the electrocatalytic performances for both HER and SOR. This work not only provides a novel strategy to prepare an efficient and stable nanofoam electrocatalyst for hydrogen production but also highlights the potential application of oxide heterojunction electrocatalysts in treating sulfur-containing waste liquid.

Single Atom Catalysts Based on Earth-Abundant Metals for Energy-Related Applications

Anthropogenic activities related to population growth, economic development, technological advances, and changes in lifestyle and climate patterns result in a continuous increase in energy consumption. At the same time, the rare metal elements frequently deployed as catalysts in energy related processes are not only costly in view of their low natural abundance, but their availability is often further limited due to geopolitical reasons. Thus, electrochemical energy storage and conversion with earth-abundant metals, mainly in the form of single-atom catalysts (SACs), are highly relevant and timely technologies. In this review the application of earth-abundant SACs in electrochemical energy storage and electrocatalytic conversion of chemicals to fuels or products with high energy content is discussed. The oxygen reduction reaction is also appraised, which is primarily harnessed in fuel cell technologies and metal-air batteries. The coordination, active sites, and mechanistic aspects of transition metal SACs are analyzed for two-electron and four-electron reaction pathways. Further, the electrochemical water splitting with SACs toward green hydrogen fuel is discussed in terms of not only hydrogen evolution reaction but also oxygen evolution reaction. Similarly, the production of ammonia as a clean fuel via electrocatalytic nitrogen reduction reaction is portrayed, highlighting the potential of earth-abundant single metal species.

Materials Containing Single-, Di-, Tri-, and Multi-Metal Atoms Bonded to C, N, S, P, B, and O Species as Advanced Catalysts for Energy, Sensor, and Biomedical Applications

Modifying the coordination or local environments of single-, di-, tri-, and multi-metal atom (SMA/DMA/TMA/MMA)-based materials is one of the best strategies for increasing the catalytic activities, selectivity, and long-term durability of these materials. Advanced sheet materials supported by metal atom-based materials have become a critical topic in the fields of renewable energy conversion systems, storage devices, sensors, and biomedicine owing to the maximum atom utilization efficiency, precisely located metal centers, specific electron configurations, unique reactivity, and precise chemical tunability. Several sheet materials offer excellent support for metal atom-based materials and are attractive for applications in energy, sensors, and medical research, such as in oxygen reduction, oxygen production, hydrogen generation, fuel production, selective chemical detection, and enzymatic reactions. The strong metal-metal and metal-carbon with metal-heteroatom (i.e., N, S, P, B, and O) bonds stabilize and optimize the electronic structures of the metal atoms due to strong interfacial interactions, yielding excellent catalytic activities. These materials provide excellent models for understanding the fundamental problems with multistep chemical reactions. This review summarizes the substrate structure-activity relationship of metal atom-based materials with different active sites based on experimental and theoretical data. Additionally, the new synthesis procedures, physicochemical characterizations, and energy and biomedical applications are discussed. Finally, the remaining challenges in developing efficient SMA/DMA/TMA/MMA-based materials are presented.

Iron Active Center Coordination Reconstruction in Iron Carbide Modified on Porous Carbon for Superior Overall Water Splitting

In this work, a novel liquid nitrogen quenching strategy is engineered to fulfill iron active center coordination reconstruction within iron carbide (Fe3C) modified on biomass-derived nitrogen-doped porous carbon (NC) for initiating rapid hydrogen and oxygen evolution, where the chrysanthemum tea (elm seeds, corn leaves, and shaddock peel, etc.) is treated as biomass carbon source within Fe3C and NC. Moreover, the original thermodynamic stability is changed through the corresponding force generated by liquid nitrogen quenching and the phase transformation is induced with rich carbon vacancies with the increasing instantaneous temperature drop amplitude. Noteworthy, the optimizing intermediate absorption/desorption is achieved by new phases, Fe coordination, and carbon vacancies. The Fe3C/NC-550 (550 refers to quenching temperature) demonstrates outstanding overpotential for hydrogen evolution reaction (26.3 mV at -10 mA cm-2) and oxygen evolution reaction (281.4 mV at 10 mA cm-2), favorable overall water splitting activity (1.57 V at 10 mA cm-2). Density functional theory (DFT) calculations further confirm that liquid nitrogen quenching treatment can enhance the intrinsic electrocatalytic activity efficiently by optimizing the adsorption free energy of reaction intermediates. Overall, the above results authenticate that liquid nitrogen quenching strategy open up new possibilities for obtaining highly active electrocatalysts for the new generation of green energy conversion systems.

Porous Flower-Like Nanoarchitectures Derived from Nickel Phosphide Nanocrystals Anchored on Amorphous Vanadium Phosphate Nanosheet Nanohybrids for Superior Overall Water Splitting

Transition metal phosphides (TMPs) and phosphates (TM-Pis) nanostructures are promising functional materials for energy storage and conversion. Nonetheless, controllable synthesis of crystalline/amorphous heterogeneous TMPs/TM-Pis nanohybrids or related nanoarchitectures remains challenging, and their electrocatalytic applications toward overall water splitting (OWS) are not fully explored. Herein, the Ni2P nanocrystals anchored on amorphous V-Pi nanosheet based porous flower-like nanohybrid architectures that are self-supported on carbon cloth (CC) substrate (Ni2P/V-Pi/CC) are fabricated by conformal oxidation and phosphorization of pre-synthesized NiV-LDH/CC. Due to the unique microstructures and strong synergistic effects of crystalline Ni2P and amorphous V-Pi components, the obtained Ni2P/V-Pi/CC owns abundant active sites, suitable surface/interface electronic structure and optimized adsorption-desorption of reaction intermediates, resulting in outstanding electrocatalytic performances toward hydrogen and oxygen evolution reactions in alkaline media. Correspondingly, the assembled Ni2P/V-Pi/CC||Ni2P/V-Pi/CC electrolyzer only needs an ultralow cell voltage (1.44 V) to deliver 10 mA cm-2 water-splitting currents, exceeding its counterparts, recently reported bifunctional catalysts-based devices, and Pt/C/CC||IrO2/CC pairs. Moreover, the Ni2P/V-Pi/CC||Ni2P/V-Pi/CC manifests remarkable stability. Also, such device shows a certain prospect for OWS in acidic media. This work may spur the development of TMPs/TMPis-based nanohybrid architectures by combining structure and phase engineering, and push their applications in OWS or other clean energy options.

Rational Construction of 3D Self-Supported MOF-Derived Cobalt Phosphide-Based Hollow Nanowall Arrays for Efficient Overall Water Splitting At large Current Density

Developing efficient nonprecious bifunctional electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER) in the same electrolyte with a low overpotential and large current density presents an appealing yet challenging goal for large-scale water electrolysis. Herein, a unique 3D self-branched hierarchical nanostructure composed of ultra-small cobalt phosphide (CoP) nanoparticles embedded into N, P-codoped carbon nanotubes knitted hollow nanowall arrays (CoPʘNPCNTs HNWAs) on carbon textiles (CTs) through a carbonization-phosphatization process is presented. Benefiting from the uniform protrusion distributions of CoP nanoparticles, the optimum CoPʘNPCNTs HNWAs composites with high abundant porosity exhibit superior electrocatalytic activity and excellent stability for OER in alkaline conditions, as well as for HER in both acidic and alkaline electrolytes, even under large current densities. Furthermore, the assembled CoPʘNPCNTs/CTs||CoPʘNPCNTs/CTs electrolyzer demonstrates exceptional performance, requiring an ultralow cell voltage of 1.50 V to deliver the current density of 10 mA cm-2 for overall water splitting (OWS) with favorable stability, even achieving a large current density of 200 mA cm-2 at a low cell voltage of 1.78 V. Density functional theory (DFT) calculation further reveals that all the C atoms between N and P atoms in CoPʘNPCNTs/CTs act as the most efficient active sites, significantly enhancing the electrocatalytic properties. This strategy, utilizing 2D MOF arrays as a structural and compositional material to create multifunctional composites/hybrids, opens new avenues for the exploration of highly efficient and robust non-noble-metal catalysts for energy-conversion reactions.

Strain Engineering for Electrocatalytic Overall Water Splitting

Strain engineering is a novel method that can achieve superior performance for different applications. The lattice strain can affect the performance of electrochemical catalysts by changing the binding energy between the surface-active sites and intermediates and can be affected by the thickness, surface defects and composition of the materials. In this review, we summarized the basic principle, characterization method, introduction strategy and application direction of lattice strain. The reactions on hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are focused. Finally, the present challenges are summarized, and suggestions for the future development of lattice strain in electrocatalytic overall water splitting are put forward.

Heterojunction-Induced Rapid Transformation of Ni3+/Ni2+ Sites which Mediates Urea Oxidation for Energy-Efficient Hydrogen Production

Water electrolysis is an environmentally-friendly strategy for hydrogen production but suffers from significant energy consumption. Substituting urea oxidation reaction (UOR) with lower theoretical voltage for water oxidation reaction adopting nickel-based electrocatalysts engenders reduced energy consumption for hydrogen production. The main obstacle remains strong interaction between accumulated Ni3+ and *COO in the conventional Ni3+-catalyzing pathway. Herein, a novel Ni3+/Ni2+ mediated pathway for UOR via constructing a heterojunction of nickel metaphosphate and nickel telluride (Ni2P4O12/NiTe), which efficiently lowers the energy barrier of UOR and avoids the accumulation of Ni3+ and excessive adsorption of *COO on the electrocatalysts, is developed. As a result, Ni2P4O12/NiTe demonstrates an exceptionally low potential of 1.313 V to achieve a current density of 10 mA cm-2 toward efficient urea oxidation reaction while simultaneously showcases an overpotential of merely 24 mV at 10 mA cm-2 for hydrogen evolution reaction. Constructing urea electrolysis electrolyzer using Ni2P4O12/NiTe at both sides attains 100 mA cm-2 at a low cell voltage of 1.475 V along with excellent stability over 500 h accompanied with nearly 100% Faradic efficiency.

Mn Doping and P Vacancy Induced Fast Phase Reconstruction of FeP for Enhanced Electrocatalytic Oxygen Evolution Reaction in Alkaline Seawater

Due to the shortage of pure water resources, seawater electrolysis is a promising strategy to produce green hydrogen energy. To avoid chlorine oxidation reactions (ClOR) and the production of more corrosive hypochlorite, enhancing OER electrocatalyst activity is the key to solving the above problem. Considering that transition metal phosphides (TMPs) are promising OER eletrocatalysts for seawater splitting, a method to regulate the electronic structure of FeP by introducing Mn heteroatoms and phosphorus vacancy on it (Mn-FePV) is developed. As an OER electrocatalyst in seawater solution, the synthesized Mn-FePV achieves extremely low overpotentials (η500 = 376, η1000 = 395 mV). In addition, the Pt/C||Mn-FePV couple only requires the voltage of 1.81 V to drive the current density of 1000 mA cm-2 for overall seawater splitting. The density functional theory (DFT) calculation shows that Mn-FePV (0.21 e-) has more charge transfer number compared with FeP (0.17 e-). In-situ Raman analysis shows that phosphorus vacancy and Mn doping can synergistically regulate the electronic structure of FeP to induce rapid phase reconstruction, further improving the OER performance of Mn-FePV. The new phase species of FeOOH is confirmed to can enhance the adsorption kinetics of OER intermediates.

In Situ Fast Construction of Ni3S4/FeS Catalysts on 3D Foam Structure Achieving Stable Large-Current-Density Water Oxidation

By increasing the content of Ni3+, the catalytic activity of nickel-based catalysts for the oxygen evolution reaction (OER), which is still problematic with current synthesis routes, can be increased. Herein, a Ni3+-rich of Ni3S4/FeS on FeNi Foam (Ni3S4/FeS@FNF) via anodic electrodeposition to direct obtain high valence metal ions for OER catalyst is presented. XPS showed that the introduction of Fe not only further increased the Ni3+ concentration in Ni3S4/FeS to 95.02%, but also inhibited the dissolution of NiOOH by up to seven times. Furthermore, the OER kinetics is enhanced by the combination of the inner Ni3S4/FeS heterostructures and the electrochemically induced surface layers of oxides/hydroxides. Ni3S4/FeS@FNF shows the most excellent OER activity with a low Tafel slope of 11.2 mV dec-1 and overpotentials of 196 and 445 mV at current densities of 10 and 1400 mA cm-2, respectively. Furthermore, the Ni3S4/FeS@FNF catalyst can be operated stably at 1500 mA cm-2 for 200 h without significant performance degradation. In conclusion, this work has significantly increased the high activity Ni3+ content in nickel-based OER electrocatalysts through an anodic electrodeposition strategy. The preparation process is time-saving and mature, which is expected to be applied in large-scale industrialization.

Heteronuclear Dual Metal Atom Electrocatalysts for Water-Splitting Reactions

Hydrogen is considered a promising substitute for traditional fossil fuels because of its widespread sources, high calorific value of combustion, and zero carbon emissions. Electrocatalytic water-splitting to produce hydrogen is also deemed to be an ideal approach; however, it is a challenge to make highly efficient and low-cost electrocatalysts. Single-atom catalysts (SACs) are considered the most promising candidate to replace traditional noble metal catalysts. Compared with SACs, dual-atom catalysts (DACs) are capable of greater attraction, including higher metal loading, more versatile active sites, and excellent catalytic activity. In this review, several general synthetic strategies and structural characterization methods of DACs are introduced, and recent experimental advances in water-splitting reactions are discussed. The authors hope that this review provides insights and inspiration to researchers regarding DACs in electrocatalytic water-splitting.

Universal Sublimation Strategy to Stabilize Single-Metal Sites on Flexible Single-Wall Carbon-Nanotube Films with Strain-Enhanced Activities for Zinc-Air Batteries and Water Splitting

Single-metal-site catalysts have recently aroused extensive research in electrochemical energy fields such as zinc-air batteries and water splitting, but their preparation is still a huge challenge, especially in flexible catalyst films. Herein, we propose a sublimation strategy in which metal phthalocyanine molecules with defined isolated metal-N4 sites are gasified by sublimation and then deposited on flexible single-wall carbon nanotube (SWCNT) films by means of π-π coupling interactions. Specifically, iron phthalocyanine anchored on the SWCNT film prepared was directly used to boost the cathodic oxygen reduction reaction of the zinc-air battery, showing a high peak power density of 247 mW cm-2. Nickel phthalocyanine and cobalt phthalocyanine were, respectively, stabilized on SWCNT films as the anodic and cathodic electrocatalysts for water splitting, showing a low potential of 1.655 V at 10 mA cm-2. In situ Raman spectra and theoretical studies demonstrate that highly efficient activities originate from strain-induced metal phthalocyanine on SWCNTs. This work provides a universal preparation method for single-metal-site catalysts and innovative insights for electrocatalytic mechanisms.

Multi-Omics Study on Molecular Mechanisms of Single-Atom Fe-Doped Two-Dimensional Conjugated Phthalocyanine Framework for Photocatalytic Antibacterial Performance

Currently, photocatalysis of the two-dimensional (2D) conjugated phthalocyanine framework with a single Fe atom (CPF-Fe) has shown efficient photocatalytic activities for the removal of harmful effluents and antibacterial activity. Their photocatalytic mechanisms are dependent on the redox reaction-which is led by the active species generated from the photocatalytic process. Nevertheless, the molecular mechanism of CPF-Fe antimicrobial activity has not been sufficiently explored. In this study, we successfully synthesized CPF-Fe with great broad-spectrum antibacterial properties under visible light and used it as an antibacterial agent. The molecular mechanism of CPF-Fe against Escherichia coli and Salmonella enteritidis was explored through multi-omics analyses (transcriptomics and metabolomics correlation analyses). The results showed that CPF-Fe not only led to the oxidative stress of bacteria by generating large amounts of h+ and ROS but also caused failure in the synthesis of bacterial cell wall components as well as an osmotic pressure imbalance by disrupting glycolysis, oxidative phosphorylation, and TCA cycle pathways. More surprisingly, CPF-Fe could disrupt the metabolism of amino acids and nucleic acids, as well as inhibit their energy metabolism, resulting in the death of bacterial cells. The research further revealed the antibacterial mechanism of CPF-Fe from a molecular perspective, providing a theoretical basis for the application of CPF-Fe photocatalytic antibacterial nanomaterials.

Designing Different Heterometallic Organic Frameworks by Heteroatom and Second Metal Doping Strategies for the Electrocatalytic Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) are considered one of the most significant electrocatalysts for the sluggish oxygen evolution reaction (OER). Hence, a series of novel N,S-codoped Ni-based heterometallic organic framework (HMOF) (NiM-bptz-HMOF, M = Co, Zn, and Mn; bptz = 2,5-bis((3-pyridyl)methylthio)thiadiazole) precatalysts are constructed by the heteroatom and second metal doping strategies. The effective combination of the two strategies promotes electronic conductivity and optimizes the electronic structure of the metal. By regulation of the type and proportion of metal ions, the electrochemical performance of the OER can be improved. Among them, the optimized Ni6Zn1-bptz-HMOF precatalyst exhibits the best performance with an overpotential of 268 mV at 10 mA cm-2 and a small Tafel slope of 72.5 mV dec-1. This work presents a novel strategy for the design of modest heteroatom-doped OER catalysts.

Rational Manipulation of Active CNT Encapsulated Fe Doped NiCoP Nanoparticles In Situ Grown in Hierarchically Carbonized Wood for High-Current-Density Water Splitting

Precise morphology design and electronic structure regulation are critically significant to promote catalytic activity and stability for electrochemical hydrogen production at high current density. Herein, the carbon nanotube (CNT) encapsulated Fe-doped NiCoP nanoparticles is in-situ grown in hierarchical carbonized wood (NCF0.5 P@CNT/CW) for water splitting. Coupling merits of porous carbonized wood (CW) substrate, CNT encapsulating and Fe doping, the NCF0.5 P@CNT/CW features remarkable and durable electrocatalytic activity. The overpotentials of NCF0.5 P@CNT/CW at 50 mA cm-2 mV and 205 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) and features high current density of 800 mA cm-2 within 300 mV for both OER and HER. Moreover, NCF0.5 P@CNT/CW displays outstanding overall water splitting performance (η50 = 1.62 V and η100 = 1.67 V), outperforming Pt/C║RuO2 (η50 = 1.74 V), and can achieve the current density of 700 mA cm-2 at a lower cell voltage of 1.78 V. Overpotential is only 4.0 % decay after 120 h measurement at 50 mA cm-2 . Density functional theory (DFT) calculations reveals Fe doping optimizes the binding energy and Gibbs free energy of intermediates, and regulates d-band center of NCF0.5 P@CNT/CW. Such synergistic strategy of morphology manipulation and electronic structure optimization provides a spark for developing effective and robust bifunctional catalysts.

Novel mixed heterovalent (Mo/Co)Ox-zerovalent Cu system as bi-functional electrocatalyst for overall water splitting

A novel hybrid ternary metallic electrocatalyst of amorphous Mo/Co oxides and crystallized Cu metal was deposited over Ni foam using a one-pot, simple, and scalable solvothermal technique. The chemical structure of the prepared ternary electrocatalyst was systematically characterized and confirmed via XRD, FTIR, EDS, and XPS analysis techniques. FESEM images of (Mo/Co)Ox-Cu@NF display the formation of 3D hierarchical structure with a particle size range of 3-5 µm. The developed (Mo/Co)Ox-Cu@NF ternary electrocatalyst exhibits the maximum activity with 188 mV and 410 mV overpotentials at 50 mA cm-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Electrochemical impedance spectroscopy (EIS) results for the (Mo/Co)Ox-Cu@NF sample demonstrate the minimum charge transfer resistance (Rct) and maximum constant phase element (CPE) values. A two-electrode cell based on the ternary electrocatalyst just needs a voltage of about 1.86 V at 50 mA cm-2 for overall water splitting (OWS). The electrocatalyst shows satisfactory durability during the OWS for 24 h at 10 mA cm-2 with an increase of only 33 mV in the cell potential.

Engineering organic polymers as emerging sustainable materials for powerful electrocatalysts

Cost-effective and high-efficiency catalysts play a central role in various sustainable electrochemical energy conversion technologies that are being developed to generate clean energy while reducing carbon emissions, such as fuel cells, metal-air batteries, water electrolyzers, and carbon dioxide conversion. In this context, a recent climax in the exploitation of advanced earth-abundant catalysts has been witnessed for diverse electrochemical reactions involved in the above mentioned sustainable pathways. In particular, polymer catalysts have garnered considerable interest and achieved substantial progress very recently, mainly owing to their pyrolysis-free synthesis, highly tunable molecular composition and microarchitecture, readily adjustable electrical conductivity, and high stability. In this review, we present a timely and comprehensive overview of the latest advances in organic polymers as emerging materials for powerful electrocatalysts. First, we present the general principles for the design of polymer catalysts in terms of catalytic activity, electrical conductivity, mass transfer, and stability. Then, the state-of-the-art engineering strategies to tailor the polymer catalysts at both molecular (i.e., heteroatom and metal atom engineering) and macromolecular (i.e., chain, topology, and composition engineering) levels are introduced. Particular attention is paid to the insightful understanding of structure-performance correlations and electrocatalytic mechanisms. The fundamentals behind these critical electrochemical reactions, including the oxygen reduction reaction, hydrogen evolution reaction, CO2 reduction reaction, oxygen evolution reaction, and hydrogen oxidation reaction, as well as breakthroughs in polymer catalysts, are outlined as well. Finally, we further discuss the current challenges and suggest new opportunities for the rational design of advanced polymer catalysts. By presenting the progress, engineering strategies, insightful understandings, challenges, and perspectives, we hope this review can provide valuable guidelines for the future development of polymer catalysts.

Revealing the Orbital Interactions between Dissimilar Metal Sites during Oxygen Reduction Process

A FeCo/DA@NC catalyst with the well-defined FeCoN6 moiety is customized through a novel and ultrafast Joule heating technique. This catalyst demonstrates superior oxygen reduction reaction activity and stability in an alkaline environment. The power density and charge-discharge cycling of znic-air batteries driven by FeCo/DA@NC also surpass those of Pt/C catalyst. The source of the excellent oxygen reduction reaction activity of FeCo/DA@NC originates from the significantly changed charge environment and 3d orbital spin state. These not only improve the bonding strength between active sites and oxygen-containing intermediates, but also provide spare reaction sites for oxygen-containing intermediates. Moreover, various in situ detection techniques reveal that the rate-determining step in the four-electron oxygen reduction reaction is *O2 protonation. This work provides strong support for the precise design and rapid preparation of bimetallic catalysts and opens up new ideas for understanding orbital interactions during oxygen reduction reactions.

Electronic Structure Regulated Nickel-Cobalt Bimetal Phosphide Nanoneedles for Efficient Overall Water Splitting

Transition metal phosphides (TMPs) have been widely studied for water decomposition for their monocatalytic property for anodic or cathodic reactions. However, their bifunctional catalytic activity still remains a major challenge. Herein, hexagonal nickel-cobalt bimetallic phosphide nanoneedles with 1-3 μm length and 15-30 nm diameter supported on NF (NixCo2-xP NDs/NF) with adjusted electron structure have been successfully prepared. The overall alkaline water electrolyzer composed of the optimal anode (Ni0.67Co1.33P NDs/NF) and cathode (Ni1.01Co0.99P NDs/NF) provide 100 mA cm-2 at 1.62 V. Gibbs Free Energy for reaction paths proves that the active site in the hydrogen evolution reaction (HER) is Ni and the oxygen evolution reaction (OER) is Co in NixCo2-xP, respectively. In the HER process, Co-doping can result in an apparent accumulation of charge around Ni active sites in favor of promoting HER activity of Ni sites, and ΔGH* of 0.19 eV is achieved. In the OER process, the abundant electron transfer around Co-active sites results in the excellent ability to adsorb and desorb *O and *OOH intermediates and an effectively reduced ∆GRDS of 0.37 eV. This research explains the regulation of electronic structure change on the active sites of bimetallic materials and provides an effective way to design a stable and effective electrocatalytic decomposition of alkaline water.

Heterogeneous Co-Ni phosphide with active sites for water dissociation and efficient hydrogen evolution reaction

The construction of highly active and stable transition phosphide-based materials is widely regarded as an alternative approach to the use of Pt-based catalysts in the field of electrocatalytic hydrogen evolution. Herein, self-supported heterostructure Co-Ni phosphides (denoted as CoxNi1-x-P) were synthesized with different metal ratios by a low temperature electrodeposition strategy. Impressively, the optimized heterogeneous Co0.5Ni0.5-P nanocomposites displayed outstanding hydrogen evolution performance, with low overpotentials of 67 mV and 181 mV to deliver current densities of 10 mA cm-2 and 100 mA cm-2 in alkaline electrolyte. X-ray photoelectron spectroscopy revealed the optimized electronic structure of Co0.5Ni0.5-P, which led to an improvement in the conductivity. Density functional theory calculations demonstrated that the Co0.5Ni0.5-P heterostructure could provide a more optimal water-dissociation-related Volmer process for hydrogen evolution reaction (HER), in which water molecules could be easily activated on Co0.5Ni0.5-P with a low energy barrier. Moreover, the downshift of the d-band center confirmed the optimized H adsorption, further accelerating the HER kinetics.

Modulating Electronic Structures of Bimetallic Co-Fe Sulfide Ultrathin Nanosheet Supported on g-C3N4 Promoting Electrocatalytic Hydrogen Evolution Performance

Heteroatom doping is a possible way to regulate the catalytic capability of electrocatalysts for hydrogen evolution reaction (HER). This work focuses on the development of bimetallic Cobalt-Iron sulfide ultrathin nanosheets supported on the graphitic carbon nitride (g-C3N4) catalyst as efficient HER electrocatalysts (CoS2/FeS2/CN) with good stability at wide pH value. The ultrathin nanosheet exposes more active sites and enhances the catalyst activity. Electrochemical experiments demonstrate that adding g-C3N4 and Fe to CoS2 increases its catalytic activity and stability. Furthermore, g-C3N4 and Fe co-doped with CoS2 can modulate electronic structures on the interface. The CoS2/FeS2/CN exhibits outstanding HER performance, reaching a current density of 10 mA cm-2 with overpotentials of only 76.5 mV in an acidic solution and 175.6 mV in an alkaline solution. It also demonstrates exceptional durability, superior to commercial platinum/carbon catalysts. This work introduces a promising approach for designing low-cost, high-performance HER electrocatalysts with a wide pH range.

Conductive Covalent Organic Frameworks of Polymetallophthalocyanines as a Tunable Platform for Electrocatalysis

Developing an electrocatalyst platform that can control the interplay among activity, selectivity, and stability at atomic precision remains a grand challenge. Here, we have synthesized highly crystalline polymetallophthalocyanines (pMPcs, M = Fe, Co, Ni, and Cu) through the annulation of tetracyanobenzene in the presence of transition metals. The conjugated, conductive, and stable backbones with precisely installed metal sites render pMPcs a unique platform in electrochemical catalysis, where tunability emerges from long-range interactions. The construction of pCoNiPc with a Co and Ni dual-site integrates the advantageous features of pCoPc and pNiPc in electrocatalytic CO2 reduction through electronic communication of the dual-site with an unprecedented long atomic separation of ≥14 chemical bonds. This integration provides excellent activity (current density, j = -16.0 and -100 mA cm-2 in H-type and flow cell, respectively), selectivity (CO Faraday efficiency, FECO = 94%), and stability (>10 h), making it one of the best-performing reticular materials.

Direct Assembly of Metal-Phenolic Network Nanoparticles for Biomedical Applications

Coordination assembly offers a versatile means to developing advanced materials for various applications. However, current strategies for assembling metal-organic networks into nanoparticles (NPs) often face challenges such as the use of toxic organic solvents, cytotoxicity because of synthetic organic ligands, and complex synthesis procedures. Herein, we directly assemble metal-organic networks into NPs using metal ions and polyphenols (i.e., metal-phenolic networks (MPNs)) in aqueous solutions without templating or seeding agents. We demonstrate the role of buffers (e.g., phosphate buffer) in governing NP formation and the engineering of the NP physicochemical properties (e.g., tunable sizes from 50 to 270 nm) by altering the assembly conditions. A library of MPN NPs is prepared using natural polyphenols and various metal ions. Diverse functional cargos, including anticancer drugs and proteins with different molecular weights and isoelectric points, are readily loaded within the NPs for various applications (e.g., biocatalysis, therapeutic delivery) by direct mixing, without surface modification, owing to the strong affinity of polyphenols to various guest molecules. This study provides insights into the assembly mechanism of metal-organic complexes into NPs and offers a simple strategy to engineer nanosized materials with desired properties for diverse biotechnological applications.

Microstructure and Oxygen Evolution Property of Prussian Blue Analogs Prepared by Mechanical Grinding

Solvent-free mechanochemical synthesis of efficient and low-cost double perovskite (DP), like a cage of Prussian blue (PB) and PB analogs (PBAs), is a promising approach for different applications such as chemical sensing, energy storage, and conversion. Although the solvent-free mechanochemical grinding approach has been extensively used to create halide-based perovskites, no such reports have been made for cyanide-based double perovskites. Herein, an innovative solvent-free mechanochemical synthetic strategy is demonstrated for synthesizing Fe4[Fe(CN)6]3, Co3[Fe(CN)6]2, and Ni2[Fe(CN)6], where defect sites such as carbon-nitrogen vacancies are inherently introduced during the synthesis. Among all the synthesized PB analogs, the Ni analog manifests a considerable electrocatalytic oxygen evolution reaction (OER) with a low overpotential of 288 mV to obtain the current benchmark density of 20 mA cm-2. We hypothesize that incorporating defects, such as carbon-nitrogen vacancies, and synergistic effects contribute to high catalytic activity. Our findings pave the way for an easy and inexpensive large-scale production of earth-abundant non-toxic electrocatalysts with vacancy-mediated defects for oxygen evolution reaction.