Cathodic Activation of Titania-Fly Ash Cenospheres for Efficient Electrochemical Hydrogen Production: A Proposed Solution to Treat Fly Ash Waste

Fly ash (FA) is a waste product generated in huge amounts by coal-fired electric and steam-generating plants. As a result, the use of FA alone or in conjunction with other materials is an intriguing study topic worth exploring. Herein, we used FA waste in conjunction with titanium oxide (TiO2) to create (FA-TiO2) nanocomposites. For the first time, a cathodic polarization pre-treatment regime was applied to such nanocomposites to efficiently produce hydrogen from an alkaline solution. The FA-TiO2 hybrid nanocomposites were prepared by a straightforward solvothermal approach in which the FA raw material was mixed with titanium precursor in dimethyl sulfoxide (DMSO) and refluxed during a given time. The obtained FA-TiO2 hybrid nanocomposites were fully characterized using various tools and displayed a cenosphere-like shape. The synthesized materials were tested as electrocatalysts for the hydrogen evolution reaction (HER) in 0.1 M KOH solution in the dark, employing various electrochemical techniques. The as-prepared (unactivated) FA-TiO2 exhibited a considerable HER electrocatalytic activity, with an onset potential (EHER) value of −144 mV vs. RHE, a Tafel slope (−bc) value of 124 mV dec−1 and an exchange current density (jo) of ~0.07 mA cm−2. The FA-TiO2′s HER catalytic performance was significantly enhanced upon cathodic activation (24 h of chronoamperometry measurements performed at a high cathodic potential of −1.0 V vs. RHE). The cathodically activated FA-TiO2 recorded HER electrochemical kinetic parameters of EHER = −28 mV, −bc = 115 mV dec−1, jo = 0.65 mA cm−2, and an overpotential η10 = 125 mV to yield a current density of 10 mA cm−2. Such parameters were comparable to those measured here for the commercial Pt/C under the same experimental conditions (EHER = −10 mV, −bc = 113 mV dec−1, jo = 0.88 mA cm−2, η10 = 110 mV), as well as to the most active electrocatalysts for H2 generation from aqueous alkaline electrolytes.

Metal nitride-based nanostructures for electrochemical and photocatalytic hydrogen production

The over-dependence on fossil fuels is one of the critical issues to be addressed for combating greenhouse gas emissions. Hydrogen, one of the promising alternatives to fossil fuels, is renewable, carbon-free, and non-polluting gas. The complete utilization of hydrogen in every sector ranging from small to large scale could hugely benefit in mitigating climate change. One of the key aspects of the hydrogen sector is its production via cost-effective and safe ways. Electrolysis and photocatalysis are well-known processes for hydrogen production and their efficiency relies on electrocatalysts, which are generally noble metals. The usage of noble metals as catalysts makes these processes costly and their scarcity is also a limiting factor. Metal nitrides and their porous counterparts have drawn considerable attention from researchers due to their good promise for hydrogen production. Their properties such as active metal centres, nitrogen functionalities, and porous features such as surface area, pore-volume, and tunable pore size could play an important role in electrochemical and photocatalytic hydrogen production. This review focuses on the recent developments in metal nitrides from their synthesis methods point of view. Much attention is given to the emergence of new synthesis techniques, methods, and processes of synthesizing the metal nitride nanostructures. The applications of electrochemical and photocatalytic hydrogen production are summarized. Overall, this review will provide useful information to researchers working in the field of metal nitrides and their application for hydrogen production.

Critical Review, Recent Updates on Zeolitic Imidazolate Framework-67 (ZIF-67) and Its Derivatives for Electrochemical Water Splitting

Design and construction of low-cost electrocatalysts with high catalytic activity and long-term stability is a challenging task in the field of catalysis. Metal-organic frameworks (MOF) are promising candidates as precursor materials in the development of highly efficient electrocatalysts for energy conversion and storage applications. This review starts with a summary of basic concepts and key evaluation parameters involved in the electrochemical water-splitting reaction. Then, different synthesis approaches reported for the cobalt-based Zeolitic imidazolate framework (ZIF-67) and its derivatives are critically reviewed. Additionally, several strategies employed to enhance the electrocatalytic activity and stability of ZIF-67-based electrocatalysts are discussed in detail. The present review provides a succinct insight into the ZIF-67 and its derivatives (oxides, hydroxides, sulfides, selenides, phosphide, nitrides, telluride, heteroatom/metal-doped carbon, noble metal-supported ZIF-67 derivatives) reported for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting applications. Finally, this review concludes with the associated challenges and the perspectives on developing the best economic, durable electrocatalytic materials.

Synthesis and electrochemical properties of a nickel(II) thiacalix[4]arene-based electrocatalyst for the hydrogen evolution reaction

Thiacalix[4]arene with four sulfur-bridging atoms and four hydroxy coordinating groups in the lower rim is a promising candidate for powerful electrocatalytic hydrogen production. However, there are only a few examples reported in the literature where conjugated thiacalix[4]arene is used as a pre-catalyst for the hydrogen evolution reaction. The present work evaluates the electrocatalytic performance of a nickel(II) coordination complex based on generic thiacalix[4]arene. The coordination complex is characterized by cyclic voltammetry, where two reduction peaks are observed at −0.57 and −1.33 V for the two redox couples Ni2+/Ni+ and Ni+/Ni, respectively. The proton reduction occurs at the second peak potential with a slight shift at about −1.0 V, with increasing peak currents directly related to the number of acetic acid equivalents. The maximum peak current was observed to be at about 67 µA for 20 equiv. of acetic acid.

Mo2C@C nanofibers film as durable self-supported electrode for efficient electrocatalytic hydrogen evolution

Self-supported electrocatalytic thin films consist 3D conducting network and well-embedded electrocatalysts, which endows the advantage in mass flow kinetics and durability for large-scale water splitting. Synthesis of such self-supported electrode still remains a big challenge due to the difficulty in the control over the 3D conducting network and the simultaneous growth of catalyst with well attachment on the conducting fibers. Herein, a self-supported Mo₂C@carbon nanofibers (Mo₂C@C NF) film has been successfully fabricated with outstanding electrocatalytic performance under optimized pyrolysis temperature and precursors mass ratio conditions. During the carbonation process, the Mo₂C nanoparticles (∼16 nm) were simultaneously grown and well dispersed on the inter-connected carbon nanofibers, which formed a 3D conducting network. The as-formed 3D carbon network was strong enough to support direct electrocatalytic application without additional ink or supporting substrates. This particular electrode structure facilitated easy access to the active catalytic sites, electron transfer, and hydrogen diffusion, resulting in the high hydrogen evolution reaction activity. A low overpotential of 86 mV was needed to achieve 10 mA cm^-2current density with outstanding kinetics metric (Tafel 43 mV dec^-1) in 1 M KOH. Additionally, the self-supported Mo₂C@C NF film, a binder-free electrode, exhibited extraordinary stability of more than 340 h.

Mesoporous Ti0.5Cr0.5N for trace H2S detection with excellent long-term stability

Efficient, accurate and reliable detection and monitoring of H₂S is of significance in a wide range of areas: industrial production, medical diagnosis, environmental monitoring, and health screening. However the rapid corrosion of commercial platinum-on-carbon (Pt/C) sensing electrodes in the presence of H₂S presents a fundamental challenge for fuel cell gas sensors. Herein we report a solution to the issue through the design of a sensing electrode, which is based on Pt supported on mesoporous titanium chromium nitrides (Pt/Ti_0.5Cr_0.5N). Its desirable characteristics are due to its high electrochemical stability and strong metal-support interactions. The Pt/Ti_0.5Cr_0.5N-based sensors exhibit a much smaller attenuation (1.3%) in response to H₂S than Pt/C-sensor (40%), after 2 months sensing test. Furthermore, the Pt/Ti_0.5Cr_0.5N-based sensors exhibit negligible cross response to other interfering gases compared with hydrogen sulfide. Results of density functional theory calculation also verify the excellent long-term stability and selectivity of the gas sensor. Our work hence points to a new sensing electrode system that offers a combination of high performance and stability for fuel-cell gas sensors.

Mo,Fe-codoped metal phosphide nanosheets derived from Prussian blue analogues for efficient overall water splitting

Designing non-precious electrocatalysts with multiple active centers and durability toward overall water splitting is of great significance for storing renewable energy. This study reports a low-cost Mo, Fe codoped NiCoP_x electrocatalysts derived from Co-Fe Prussian blue analogue and following phosphorization process. Benefitted from the optimized electronic configuration, hierarchical structure and abundant active sites, the Mo,Fe-NiCoP_x/NF electrode has shown competitive oxygen evolution reaction (ƞ₁₀ = 197 mV) and hydrogen evolution reaction performance (ƞ₁₀ = 99 mV) when the current density is 10 mA cm^-2 in 1 M KOH solution. Moreover, the integrated water splitting device assembled by Mo,Fe-NiCoP_x/NF as both anode and cathode only needs a voltage of 1.545 V to reach 10 mA cm^-2. Density functional theory results further confirm that the Mo, Fe codoped heterostructure can synergistically optimize the d-band center and Gibbs free energy during electrocatalytic processes, thus accelerating the kinetics of electrochemical water splitting. This work demonstrates the importance of rational combination of metal doping and interface engineering for advanced catalytic materials.

Ultrafine Mo2C nanoparticles embedded in an MOF derived N and P co-doped carbon matrix for an efficient electrocatalytic oxygen reduction reaction in zinc-air batteries

Exploring high-activity electrocatalysts for an oxygen reduction reaction (ORR) is of great significance for a variety of renewable energy conversion and storage technologies. Here, ultrafine Mo₂C nanoparticles assembled in N and P-co-doped carbon (Mo₂C@NPC) was developed from ZIF-8 encapsulated molybdenum-based polyoxometalates (PMo₁₂) as a highly efficient ORR electrocatalyst and shows excellent performance for zinc-air batteries. The well distribution of the PMo₁₂ in ZIF-8 results in the formation of ultrafine Mo₂C nanocrystallites encapsulated in a porous carbon matrix after pyrolysis. Significantly, from experimental and theoretical investigations, the highly porous structure, highly dispersed ultrafine Mo₂C and the N and P co-doping in the Mo₂C@NPC lead to the remarkable ORR activity with an onset potential of ∼1.01 V, a half-wave potential of ∼0.90 V and a Tafel slope of 51.7 mV dec^-1 at 1600 rpm in 0.1 M KOH. In addition, the Mo₂C@NPC as an ORR catalyst in zinc-air batteries achieved a high power density of 266 mW cm^-2 and a high specific capacity of 780.9 mA h g^-1, exceeding that driven by commercial Pt/C. Our results revealed that the porous architecture and ultrafine Mo₂C nanocrystallites of the electrocatalysts could facilitate mass transport and increase the accessibility of active sites, thus optimizing their performances in an ORR. The present study provides some guidelines for the design and synthesis of efficient nanostructured electrocatalysts.

Phase controlled synthesis of transition metal carbide nanocrystals by ultrafast flash Joule heating

Nanoscale carbides enhance ultra-strong ceramics and show activity as high-performance catalysts. Traditional lengthy carburization methods for carbide syntheses usually result in coked surface, large particle size, and uncontrolled phase. Here, a flash Joule heating process is developed for ultrafast synthesis of carbide nanocrystals within 1 s. Various interstitial transition metal carbides (TiC, ZrC, HfC, VC, NbC, TaC, Cr2C3, MoC, and W2C) and covalent carbides (B4C and SiC) are produced using low-cost precursors. By controlling pulse voltages, phase-pure molybdenum carbides including β-Mo2C and metastable α-MoC1-x and η-MoC1-x are selectively synthesized, demonstrating the excellent phase engineering ability of the flash Joule heating by broadly tunable energy input that can exceed 3000 K coupled with kinetically controlled ultrafast cooling (>104 K s-1). Theoretical calculation reveals carbon vacancies as the driving factor for topotactic transition of carbide phases. The phase-dependent hydrogen evolution capability of molybdenum carbides is investigated with β-Mo2C showing the best performance.

Highly efficient catalysts of ruthenium clusters on Fe3O4/MWCNTs for the hydrogen evolution reaction

Anchoring of Ru onto the Fe3O4/MWCNTs composite, its HER mechanism and DFT calculated change in Gibbs free energy of adsorbed hydrogen are described.

Molybdenum and cobalt co-doped VC nanoparticles encapsulated in nanocarbon as efficient electrocatalysts for the hydrogen evolution reaction

Molybdenum and cobalt co-doped VC nanoparticles encapsulated in nanocarbon (Mo, Co-VC@C) are successfully synthesized via a one-pot calcination route, exhibiting excellent electrocatalytic HER performance.

Structural Transformation of Heterogeneous Materials for Electrocatalytic Oxygen Evolution Reaction

Electrochemical water splitting for hydrogen generation is a promising pathway for renewable energy conversion and storage. One of the most important issues for efficient water splitting is to develop cost-effective and highly efficient electrocatalysts to drive sluggish oxygen-evolution reaction (OER) at the anode side. Notably, structural transformation such as surface oxidation of metals or metal nonoxide compounds and surface amorphization of some metal oxides during OER have attracted growing attention in recent years. The investigation of structural transformation in OER will contribute to the in-depth understanding of accurate catalytic mechanisms and will finally benefit the rational design of catalytic materials with high activity. In this Review, we provide an overview of heterogeneous materials with obvious structural transformation during OER electrocatalysis. To gain insight into the essence of structural transformation, we summarize the driving forces and critical factors that affect the transformation process. In addition, advanced techniques that are used to probe chemical states and atomic structures of transformed surfaces are also introduced. We then discuss the structure of active species and the relationship between catalytic performance and structural properties of transformed materials. Finally, the challenges and prospects of heterogeneous OER electrocatalysis are presented.

Fe2P encapsulated in carbon nanowalls decorated with well-dispersed Fe3C nanodots for efficient hydrogen evolution and oxygen reduction reactions

The development of cost-effective, high-efficiency bifunctional electrocatalysts as alternatives to the state-of-the-art Pt-based materials toward the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) is of great significance but still challenging. Herein, an advanced bifunctional electrocatalyst is presented, composed of Fe₂P encapsulated in carbon nanowalls decorated with well-dispersed Fe₃C nanodots (denoted as Fe₂P@Fe₃C/CNTs), which is achieved by a novel "inside-out" gas-solid reaction protocol. When functioning as a cathodic catalyst for water splitting, the Fe₂P@Fe₃C/CNT catalyst needs an ultralow overpotential of 83 mV to deliver a current density of 10 mA cm^-2, shows a small Tafel slope of 53 mV dec^-1 and ensures long-term stability for over 200 h in an alkaline electrolyte. Notably, the Fe₂P@Fe₃C/CNT catalyst exhibits an extremely impressive ORR performance with an onset potential (E_onset) of 1.060 V and a half-wave potential (E_1/2) of 0.930 V, excellent stability (≈94% activity retention after 36 000 s), and a strong methanol resistance ability, even far outperforming commercial Pt/C (E_onset = 0.955 V, E_1/2 = 0.825 V, ≈75% activity retention after less than 3500 s). Such outstanding HER and ORR performances are mainly ascribed to the improved corrosion resistance of the unique Fe₂P@C core-shell structures, the abundant catalytically active sites of ultrasmall Fe₃C nanodots incorporated in carbon nanowalls, and the good electrical conductivity of 2D graphitic carbon nanotubes used as a support.

Nanostructured Metal Borides for Energy-Related Electrocatalysis: Recent Progress, Challenges, and Perspectives

The discovery of durable, active, and affordable electrocatalysts for energy-related catalytic applications plays a crucial role in the advancement of energy conversion and storage technologies to achieve a sustainable energy future. Transition metal borides (TMBs), with variable compositions and structures, present a number of interesting features including coordinated electronic structures, high conductivity, abundant natural reserves, and configurable physicochemical properties. Therefore, TMBs provide a wide range of opportunities for the development of multifunctional catalysts with high performance and long durability. This review first summarizes the typical structural and electronic features of TMBs. Subsequently, the various synthetic methods used thus far to prepare nanostructured TMBs are listed. Furthermore, advances in emerging TMB-catalyzed reactions (both theoretical and experimental) are highlighted, including the hydrogen evolution reaction, the oxygen evolution reaction, the oxygen reduction reaction, the carbon dioxide reduction reaction, the nitrogen reduction reaction, the methanol oxidation reaction, and the formic acid oxidation reaction. Finally, challenges facing the development of TMB electrocatalysts are discussed, with focus on synthesis and energy-related catalytic applications, and some potential strategies/perspectives are suggested as well, which will profit the design of more efficient TMB materials for application in future energy conversion and storage devices.

Hierarchical molybdenum-doped NiCoP@carbon microspheres: a highly-efficient electrocatalyst for the hydrogen evolution reaction

Herein, we present hierarchical Mo-doped NiCoP@carbon microspheres that exhibit a noticeable enhancement in catalytic activity and fast kinetics for hydrogen evolution. An overpotential of 74.6 mV at 10 mA cm^-2 and 54.9 mV dec^-1 can be achieved. These results demonstrated the excellent electrochemical properties arising from the intrinsic characteristics of elemental doping and morphology control. We believe that this work opens a new avenue to fabricating TMD-based catalysts via the engineering of transition metal compounds.

One-Pot Crystallization of 2D and 3D Cobalt-Based Metal-Organic Frameworks and Their High-Performance Electrocatalytic Oxygen Evolution

Developing highly competent and low-cost earth-plentiful metal-based oxygen evolution reaction (OER) electrocatalysts is critical for future electrochemical conversion technologies and renewable energy systems. Herein, two cobalt-based metal-organic frameworks (Co-MOFs), [Co₆(btc)₂(DMF)₆(HCOO)₆] (2D, MOF 1) and [Co₃(CHOO)₉{DMA}₃] (3D, MOF 2), where btc = 1,3,5-benzenetricarboxylic acid, DMF = N,N-dimethylformamide, and DMA = dimethylamine, have been crystallized under hydrothermal conditions, from a single reaction. MOF 1 shows an extraordinary OER performance with 175 mV overpotential to attain 10 mA cm^-2 current density with a low Tafel slope value of 80 mV dec^-1, whereas MOF 2 achieves 10 mA cm^-2 current density at 389 mV overpotential. Two different architecture-based MOFs have been synthesized from a single solution for the very first time. Also, the OER activity of MOF 1 overpowers the commercially used RuO₂ and surpasses most of the reported OER electrocatalysts. Post OER characterization of MOF 1 revealed the in situ formation of Co(OH)₂ and CoOOH, acting as active sites for the OER process.

Molybdenum nitrides from structures to industrial applications

Owing to their remarkable characteristics, refractory molybdenum nitride (MoN x )-based compounds have been deployed in a wide range of strategic industrial applications. This review reports the electronic and structural properties that render MoN x materials as potent catalytic surfaces for numerous chemical reactions and surveys the syntheses, procedures, and catalytic applications in pertinent industries such as the petroleum industry. In particular, hydrogenation, hydrodesulfurization, and hydrodeoxygenation are essential processes in the refinement of oil segments and their conversions into commodity fuels and platform chemicals. N-vacant sites over a catalyst’s surface are a significant driver of diverse chemical phenomena. Studies on various reaction routes have emphasized that the transfer of adsorbed hydrogen atoms from the N-vacant sites reduces the activation barriers for bond breaking at key structural linkages. Density functional theory has recently provided an atomic-level understanding of Mo–N systems as active ingredients in hydrotreating processes. These Mo–N systems are potentially extendible to the hydrogenation of more complex molecules, most notably, oxygenated aromatic compounds.

Strategies and Perspectives to Catch the Missing Pieces in Energy-Efficient Hydrogen Evolution Reaction in Alkaline Media

Transition metal hydroxides (M-OH) and their heterostructures (X|M-OH, where X can be a metal, metal oxide, metal chalcogenide, metal phosphide, etc.) have recently emerged as highly active electrocatalysts for hydrogen evolution reaction (HER) of alkaline water electrolysis. Lattice hydroxide anions in metal hydroxides are primarily responsible for observing such an enhanced HER activity in alkali that facilitate water dissociation and assist the first step, the hydrogen adsorption. Unfortunately, their poor electronic conductivity had been an issue of concern that significantly lowered its activity. Interesting advancements were made when heterostructured hydroxide materials with a metallic and or a semiconducting phase were found to overcome this pitfall. However, in the midst of recently evolving metal chalcogenide and phosphide based HER catalysts, significant developments made in the field of metal hydroxides and their heterostructures catalysed alkaline HER and their superiority have unfortunately been given negligible attention. This review, unlike others, begins with the question of why alkaline HER is difficult and will take the reader through evaluation perspectives, trends in metals hydroxides and their heterostructures catalysed HER, an understanding of how alkaline HER works on different interfaces, what must be the research directions of this field in near future, and eventually summarizes why metal hydroxides and their heterostructures are inevitable for energy-efficient alkaline HER.

Transition metal-based catalysts for electrochemical water splitting at high current density: current status and perspectives

As a clean energy carrier, hydrogen has priority in decarbonization to build sustainable and carbon-neutral economies due to its high energy density and no pollutant emission upon combustion. Electrochemical water splitting driven by renewable electricity to produce green hydrogen with high-purity has been considered to be a promising technology. Unfortunately, the reaction of water electrolysis always requires a large excess potential, let alone the large-scale application (e.g., >500 mA cm^-2 needs a cell voltage range of 1.8-2.4 V). Thus, developing cost-effective and robust transition metal electrocatalysts working at high current density is imperative and urgent for industrial electrocatalytic water splitting. In this review, the strategies and requirements for the design of self-supported electrocatalysts are summarized and discussed. Subsequently, the fundamental mechanisms of water electrolysis (OER or HER) are analyzed, and the required important evaluation parameters, relevant testing conditions and potential conversion in exploring electrocatalysts working at high current density are also introduced. Specifically, recent progress in the engineering of self-supported transition metal-based electrocatalysts for either HER or OER, as well as overall water splitting (OWS), including oxides, hydroxides, phosphides, sulfides, nitrides and alloys applied in the alkaline electrolyte at large current density condition is highlighted in detail, focusing on current advances in the nanostructure design, controllable fabrication and mechanistic understanding for enhancing the electrocatalytic performance. Finally, remaining challenges and outlooks for constructing self-supported transition metal electrocatalysts working at large current density are proposed. It is expected to give guidance and inspiration to rationally design and prepare these electrocatalysts for practical applications, and thus further promote the practical production of hydrogen via electrochemical water splitting.

Cr-Doped CoP Nanorod Arrays as High-Performance Hydrogen Evolution Reaction Catalysts at High Current Density

Developing highly efficient, low-cost electrocatalysts with long-time stability at high current density working conditions for hydrogen evolution reaction (HER) remains a great challenge for the large-scale commercialization of hydrogen production from water electrolysis. Herein, the Cr-doped CoP nanorod arrays on carbon cloth (Cr-CoP-NR/CC) is reported as high performance HER catalysts with overpotentials of 38 and 209 mV at the HER current densities of 10 and 500 mA cm^-2 , respectively, outperforming the performance of the commercial Pt/C at high current density. And its HER performance shows almost no loss after 20 h working at 500 mA cm^-2 . The high performance is attributed to the Cr doping, which optimizes the hydrogen binding energy of CoP and prevents its oxidation. The nanorod array structure helps the escaping of the generated hydrogen gas, which is suitable for working at high current density. The obtained Cr-CoP-NR/CC catalyst shows the potential to replace the costly Pt-based HER catalysts in the water electrolyzer.

Graphene-Like Hydrogen-Bonded Melamine-Cyanuric Acid Supramolecular Nanosheets as Pseudo-Porous Catalyst Support

Behaving as structural protectors and electronic modulators, catalyst supports such as graphene derivatives are generally constructed by covalent bonds. Here, hydrogen-bonded ultrathin nanosheets are reported as a new type of catalyst support. Melamine (M) and cyanuric acid (CA) molecules self-assemble to form the graphite-like hydrogen-bonded co-crystal M-CA, which can be easily exfoliated by ultrasonic treatment to yield ultrathin nanosheets with thickness of ≈1.6 nm and high stability at pH = 0. The dynamic nanosheets form adaptive defects/pores in the synthetic process of CoP nanoparticles, giving embedded composite with high hydrogen evolution activity (overpotential of 66 mV at 10 mA cm^-2 ) and stability. Computational calculations, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy unveil the electron modulation effects of the nanosheets. This pseudo-porous catalyst support also can be applied to other metal phosphides.

Boosting Selective Nitrogen Reduction via Geometric Coordination Engineering on Single-Tungsten-Atom Catalysts

Atomic interface regulation that can efficiently optimize the performance of single-atom catalysts (SACs) is a worthwhile research topic. The challenge lies in deeply understanding the structure-properties correlation based on control of the coordination chemistry of individual atoms. Herein, a new kind of W SACs with oxygen and nitrogen coordination (W-NO/NC) and a high metal loading over 10 wt% is facilely prepared by introducing an oxygen-bridged [WO₄ ] tetrahedron. The local structure and coordination environment of the W SACs are confirmed by high-angle annular dark-field scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure. The catalyst shows excellent selectivity and activity for the electrochemical nitrogen reduction reaction (NRR). Density functional theory calculation reveals that unique electronic structures of the N and O dual-coordinated W sites optimize the binding energy of the NRR intermediate, resulting in facilitating the electrocatalytic NRR. This work opens an avenue toward exploring the correlation between coordination structure and properties.

Plasma-Engraved Co2N Nanostructures toward High-Performance Alkaline Hydrogen Evolution

Hydrogen generated by electrochemical water splitting is an attractive alternative to fossil fuels. Herein, we developed hollow-like Co₂N nanoarrays that serve as electrocatalysts for the hydrogen evolution reaction (HER) with surface engineering by argon plasma. The argon plasma-engraved Co₂N nanoarrays (Ar-Co₂N/CC) represent a dramatic catalytic performance for the HER with an overpotential of 34 mV at a current density of 10 mA cm^-2 in an alkaline electrolyte, as well as outstanding durability of 240 h. Characterization experiments and density functional theory (DFT) calculations suggest that the enhanced HER activity is due to the rational coordination environment of Co, which can be tuned by Ar plasma engraving. Based on our research, one new view for conducting exceptional catalyst surface modification engineering via plasma engraving might be established.

Transition-Metal Carbides as Hydrogen Evolution Reduction Electrocatalysts: Synthetic Methods and Optimization Strategies

With the strengths of zero carbon emission and high gravimetric energy density, hydrogen energy is recognized as a primary choice for future energy supply. Electrochemical water splitting provides a promising strategy for effective and sustainable hydrogen production through renewable electricity, and one of the immediate challenges toward its large-scale application is the availability of low-cost and efficient electrocatalysts for the hydrogen evolution reaction (HER). Given the enormous efforts in the exploration of potential transition-metal carbide (TMC) electrocatalysts, this review aims to summarize the recent advances in synthetic methods and optimization strategies of TMC electrocatalysts. Additionally, the perspectives for the development of novel efficient TMC-based catalysts are also proposed.

Tellurium Triggered Formation of Te/Fe-NiOOH Nanocubes as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting

The electrocatalyzed oxygen and hydrogen evolution reactions (OER/HER) are the key constituents of water splitting toward hydrogen production over electrolysis. The development of stable non-noble nanomaterials as bifunctional OER/HER electrocatalysts is the foremost bottleneck to commercial applications. Herein, the fabrication of Te-modulated FeNiOOH nanocubes (NCs) by a novel tailoring approach is reported, and the doping of Te superbly modulated the local electronic structures of Fe and Ni. The Te/FeNiOOH-NC catalyst displays better mass and electron transfer ability, exposure of plentiful OER/HER edge active centers on the surface, and a modulated electronic structure. Accordingly, the as-made Te/FeNiOOH-NC catalyst reveals robust OER activity (overpotential of 0.22 V@10 mA cm^-2) and HER activity (overpotential of 0.167 V@10 mA cm^-2) in alkaline media. Considerably, this bifunctional catalyst facilitates a high-performance alkaline water electrolyzer with a cell voltage of 1.65 V at 10 mA cm^-2. This strategy opens up a new way for designing and advancing the tellurium dopant nanomaterials for various applications.

Co2N/Co2Mo3O8 Heterostructure as a Highly Active Electrocatalyst for an Alkaline Hydrogen Evolution Reaction

The development of earth-abundant electrocatalysts with high intrinsic activity, abundant active sites, and good electrical conductivity is of vital importance for the market penetration of clean energy technologies. We herein report a facile synthesis of a self-supported Co₂N/CoN/Co₂Mo₃O₈ heterostructured catalyst on cobalt foam (CF) by a hydrothermal process followed by nitridation treatment. Our first-principles calculations revealed that Co₂Mo₃O₈ and Co₂N could work in concert to provide active sites for an alkaline hydrogen evolution reaction (HER). The hierarchical and nanoporous architecture of the Co₂N/CoN/Co₂Mo₃O₈ catalyst ensured an abundance of accessible active sites. The direct growth of metalloid Co_xN nanoparticles on the defective Co₂Mo₃O₈ substrate endowed the catalyst with good electrical conductivity. As a consequence, the Co₂N/CoN/Co₂Mo₃O₈/CF catalyst showed extraordinarily high activity and good stability toward the alkaline HER, outperforming most existing non-precious electrocatalysts. In particular, it exhibited a comparable catalytic performance to the commercial Pt/C catalyst at a current density of 100 mA cm^-2.

Stable and Antisintering Tungsten Carbides with Controllable Active Phase for Selective Cleavage of Aryl Ether C-O Bonds

Transition-metal carbides are important materials in heterogeneous catalysis. It remains challenging yet attractive in nanoscience to construct the active phase of carbide catalysts in a controllable manner and keep a sintering-resistant property in redox reactions, especially hydroprocessing. In this work, an integrated strategy was presented to synthesize stable and well-defined tungsten carbide nanoparticles (NPs) by assembling the metal precursor onto carbon nanotubes (CNTs), wrapping a thin polymeric layer, and following a controlled carburization. The polymer served as a soft carbon source to modulate the metal/carbon ratio in the carbides and introduced amorphous carbons around the carbides to prevent the NPs from sintering. The as-built p-W_xC/CNT displayed high stability in the hydrogenolysis of aryl ether C-O bond in guaiacol for more than 150 h. Its activity was more than two and six times higher than those prepared via typical temperature-programmed reduction with gaseous carbon (W_xC/CNT-TPR) and carbothermal reduction with intrinsic carbon support (W_xC/CNT-CTR), respectively. Our p-W_xC/CNT catalyst also achieved high efficiency for selective cleavage of the aryl ether C-O bonds in lignin-derived aromatic ethers, including anisole, dimethoxylphenol, and diphenyl ether, with a robust lifespan.