Bimetallic Nickel Cobalt Sulfide as Efficient Electrocatalyst for Zn-Air Battery and Water Splitting

The synergistic effect of NiCo2S4 and carbon nanosheets for supercapacitor: Enhanced adsorption/desorption of OH- on Ni and Co active sites

To improve the low energy density, low conductivity, and poor cycling stability of NiCo2S4 in supercapacitors, a two-step hydrothermal method was used to prepare a composite material of NiCo2S4 and carbon nanosheets (NiCo2S4/CNs). The electrochemical tests revealed a high specific capacitance of 1576 F g-1 at 1 A/g for the composite, and the NiCo2S4/CNs//AC asymmetric supercapacitor showed a energy density of 49.7 Wh kg-1 at 818 W kg-1. This study confirmed the phase transformation of NiCo2S4 during charge/discharge in alkaline solution through ex-situ X-ray diffraction (ex-situ XRD) for the first time, and proposed a potential reaction pathway. Moreover, Density Functional Theory (DFT) confirmed that the NiCo2S4/CNs heterostructure enhances OH- adsorption/desorption on Ni and Co active sites and improves electronic conductivity. In conclusion, this study advances the application of transition metal sulfide in high-performance energy storage.

A seed-like structured Mo@ZrS2 catalyst on graphene nanosheets for boosting the performance of rechargeable Zn-air batteries

Novel composite materials are being studied by researchers for energy storage and renewable energy applications. Here, a seed-like Mo-doped ZrS2 catalyst was developed on a reduced graphene oxide (rGO) surface by an annealing and hydrothermal method. Using photoelectron spectroscopy, scanning microscopy, and X-ray diffraction analyses, the structure of Mo@ZrS2/rGO and the impact of heteroatoms are demonstrated, providing insight into the catalyst. Furthermore, it is demonstrated that Mo@ZrS2/rGO has been utilized as an efficient energy storage electrocatalyst by offering a very low half-wave potential of 0.80 V for the oxygen reduction reaction in an alkaline solution. Furthermore, Zn-air batteries with a high-power density of 128.6 mW cm-2 and exceptional cycling stability are demonstrated by the developed array electrocatalyst. Ultimately, the research findings suggest novel perspectives on the structure of ZrS2 nanoseeds created by Mo surface doping, promote the usage of Zn-air batteries in practical scenarios, and offer a fascinating idea for creating a redox electrocatalyst.

Unraveling the Impact of Curvature on Electrocatalytic Performance of Carbon Materials: A State-of-the-Art Review

Curvature of carbon materials has gained significant attention as catalysts due to their distinctive properties and potential applications. This review comprehensively summarizes how the bending of carbon materials can improve electrocatalytic performance, with special attention to the applications of various bent carbon materials (such as carbon nanotubes, graphene, and fullerene) in electrocatalysts and a large number of related density functional theory (DFT) theoretical calculations. Extensive mechanism research has provided a wealth of evidence indicating that the curvature of carbon materials has a profound impact on catalytic activity. This improvement in catalytic performance by curved carbon materials is attributed to factors like a larger active surface area, modulation of electronic structure, and better dispersal of catalytic active sites. A comprehensive understanding and utilization of these effects enable the design of highly efficient carbon-based catalysts for applications in energy conversion, environmental remediation, and chemical synthesis.

Ruthenium-doped cobalt sulphide electrocatalyst derived from a ruthenium-cobalt Prussian blue analogue (RuCo-PBA) for an enhanced hydrogen evolution reaction (HER)

The designing of efficient electrocatalysts for hydrogen generation is essential for the practical application of water-splitting devices. With numerous electrochemical advantages, transition metal sulphides are regarded as the most promising candidates for catalysing the hydrogen evolution reaction (HER) in acidic media. In the present study, Ru-doped cobalt sulphide nanosheets, termed Co9S8/Ru@t (t = 24 h, 48 h, and 72 h), were obtained by varying the reaction time from 24 h to 72 h from a RuCo-PBA precursor. The role of the time period for the synthesis of Co9S8/Ru@48h is vital in increasing the number of electroactive sites and optimising the hydrogen adsorption-desorption phenomena leading to an increment in the HER activity. The electrochemical outcomes demonstrate that the optimized Co9S8/Ru@48h requires a low overpotential of just 94 mV to produce 10 mA cm-2 current density, and also exhibits a lower Tafel slope value of 84 mV dec-1 defining its faster reaction kinetics. The as-synthesized Co9S8/Ru@48h was stable for up to 20 h of constant electrolysis signifying its outstanding durability. The optimized synthetic approach and impressive electrochemical results make Co9S8/Ru@48h a suitable alternative to noble-metal-based electrocatalysts for the HER.

Nanosheet-assembled transition metal sulfides nanoflowers derived from CoMo-MOF for efficient oxygen evolution reaction

Oxygen evolution reaction (OER) is a multi-electron transfer process, whose intrinsic sluggish dynamic restricts the whole process of overall water splitting (OWS). To address this issue, a porous transition metal sulfide (TMS) catalyst with rich heterojunctions was prepared by vulcanization and trace Fe doping of CoMo-based metal-organic framework (MOF). In this work, the nanoflower composed of ultrathin 2D nanosheets anchored on a nickel foam presents a layered interface that contributes to the exposure of active regions. The resulting electrode denoted as Fe@CoMo2S4/Ni3S2/NF required a low overpotential (η10 = 167 mV @ 10 mA cm-2, η50 = 260 mV @ 50 mA cm-2) in 1.0 M KOH for OER and a small cell voltage (E = 1.513 V @ 10 mA cm-2) to power OWS when coupled with commercial Pt/C. It also exhibited splendid morphological and chemical stability with virtually invariant polarization curve and flower-like appearance after 1000 CV cycles, as well as long-term durability over 100 h with a constant current density of 10 mA cm-2. This work revealed the multi-anionic regulation mechanism in the surface reconstruction of sulfide electrocatalysts, and verified that Co/Mo/Ni-based oxysulfide was the true active substance of OER, which inspired the understanding and design of multi-anionic regulated electrocatalysts.

Unraveling the Mechanism of Cooperative Redox Chemistry in High-Efficient Zn2+ Storage of Vanadium Oxide Cathode

The inferior capacity and cyclic durability of V2 O5 caused by inadequate active sites and sluggish kinetics are the main problems to encumber the widespread industrial applications of vanadium-zinc batteries (VZBs). Herein, a cooperative redox chemistry (CRC) as "electron carrier" is proposed to facilitate the electron-transfer by capturing/providing electrons for the redox of V2 O5 . The increased oxygen vacancies in V2 O5 provoked in situ by CRC offers numerous Zn2+ storage sites and ion-diffusion paths and reduces the electrostatic interactions between vanadium-based cathode and intercalated Zn2+ , which enhance Zn2+ storage capability and structural stability. The feasibility of this strategy is fully verified by some CRCs. Noticeably, VZB with [Fe(CN)6 ]3- /[Fe(CN)6 ]4- as CRC displays conspicuous specific capacity (433.3 mAh g-1 ), ≈100% coulombic efficiency and superb cyclability (≈3500 cycles without capacity attenuation). Also, the mechanism and selection criteria of CRC are specifically unraveled in this work, which provides insightful perspectives for the development of high-efficiency energy-storage devices.

Hollow N-doped carbon nano-mushroom encapsulated hybrid Ni3S2/Fe5Ni4S8 particle anchored to the inner wall of porous wood carbon for efficient oxygen evolution electrocatalysis

Structural design and morphology engineering are considered significant strategies to boost the catalytic performance of electrocatalysts toward the oxygen evolution reaction. Inspired by the natural porosity and abundant functional groups, herein, hollow N-doped carbon nano-mushroom (NCNM) encapsulated hybrid sulfide particles rooted into a carbonized wood (CW) framework were prepared through simple impregnation followed by calcination. The as-prepared self-supporting electrodes present ultrahigh activity and robust stability. Among them, the NiFeS14@NCNM/CW catalyst yields incredible OER activity with an extraordinarily low overpotential of 147 and 250 mV to reach 10 and 50 mA cm-2, respectively, superior to most of the state-of-the-art wood-derived electrocatalysts. Additionally, a steady OER current density is maintained without obvious attenuation after continuous operation for 24 h. The superior electrocatalytic performance of NiFeS14@NCNM/CW is attributed to the synergistic effect of hybridization between Ni3S2 and Fe5Ni4S8, the coordination of one-dimensional (1D) NCNMs and hierarchical three-dimensional (3D) porous CW, modified electronic states by N and S doping, a large electrochemical surface area, and low activation energy. This research provides a novel approach to industrial-scale conversion of abundant biomass into efficient binder-free electrocatalysts for energy-related applications.

CDs Regulated Sulfur-Based Flexible Electrode with Range pH Values for Efficient and Durable Hydrogen Evolution Reaction

How to mildly structure a high intrinsic activity and stable catalytic electrode to realize long-term catalytic water splitting to produce hydrogen at a wide range of pH values at industrial high current is a challenge. Herein, this work creatively proposes to prepare industrial-grade catalytic electrodes with high efficiency and stability at high current density through carbon quantum dots (CDs) modification nickel sulfide on hydrophilic flexible filter paper via one-step mild chemical plating (denoted as CDs-Ni3 S2 @HFP). The intrinsic activity and surface area, electron transfer ability, and corrosion resistance of Ni3 S2 material are increased due to the regulation, homogenous, and high concentration doping of CDs. The overpotential of the flexible catalytic electrode is only 30, 35, and 87 mV in 1 m KOH, simulated seawater (1 m KOH + 0.5 m NaCl), and neutral electrolyte (0.5 m PBS) at a current density of 10 mA cm-2 . More attractively, the CDs-Ni3 S2 @HFP electrode achieves over 500 h of efficient and stable catalysis at industrial high current density (500 mA cm-2 ). Due to the advantages of mild, universal, and large-area preparation of catalytic materials, this work provides technical support for flexible catalytic electrodes in efficient catalysis toward water splitting, energy storage, and device preparation.

Rapid preparation of high efficiency hydrogen evolution catalyst with hydrophilicity

The electrolytic water method is an outstanding hydrogen production process because of its high stability and no restriction. A low-priced and efficient catalyst for electro-deposition of Ni-Co microspheres and nanoclusters on carbon steel (Ni-Co/CS) has been prepared by the dynamic hydrogen bubble template. In the 6 M KOH solution, Ni-Co/CS only requires an overpotential of 48 mV to provide a current density of 50 mA cm-2. At the same time, it also has a large electrochemically active specific surface area (ECSA) and a hydrophilic surface. In addition, the study about the influence of carbon steel (CS) on Ni-Co coatings and the comparison experiment for different base materials has been completed. The results prove that CS is an excellent base material for hydrogen production. It can help the Ni-Co catalyst to have a stable electrolysis in 6 M KOH for 500 h. The above properties of Ni-Co/CS catalyst make it a new choice of hydrogen production by electrolysis of water in practical applications.

Sustainable zinc-air battery chemistry: advances, challenges and prospects

Sustainable zinc-air batteries (ZABs) are considered promising energy storage devices owing to their inherent safety, high energy density, wide operating temperature window, environmental friendliness, etc., showing great prospect for future large-scale applications. Thus, tremendous efforts have been devoted to addressing the critical challenges associated with sustainable ZABs, aiming to significantly improve their energy efficiency and prolong their operation lifespan. The growing interest in sustainable ZABs requires in-depth research on oxygen electrocatalysts, electrolytes, and Zn anodes, which have not been systematically reviewed to date. In this review, the fundamentals of ZABs, oxygen electrocatalysts for air cathodes, physicochemical properties of ZAB electrolytes, and issues and strategies for the stabilization of Zn anodes are systematically summarized from the perspective of fundamental characteristics and design principles. Meanwhile, significant advances in the in situ/operando characterization of ZABs are highlighted to provide insights into the reaction mechanism and dynamic evolution of the electrolyte|electrode interface. Finally, several critical thoughts and perspectives are provided regarding the challenges and opportunities for sustainable ZABs. Therefore, this review provides a thorough understanding of the advanced sustainable ZAB chemistry, hoping that this timely and comprehensive review can shed light on the upcoming research horizons of this prosperous area.

Recent advances of bifunctional catalysts for zinc air batteries with stability considerations: from selecting materials to reconstruction

With the growing depletion of traditional fossil energy resources and ongoing enhanced awareness of environmental protection, research on electrochemical energy storage techniques like zinc-air batteries is receiving close attention. A significant amount of work on bifunctional catalysts is devoted to improving OER and ORR reaction performance to pave the way for the commercialization of new batteries. Although most traditional energy storage systems perform very well, their durability in practical applications is receiving less attention, with issues such as carbon corrosion, reconstruction during the OER process, and degradation, which can seriously impact long-term use. To be able to design bifunctional materials in a bottom-up approach, a summary of different kinds of carbon materials and transition metal-based materials will be of assistance in selecting a suitable and highly active catalyst from the extensive existing non-precious materials database. Also, the modulation of current carbon materials, aimed at increasing defects and vacancies in carbon and electron distribution in metal-N-C is introduced to attain improved ORR performance of porous materials with fast mass and air transfer. Finally, the reconstruction of catalysts is introduced. The review concludes with comprehensive recommendations for obtaining high-performance and highly-durable catalysts.

Bimetallic Organic Framework-Decorated Leaf-like 2D Nanosheets as Flexible Air Cathode for Rechargeable Zn-air Batteries

Exploring highly active and robust self-supporting air electrodes is the key for flexible Zn-air batteries (FZABs). Therefore, we report a novel 3D structural bimetal-based self-supporting electrode consisting of hybrid Cu, Co nanoparticles co-modified nitrogen-doped carbon nanosheets on carbon cloth (Cu, Co NPs@NCNSs/CC), which displays excellent electrochemical activity and durability of the oxygen reduction/evolution reaction (ORR/OER). The Cu, Co NPs@NCNSs/CC exhibits a half-wave potential of 0.863 V toward ORR and an overpotential of 225 mV at 10 mA cm^-2 toward OER, owing to its exposed bimetallic sites accelerating the kinetic reaction. In addition, the density functional theory calculation proves that the synergistic effect of CuCo sites favors ORR and OER. Hence, the FZABs based on Cu, Co NPs@NCNSs/CC achieve a larger open-circuit potential (1.45 V), higher energy density (130.10 mW cm^-2 ), and outstanding cycling stability. All remarkable results demonstrate valuable enlightenment for seeking advanced energy materials of portable and wearable electronics.

Etching-Induced Surface Reconstruction of NiMoO4 for Oxygen Evolution Reaction

Rational reconstruction of oxygen evolution reaction (OER) pre-catalysts and performance index of OER catalysts are crucial but still challenging for universal water electrolysis. Herein, we develop a double-cation etching strategy to tailor the electronic structure of NiMoO4, where the prepared NiMoO4 nanorods etched by H2O2 reconstruct their surface with abundant cation deficiencies and lattice distortion. Calculation results reveal that the double cation deficiencies can make the upshift of d-band center for Ni atoms and the active sites with better oxygen adsorption capacity. As a result, the optimized sample (NMO-30M) possesses an overpotential of 260 mV at 10 mA cm-2 and excellent long-term durability of 162 h. Importantly, in situ Raman test reveals the rapid formation of high-oxidation-state transition metal hydroxide species, which can further help to improve the catalytic activity of NiMoO4 in OER. This work highlights the influence of surface remodification and shed some light on activating catalysts.

Inner Co Synergizing Outer Ru Supported on Carbon Nanotubes for Efficient pH-Universal Hydrogen Evolution Catalysis

Exploring highly active but inexpensive electrocatalysts for the hydrogen evolution reaction (HER) is of critical importance for hydrogen production from electrochemical water splitting. Herein, we report a multicomponent catalyst with exceptional activity and durability for HER, in which cobalt nanoparticles were in-situ confined inside bamboo-like carbon nanotubes (CNTs) while ultralow ruthenium loading (~ 2.6 µg per electrode area ~ cm^-2) is uniformly deposited on their exterior walls (Co@CNTsǀRu). The atomic-scale structural investigations and theoretical calculations indicate that the confined inner Co and loaded outer Ru would induce charge redistribution and a synergistic electron coupling, not only optimizing the adsorption energy of H intermediates (ΔG_H*) but also facilitating the electron/mass transfer. The as-developed Co@CNTsǀRu composite catalyst requires overpotentials of only 10, 32, and 63 mV to afford a current density of 10 mA cm^-2 in alkaline, acidic and neutral media, respectively, representing top-level catalytic activity among all reported HER catalysts. The current work may open a new insight into the rational design of carbon-supported metal catalysts for practical applications.

Electric-Field-Treated Ni/Co3O4 Film as High-Performance Bifunctional Electrocatalysts for Efficient Overall Water Splitting

HIGHLIGHTS

A novel physical approach is proposed to enhance the electrocatalytic performance by electric field. Under the action of electric field, some stable conductive filaments consisting of oxygen vacancies are formed in the Ni/Co₃O₄ film, which remarkably reduces the system resistivity. The electric-field-treated Ni/Co₃O₄ material exhibits significantly superior activity and stability as a bifunctional electrocatalyst for overall water splitting, and its performance exceeds the state-of-the-art electrocatalysts.

ABSTRACT

Rational design of bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with excellent activity and stability is of great significance, since overall water splitting is a promising technology for sustainable conversion of clean energy. However, most electrocatalysts do not simultaneously possess optimal HER/OER activities and their electrical conductivities are intrinsically low, which limit the development of overall water splitting. In this paper, a strategy of electric field treatment is proposed and applied to Ni/Co₃O₄ film to develop a novel bifunctional electrocatalyst. After treated by electric field, the conductive channels consisting of oxygen vacancies are formed in the Co₃O₄ film, which remarkably reduces the resistance of the system by almost 2 × 10⁴ times. Meanwhile, the surface Ni metal electrode is partially oxidized to nickel oxide, which enhances the catalytic activity. The electric-field-treated Ni/Co₃O₄ material exhibits super outstanding performance of HER, OER, and overall water splitting, and the catalytic activity is significantly superior to the state-of-the-art noble metal catalysts (Pt/C, RuO₂, and RuO₂ ǁ Pt/C couple). This work provides an effective and feasible method for the development of novel and efficient bifunctional electrocatalyst, which is also promising for wide use in the field of catalysis. [Image: see text]

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40820-022-00889-3.

Construction of Petal-Like Ag NWs@NiCoP with Three-Dimensional Core-Shell Structure for Overall Water Splitting

High-efficiency, good electrical conductivity and excellent performance electrocatalysts are attracting growing attention in the field of overall water splitting. In order to achieve the desirable qualities, rational construction of the structure and chemical composition of electrocatalysts is of fundamental importance. Herein, petal-like structure Ni0.33Co0.67P shells grown on conductive silver nanowires (Ag NWs) cores as bifunctional electrocatalysts for overall water splitting were synthesized through a facile hydrothermal method and phosphorization. The resultant three-dimensional core-shell petal-like structure Ag NWs@Ni0.33Co0.67P possesses excellent catalytic activities in alkaline conditions with the overpotential of 259 mV for the oxygen evolution reaction (OER), 121 mV for the hydrogen evolution reaction (HER) and a full cell voltage of 1.64 V to reach the current density of 10 mA cm-2. Highly conductive Ag NWs as cores and high surface area petal-like Ni0.33Co0.67P as shells can endow outstanding catalytic performance for the bifunctional electrocatalyst. Thus, the synthetic strategy of the three-dimensional core-shell structure Ag NWs@Ni0.33Co0.67P considerably advances the practice of Ag NWs toward electrocatalysts.

Nanoscale Heterogeneities of Non-Noble Iron-Based Metallic Glasses toward Efficient Water Oxidation at Industrial-Level Current Densities

Scaling up the production of cost-effective electrocatalysts for efficient water splitting at the industrial level is critically important to achieve carbon neutrality in our society. While noble-metal-based materials represent a high-performance benchmark with superb activities for hydrogen and oxygen evolution reactions, their high cost, poor scalability, and scarcity are major impediments to achieve widespread commercialization. Herein, a flexible freestanding Fe-based metallic glass (MG) with an atomic composition of Fe₅₀Ni₃₀P₁₃C₇ was prepared by a large-scale metallurgical technique that can be employed directly as a bifunctional electrode for water splitting. The surface hydroxylation process created unique structural and chemical heterogeneities in the presence of amorphous FeOOH and Ni₂P as well as nanocrystalline Ni₂P that offered various active sites to optimize each rate-determining step for water oxidation. The achieved overpotentials for the oxygen evolution reaction were 327 and 382 mV at high current densities of 100 and 500 mA cm^-2 in alkaline media, respectively, and a cell voltage of 1.59 V was obtained when using the MG as both the anode and the cathode for overall water splitting at a current density of 10 mA cm^-2. Theoretical calculations unveiled that amorphous FeOOH makes a significant contribution to water molecule adsorption and oxygen evolution processes, while the amorphous and nanocrystalline Ni₂P stabilize the free energy of hydrogen protons (ΔG_H*) in the hydrogen evolution process. This MG alloy design concept is expected to stimulate the discovery of many more high-performance catalytic materials that can be produced at an industrial scale with customized properties in the near future.

Multi-Strategy Architecture of High-Efficiency Electrocatalysts for Underwater Zn-H2 O2 Batteries with Superior Power Density of 442 mW cm-2

A facile multistage regulated strategy is reported to synthesize ZnCo-NC based carbon nanotubes including DMEA induced crystallization, Zn ion activation, and magnetic control growth of carbon nanotubes. Uniform Co distribution and the modulation of Zn, and their catalytic properties are carefully investigated by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. The surface contents of pyridinic N for the oxygen-reduction reaction (ORR) and the surface contents of CoN_x and high valence Co for the oxygen-evolution reaction (OER) can be significantly modulated by Zn content in precursors and the sintering temperature. Furthermore, the catalyst also contains high specific surface areas, high porosity, and high electrochemical active surfaces. Therefore, the ZnCo-NC based catalyst exhibits outstanding bifunctional electrocatalytic activities for ORR with a high E_onset (1.02 V) and E_1/2 (0.91 V) and OER with low E_j=10 (1.56 V), better than Pt/C and RuO₂ . Importantly, the ZnCo-NC based Zn-H₂ O₂ batteries achieve the superior power density of 442 mW cm^-2 , much higher than 238 and 198 mW cm^-2 of Zn-air batteries with ZnCo-NC based catalyst and Pt/C respectively. More importantly, the high-power Zn-H₂ O₂ batteries can work well in underwater conditions while Zn-air batteries are out of work.

Boosting the OER Performance of Nitrogen-Doped Ni Nanoclusters Confined in an Amorphous Carbon Matrix

Nanoclusters are ideal electrocatalysts due to their high surface activity. However, their high activities also lead to serious agglomeration and performance attenuation during the catalytic process. Here, highly dispersed Ni nanoclusters (∼3 nm) confined in an amorphous carbon matrix are successfully fabricated by pulsed laser deposition, followed by rapid temperature annealing treatment. Then, the Ni nanoclusters are further doped with nitrogen element through a clean N₂ radio frequency plasma technology. It is found that the nitrogen-doped Ni nanoclusters obtained under optimized conditions showed superior OER performance with a very low overpotential of 240 mV at a current density of 10 mA/cm², together with good stability. The excellent OER performance of the nanoclusters can be attributed to the unique confined structure and nitrogen doping, which not only provide more active sites but also improve the conductivity. Our work provides a controllable method for the construction of a novel confined structure with controllable nitrogen doping, which can be used as a high-efficiency OER electrocatalyst.

Cu/Co/CoS2 embedded in S,N doped carbon as highly-efficient oxygen reduction and evolution electrocatalyst for rechargeable zinc-air batteries

In order to improve the retarded oxygen reduction and evolution reaction (ORR/OER) in rechargeable metal-air cells in electrochemical energy conversion systems, constructing multiphase nanostructured catalysts is an alternative strategy, where...

Hierarchically structured nickel/molybdenum nitride heterojunctions as superior bifunctional electrodes for overall water splitting

A 3D hierarchical heterostructure of intermetallic compound heterojunctions is first rationally designed and presented as a highly-active bifunctional electrode for water splitting.

Hydrogen evolution, electron-transfer, and hydride-transfer reactions in a nickel-iron hydrogenase model complex: a theoretical study of the distinctive reactivities for the conformational isomers of nickel-iron hydride

Hydrogen fuel is a promising alternative to fossil fuel. Therefore, efficient hydrogen production is crucial to elucidate the distinctive reactivities of metal hydride species, the intermediates formed during hydrogen activation/evolution in the presence of organometallic catalysts. This study uses density functional theory (DFT) to investigate the isomerizations and reactivities of three nickel-iron (NiFe) hydride isomers synthesized by mimicking the active center of NiFe hydrogenase. Hydride transfer within these complexes, rather than a chemical reaction between the complexes, converts the three hydrides internally. Their reactivities, including their electron-transfer, hydride-transfer and proton-transfer reactions, are investigated. The bridging hydride complex exhibits a higher energy level for the highest occupied molecular orbital (HOMO) than the terminal hydride during the electron-transfer reaction. This energy level indicates that the bridging hydride is more easily oxidized and is more susceptible to electron transfer than the terminal hydride. Regarding the hydride-transfer reaction between the NiFe hydride complex and methylene blue, the terminal hydrides exhibit larger hydricity and lower reaction barriers than the bridging hydride complexes. The results of energy decomposition analysis indicate that the structural deformation energy of the terminal hydride in the transition state is smaller than that of the bridging hydride complex, which lowers the reaction barrier of hydride transfer in the terminal hydride. To produce hydrogen, the rate-determining step is represented by the protonation of the hydride, and the terminal hydrides are thermodynamically and kinetically superior to the bridging ones. The differences in the reactivities of the hydride isomers ensure the precise control of hydrogen, and the theoretical calculations can be applied to design catalysts for hydrogen activation/production.

Hydrothermal temperature-driven evolution of morphology and electrocatalytic properties of hierarchical nanostructured CoFe-LDHs as highly efficient electrocatalysts for oxygen evolution reactions

The development of economical and efficient oxygen evolution reaction (OER) catalysts plays an important part in electrochemical water oxidation, and it has been known that the electrocatalytic performance of these materials is closely related to their micromorphology at micro/nanometer scales. Herein, we report a unique hierarchical nanosheet-nanowire structure of a CoFe layered double hydroxide (LDH) electrocatalyst directly grown on conductive nickel foam (NF) by optimizing the hydrothermal temperature of the reaction. The hydrothermal temperature is decisive in driving the formation of the wire-in-sheet morphology of CoFe-LDH, while the hydrothermal time has almost no effect on the morphology of the electrocatalyst. The possible mechanism of the morphological evolution has been proposed. The wire-in-sheet nanoarray of CoFe-LDH provides a higher number of active sites, more intricate transmission networks and improved electronic conductivity, resulting in enhanced electrocatalytic performance. Consequently, the resultant CoFe-LDH exhibits superior OER performance: a low overpotential of 242 mV at 100 mA cm^-2 (η = 242 mV@100 mA cm^-2), with an exceedingly small Tafel slope of 41 mV dec^-1, as well as an ultra-long durability (97 h) in 1 M KOH electrolyte. Therefore, the design of a unique hierarchical nanostructure by tuning the reaction conditions may open up a new avenue for high-performance OER electrocatalysts.

Double shelled hollow CoS2@MoS2@NiS2 polyhedron as advanced trifunctional electrocatalyst for zinc-air battery and self-powered overall water splitting

Electrocatalysts play important role in various energy conversion and storage devices. The catalytic performance of electrocatalysts can be enhanced through the increasement of intrinsic catalytic activity by optimizing electronic structure and the improvement of exposed active sites by designing proper nanostructures. In this work, CoS₂@MoS₂@NiS₂ nano polyhedron with double-shelled structure was prepared using metal organic framework as a precursor. Due to the rational integration of multifunctional active center, the strong electronic interaction of the various component, the high electrochemical surface area and shortened mass transport induced by the special structure, CoS₂@MoS₂@NiS₂ exhibits high catalytic activity for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Specifically, low overpotentials of 156 and 200 mV was achieved to deliver a current density of 10 mA cm^-2 for HER and OER, and a high half-wave potential of 0.80 V was observed for ORR. More importantly, the Zn-air battery assembled by CoS₂@MoS₂@NiS₂ exhibits a high-power density of 80.28 mW cm^-2 and could effectively drive overall water splitting. This work provides a new platform for designing multifunctional catalysts with high activity for energy conversion and storage.

An electrochemical immunosensor for the detection of carcinoembryonic antigen based on Au/g-C3N4 NSs-modified electrode and CuCo/CNC as signal tag

Carcinoembryonic antigen levels in the human body reflect the conditions associated with a variety of tumors and can be used for the identification, development, monitoring, and prognosis of lung cancer, colorectal cancer, and breast cancer. In this study, an amperometric immunosensor with CuCo/carbon nanocubes (CuCo/CNC) as the signal label is constructed. The bimetal-doped carbon skeleton structure has a high specific surface area and exhibits good electrocatalytic activity. In addition, Au/g-C₃N₄ nanosheets (Au/g-C₃N₄ NSs) are used to modify the substrate platform, facilitating the loading of more capture antibodies. The reaction mechanism was explored through electrochemical methods, X-ray powder diffraction, X-ray photoelectron spectroscopy, and other methods. Kinetic studies have shown that CuCo/CNC have good peroxidase-like activity. In addition, the electrocatalytic reduction ability of CuCo/CNC on hydrogen peroxide can be monitored using amperometric i-t curve (- 0.2 V, vs. SCE), and the response current value is positively correlated with the CEA antigen concentration. The prepared electrochemical immunosensor has good selectivity, precision, and stability. The dynamic range of the sensor was 0.0001-80 ng/mL, and the detection limit was 0.031 pg/mL. In addition, the recovery and relative standard deviation in real serum samples were 97.7-103 % and 3.25-4.13 %, respectively. The results show that the sensor has good analytical capabilities and can provide a new method for the clinical monitoring of CEA.

Defect-Engineered 3D hierarchical NiMo3S4 nanoflowers as bifunctional electrocatalyst for overall water splitting

The design and construction of bifunctional electrocatalysts with high activity and durability is essential for overall water splitting. Herein, a unique 3D hierarchical NiMo₃S₄ nanoflowers with abundant defects and reactive sites were grown directly on carbon textiles (NiMo₃S₄/CTs) using a facile hydrothermal synthesis method. The defect-rich NiMo₃S₄ nanoflakes, prepared by doping Ni²⁺ in the lattice of Mo-S, displays extended d-spacing of (002) crystal plane, resulting in the electrocatalytic activity of hydrogen evolution and oxygen evolution reaction (HER and OER) was improved under alkaline conditions. The self-supported NiMo₃S₄/CTs electrode delivers a small overpotential of 149.5 mV for HER and 126.2 mV for OER at 10 mA cm^-2, respectively. Based on detailed structure analysis and density functional theory (DFT) calculations, the excellent HER and OER activities can be attributed to the unique structure of the nanoflowers, where the metallic characteristics for Ni-doped Mo-S lead to the enhancement of intrinsic conductivity and the rich abundance of Ni³⁺ active sites. As a result, the NiMo₃S₄/CTs as efficient bifunctional electrocatalysts for overall water-splitting was performed in alkaline electrolyte, where the system required only 1.55, 1.66 and 1.76 V to deliver current densities of 10, 50 and 100 mA cm^-2, respectively. This study provides a new method for improving the electrocatalysis properties of transition metal sulfides by metal-ion doping to generate more active defect sites, thus promoting the development of non-noble-metal electrocatalysts for overall water splitting.

A generalizable strategy for constructing ultralight three-dimensional hierarchical network heterostructure as high-efficient microwave absorber

Using previous models and theories to construct and develop high-efficient microwave absorbers (MAs) should be a strategic and effective ways to optimize the electromagnetic wave attenuation. Herein, the ultralow density and flexible graphene oxide foam (GOF) and reduced graphene oxide foam (RGOF)/MoS₂ nanosheets were designed and fabricated by the method of chemical vapor deposition and hydrothermal reaction. The obtained GOF and RGOF/MoS₂ samples exhibited very excellent microwave absorption properties while their densities were merely 0.0082 and 0.0084 g•cm^-3, respectively. More importantly, benefiting from the excellent synergistic effect between RGOF and MoS₂, the designed RGOF/MoS₂ well inherited the combined advantages of GOF and MoS₂ in terms of strong absorption abilities, broad absorption bandwidth and thin matching thicknesses. The values of minimum reflection loss and effective frequency bandwidth for RGOF/MoS₂ sample could reach up to -62.92 dB with the matching thickness of 2.27 mm and 4.48 GHz with the matching thickness of 2.12 mm, which were very desirable for high-performance MAs. Moreover, the obtained results indicated that the microwave absorption properties of RGOF/MoS₂ sample could be further optimized by regulating the MoS₂ content. Therefore, a new and effective strategy was proposed to develop high efficiency MAs with ultra-lightweight, wide-band, thin thickness and strong absorption capabilities.

Comparative study on supercapacitive and oxygen evolution reaction applications of hollow nanostructured cobalt sulfides

Due to the diversity of sulfur valence in cobalt-based sulfides, it is difficult to control the crystal phase and composition of the products during synthesis. Herein, a one-pot hydrothermal method is reported to self-assemble the cobalt sulfides (CoS₂, Co₉S₈and Co₃S₄) with hollow nanostructures. The whole preparation process is simple and mild, avoiding high temperature calcination. The performances of the three kinds of cobalt sulfide in superior supercapacitors and electrocatalytic oxygen evolution performance applications follow the order of CoS₂ > Co₉S₈ > Co₃S₄. Further analysis demonstrates that the performance difference in these cobalt sulfides may be attributed to three factors: the presence ofS22-,the coordination environment of Co and the presence of continuous network of Co-Co bonds. The distinctive electrochemical performance of CoS₂and Co₉S₈may help us to better understand the excellent electrochemical activity of metal polysulfides and metal sulfides after doping or alloying. Therefore, this work may provide a reference in understanding and designing the electrode materials for highly efficient applications in the fields of energy storage and conversion.

An Extreme Energy-Saving Carbohydrazide Oxidization Reaction Directly Driven by Commercial Graphite Paper in Alkali and Near-Neutral Seawater Electrolytes

The energy-saving anode with low oxidization potential has been an intriguing pursue for earth-abundant seawater electrolysis. In this paper, we first introduced a superior energy-saving carbohydrazide oxidization reaction catalysis system in the anode section, which can be driven by commercial graphite paper with good durability. Combining this catalysis reaction and common graphite paper, the lowest anodic potentials 0.63 V (vs RHE) and 1.09 V (vs RHE) were obtained for driving a 10 mA/cm² current density in alkali and near-neutral seawater electrolytes, respectively, outperforming all the as-reported alkali or near-neutral seawater catalysts accordingly to the best of our knowledge.

Ultrathin and Highly Crumpled/Porous CoP Nanosheet Arrays Anchored on Graphene Boosts the Capacitance and Their Synergistic Effect toward High-Performance Battery-Type Hybrid Supercapacitors

Constructing novel electrode materials with supernal specific capacitance and cycle stability is important for the practical applications of supercapacitors. Herein, ultrathin and highly crumpled CoP/reduced graphene oxide (rGO) nanosheet arrays are grown on nickel foam (NF) through a hydrothermal-phosphidation route. Benefitting from the synergistic effects of CoP with large specific capacity and rGO with high conductivity and ultrathin nanosheet arrays structure, CoP/rGO shows extraordinary electrochemical performance. The CoP/rGO electrode possesses a superior specific capacity of 1438.0 C g^-1 (3595.0 F g^-1) at 1 A g^-1, which is 3.43, 2.05, and 2.26 times larger than those of Co(OH)₂/rGO, Co₃O₄/rGO, and bare CoP. In particular, the CoP/rGO nanosheet arrays show the highest specific capacities among the monometallic phosphide-based nanostructures reported so far. The CoP/rGO retains 1198.9 C g^-1 (2997.2 F g^-1) at 10 A g^-1, revealing the outstanding rate capability of 83%. Theoretical calculations reveal that rGO can adequately reduce the absorption energy of OH^- on CoP, which makes CoP/rGO have strong adsorption capacity of OH^-, resulting in boosting electrochemical performance. A hybrid supercapacitor of CoP/rGO/NF//AC was designed, which presents a superior energy density of 43.2 Wh kg^-1 at a power density of 1010.5 W kg^-1. After 10 000 cycles, the CoP/rGO/NF//AC supercapacitor reveals excellent cycling durability with a capacitance retention of 89%. This work provides a new insight into the design of high-performance electrode materials by combining high capacitive metal phosphides with conductive carbon, which is of great significance for energy storage systems.

Synergistic photocatalytic activity of a combination of carbon nanotubes-graphene-nickel foam nanocomposites enhanced by dielectric barrier discharge plasma technology for water purification

Degradation activity of plasma catalysis between dielectric barrier discharge (DBD) and carbon nanotubes-graphene-nickel foam (CNTs-G-Ni_f) has been studied in treatment of dye wastewater. CNTs-G-Ni_f was prepared through a two-step chemical vapor deposition (CVD) approach. The composite has been characterized by different techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy. SEM results showed that the Ni_f as the growth substrate was evenly wrapped by G and then CNTs were successfully grown on G as the support. The growth mechanism of composite was proposed. The possible coupled catalytic mechanism between DBD and CNTs-G-Ni_f were addressed. In addition, the modification on G-Ni_f was found by SEM during the discharge process in liquid phase. And the modification mechanism of DBD plasma (DBDP) acting on composites was discussed. Finally, by means of analyses of ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS), the general degradation pathway and stepwise degradation pathways of alizarin green (AG) were proposed in detail.

Electrocatalytic Oxygen Evolution by Hierarchically Structured Cobalt-Iron Composites

The development of scalable routes to highly active and efficient oxygen evolution reaction (OER) electrocatalysts based on earth-abundant materials is crucial for post-fossil fuel energy schemes. Here, we demonstrate how commercial copper foam electrodes can be functionalized for water oxidation using a facile electrodeposition process. The resulting composite electrode features hierarchically structured cobalt-iron-based catalyst particles, which offer channel-like structures for the transport of electrolyte and release of oxygen gas bubbles. We report high electrocatalytic OER performance as demonstrated by high current densities at low overpotentials (293 mV at j = 50 mA cm^-2) and long-term stability under technologically relevant alkaline conditions (>24 h in 1.0 M aqueous KOH).

Interface Engineering via Ti3C2Tx MXene Electrolyte Additive toward Dendrite-Free Zinc Deposition

Zinc metal batteries have been considered as a promising candidate for next-generation batteries due to their high safety and low cost. However, their practical applications are severely hampered by the poor cyclability that caused by the undesired dendrite growth of metallic Zn. Herein, Ti₃C₂T_x MXene was first used as electrolyte additive to facilitate the uniform Zn deposition by controlling the nucleation and growth process of Zn. Such MXene additives can not only be absorbed on Zn foil to induce uniform initial Zn deposition via providing abundant zincophilic-O groups and subsequently participate in the formation of robust solid-electrolyte interface film, but also accelerate ion transportation by reducing the Zn²⁺ concentration gradient at the electrode/electrolyte interface. Consequently, MXene-containing electrolyte realizes dendrite-free Zn plating/striping with high Coulombic efficiency (99.7%) and superior reversibility (stably up to 1180 cycles). When applied in full cell, the Zn-V₂O₅ cell also delivers significantly improved cycling performances. This work provides a facile yet effective method for developing reversible zinc metal batteries.

A nitrogen-doped NiCo2S4/CoO hollow multi-layered heterostructure microsphere for efficient oxygen evolution in Zn-air batteries

Exploring highly effective and low-cost non-noble metal-based electrocatalysts for oxygen evolution reaction (OER) is critical for renewable energy conversion and metal-air batteries. Herein, a novel and high-efficient OER catalyst was reported with nitrogen-doped oxide/sulfide heterostructures (named N-NiCo2S4/CoO microsphere). The N-NiCo2S4/CoO microsphere was synthesized by annealing NiCo-BTC MOF to a multi-layered hollow structure of NiCo2O4 microspheres, followed by the direct vulcanization in the presence of NH4HCO3, resulting in an oxide/sulfide heterojunction. Benefiting from the nitrogen doping, the abundant multi-layered hollow heterostructure and the interfaces between multiple components, the N-NiCo2S4/CoO microsphere exhibited excellent OER activity with a low overpotential of 227 mV at 10 mA cm-2. The Zn-air battery based on the N-NiCo2S4/CoO + Pt/C catalyst displayed excellent cycling stability after 900 cycles at a large current density of 5 mA cm-2, where the commercial RuO2 + Pt/C-based battery exhibited a big drop after only 30 cycles, suggesting its great application prospects as power source devices.

Cu/S-Occupation Bifunctional Oxygen Catalysts for Advanced Rechargeable Zinc-Air Batteries

The design and synthesis of low-cost and highly efficient bifunctional catalysts is an inevitable path for rechargeable zinc-air batteries (rZABs). In this work, double-carbon co-supported Co-based oxide with the Cu and S substitutions are synthesized by a one-step hydrothermal method and formed a unique honeycomb structure. As expected, the (Cu, Co)₃OS₃@CNT-C₃N₄ exhibits high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity with low overpotential (0.86 V), high power density (215 mW cm^-2), and long-term discharge stability (115 h). The (Cu, Co)₃OS₃@CNT-C₃N₄-based rZAB also shows a stronger charge-discharge durability with a very low voltage gap of merely 0.5 V than that of Pt/C+RuO₂. The high catalytic performances are attributed to these following reasons: (i) the porous morphology and hierarchical structure with plentiful "catalytic buffer", which accelerates the mass transfer; (ii) a high-speed electronic transmission network established by C₃N₄ and carbon nanotube (CNT), enhancing the conductivity; (iii) the strong synergistic effect between (Cu, Co)₃OS₃@CNT and C₃N₄, which improves the kinetics of ORR/OER; and (iv) the controllable occupation of Cu ions and S ions, which effectively regulates the CoO₆ surface and increases the active site density. This work not only offers a promising ORR/OER electrode for rZAB but also provides a new pathway to understand the improvement mechanism for catalysts by the bi-ion substitutions.

Defect Engineering of Molybdenum-Based Materials for Electrocatalysis

Molybdenum-based electrocatalysts have been widely applied in electrochemical energy conversion reactions. The essential roles of defects, including doping, vacancies, grain boundaries, and dislocations in improving various electrocatalytic performances have been reported. This review describes the latest development of defect engineering in molybdenum-based materials for hydrogen evolution, oxygen reduction, oxygen evolution, and nitrogen reduction reactions. The types of defects, preparation methods, characterization techniques, and applications of molybdenum-based defect materials are elucidated. Finally, challenges and future research directions for these types of materials are also discussed.

Interface Engineering of CoS/CoO@N-Doped Graphene Nanocomposite for High-Performance Rechargeable Zn-Air Batteries

Low cost and green fabrication of high-performance electrocatalysts with earth-abundant resources for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial for the large-scale application of rechargeable Zn-air batteries (ZABs). In this work, our density functional theory calculations on the electrocatalyst suggest that the rational construction of interfacial structure can induce local charge redistribution, improve the electronic conductivity and enhance the catalyst stability. In order to realize such a structure, we spatially immobilize heterogeneous CoS/CoO nanocrystals onto N-doped graphene to synthesize a bifunctional electrocatalyst (CoS/CoO@NGNs). The optimization of the composition, interfacial structure and conductivity of the electrocatalyst is conducted to achieve bifunctional catalytic activity and deliver outstanding efficiency and stability for both ORR and OER. The aqueous ZAB with the as-prepared CoS/CoO@NGNs cathode displays a high maximum power density of 137.8 mW cm^-2, a specific capacity of 723.9 mAh g^-1 and excellent cycling stability (continuous operating for 100 h) with a high round-trip efficiency. In addition, the assembled quasi-solid-state ZAB also exhibits outstanding mechanical flexibility besides high battery performances, showing great potential for applications in flexible and wearable electronic devices.

Exploiting a High-Performance "Double-Carbon" Structure Co9S8/GN Bifunctional Catalysts for Rechargeable Zn-Air Batteries

Rational synthesis of bifunctional electrocatalysts with high performance and strong durability is highly demanded rechargeable metal-air battery. In this work, ZIF-derived Co₉S₈/C coated with conductive graphene nanosheet (Co₉S₈/GN) was synthesized by a simple solvothermal method and formed a stable double-carbon structure. As expected, the prepared Co₉S₈/GN catalyst exhibits a high catalytic activity (ΔE: 0.88 V) and long-term durability toward both oxygen reduction reaction and oxygen evolution reaction (ORR and OER), which is even superior to the Pt/C + Ir/C mixture (0.91 V). In addition, the Zn-air battery with the Co₉S₈/GN catalyst showed higher power density (186 mW cm^-2) and more stable charge-discharge cycling performances (2000 cycles) than the Pt/C + Ir/C (118 mW cm^-2). Based on these analysis results, the favorable catalytic performance of ORR/OER should be illustrated by the following reasons: (i) large specific surface area and unique mesoporous structure, providing abundant active sites; (ii) good conductivity, accelerating the electrons transfer; and (iii) the unique stable "double-carbon" structures (metal-S-C-C), preventing the agglomeration of metal sulfide, building new quick transfer pathway, and forming the strong electron coupling ability and synergistic effect.