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

Phase Engineering of Iron-Cobalt Sulfides for Zn-Air and Na-Ion Batteries

Rechargeable batteries are promising platforms for sustainable development of energy conversion and storage technologies. Highly efficient multifunctional electrodes based on bimetallic sulfides for rechargeable batteries are extremely desirable but still challenging to tailor with controllable phase and structure. Here, we report a colloidal strategy to fabricate FeCo-based bimetallic sulfides on reduced graphene oxide (rGO), which are expected to display highly efficient oxygen electrocatalysis and sodium storage performances. Specifically, as-screened FeCo₈S₈ nanosheets (NSs) on rGO originating from suitable tailoring of the Co₉S₈ matrix with Fe at the atomic level exhibited a very low potential difference (0.77 V) at 10 mA cm^-2 and negligible voltage loss after 200 cycles as an air electrode for Zn-air batteries. For Na-ion batteries, FeCo₈S₈ NS/rGO demonstrated a superior high-rate capability (188 mAh g^-1 at 20 A g^-1) with long-term cycling stability. The bifunctional electrocatalytic property and sodium storage performance are attributed to not only the synergistic effect of Fe/Co but also the optimized catalytic activity and ion transport ability by the in situ rGO hybrid. This work demonstrates the potential applications of FeCo-based bimetallic sulfides as efficient electrode materials for both rechargeable Zn-air and Na-ion batteries.

Recent Advances on Self-Supported Arrayed Bifunctional Oxygen Electrocatalysts for Flexible Solid-State Zn-Air Batteries

Flexible solid-state Zn-air batteries have been rapidly developed benefiting from the uprising demand for wearable electronic devices, wherein the air electrode integrated with efficient bifunctional oxygen electrocatalysts plays an important role to achieve high performance. Binder-free self-supported bifunctional catalysts can provide large active surface area, fast electron transport path, easy ion diffusion, and excellent structural stability and flexibility, thus acting as promising flexible air cathodes. In this review, recent advances on the application of nanoarrayed electrocatalysts as air cathodes in flexible Zn-air batteries are reviewed. Especially, various types of bifunctional oxygen electrocatalysts, including carbonaceous material arrays, transition metal compound arrays, transition metal/carbon arrays, transition metal compound/carbon arrays, and other hybrid arrays, are discussed. The applications of flexible Zn-air batteries with two configurations (i.e., planar stacks and cable fibers) are also introduced. Finally, perspectives on the optimization of arrayed air cathodes for future development to achieve high-performance flexible Zn-air batteries are shared.

MOFs derived Co/Cu bimetallic nanoparticles embedded in graphitized carbon nanocubes as efficient Fenton catalysts

In this work, Cu-Co bimetallic nanoparticles embedded carbon nanocubes (Cu_xCo_10-x/CNC) are synthesized by direct carbonization of Cu-Co bimetal ZIF. The ratio of Cu and Co nanoparticles in Cu_xCo_10-x/CNC as well as morphology, pore structure and graphitization degree of carbon substrates can be tuned by adjusting the molar ratio of Cu/Co (0:10, 1:9, 2:8, 3:7, 4:6 and 5:5) in ZIF precursors. The Fenton catalytic performances of Cu_xCo_10-x/CNC are further studied by degrading a typical azo dye, Acid Orange II (AOII). The results show the Cu_xCo_10-x/CNC with a Cu/Co ratio of 4/6 display the highest catalytic activity with faster dye degradation rate than other catalysts, which may be ascribed to the synergetic effects of optimized ratio of Cu/Co bimetals, high surface area and graphitized carbon framework. The stability and reusability of the catalyst has been investigated, showing a good performance after five consecutive runs. The catalysts prepared in this study can be used as an attractive alternative in heterogeneous Fenton chemistry and wastewater treatment.

Keggin-Type Polyoxometalate-Based ZIF-67 for Enhanced Photocatalytic Nitrogen Fixation

The photocatalytic reduction of N₂ to NH₃ is considered a promising strategy to alleviate human need for accessible nitrogen and environmental pollution, for which developing a photocatalyst is an effective method to complete the transformation of this process. We firstly design a series of highly efficient and stable polyoxometalates (POMs)-based zeolitic imidazolate framework-67 (ZIF-67) photocatalysts for N₂ reduction. ZIF-67 can effectively fix N₂ owing to its porosity. Integration of POMs cluster contributes enormous advantages in terms of broadening the absorption spectrum to improve sunlight utilization, enhance the stability of the materials, effectively inhibit the recombination of photo-generated electron-hole pairs, and reduce charge-transfer impedance. POMs can absorb light to convert into reduced POMs, which have stronger reducing ability to provide ample electrons to reduce N₂ . The reduced POMs can recover their oxidation state through contact with an oxidant, which forms a self-recoverable and recyclable photocatalytic fixing N₂ system. The photocatalytic activity enhances with the increasing number V substitutions in the POMs. Satisfactorily, ZIF-67@K₁₁ [PMo₄ V₈ O₄₀ ] (PMo₄ V₈ ) displays the most significant photocatalytic N₂ activity with a NH₃ yield of 149.0 μmol L^-1  h^-1 , which is improved by 83.5 % (ZIF-67) and 78.9 % (PMo₄ V₈ ). The introduction of POMs provides new insights for the design of high-performance photocatalyst nanomaterials to reduce N₂ .

Interfacial Engineering of NiO/NiCo2O4 Porous Nanofibers as Efficient Bifunctional Catalysts for Rechargeable Zinc-Air Batteries

To meet the crucial demand of regenerative Zn-air (ZA) batteries, low cost, highly efficient, and durable electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are needed to replace the noble metal. Herein, porous NiO/NiCo₂O₄ nanofibers with superior electrocatalytic performance are synthesized by a facile electrospinning strategy with precursor transition metal salts in nonstoichiometric ratio, which confers the heterostructured NiO/NiCo₂O₄ with abundant interface-related active sites and electronic transmission channels. Density functional calculation results reveal the chemical bonds easily form between NiO and NiCo₂O₄ to facilitate the charge transfer, while X-ray absorption fine spectroscopy and X-ray photoelectron spectroscopy results demonstrate there are abundant Ni³⁺ and Co³⁺ species in NiO/NiCo₂O₄ due to the interfacial engineering. As a result, the NiO/NiCo₂O₄ porous nanofibers exhibit highly efficient and durable performances of OER and ORR in KOH solution, including a lower overpotential of 357 mV at 10 mA cm^-2 (OER) and half-wave potential of 0.73 V (ORR) than that of the individual. What's more, the NiO/NiCo₂O₄-based ZA battery displays excellent specific capacities of 814.4 mA h g^-1, and good cycling stability of 175 h. Additionally, the flexible ZA battery displays a long cycling life of 14 h and decent flexibility. This work shows that construction of the heterostructure could provide a feasible method to optimize their electrocatalytic performance and make them widely used in power source devices.

Hierarchical Carbon Microtube@Nanotube Core-Shell Structure for High-Performance Oxygen Electrocatalysis and Zn-Air Battery

Zinc-air batteries (ZABs) hold tremendous promise for clean and efficient energy storage with the merits of high theoretical energy density and environmental friendliness. However, the performance of practical ZABs is still unsatisfactory because of the inevitably decreased activity of electrocatalysts when assembly into a thick electrode with high mass loading. Herein, we report a hierarchical electrocatalyst based on carbon microtube@nanotube core-shell nanostructure (CMT@CNT), which demonstrates superior electrocatalytic activity for oxygen reduction reaction and oxygen evolution reaction with a small potential gap of 0.678 V. Remarkably, when being employed as air-cathode in ZAB, the CMT@CNT presents an excellent performance with a high power density (160.6 mW cm-2), specific capacity (781.7 mAhg Zn -1 ) as well as long cycle stability (117 h, 351 cycles). Moreover, the ZAB performance of CMT@CNT is maintained well even under high mass loading (3 mg cm-2, three times as much as traditional usage), which could afford high power density and energy density for advanced electronic equipment. We believe that this work is promising for the rational design of hierarchical structured electrocatalysts for advanced metal-air batteries.

AgO-decorated multi-dimensional chrysanthemum-like NiCo2O4 mounted on nickel foam as a highly efficient and stable electrocatalyst for the oxygen evolution reaction

AgO nanoparticles were successfully integrated into NiCo2O4 nanosheets for enhanced electrochemical catalysis ability and stability in the oxygen evolution reaction (OER). The chrysanthemum-like NiCo2O4/AgO composites mounted on nickel foam (NF) were synthesized by a hydrothermal-calcination method. AgO upgraded the ratio of Co3+/Co2+ and thus regulated the intrinsic activity of the species. The highly hierarchical structure of NiCo2O4/AgO composed of 0D AgO nanoparticles, 1D NiCo2O4 needles, 2D NiCo2O4 nanosheets, and 3D chrysanthemum-like bundles grown on NF bestowed the high surface area and mesoporous structure for the easy evolution of O2. The Ni atoms in NiCo2O4 originating in situ from NF in the process of AgO formation produced an integrated electrode of the active component of NiCo2O4 bound on NF with a superb highway for charge transfer. AgO significantly tuned the structure and physicochemical properties of NiCo2O4. As a result, NiCo2O4/AgO/NF exhibited excellent OER performance with an overpotential of 232 mV to obtain a current density of 10 mAcm-2 in an alkaline electrolyte, and the catalyst showed a small loss of the initial catalyst activity for 50 h and over 5000 cycles. This study provides a pathway for developing high-performance OER electrocatalysts.

Enhanced OER Performances of Au@NiCo2S4 Core-Shell Heterostructure

Transition metal sulfides have attracted a lot of attention as potential oxygen evolution reaction (OER) catalysts. Bimetallic sulfide possesses superior physicochemical properties due to the synergistic effect between bimetallic cations. By introducing a metal-semiconductor interface, the physicochemical properties of transition metal sulfide can be further improved. Using the solvothermal method, Au@NiCo₂S₄ core-shell heterostructure nanoparticles (NPs) and bare NiCo₂S₄ NPs were prepared. The measurement of the OER catalytic performance showed that the catalytic activity of Au@NiCo₂S₄ core-shell heterostructure was enhanced compared to bare NiCo₂S₄ NPs. At the current density of 10 mA cm⁻², the overpotential of Au@NiCo₂S₄ (299 mV) is lower than that of bare NiCo₂S₄ (312 mV). The Tafel slope of Au@NiCo₂S₄ (44.5 mV dec⁻¹) was reduced compared to that of bare NiCo₂S₄ (49.1 mV dec⁻¹), indicating its faster reaction kinetics. Detailed analysis of its electronic structure, chemical state, and electrochemical impedance indicates that the enhanced OER catalytic performances of bare Au@NiCo₂S₄ core-shell NPs were a result of its increased proportion of high-valance Ni/Co cations, and its increased electronic conductivity. This work provides a feasible method to improve OER catalytic performance by constructing a metal-semiconductor core-shell heterostructure.

Mesoporous Ternary Nitrides of Earth-Abundant Metals as Oxygen Evolution Electrocatalyst

As sustainable energy becomes a major concern for modern society, renewable and clean energy systems need highly active, stable, and low-cost catalysts for the oxygen evolution reaction (OER). Mesoporous materials offer an attractive route for generating efficient electrocatalysts with high mass transport capabilities. Herein, we report an efficient hard templating pathway to design and synthesize three-dimensional (3-D) mesoporous ternary nickel iron nitride (Ni3FeN). The as-synthesized electrocatalyst shows good OER performance in an alkaline solution with low overpotential (259 mV) and a small Tafel slope (54 mV dec-1), giving superior performance to IrO2 and RuO2 catalysts. The highly active contact area, the hierarchical porosity, and the synergistic effect of bimetal atoms contributed to the improved electrocatalytic performance toward OER. In a practical rechargeable Zn-air battery, mesoporous Ni3FeN is also shown to deliver a lower charging voltage and longer lifetime than RuO2. This work opens up a new promising approach to synthesize active OER electrocatalysts for energy-related devices.

Bifunctional Oxygen Electrocatalyst of Mesoporous Ni/NiO Nanosheets for Flexible Rechargeable Zn-Air Batteries

One approach to accelerate the stagnant kinetics of both the oxygen reduction and evolution reactions (ORR/OER) is to develop a rationally designed multiphase nanocomposite, where the functions arising from each of the constituent phases, their interfaces, and the overall structure are properly controlled. Herein, we successfully synthesized an oxygen electrocatalyst consisting of Ni nanoparticles purposely interpenetrated into mesoporous NiO nanosheets (porous Ni/NiO). Benefiting from the contributions of the Ni and NiO phases, the well-established pore channels for charge transport at the interface between the phases, and the enhanced conductivity due to oxygen-deficiency at the pore edges, the porous Ni/NiO nanosheets show a potential of 1.49 V (10 mA cm-2) for the OER and a half-wave potential of 0.76 V for the ORR, outperforming their noble metal counterparts. More significantly, a Zn-air battery employing the porous Ni/NiO nanosheets exhibits an initial charging-discharging voltage gap of 0.83 V (2 mA cm-2), specific capacity of 853 mAh g Zn -1 at 20 mA cm-2, and long-time cycling stability (120 h). In addition, the porous Ni/NiO-based solid-like Zn-air battery shows excellent electrochemical performance and flexibility, illustrating its great potential as a next-generation rechargeable power source for flexible electronics.

ZIF-Derived Co9-xNixS8 Nanoparticles Immobilized on N-Doped Carbons as Efficient Catalysts for High-Performance Zinc-Air Batteries

Bimetallic sulfides have been attracting considerable attention because of their high catalytic activities for oxygen reduction reaction (ORR) and oxygen evolution reaction; thus, they are considered efficient catalysts for important energy conversion devices such as fuel cells and metal-air batteries. Here, the catalytic activity of a novel catalyst composed of Co_9-xNi_xS₈ nanoparticles immobilized on N-doped carbons (Co_9-xNi_xS₈/NC) is reported. The catalyst is synthesized using a Ni-adsorbed Co-Zn zeolitic imidazolate framework (ZIF) precursor (NiCoZn-ZIF). Because of the porous structure of ZIF and the high intrinsic activity of the bimetallic sulfide nanoparticles, the Co_9-xNi_xS₈/NC catalyst exhibits high half-wave potential 0.86 V versus reversible hydrogen electrode for ORR and outstanding bifunctional catalytic performance. When Co_9-xNi_xS₈/NC is applied as a cathode catalyst in zinc-air batteries, considerably higher power density of about 75 mW cm^-2 and discharge voltage are achieved compared to those of batteries with commercial Pt/C and other ZIF-derived catalysts. The zinc-air battery with the Co_9-xNi_xS₈/NC catalyst shows a high cyclability more than 170 cycles for 60 h with almost negligible decline at 10 mA cm^-2. Our work provides a new insight into the design of bimetallic sulfide composites with high catalytic activities.

Interconnected porous nanoflakes of CoMo2S4 as an efficient bifunctional electrocatalyst for overall water electrolysis

Interconnected porous nanoflakes of the bimetallic CoMo₂S₄ are synthesized and investigated as bifunctional catalysts for highly efficient overall water electrolysis.

W doping dominated NiO/NiS2 interfaced nanosheets for highly efficient overall water splitting

Constructing high-efficiency electrocatalysts is vital towards electrocatalytic water splitting, but it remains a challenge. Although Ni-based materials have drawn extensive attention as highly active catalysts, the relatively limited electroactive sites in Ni-based catalysts still remains a great issue. In order to further boost the electrocatalytic performances, heteroatom doping and interface engineering are usually adopted for modification. Here, a new strategy is developed to construct W doped NiO/NiS₂ interfaced nanosheets directly on carbon sheet, which is working as efficient and bifunctional electrocatalysts for overall water splitting. W doped NiO nanosheets are directly constructed on the carbon sheet by the hydrothermal and annealing processes. After that, W-NiO was subjected to Ar plasma assisted sulfuration treatment for forming W doped NiO/NiS₂ interfaced nanosheets. Based on systematic investigations, we find that W doping can effectively induce the modified electronic structure of Ni to boost the intrinsic activities in NiO/NiS₂. Further, forming NiO/NiS₂ nanointerfaces can also provide rich electroactive sites and boost the charge transfer rate. Consequently, W doped NiO/NiS₂ exhibits the much enhanced performances for overall water splitting. As a bifunctional electrode, W-NiO/NiS₂ demonstrates a remarkable activity with a 1.614 V cell voltage at 10 mA cm^-2 for overall water splitting.

Large-scale fabrication of 3D hierarchical MoSe2 hollow sphere arrays with like-Pacific Plate architecture for high-performance hydrogen evolution reaction

Optimizing of electrocatalytic hydrogen evolution reaction (HER) activity is still a gigantic challenge for improving catalysts in renewable energy field. In this research, large-scale neoteric three-dimensional (3D) hierarchical MoSe₂ hollow sphere arrays with like-Pacific Plate architecture were triumphantly prepared via a facile and reliable approach. The like-Pacific Plate architecture consists of many 3D MoSe₂ arrays plates with the dimensions of 10-35 μm, which is assembled by plentiful closely connecting hierarchical MoSe₂ hollow spheres with the outer diameter, hierarchically secondary unit size and shell thickness of 450 nm, 25 nm and 10 nm, respectively. The products exhibited remarkable HER performance with small Tafel slope (58.5 mV dec^-1), low overpotential (169.8 mV) at 10 mA cm^-2, high conductivity and durable stability, which are attribute to the mechanisms (1) 3D hollow framework furnishes large specific surface area, (2) hierarchical structure generates more active sites, (3) like-Pacific Plate architecture facilitates the touch of electrolyte and catalyst, and (4) closely packed arrays expedite the migration of electron. The current work indicates that unique 3D hierarchical hollow sphere arrays with like-Pacific Plate architecture will be a potential candidate for effective electrocatalytic water splitting catalyst material with energy source application.

Aldehyde reduced Co3O4 to form oxygen vacancy and enhance the electrochemical performance for oxygen evolution reaction and supercapacitors

Oxygen vacancy is a feasible approach to boost the electrochemical properties for metal oxides. In this work, a Co₃O₄ with abundant oxygen vacancy is synthesized via aldehyde reduction. After the procedure, the reduced Co₃O₄ exhibits larger electrochemical active surface areas and better electrical conductivity. These outstanding characteristics can improve its performance of catalytic and energy storage. As for catalyst of oxygen evolution reaction, the reduced Co₃O₄ delivers a smaller potential of 1.55 V versus the reversible hydrogen electrode to realize a current density of 10 mA cm^-2 and a lower Tafel slope of 71 mV dec^-1 in alkaline solution, and these values are smaller than those of pristine Co₃O₄. Especially the reduced Co₃O₄ possesses superior stability: the measurements of the polarization curves before and after 15h of stability tests basically coincide. In a supercapacitor, the positive electrode of reduced Co₃O₄ achieves about 1.7 times areal capacitance of pristine Co₃O₄ at current density of 1 mA cm^-2. Significantly, the superior cycling stability is still retained. Also, an aqueous asymmetric supercapacitor is assembled to evaluate the energy storage performance of the R-Co₃O₄. Moreover, the oxygen vacancy formation strategy for Co₃O₄ may be generally extended to other metal oxides for application in energy storage and conversion.

Co and CeO2 co-decorated N-doping carbon nanofibers for rechargeable Zn-air batteries

The heterogeneous Co and CeO₂ co-decorated N-doping carbon nanofibers (Co-CeO₂-N-C) were synthesized via the electrospinning technique. As the bifunctional electrocatalyst, Co-CeO₂-N-C nanofibers show excellent oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) performance, owing to the higher degree of graphitization of carbon, the N-doping, and the formation of an interface between Co and CeO₂. The liquid Zn-air battery based on Co-CeO₂-N-C nanofibers displays excellent specific capacities (815.9 mA h g^-1 at 5 mA cm^-2), higher open circuit voltages (1.47 V), and good cycling stability (113 h). The corresponding flexible solid state Zn-air battery shows excellent cycling stability (11 h), and good flexibility. Our finding suggests that Co-CeO₂-N-C nanofibers could serve as a new group of bifunctional electrocatalysts for OER and ORR with excellent performance, and make them promising for use in future electric vehicles, off-grid power sources, and portable electronics.

Bifunctional Electrocatalytic Activity of Nitrogen-Doped NiO Nanosheets for Rechargeable Zinc-Air Batteries

In order to improve the efficiencies and service lifetimes of rechargeable Zn-air batteries, it is necessary to develop highly efficient air electrocatalysts. In the present study, we prove that the bifunctional electrocatalytic activity in NiO nanosheets is effectively improved by the synergistic effects of N dopants and considerably porous structure. As an electrocatalyst for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), the as-prepared porous N-doped NiO nanosheets exhibit good activities with the small overpotential and ideal half-wave potential, which is superior to Ir/C electrocatalyst. Besides, it is proved that the process of HO* being oxidized to O* is the OER potential rate-determining step; also the OER electrocatalytic performance of NiO can be markedly promote by the doping of N atoms using the density functional theory calculations. Furthermore, the fabricated Zn-air battery based on the porous N-doped NiO nanosheets also exhibits superior activities, outperforming many reported NiO-based electrocatalyst materials. Two series Zn-air cells with a voltage of 2.80 V can power a red light-emitting diode, which shows their large potential for various applications.

Amorphous Bimetallic Cobalt Nickel Sulfide Cocatalysts for Significantly Boosting Photocatalytic Hydrogen Evolution Performance of Graphitic Carbon Nitride: Efficient Interfacial Charge Transfer

Noble metals usually work as the cocatalyst for photocatalytic water splitting, but their rare and expensive properties narrowed their wide development. Transition-metal sulfides have appeared to be promising non-noble metal cocatalysts in the hydrogen evolution reaction (HER) to meet future energy demands. Meanwhile, many studies focus on the fabrication of bimetallic catalysts because of their remarkably superior catalytic activity compared with monometallic substances. Herein, amorphous bimetallic cobalt nickel sulfide (CoNiS_x) was fabricated to work as a cocatalyst in the photocatalytic H₂ evolution reaction, which can couple with pristine graphitic carbon nitride (g-C₃N₄). CoNiS_x-CN exhibits a larger specific surface area compared with g-C₃N₄, making it possess more reaction active sites. Moreover, the contacted interface in the CoNiS_x-CN composite photocatalyst contributes to higher separation efficiency of photogenerated carriers, which was proved by experimental and theoretical calculations. More importantly, the theoretical calculation also verified that CoNiS_x-CN has relatively closer Gibbs free energy to zero than pure g-C₃N₄ and corresponding monometallic cocatalyzed g-C₃N₄. Therefore, the prepared CoNiS_x-CN composite exhibited a dramatic photocatalytic HER performance of 2.366 μmol mg^-1 h^-1, which is about 76-fold higher in comparison with pristine g-C₃N₄ and comparable to g-C₃N₄ with Pt as a cocatalyst under 420 nm light irradiation. This study reveals a promising and efficient bimetallic cocatalyst for the photocatalytic H₂ evolution reaction.

Bifunctional Electrocatalysts Based on Mo-Doped NiCoP Nanosheet Arrays for Overall Water Splitting

Rational design of efficient bifunctional electrocatalysts is highly imperative but still a challenge for overall water splitting. Herein, we construct novel freestanding Mo-doped NiCoP nanosheet arrays by the hydrothermal and phosphation processes, serving as bifunctional electrocatalysts for overall water splitting. Notably, Mo doping could effectively modulate the electronic structure of NiCoP, leading to the increased electroactive site and improved intrinsic activity of each site. Furthermore, an electrochemical activation strategy is proposed to form Mo-doped (Ni,Co)OOH to fully boost the electrocatalytic activities for oxygen evolution reaction. Benefiting from the unique freestanding structure and Mo doping, Mo-doped NiCoP and (Ni,Co)OOH show the remarkable electrochemical performances, which are competitive among current researches. In addition, an overall water splitting device assembled by both electrodes only requires a cell voltage of 1.61 V to reach a current density of 10 mA cm-2. Therefore, this work opens up new avenues for designing nonprecious bifunctional electrocatalysts by Mo doping and in situ electrochemical activation.

Effect of Ion Diffusion in Cobalt Molybdenum Bimetallic Sulfide toward Electrocatalytic Water Splitting

The electrocatalyst comprising two different metal atoms is found suitable for overall water splitting in alkaline medium. Hydrothermal synthesis is an extensively used technique for the synthesis of various metal sulfides. Time-dependent diffusion of the constituting ions during hydrothermal synthesis can affect the crystal and electronic structure of the product, which in turn would modulate its electrocatalytic activity. Herein, cobalt molybdenum bimetallic sulfide was prepared via hydrothermal method after varying the duration of reaction. The change in crystal structure, amount of Co-S-Mo moiety, and electronic structure of the synthesized materials were thoroughly investigated using different analytical techniques. These changes modulated the charge transfer at the electrode-electrolyte interface, as evidenced by electrochemical impedance spectroscopy. The Tafel plots for the prepared materials were investigated considering a less explored approach and it was found that different materials facilitated different electrocatalytic pathways. The product obtained after 12 h reaction showed superior catalytic activity in comparison to the products obtained from 4, 8, and 16 h reaction, and it surpassed the overall water splitting activity of the RuO₂-Pt/C couple. This study demonstrated the ion diffusion within the bimetallic sulfide during hydrothermal synthesis and change in its electrocatalytic activity due to ion diffusion.

Solvothermally Doping NiS2 Nanoparticles on Carbon with Ferric Ions for Efficient Oxygen Evolution Catalysis

Exploring efficient non-precious metal based electrocatalysts for the oxygen evolution reaction (OER) is a prerequisite to implement the widespread application of a water electrolyzer and metal-air batteries. Herein, Fe-doped NiS2 nanoparticles on a carbon matrix (Fe-NiS2/C) are facilely prepared via a two-step solvothermal process, where Ni-containing metal organic frameworks (Ni-MOFs) are vulcanized in situ and carbonized by a solvothermal method to form abundant NiS2 nanoparticles homogeneously distributed on a carbon matrix (NiS2/C), followed by doping with ferric ions via a similar solvothermal treatment. The resulting Fe-NiS2/C nanoparticle composites show a rougher surface than the NiS2/C parent, likely due to the formation of more structural defects after ferric ion doping, which maximizes the exposure of active sites. Moreover, ferric ion doping can also regulate the surface electronic state to reduce the activation energy barrier for OER on NiS2/C sample. With these merits, the best sample Fe-NiS2/C-30 only requires a potential of +1.486 V (vs. RHE) to reach an OER current density of 10 mA cm−2 and can retain 96.85% of its initial current after continuous working for about 10 h in 1.0 M KOH aqueous solution, along with a small Tafel slope of 45.66 mV/dec, outperforming a commercial RuO2 catalyst. The results in this work enrich the method to tailor the catalytic activity of transition metal sulfides for electrochemical energy technologies.

Engineering Ternary Copper-Cobalt Sulfide Nanosheets as High-performance Electrocatalysts toward Oxygen Evolution Reaction

The rational design and development of the low-cost and effective electrocatalysts toward oxygen evolution reaction (OER) are essential in the storage and conversion of clean and renewable energy sources. Herein, a ternary copper-cobalt sulfide nanosheets electrocatalysts (denoted as CuCoS/CC) for electrochemical water oxidation has been synthesized on carbon cloth (CC) via the sulfuration of CuCo-based precursors. The obtained CuCoS/CC reveals excellent electrocatalytic performance toward OER in 1.0 M KOH. It exhibits a particularly low overpotential of 276 mV at current density of 10 mA cm−2, and a small Tafel slope (58 mV decade−1), which is superior to the current commercialized noble-metal electrocatalysts, such as IrO2. Benefiting from the synergistic effect of Cu and Co atoms and sulfidation, electrons transport and ions diffusion are significantly enhanced with the increase of active sites, thus the kinetic process of OER reaction is boosted. Our studies will serve as guidelines in the innovative design of non-noble metal electrocatalysts and their application in electrochemical water splitting

Identifying the Activation of Bimetallic Sites in NiCo2 S4 @g-C3 N4 -CNT Hybrid Electrocatalysts for Synergistic Oxygen Reduction and Evolution

Hybrid materials composed of transition-metal compounds and nitrogen-doped carbonaceous supports are promising electrocatalysts for various electrochemical energy conversion devices, whose activity enhancements can be attributed to the synergistic effect between metallic sites and N dopants. While the functionality of single-metal catalysts is relatively well-understood, the mechanism and synergy of bimetallic systems are less explored. Herein, the design and fabrication of an integrated flexible electrode based on NiCo₂ S₄ /graphitic carbon nitride/carbon nanotube (NiCo₂ S₄ @g-C₃ N₄ -CNT) are reported. Comparative studies evidence the electronic transfer from bimetallic Ni/Co active sites to abundant pyridinic-N in underlying g-C₃ N₄ and the synergistic effect with coupled conductive CNTs for promoting reversible oxygen electrocatalysis. Theoretical calculations demonstrate the unique coactivation of bimetallic Ni/Co atoms by pyridinic-N species (a Ni, Co-N₂ moiety), which simultaneously downshifts their d-band center positions and benefits the adsorption/desorption features of oxygen intermediates, accelerating the reaction kinetics. The optimized NiCo₂ S₄ @g-C₃ N₄ -CNT hybrid manifests outstanding bifunctional performance for catalyzing oxygen reduction/evolution reactions, highly efficient for realistic zinc-air batteries featuring low overpotential, high efficiency, and long durability, superior to those of physical mixed counterparts and state-of-the-art noble metal catalysts. The identified bimetallic coactivation mechanism will shed light on the rational design and interfacial engineering of hybrid nanomaterials for diverse applications.

Nitrogen-Doped Sponge Ni Fibers as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction

Controllable synthesis of highly active micro/nanostructured metal electrocatalysts for oxygen evolution reaction (OER) is a particularly significant and challenging target. Herein, we report a 3D porous sponge-like Ni material, prepared by a facile hydrothermal method and consisting of cross-linked micro/nanofibers, as an integrated binder-free OER electrocatalyst. To further enhance the electrocatalytic performance, an N-doping strategy is applied to obtain N-doped sponge Ni (N-SN) for the first time, via NH₃ annealing. Due to the combination of the unique conductive sponge structure and N doping, the as-obtained N-SN material shows improved conductivity and a higher number of active sites, resulting in enhanced OER performance and excellent stability. Remarkably, N-SN exhibits a low overpotential of 365 mV at 100 mA cm^-2 and an extremely small Tafel slope of 33 mV dec^-1, as well as superior long-term stability, outperforming unmodified sponge Ni. Importantly, the combination of X-ray photoelectron spectroscopy and near-edge X-ray adsorption fine structure analyses shows that γ-NiOOH is the surface-active phase for OER. Therefore, the combination of conductive sponge structure and N-doping modification opens a new avenue for fabricating new types of high-performance electrodes with application in electrochemical energy conversion devices.

The oxygen evolution reaction enabled by transition metal phosphide and chalcogenide pre-catalysts with dynamic changes

Dynamic morphological, structural and compositional changes will occur when transition metal phosphides and chalcogenides are used to catalyze the oxygen evolution reaction, which can substantially enhance their electrocatalytic performance.