NiCo2S4 sub-micron spheres: an efficient non-precious metal bifunctional electrocatalyst

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

A flexible wearable self-supporting hybrid supercapacitor device based on hierarchical nickel cobalt sulfide@C electrode

A flexible wearable electrode consisting of nickel-cobalt sulfide (NCS) nanowires was fabricated in this study. Self-supporting NCS was grown in situ on porous carbon nanofibers without a binder as a novel material for supercapacitor electrodes. The NCS nanowires were grown using cyclic voltammetry electrodeposition, which proved to be a fast and environmentally friendly method with good controllability of the material structure. One-dimensional carbon nanofibers (C) have high surface-area-to-volume ratios, short ion transmission distances, excellent mechanical strengths, and remarkable flexibilities. Moreover, the NCS@C flexible electrode exhibited a synergetic effect with the active compounds, and the dense active sites were uniformly distributed across the entire surface of the carbon fibers, enabling rapid electron transport and enhancing the electrochemical properties of the NCS@C nanowires. The NCS@C achieved specific capacitances of 334.7 and 242.0 mAh g-1 at a current density of 2 A g-1 and high current densities (up to 40 A g-1), respectively, corresponding to a 72.3% retention rate. An NCS@C-nanofilm-based cathode and an activated-carbon-based anode were used to fabricate a flexible asymmetric supercapacitor. The device exhibited high energy and power densities of 12.91 Wh kg-1 and 358 W kg-1, respectively.

Solvent-free synthesis of NiCo2S4 having the metallic nature

Nickel-cobalt sulfide (NiCo2S4) is a prominent member of bimetallic transition metal sulfides. It is being widely used for a variety of applications such as electrode material, photocatalysis, and energy storage devices (like pseudo capacitors, supercapacitors, solar cells, and fuel cells) due to its better electronic conductivity, manageable morphology, and high capacitance. This work presents the one-step solventless synthesis of NiCo2S4 sheet-like nanostructures and then explores their metallic nature. Scanning electron microscopy (SEM) and transmission electron microscopic (TEM) analysis show the sheet-like grown morphology. Few nanorods are also seen. Except for a recent study (Xia et al. 2015) that shows metallic behavior, most of the reports show that NiCo2S4 is a semiconductor with claimed bandgap between 1.21 and 2.4 eV. In this study, we observe from UV-Vis and diffuse reflectance spectroscopy (DRS) that NiCo2S4 has a specific band gap value between 2.02 and 2.17 eV. However, IV characteristics in the temperature range of 300-400 K show that NiCo2S4 is a metal with a positive temperature coefficient of resistance consistent with a recent report. Furthermore, we see the ohmic conduction mechanism. The Arrhenius plot is drawn, and the activation energy is calculated to be 3.45 meV. The metallic nature is attributed to the coupling of two metal species (nickel and cobalt), which accounts for its superior conductivity and performance in a variety of essential applications.

Interfacial Electron Redistribution of FeCo2S4/N-S-rGO Boosting Bifunctional Oxygen Electrocatalysis Performance

Developing bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for the development of zinc–air batteries (ZABs), but several challenges remain in terms of bifunctional activity. FeCo2S4/N-S-rGO was prepared by in situ homogeneous growth of bimetallic sulfide FeCo2S4 on N, S-doped reduced graphene oxide. FeCo2S4/N-S-rGO exhibits a half-wave potential of 0.89 V for ORR and an overpotential of 0.26 V at 10 mA cm−2 for OER, showing significantly bifunctional activity superior to Pt/C (0.85 V) and RuO2 (0.41 V). Moreover, the FeCo2S4/N-S-rGO assembled ZAB shows a superior specific capacity and a power density of 259.13 mW cm−2. It is demonstrated that the interfacial electron redistribution between FeCo2S4 nanoparticles and heteroatom-doped rGO matrix can efficiently improve the electrochemical performance of the catalyst. The results provide new insights into the preparation of high-capability composite catalysts combining transition metal sulfides with carbon materials for applications in ZABs.

Fe and Cu dual-doped Ni3S4 nanoarrays with less low-valence Ni species for boosting water oxidation reaction

Transition metal materials with high efficiency and durable electrocatalytic water splitting activity have attracted widespread attention among scientists. In this work, two cation co-doped Ni₃S₄ nanoarrays grown on a Ni foam support were firstly synthesized through a typical two step hydrothermal process. Cu and Fe co-doping can regulate the internal electron configuration of the material, thus reducing the activation energy of the active species. Moreover, density functional theory calculations demonstrate that a low Ni²⁺ amount improves the adsorption energy of H₂O, which facilitates the formation and reaction of intermediate species in the water splitting process. The experimental results indicate that the Cu and Fe co-doped Ni₃S₄ material has superior electrochemical activity for water oxidation reaction to pure Ni₃S₄, Fe doped Ni₃S₄ and Cu doped Ni₃S₄. The Fe-Cu-Ni₃S₄ material displays a significantly enhanced electrocatalytic performance with low overpotentials of 230 mV at 50 mA cm^-2 and 260 mV at 100 mA cm^-2 for the oxygen evolution reaction under alkaline conditions. It's worth noting that when Fe-Cu-Ni₃S₄ was used as the anode and cathode, a small cell voltage of 1.59 V at 10 mA cm^-2 was obtained to achieve stable overall water splitting. Our work will afford a novel view and guidance for the preparation and application of efficient and environmentally friendly water splitting catalysts.

Enhancing bifunctional catalytic activity of cobalt-nickel sulfide spinel nanocatalysts through transition metal doping and its application in secondary zinc-air batteries

Developing large-scale and high-performance OER (oxygen evolution reaction) and ORR (oxygen reduction reaction) catalysts have been a challenge for commercializing secondary zinc-air batteries. In this work, transition metal-doped cobalt-nickel sulfide spinels are directly produced via a continuous hydrothermal flow synthesis (CHFS) approach. The nanosized cobalt-nickel sulfides are doped with Ag, Fe, Mn, Cr, V, and Ti and evaluated as bifunctional OER and ORR catalyst for Zn-air battery application. Among the doped spinel catalysts, Mn-doped cobalt-nickel sulfides (Ni1.29Co1.49Mn0.22S4) exhibit the most promising OER and ORR performance, showing an ORR onset potential of 0.9 V vs. RHE and an OER overpotential of 348 mV measured at 10 mA cm-2, which is attributed to their high surface area, electronic structure of the dopant species, and the synergistic coupling of the dopant species with the active host cations. The dopant ions primarily alter the host cation composition, with the Mn(iii) cation linked to the introduction of active sites by its favourable electronic structure. A power density of 75 mW cm-2 is achieved at a current density of 140 mA cm-2 for the zinc-air battery using the manganese-doped catalyst, a 12% improvement over the undoped cobalt-nickel sulfide and superior to that of the battery with a commercial RuO2 catalyst.

One pot solvent assisted syntheses of Ag3SbS3 nanocrystals and exploring their phase dependent electrochemical behavior toward oxygen reduction reaction and visible light induced methanol oxidation reaction

A huge variety of silver based ternary sulfide semiconductors (SCs) have been considered for the sustainable advancement of renewable energy sources. Herein, we have synthesized two important classes of newly emerging semiconductor nanocrystals (NCs) Ag3SbS3 (SAS), i.e. hexagonal and monoclinic by simply tuning the solvent polarity, of which the second one has been synthesized in a phase pure NC for the first time by the thermal decomposition of silver and antimony based dithiocarbamate (∼N-CS2-M) complexes. Interestingly, these two systems exhibit two different semiconducting (SC) properties and band gaps; hexagonal SAS has a p type (Eg ∼ 1.65 eV) whereas monoclinic SAS has an n type (Eg ∼ 2.1 eV) character. For the first time ever we have designed a reducing working electrode (i.e. cathode) by modifying the rotating disc electrode (RDE) with hexagonal SAS that exhibits excellent electrochemical oxygen reduction reaction (ORR) activity (Eonset = 1.09 V vs. RHE and average number of electron transfer: 3.89) comparable to that of the highly expensive Pt/C (Eonset = 0.88 V vs. RHE and average number of electron transfer: 3.92). Density functional theory (DFT) investigation confirms the corroborations of experimental data with theoretical implications. In addition, the electrode fabricated from monoclinic SAS acts as an efficient photoanode which exhibits higher photoelectrochemical (PEC) methanol oxidation reaction (MOR) activity under illumination in alkaline medium compared to that of standard TiO2 grown on an indium tin oxide (ITO) coated glass slide. On illumination, the relative photocurrent density at the onset potential has been obtained to be 845 which is a very significant experimental output with respect to any other TiO2 or Pt@TiO2 based photocatalysts for this application. The physicochemical stability and reusability of both materials were supported by 50 hours of extended electrochemical chronoamperometric measurements and powder XRD and the TEM analyses after electrocatalysis. This study explores a possible pathway for designing simple and less expensive but catalytically efficient silver based ternary sulfide NC systems for developing an SC material to reduce the energy crisis in the near future.

Co5.47N loaded N-doped carbon as an efficient bifunctional oxygen electrocatalyst for a Zn-air battery

Highly active bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play a pivotal role in Zn-air batteries. The high cost, scarcity and instability of precious-metal-based electrocatalysts for the ORR and OER dramatically hamper their practical application in such clean-energy set-ups. Here, we report highly active Co5.47N-loaded N-doped carbon (CoNMC), prepared via the direct NH3 annealing of a millet-CoCl2 mixture, which is a cheap and mass-producible form of biomass. The optimized product shows superior ORR activity (a half-wave potential of 0.81 V vs. RHE) and electrochemical stability (a 16.5 mV negative shift of the half-wave potential after 2000 cycles) in alkaline media. Also, it shows appealing OER activity (an operating potential at 10 mA cm-2 of 1.62 V vs. RHE). This excellent electrochemical performance can be attributed to the formation of active Co5.47N nanoparticles, the large specific surface area, the abundance of nitrogen active sites, and the high graphitization degree. When assembled into a Zn-air battery, the CoNMC-based cell shows comparable performance to a Pt/C-RuO2 one.

Amorphous nickel sulfide nanoparticles anchored on N-doped graphene nanotubes with superior properties for high-performance supercapacitors and efficient oxygen evolution reaction

The rational design of a novel material system with superior properties of energy storage and conversion is a significant work. In this paper, amorphous nickel sulfide nanoparticles anchored on N-doped graphene nanotubes (N-GNTs@NSNs) were firstly synthesized by a facile electrochemical-deposition method, which can serve as free-standing robust supercapacitor electrode materials and electrocatalysts. Stemming from the disordered structure of amorphous active materials and the synergy of novel N-GNT framework materials, the as-prepared N-GNT@NSN electrode unveils prominent capacitive behaviors, including a large specific capacity of 240 mA h g-1 (2160 F g-1), decent rate capability, and outstanding cycling stability (95.8% of capacity retention after 12 000 cycles). An asymmetric supercapacitor with N-GNTs@NSNs as the positive electrode and active carbon (AC) as the negative electrode is further assembled, which shows a maximum energy density of 49.5 W h kg-1 at a power density of 800 W kg-1 and robust stability (96.6% capacity retention after 12 000 cycles). Moreover, the electrode also possesses high activities in the oxygen evolution reaction (OER), namely it can attain a current density of 10 mA cm-2 at an overpotential of 284 mV in 1 M KOH. This finding is not only important for significantly enhancing the electrochemical performances of supercapacitor electrode materials and electrocatalysts, but also lays the solid foundation for their further industrial applications in energy storage and conversion systems.

Developing Indium-based Ternary Spinel Selenides for Efficient Solid Flexible Zn-Air Batteries and Water Splitting

Exploring economic, efficient, and corrosion-resistant oxygen electrocatalysts contributes to further development of zinc-air batteries and overall water splitting. Herein, a novel ternary spinel CoIn₂Se₄ nanomaterial, with Co²⁺ and In³⁺ occupying the tetrahedral and octahedral sites of the crystalline structure, has been fabricated using a facile and environment-friendly method. Moreover, CoIn₂Se₄ nanosheets outperform pristine CoSe₂ and In₂Se₃ in catalyzing both oxygen evolution reaction (an overpotential of 315 mV at 10 mA cm^-2) and oxygen reduction reaction (an onset overpotential of 0.88 V). The reduced charge resistance and increased active site exposure ratio contribute to the superior performance of CoIn₂Se₄. Moreover, density of theory (density functional theory-DFT) calculations suggest a significantly reduced reaction energy barrier after introducing indium into the spinel, and therefore the reaction kinetics are facilitated. Based on the advantages described above, CoIn₂Se₄ is used as the air cathode for a solid flexible zinc-air battery (FZAB). The system displays an efficient performance that outperforms the Pt@Ir/C catalyst: a significantly enhanced specific capacity of 733 mAh g_Zn^-1, a high energy density of 931 Wh kg^-1, and an excellent flexibility with long cycle life performance (over 400 cycles). The CoIn₂Se₄-based two-series-connected FZABs successfully power light-emitting diode (LED) screens for over 10 h. Meanwhile, CoIn₂Se₄ also shows a significantly enhanced hydrogen evolution reaction (HER) performance than other samples. Finally, the CoIn₂Se₄-based electrolyzer shows an efficient overall water-splitting performance with high stability. This work demonstrates that the indium-based ternary selenides show promising applications in developing renewable energy and water-splitting devices.

Bifunctionally active nanosized spinel cobalt nickel sulfides for sustainable secondary zinc–air batteries: examining the effects of compositional tuning on OER and ORR activity

Nanosized cobalt nickel sulfides were prepared via a continuous hydrothermal method and evaluated as electrocatalysts, with the catalytic activity being linked to the cationic composition.

Thermally reduced mesoporous manganese MOF @reduced graphene oxide nanocomposite as bifunctional electrocatalyst for oxygen reduction and evolution

Oxygen electrocatalysis plays a crucial role in harnessing energy from modern renewable energy technologies like fuel cells and metal–air batteries. But high cost and stability issues of noble metal catalysts call for research on tailoring novel metal–organic framework (MOF) based architectures which can bifunctionally catalyze O₂ reduction and evolution reactions (ORR & OER). In this work, we report a novel manganese MOF @rGO nanocomposite synthesized using a facile self-templated solvothermal method. The nanocomposite is superior to commercial Pt/C catalyst both in material resource and effectiveness in application. A more positive cathodic peak (E_pc = 0.78 V vs. RHE), onset (E_onset = 1.09 V vs. RHE) and half wave potentials (E_1/2 = 0.98 V vs. RHE) for the ORR and notable potential to achieve the threshold current density (E_{@10 mA cm⁻²}= 1.84 V vs. RHE) for OER are features promising to reduce overpotentials during ORR and OER. Small Tafel slopes, methanol tolerance and acceptable short term stability augment the electrocatalytic properties of the as-prepared nanocomposite. Remarkable electrocatalytic features are attributed to the synergistic effect from the mesoporous 3D framework and transition metal–organic composition. Template directed growth, tunable porosities, novel architecture and excellent electrocatalytic performance of the manganese MOF @rGO nanocomposite make it an excellent candidate for energy applications.

Oxygen electrocatalysis plays a crucial role in harnessing energy from modern renewable energy technologies like fuel cells and metal–air batteries.

Preparation of Yolk-Shell-Structured Cox Fe1-x P with Enhanced OER Performance

The design and development of low-cost, highly efficient, and stable electrocatalysts to take the place of noble-metal catalysts for the oxygen evolution reaction (OER) remain a significant challenge. Herein, the synthesis of yolk-shell-structured binary transition metal phosphide Co_x Fe_1-x P with different Co/Fe ratios by phosphidation of a cobalt ferrite precursor is reported. The as-synthesized Co_x Fe_1-x P catalysts were used for the OER. All yolk-shell Co_x Fe_1-x P catalysts with different Co/Fe ratios showed much better performance than the corresponding solid catalyst. The formation of Co oxides on the catalyst surface during OER and the optimal Co/Fe ratio were found to be critical to their activity. Among the as-prepared Co_x Fe_1-x P catalysts, that with a Co/Fe ratio of 0.47/0.53 (Co_0.47 Fe_0.53 P) exhibited the best performance. Co_0.47 Fe_0.53 P has an overpotential of 277 mV at a current density of 10 mA cm^-2 , a Tafel slope of 37 mV dec^-1 , and superior stability in alkaline medium. The outstanding performance is partly ascribed to the transfer of valence electrons from Co to P and Fe. The Co_0.47 Fe_0.53 P matrix with excellent conductivity and Fe phosphate that is stable on the surface of the catalyst are also helpful for the OER performance. In addition, the yolk-shell structure of Co_0.47 Fe_0.53 P increases the contact area between electrolyte and catalyst. These characteristics of Co_0.47 Fe_0.53 P greatly improve its OER performance. This optimized binary transition metal phosphide provides a new approach for the design of nonprecious-metal electrocatalysts.

The importance of charge redistribution during electrochemical reactions: a density functional theory study of silver orthophosphate (Ag3PO4)

The charge redistribution during oxygen evolution reaction relates to the electrochemical activity as shown for Ag₃PO₄ structures.

Controlled-Size Hollow Magnesium Sulfide Nanocrystals Anchored on Graphene for Advanced Lithium Storage

Magnesium sulfide (MgS), representative of alkaline-earth metal chalcogenides (AEMCs), is a potential conversion/alloy-type electrode material for lithium ion batteries (LIBs), by virtue of its low potential, high theoretical capacity, and abundant magnesium resource. However, the limited capacity utilization and inferior rate performance still hinder its practical application, and the progress is rather slow due to the challenging fabrication technique for MgS. Herein, we report a series of controlled-size hollow MgS nanocrystals (NCs) homogeneously distributed on graphene (MgS@G), fabricated through a metal hydride framework (MHF) strategy, and its application as advanced electrode material for LIBs. The hollow structure of MgS NCs is mainly attributed to the Kirkendall effect and the escape of hydrogen atoms from metal hydride during sulfuration. The as-synthesized MgS@G demonstrates robust nanoarchitecture and admirable interactions, which ensure a spatially confined lithiation/delithiation process, optimize the dynamics of two-steps conversion/alloying reactions, and induce a synergetic pseudocapacitive storage contribution. As a result, a representative MgS@G composite delivers a largely enhanced capacity of >1208 mAh g^-1 at a current density of 100 mA g^-1 and a long-term cycle stability at a high current density of 5 A g^-1 with a capacity of 838 mAh g^-1 over 3000 cycles, indicating well-sustained structural integrity. This work presents an effective route toward the development of high-performance magnesium-based material for energy storage.

Fe2 N/S/N Codecorated Hierarchical Porous Carbon Nanosheets for Trifunctional Electrocatalysis

Construction of multifunctional highly active earth-abundant electrocatalysts on a large scale is a great challenge due to poor control over nanostructural features and limited active sites. Here, a simple methodology to tailor metal-organic frameworks (MOFs) to extract highly active multifunctional electrocatalysts on a large scale for oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution reaction (HER) is presented. The N, S codoped Fe₂ N decorated highly porous and defect-rich carbon nanosheets are grown using MOF xerogels, melamine, and polyvinylpyrollidone. The resulting catalyst exhibits excellent activity for ORR with an onset (0.92 V) and half-wave (0.81 V) potential similar to state-of-the-art Pt/C catalysts. The catalyst also shows outstanding OER and HER activities with a small overpotential of 360 mV in 1 m KOH and -123 mV in 0.5 m H₂ SO₄ at a current density of 10 mA cm^-2 , respectively. Excellent catalytic properties are further supported by theoretical calculations where relevant models are built and various possible activation sites are identified by first-principles calculations. The results suggest that the carbon atoms adjacent to heteroatoms as well as Fe₂ -N sites present the active sites for improved catalytic response, which is in agreement with the experimental results.

One-Pot Synthesis of NiCo2S4 Hollow Spheres via Sequential Ion-Exchange as an Enhanced Oxygen Bifunctional Electrocatalyst in Alkaline Solution

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are considered to be cornerstones of many energy conversion and storage technologies. It is difficult studying high-performance nonprecious materials as cost-effective bifunctional electrocatalysts for both the OER and ORR in future practical applications. In this study, NiCo₂S₄ hollow spheres (NiCo₂S₄ HSs) were fabricated via an effective and facile one-pot "green" approach in an N, N-dimethylformamide-ethylene glycol binary solution. The obtained NiCo₂S₄ HSs had a high specific surface area as well as numerous active sites and showed a remarkable catalytic performance and durability toward both the OER and ORR in an alkaline electrolyte. For the ORR, NiCo₂S₄ HSs exhibited a positive half-wave potential of 0.80 V and demonstrated outstanding stability and enhanced methanol tolerance. For the OER, NiCo₂S₄ HSs presented a low overpotential (400 mV) at a current density of 10 mA cm^-2, small Tafel slope, and excellent stability in 0.1 M KOH. Moreover, regarding the overall electrocatalytic activity, the potential difference of NiCo₂S₄ HSs was 0.83 V, surpassing that of NiCo₂S₄ nanoparticles, binary counterparts (CoS, NiS), and most highly active bifunctional catalysts described in the literature. The superior catalytic performance of NiCo₂S₄ HSs is mainly ascribed to its unique hollow structure, which increases molecular diffusion and adsorption, as well as the synergistic effect of Ni and Co, which offers richer redox reaction sites. Importantly, this strategy may facilitate the design and preparation of excellent bifunctional nonprecious metal electrocatalysts in various domains.

Hierarchical Nickel-Cobalt Dichalcogenide Nanostructure as an Efficient Electrocatalyst for Oxygen Evolution Reaction and a Zn-Air Battery

A unique three-dimensional (3D) structure consisting of a hierarchical nickel-cobalt dichalcogenide spinel nanostructure is investigated for its electrocatalytic properties at benign neutral and alkaline pH and applied as an air cathode for practical zinc-air batteries. The results show a high oxygen evolution reaction catalytic activity of nickel-cobalt sulfide nanosheet arrays grown on carbon cloth (NiCo₂S₄ NS/CC) over the commercial benchmarking catalyst under both pH conditions. In particular, the NiCo₂S₄ NS/CC air cathode shows high discharge capacity, a narrow potential gap between discharge and charge, and superior cycle durability with reversibility, which exceeds that of commercial precious metal-based electrodes. The excellent performance of NiCo₂S₄ NS/CC in water electrolyzers and zinc-air batteries is mainly due to highly exposed electroactive sites with a rough surface, morphology-based advantages of nanosheet arrays, good adhesion between NiCo₂S₄ and the conducting carbon cloth, and the active layer formed of nickel-cobalt (oxy)hydroxides during water splitting. These results suggest that NiCo₂S₄ NS/CC could be a promising candidate as an efficient electrode for high-performance water electrolyzers and rechargeable zinc-air batteries.

Recent Advances toward the Rational Design of Efficient Bifunctional Air Electrodes for Rechargeable Zn-Air Batteries

Large-scale application of renewable energy and rapid development of electric vehicles have brought unprecedented demand for advanced energy-storage/conversion technologies and equipment. Rechargeable zinc (Zn)-air batteries represent one of the most promising candidates because of their high energy density, safety, environmental friendliness, and low cost. The air electrode plays a key role in managing the many complex physical and chemical processes occurring on it to achieve high performance of Zn-air batteries. Herein, recent advances of air electrodes from bifunctional catalysts to architectures are summarized, and their advantages and disadvantages are discussed to underline the importance of progress in the evolution of bifunctional air electrodes. Finally, some challenges and the direction of future research are provided for the optimized design of bifunctional air electrodes to achieve high performance of rechargeable Zn-air batteries.

Bi-metallic boride electrocatalysts with enhanced activity for the oxygen evolution reaction

Rational design and understanding of the intrinsic mechanism are critical to develop highly active and durable electrocatalysts. In this study, a series of bi-metallic boride catalysts based on Ni and Co were prepared, and their activities were evaluated. The synthesised Co-10Ni-B catalyst exhibited excellent activity for water splitting in a 1 M KOH electrolyte. The overpotential was 330 mV at a current density of 10 mA cm-2, better than previously reported mono-metallic borides and even IrO2. The synergistic effect of Co and Ni was proved by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The facile formation of critical intermediates CoOOH and NiOOH during the catalytic processes and a significant increase in surface area owing to the introduction of a second metal into mono-metallic boride were attributed to the superior catalytic performance of catalysts for the oxygen evolution reaction. A Co-10Ni-B-sp catalyst with a higher surface area than the Co-10Ni-B catalyst was also synthesised to evaluate the effect of a high surface area on the catalytic activity. A lower overpotential of 310 mV at a current density of 10 mA cm-2 was achieved.

NiCo2 S4 Materials for Supercapacitor Applications

Cobalt-nickel sulfide (NiCo₂ S₄ ) shows extensive potential for innovative photoelectronic and energetic materials owing to distinctive physical and chemical properties. In this review, representative strategies for the fabrication and application of NiCo₂ S₄ and composite nanostructures are outlined for supercapacitors, with the aim of promoting the development of NiCo₂ S₄ and their composites in the supercapacitor field through an analysis and comparison of diverse nanostructures. A brief introduction into the structures, properties, and morphologies are presented. Further prospects and promising developments of the materials in the supercapacitor field are also proposed.

Unveiling the Catalytic Origin of Nanocrystalline Yttrium Ruthenate Pyrochlore as a Bifunctional Electrocatalyst for Zn-Air Batteries

Zn-air batteries suffer from the slow kinetics of oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER). Thus, the bifunctional electrocatalysts are required for the practical application of rechargeable Zn-air batteries. In terms of the catalytic activity and structural stability, pyrochlore oxides (A₂[B_2-xA_x]O_7-y) have emerged as promising candidates. However, a limited use of A-site cations (e.g., lead or bismuth cations) of reported pyrochlore catalysts have hampered broad understanding of their catalytic effect and structure. More seriously, the catalytic origin of the pyrochlore structure was not clearly revealed yet. Here, we report the new nanocrystalline yttrium ruthenate (Y₂[Ru_2-xY_x]O_7-y) with pyrochlore structure. The prepared pyrochlore oxide demonstrates comparable catalytic activities in both ORR and OER, compared to that of previously reported metal oxide-based catalysts such as perovskite oxides. Notably, we first find that the catalytic activity of the Y₂[Ru_2-xY_x]O_7-y is associated with the oxidations and corresponding changes of geometric local structures of yttrium and ruthenium ions during electrocatalysis, which were investigated by in situ X-ray absorption spectroscopy (XAS) in real-time. Zn-air batteries using the prepared pyrochlore oxide achieve highly enhanced charge and discharge performance with a stable potential retention for 200 cycles.

Facile Growth of Caterpillar-like NiCo2S4 Nanocrystal Arrays on Nickle Foam for High-Performance Supercapacitors

Ternary cobalt nickel sulfide as a novel and efficient electrode material in supercapacitors has recently gained extensive interests. Herein, we first report a highly conductive caterpillar-like NiCo₂S₄, composed of NiCo₂S₄ nanosheet core and nanowire shell grown on Ni foam via a facile and cost-effective chemical liquid process. Growth mechanism of the NiCo₂S₄ nanosheets@nanowires (NSNWs) structure was also investigated in detail by analyzing time-dependent experimental as well as the amount of additive ammonium fluoride in solution. Furthermore, the electrochemical measurements were performed among three different morphologies of NiCo₂S₄ including nanosheets, nanosheets@nanoparticles, and NSNWs structure, which were obtained from different reaction stages. Because the NSNWs structure has relatively high electroactive surface area, conductivity, and effective electron transport pathways, the as-prepared NiCo₂S₄ NSNWs structure comparing with two other morphologies exhibits the maximum specific capacity of 1777 F/g at 1 A/g and the highest capacitance retention (83% after 3000 cycles) at a high scan rate of 10 A/g with a mass loading density of 4.0 mg/cm². These results indicate that the NiCo₂S₄ NSNWs structure has great potential in supercapacitors.

In situ template synthesis of hollow nanospheres assembled from NiCo2S4@C ultrathin nanosheets with high electrochemical activities for lithium storage and ORR catalysis

NiCo₂S₄@C ultrathin nanosheet assembled hollow nanospheres are fabricated by synchronized sulfurization and in situ template removal, and exhibit high electrochemical activities for lithium storage and ORR catalysis.

S, N-Co-Doped Graphene-Nickel Cobalt Sulfide Aerogel: Improved Energy Storage and Electrocatalytic Performance

Metal sulfides are commonly used in energy storage and electrocatalysts due to their redox centers and active sites. Most literature reports show that their performance decreases significantly caused by oxidation in alkaline electrolyte during electrochemical testing. Herein, S and N co-doped graphene-based nickel cobalt sulfide aerogels are synthesized for use as rechargeable alkaline battery electrodes and oxygen reduction reaction (ORR) catalysts. Notably, this system shows improved cyclability due to the stabilization effect of the S and N co-doped graphene aerogel (SNGA). This reduces the rate of oxidation and the decay of electronic conductivity of the metal sulfides materials in alkaline electrolyte, i.e., the capacity decrease of CoNi₂S₄/SNGA is 4.2% for 10 000 cycles in a three-electrode test; the current retention of 88.6% for Co-S/SNGA after 12 000 s current-time chronoamperometric response in the ORR test is higher than corresponding Co-S nanoparticles and Co-S/non-doped graphene aerogels. Importantly, the results here confirm that the Ni-Co-S ternary materials behave as an electrode for rechargeable alkaline batteries rather than supercapacitors electrodes in three-electrode test as commonly described and accepted in the literature. Furthermore, formulas to evaluate the performance of hybrid battery devices are specified.