Effect of reaction temperature on the amorphous-crystalline transition of copper cobalt sulfide for supercapacitors

Preparation and Hydrogen Production Application of Core-Shell Heterojunction Photocatalyst (PbS/ZnO)@CuS

This study employed a hydrothermal method to coat CuS onto PbS quantum dots loaded with ZnO, resulting in a core-shell-structured (PbS/ZnO)@CuS hetero-structured photocatalyst. The sulfide coating enhanced the photocatalyst's absorption in the near-infrared to visible light range and effectively reduced electron-hole (h+) pair recombination during photocatalytic processes. Electron microscopy analysis confirmed the successful synthesis of this core-shell structure using polyvinylpyrrolidone (PVP); however, the spatial hindrance effect of PVP led to a disordered arrangement of the CuS lattice, facilitating electron-hole recombination. Comprehensive analyses using transmission electron microscopy (TEM), photoluminescence (PL), and Brunauer-Emmett-Teller (BET) methods revealed that the (PbS/ZnO)@CuS photocatalyst synthesized at a hydrothermal temperature of 170 °C exhibited optimal hydrogen production efficiency. After conducting a photocatalytic reaction for 5 h in a mixed aqueous solution containing 0.25 M Na2S + Na2SO3 as a sacrificial agent, a hydrogen production rate of 3473 μmol·g-1·h-1 was achieved.

Exchanging Anion in CuCo-Carbonate Double Hydroxide for Faradaic Supercapacitors: A Case Study

A systematic synthetic method involving the anion exchange process was designed and developed to fabricate the superior functioning three-dimensional (3-D) urchin-architectured copper cobalt oxide (CuCo₂O₄; CCO) and copper cobalt sulfide (CuCo₂S₄; CCS) electrode materials from copper-cobalt carbonate double hydroxide [(CuCo)₂(CO₃)(OH)₂; CCH]. The effective tuning of chemical, crystalline, and morphological properties was achieved during the derivatization process of CCH, based on the anion exchange effect and phase transformation without altering the 3-D spatial assembly. Benefiting from morphological and structural advantages, CCO and CCS exhibited superior electrochemical activity with capacity values of 1508 and 2502 C g^-1 at 10 A g^-1 to CCH (1182 C g^-1 at 10 A g^-1). The thermal treatment of CCH has generated a highly porous nature in nanospikes of 3-D urchin CCO structures, which purveys betterment in electrochemical phenomena than pristine smooth-surfaced CCH. Meanwhile, the sulfurization reaction induced the anion effect to a greater extent in the CCS morphology, resulting in hierarchical 3-D urchins formed by 1-D nanospikes constituting coaxially swirled 2-D nanosheets with high exposure of active sites, specific surface areas, and 3-D electron/ion transportation channels. The asymmetric supercapacitor was constructed with a superior CCS electrode as a cathode and an activated carbon electrode as an anode, showing a high specific capacity of 287.35 C g^-1 at 7 A g^-1 and durability for 5000 cycles with 94.2% retention at a high current density of 30 A g^-1. The ultrahigh energy and power density of 135.3 W h kg^-1 (10 A g^-1) and 44.35 kW kg^-1 (30 A g^-1) were harvested during the PC device performance. Our finding proposes an idea about the importance of anions and phase transformation as a versatile tool for engineering high-functioning electrode materials and their endeavor toward overwhelming the major demerit of SCs by aggrandizing the energy density value and rate performance.

Construction of Ti3C2Tx MXene wrapped urchin-like CuCo2S4 microspheres for high-performance asymmetric supercapacitors

Copper cobalt sulfide (CuCo₂S₄) nanomaterials are regarded as promising electrode materials for high-performance supercapacitors due to their abundant redox states and considerable theoretical capacities. However, the intrinsic poor electrical conductivity, sluggish reaction kinetics and insufficient number of electroactive sites of these materials are huge barriers to realize their practical applications. In this study, a facile two-step strategy to engineer a hierarchical 3D porous CuCo₂S₄/MXene composite electrode is presented for enhanced storage properties. This well-constructed CuCo₂S₄/MXene composite not only provides abundant active sites for the faradaic reaction, but also offers more efficient pathways for rapid electron/ion transport and restricts the volumetric expansion during the charge/discharge process. When evaluated in a 3 M KOH electrolyte, the CuCo₂S₄/MXene-3 electrode exhibits a specific capacity of 1351.6 C g^-1 at 1 A g^-1 while retaining excellent cycling stability (95.2% capacity retention at 6 A g^-1 after 10 000 cycles). Additionally, the solid-state asymmetric supercapacitor (ASC) CuCo₂S₄/MXene//AC device displays an energy density of 78.1 W h kg^-1 and a power density of 800.7 W kg^-1. Two ASC devices connected in series can illuminate a blue LED indicator for more than 20 min, demonstrating promising prospects for practical applications. These electrochemical properties indicate that the high-performance CuCo₂S₄/MXene composites are promising electrode materials for advanced asymmetric supercapacitors.

Preparation of CuS/PbS/ZnO Heterojunction Photocatalyst for Application in Hydrogen Production

A hexagonal wurtzite ZnO photocatalyst was prepared via a precipitation method. CuS nanoparticles (NPs) and PbS quantum dots (QDs) were loaded onto ZnO via a hydrothermal method to obtain a CuS/PbS/ZnO heterojunction photocatalyst. The CuS/PbS/ZnO photocatalyst obtained via the abovementioned method has significant absorption capabilities in the ultraviolet to near-infrared spectral regions, and effectively reduced the recombination of electron–hole pairs during a photocatalytic reaction. Electron microscope images showed that in the CuS/PbS/ZnO photocatalyst prepared at 130 °C, the particle size of the PbS QDs was approximately 5.5–5.7 nm, and the bandgap determined from the Tauc plot was 0.84 eV; this catalyst demonstrated the best water splitting effect. Furthermore, after adding a 0.25 M mixed solution of Na2S and Na2SO3 as the sacrificial reagent in photocatalysis for 5 h, the hydrogen production efficiency from water splitting reached 6654 μmol g−1 h−1.

Recent Progress of Metal Sulfide Photocatalysts for Solar Energy Conversion

Artificial photosynthetic solar-to-chemical cycles enable an entire environment to operate in a more complex, yet effective, way to perform natural photosynthesis. However, such artificial systems suffer from a lack of well-established photocatalysts with the ability to harvest the solar spectrum and rich catalytic active-site density. Benefiting from extensive experimental and theoretical investigations, this bottleneck may be overcome by devising a photocatalytic platform based on metal sulfides with predominant electronic, physical, and chemical properties. These tunable properties can endow them with abundant active sites, favorable light utilization, and expedited charge transportation for solar-to-chemical conversion. Here, it is described how some vital lessons extracted from previous investigations are employed to promote the further development of metal sulfides for artificial photosynthesis, including water splitting, CO₂ reduction, N₂ reduction, and pollutant removal. Their functions, properties, synthetic strategies, emerging issues, design principles, and intrinsic functional mechanisms for photocatalytic redox reactions are discussed in detail. Finally, the associated challenges and prospects for the utilization of metal sulfides are highlighted and future development trends in photocatalysis are envisioned.

Amorphous/Crystalline Heterophase Ruthenium Nanosheets for pH-Universal Hydrogen Evolution

To design and synthesize heterophase noble-metal materials is of crucial importance owing to their unique structure and apparent properties. Ruthenium (Ru) is one of the most active candidates for hydrogen evolution reaction because of its low price compared with other precious metals, which is favorable for industrial hydrogen cycle operation. In this study, free-standing amorphous/crystalline Ru nanosheets are facilely synthesized through a controlled annealing method. Charge redistribution occurs at the phase interface because of the work function difference between amorphous and crystalline domains. The resulting structure and property are conductive to the adsorption and dissociation of water molecules, associated with optimized hydrogen interaction and enhanced binding between Ru atoms. Accordingly, electrochemical measurements demonstrate that the amorphous/crystalline heterophase Ru exhibits improved hydrogen evolution efficiency as compared with pure amorphous Ru and pure crystalline Ru, at pH-universal conditions. Specifically, only 16.7 mV overpotential is required to reach 10 mA cm^-2 in 1.0 m KOH. Meanwhile, the heterophase structure displays a higher stability during operation than pure amorphous and crystalline structures. This study demonstrates the importance of phase engineering, broadens the Ru-based material family, and provides more insights for developing efficient metal materials.

Interface Engineering of CoP3/Ni2P for Boosting the Wide pH Range Water-Splitting Activity

Developing electrocatalysts with low price, high energy efficiency, and universal pH value for hydrogen/oxygen evolution reaction (HER and OER) is very important for the wide application of electrochemical water splitting in hydrogen production. The results of density functional theory show that the interface region of CoP3/Ni2P heterostructures can significantly boost all of the catalytic performances. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were used to confirm the abundant structural defects and the corresponding adjustment of the electronic state, thus ameliorating the activation energy, conductivity, and active area of the catalyst. Benefiting from these, CoP3/Ni2P heterostructures exhibit superior performance of both HER and OER in a wide pH range. CoP3/Ni2P can also be used for water splitting (1.557 V at 10 mA cm-2) more than 40 h, superior to benchmark pairs of Pt/C and RuO2 on Ni foam.

High Magnetic Field-Engineered Bunched Zn-Co-S Yolk-Shell Balls Intercalated within S, N Codoped CNT/Graphene Films for Free-Standing Supercapacitors

The abundant mass and charge transfer involved in Faradaic redox reactions are largely determined by microstructures including the surface area and porosity, elemental composition and electrical conductivity of bimetallic sulfides. Here, a high magnetic field (HMF) was introduced to tune these intrinsic characters for superior supercapacitor electrodes. We developed a novel HMF-controlled anion-exchange methodology to prepare the one-dimensional (1D) bunched Zn-Co-S yolk-shell balls (ZCS^6T BYSBs). The HMF-induced directional growth and alignment of Zn_0.76Co_0.24S drive the directional 1D assembly. The as-obtained ZCS^6T BYSBs possess larger surface area/pore volume, higher crystallinity and electrical conductivity, richer electroactive elements, and favorable axial electron and ion transport because of HMF-enhanced favorable ion diffusion and exchange kinetics. Flexible S, N codoped carbon nanotubes/graphene films embedded with the ZCS^6T BYSBs (CZS^6T/CNTs/SNGS) were fabricated by vacuum filtration and one-step S, N codoping and reduction of graphene oxides to improve structural stability and charge transport. The CZS^6T/CNTs/SNGS electrode displayed impressive enhanced specific capacitance and rate capability with 78.7% capacitance retention at 30 A g^-1. Furthermore, the CZS^6T/CNTs/SNGS//CNTs/SNGS asymmetric supercapacitor delivered remarkable cycling stability with a high energy density of 41.1 W h kg^-1 at a large power density of 9022 W kg^-1.

Recent Progress in Two-Dimensional Layered Double Hydroxides and Their Derivatives for Supercapacitors

High-performance supercapacitors have attracted great attention due to their high power, fast charging/discharging, long lifetime, and high safety. However, the generally low energy density and overall device performance of supercapacitors limit their applications. In recent years, the design of rational electrode materials has proven to be an effective pathway to improve the capacitive performances of supercapacitors. Layered double hydroxides (LDHs), have shown great potential in new-generation supercapacitors, due to their unique two-dimensional layered structures with a high surface area and tunable composition of the host layers and intercalation species. Herein, recent progress in LDH-based, LDH-derived, and composite-type electrode materials targeted for applications in supercapacitors, by tuning the chemical/metal composition, growth morphology, architectures, and device integration, is reviewed. The complicated relationships between the composition, morphology, structure, and capacitive performance are presented. A brief projection is given for the challenges and perspectives of LDHs for energy research.

Hydrothermal Synthesis of Cobalt Ruthenium Sulfides as Promising Pseudocapacitor Electrode Materials

In this paper, we report the successful synthesis of cobalt ruthenium sulfides by a facile hydrothermal method. The structural aspects of the as-prepared cobalt ruthenium sulfides were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. All the prepared materials exhibited nanocrystal morphology. The electrochemical performance of the ternary metal sulfides was investigated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy techniques. Noticeably, the optimized ternary metal sulfide electrode exhibited good specific capacitances of 95 F g−1 at 5 mV s−1 and 75 F g−1 at 1 A g−1, excellent rate capability (48 F g−1 at 5 A g−1), and superior cycling stability (81% capacitance retention after 1000 cycles). Moreover, this electrode demonstrated energy densities of 10.5 and 6.7 Wh kg−1 at power densities of 600 and 3001.5 W kg−1, respectively. These attractive properties endow proposed electrodes with significant potential for high-performance energy storage devices.

Supercapacitive Performances of Ternary CuCo2S4 Sulfides

Currently, ternary CuCo₂S₄ sulfides are intensively investigated as electrode materials for electrochemical capacitors due to their low cost, high conductivity, and synergistic effect. The research of CuCo₂S₄ materials for energy storage has gradually grown from 2016. The supercapacitive performances of CuCo₂S₄ electrodes for electrochemical capacitors are briefly reviewed in this work. The structure, morphology, and particle size of CuCo₂S₄ are related to the synthesis conditions and electrochemical performances. The thin films of CuCo₂S₄ nanostructures deposited on conductive substrates and their composites both show better properties than single CuCo₂S₄. CuCo₂S₄ and its composites reveal large potential for asymmetric capacitors, delivering high energy densities. However, there is still much new space remaining for future research. The possible development directions, challenges, and opportunities for CuCo₂S₄ materials are also discussed.

Synthesis of single-phase CuCo2-xNixS4 for high-performance supercapacitors

Developing safe, efficient and environment-friendly energy storage systems continues to inspire researchers to synthesize new electrode materials. Doping or substituting host material by some guest elements has been regarded as an effective way to improve the performance of supercapacitors. In this work, single-phase CuCo_2-xNi_xS₄ materials were synthesized by a facile two-step hydrothermal method, where Co in CuCo₂S₄ was substituted by Ni. Cobalt could be easily substituted with Ni in a rational range to keep its constant phase. But, a high content of Ni resulted in a multi-phase composite. Among a series of CuCo_2-xNi_xS₄ materials with different Ni/Co mole ratios, CuCo_1.25Ni_0.75S₄ material presented a significantly high specific capacitance (647 F g^-1 or 272 C g^-1 at 1 A g^-1) and the best cycling stability (∼98% specific capacitance retention after 10,000 charge-discharge cycles), which was mainly due to the modified composition, specific single phase, higher electroconductivity, more electroactive sites and the synergistic effect between Ni and Co. Moreover, the assembled asymmetric capacitor using CuCo_1.25Ni_0.75S₄ as a positive electrode and activated carbon as a negative electrode delivered a high energy density of 31.8 Wh kg^-1 at the power density of 412.5 W kg^-1. These results demonstrated that ternary metal sulfides of CuCo_2-xNi_xS₄ are promising electrode materials for high-performance supercapacitors.

The Synthesis of NiCo2O4-MnO2 Core-Shell Nanowires by Electrodeposition and Its Supercapacitive Properties

Hierarchical composite films grown on current collectors are popularly reported to be directly used as electrodes for supercapacitors. Highly dense and conductive NiCo2O4 nanowires are ideal backbones to support guest materials. In this work, low crystalline MnO2 nanoflakes are electrodeposited onto the surface of NiCo2O4 nanowire films pre-coated on nickel foam. Each building block in the composite films is a NiCo2O4-MnO2 core-shell nanowire on conductive nickel foam. Due to the co-presence of MnO2 and NiCo2O4, the MnO2@NiCo2O4@Ni electrode exhibits higher specific capacitance and larger working voltage than the NiCo2O4@Ni electrode. It can have a high specific capacitance of 1186 F·g-1 at 1 A·g-1. When the core-shell NiCo2O4-MnO2 composite and activated carbon are assembled as a hybrid capacitor, it has the highest energy density of 29.6 Wh·kg-1 at a power density of 425 W·kg-1 with an operating voltage of 1.7 V. This work shows readers an easy method to synthesize composite films for energy storage.

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

Core-Shell Structured Cobalt Sulfide/Cobalt Aluminum Hydroxide Nanosheet Arrays for Pseudocapacitor Application

The direct synthesis of nanostructured electrode materials on three-dimensional substrates is important for their practical application in electrochemical cells without requiring the use of organic additives or binders. In this study, we present a simple two-step process to synthesize a stable core-shell structured cobalt sulfide/cobalt aluminum hydroxide nanosheet (LDH-S) for pseudocapacitor electrode application. The cobalt aluminum layered double hydroxide (CoAl-LDH) nanoplates were synthesized in basic aqueous solution with a kinetically-controlled thickness. Owing to the facile diffusion of electrolytes through the nanoplates, thin CoAl-LDH nanoplates have higher specific capacitance values than thick nanoplates. The as-grown CoAl-LDH nanoplates were transformed into core-shell structured LDH-S nanosheets by a surface modification process in Na2 S aqueous solution. The chemically robust cobalt sulfide (CoS) shell increased the electrochemical stability compared to the sulfide-free CoAl-LDH electrodes. The LDH-S electrodes exhibited high electrochemical performance in terms of specific capacitance and rate capability with a galvanostatic discharge of 1503 F g-1 at a current density of 2 A g-1 and a specific capacitance of 91 % at 50 A g-1 .

Research Progress of NiMn Layered Double Hydroxides for Supercapacitors: A Review

The research on supercapacitors has been attractive due to their large power density, fast charge/discharge speed and long lifespan. The electrode materials for supercapacitors are thus intensively investigated to improve the electrochemical performances. Various transition metal layered double hydroxides (LDHs) with a hydrotalcite-like structure have been developed to be promising electrode materials. Earth-abundant metal hydroxides are very suitable electrode materials due to the low cost and high specific capacity. This is a review paper on NiMn LDHs for supercapacitor application. We focus particularly on the recent published papers using NiMn LDHs as electrode materials for supercapacitors. The preparation methods for NiMn LDHs are introduced first. Then, the structural design and chemical modification of NiMn LDH materials, as well as the composites and films derived from NiMn LDHs are discussed. These approaches are proven to be effective to enhance the performance of supercapacitor. Finally, the reports related to NiMn LDH-based asymmetric supercapacitors are summarized. A brief discussion of the future development of NiMn LDHs is also provided.