CoS2 hollow nanocubes derived from Co-Co Prussian blue analogue: High-performance electrode materials for supercapacitors

Highly selective regulation of non-radical and radical mechanisms by Co cubic assembly catalysts for peroxymonosulfate activation

Peroxymonosulfate (PMS) activation on efficient catalysts is a promising strategy to produce sulfate radical (SO4-) and singlet oxygen (1O2) for the degradation of refractory organic pollutants. It is a great challenge to selectively generate these two reactive oxygen species, and the regulation mechanism from non-radical to radical pathway and vice versa is not well established. Here, we report a strategy to regulate the activation mechanism of PMS for the selective generation of SO4- and 1O2 with 100 % efficiency by sulfur-doped cobalt cubic assembly catalysts that was derived from the Co-Co Prussian blue analog precursor. This catalyst showed superior catalytic performance in activating PMS with normalized reaction rate increased by 87 times that of the commercial Co3O4 nanoparticles and had much lower activation energy barrier for the degradation of organic pollutant (e.g., p-chlorophenol) (18.32 kJ⋅mol-1). Experimental and theoretical calculation results revealed that S doping can regulate the electronic structure of Co active centers, which alters the direction of electron transfer between catalyst and PMS. This catalyst showed a strong tolerance to common organic compounds and anions in water, wide environmental applicability, and performed well in different real-water systems. This study provides new opportunities for the development of metal catalyst with metal-organic frameworks structure and good self-regeneration ability geared specifically towards PMS-based advanced oxidation processes applied for water remediation.

Rapid Construction of Double Crystalline Prussian Blue Analogue Hetero-Superstructure

The controllable construction of complex metal-organic coordination polymers (CPs) merits untold scientific and technological potential, yet remains a grand challenge of one-step construction and modulating simultaneously valence states of metals and topological morphology. Here, a thiocyanuric acid (TCA)-triggered strategy is presented to one-step rapid synthesis a double-crystalline Prussian blue analogue hetero-superstructure (PBA-hs) that comprises a Co3[Fe(CN)6]2 cube overcoated with a KCo[Fe(CN)6] shell, followed by eight self-assembled small cubes on vertices. Unlike common directing surfactants, TCA not only acts as a trigger for the fast growth of KCo[Fe(CN)6] on the Co3[Fe(CN)6]2 phase resulting in a PBA-on-PBA hetero-superstructure, but also serves as a flange-like bridge between them. By combining experiments with simulations, a deprotonation-induced electron transfer (DIET) mechanism is proposed for formation of second phase in PBA-hs, differing from thermally and photo-induced electron transfer processes. To prove utility, the calcined PBA-hs exhibits enhanced oxygen evolution reaction performance. This work provides a new method to design of novel CPs for enriching chemistry and material science. This work offers a practical approach to design novel CPs for enriching chemistry and material science.

From lab to field: Prussian blue frameworks as sustainable cathode materials

Prussian blue and Prussian blue analogues have attracted increasing attention as versatile framework materials with a wide range of applications in catalysis, energy conversion and storage, and biomedical and environmental fields. In terms of energy storage and conversion, Prussian blue-based materials have emerged as suitable candidates of growing interest for the fabrication of batteries and supercapacitors. Their outstanding electrochemical features such as fast charge-discharge rates, high capacity and prolonged cycling life make them favorable for energy storage application. Furthermore, Prussian blue and its analogues as rechargeable battery anodes can advance significantly by the precise control of their structure, morphology, and composition at the nanoscale. Their tunable structural and electronic properties enable the detection of many types of analytes with high sensitivity and specificity, and thus, they are ideal materials for the development of sensors for environmental detection, disease trend monitoring, and industrial safety. Additionally, Prussian blue-based catalysts display excellent photocatalytic performance for the degradation of pollutants and generation of hydrogen. Specifically, their excellent light capturing and charge separation capabilities make them stand out in photocatalytic processes, providing a sustainable option for environmental remediation and renewable energy production. Besides, Prussian blue coatings have been studied particularly for corrosion protection, forming stable and protective layers on metal surfaces, which extend the lifespan of infrastructural materials in harsh environments. Prussian blue and its analogues are highly valuable materials in healthcare fields such as imaging, drug delivery and theranostics because they are biocompatible and their further functionalization is possible. Overall, this review demonstrates that Prussian blue and related framework materials are versatile and capable of addressing many technical challenges in various fields ranging from power generation to healthcare and environmental management.

Construction of CoNi2S4/Co9S8@Co4S3 nanocubes derived from Ni-Co prussian blue analogues@cobalt carbonate hydroxide core-shell heterostructure for asymmetric supercapacitor

Construction of Prussian blue analogues (PBAs) with heterostructure is beneficial to preparing PBAs derivatives with superior electrochemical performance. In this work, the core-shell nanostructured nanocubes composed of nickel hexacyanocobalt PBA (NiCo-PBA)@cobalt carbonate hydroxide (CCH) are synthesized through an in-situ epitaxial growth strategy, and the formation mechanisms of coating are carefully validated and specifically discussed. Then, the precursors are successfully transformed into hierarchical CoNi2S4/Co9S8@Co4S3 via the gas-phase vulcanization method. Benefiting from the intriguing heterostructure and multicomponent sulfides, the CoNi2S4/Co9S8@Co4S3-80 electrode exhibits a high specific capacity of 799 ± 16C/g (specific capacitance of 1595 ± 31F/g) at 1 A/g, ultra-high capacity retention of 80 % at a high current density of 20 A/g. The assembled asymmetric supercapacitor (ASC) device delivers a high energy density of 43.3 Wh kg-1 at a power density of 899 W kg-1 and exhibits superior cycling stability with the capacity retention of 88 % after 5,000 cycles. Subsequently, the fabricated all-solid-state ASC device shows an excellent energy density of 36.4 Wh kg-1 with a power density of 824 W kg-1. This work proposing rational design of combining multicomponent sulfides and core-shell heterostructure based on PBA nanocubes opens up a novel route for developing asymmetric supercapacitor electrode materials with superior performance.

Direct Synthesis of Mn3[Fe(CN)6]2·nH2O Nanosheets as Novel 2D Analog of Prussian Blue and Material for High-Performance Metal-Ion Batteries

Rechargeable metal-ion batteries (RMIBs) are prospective highly effective and low-cost devices for energy storage. Prussian blue analogues (PBAs) have become a subject of significant interest for commercial applications owing to their exceptional specific capacity and broad operational potential window as cathode materials for rechargeable metal-ion batteries. However, the limiting factors for its widespread use are its poor electrical conductivity and stability. The present study describes the direct and simple synthesis of 2D nanosheets of MnFCN (Mn₃[Fe(CN)₆]₂·nH₂O) on nickel foam (NF) via a successive ionic layer deposition (SILD) method, which provided more ion diffusion and electrochemical conductivity. MnFCN/NF exhibited exceptional cathode performance for RMIBs, delivering a high specific capacity of 1032 F/g at 1 A/g in an aqueous 1M NaOH electrolyte. Additionally, the specific capacitance reached the remarkable levels of 327.5 F/g at 1 A/g and 230 F/g at 0.1 A/g in 1M Na₂SO₄ and 1M ZnSO₄ aqueous solutions, respectively.

Sea urchin-like CoNi2S4materials derived from nickel hexamyanocobaltate for high-performance asymmetric hybrid supercapacitor

In this work, a mild chemical precipitation method and simple hydrothermal treatment of the nickel hexamyanocobaltate precursor strategy are developed to prepare a sea urchin-like CoNi₂S₄compound with remarkable specific capacity and excellent cycling stability. The prepared CoNi₂S₄has an outstanding specific capacity of 149.1 mA h g^-1at 1 A g^-1and an initial capacity of 83.1% after 3000 cycles at 10 A g^-1. Moreover, the porous carbon nanospheres (PCNs) with exhibit cycling stability (94.7% of initial specific capacity after 10 000 cycles at 10 A g^-1) are selected as negative electrode to match CoNi₂S₄positive electrode for assembly of CoNi₂S₄//PCNs asymmetric supercapacitor (ASC). Satisfactorily, the as-assembled CoNi₂S₄//PCNs ASC exhibits an impressive energy density of 41.6 Wh kg^-1at 797 W kg^-1, as well as the suitable capacity retention of 82.8% after 10 000 cycles. The superior properties of the device demonstrated that the as-prepared material is potential energy storage material.

Bimetallic CoSe2/FeSe2 hollow nanocuboids assembled by nanoparticles as a positive electrode material for a high-performance hybrid supercapacitor

Design and fabrication of impressive and novel electrode materials for energy storage devices, especially supercapacitors, are of great importance. Herein, bimetallic CoSe₂/FeSe₂ hollow nanocuboid nanostructures derived from Co/Fe-Prussian Blue analogues (denoted as CoSe₂/FeSe₂ HNCs) are successfully designed and fabricated as a remarkable positive electrode material for high-performance supercapacitors. The bimetallic CoSe₂/FeSe₂ HNC nanostructures can have increased active sites and short electron-ion diffusion pathways. Bimetallic CoSe₂/FeSe₂ HNCs@NiF as a positive electrode showed efficient supercapacitive properties with a great specific capacity of 332.75 mA h g^-1 (1197.90 C g^-1) at 1 A g^-1, retaining 80.61% of its initial capacity at 20 A g^-1, considerable longevity (91.47% of its initial capacity after 10 000 cycles) and an excellent coulombic efficiency of 98.49%. Also, the designed and fabricated CoSe₂/FeSe₂ HNCs@NiF||AC@NiF hybrid supercapacitor device using bimetallic CoSe₂/FeSe₂ HNCs@NiF (positive electrode) and activated carbon@NiF (AC, negative electrode) exhibited an efficient energy density of 63.62 W h kg^-1 and a superior durability of 91.14% after 10 000 cycles.

Co-Precipitation Synthesis of Co3[Fe(CN)6]2·10H2O@rGO Anode Electrode for Lithium-Ion Batteries

Rechargeable lithium-ion batteries (LIBs) are known to be practical and cost-effective devices for storing electric energy. LIBs have a low energy density, which calls for the development of new anode materials. The Prussian blue analog (PBA) is identified as being a candidate electrode material due to its facile synthesis, open framework structures, high specific surface areas, tunable composition, designable topologies and rich redox couples. However, its poor electrical conductivity and mechanical properties are the main factors limiting its use. The present study loaded PBA (Co3[Fe(CN)6]·10H2O) on graphene oxide (Co-Fe-PBA@rGO) and then conducted calcination at 300 °C under the protection of nitrogen, which reduced the crystal water and provided more ion diffusion pathways. As a result, Co-Fe-PBA@rGO showed excellent performance when utilized as an anode in LIBs, and its specific capacities were 546.3 and 333.2 mAh g-1 at 0.1 and 1.0 A g-1, respectively. In addition, the electrode also showed excellent performance in the long-term cycle, and its capacity reached up to 909.7 mAh g-1 at 0.1 A g-1 following 100 cycles.

NiCoSe4nanoparticles derived from nickel-cobalt Prussian blue analogues on N-doped reduced graphene oxide for high-performance asymmetric supercapacitors

Synthesis of NiHCCo precursors via simple co-precipitation and nickel-cobalt tetraselenide composites grown on nitrogen-doped reduced graphene oxide (NiCoSe₄/N-rGO) were fabricated using solvothermal method. The introduction of N-rGO used as a template effectively prevented agglomeration of NiCoSe₄nanoparticles and provided more active sites, which greatly increased the electrochemical and electrical conductivity for NiCoSe₄/N-rGO. NiCoSe₄/N-rGO-20 presents a remarkably elevated specific capacity of 120 mA h g^-1under current density of 1 A g^-1. NiCoSe₄/N-rGO-20 demonstrates an excellent cycle life and achieves a remarkable 83% retention rate over 3000 cycles with 10 A g^-1. NiCoSe₄/N-rGO-20//N-rGO asymmetric supercapacitor was constructed based on the NiCoSe₄/N-rGO-20 as an anode, N-rGO as cathode by using 2 mol l^-1KOH as an electrolyte. NiCoSe₄/N-rGO-20//N-rGO ASC demonstrates an ultra-big energy density of 14 Wh kg^-1and good circulation stability in the power density of 902 W kg^-1. It is doubled in comparison to the NiCoSe₄/N-rGO-20//rGO asymmetric supercapacitor (7 Wh kg^-1). The NiCoSe₄/N-rGO-20//N-rGO ASC capacity retention is still up to 93% over 5000 cycles (5 A g^-1). The results reveal that this device would be a prospective cathode material of supercapacitors in actual applications.

High-rate transition metal-based cathode materials for battery-supercapacitor hybrid devices

With the rapid development of portable electronic devices, electric vehicles and large-scale grid energy storage devices, there is a need to enhance the specific energy density and specific power density of related electrochemical devices to meet the fast-growing requirements of energy storage. Battery-supercapacitor hybrid devices (BSHDs), combining the high-energy-density feature of batteries and the high-power-density properties of supercapacitors, have attracted mass attention in terms of energy storage. However, the electrochemical performances of cathode materials for BSHDs are severely limited by poor electrical conductivity and ion transport kinetics. As the rich redox reactions induced by transition metal compounds are able to offer high specific capacity, they are an ideal choice of cathode materials. Therefore, this paper reviews the currently advanced progress of transition metal compound-based cathodes with high-rate performance in BSHDs. We discuss some efficient strategies of enhancing the rate performance of transition metal compounds, including developing intrinsic electrode materials with high conductivity and fast ion transport; modifying materials, such as inserting defects and doping; building composite structures and 3D nano-array structures; interfacial engineering and catalytic effects. Finally, some suggestions are proposed for the potential development of cathodes for BSHDs, which may provide a reference for significant progress in the future.

Core-shell shaped Ni2CoHCF@PPy microspheres from prussian blue analogues for high performance asymmetric supercapacitors

Recently, prussian blue analogues (PBAs), as the most classical class of metal-organic frameworks, have been widely studied by scientists. Nevertheless, the inferior conductivity of PBAs restricts the application in supercapacitors. In this work, nickel cobalt hexacyanoferrate (Ni₂CoHCF) had been produced via a simple co-precipitation approach and coated with polypyrrole on its surface. The conductivity of PBAs was improved by the polypyrrole coating. The Ni₂CoHCF@PPy-400 microspheres were demonstrated to the outstanding specific capacity of 82 mAh g^-1at 1 A g^-1. After 3000 cycles, the Ni₂CoHCF@PPy-400 microspheres had a long cycle life and 86% specific capacity retention rate at 5 A g^-1. Additionally, it was coupled with activated carbon to build high performance asymmetric supercapacitor (Ni₂CoHCF@PPy-400//AC), which displayed a high energy density of 21.7 Wh kg^-1at the power density of 888 W kg^-1and good cycle stability after 5000 cycles (a capacity retention rate of 85.2%). What is more, the results reveal that the Ni₂CoHCF@PPy-400 microspheresare a prospective candidate for exceptional energy storage devices.

Enhanced Supercapacitor Performance Using a Co3 O4 @Co3 S4 Nanocomposite on Reduced Graphene Oxide/Ni Foam Electrodes

To avoid an enormous energy crisis in the not-too-distant future, it be emergent to establish high-performance energy storage devices such as supercapacitors. For this purpose, a three-dimensional (3D) heterostructure of Co₃ O₄ and Co₃ S₄ on nickel foam (NF) that is covered by reduced graphene oxide (rGO) has been prepared by following a facile multistep method. At first, rGO nanosheets are deposited on NF under mild hydrothermal conditions to increase the surface area. Subsequently, nanowalls of cobalt oxide are electro-deposited on rGO/Ni foam by applying cyclic-voltammetry (CV) under optimized conditions. Finally, for the synthesis of Co₃ O₄ @Co₃ S₄ nanocomposite, the nanostructure of Co₃ S₄ was fabricated from Co₃ O₄ nanowalls on rGO/NF by following an ordinary hydrothermal process through the sulfurization for the electrochemical application. The samples are characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The obtained sample delivers a high capacitance of 13.34 F cm^-2 (5651.24 F g^-1 ) at a current density of 6 mA cm^-2 compared to the Co₃ O₄ /rGO/NF electrode with a capacitance of 3.06 F cm^-2 (1230.77 F g^-1 ) at the same current density. The proposed electrode illustrates the superior electrochemical performance such as excellent specific energy density of 85.68 W h Kg^-1 , specific power density of 6048.03 W kg^-1 and a superior cycling performance (86% after 1000 charge/discharge cycles at a scan rate of 5 mV s^-1 ). Finally, by using Co₃ O₄ @Co₃ S₄ /rGO/NF and the activated carbon-based electrode as positive and negative electrodes, respectively, an asymmetric supercapacitor (ASC) device was assembled. The fabricated ASC provides an appropriate specific capacitance of 79.15 mF cm^-2 at the applied current density of 1 mA cm^-2 , and delivered an energy density of 0.143 Wh kg^-1 at the power density of 5.42 W kg^-1 .

Phosphorus-Doped Metal-Organic Framework-Derived CoS2 Nanoboxes with Improved Adsorption-Catalysis Effect for Li-S Batteries

Lithium-sulfur (Li-S) batteries are regarded as one of the most promising next-generation battery technologies owing to their ultrahigh energy density up to 2600 W h kg^-1 and low cost. However, major challenges still remain in the application of Li-S batteries, such as shuttle effect and sluggish redox kinetics. Herein, it is demonstrated that phosphorus doping can not only significantly improve the polysulfide adsorption but also enhance the catalysis effects of metal-organic framework-derived CoS₂ nanoboxes in Li-S batteries. Consequently, a modified separator integrated with P-CoS₂ and carbon nanotubes effectively suppresses the polysulfide shuttle and propels the redox kinetics of polysulfides, thus promising higher specific discharge capacity, better rate, and stable cycle performance. Even under the high sulfur loading condition (4.8 mg cm^-2), the areal discharge capacity of the cell with the functional separator can still remain at 4.5 mA h cm^-2 after 100 cycles at 0.2 C. More importantly, this work may encourage more effort on anion doping for engineering the polar surface of transition-metal compounds to further mediate the interfacial redox chemistry between transition-metal compounds and polysulfides in Li-S batteries.

Advanced materials and technologies for supercapacitors used in energy conversion and storage: a review

Supercapacitors are increasingly used for energy conversion and storage systems in sustainable nanotechnologies. Graphite is a conventional electrode utilized in Li-ion-based batteries, yet its specific capacitance of 372 mA h g−1 is not adequate for supercapacitor applications. Interest in supercapacitors is due to their high-energy capacity, storage for a shorter period and longer lifetime. This review compares the following materials used to fabricate supercapacitors: spinel ferrites, e.g., MFe2O4, MMoO4 and MCo2O4 where M denotes a transition metal ion; perovskite oxides; transition metals sulfides; carbon materials; and conducting polymers. The application window of perovskite can be controlled by cations in sublattice sites. Cations increase the specific capacitance because cations possess large orbital valence electrons which grow the oxygen vacancies. Electrodes made of transition metal sulfides, e.g., ZnCo2S4, display a high specific capacitance of 1269 F g−1, which is four times higher than those of transition metals oxides, e.g., Zn–Co ferrite, of 296 F g−1. This is explained by the low charge-transfer resistance and the high ion diffusion rate of transition metals sulfides. Composites made of magnetic oxides or transition metal sulfides with conducting polymers or carbon materials have the highest capacitance activity and cyclic stability. This is attributed to oxygen and sulfur active sites which foster electrolyte penetration during cycling, and, in turn, create new active sites.

Metal-organic Framework-driven Porous Cobalt Disulfide Nanoparticles Fabricated by Gaseous Sulfurization as Bifunctional Electrocatalysts for Overall Water Splitting

Both high activity and mass production potential are important for bifunctional electrocatalysts for overall water splitting. Catalytic activity enhancement was demonstrated through the formation of CoS2 nanoparticles with mono-phase and extremely porous structures. To fabricate porous structures at the nanometer scale, Co-based metal-organic frameworks (MOFs), namely a cobalt Prussian blue analogue (Co-PBA, Co3[Co(CN)6]2), was used as a porous template for the CoS2. Then, controlled sulfurization annealing converted the Co-PBA to mono-phase CoS2 nanoparticles with ~ 4 nm pores, resulting in a large surface area of 915.6 m2 g-1. The electrocatalysts had high activity for overall water splitting, and the overpotentials of the oxygen evolution reaction and hydrogen evolution reaction under the operating conditions were 298 mV and -196 mV, respectively, at 10 mA cm-2.