High-performance flexible supercapacitor enabled by Polypyrrole-coated NiCoP@CNT electrode for wearable devices

In-Situ Preparation of Iron(II)-phthalocyanine@multi-Walled-CNTs Nanocomposite for Quasi-Solid-State Flexible Symmetric Supercapacitors with Long Cycling Life

The construction of supercapacitor electrode materials with exceptional performance is crucial to the commercialisation of flexible supercapacitors. Here, a novel in-situ precipitation technique was applied for constructing iron(II)-phthalocyanine (FePc) based nanocomposite as the electrode material in quasi-solid-state flexible supercapacitors. The highly redox-active FePc nanostructures were grown in the multi-walled-CNTs (MWCNTs) networks, which shows convenient electron/electrolyte ion transport pathways along with outstanding structural stability, leading to high energy storage and long cycling life. The electrode of FePc@MWCNTs delivered a higher specific capacity than that of individual MWCNTs and FePc. The quasi-solid-state symmetric flexible device that was constructed using FePc@MWCNTs electrode demonstrated impressive performance with a maximum energy density of 29.7 Wh kg-1 and a maximum power density of 4000 W kg-1. Moreover, the device demonstrated superior durability and flexibility, as evidenced by its exceptional cyclic stability (111.3 %) even after 30000 cycles at 8 A g-1. These results reveal that the FePc@MWCNTs nanocomposite prepared by this simple in-situ precipitation method is promising as electrode material for next-generation flexible wearable power sources.

MOF-Derived Transition Metal Phosphides for Supercapacitors

Transition metal phosphides (TMPs) in supercapacitors (SCs) applications are increasingly attracting attention because of their exceptional electrochemical performance. MOF-derived TMPs, possess high specific surface areas, rich pore structure, and controllable chemical compositions, offering promising opportunities for supercapacitor applications. There is a wide variety of MOF-derived TMPs, and they exhibit different properties in SCs. This work mainly categorizes MOF-derived TMPs (FexP, CoxP, NixP, NixCoyP, CuxP), and then outlines the latest research advancements regarding their use as electrode materials in SCs, including the latest results of synthesis methods and structural modulation. Subsequently, the applications of MOF-derived TMPs as electrode materials in SCs are discussed. At the end, perspectives of future developments and key challenges in the applications of MOF-derived TMPs in SCs are highlighted, with the aim of providing guidance for future research.

Overview of recent developments in carbon-based nanocomposites for supercapacitor applications

Energy storage devices are recognized as environmentally friendly technologies. Supercapacitors, known for their high cycle stability, have been proposed as potential alternatives to fossil fuels. Recent studies have focused on selecting suitable electrode materials to achieve energy storage systems with high stability, high specific capacity, and biocompatibility. In particular, carbon-based electrode materials, such as graphene oxide, activated carbon, carbon nanotubes, and carbon-based quantum dots, have attracted considerable attention due to their intrinsic properties, such as high conductivity and stability. However, carbon materials alone exhibit limitations, such as low energy density and low specific capacitance. To address this limitation, the synergistic effect of carbon materials has been combined with other electroactive materials to develop electrode materials with enhanced supercapacitor properties. The present study also investigates the supercapacitor performance of carbon-based nanocomposites. It examines the effect of each carbon material (AC, CNT, GO, rGO) on improving the performance of other electroactive materials, including metal oxides, metal sulfides, MXenes, MOFs, and conductive polymers. This study provides valuable insights for further studies on carbon-based electrode materials for supercapacitor applications.

A FeCo-Se@NiCo-PO4 Electrode Designed by Hierarchical Strategy for Supercapacitors and NiCo//Bi Batteries

In this work, FeCo-Se and NiCo-PO4 were electrodeposited on nickel foam (NF) successively to prepare a cathode material for asymmetric supercapacitors (ASCs) and NiCo//Bi batteries. FeCo-Se@NiCo-PO4 combines the advantages of transition metal selenides (TMSs) and transition metal phosphates (TMPs). FeCo-Se electrodeposited in the underlying layer can facilitate electron transfer for higher conductivity. NiCo-PO4 in the outer layer can facilitate OH- ions diffusion because TMPs can be intercalated into ions readily and the outer robust P-O bond of TMPs can stabilize the structure. Precisely because the hierarchical structure maximizes the synergy between FeCo-Se and NiCo-PO4, FeCo-Se@NiCo-PO4 delivers a rapid electron/ion transfer capability and superior electrochemical performance. The FeCo-Se@NiCo-PO4 exhibits a high specific capacitance of 2221.5 F g-1 (888.6 C g-1) at 1 A g-1. Its aqueous ASC shows specific capacitance of 115.8 F g-1 at 1 A g-1 and all-solid-state ASC presents high reversibility. Its aqueous NiCo//Bi battery has superior durability of about 60% capacity retention and 98% Coulombic efficiency after 2300 cycles. And its all-solid-state NiCo//Bi battery possesses a higher energy density and power density.

Multi-layered heterogeneous interfaces created in Co0.85Se@Ni3S4/NF to enhance supercapacitor performances by multi-step alternating electrodeposition

Heterogeneous interface construction is of far-reaching significance to optimize the electrochemical performance of electrodes. Herein, a multi-step alternating electrochemical deposition (MAED) method is proposed to alternately deposit Co0.85Se and Ni3S4 nanosheets on a nickel foam (NF), forming a special alternate layer-by-layer structure with multi-layered heterogeneous interfaces. The creation of the multi-layered heterogeneous interfaces provides a large interfacial area for redox reactions with optimum interstitials facilitating ion diffusion, thus greatly improving the electrochemical energy storage efficiency. With the increase in the layer number, the material exhibits increasingly better energy storage performance, and 8L-Co0.85Se@Ni3S4/NF exhibits the highest specific capacitances of 2508 F g-1 and 1558 F g-1 at a scan rate of 2 mV s-1 and a current density of 1 A g-1. The 8L-Co0.85Se@Ni3S4/NF//polypyrrole (PPy)/NF asymmetric supercapacitor provides a maximum operation potential window of 1.55 V and energy densities of 76.98 and 35.74 W h kg-1 when the power densities are 775.0 and 15 500 W kg-1, respectively, superior to most of the related materials reported. Through MAED, the deposited phase and the layer number can be accurately controlled, thus providing an efficient strategy for interface construction so as to increase the electrochemical activity of the energy storage materials.

Promoted OH- adsorption and electron-transfer kinetics by electrospinning mono-disperse NiCo2S4 nanocrystals within porous CNFs for solid asymmetric supercapacitors

Bimetallic sulfide NiCo2S4 has been regarded as a potential supercapacitor electrode material with excellent electrochemical performance. However, the origin of its high specific capacity is little studied, and the design of a rational structure still remains a challenge to exert its intrinsic advantage. In this work, the advantage of NiCo2S4 over NiS and CoS is explained by density functional theory calculation from the aspects of energy band, density of electronic states and OH- adsorption energy. It is proved that the synergistic effect of Ni and Co in NiCo2S4 can reduce its OH- adsorption energy and provide more active electrons near the Fermi level, thus promoting electrochemical reaction kinetics in supercapacitors. Then, a simple electrospinning method is used to in-situ load mono-disperse NiCo2S4 nanocrystals within amorphous carbon nanofibers, obtaining a porous, lotus-leaf-stem-like one-dimensional nanocomposite of NiCo2S4/CNF. Ex-situ XPS characterization confirms that the proportion of metal ions involved in electrochemical reactions and the number of transferred electrons in NiCo2S4/CNF during the redox reaction are significantly higher than those in mono-metallic sulfides (NiS/CNF and CoS/CNF), verifying the calculation results. With its boosting reaction kinetics, the NiCo2S4/CNF gives the specific capacity of 757.97C g-1 at 1 A/g and the capacity retention of 95.15 % after 10,000 cycles at 5 A/g, both greater than NiS/CNF and CoS/CNF. The NiCo2S4/CNF, as the positive electrode, and activated carbon, as the negative electrode, are assembled into liquid-state and solid-state asymmetric supercapacitor (ASC) devices, and both show high power density (760.6 W kg-1 for liquid-state device and 1067.4 W kg-1 for solid-state device), high energy density (52.25 Wh kg-1 for liquid-state device and 48.54 Wh kg-1 for solid-state device) and great cycle stability. Moreover, the solid-state ASC device possesses excellent low temperature capacity and reversibility, further demonstrating the wide application potential of the NiCo2S4/CNF composite.

Porous prussian blue analogs derived nickel-iron bimetallic phosphide nanocubes on conductive hollow mesoporous carbon nanospheres for stable and flexible high-performance supercapacitor electrode

Nickel-iron bimetallic phosphide (Ni-Fe-P) is the ideal battery-type materials for supercapacitor in virtue of high theoretical specific capacitance. Nevertheless, its actual adhibition is astricted on account of inferior rate capability and cyclic stability. Herein, we constructed hierarchical core-shell nanocomposites with hollow mesoporous carbon nanospheres (HMCS) packaged via prussian blue analogs derived Ni-Fe-P nanocubes (Ni-Fe-P@HMCS), as a positive electrode for hybrid supercapacitor (HSC). Profiting from the cooperative effects of Ni-Fe-P nanocubes with small size and good dispersibility, and HMCS with continuously conductive network, the Ni-Fe-P@HMCS composite electrode with abundantly porous architectures presents an ultrahigh gravimetric specific capacity for 739.8 C g-1 under 1 A g-1. Specially, the Ni-Fe-P@HMCS electrode presents outstanding rate capability of 78.4% (1 A g-1 to 20 A g-1) and cyclic constancy for 105% after 5000 cycles. Density functional theory implies that the composite electrode possesses higher electrical conductivity than bare Ni-Fe-P electrode by reason of the incremental charge density, and the electrons transferring from NiFe3P4 to HMCS layers. Additionally, the assembled Ni-Fe-P@HMCS//HMCS HSC facility delivers the high energy density for 64.1 Wh kg-1, remarkable flexibility and mechanical stability. Thus, this work proffers a viable and efficacious measure to construct ultra-stability electrode for high-performance portable electronic facilities.

High energy density flexible Zn-ion hybrid supercapacitors with conductive cotton fabric constructed by rGO/CNT/PPy nanocomposite

Fiber-shaped energy-storage devices for high energy and power density are crucial to power wearable electronics. In this work, reduced graphene oxide/carbon nanotubes/polypyrrole (GCP-op) cotton fabric with the optimal performance is prepared via a facile and cost-effective dipping-drying together with chemical polymerization approach. The structural characterizations confirm that the GCP-op cotton fabric has been successfully attached with numerous nanoparticles and carbon nanotubes, which can serve as a channel for electronical transfer. And GCP-op cotton fabric electrode displays admirable areal specific capacitance with 8397 mF cm-2at 1 mA cm-2. By combining GCP-op cathode with zinc anode, a GCP-op//PAM/ZnCl2//Zn flexible Zn-ion hybrid supercapacitor (FZHSC) is produced with 2 M polyacrylamide/ZnCl2(PAM/ZnCl2) hydrogel as the gel electrolyte. The FZHSC has superior cycle stability of 88.2%, outstanding energy density of up to 158μWh cm-2and power density at 0.5 mW cm-2. The remarkable performance proves that PPy-based material can provide more options for design and fabricate high energy flexible Zn-ion hybrid supercapacitors.

A Membrane-Free Decoupled Water Electrolyzer Operating at Simulated Fluctuating Renewables with Tri-Functional NiCo-P Electrode

Water electrolysis has been considered a promising technology for the conversion of renewables to hydrogen. However, preventing mixing of products (H2 and O2 ) and exploring cost-efficient electrolysis components remains challenging for conventional water electrolyzers. Herein, we designed a membrane-free decoupled water electrolysis system by using graphite felt supported nickel-cobalt phosphate (GF@Nix Coy -P) material as a tri-functional (redox mediator, hydrogen evolution reaction (HER), oxygen evolution reaction (OER)) electrode. The versatile GF@Ni1 Co1 -P electrode obtained by a one-step electrodeposition not only exhibits high specific capacity (176 mAh g-1 at 0.5 A g-1 ) and long cycle life (80 % capacity retention after 3000 cycles) as a redox mediator, but also has relatively outstanding catalytic activities for HER and OER. The excellent properties of the GF@Nix Coy -P electrode endow this decoupled system with more flexibility for H2 production by fluctuating renewable energies. This work provides guidance for multifunctional applications of transition metal compounds between energy storage and electrocatalysis.

Anti-Shedding Nickel-Protection-Layer Boosting an Ultrahigh Loading Carbon Fiber@Co-NiSx Electrode to Deliver Superior Areal/Volumetric/Gravimetric Capacitance

Challenges remain to show good capacitive performance while achieving high loadings of active materials for supercapacitors. Trying to realize this version, a nickel-protecting carbon fiber paper@Co-doped NiSx (Ni-CP@Co-NiSx) electrode with high specific gravimetric, areal, and volumetric capacitance is reported in this work. This free-standing electrode is prepared by an electroplating-hydrothermal-electroplating (EHE) three-step method to achieve a high loading of almost 26.7 mg cm-2. The cobalt-doping and nickel-protection strategies effectively decrease the impedance and inhibit the active material dropping from the electrode resulting from the expansion stress, which endows the Ni-CP@Co-NiSx electrode with a high rate and good cycling performance, especially with an ultrahigh specific areal/volumetric/gravimetric capacitance of 53.3 F cm-2/2807 F cm-3/1997 F g-1 at 5 mA cm-2, respectively. Employing activated carbon functionalized with riboflavin (AC/VB2) as a negative electrode, the asymmetric supercapacitor device delivers a very high energy density of up to 60.4 W h kg-1. This work demonstrates that electrodes with a high loading density and excellent performance can be obtained by the combination of the EHE method to adjust the internal conductivity and external structural stability.

Applications of MXene-Containing Polypyrrole Nanocomposites in Electrochemical Energy Storage and Conversion

The atomically thick two-dimensional (2D) materials are at the forefront of revolutionary technologies for energy storage devices. Due to their fascinating physical and chemical features, these materials have gotten a lot of attention. They are particularly appealing for a wide range of applications, including electrochemical storage systems, due to their simplicity of property tuning. The MXene is a type of 2D material that is widely recognized for its exceptional electrochemical characteristics. The use of these materials in conjunction with conducting polymers, notably polypyrrole (PPy), has opened new possibilities for lightweight, flexible, and portable electrodes. Therefore, herein we report a comprehensive review of recent achievements in the production of MXene/PPy nanocomposites. The structural-property relationship of this class of nanocomposites was taken into consideration with an elaborate discussion of the various characterizations employed. As a result, this research gives a narrative explanation of how PPy interacts with distinct MXenes to produce desirable high-performance nanocomposites. The effects of MXene incorporation on the thermal, electrical, and electrochemical characteristics of the resultant nanocomposites were discussed. Finally, it is critically reviewed and presented as an advanced composite material in electrochemical storage devices, energy conversion, electrochemical sensors, and electromagnetic interference shielding.

Polypyrrole-coated Boron-doped Nickel-Cobalt sulfide on electrospinning carbon nanofibers for high performance asymmetric supercapacitors

Although nickel-cobalt bimetallic sulfides have been widely studied for supercapacitor electrodes, how to obtain high specific capacity and cycle stability is still an important challenge. Here, an efficient chemical redox method is used to adjust the crystal and electronic structure of cobalt-nickel sulfide (NCS) via B doping, combined with electrospinning technology and conductive polymer polypyrrole (PPy) coating to facilitate faraday redox reactions and obtain high energy density electrode materials. The resulting composite with boron-doped nickel-cobalt sulfide on electrospinned carbon nanofibers with polypyrrole-coating (PPy@B-NCS/CNF) has a high specific capacity (751.61C/g at 1 A/g) and good cycle stability (82.49 % retention after 4000 cycles at 5 A/g). With PPy@B-NCS/CNF as the positive electrode and activated carbon as the negative electrode, an asymmetric supercapacitor (ASC) is prepared. It has excellent electrochemical properties with a power density of 65.58 Wh kg^-1 and an energy density of 819.72 W kg^-1. The low-temperature performance test shows high reversibility, which provides the possibility for the development of low-temperature electrolytes. Finally, density functional theory (DFT) explains that B-doped NCS has better electrochemical properties from the energy band and state density.

Towards advanced aqueous zinc battery by exploiting synergistic effects between crystalline phosphide and amorphous phosphate

High-performance aqueous zinc batteries are expected to be realized, rooting from component synergistic effects of the hierarchical composite electrode materials. Herein, hierarchical crystalline Ni-Co phosphide coated with amorphous phosphate nanoarrays (C-NiCoP@A-NiCoPO₄) self-supporting on the Ni foam are constructed as cathode material of an aqueous zinc battery. In this unique core-shell structure, the hexagonal phosphide with high conductivity offers ultra-fast electronic transmission and amorphous phosphate with high stability, and open-framework provides more favorable ion diffusivity and a stable protective barrier. The synergistic effects of this intriguing core-shell structure endow the electrode material with outstanding reaction kinetics and structural stability, which is theoretically confirmed by density functional theory (DFT) calculations. As a result, the C-NiCoP@A-NiCoPO₄ electrode exhibits a higher specific capacity of 350.6 mA h g^-1 and excellent cyclic stability with 92.6% retention after 10 000 cycles. Moreover, the C-NiCoP@A-NiCoPO₄ is coupled with Zn anode to assemble an aqueous pouch battery that delivers ultra-high energy density (626.33 W h kg^-1 at 1.72 kW kg^-1) with extraordinary rate performance (452.05 W h kg^-1 at 33.56 kW kg^-1). Moreover, the corresponding quasi-solid flexible battery with polyacrylamide hydrogel electrolyte exhibits favorable durability under frequent mechanical strains, which indicates the great promise of crystalline/amorphous hierarchical electrodes in the field of energy storage.