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Transition-metal complexes, with their reversible redox properties, are the basis for electrochemical energy storage devices, such as rechargeable batteries and supercapacitors. In order to comprehend the variation in the electronic properties and electrochemical activity of concurrent transition elements existing in identical coordination environments, we have pursued extensive multi-method spectroscopic and electrochemical studies of three new isostructural 3d–4d mixed metal polyoxometalate complexes, viz. the vanado-molybdate Na2(NH4)5[(Mo VI2 O5)2(VIIIO2){O3P-C(O)(CH2-4-C5NH4)-PO3}2]·10H2O (1a), the chromo-molybdate (NH4)5[H2(Mo VI2 O5)2(CrIIIO2){O3P-C(O)(CH2-4-C5NH4)-PO3}2]·10H2O (2a) and the mangano-molybdate Na(NH4)6[H(Mo VI2 O5)2(MnIIIO2){O3P-C(O)(CH2-4-C5NH4)-PO3}2]·9H2O (3a). The structures of the polyanions have been determined using single-crystal X-ray diffraction studies, with all three polyanionic complexes having identical connectivity and arrangement in the solid state. The electronic properties of the complexes have been analyzed thoroughly with electronic paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) methods. EPR measurements for all three complexes are characterized by Δms = ±2 signals (forbidden half-field transition) at 77 K in the solid state, confirming the existence of VIII, S = 1 for 1a; CrIII, S = 3/2 for 2a; MnIII, S = 2 for 3a. Using our original and novel binder-free approach to prepare supercapacitor electrodes, thorough comparative electrochemical energy storage studies have been performed on the three compounds using cyclic voltammetry, galvanostatic charge–discharge, electrochemical impedance spectroscopy, and cycling stability tests. The oxo-vanado-molybdate complex has shown superior electrochemical performance, with respect to specific capacitances, energy density, power density, and electrochemical stability, followed by the oxo-chromo-molybdate and the oxo-mangano-molybdate complexes. The electrochemical performance trend has been corroborated by DFT calculations at the B3LYP-D3(BJ)/Def2-SVP level of theory in a polar medium, with the same order of promptness of undergoing oxidation of the polyanionic species [M2+ to M3+; M = V (1′) > Cr (2′) > Mn (3′); ΔG(M3+–M2+) = +48 (1′), +58 (2′), +81 (3′) kcal mol−1], corroborating the experimental observations. The oxidation of these anionic species from 8-(MII) to 7-(MIII) has thus been computed to be highly favourable in the gas phase [∼−328 (1′), −311 (2′), and −309 (3′) kcal mol−1]. The studies on the HOMO–LUMO structures of the complexes also corroborate the trend of the electrochemical behavior. [ABSTRACT FROM AUTHOR]

To date, bimetallic phosphates have not been much explored in supercapacitor applications. Interestingly, these materials hold significant potential for energy storage due to the existence of multiple oxidation states and the presence of numerous phosphorus atoms, each contributing to a substantial number of electron clouds. Present research emphasises the electrochemical performance of composite electrodes consisting of multiwalled carbon nanotubes (MWCNTs) and cobalt iron phosphate (Co3Fe4(PO4)6). These electrodes have demonstrated exceptional performance and achieved a specific capacity of 3320 C g−1 (3688 F g−1) at a scan rate of 5 mV s−1 maintaining 87% stability even after 5000 cyclic voltammetry (CV) cycles. The electrochemical active surface area (ECSA) was estimated to be 700 cm2 for Co3Fe4(PO4)6 and 1625 cm2 for MWCNTs/Co3Fe4(PO4)6 composites. Additionally, the designed and tested large-area (10 × 4 cm2) liquid-configured symmetric device exhibited an impressive energy density (ED) of 52.3 W h kg−1 and power density (PD) of 3.5 kW kg−1. The fabricated device showed outstanding stability with a 98% capacity retention after 5000 cycles, reflecting the remarkable durability of the designed system. To illustrate its real-world applicability, the constructed device underwent a 10 s charging to power a DC fan for 110 s. [ABSTRACT FROM AUTHOR]

Increasing demand for microelectronic devices necessitates the development of highly flexible energy storage technologies with a wide operating voltage. Thus, flexible electrodes and their devices with the requisite mechanical and electrochemical characteristics have prime importance. In this regard, the present article demonstrates the feasibility of designing a flexible all-solid-state supercapacitor using a chemically grown Ni3P2O8 lamellar microstructured electrode embedded with carboxy methyl cellulose-Na2SO4 (CMC-Na2SO4) gel electrolyte. The formed symmetric device impressively exhibited a maximum working voltage window of 1.8 V with a high specific energy of 44.7 W h kg−1 and specific power of 3.3 kW kg−1 along with prolonged cycle life. Also, the device's high deformation tolerance (95%) when bent at 170° with a flashing light-emitting diode (LED) working demonstration showcases its viability for advanced flexible energy storage applications. [ABSTRACT FROM AUTHOR]

World-wide increased energy consumption and demand has made the need for high-energy storage systems with longer life critical. Hence, extensive research and development on supercapacitors based on efficient and robust electrode materials are being carried out. Many materials based on metal oxides, polymers, carbon-based materials, metal chalcogenides, and their composites have been explored to reach saturation to obtain the milestone capacitance value and hence, energy density. Recently, metal phosphides (MPs) have arisen as a unique class of potential candidates for high performance supercapacitors owing to their inherent semi-metallic nature and high electrical conductivity. Aid of fascinating properties, MPs has driven to reach demarcation outlined by champion metal oxides based electrodes for supercapacitor applications. We systematically reviewed the phosphides family including the phosphides of nickel, cobalt, iron, and copper to give a clear understanding in terms of their versatile applications in supercapacitors. Aside from mono-metal phosphides, an extensive analysis of the literature about multi-metal phosphides with combined benefits from two or more metal species has been explored. To further exploit their interesting properties, MPs have been integrated with other functionally active materials to form binary composites or even ternary composites. The present review article concentrates on modern research findings in the synthetic routes of nanostructured MPs in correlation with the structures, morphologies, and their electrochemical energy storage performances. Subsequently, the importance of strong synergistic effects existing in MP-based nanocomposites is highlighted along with some representative works on the fabrication of symmetric/asymmetric supercapacitor devices. Finally, the current challenges and future opportunities in real energy technology device applications of MPs are proposed. [ABSTRACT FROM AUTHOR]

Simple, cost-effective, and room-temperature electroless reduction process has been advanced to anchor silver nanoparticles over multi-walled carbon nanotubes (MWCNTs). The impact of conductive MWCNT/Ag films with respect to structural, morphological, and electrochemical assets has been investigated. The highly stable and large surface area-aligned MWCNTs combined with conductive silver nanoparticles boost the electrochemical performance of the composite electrode. Remarkable specific capacitance of 757 F g−1 along with a significant cyclic stability of 83% over 3000 cycles has been achieved due to the strong synergistic effect between MWCNTs and silver nanoparticles. Moreover, the electrode exhibits a maximum specific energy and power of 60.7 W h kg−1 and 3.3 kW kg−1, respectively. This superior finding obviously offers future pathways for development in the energy storage field. [ABSTRACT FROM AUTHOR]

Hierarchical one-dimensional CdS nanowires (NWs)/CdSe core–shell heterostructure have been synthesized through a simple wet chemical approach. The formation of CdS NWs/CdSe core–shell has been confirmed by structural, morphological and optical analysis. Fascinatingly, an optimum number of CdSe nanoparticles loaded over CdS NWs boosts the device efficiency five times than a bare CdS NW structure. The effect of increasing the CdSe SILAR cycles over CdS induces red shift in the absorption wavelength along with quenching, which leads to the negative shift of the conduction band, and hence, increases the electron lifetime, as analyzed by photoluminescence, external quantum efficiency and Mott–Schottky studies. Interestingly, a discrepancy has been found in the device performance with an increase in the SILAR cycles beyond the optimum condition. The reason behind this fact has been investigated through different comprehensive photoelectrochemical characterization and discussed herein. [ABSTRACT FROM AUTHOR]

An easy ion-exchange route has been utilized to replace the toxic cadmium ions with silver in cadmium sulfide nanowires to obtain silver sulfide nanowires as the end product, since direct deposition of such novel morphology of silver sulfide has been a great challenge. This work is based on the concept of negative free energy of formation. In course of this process, CdS, CdS/Ag2S core–shell heterostructures and Ag2S nanowires as the end product were obtained, and has been employed as supercapacitor electrodes where completely converted Ag2S nanowires showed a highest capacitance of 268.4 F g−1 at 1.5 mA cm−2 current density in 1.5 M NaOH electrolyte. This simple approach promises a new electrode engineering technique just by controlling the reaction stage in the cationic source where a less toxic end product can be obtained with enhanced performance. [ABSTRACT FROM AUTHOR]

An attempt has been made to synthesize multiwall carbon nanotubes (MWCNTs), Fe2O3, and Fe2O3/MWCNTs through a simple and cost-effective route onto a stainless steel (SS) substrate. The ‘dip and dry’ coating method was used for MWCNTs, whereas the successive ionic layer adsorption and reaction (SILAR) method was used for Fe2O3. Thin films were characterized using structural and surface morphological studies by XRD, FESEM and TEM. The results showed that hematite Fe2O3 nanoparticles with a particle size of less than 10 nm were uniformly coated onto the surface of MWCNTs. Comparative investigations have been made for MWCNTs, Fe2O3, and Fe2O3/MWCNTs as supercapacitive electrode materials with the aid of scan rate, cyclic voltammetry, charge–discharge studies, energy density, power density and stability. Among the three thin films, the Fe2O3/MWCNTs hybrid thin film electrode exhibited a high specific capacitance of 431 F g−1 at a scan rate of 5 mV s−1 and a high energy density of 38 W h kg−1 with a power density of 800 W kg−1 in 1 M Na2SO3 electrolyte solution. The detailed analyses are reported herein. [ABSTRACT FROM AUTHOR]

Cadmium hydroxide [Cd(OH)2] nanowires have been successfully synthesized by a simple chemical bath deposition method (CBD) on a stainless-steel (SS) substrate. By scanning electron microscopy (SEM), the surface architecture displays the formation of bundles of high-surface-area nanowires (NWs), which are beneficial for supercapacitor applications. Cd(OH)2 NWs on SS has been tested as an electrode material for supercapacitor applications via electrochemical studies by cyclic voltammetry, charge–discharge and electrochemical impedance spectroscopy techniques in an aqueous electrolyte. Electrochemical data confirm that Cd(OH)2 NWs thin-film electrode exhibits supercapacitor behavior, having a yield of 267 F g−1 specific capacitance at 5 mV s−1 scan rate with excellent cycling life (86% capacitance retention over 1000 cycles). A Cd(OH)2/Cd(OH)2 symmetric supercapacitor device provides a maximum energy density of 11.09 W h kg−1 and a power density of 799 W kg−1 at a current density of 0.84 A g−1. The present work demonstrates that Cd(OH)2 NWs thin film is a promising, low-cost alternative material prepared by a simple, binder-free chemical bath deposition method (CBD) for supercapacitor applications. [ABSTRACT FROM AUTHOR]

This paper reports the novel synthesis of uranyl sulphide (UO2S) thin films using the successive ionic layer adsorption and reaction (SILAR) technique at room temperature. Cationic exchange reaction was used to convert uranyl sulphide (UO2S) to silver sulphide (Ag2S). The influence of concentration variation on the structural and optical properties of UO2S and Ag2S thin films was investigated. The structural, surface morphological, elemental analysis and optical absorption studies were performed. Structural studies revealed that all the deposited films were nano-sized and amorphous in nature. Surface morphology showed that all the grains were spherical and granular in nature and grains got conglomerated to form a large particle. Also, the variations of the optical band gap and the width of the tail of localized states were represented as a function of various parameters. [ABSTRACT FROM AUTHOR]

Increasing demand for microelectronic devices necessitates the development of highly flexible energy storage technologies with a wide operating voltage. Thus, flexible electrodes and their devices with the requisite mechanical and electrochemical characteristics have prime importance. In this regard, the present article demonstrates the feasibility of designing a flexible all-solid-state supercapacitor using a chemically grown Ni 3 P 2 O 8 lamellar microstructured electrode embedded with carboxy methyl cellulose-Na 2 SO 4 (CMC-Na 2 SO 4 ) gel electrolyte. The formed symmetric device impressively exhibited a maximum working voltage window of 1.8 V with a high specific energy of 44.7 W h kg -1 and specific power of 3.3 kW kg -1 along with prolonged cycle life. Also, the device's high deformation tolerance (95%) when bent at 170° with a flashing light-emitting diode (LED) working demonstration showcases its viability for advanced flexible energy storage applications.

Next-generation portable flexible electronic appliances require liquid-free energy storage supercapacitor devices to eliminate leakage and to support mechanical bending that is compatible with roll-to-roll technologies. Hence, a state-of-the-art process is presented to design a solid-state, wide-potential and flexible supercapacitor through the use of nano-pebbles of cerium selenide via a simple successive ionic layer adsorption and reaction (SILAR) method that could allow an industry scalable route. We strongly believe that this is the first approach amongst physical and chemical routes not only for synthesizing cerium selenide in thin-film form but also using it for device-grade supercapacitor applications. The designed solid-state symmetric supercapacitor assembled from cerium selenide electrodes sandwiched by PVA-LiClO 4 gel electrolyte attains a wide potential window of 1.8 V with capacitance of 48.8 F g -1 at 2 mV s -1 and reveals excellent power density of 4.89 kW kg -1 at an energy density of 11.63 W h kg -1 . The formed device is capable of 87% capacitive retention even at a mechanical bending angle of 175°. Lighting up a strip of 21 parallel connected red LEDs clearly demonstrates the practical use of the designed symmetric solid-state supercapacitor, aiming towards the commercialization of the product in the future., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)