Electrolytes have been identified as some of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) is discussed. The challenges in producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim of improving ESs' energy density without sacrificing existing advantages (e.g., a high power density and a long cycle-life) (507 references).

A review of electrolyte materials and compositions for electrochemical supercapacitors

Electrolytes are one of the vital constituents of electrochemical energy storage devices and their physical and chemical properties play an important role in these devices' performance, including capacity, power density, rate performance, cyclability and safety. This article reviews the current state of understanding of the electrode–electrolyte interaction in supercapacitors and battery–supercapacitor hybrid devices. The article discusses factors that affect the overall performance of the devices such as the ionic conductivity, mobility, diffusion coefficient, radius of bare and hydrated spheres, ion solvation, viscosity, dielectric constant, electrochemical stability, thermal stability and dispersion interaction. The requirements needed to design better electrolytes and the challenges that still need to be addressed for building better supercapacitive devices for the competitive energy storage market have also been highlighted.

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Electrolyte selection for supercapacitive devices: a critical review

In this paper, we wish to present an overview of the research carried out in our laboratories with low-cost transition metal oxides (manganese dioxide, iron oxide and vanadium oxide) as active electrode materials for aqueous electrochemical supercapacitors. More specifically, the paper focuses on the approaches that have been used to increase the capacitance of the metal oxides and the cell voltage of the supercapacitor. It is shown that the cell voltage of an electrochemical supercapacitor can be increased significantly with the use of hybrid systems. The most relevant associations are Fe3O4 or activated carbon as the negative electrode and MnO2 as the positive. The cell voltage of the Fe3O4/MnO2 device is 1.8 V and this value was increased to 2.2 V by using activated carbon instead of Fe3O4. These two systems have shown superior behavior compared to a symmetric MnO2/MnO2 device which only works within a 1 V potential window in aqueous K2SO4. Furthermore, the activated carbon/MnO2 hybrid device exhibits a real power density of 605 W/kg (maximum power density =19.0 kW/kg) with an energy density of 17.3 Wh/kg. These values compete well with those of standard electrochemical double layer capacitors working in organic electrolytes.

http://ma.ecsdl.org/content/MA2005-01/35/1348.full.pdf

Nanostructured Transition Metal Oxides for Aqueous Hybrid Electrochemical Supercapacitors

Advanced electrical energy storage technology is a game changer for a clean, sustainable, and secure energy future because efficient utilization of newable energy hinges on cost-effect and efficient energy storage. Further, the viability of many emerging technologies depends on breakthroughs in energy storage technologies, including electric vehicles (EVs) or hybrid electric vehicles (HEVs) and smart grids. Lithium-ion batteries (LIBs), a great success in the portable electronics sector, are believed also the most promising power sources for emerging technologies such as EVs and smart grids. To date, however, the existing LIBs (with LiCoOx cathode and graphite anode) are still unable to meet the strict requirements for safety, cycling stability, and rate capability. The development of advanced anode materials, which can overcome the shortcomings of graphite anode (such as formation of dendritic lithium during charge and undesirable solid electrolyte interface), is of critical importance to enhancing the cycling stability and operational safety of LIBs. Lithium titanate (Li4Ti5O12) has recently attracted considerable attentions as a potential anode material of LIBs for high power applications due to several outstanding features, including a flat charge/discharge plateaus (around 1.55 V vs. Li/Li+) because of the two-phase lithium insertion/extraction mechanism and minimum chance for the formation of SEI and dendritic lithium, dramatically enhance the potential for high rate capability and safety. In addition, there is almost no volume change during the lithium insertion and extraction processes, ensuring a high cycling stability and long operational life. However, the electronic conductivity of Li4Ti5O12 is relatively low, resulting in large polarization lose, more so at higher cycling rates, and poor rate performance. Currently, considerable research efforts have been devoted to improving the performance of Li4Ti5O12 at fast charge/discharge rates, and some important progresses have been made. In this review, we first present a general overview of the structural features, thermodynamic properties, transport properties, and the electrochemical behavior of Li4Ti5O12 under typical battery operating conditions. We then provide a comprehensive review of the recent advancements made in characterization, modification, and applications of Li4Ti5O12 electrodes to LIBs, including nanostructuring, surface coating, morphological optimization, doping, and rational design of composite electrodes. Finally, we highlight the critical challenges facing us today and future perspectives for further development of Li4Ti5O12-based electrodes. It is hoped that this review may provide some useful guidelines for rational design of better electrodes for advanced LIBs.

A comprehensive review of Li4Ti5O12-based electrodes for lithium-ion batteries: The latest advancements and future perspectives

Photocatalytic water splitting using sunlight is a promising technology capable of providing high energy yield without pollutant byproducts. Herein, we review various aspects of this technology including chemical reactions, physiochemical conditions and photocatalyst types such as metal oxides, sulfides, nitrides, nanocomposites, and doped materials followed by recent advances in computational modeling of photoactive materials. As the best-known catalyst for photocatalytic hydrogen and oxygen evolution, TiO2 is discussed in a separate section, along with its challenges such as the wide band gap, large overpotential for hydrogen evolution, and rapid recombination of produced electron-hole pairs. Various approaches are addressed to overcome these shortcomings, such as doping with different elements, heterojunction catalysts, noble metal deposition, and surface modification. Development of a photocatalytic corrosion resistant, visible light absorbing, defect-tuned material with small particle size is the key to complete the sunlight to hydrogen cycle efficiently. Computational studies have opened new avenues to understand and predict the electronic density of states and band structure of advanced materials and could pave the way for the rational design of efficient photocatalysts for water splitting. Future directions are focused on developing innovative junction architectures, novel synthesis methods and optimizing the existing active materials to enhance charge transfer, visible light absorption, reducing the gas evolution overpotential and maintaining chemical and physical stability.

https://www.mdpi.com/1420-3049/21/7/900/pdf

Photocatalytic Water Splitting—The Untamed Dream: A Review of Recent Advances

Synthesis of Bi2WO6 nanoparticles and its electrochemical properties in different electrolytes for pseudocapacitor electrodes

The electrical and dielectric properties of FeVO 4 nanoparticles were studied at different temperatures from ambient to 200 °C. The samples were prepared by simple co-precipitation method using ferric nitrate and ammonium metavanadate as the starting precursors. The powder X-ray diffraction pattern inferred the single phase formation and triclinic structure of FeVO 4 . The morphology of the particles was elucidated from SEM studies. Detailed studies on the electrical and dielectric properties of the compound were carried out by using solid state impedance spectroscopy. A maximum dc conductivity of 4.65×10 −5  S cm −1 was observed at the measuring temperature of 200 °C. The calculated activation energy from dc conductivity was found to be 0.28 eV. It was evident that the electrical transport process in the system was due to the hopping mechanism. The detailed dielectric studies were also carried out.

Synthesis, electrical and dielectric properties of FeVO4 nanoparticles

Effect of pH on the sonochemical synthesis of BiPO4 nanostructures and its electrochemical properties for pseudocapacitors

Sub-micron sized polyhedral shaped NiMn2O4 particles were successfully prepared by a glycine assisted solution combustion method. The phase purity and the presence of functional groups in NiMn2O4 were revealed through X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR), respectively. The formation of polyhedral shaped particles was inferred by field emission scanning electron microscopy (FE-SEM). The negative temperature coefficient of resistance (NTCR) behaviour of NiMn2O4 was observed using a solid state impedance analyser in the measured temperature range between 30 and 180 °C. Further, electrochemical studies revealed that NiMn2O4 stores the charge through intercalation rather than by a capacitive mechanism. The electrode stores 91% of the specific capacitance by intercalation and 9% by a capacitive mechanism. Also, NiMn2O4 possesses a specific capacitance of 202 F g−1 at 0.5 mA cm−2 in 1 M Na2SO4 electrolyte and exhibits excellent cyclic stability over 15 000 cycles. Similarly, the fabricated asymmetric device (FeVO4‖NiMn2O4) also delivers good specific capacitance (50 F g−1 at 1 mV s−1) and cyclic stability.

V.D. Nithya

Papers

Synthesis of Bi2WO6 nanoparticles and its electrochemical properties in different electrolytes for pseudocapacitor electrodes

Synthesis, electrical and dielectric properties of FeVO4 nanoparticles

Effect of pH on the sonochemical synthesis of BiPO4 nanostructures and its electrochemical properties for pseudocapacitors

Studies on the electrochemical intercalation/de-intercalation mechanism of NiMn2O4 for high stable pseudocapacitor electrodes

Investigations on the temperature dependent electrical and magnetic properties of NiTiO3 by molten salt synthesis