Electrochemical capacitors, also called supercapacitors, store energy using either ion adsorption (electrochemical double layer capacitors) or fast surface redox reactions (pseudo-capacitors). They can complement or replace batteries in electrical energy storage and harvesting applications, when high power delivery or uptake is needed. A notable improvement in performance has been achieved through recent advances in understanding charge storage mechanisms and the development of advanced nanostructured materials. The discovery that ion desolvation occurs in pores smaller than the solvated ions has led to higher capacitance for electrochemical double layer capacitors using carbon electrodes with subnanometre pores, and opened the door to designing high-energy density devices using a variety of electrolytes. Combination of pseudo-capacitive nanomaterials, including oxides, nitrides and polymers, with the latest generation of nanostructured lithium electrodes has brought the energy density of electrochemical capacitors closer to that of batteries. The use of carbon nanotubes has further advanced micro-electrochemical capacitors, enabling flexible and adaptable devices to be made. Mathematical modelling and simulation will be the key to success in designing tomorrow's high-energy and high-power devices.

https://hal.archives-ouvertes.fr/hal-02417326/document

Materials for electrochemical capacitors

In this critical review, metal oxides-based materials for electrochemical supercapacitor (ES) electrodes are reviewed in detail together with a brief review of carbon materials and conducting polymers. Their advantages, disadvantages, and performance in ES electrodes are discussed through extensive analysis of the literature, and new trends in material development are also reviewed. Two important future research directions are indicated and summarized, based on results published in the literature: the development of composite and nanostructured ES materials to overcome the major challenge posed by the low energy density of ES (476 references).

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A review of electrode materials for electrochemical supercapacitors

By use of the membrane-templated synthesis route, hydrous RuO2 (RuO2·xH2O) nanotubular arrayed electrodes were successfully synthesized by means of the anodic deposition technique. The desired three-dimensional mesoporous architecture of RuO2·xH2O nanotubular arrayed electrodes with annealing in air at 200 °C for 2 h simultaneously maintained the facility of electrolyte penetration, the ease of proton exchange/diffusion, and the metallic conductivity of crystalline RuO2, exhibiting unexpectedly ultrahigh power characteristics with its frequency “knee” reaching ca. 4.0−7.8 kHz, 20−40 times better than that of RuO2 single crystalline, arrayed nanorods. The specific power and specific energy of annealed RuO2·xH2O nanotubes measured at 0.8 V and 4 kHz is equal to 4320 kW kg-1 and 7.5 W h kg-1, respectively, demonstrating the characteristics of next generation supercapacitors.

Design and Tailoring of the Nanotubular Arrayed Architecture of Hydrous RuO2 for Next Generation Supercapacitors

The performance of a supercapacitor can be characterized by a series of key parameters, including the cell capacitance, operating voltage, equivalent series resistance, power density, energy density, and time constant. To accurately measure these parameters, a variety of methods have been proposed and are used in academia and industry. As a result, some confusion has been caused due to the inconsistencies between different evaluation methods and practices. Such confusion hinders effective communication of new research findings, and creates a hurdle in transferring novel supercapacitor technologies from research labs to commercial applications. Based on public sources, this article is an attempt to inventory, critique and hopefully streamline the commonly used instruments, key performance metrics, calculation methods, and major affecting factors for supercapacitor performance evaluation. Thereafter the primary sources of inconsistencies are identified and possible solutions are suggested, with emphasis on device performance vs. material properties and the rate dependency of supercapacitors. We hope, by using reliable, intrinsic, and comparable parameters produced, the existing inconsistencies and confusion can be largely eliminated so as to facilitate further progress in the field.

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Supercapacitors performance evaluation

2D transition metal carbides and nitrides, known as MXenes, are an emerging class of 2D materials with a wide spectrum of potential applications, in particular in electrochemical energy storage. The hydrophilicity of MXenes combined with their metallic conductivity and surface redox reactions is the key for high-rate pseudocapacitive energy storage in MXene electrodes. However, symmetric MXene supercapacitors have a limited voltage window of around 0.6 V due to possible oxidation at high anodic potentials. In this study, the fact that titanium carbide MXene (Ti3C2Tx) can operate at negative potentials in acidic electrolyte is exploited, to design an all-pseudocapacitive asymmetric device by combining it with a ruthenium oxide (RuO2) positive electrode. This asymmetric device operates at a voltage window of 1.5 V, which is about two times wider than the operating voltage window of symmetric MXene supercapacitors, and is the widest voltage window reported to date for MXene-based supercapacitors. The complementary working potential windows of MXene and RuO2, along with proton-induced pseudocapacitance, significantly enhance the device performance. As a result, the asymmetric devices can deliver an energy density of 37 µW h cm−2 at a power density of 40 mW cm−2, with 86% capacitance retention after 20 000 charge–discharge cycles. These results show that pseudocapacitive negative MXene electrodes can potentially replace carbon-based materials in asymmetric electrochemical capacitors, leading to an increased energy density.

Tongchang Liu

Papers

Self-discharge and potential recovery phenomena at thermally and electrochemically prepared RuO2 supercapacitor electrodes