This tutorial review provides a brief summary of recent research progress on carbon-based electrode materials for supercapacitors, as well as the importance of electrolytes in the development of ...supercapacitor technology. The basic principles of supercapacitors, the characteristics and performances of various nanostructured carbon-based electrode materials are discussed. Aqueous and non-aqueous electrolyte solutions used in supercapacitors are compared. The trend on future development of high-power and high-energy supercapacitors is analyzed.
Electric double-layer capacitors are a family of electrochemical energy storage devices that offer a number of advantages, such as high power density and long cyclability. In recent years, research ...and development of electric double-layer capacitor technology has been growing rapidly, in response to the increasing demand for energy storage devices from emerging industries, such as hybrid and electric vehicles, renewable energy, and smart grid management. The past few years have witnessed a number of significant research breakthroughs in terms of novel electrodes, new electrolytes, and fabrication of devices, thanks to the discovery of innovative materials (
e.g.
graphene, carbide-derived carbon, and templated carbon) and the availability of advanced experimental and computational tools. However, some experimental observations could not be clearly understood and interpreted due to limitations of traditional theories, some of which were developed more than one hundred years ago. This has led to significant research efforts in computational simulation and modelling, aimed at developing new theories, or improving the existing ones to help interpret experimental results. This review article provides a summary of research progress in molecular modelling of the physical phenomena taking place in electric double-layer capacitors. An introduction to electric double-layer capacitors and their applications, alongside a brief description of electric double layer theories, is presented first. Second, molecular modelling of ion behaviours of various electrolytes interacting with electrodes under different conditions is reviewed. Finally, key conclusions and outlooks are given. Simulations on comparing electric double-layer structure at planar and porous electrode surfaces under equilibrium conditions have revealed significant structural differences between the two electrode types, and porous electrodes have been shown to store charge more efficiently. Accurate electrolyte and electrode models which account for polarisation effects are critical for future simulations which will consider more complex electrode geometries, particularly for the study of dynamics of electrolyte transport, where the exclusion of electrode polarisation leads to significant artefacts.
A comprehensive review of molecular simulations of electric double-layer capacitors using various electrolyte types at porous and non-porous electrodes.
Supercapacitors, which are attracting rapidly growing interest from both academia and industry, are important energy‐storage devices for acquiring sustainable energy. Recent years have seen a number ...of significant breakthroughs in the research and development of supercapacitors. The emergence of innovative electrode materials (e.g., graphene) has clearly provided great opportunities for advancing the science in the field of electrochemical energy storage. Conversely, smart configurations of electrode materials and new designs of supercapacitor devices have, in many cases, boosted the electrochemical performance of the materials. We attempt to summarize recent research progress towards the design and configuration of electrode materials to maximize supercapacitor performance in terms of energy density, power density, and cycle stability. With a brief description of the structure, energy‐storage mechanism, and electrode configuration of supercapacitor devices, the design and configuration of symmetric supercapacitors are discussed, followed by that of asymmetric and hybrid supercapacitors. Emphasis is placed on the rational design and configuration of supercapacitor electrodes to maximize the electrochemical performance of the device.
Charged and ready to go: In the past few years, significant breakthroughs in the development of supercapacitors as energy‐storage devices is promoted by the emergence of innovative electrode materials (e.g., graphene) and driven by rapidly increasing demands for high‐performance energy‐storage devices (see picture; ASC/SSC=asymmetric/symmetric supercapacitor.
In this work, conducting polymers poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline (PANi), and polypyrrole (PPy) were directly coated on the surface of reduced graphene oxide (RGO) sheets via ...an in situ polymerization process to prepare conducting-polymer-RGO nanocomposites with different loadings of the conducting polymers. Experiment results showed that ethanol played an important role in achieving a uniform coating of the polymers on RGO sheets. The electrochemical capacitive properties of the composite materials were investigated by using cycle voltammetry and charge/discharge techniques. The composite consisting of RGO and PANi (RGO-PANi) exhibited a specific capacitance of 361 F/g at a current density of 0.3 A/g. The composites consisting of RGO and PPy (RGO-PPy) and PEDOT (RGO-PEDOT) displayed specific capacitances of 248 and 108 F/g, respectively, at the same current density. More than 80% of initial capacitance retained after 1000 charge/discharge cycles, suggesting a good cycling stability of the composite electrodes. The good capacitive performance of the conducting–polymer-RGO composites is contributed to the synergic effect of the two components.
The oxygen reduction reaction (ORR) is one of the most important electrochemical reactions in energy conversion and storage technologies, such as fuel cells and metal-air batteries. However, the ...sluggish kinetics of the ORR is a key factor limiting the performance of these energy storage and conversion devices. Perovskite oxides are a promising family of electrocatalysts for the ORR because of their unique physical and chemical properties, such as variable crystal structure and non-stoichiometric chemistry. Studies have shown that the catalytic properties of perovskite oxides in the ORR are largely related to oxygen vacancies, which alter their electronic and crystal structures and surface chemistry. This review summarizes recent research advances on understanding the role of oxygen vacancies of the ABO
3
perovskite oxides in catalyzing the ORR. With a brief introduction of perovskite oxides, approaches to creating oxygen vacancies in the ABO
3
perovskite oxides and the role of oxygen vacancies in improving their catalytic performance for the ORR are discussed. Research perspectives in this important area are highlighted.
The oxygen reduction reaction (ORR) is one of the most important electrochemical reactions in energy conversion and storage technologies, such as fuel cells and metal-air batteries.
Chemically derived graphene holds great promise as an electrode material for electrochemical energy storage owing to its unique physical and chemical properties. Recent years have witnessed ...tremendous research breakthroughs in the field of graphene-based materials for electrochemical capacitors. This article presents a review of the latest developments in the functionalization of chemically derived graphene for improving its electrocapacitive properties. Beginning with a brief description of supercapacitors, graphene, and chemically derived graphene, we discuss the preparation, electrocapacitive properties, and drawbacks of chemically derived graphene and its derivatives, followed by a discussion on how to functionalize chemically derived graphene for improving its double-layer capacitance and pseudocapacitance. Emphasis is made on comparing and highlighting demonstrated approaches to functionalizing chemically derived graphene. Future research towards developing advanced electrochemical capacitors, perspectives and challenges are outlined.
This review summarizes the latest developments in the functionalization of chemically derived graphene for improving its electrocapacitive performance.
We report a facile and green route to reduce graphite oxide using urea as the reducing agent. UV-Vis, XRD and XPS results revealed that urea was an effective reducing agent in removing ...oxygen-containing groups from graphite oxide for restoring the conjugated electronic structure of graphene. The obtained reduced graphene oxide was tested as supercapacitor electrode. Gravimetric capacitances of 255 and 100 F g super(-1) at current densities of 0.5 and 30 A g super(-1), respectively, were observed. The reduced graphene oxide paper showed a volumetric capacitance of 196 F cm super(-3). After 1200 continuous cycles of galvanostatic charge-discharge, the electrode retained 93% of its capacitance. The electrocapacitive performance of the urea-reduced graphene oxide outperformed most of the previously reported graphene-based electrode materials, which were reduced by hydrazine or NaBH sub(4). The present reduction method holds great promise for mass production of soluble graphene-based materials for electrochemical energy storage.
A series of Ce-incorporated SBA-15 mesoporous materials were synthesized through direct hydrothermal synthesis method and further impregnated with 12 wt.% Ni. The samples were characterized by ...ICP-AES, XRD, N
2 physisorption, XPS, TPR, H
2 chemisorption, TGA, temperature-programmed hydrogenation (TPH) and TEM measurements. The low-angle XRD and N
2 physisorption results showed the Ce successfully incorporated into the framework of SBA-15. The catalytic properties of these catalysts were investigated in methane reforming with CO
2. The Ce/Si molar ratio had a significant influence on the catalytic performance. The highest catalytic activity and long-term stability were obtained over the Ni/Ce-SBA-15 (Ce/Si = 0.04) sample. The improved catalytic behavior could be attributed to the cerium impact in the framework of SBA-15, where cerium promoted the dispersion of nano-sized Ni species and inhibited the carbon formation. In comparison with the effect of CeO
2 crystallites in SBA-15, cerium in the framework of SBA-15 promoted the formation of the nickel metallic particles with smaller size. The XRD and TGA results exhibited that carbon deposition was responsible for activity loss of Ni/SBA-15 and Ni/Ce-SBA-15 (Ce/Si = 0.06) catalysts. TEM results showed that the hexagonal mesopores of SBA-15 were still kept intact after reaction and the pore walls of SBA-15 prevented the aggregation of nickel.
This review provides a summary of recent research progress towards biomass-derived carbon electrode materials, including specific cellulose-, lignin- and hemicellulose-derived carbon electrode ...materials, for supercapacitors. Various lab-scale methods for preparing biomass-derived carbons, including carbonisation and/or activation conditions are discussed. Control over the pore structure, electrical conductivity, and surface functional groups of biomass-derived carbons for enhancing electrocapacitive performance is analysed. Emphasis is made on discussing cellulose-, lignin- and hemicellulose-derived carbon electrode materials for supercapacitor applications. Future research trends in this field are projected.
Accurate models of the free energies of ions in solution are crucially important. They can be used to predict and understand the properties of electrolyte solutions in the huge number of important ...applications where these solutions play a central role such as electrochemical energy storage. The Born model, developed to describe ion solvation free energies, is widely considered to be critically flawed as it predicts a linear response of water to ionic charge, which fails to match water's supposed intrinsic preference to solvate anions over cations. Here, we demonstrate that the asymmetric response observed in simulation is the result of an arbitrary choice of the oxygen atom to be the centre of a water molecule. We show that an alternative and reasonable choice, which places the centre 0.5 Å towards the hydrogen atoms, results in a linear and charge symmetric response of water to ionic charge for a classical water model consistent with the Born model. Therefore, this asymmetry should be regarded as a property of the specific short-range repulsive interaction not an intrinsic electrostatic property of water and so the fact that the Born model does not reproduce it is not a limitation of this approach. We also show that this new water centre results in a more reasonable surface potential contribution to the solvation free energies.
The solvation free energies of ions in water are consistent with the Born linear response model if the centre on which the ion-water repulsion force acts is moved from the oxygen atom towards the hydrogens.
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