Energy storage mechanism of electrical double-layer capacitors (EDLCs)

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Introduction

Due to rapid depletion and high, consumption of fossil fuels and other natural resources there has been increased development and investigation of renewable and clean energy conversion/storage technology that can meet present energy and power consumption demands. Meanwhile, conventional energy storage devices like batteries which have high energy density with relatively low power output and conventional capacitors with low energy output and high power densities are not capable of meeting the increase in energy demand of future systems. In order to make effective use of renewable energy technology, it is important to develop high-performance, low-cost and environmental-friendly energy conversion and storage technology. Fuel cells and electrochemical capacitors are promising systems for future electrochemical energy conversion and storage [1–3]. However, the performance of these systems is related to the material properties. Therefore improving and optimizing the properties of these materials for energy storage technologies has become essential recently in scientiﬁc communities.

The basic principle of electrochemical capacitors

Electrochemical capacitors (ECs) are energy storage devices like batteries, but uses different storage mechanisms. Batteries store energy chemically (bulk phenomenon), while ECs store energy physically through dielectric polarization or electronic double layer of ions and electron (surface phenomenon) [1–4]. Using chemical storage allows batteries to store relatively large amounts of energy compared with ECs, but have relatively short cycle lifetime, slow power delivery or uptake and thermal management are issues related with batteries. ECs are power devices that can be fully charged or discharged in seconds; as a consequence, they have low energy density than batteries, but have a much higher power delivery or uptake that can be achieved in shorter times [1]. Figure 2.1 is the Ragone plot which shows the various comparisons within the energy systems. It shows that batteries usually exhibit higher energy densities compared to ECs but have low power densities.

Energy storage mechanism of electrical double-layer capacitors (EDLCs).

Generally ECs can be classified into two types based on their energy storage mechanism: pseudocapacitor which is due fast and reversible redox processes and electric double-layer capacitor from pure electrostatic charge accumulation at the electrode interface as shown in figure 2.2. ECs can be viewed as two reactive porous parallel plates, or electrodes, immersed in an electrolyte, with a voltage potential applied across the two collectors. A porous dielectric separator between the two electrodes prevents the charge from moving between the two electrodes. A schematic diagram of porous electrodes and electrolytic solution is shown in figure 2.3.

The energy storage mechanism of pseudo-capacitance.

Double layer capacitors are complemented by capacitors based on pseudocapacitance, which arises in some electro-sorption processes and in redox reaction at electrode surfaces. In contrast with double layer capacitance which arises from potential-dependence of surface density and stores charges electrostatically (non-Faradaic); pseudocapacitance arises from thermodynamic conditions and is due to charge acceptance ( ∆q) and a change in voltage ( ∆V) [1]. It is Faradaic in origin, involving the passage of charges across the double layer so that a derivative ) ( ) ( V d q d C ∆ ∆ = is equivalent to the measured capacitance and referred to as pseudocapacitance. Three types of Faradaic processes occur for pseudocapacitors namely: reversible adsorption, redox reactions of transition metal oxides, and reversible electrochemical doping and undoping in polymer based electrodes [1]. The most commonly studied materials for the pseudocapacitor electrodes are transition metal oxides [4,6,23,24], conducting polymers [25–28], transition metal nitrides [29,30] and carbon materials enriched with oxygen/nitrogen containing functional groups [31–33]. In contrast to the mechanism of an EDLC, Faradaic charge transfer takes place in the pseudocapacitor electrode. However, pseudo-capacitance has demonstrated extended working voltage and can provide higher capacitance than EDLCs. The major disadvantage of the pseudocapacitive system is their surface which are higher affine to degradation and redox-dependent solid state kinetics, leading to low power performance due to poor electrical conductivity and lack of stability during cycling, compared to pure EDLCs systems [4].

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Chapter 1 Introduction
1.1 General motivation
1.2 Aims and objectives
References
Chapter 2 Literature Review
2.1 The basic principle of electrochemical capacitors
2.2 Principle of energy storage
2.2.1 Energy storage mechanism of electrical double-layer capacitors (EDLCs)
2.2.2 The energy storage mechanism of pseudo-capacitance.
2.3 Testing an electrochemical cell
2.4 Fabrication of cell for a two electrode configuration
2.5 Evaluation of electrode material for electrochemical capacitors
2.5.1 Cyclic voltammetry and galvanostatic chronopotentiometry (GCP)
2.5.2 Galvanostatic charge/discharge (GCD)
2.5.3 Electrochemical impedance spectroscopy (EIS)
2.6 Advantages and applications of ECs
2.7 Application of Electrochemical capacitors
2.8 Electrode materials
2.8.1 Carbon materials:
2.8.2 Conducting polymers (CPs)
2.8.3 Transition metal oxides (TMO)
2.8.4 Composite materials
2.9 Electrolytes
2.10 Graphene based materials for electrochemical capacitors
2.11 Properties of graphene
References
Chapter 3 Characterization and growth techniques
Introduction
3.1 Chemical vapor deposition (CVD)
3.1.1 Aqueous chemical growth technique (ACG)
3.1.2 The microwave (MW) technology
3.1.3 Successive Ionic Layer Adsorption and Reaction (SILAR)
3.2 Materials Characterization
3.2.1 Raman Analysis
3.2.2 Morphological Analysis
3.2.3 Crystallinity and Qualitative Phase Analysis
3.2.4 Gas Adsorption Analysis
3.2.5 Electrochemical Analysis
Reference
Chapter 4 Result and discussions
4.1 Interaction between graphene foam and silver metal particles
Introduction
Result and discussions
Summary
References
4.2 Adsorption between Nickel foam graphene (NF-G) and Nickel oxide (NiO)
Introduction
Results and discussions
Summary
References
4.3 Aqueous chemical growth technique (ACG) of simonkolleite microplatelets on nickel foamgraphene
Introduction
Results and discussions
Summary
References
4.4 Microwave synthesis of manganese oxide (MnO2) nanostructure on nickel foam-graphene as electrode for electrochemical capacitors
Introduction
Results and discussions
Summary
References
4.5 Symmetric electrochemical capacitor based on graphene foam (GF) and manganese oxide (MnO2)
Introduction
Results and discussions
Summary
References
4.6 Non-covalent functionalization of graphene foam with pyrene carboxylic acid (PCA) as electrode for electrochemical storage
Results and discussions
Introduction
Summary
References
Chapter 5 General Conclusions
Chapter 6 Future work