Charge storage mechanisms in electrochemical capacitors: Effects of electrode properties on performance

doi:10.1016/j.jpowsour.2016.03.073

Journal of Power Sources

Volume 326, 15 September 2016, Pages 613-623

https://doi.org/10.1016/j.jpowsour.2016.03.073 Get rights and content

Highlights

•
Step potential electrochemical spectroscopy is used to evaluate material behaviour.
•
A range of common electrochemical capacitor materials have been examined.
•
Double layer and pseudo-capacitance processes are differentiated.
•
Differences in charge storage processes for various materials are explained.

Abstract

The capacitive behaviour of four commonly studied electrochemical capacitor systems has been analyzed using the step potential electrochemical spectroscopy (SPECS) method. Electrode-electrolyte combinations with different charge storage mechanisms were characterized, including activated carbon in aqueous (H₂SO₄) and organic (TEABF₄ in acetonitrile) electrolytes, manganese dioxide (Na₂SO₄) and anhydrous ruthenium oxide (H₂SO₄). The SPECS method was used to separate the charge storage contributions from double layer capacitance (C_DL) and diffusion-limited pseudo-capacitance (C_D) at scan rates ranging from 0.08 to 125 mV/s. The relative contributions from each process are related to the physicochemical properties of the electrode. Additionally, the effects of these electrode properties on the overall performance of each system, in terms of specific power and energy, are identified.

Introduction

Electrochemical capacitors (ECs) are a promising energy storage technology for addressing many of the problems associated with the transition from fossil fuel based energy to renewable energy technologies. In particular, they can be used for mitigating the variable energy supply of some renewable energy sources and can be used in energy harvesting or regenerative energy technologies [1], [2].

ECs have a number of advantages. They can be charged and discharged relatively quickly making them ideal for a range of energy harvesting applications [3], they are highly efficient (>98% [4]), have good cyclability (>10⁵ cycles [4]) and are relatively non-hazardous [5]. The development of EC devices with performance characteristics tailored to specific applications is necessary to ensure the uptake of this technology. However, current progress in EC development is limited by our understanding of how certain performance characteristics are governed by electrode and electrolyte properties. Identifying this relationship between material properties and device performance will lead to the development of devices with improved performance.

An EC consists of two solid electrodes separated by an electrolyte. When a potential is applied to the electrodes, ions in solution accumulate at the surface of the charged electrode, forming an electrical double layer (EDL) [6]. Charge stored via this mechanism (known as double layer capacitance) is restricted to the surface of an electrode and is therefore determined predominantly by the surface area of the electrode accessible by the electrolyte. In addition to double layer capacitance, some materials store charge via fast, reversible redox reactions, known as pseudo-capacitance [6], [7]. These reactions are not limited to the surface of the electrode so the bulk of the electrode can be accessed, leading to greater charge storage.

Double layer capacitance occurs in all electrodes; however, only some materials exhibit pseudo-capacitance. The performance of an EC, in terms of total capacitance, power and energy, is determined predominantly by the relative contributions to charge storage from double layer and pseudo-capacitive processes. Double layer capacitance is a fast process because charge is stored at the surface via electrostatic forces, unlike pseudo-capacitance which is limited by the kinetics of charge transfer and diffusional processes. Therefore, double layer capacitors (EDLCs) often have high power, but low energy compared to pseudo-capacitors as they cannot sustain this discharge for extended times [8].

The relative charge storage contributions from each mechanism in an electrode are determined by the physicochemical properties of the electrode material, such as chemical composition, crystal structure, surface area and porosity, among others. It is therefore necessary to understand how such properties can be tailored to optimise EC performance for applications.

EC electrodes are commonly made of one of three materials; i.e., activated carbon, ruthenium oxides or manganese dioxide. Each of these materials has a different charge storage mechanism which is determined by the properties of the material.

Activated carbon is an EDLC electrode and is the most commonly used material in commercial ECs. The extremely high surface area attainable for activated carbon (up to 3000 m²/g [9]) leads to a relatively high capacitance (∼100 F/g (organic electrolyte) and 200 F/g (aqueous)), despite charge storage being limited to the electrode-electrolyte interface. Due to double layer capacitance being the primary charge storage method, activated carbon electrodes generally have high specific power but low energy [6].

Ruthenium oxide is the prototypical pseudo-capacitor in which charge is stored via both double layer and pseudo-capacitance [7]. This pseudo-capacitance arises through the insertion of an electron from the external circuit to reduce Ru(IV) to Ru(III) coupled with the subsequent insertion of a proton from the electrolyte to maintain charge neutrality. This proton insertion process is very facile due to the high conductivity of ruthenium oxide, hence ruthenium oxide-based electrodes have exhibited some of the highest reported capacitance values, up to 900 F/g [10], [11]. The high conductivity and high specific capacitance of ruthenium oxide electrodes means that they generally exhibit both high specific energy and power [12]. However, despite the high performance of ruthenium oxide electrodes, their commercial potential is impeded by their high cost and toxicity [12].

Manganese dioxide has been identified as a promising alternative to ruthenium oxide as it is inexpensive, readily available and non-hazardous. Similarly to ruthenium oxide, manganese dioxide stores charge via a pseudo-capacitive cation insertion mechanism. In manganese dioxide, cation insertion can involve either protons or metal cations from the electrolyte, such as Na⁺. Due to their smaller size, proton insertion is often a more facile process and accounts for a large proportion of charge storage, however, in large tunnel and layered manganese dioxide phases (such as the birnessite phase), there is likely to be a more significant contribution from metal cation insertion. Despite manganese dioxide having a higher theoretical capacitance than ruthenium oxide, the reported capacitance of manganese dioxide electrodes have not yet surpassed that of ruthenium oxide, with the exception of some extremely thin electrodeposited films [13].

To improve the performance of these EC electrode materials, it is necessary to identify how their different charge storage mechanisms determine specific performance characteristics (such as power and energy), and furthermore, identify the extent to which the charge storage mechanism is governed by the properties of the electrode material. This will lead to the synthesis of electrode materials with material properties optimized for specific performance requirements. However, it has previously been difficult to experimentally characterize the different charge storage processes occurring at an electrode. This is due to the inability of conventional electrochemical techniques to separate the charge storage contributions from double layer and pseudo-capacitive processes.

In this work, the performance of commonly used EC electrode materials is compared and evaluated in terms of their material properties and different charge storage mechanisms. The charge storage mechanisms of activated carbon, manganese dioxide (birnessite) and hydrous ruthenium oxide (RuO₂.nH₂O) are characterized using the step potential electrochemical spectroscopy (SPECS) method which allows the contributions from double layer and pseudo-capacitive processes to be separated [14].

Section snippets

Activated carbon

The activated carbon used in this work was prepared in a two stage process. The first stage involved the pyrolysis of coconut husks at 500 °C under a nitrogen atmosphere for 3 h. After cooling the resultant char was milled in a zirconia mill to produce a material with a mean particle size of ∼20 μm. In the second stage, activation of the char was then carried out with the addition of a small volume of concentrated H₃PO₄, with the resultant suspension again pyrolyzed at 700 °C under a nitrogen

Surface area

Gas adsorption analysis was used to determine the surface area of each sample. The total surface areas were calculated using the linearized BET isotherm and the results shown in Table 3. Activated carbon has a very high surface area, as is typical for this material [17], [18]. The reported surface areas for Birnessite vary depending on the synthesis conditions used, with surface areas ranging from 5 to 230 m²/g being reported [19], [20], [21]. The Birnessite phase synthesized here has a surface

Conclusions

Here it is demonstrated that the step potential electrochemical spectroscopy (SPECS) technique can be used to characterize the different charge storage mechanisms in a range of commonly used electrochemical capacitor systems. The total charge storage in each system was separated into its double layer components C_DL1 and C_DL2 (representing geometric and porous double layer capacitance, respectively) and diffusional (pseudo-capacitive) processes C_D. The relative contributions from these processes

Acknowledgements

MFD would like to acknowledge the University of Newcastle for the provision of a PhD scholarship. The authors would also like to acknowledge Prof. Wataru Sugimoto (Shinshu University, Japan) for providing the hydrous ruthenium oxide which was studied in this work.

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Charge storage mechanisms in electrochemical capacitors: Effects of electrode properties on performance

Highlights

Abstract

Introduction

Section snippets

Activated carbon

Surface area

Conclusions

Acknowledgements

Int. J. Hydrogen Energy

Carbon

Electrochimica Acta

J. Power Sources

Carbon

Bioresour. Technol.

Mater. Lett.

Mater. Charact.

Electrochimica Acta

Electrochimica Acta

J. Power Sources

Carbon

Electrochimica Acta

Carbon

Electrochimica Acta

Electrochimica Acta

Electrochemical capacitors for energy management

Sci. Mag.

Electrochem. Soc. Interface

Energy & Environ. Sci.

Chem. Soc. Rev.

Supercapacitors Materials, Systems, and Applications

Chem. Soc. Rev.

J. Electrochem. Soc.