Electrode mass ratio impact on electrochemical capacitor performance
Introduction
The alternative energy storage technologies with cost-effective, environmental friendliness and sustainability of the devices are strongly needed in order to reduce the fossil fuel consumption as well as to reduce the environmental pollution [1,2]. The effective methods to store electrical energy for use on demand are critical issues [3,4]. Electrochemical capacitor have been recognised as promising next generation energy storage devices due to their ultra-fast charging, very high power density, and long cycle life as compared than most widely used lithium ion batteries in the commercial applications [5,6]. At the same time, low energy density and high production cost of electrochemical capacitor as compared with lithium ion batteries are restricted their practical applications, which need to be addressed [7,8]. These drawbacks can be mitigated by developing a new class of high performance premium grade carbon which consists of a combination of materials produced from abundant, cheap, environmentally friendliness and sustainable resources with low processing costs [9,10].
Electrochemical capacitors with carbon-carbon based electrodes have been developed and extensively studied [[11], [12], [13], [14]]. Although the energy density of the electrochemical capacitor is lower than LiB, the commercial electrochemical capacitor with currently available energy density have been widely accepted and demonstration for various applications such as emergency doors opening, energy recovery, flash/solar lights, memory backup, regenerative braking and transport sectors (Buses, tramways, hybrid cars) etc. [[15], [16], [17], [18]]. Significant research and development on enhancement of the energy density by using advanced nanostructured carbon-carbon based electrochemical capacitor at a premium price have been already targeted [[19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]].
However, the high specific capacitance of electrodes and the potential working window of electrolytes couldn't always be fully utilised when the electrode assembled into electrochemical capacitor device. In a full electrochemical capacitor device, the positive and negative electrodes are connected in series and the storage of electric charge via formation of EDLCs on the two electrodes is basically equal as per equation (1) [31] and the potential windows of the two electrochemical capacitor electrodes are determined by the specific capacitance of the electrodes and mass of the two electrodes as per equation (2) [31].where, Q+ and Q- represent the electric charge on the positive and negative electrode, U+ and U- are voltage windows of the two electrodes, C+ and C- are the specific capacitance of the two electrodes, and m+ and m- are the mass of the two electrochemical capacitor electrodes, respectively.
The positive and negative electrochemical capacitor electrodes are generally composed of the same carbon material with same active mass loading and this system is so called “equal electrochemical capacitor configuration. In such equal mass configuration, one electrode will reach the potential limit faster than other electrode because the cation and anions of the electrolyte, particularly non-aqueous electrolyte, are different in ionic sizes which have different adsorption and ionic transportation on the porous surface of positive and negative electrodes. For example, non-aqueous electrolytes i.e. 1 M tetraethylammonium tetrafluoroborate (1 M TEABF4/AN) as an organic salt dissolved in acetonitrile was used as a common organic electrolyte and the bare ionic size of the cation (TEA+) is around 0.68 nm and the size of the anion (BF4−) is around 0.48 nm [32,33]. In such a case, the electric charge storage and utilisation of active carbon electrode for the electric double layer formation is also different. Furthermore, the assignment of potential ratio between the positive electrode and negative electrode may influence significantly on the degradation of electrolyte and it may affect the charging and discharging profile [[31], [32], [33], [34], [35]]. In order to increase the specific interfacial area between electrochemical capacitor electrode and electrolyte, the mass ratio of the electrochemical capacitor electrode plays a vital role in the utilization of active carbon and life cycle of electrochemical capacitor. For example, Waterhouse [21] performed a preliminary investigation of a different approach to increase the operating voltage of the EDLC by using ElectroFlex activated carbon as electrode films and he found that the operating life and potential window of the electrochemical capacitor can be extended by adjusting the mass ratio of activated carbon electrodes. Thus, smart adjustment in the active mass ratio of two electrochemical capacitor electrodes, the potential window of each electrode can be utilized completely and then the operating voltage of the electrochemical capacitor device can be uplifted [[33], [34], [35], [36]]. Therefore, the potential window of the electrochemical capacitor should be optimized to achieve the maximum energy density of electrochemical capacitor for practical implementation.
The novelty of the present study is to emphasise the better electrochemical capacitor performance of unequal mass configurations of activated carbon fibres based electrodes using organic electrolyte (i.e. 1 M TEABF4/AN) at 3.0 V as compared to commercial Kuraray YP-50 carbon. Smart adjustment in the mass ratio of the positive and negative electrodes, the available electrochemical stability windows of both electrodes can be extended; therefore, the operational voltage of the cell can be increased. This strategy has been successfully demonstrated by using two different carbon electrodes such as indigenous activated carbon fibres derived from waste cotton and Kuraray YP-50 (Kuraray, Japan) commercial carbon derived from coconut shell. Schematic representation of equal and unequal mass configurations of carbon/carbon based electrochemical capacitor electrodes and their mechanism are illustrated in Fig. 1.
Section snippets
Activated porous carbon as electrode material
In this study, two different carbons such as indigenous activated porous carbon fibres and commercial Kuraray YP-50 were used as electrode materials. The indigenous high surface area porous carbon fibres derived from waste cotton was obtained by pre-carbonisation followed by chemical activation. Briefly, the waste cotton obtained from cotton industry (Jansons Industries Ltd, Tiruchengode, Tamilnadu, India) was pre-carbonized by using a tubular furnace under N2 flow at 600 °C for 1 h with a rate
Textural characteristics of the carbon electrodes
The textural properties such as specific surface area (BET), specific pore volume and average pore size of the activated carbon fibres and commercial Kuraray YP-50 were represented in Table 1. From the table, it is clearly observed that textural properties of activated carbon fibres showed almost similar specific surface area, pore volume and pore size as compared to that of commercial YP-50 carbon. The both samples are having predominant micropores with high surface area and narrow pore size
Conclusions
The equal/unequal mass configurations of carbon/carbon based electrochemical capacitor using organic electrolyte (i.e. 1 M TEABF4/AN) were successfully examined in order to confirm the influence of different electrode mass loading on the electrochemical performance in terms of potential window stability and durability of the electrochemical capacitor. The energy densities of two different electrochemical capacitor cells were compared at identical conditions and validated by practical
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
This research work was supported by technical research centre (TRC) project (Ref. No. AI/1/65/ARCI/2014 (c)) sponsored by Department of Science and Technology (DST), Govt. of India, New Delhi, India.
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