Iodide ion containing ionic liquid mixture based asymmetrical capacitor performance

Highlights

• Iodide containing ionic liquid mixture was used to enhance system specific energy.

• Iodide ion specific adsorption and redox reactions increased system specific energy.

• Increase in cell capacitance 5 F g^-1 and specific energy 3.5 W h kg^-1 was achieved.

• Iodide based ionic liquid mixture lowered system coulombic and energy efficiencies.

Abstract

Development of high efficiency energy storage systems is increasingly important as these systems enable utilize energy from renewable sources and reduce greenhouse gas evolution caused by fuel combustion technologies in the same time. Electricity storage in the supercapacitor containing neat 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) ionic liquid or mixture of ionic liquids (EMImBF₄ with addition of 5 wt% 1-ethyl-3-methylimidazolium iodide (EMImI)) has been studied in the two- and three-electrode systems using high porosity carbon electrode material with specific surface area of 2090 m² g^-1, micropore surface area of 2060 m² g^-1 and total pore volume of 1.085 cm³ g^-1. Based on electrochemical characterization data (cyclic voltammetry, electrochemical impedance spectroscopy, constant current charge/discharge and constant power discharge), the asymmetrical capacitor with ionic liquid mixture of 5 wt% EMImI in EMImBF₄ shows increase in cell capacitance 5 F g^-1 and specific energy about 3.5 W h kg^-1 as well as in specific power, compared with the data for symmetrical capacitor system based on neat EMImBF₄. However, electrochemical stability, i.e. the potential region of ideal polarizability for the ionic liquid mixture based capacitor is lower and therefore the coulombic and energy efficiencies calculated are also smaller.

Introduction

The increased global energy demand and pollution of environment due to the consumption of fossil fuels require utilization of energy generated from renewable energy sources such as wind, solar and tidal power. As the contribution of these energy resources grows the energy storage systems with high specific energy, efficiency and long cycle life become increasingly important. Energy stored in these systems can be used at times of high energy demand and same systems can be used for energy storage when energy is in excess. In this regard supercapacitors (SCs) are considered as the most promising short-term energy storage devices due to their very high power density, short characteristic time constant, excellent coulombic reversibility (98% or higher), high energy efficiency (92–94%), long cycle life (over 10⁶ cycles) and wide operation temperature range [1], [2], [3], [4], [5].

In SCs, the energy storage is realized by one or two charge storage mechanisms, i.e. by electrical double layer capacitance and pseudocapacitance [6], [7], [8], [9]. In the case of electrical double layer capacitance the charge is stored at an electrode│electrolyte interface, i.e. in the electrical double layer, where the adsorption of ions is mainly based on the electrostatic interactions and charge storage is proportional to the electrochemically available surface area of an electrode material. Pseudocapacitance is associated with fast faradaic charge transfer processes at the electrode│electrolyte interface and enhancement of capacitance can be achieved using transition metal oxides, nitrogen enriched carbons, conducting polymers etc. as the electrochemically active (faradaic) electrode materials. However, pseudocapacitance of the electrode materials is often limited by slow diffusion (mass transfer) and therefore the redox active electrolytes based on transition metal ions, halide ions, quinones, phenylamines etc. are more promising substances to enhance the specific energy of SCs [7,8,[10], [11], [12], [13], [14], [15], [16]].

The specific energy depends strongly on the so-called potential window, which in turn depends the electrolyte used. For aqueous electrolytes based SCs the electrochemical stability potential region is about 1.0–1.2 V, while the organic electrolytes and ionic liquids (ILs) based SCs have the potential windows 2.7–3.0 V and 3.5–4.0 V, respectively. Therefore, the SCs based on aqueous redox active electrolytes [10], [11], [12], [13], [14], [15], [16] and ILs with redox active additions [17], [18], [19], [20], [21] have received some attention. Many ILs have lower conductivity, higher viscosity and thus, narrower low temperature operation limit compared to aqueous and non-aqueous electrolytes. However, compared with volatile aqueous and organic electrolytes, ILs are much safer in SCs applications, especially in the case of high current densities [22,23]. All mentioned components, i.e. electrode materials (capacitive, pseudocapacitive or redox materials) and electrolyte (concentration and composition), prober selection and optimization is crucial for developing energy storage systems with high energy and power characteristics as well as high faradaic efficiency [24].

Influence of the I^- anions addition on the capacitance (pseudocapacitance), specific energy and specific power values as well as characteristic time constant values has been tested in aqueous [[11], [12], [13],16] as well as in ILs based SCs [17,19,21,25]. However, the three-electrode configuration has not been used for IL system with addition of I^- anions. Therefore, in the present work 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) with addition of 5 wt% 1-ethyl-3-methylimidazolium iodide (EMImI) was chosen to study influence of I^- anions addition on the electrochemical properties of microporous-mesoporous carbon (MMPC) material surface electrochemical characteristics in a three-electrode system. 5 wt% of EMImI in EMImBF₄ mixture was selected due to the optimal operation at room temperature as its melting point is 19.5 °C [25], and to avoid irreversible surface blocking of MMPC with the I^- oxidation intermediates. Afterward the two-electrode asymmetrical SCs systems were completed and influence of I^- anions addition on SCs cells capacitance, characteristic time constant, electrochemical stability potential region, coulombic and energy efficiency, and specific energy and specific power characteristics were established. The electrochemical behaviour in two-electrode and three-electrode systems has been characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), constant current charge/discharge (CC) and constant power (CP) discharge methods.