Electric Double-Layer Capacitance on Carbon Electrodes in Concentrated Aqueous Electrolytes of Alkaline Metal Salts
Authors: Urai Hirotaka , Kondo Yasuyuki , Katayama Yu , Yamada Yuki 
Published: December 2024

Introduction:

Electric double layer capacitors (EDLCs), which store electricity by adsorption and desorption of ions, are commercialized for mobile devices and vehicles due to their high-rate capability and long cycle life. However, the lower energy densities of EDLCs than batteries limit their applications. Thus, much attention has been paid to improving their energy densities (E), described by the equation E = 1/2 CV2, where C is the capacitance, and V is the operating cell voltage1). Here, we focused on aqueous electrolytes, which are superior to organic electrolytes used in most commercial EDLCs, in terms of safety, cost, ionic conductivity, and permittivity2). However, the narrow potential window by theoretical water splitting voltage limits the operating voltage of aqueous EDLCs. Recently, highly concentrated aqueous electrolytes, referred to as hydrate-melts3), have been developed, enabling high-voltage operation of aqueous energy-storage devices, including aqueous EDLCs. In this context, it is desired to investigate the influences of electrolyte compositions on double layer capacitances in concentrated aqueous electrolytes. In most previous studies, the performance evaluation, including capacitance in each electrolyte, was performed with activated carbon electrodes used for commercial EDLCs, demonstrating the feasibility of relatively concentrated aqueous electrolytes for EDLCs4). However, the capacitance of activated carbon electrodes may be affected by various non-intrinsic factors including viscosity. Therefore, capacitance evaluation, eliminating various effects of non-EDL related factors, is required to gain a fundamental understanding of the electric double layer in highly concentrated aqueous electrolytes as well as establish an electrolyte design strategy to improve energy density.
In this study, we studied the EDL capacitance in concentrated aqueous electrolytes. To exclude the influence of electrolyte viscosity, we used a flat carbon plate as a model electrode and evaluated the EDL capacitance by electrochemical impedance spectroscopy (EIS). The EDL capacitance was evaluated in aqueous electrolytes with various concentrations of alkali-metal salts to gain basic insights into the design of aqueous electrolytes for EDLCs.

Experimental:

EIS was performed in the frequency range of 100 kHz–1 Hz at 30°C with three-electrode cells (with glassy carbon, Pt mesh, and Ag/AgCl as a working, counter, and reference electrodes, respectively) to evaluate the capacitances in 100 Hz at various potentials in aqueous electrolytes. KN(SO2F)2 (KFSI) with concentrations ranging from 1.1 mol kg-1 (m) to 30.8 m, 61.7 m K(N(SO2F)2)0.55(SO3CF3)0.45 (K(FSI)0.55(OTf)0.45), 21 m NaFSI, 21 m LiN(CF3SO2)2 (LiTFSI) were used as electrolytes.

Results & Discussion:

The EIS results show that the capacitance of glassy carbon in KFSI/H2O reached its maximum at 22.2 m (Fig.1a). Further increasing the salt concentration up to 61.7 m K(FSI)0.55(OTf)0.45/H2O monotonically decreased the capacitance. This trend is different from those reported for activated carbon electrodes, at which the capacitance monotonically increased with increasing concentration5). This difference may result from the electrolyte viscosity, which can differently affect the flat glassy carbon and porous activated carbon. We also studied various alkaline-metal salt electrolytes at the same 21 m concentrations (Fig.1b). The capacitance at glassy carbon electrodes increased in the order of LiTFSI<NaFSI<KFSI, suggesting that larger cations result in higher capacitances. In the presentation, we will discuss how the electrolyte components and concentrations dominate EDL structures and capacitances on carbon electrodes in aqueous electrolytes.

Acknowledgement:

This study was partially supported by JSPS KAKENHI Grant Number JP 24H00483 and Mitsubishi Foundation.

References:

1) C. Zhong, et al.
CRC Press, 361 (2016).
2) C. Zhong, et al.
Chem. Soc. Rev., 44, 7484-7539 (2015).
3) S. Ko, et al.
Electrochem. Commun., 116, 106764 (2020).
4) X. Tian, et al. ChemSusChem, 14, 2501–2515 (2021).
5) P. Lannelongue, et al.
J. Electrochem. Soc., 165, A657 (2018).

Figure 1