Abstract
The utilizable capacitance of Electrochemical Double Layer Capacitors (EDLCs) is a function of the frequency at which they are operated and this is strongly dependent on the construction and physical parameters of the device. We simulate the dynamic behavior of an EDLC using a spatially resolved model based on the porous electrode theory. The model of Verbrugge and Liu (J. Electrochem. Soc. 152, D79 (2005)) was extended with a dimension describing the transport into the carbon particle pores. Our results show a large influence of the electrode thickness (Le), separator thickness (Ls) and electrolyte conductivity (κ) on the performance of EDLCs. In agreement with experimental data, the time constant was an increasing function of Le and Ls and a decreasing function of κ. The main limitation was found to be on the scale of the whole cell, while transport into the particles became a limiting factor only if the particle size was unrealistically large. The results were generalized into a simplified relation allowing for a quick evaluation of performance for the design of new devices. This work provides an insight into the performance limitation of EDLCs and identifies the critical parameters to consider for both systems engineers and material scientists.
Highlights
Electrochemical Double Layer Capacitors (EDLCs) store energy by the adsorption of ions from an electrolyte, storing the ions in the electrochemical double layer of a charged electrode with a very large surface area
Constant current charge – constant voltage discharge (CCCCVD).—Before proceeding to the performance evaluation of supercapacitors, let us compare the predictions of our model to the experimental data of Verbrugge and Liu (2005).[13]
The performance was described by the time constant τ0, which characterizes the decrease of the capacitance at high frequencies
Summary
The two phases of a porous medium, i.e., the carbon solid phase and electrolyte liquid phase, are assumed to be perfectly interpenetrating and are volume-averaged in this approach. This assumption is valid as long as the size of pores is significantly smaller than the dimensions of the electrode or separator. A discussion by Verbrugge and Liu (2005) confirmed the validity of this approach for the present system. This assumption enables us to treat the transport into the particles as effectively one-dimensional
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