(Invited) Dynamic Behaviour of Electric Double Layer Capacitors

Electrical double layer capacitors (EDLCs) store energy by accumulating ions in electrolyte at electrode-electrolyte interface in the form of a double layer (DL) in porous electrodes.  As electrochemical reactions are not involved, EDLCs find use in devices requiring energy at high rates, such as in hybrid electric vehicles, high power electric tools, and emergency power banks.  Highly non-linear decrease in cell voltage with time and decreased availability of stored charge at high discharge rates, observed experimentally, are major technical issues though.  We present here results of our probe into these limitations by requiring a transport model to quantitatively explain a large body of experimental data available on variation of cell voltage with discharge time at different electrolyte concentrations and discharge currents, charge redistribution and voltage recovery of EDLC during relaxation, and other characteristics.  The model utilizes experimentally measured variation of electrolyte conductivity with salt concentration to make predictions for EDLC characteristics.  In order for the model to consistently explain all the known experimental findings, it requires that (i) the double layer capacitance must vary with local potential difference and electrolyte concentration, (ii) it must decrease with an increase in local voltage drop, (iii) it must vary with local salt concentration in the same manner as bulk conductivity does, and (iv) ionic conductivity in pores of carbon electrodes must be an order of magnitude smaller than the reduced conductivity of electrolyte in porous media, as captured by the correlations available in the literature.  While it is known that double layer capacitance in porous media is understood only poorly, to the best of our knowledge, this is the first time that model based probe has clearly and ambiguously brought out unexpected dependence of capacitance on various parameters.