Electrochemical impedance spectroscopy (EIS) consists of plotting so-called Nyquist plots representing negative of the imaginary versus the real parts of the complex impedance of individual electrodes or electrochemical cells. To date, interpretations of Nyquist plots have been based on physical intuition and/or on the use of equivalent RC circuits. However, the resulting interpretations are not unique and have often been inconsistent in the literature. This study aims to provide unequivocal physical interpretations of electrochemical impedance spectroscopy (EIS) results for electric double layer capacitor (EDLC) electrodes and devices. To do so, a physicochemical transport model was used for numerically reproducing Nyquist plots accounting for (i) electric double layer (EDL) formation at the electrode/electrolyte interface, (ii) charge transport in the electrode, and (iii) ion electrodiffusion in binary and symmetric electrolytes. Typical Nyquist plots of EDLC electrodes were reproduced numerically for different electrode conductivity and thickness, electrolyte domain thickness, as well as ion diameter, diffusion coefficient, and concentrations. The electrode resistance, electrolyte resistance, and the equilibrium differential capacitance were identified from Nyquist plots without relying on equivalent RC circuits. The internal resistance retrieved from the numerically generated Nyquist plots was comparable to that retrieved from the “IR drop” in numerically simulated galvanostatic cycling. Furthermore, EIS simulations were performed for EDLC devices, and similar interpretations of Nyquist plots were obtained. Finally, these results and interpretations were confirmed experimentally using EDLC devices consisting of two identical activated-carbon electrodes in both aqueous and nonaqueous electrolytes.

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This study aims to investigate the effect of electrode composition on heat generation in electric double layer capacitor (EDLC) electrodes under galvanostatic cycling. EDLCs consist of two identical electrodes usually made of a mixture of (i) carbon-based material, (ii) binder, and (iii) other conductive additives. These constituents were found to influence the capacitance and internal resistance of the device and the heat generation rate in the positive and negative electrodes. Indeed, the heat generation rate was measured using an isothermal calorimeter in both electrodes of five EDLC devices with different electrode compositions but the same electrolyte consisting of 1 M LiPF 6 in EC:DMC. The reversible heat generation rates at the positive and negative electrodes were nearly identical in absence of CMC. However, in devices containing CMC, the reversible heat generation rate in the positive electrode was significantly larger than that in the negative electrode. Such asymmetric heating was attributed to asymmetry in the charging mechanism due to the overscreening effect caused by interactions between the anionic functional groups of CMC and the cations at the negative electrode. Electric double layer capacitors (EDLCs) have received significant attention in recent years for electrical energy storage applications in particular those requiring rapid charging/discharging, such as regenerative braking in electric vehicles, 1 smart grids, 2 and renewable energy harvesting systems.3-5 Indeed, EDLCs can provide higher power density, higher cycle efficiency, and longer lifetime than batteries. 4,6 EDLC devices consist typically of two carbon-based electrodes partitioned by a separator immersed in aqueous or organic electrolytes. EDLCs store electrical energy in the electrical double layer of ions forming at electrode/electrolyte interface.Heat generation in EDLC is a major concern since these devices are usually cycled under high current density resulting in excessive temperature rise.7 This, in turn, can lead to (i) accelerated cell aging, [8][9][10] (ii) increased self-discharge rates, 9,11 and possibly (iii) electrolyte decomposition and evaporation. 9,12 Heat generation in EDLCs can be classified into irreversible and reversible heat generation rates. 7,[13][14][15][16][17] Irreversible heat generation has been attributed, both theoretically and experimentally, to Joule heating. 7,9,11,[13][14][15][16][17][18][19][20] It is constant throughout the EDLC cell and equal to the product of the EDLC internal resistance and the square of the imposed current. 7,[14][15][16] On the other hand, reversible heat generation rate in the entire device was theoretically and experimentally found to be exothermic during charging and endothermic during discharging.7,14-16 The time-averaged reversible heat generation over a charging step was shown to be proportional to the imposed current under galvanostatic cycling.7,14-16 Experimentally, reversible heat generation has been assumed to be identical in both the positive and negative e...

This study aims to investigate the effect of the potential window on heat generation in carbon‐based electrical double layer capacitors (EDLCs) with ionic‐liquid (IL)‐based electrolytes using in operando calorimetry. The EDLCs consisted of two identical activated‐carbon electrodes with either neat 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethane‐sulfonyl)imide ([Pyr14][TFSI]) electrolyte or 1.0 m [Pyr14][TFSI] in propylene carbonate (PC) as electrolyte. The instantaneous heat generation rate at each electrode was measured under galvanostatic cycling for different potential windows ranging from 1 to 4 V. First, the heat generation rates at the positive and negative electrodes differed significantly in neat IL owing to the differences in the ion sizes and diffusion coefficients. However, these differences were minimized when the IL was diluted in PC. Second, for EDLC in neat [Pyr14][TFSI] at high potential window (4 V), a pronounced endothermic peak was observed at the beginning of the charging step at the positive electrode owing to TFSI− intercalation in the activated carbon. On the other hand, for EDLC in 1.0 m [Pyr14][TFSI] in PC at potential window above 3 V, an endothermic peak was observed only at the negative electrode owing to the decomposition of PC. Third, for both neat and diluted [Pyr14][TFSI] electrolytes, the irreversible heat generation rate increased with increasing potential window and exceeded Joule heating. This was attributed to the effect of potential‐dependent charge redistribution resistance. A further increase in the irreversible heat generation rate was observed for the largest potential windows owing to the degradation of the PC solvent. Finally, for both types of electrolyte, the reversible heat generation rate increased with increasing potential window because of the increase in the amount of ion adsorbed/desorbed at the electrode/electrolyte interface.

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