Effects of Constituent Materials on Heat Generation in Individual EDLC Electrodes

Abstract

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...

In Operando Calorimetric Measurements for Activated Carbon Electrodes in Ionic Liquid Electrolytes under Large Potential Windows

“…In addition, EDL formation by ions with smaller diameter and/or larger valency resulted in larger reversible heat generation rate during charging . Differences in heat generation rates at each electrode can also be a result of interactions between ions and the electrode constituents, as observed experimentally for EDLCs . Indeed, the reversible heat generation rate at the negative AC‐based electrode containing carboxymethyl cellulose (CMC) as a binder and thickening agent was endothermic at the beginning of the charging step .…”

“…Moreover, it is important to note that even if the electrodes of the cell are identical in composition and mass loading, the heat generation rate may be different at the positive and negative electrodes . This difference can be attributed to differences in the valency, size, and/or diffusion coefficient of the anions and cations, as predicted numerically .…”

“…Electrochemical experiments such as cyclic voltammetry and galvanostatic cycling are very informative methods to characterize oxidation and reduction reactions. However, to provide supplementary information and to gain insight into the nature of these reactions, complimentary in operando calorimetric measurements have been proposed . Moreover, considering that EDLCs are designed to provide high current over a wide potential window and given the large cell resistance of IL‐based electrolytes, it is essential to pay close attention to the heat generated for thermal management and safety purposes.…”

“…The present study aims to identify the thermal signatures of the different physicochemical phenomena taking place in EDLCs using neat IL electrolyte and IL diluted in organic solvent under galvanostatic cycling. To do so, the heat generation rate was measured for different potential windows using our newly developed in operando calorimeter . Particular attention was paid to ion intercalation and electrolyte decomposition under large potential windows.…”

“…Heat generation in EDLCs is a result of several interfacial, transport, and electrochemical processes . It can be divided into irreversible and reversible heat generation rates . The instantaneous irreversible heat generation rate in EDLCs under Δ ψ s =1 V has been reported to be mainly owing to Joule heating (in W) and expressed as, …”