(Invited) Influence of Current Collector on the Long-Term Performance of Electrochemical Capacitors
Authors: Lota Grzegorz , Wojciechowski Jarosław , Znaniecki Szymon , Szwabińska Katarzyna , Fic Krzysztof , Skrzypczak Andrzej 
Published: December 2020

Electrochemical capacitors store the energy by the formation of the electrical double-layer at the electrode/electrolyte interface. These devices are characterized by high power density, short charge/discharge time and excellent cycling stability. The main goal of supercapacitor development nowadays is to increase their energy density and mentioned cycling stability. The former parameter could be improved by increasing the capacitance or operating voltage of the system, while the second by preserving current collectors from electrochemical corrosion phenomena. The increase of capacitance could be realized by the so-called pseudocapacitance effect, which can originate from the quick faradaic reactions at the electrode/electrolyte interface. These reactions can be induced by the appropriate modification of electrode in order to enforce the pseudocapacitive effect [1-4]. Electrochemical capacitors are composed of two electrodes separated by a separator and soaked in the electrolyte solution. Electrode material (mainly activated carbon) is coated on the surface of the current collectors made either of metals or stainless steel. In such a case, current collectors are exposed to corrosive electrolytes and can be subjected to corrosion processes [5-7]. There are several methods that are utilized to protect metals and steel from the harmful effects of electrochemical corrosion. One of them is the utilization of ionic liquids (ILs), which are added to an aggressive aqueous environment [8]. They act as corrosion inhibitors. This form of protection is being discussed in the scientific literature from about 20 years. A majority of articles describes the ability of this class of compounds to provide anti-corrosive protection for metal and steel. For these materials, an inhibitive influence on corrosion processes has been documented for dozens of ILs, vastly diversified in their chemical structure. A list of almost 90 investigated substances can be found in the literature [8]. Ionic liquids (ILs) are organic compounds which exhibit ionic structure and melting points below 100 degrees of Celsius. Due to their numerous advantageous properties, from some decades ILs stay in the scope of intense scientific interest, what results in the extensive research on possible ways to utilize them industrially. It covers various fields of IL application, for example as a solvent, an electrolyte, a catalyst, as a pharmaceutical or as corrosion inhibitors.
In this work, the influence of ionic liquids additives on the performance of electrochemical capacitors is presented. Strictly, ionic liquids have been added to aqueous 1 M Na2SO4 electrolyte to inhibit corrosion processes on the surface of 316L stainless steel current collectors, which also influence the performance of electrochemical capacitors. Ionic liquids compounds were based on a common 2,5-dihydroxybenzenesulfonic anion. Electrochemical measurements were performed for a 2,5-dihydroxybenzenesulfonic acid, introduced to the electrolyte solution in 4 different concentrations ranging from 0,001% to 1%. Analogous tests were also conducted for 0,1% addition of 6 chosen ILs. All of ILs used in the research were composed of 2,5-dihydroxybenzenesulfonic anion and alkyldimethylammonium cation. The alkyl substituent of this cation varied, including n-ethyl, n-butyl, n-hexyl, cyclohexyl, n-octyl and n-decyl groups. The performed analysis proved an inhibitive effect of investigated compounds on the corrosion processes. The highest effectivities were observed for the ILs with bulkier alkyl substituent (i.e. containing cyclohexyl, n-octyl and n-decyl groups). On the basis of potentiodynamic polarization tests, a mixed corrosion inhibition mechanism was identified, with a predominant influence on the anodic process. Moreover, particular ionic liquids with the best anti-corrosive properties were used as additives to the electrolyte solution in electrochemical capacitors at a concentration of 0.1%. The results of electrochemical tests indicate a significant effect of the addition of ionic liquids on the working parameters of the electrochemical capacitors systems.
This work was financially supported by the National Science Centre of Poland granted on the basis of the decision number 2018/31/B/ST8/01619 and by European Research Council (ERC) within ERC-StG-2017 project (grant agreement No. 759603) under the European Union’s Horizon 2020 research and innovation programme.

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