energies Article A Multi-Channel Fast Impedance Spectroscopy Instrument Developed for Quality Assurance of Super-Capacitors Farhan Farooq , Asad Khan, Seung June Lee, Mohammad Mahad Nadeem and Woojin Choi *   Citation: Farooq, F.; Khan, A.; Lee, S.J.; Mahad Nadeem, M.; Choi, W. A Multi-Channel Fast Impedance Spectroscopy Instrument Developed for Quality Assurance of Super-Capacitors. Energies 2021, 14, 1139. https://doi.org/10.3390/ en14041139 Academic Editor: Mojtaba Mirzaeian Received: 1 February 2021 Accepted: 18 February 2021 Published: 21 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Electrical Engineering, Soongsil University, Seoul 06978, Korea; farhanfarooq93@gmail.com (F.F.); asadkhan734b@gmail.com (A.K.); coql77@naver.com (S.J.L.); mahad.baig97@gmail.com (M.M.N.) * Correspondence: cwj777@ssu.ac.kr; Tel.: +82-10-2701-3823 Abstract: Conventional experimental methods for testing the performance of super-capacitors include the measurement of capacitance through charge and discharge, measurement of equivalent series resistance (ESR) and measurement of self-discharge and the equivalent circuit model (ECM) by electrochemical impedance spectroscopy (EIS). However, the methods are not suitable for the mass production line of supercapacitors since they require a long time for the test and several kinds of different instrument. EIS is an attractive method to evaluate the performance of supercapacitors except that it takes a long time for a single test. In this paper a fast EIS instrument suitable for quality assurance for the mass production of supercapacitors is proposed. In order to reduce the time for the test, a multi-sine sweeping method is used for the EIS test and the results are analyzed by extracting the parameters of the ECM to evaluate the performance of the supercapacitors. The proposed instrument is developed to have multi-channel to further decrease the time for the test with a supercapacitor. It is also presented as to how the extracted parameter values of the ECM can be used to evaluate the performance of the supercapacitor. Keywords: electrochemical impedance spectroscopy; equivalent circuit analysis; super-capacitors; digital lock-in amplifier; multi sine super imposed method; multi-channel 1. Introduction A supercapacitor is also known as an electrochemical double-layer capacitor (EDLC). The EDLC super-capacitor differs from a traditional capacitor as its electrodes are com- posed of a porous conductor such as activated carbon, which has a huge surface area, and it accumulates and keeps the charges on the thin layer of the electrode/electrolyte interface via electrostatic force or non-faradic effect, so that it possesses a huge capacitance (>100 F/g), high power density and long cycle life of more than 100,000 cycles [1,2]. These characteristics make it suitable for new and renewable energy source applications requiring high power output such as wind power generation systems, photovoltaic power generation systems, electric vehicles, railways, and uninterruptable power supply (UPS) systems [3,4]. Typically, the systems require modules that are composed of multiple supercapacitor cells connected in series and parallel. When the cells with uneven performance are used in the supercapacitor module, the performance of the module cannot be maximized, and balanc- ing problems may arise which can cause a failure at the time of application [5–7]. Therefore, the defective cells should be selectively removed at the time of production. The capacitance of the supercapacitor may vary depending on several factors such as the electrode and the electrolyte materials. The compatibility of the electrolyte with the electrode material plays a crucial role in the development of a supercapacitor because the electric double layer is established at the electrode/electrolyte interface. The voltage of a super-capacitor depends on the stability of the electrolyte [8,9]. Typical factors that determine the performance of a supercapacitor include capacitance, equivalent series resistance (ESR), and self-discharge rate [7,10,11]. According to several studies, little is known about why electrochemical mechanisms cause gradual loss of voltage in charged supercapacitors stored at open circuit Energies 2021, 14, 1139. https://doi.org/10.3390/en14041139 https://www.mdpi.com/journal/energies