Fabrication and Electrochemical Evaluation of Organic Secondary Battery Capable of Charging and Discharging in Neutral Solution

Abstract

There are attractive attentions on organic secondary batteries as new energy storage devices, in which organic compounds as electrode active materials supported on a porous carbon. Dilute sulfuric acid and organic solvents were used as electrolytes1,2). On the other hand, an organic electrolyte has a risk of ignition, and an acidic electrolyte is low safety when leaked.In this study, we newly fabricated an organic secondary battery that can be charged and discharged with a non-flammable, non-poisonous neutral aqueous solution, and evaluated its electrochemical performance.4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl-benzoate (TEMPO-benzoate) was used as the cathode active material, and 9,10-anthraquinone (AQ) was used as the anode active material. Both active materials are hydrophobic and expected to overcome the problem of elution of the active material into the electrolyte.Firstly, carbon cloth was immersed in a slurry of acetylene black and polyvinylidene fluoride (PVdF) ultrasonically dispersed in N-methyl-2-pyrrolidone (NMP) and dried at 60 ° C for 48 hours. Next, each active material solution was dropped, dried under reduced pressure for 4 hours to adsorb each active material, and a porous carbon electrode was produced. For single electrode evaluation, cyclic voltammetry was performed at a scanning speed of 10 mV s-1. The as-fabricated electrodes were used as the working electrode, a platinum mesh was used as the counter electrode, a saturated KCl/Ag/AgCl electrode (SSE) was used as the reference electrode, and a 1 mol/dm3 KCl aqueous solution was used as the electrolyte. An organic secondary battery was fabricated using the as-fabricated electrode, and electrochemical evaluation was performed by a charge and discharge test using a reference electrode. A cellophane was used for the separator.Figure 1 (A) shows the reaction mechanism of the active materials of both electrodes used in this study. Figure 1 (B) and (C) show the cyclic voltammograms of AQ and TEMPO-benzoate. It was found that AQ showed an anodic peak near -0.65 V vs. SSE and a cathodic peak near -0.8 V vs. SSE (Fig. 1(B)). The anodic current and cathodic current is related to the two-electron transfer reaction between quinone and hydroquinone shown on the left side of Fig.1 (A). TEMPO-benzoate showed an anodic peak near 0.85 V vs. SSE and a cathodic peak near 0.65 V vs. SSE (Fig. 1(C)) due to the one-electron transfer reaction between the NO radical and the NO cation on TEMPO-benzoate shown on the right side of Fig. 1 (A). The peak appeared near 1.1 V vs. SSE in Fig. 1 (C) is considered to be due to chlorine gas generation on the electrode surface by chloride ions in the electrolyte.Figure1 (D) shows the results of a charge and discharge test performed by applying a constant current of 820 mA to an organic secondary battery. The cut-off voltage were 2 V and 0.4 V, respectively. A large irreversible capacity appeared during the first charge. The potentials of the two electrodes during charging and discharging using the reference electrode suggested that reduction of dissolved oxygen in the negative electrode and oxygen in the porous carbon pores and side reactions were caused. On the other hand, it was confirmed that charge and discharge was performed stably repeatedly after the first discharge. We also tried to improve cycling characteristics by suppressing irreversible capacity by performing thorough degassing of porous carbon and degassing of electrolyte, and assembling cells in an inert gas atmosphere. References 1) T. Tomai et al., Scientific reports, 4, 3591 (2014).2) B. Huskinson et al., Nature, 505, 195-198 (2014). Acknowledgments This work was partially supported by JST-ASTEP Grant Number JPMJTS1513, JSPS Grant Number 17H02162 and Private University Research Branding Project (2017-2019) from Ministry of Education, Culture, Sports, Science and Technology, and Tokyo University of Science Grant for President's Research Promotion． Figure 1