Improvement in Cycling Stability of Aqueous Organic Redox Capacitor Via Interface Control

Abstract

Because of long lifetime, high power capability and relatively high energy density, redox capacitor can be recognized as the promising energy storage system for smart grids that connect with renewable energy sources. We demonstrated the metal-free aqueous redox capacitors using inexpensive low-molecular-weight quinonic organic compounds as redox active materials, which facilitate charge/discharge by proton insertion/extraction in aqueous electrolyte [1]. However, the cycling stability has been a problem. The capacity degradation during charge-discharge cycling is mainly caused by the dissolution to the electrolyte, accompanying the crystal growth via reprecipitation. In this study, we aimed at the suppression of the capacity degradation in the aqueous redox capacitor by coating the electrode surface with Nafion film, a proton conducting polymer, to avoid a direct contact with the electrolyte. In the experiment, we prepared the composite of AQ and nanoporous activated carbon (Maxsorb® MSC-30, Kansai Coke and Chemicals Co., Ltd.). To absorb AQ on the surface of the carbon, we soaked the carbon in acetone dissolving AQ, and then evaporated the acetone at 70°C to obtain an organic active material/nanoporous activated carbon composite. The weight ratio of the carbon material to the organic material was fixed at the ratio 3:7. The electrode were prepared by mixing the composites with the conductive carbon (DENKA BLACK®FX-35, DENKI KAGAKU KOGYO Co., Ltd.) and polytetrafluoroethylene (PTFE), as a binder. The weight ratio of the composites to the conductive carbon to the PTFE was 8:1:1. The electrodes, prepared in the form of pressed pellets (φ 7 mm), were attached to Au mesh current collectors. To coat the electrode with Nafion, we immersed the electrode in Nafion aqueous dispersion, and then dried it in air for 1 hour at 60°C followed by a calcination treatment for 30 mins at 120°C. For the electrochemical measurements, aqueous H2SO4 solution (0.5 M), an Ag/AgCl electrode and a carbon electrode (the nanoporous carbon to the conductive carbon to PTFE weight ratio was 8:1:1) were employed as the electrolytes, the reference electrode and counter electrode, respectively. The counter carbon electrode was more than ten times heavier than the working electrode. Before and during the usage, N2 gas was bubbled through the aqueous H2SO4 solution (0.5 M) to remove the dissolved oxygen. We initiated the galvanostatic measurements in ambient conditions at the current of 5 C (1 C is 257 mAh g-1) charge/discharge rate based on the amount and ideal capacity of AQ. The cut-off potential for the working electrodes was -0.3 to 0.5 V vs. Ag/AgCl. We compared the cycling stabilities between the electrodes with and without the Nafion film coating. In the case of the electrode without the Nafion film, the capacity reached the maximum at about 100 cycle, then gradually deteriorated. In contrast, in the case of the electrodes coated with Nafion, the capacity degradation was not observed. For SEM observation, compared to the electrode before charge-discharge operation, significant crystal growth of AQ crystals was observed in the electrode without the Nafion film after 1000 cycles. The enlargement in the crystal size of redox-active materials elongates diffusion length of the ions/electrons, and causes the loss of the contact between quinonic organic materials and the carbon material responsible for electron conductivities. These are supposed to be the main reason for the capacity degradation by charge-discharge operation. On the other hand, the surface morphology of the AQ electrode coated with Nafion didn't show obvious change even after 1000 cycles. This should result from the suppression of the elution of AQ to the electrolyte and its crystal growth by Nafion coating, preventing the direct contact of AQ and the electrolytes. In conclusion, by Nafion coating, irreversible morphological changes of organic-based electrodes in aqueous redox capacitors was suppressed, and the cycle stability was enhanced. Such interface control will be an important design guideline in the electric storage device which uses an organic material system.