Room Temperature, Hybrid Na-Based Flow Batteries with Organic Catholytes and Multi-Electron Transfer Redox Reactions

Abstract

Redox flow batteries (RFBs) have attracted significant interest lately for use in grid-scale electrochemical energy storage. One of the most studied RFB systems is the all vanadium RFB. Due to a phenomena known as crossover, RFBs suffer from rapid capacity decay in which the catholyte and anolyte become mixed during cycles. In addition, the energy density of RFBs is low. To increase the cell voltage and thus energy density, organic electrolytes can be used to replace aqueous electrolytes. However, to date the organic electrolyte based RFBs have not offered higher energy densities due to the low solubility of ions in electrolytes. Recently, we have introduced a new concept of hybrid Na-based flow batteries (HNFBs) with a molten Na alloy anode in conjunction with a flowing catholyte separated by a solid Na-ion exchange membrane. This new type of room temperature flow batteries address low energy density and crossover issues simultaneously. In this study we have investigated the redox reaction mechanism of an organic catholyte made of vanadium acetylacetonate, V(acac)3, in acetonitrile solvent. We demonstrate that with the newly invented HNFB the V(acac)3 catholyte can provide multi-electron transfer redox reactions per vanadium ion to enhance the energy density of the HNFB while possessing tolerance to the impurity water in the system. In conventional RFBs the impurity water reacts with oxidized V(acac)3 to become vanadyl acetylacentonate, VO(acac)2, leading to gradual degradation of the cell. This study, for the first time, shows that the reaction between the impurity water and oxidized V(acac)3 can be utilized as an energy storage mechanism enabled by HNFBs to enhance the energy density of the cell.