Boron-Based Anolyte for Non-Aqueous Redox Flow Batteries

Abstract

Redox flow batteries (RFBs) that employ dissolved electroactive materials to store energy have attracted growing attention because of their great potential to store renewable but intermittent solar and wind energy. The unique technological merits of decoupled energy and power leads to high flexibility and scalability in design for the RFBs to satisfy a wide range of energy/power applications[1,2]. In established traditional RFBs, the electroactive species are aqueous solutions of redox-active inorganic transition-metal salts including all-vanadium, iron-chromium and zinc-bromine RFBs. Despite immense progress, the attainable cell voltages in these aqueous systems are largely limited by the narrow electrochemical stability window of water (1.5V). As a result, aqueous RFBs suffer from low energy densities and in turn require more electroactive materials and supporting salts to achieve high storage capacity[3,4]. To address these limitations, recent research is shifting to non-aqueous RFBs (NARFBs) that utilize non-aqueous solvents, taking advantage of the wider electrochemical windows over ~5 V depending on the organic solvent employed. A variety of electro-active species including organic redox-active molecules and organometallic complexes have been used in NARFBs[5,6]. Here, we will introduce our research efforts on the development of boron-based redox active anode molecules for non-aqueous redox flow batteries. We have investigated the electrochemical properties of boron-based anode molecules with different substitution groups and the cyclic voltammetry (CV) results indicate that the boron-based redox active molecule with di-tert-butyl substituted (DTBF2) exhibits superior electrochemical reversibility and stability and low half wave potential of -1.82V. When paired with the catholyte 2,5-ditert-butyl-1-methoxy-4-[2′-methoxyethoxy]benzene (DBMMB), the cell voltage can reach up to 2.56V. The built redox flow cell demonstrates an energy efficiency of 80% and capacity retention of 99.3% per cycle, as shown in figure 2. These initial results proved that this readily synthesized boron-based redox active anode is promising anolyte for NARFBs.More experimental data and results will be disclosed during the meeting. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R)