Organic Redox Species in Aqueous Flow Batteries: Redox Potentials, Chemical Stability and Solubility

Denes Konya,Anders Bentien,Emil Dražević,Kristina Wedege

doi:10.1038/srep39101

Abstract

Organic molecules are currently investigated as redox species for aqueous low-cost redox flow batteries (RFBs). The envisioned features of using organic redox species are low cost and increased flexibility with respect to tailoring redox potential and solubility from molecular engineering of side groups on the organic redox-active species. In this paper 33, mainly quinone-based, compounds are studied experimentially in terms of pH dependent redox potential, solubility and stability, combined with single cell battery RFB tests on selected redox pairs. Data shows that both the solubility and redox potential are determined by the position of the side groups and only to a small extent by the number of side groups. Additionally, the chemical stability and possible degradation mechanisms leading to capacity loss over time are discussed. The main challenge for the development of all-organic RFBs is to identify a redox pair for the positive side with sufficiently high stability and redox potential that enables battery cell potentials above 1 V.

Highlights

The widespread interest in replacing fossil fuels with renewable energy sources is a challenging task motivated by a need for energy independence and carbon footprint reduction
Anthraquinone-2,7-disulfonic acid (AQDS(2,7)) and bromine in an acidic environment result in a cell voltage of 0.9 V, while 2,6-dihydroxy anthraquinone (AQDH(2,6)) and potassium ferricyanide in an alkaline environment results in a cell potential of 1.2 V11,12
The results show that all-quinone aqueous redox flow batteries (RFBs) with cell potential above 1.0 V is attainable by addition of electron withdrawing groups on hydroquinones and electron donating groups on anthraquinones

Summary

Introduction

The widespread interest in replacing fossil fuels with renewable energy sources is a challenging task motivated by a need for energy independence and carbon footprint reduction. The cost of wind and photovoltaics electricity is still decreasing and will be integrated even more in a future fully renewable-based utility grid This integration is faced with major challenges with respect to stability and security of supply because of the mismatch between electricity production and demand[1]. The maximum allowable cost of organic redox species was studied in a scenario where a system capital cost for an aqueous RFB of $120 kWh−1 was targeted[9]. An interesting point here is that the redox species maximum allowable cost scales with the cell potential with a power greater than one This is because the energy density increases with cell potential, and cycle efficiency for a given internal flow cell resistance. Discharge current densities in these RFBs are almost twice those observed for optimised vanadium RFBs, the maximum discharge power densities are about the same (0.4–0.6 W cm−2) because of the lower cell potentials[18]

Methods

Results

Conclusion