Importance of Controlling Cycling Conditions to Improve the Lifetime of Anthraquinone-Based Negolytes with a Potentially Low Cost of Manufacture

Abstract

Organic redox flow batteries (RFB) in general and anthraquinones in specific have shown a great promise for developing cost-effective alternatives to vanadium RFBs. Two main hurdles, however, have been the fast fade rate and a high estimated cost of manufacture (COM) that have slowed down the scale up of organic RFBs. In this study, 1,8-dihydroxy-2,7-dicarboxymethyl-9,10-anthraquinone (1,8-DCDHAQ), 1,5-dihydroxy-2,6-dicarboxymethyl-9,10-anthraquinone (1,5-DCDHAQ) and 1,3,5,7-tetrahydroxyanthraquinone (THAQ) have been explored. These negolytes are expected to have a reasonably low COM due to their simple synthesis route. Additionally, these electroactive species have a low fade rate. Nevertheless, the fade rates are not adequately low for scaleup and commercialization purposes. Therefore, the effect of various factors including the electrolyte flow rate, cutoff voltages, SOC limits, and OH- concentration on the cycling performance have been explored in order to further reduce the fade rate.Our experimental analysis suggests that: (1) the electrolyte flow rate and the extent of mixing in the electrolyte reservoir may lead to inaccurate capacity utilizations and fade rates, (2) increasing the OH- concentration significantly lowers the fade rate and the effect continues even at concentrations higher than 1.0 M, (3) choosing an unnecessarily low discharge cutoff voltage has a detrimental effect on the fade rate of anthraquinones, (4) SOC limit is a very important tool that significantly lowers the fade rate. Through controlling these cycling conditions, the lifetime can be improved by at least a factor of 10. Under properly chosen conditions, the fade rate of 1,5-DCDHAQ and THAQ remains < 0.001%/day (< 0.4%/year). To further verify the effect of the operating conditions, cyclic voltammetry and HPLC techniques were used to identify and/or characterize the side products in the electrolyte. COM of 1,5-DCDHAQ and THAQ at scale are estimated to be 27.99 and 47.44 $/kAh (or 24.13 and 37.95 $/kWh), respectively. 1,8-DCDHAQ and 1,5-DCDHAQ are expected to have a similar COM.Due to the expected low COM for the abovementioned anthraquinones and their very low fade rate under optimum cycling conditions explored in this study, these molecules now present a distinctive prospect for developing and commercializing an affordable and long lifetime organic RFB chemistry.