(103e) Investigating Cyclability of Aqueous Redox Flow Battery Electroactive Species at High Concentrations

Structure-function relationships for electroactive species have been useful in the development of redox flow batteries for grid level storage, particularly in terms of energy density. One of the remaining challenges is systematically determining how molecular structure and electrolyte composition impact stability. In this paper, we describe our efforts to understand the cyclability of various commercially available redox active molecules as a function of their concentration in aqueous media. The cyclability is assessed using cyclic bulk electrolysis and the decay products were characterized using cyclic voltammetry, UV-Vis, NMR and Raman spectroscopy. Quinones have emerged as very promising electroactive species for aqueous redox flow batteries. For anthraquinone 2-monosulfate (AQDS-2), a well-studied quinone analog, we found that at low concentrations the capacity fade is slow. In particular at 8% [50mM] and 25% [160mM] of max solubility, the decay rates are 0.005% and 0.024% per cycle, respectively. At 50% of the maximum solubility [320mM], the decay rate increased significantly to 0.5% of total material per cycle. These results are consistent with recently published reports which suggest the degradation is due to dimerization or higher order polymerization reactions. Flavins such as riboflavin-5'-monophosphate, and metal coordination complexes including ferrocenium and cobalt terpyridine derivatives displayed better capacity retention. For example, the riboflavin at 50% [50mM] of its solubility limit exhibited a decay rate of 0.014% per cycle in simple basic media. The addition of nicotinamide, a hygroscopic agent, improved the performance. Spectroscopic analysis of the spent solutions provides some insight regarding the degradation mechanisms. These and other results will be presented in the paper.