High Energy Density, Asymmetric, Nonaqueous Redox Flow Batteries without a Supporting Electrolyte

Abstract

Energy density in nonaqueous redox flow batteries (RFBs) is often limited by the modest solubility of the redox-active organic molecules (ROMs). In addition, the lack of a separator that prevents ROMs from crossing between anolyte and catholyte solutions necessitates the use of 1:1 mixtures of two ROMs in both the anolyte and catholyte solutions in symmetric RFBs, further limiting concentrations. We show that permanently cationic oligomers of viologen, tris(dialkylamino)cyclopropenium, and phenothiazine molecules have high solubility in acetonitrile and cross over an anion exchange membrane at slow to undetectable rates, enabling the creation of asymmetric RFBs with low crossover. No added supporting electrolyte is necessary, with only the PF 6 − counteranions of the ROMs crossing the membrane during charge/discharge. An oligomeric viologen + oligomeric cyclopropenium RFB at 1.0 M (redox equivalents) has a voltage of 1.66 V and a theoretical energy density of 22.2 Wh/L, one of the highest reported for nonaqueous RFBs.

A Physical Organic Chemistry Approach to Developing Cyclopropenium-Based Energy Storage Materials for Redox Flow Batteries

“…3,36,48 Ultimately, balancing all these properties enabled the design of a polycationic oligomer-based nonaqueous RFB with an energy density of 22.2 Wh/L, comparable to state-of-the-art aqueous systems. 47 Many challenges and opportunities remain going forward. For instance, we are working to develop structure−property relationship models that predict multiple properties (e.g, solubility/crossover rates or potential/stability) simultaneously.…”

“…As one example, we show how optimization of redox potential, guided by Hammett σ p -values, is used to achieve an RFB with an unprecedentedly high cell potential of 3.2 V. However, these studies also reveal that the target properties have complex and often opposing interrelationships. For instance, high potentials are often achieved at the expense of cycling stability. ,, Ultimately, balancing all these properties enabled the design of a polycationic oligomer-based nonaqueous RFB with an energy density of 22.2 Wh/L, comparable to state-of-the-art aqueous systems …”

“…46 Based on these promising data, two polycationic, highly soluble trimers (25 and VIII) were designed for deployment in a high concentration asymmetric flow battery separated by FAPQ-375-PP (Figure 23). 47 Both contain solubilizing substituents (ethyl/propyl on 25 and ethers on VIII) and both cycle between a trication and a hexacation. Trimers 25 and VIII were cycled in an asymmetric flow battery at 0.33 M in MeCN (Figure 23).…”

Identifying structure-function relationships to modulate crossover in nonaqueous redox flow batteries

“…These two half-cells are connected by a separator, which must enable rapid diffusion of charge-balancing supporting ions while impeding crossover of the redox-active molecules. [16][17][18] Over the past decade, there has been considerable progress in the discovery of ROMs that possess the molecular properties required for non-aqueous RFBs, including high (catholyte) and low (anolyte) redox potentials as well as high solubility and stability to redox cycling. [19][20][21][22][23] However, crossover of the anolyte and catholyte between the two battery half cells remains a major challenge in non-aqueous media.…”

“…These two half-cells are connected by a separator, which must enable rapid diffusion of charge-balancing supporting ions while impeding crossover of the redox-active molecules. 16–18…”