Aqueous Organic Redox Flow Battery Materials Providing High Cycling Stability

Redox flow batteries (RFB), which have advantages in terms of size and capacity scalability, have been investigated mainly using inorganic materials, whereas organic materials have been comparatively overlooked. In recent years, organic redox flow batteries (ORFB) using organic materials have been receiving much attention, particularly because they are composed of earth-abundant elements. Recently, Gordon and Aziz et al. have developed viologen (1) and ferrocene (2) derivatives bearing trimethylammoniopropyl groups as aqueous ORFB materials.¹⁾ The reduction potential of 1 and oxidation potential of 2 are -0.358 V and +0.390 V (vs. SHE) respectively, and feature several advantages: high solubility in water (up to 2.0 M), applicability to neutral pH systems, and high chemical and electrochemical stability. An ORFB comprised of these materials (Fig. 1) has marked very high cycling stability up to 99.9989%/cycle at 0.75-1.00 M.

Figure 1. An illustration of a RFB using 1 and 2