Mono-methyl viologen: a promising anolyte for alkaline aqueous redox flow batteries

Abstract

An Aqueous Redox Flow Battery (ARFB) has emerged as a sustainable option for large-scale energy storage systems due to its relatively low cost and abundant raw materials. However, there exists a pressing need to enhance its power density and energy efficiency by addressing limitations such as low stability and poor solubility of redox-active species. In this study, we present a systematic analysis of an asymmetric viologen-based negolyte named MMV (1-methyl-4,4′-bipyridinium iodide) and its detailed comparative study with N,N′-dimethyl-4,4′-bipyridinium diiodide (dimethyl viologen (DMV)). Following extensive electrochemical characterization, we constructed an alkaline MMV battery paired with the potassium ferro/ferricyanide posolyte at a higher concentration (0.8 M), yielding a cell voltage of 1.25 V, nearly equivalent to those of vanadium RFBs. Remarkably, long-term cycling with 97% coulombic efficiency (CE) and 71% energy efficiency (EE) was achieved at a current density of 60 mA cm⁻². Furthermore, we observed 99% capacity retention over 100 cycles and 75% capacity retention over 500 charge/discharge cycles. Additionally, we performed a detailed Density Functional Theory (DFT) analysis of MMV. This study underscores the importance of inhibiting dimer formation in viologen derivatives and positions MMV as a promising negolyte surpassing DMV.