Investigating Transport through Separator Membranes in Aqueous Organic Redox Flow Batteries Using NMR Spectroscopy

Abstract

Electricity and heat generation contribute 25% of greenhouse gas emissions globally.1 Redox flow batteries (RFBs) are prospective devices for long-duration energy storage, which is required to integrate more renewable energy sources onto the electricity grid.2 RFBs have a modular design with decoupled power and energy ratings, allowing them to be scaled to suit grid-level energy storage requirements. The development of aqueous organic redox flow batteries (AORFBs), such as quinone-based systems, is gaining momentum because they are potentially cheaper, safer and more sustainable than vanadium-based RFBs.3-5 However, the crossover of redox-active species through the separator membrane can lead to irreversible capacity fade, limiting their lifetime and economic viability.6 It has previously been demonstrated that in situ spectroscopic techniques are powerful tools for determining reaction mechanisms in redox flow batteries.5,7 Here, we explore how solution-state in situ nuclear magnetic resonance (NMR) spectroscopy and solid-state NMR spectroscopy can be used to study the crossover of electrolytes in AORFBs. We demonstrate that in situ solution NMR spectroscopy can be used to characterise transport in operating AORFBs with high temporal resolution and minimal system disturbance. This method can therefore be applied to investigate how crossover is governed by structure-property relationships and the charging protocols used. Furthermore, polymer-electrolyte interactions within the membrane can be probed using complementary solid-state NMR studies. Together, these fundamental studies will ultimately advance our understanding of electrolyte crossover, so that improved separator membranes can be developed.