Membraneless biphasic redox flow batteries: Interfacial effects and generalisation of the chemistry

The wider adoption of redox flow batteries (RFBs) is hindered partly by the high cost of ion-exchange membranes. Membrane-free batteries have recently emerged as a potential alternative to traditional RFBs, with the redox anolyte and catholyte confined to immiscible aqueous and non-aqueous phases. The overall performance of the “interfacial” batteries reported to date has been inferior to conventional RFBs. Here we report a cell with an all-aqueous biphasic interface (ABI), using a range of redox mediators. Phase separation with the ABI is achieved by using solutions of monovalent salts (LiCl and lithium bis(trifluoromethanesulfonyl)imide). Interfacial properties play an important role in determining the performance of ABI batteries: optimisation of the (liquid) interfacial area and the inclusion of a common-ion, such as Li⁺, are shown to be key factors dictating cell performance.