Here we present a multiple ion-exchange membrane (IEM) cell design for redox flow batteries (RFBs) that can generally accommodate redox pair combinations with any mixed ion charges. This design also allows flexible choice of electrolytes such as an acid electrolyte at one electrode while a base in the other. More specifically, the double-IEM cell with one anion-exchange membrane (AEM), one cation-exchange membrane (CEM), and a middle electrolyte in between can work with all redox pairs except for the case of two hybrid redox pairs (i.e., an anion–cation pair vs. an anion–cation pair). For the combination of two hybrid pairs, a triple-IEM cell with three membranes (CEM/AEM/CEM or AEM/CEM/AEM) and two middle electrolytes is needed. The double- and triple-IEM cells bring unprecedented freedom in choosing redox pairs and supporting electrolytes. Of particular importance, two featured aqueous RFBs are demonstrated here: (1) ultra-high voltage zinc–cerium RFB with 3.08 V standard cell voltage, the highest among all known aqueous RFBs; and (2) ultra-low cost sulfur–iron RFB with 1.22 V standard cell voltage, with two highly available elements (iron and sulfur are the 1st and 5th most produced elements worldwide, respectively).
