Flow batteries are promising for long-duration grid-scale energy storage. Future terawatt-scale
deployment of flow batteries will require substantial capital cost reduction, particularly low-cost
electrolytes and hydrocarbon ion exchange membranes. However, integration of hydrocarbon
membranes with novel flow battery chemistries in commercial-scale stacks is yet to be demonstrated.
Here we report the pilot scale synthesis and roll-to-roll manufacturing of sulfonated poly(ether ether
ketone) (SPEEK) membranes and demonstrate their high hydroxide conductivity and chemical stability
in kW-scale alkaline-based flow batteries. After exposure to a 5 mol L-1 NaOH solution at 60 °C for 41 days,
the SPEEK membrane still enabled a stable alkaline zinc-iron flow battery performance for more than 650
cycles (more than 650 hours) at high current densities (80 to 160 mA cm-2
). Furthermore, the membrane
was integrated in flow battery stacks with power up to 4000 W, which demonstrated a high energy
efficiency of 85.5% operated at 80 mA cm-2 and long term stable operation over 800 h as well as
substantial cost savings relative to Nafion membranes. This work illustrates a potential pathway for
manufacturing and upscaling of next-generation cost-effective flow batteries based on low-cost
hydrocarbon membranes developed in past decades to translate to large scale applications for longduration energy storage.