Grid-scale storage of electricity is vital in energy scenarios with a high share of renewable electricity generation, such as wind and solar power. Redox flow batteries are particularly suited for intra-day time-shifting storage applications, yet investment costs need to be lowered for economic viability of the technology.

Vanadium redox flow batteries (VFB) use vanadium-ions in different oxidation states as redox-active species in the two electrolytes, thus irreversible contamination of electrolytes by cross-mixing is avoided. They offer independent scalability of energy and power, safe operation, long cycle-life and deep discharge capability. In the current state of the art, either a cation exchange membrane (CEM) or an anion exchange membrane (AEM) is used as a polymer electrolyte in VFBs. CEMs offer high conductivity, yet vanadium barrier properties are poor, and vice-versa for AEMs. Furthermore, with these membrane types, capacity fading is rather pronounced, which is related to the asymmetric transport properties of vanadium-ions through these materials. Therefore, amphoteric ion exchange membranes (AIEMs) were designed, which contain both anion and cation exchange groups, to balance transport effects. On the device level (cf. Figure), the superior capacity retention using this type of membrane is clearly demonstrated. Currently, the technology is further developed in the framework of a Bridge-Discovery project funded by the National Science Foundation (SNF) for scale-up and industrial implementation.

Discharge capacity of a vanadium redox flow cell over 80 charge-discharge cycles at a current density of  40 mA/cm2, comparing an cation exchange membrane (CEM, Nafion NR212), an anion exchange membrane (AEM, Fumatech FAP-450), and an amphoteric ion exchange membrane (AIEM) developed at PSI.

 

The research is being conducted within the Energy and Environment Research Division (ENE) at the Paul Scherrer Institute (PSI), headed by the ESC Member Prof. Thomas Schmidt. Schmidt is also Professor at the Laboratory of Physical Chemistry (LPC) at ETH Zurich’s Department of Chemistry and Applied Biosciences (D-CHAB).

Further information about the research can be found in

Oldenburg, F.J., Schmidt, T.J., and Gubler, L., “Tackling capacity fading in vanadium flow batteries with amphoteric membranes”, J. Power Sources 368 (2017), 68-72, doi: 10.1016/j.jpowsour.2017.09.051