There are two electrodes in every battery. One electrode, called the cathode, connects to the positive end of the battery and is where the electric current leaves the battery. The other electrode, the anode, connects to the negative end of the battery and is where the electric current enters the battery. The flow of these electrons through an external circuit is what produces the electric current. However, for every charge that flows in the external circuit, a similar current carried by an ion inside of the battery needs to exist. These ions are transferred through a membrane or separator.
The separator sits between the anode and the cathode inside the battery. Without a separator, the two electrodes would come into contact, which would prevent the battery from working properly.
One of the three energy storage concepts being explored by JCESR is the redox flow battery, which stores energy in liquids instead of solids and appears to be well suited for grid application. One liquid is for the cathode (catholyte), and one is for the anode (anolyte). Each liquid contains molecules designed to activate the reduction-oxidation (redox) reactions necessary for energy storage and electricity generation. These liquids are passed through their respective electrode compartments, while charge-balancing ions flow through a porous separator between the two compartments.
There are two types of redox flow batteries: one with a water-based (aqueous) electrolyte and one with a non-aqueous electrolyte. Little research has been done on the non-aqueous type, despite their great potential, because of the lack of a suitable separator that permits proper ion flow between electrode compartments while preventing the detrimental mixing of the anolyte and catholyte.
Changing the paradigm, a JCESR response to this challenge has been the pursuit of a size-selective strategy for these batteries, where the focus is now on the design of large redox active polymer molecules for the liquids while using a simple and inexpensive porous separator to prevent mixing of the anolyte and catholyte. As shown in the figure, this scheme allows the unimpeded flow of charge-balancing ions through the separator membrane pores, while keeping the redox active polymers in their own electrode compartment for delivering energy.
Our recent publication in the Journal of the American Chemical Society reports on experimental work demonstrating the feasibility of this approach. Through a JCESR Sprint, we are now investigating new ideas to improve redox active polymers: from increasing their size to control unwanted mixing, to enhancing their flow properties, energy density, and ability to transfer charge. Redox active polymers, an innovation enabled by JCESR, are strong new players in the energy storage field.