One way to drive down the cost of flow batteries is to eliminate the fluorinated polymer membranes altogether and come up with a high-performing yet cheaper alternative such as AquaPIMs, said Miranda Baran, a graduate student researcher in Helms' research group.
Initial polymer membranes for aqueous alkaline systems were modified with an exotic chemical called an "amidoxime" that allows ions to quickly travel between the anode and cathode. Later, while evaluating AquaPIM membrane performance and compatibility with different grid battery chemistries, the researchers found these lead to remarkably stable alkaline cells. The AquaPIM prototypes also retained the integrity of the charge-storing materials in the cathode as well as in the anode.
Baran and her collaborators then tested how an AquaPIM membrane would perform with an aqueous alkaline electrolyte. The polymer-bound amidoximes are stable, which was a surprise as organic materials are not typically stable at high pH. This prevented the AquaPIM membrane pores from collapsing, allowing them to stay conductive without any loss in performance over time. This compared to the pores of a commercial fluoro-polymer membrane collapsed as expected.
Simulating the structures of AquaPIM membranes using computational resources at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC) found that the structure of the polymers making up the membrane were significantly resistant to pore collapse under highly basic conditions in alkaline electrolytes.
This then led to a model that tied the performance of the battery to the performance of various membranes. This model could predict the lifetime and efficiency of a flow battery without having to build an entire device. They also showed that similar models could be applied to other battery chemistries and their membranes.
"Typically, you'd have to wait weeks if not months to figure out how long a battery will last after assembling the entire cell. By using a simple and quick membrane screen, you could cut that down to a few hours or days," Helms said.
The researchers next plan to apply AquaPIM membranes across a broader scope of aqueous flow battery chemistries, from metals and inorganics to organics and polymers. They also anticipate that these membranes are compatible with other aqueous alkaline zinc batteries, including batteries that use either oxygen, manganese oxide, or metal-organic frameworks as the cathode.