Membrane enables low cost flow battery for grid storage
Researchers in Italy and the US have developed a low cost membrane technology that can be used for a flow battery in energy storage systems.
The team from Istituto Italiano di Tecnologia, the Berkeley Lab, UC Berkeley and the Massachusetts Institute of Technology used a class of polymers known as AquaPIMs (aqueous-compatible polymers of intrinsic microporosity) for long-lasting and low-cost aqueous grid batteries that use materials such as zinc, iron, and water. The team also developed a simple model showing how different battery membranes impact the lifetime of the flow battery, which is expected to accelerate early stage R&D for such technologies, particularly in the search for a suitable membrane for different battery chemistries.
“Our AquaPIM membrane technology is well-positioned to accelerate the path to market for flow batteries that use scalable, low-cost, water-based chemistries,” said Brett Helms, a principal investigator in the Joint Center for Energy Storage Research (JCESR) and staff scientist at Berkeley Lab’s Molecular Foundry who led the study. “By using our technology and accompanying empirical models for battery performance and lifetime, other researchers will be able to quickly evaluate the readiness of each component that goes into the battery, from the membrane to the charge-storing materials. This should save time and resources for researchers and product developers alike.”
Most grid battery chemistries have highly alkaline (or basic) electrodes with a positively charged cathode on one side, and a negatively charged anode on the other side. But current state-of-the-art membranes are designed for acidic chemistries, such as the fluorinated membranes found in fuel cells, but not for alkaline flow batteries. Fluorinated polymer membranes are also expensive. According to Helms, they can make up 15% to 20% of the battery’s cost, which can run in the range of $300/kWh.
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.
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