An AquaPIM flow battery membrane. Photo: Marilyn Sargent/Berkeley Lab.
An AquaPIM flow battery membrane. Photo: Marilyn Sargent/Berkeley Lab.

How do you store renewable energy so it's there when you need it, even when the sun isn't shining or the wind isn't blowing? Giant batteries designed for the electrical grid, called flow batteries, could be the answer; these batteries comprise two tanks of liquid electrolyte, which generate electricity by flowing either side of a polymer membrane. But utilities have yet to find a cost-effective flow battery that can reliably power thousands of homes over a lifecycle of 10 to 20 years.

Now, a battery membrane technology developed by researchers at the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) may point to a solution.

As reported in a paper in Joule, the researchers developed a versatile yet affordable battery membrane – from a class of polymers known as AquaPIMs. This class of polymers makes long-lasting and low-cost grid batteries possible based solely on readily available materials such as zinc, iron and water.

The team also developed a simple model showing how different battery membranes impact the lifetime of the battery. This model is expected to accelerate early stage R&D for flow-battery 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 batteries utilize highly alkaline (or basic) electrodes – a positively charged cathode on one side and a negatively charged anode on the other side. But current state-of-the-art polymer membranes, such as the fluorinated membranes found in fuel cells, are designed for acidic chemistries, not for alkaline flow batteries. Fluorinated polymer membranes are also expensive: according to Helms, they can account for 15–20% of a flow 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, said Miranda Baran, a graduate student researcher in Helms' research group and the paper's lead author. Baran is also a PhD student in the Department of Chemistry at the University of California, Berkeley.

Helms and co-authors discovered the AquaPIM technology – which stands for ‘aqueous-compatible polymers of intrinsic microporosity’ – while developing polymer membranes for aqueous alkaline (or basic) systems. This was part of a collaboration with co-author Yet-Ming Chiang, a principal investigator in JCESR and professor of materials science and engineering at the Massachusetts Institute of Technology (MIT).

Through these early experiments, the researchers learned that membranes modified with an exotic chemical called an ‘amidoxime’ allowed ions to travel quickly between the anode and cathode. Later, while evaluating AquaPIM membrane performance and compatibility with different grid battery chemistries – for example, one experimental setup used zinc as the anode and an iron-based compound as the cathode – the researchers discovered that AquaPIM membranes lead to remarkably stable alkaline cells.

In addition, they found that the AquaPIM prototypes retained the integrity of the charge-storing materials in the cathode as well as in the anode. When the researchers characterized the membranes at Berkeley Lab's Advanced Light Source (ALS), they found that these properties were universal across AquaPIM variants.

Baran and her collaborators then tested how an AquaPIM membrane would perform with an aqueous alkaline electrolyte. In this experiment, they discovered that polymer-bound amidoximes are stable under alkaline conditions – a surprising result considering that organic materials are not typically stable at high pH.

Such stability prevented the AquaPIM membrane pores from collapsing, thus allowing them to stay conductive without any loss in performance over time. In contrast, the pores of a commercial fluoro-polymer membrane collapsed as expected, to the detriment of its ion transport properties.

This behavior was further corroborated with theoretical studies by Artem Baskin, a postdoctoral researcher working with David Prendergast, who is the acting director of Berkeley Lab's Molecular Foundry and a principal investigator in JCESR, along with Chiang and Helms. Baskin simulated structures of AquaPIM membranes using computational resources at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC) and found that the structure of the polymers making up the membrane ensured they were significantly resistant to pore collapse under the highly basic conditions in alkaline electrolytes.

While evaluating AquaPIM membrane performance and compatibility with different grid battery chemistries, the researchers developed 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. The researchers 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 will be compatible with other aqueous alkaline zinc batteries, including batteries that use oxygen, manganese oxide or metal-organic frameworks as the cathode.

This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.