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Improved Flow Batteries soon to Power Grids

Today, batteries play an important role in powering electrical devices, tools and cars. Soon, they might even be able to supply energy to entire girds. The reason for this positive development is large battery devices called flow batteries which use tanks of electrolytes capable of storing enough electricity to power thousands of homes for many hours. However, most flow batteries rely on vanadium, a somewhat rare and expensive metal, and alternatives are short-lived and toxic which is why scientists still have to find a cost-effective battery that can reliably power thousands of homes throughout a lifecycle of 10 to 20 years.

Now (2019), scientists at the Berkeley National Laboratory have developed a battery membrane technology from a class of polymers known as AquaPIMs. This class of polymers were turned into long-lasting and low-cost grid batteries which were based solely on readily available materials such as zinc, iron, and water. The team also designed a simple model showing how different battery membranes impacted the lifetime of the battery, which should accelerate early stage R&D for flow-battery technologies, particularly in the search for a suitable membrane for different battery chemistries.

The AquaPIM technology – which stands for “aqueous-compatible polymers of intrinsic microporosity” – was discovered while developing polymer membranes for aqueous alkaline (or basic) systems as part of a collaboration the Massachusetts Institute of Technology (MIT).

Through these early experiments, the researchers found that membranes modified with an exotic chemical called an “amidoxime” allowed ions to quickly travel between the anode and cathode.

Later, while they evaluated the 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.

Research on flow batteries has been carried out for many decades. In 2012, scientists designed a redox flow battery which included a catholyte, an anolyte, and an anion exchange membrane disposed between the catholyte and the anolyte, wherein at least one of the catholyte and the anolyte is an organic electrolyte solution including a non-aqueous solvent, a support electrolyte, and a metal-ligand coordination compound, wherein the metal-ligand coordination compound was dissolved in an electrolyte solution while the metal-ligand coordination compound was in an atom state with Zero oxidation. The metal in the metal-ligand coordination compound was selected from the group consisting of nickel (Ni), cobalt (Co), iron (Fe), ruthenium (Ru), Zinc (Zn), manganese (Mn), yttrium (Y), zirconium (Zr), titanium (Ti), chromium (Cr), magnesium (Mg), cerium (Ce), and copper (Cu).

In 2016, a battery was developed which used polyoxometalates as the electrolytes for one or both the anode portion and cathode portion of a flow battery. In one embodiment, the cathode reservoir contained a polyoxometalate material and the anode reservoir also contained a polyoxometalate material. In another embodiment, the polyoxometalate material used in the cathode portion was similar to the polyoxometalate material used in the anode portion. Representative polyoxometalate ions suitable as an electrolyte comprised those including a Keggin anion, a Lundquist anion, a Wells Dawson anion and a mixed addenda anion. The polyoxometalate material consisted of an aqueous material or a non-aqueous material. In a further embodiment of the invention, the polyoxometalate material was an alkaline material (e.g., a polyoxometalate anion including alkali metal or alkaline earth metal cations for aqueous salts) or a polyoxometalate anion.

The advantages of the new flow battery design are numerous: the experiments showed that under alkaline conditions, polymer-bound amidoximes are stable, which was 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, whereas the pores of a commercial fluoro-polymer membrane collapsed as expected, to the detriment of its ion transport properties.
Next, the researchers want to apply AquaPIM membranes across a broader scope of aqueous flow battery chemistries, from metals and inorganics to organics and polymers. They believe that these membranes are compatible with other aqueous alkaline zinc batteries, including batteries that use oxygen, manganese oxide, or metal-organic frameworks as the cathode.