How do you store renewable energy so that it is available when you need it, even when the sun is not shining or the wind is not blowing? Giant batteries designed for the power grid - called flow batteries, which store electricity in liquid electrolyte tanks - could be the solution, but so far utilities have not found a viable battery yet can reliably feed thousands of life cycles from 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 be a solution.
As reported in the Joule Journal, researchers have developed a versatile and affordable battery membrane from a class of polymers known as AquaPIM. This class of polymers allows for low-cost, durable network batteries based solely on readily available materials such as zinc, iron and water. The team also developed a simple model showing the impact of different battery membranes on battery life, which should accelerate the early stages of R & D on DC battery technologies, in particular in the search for a membrane adapted to the different types of chemical composition of the batteries.
"Our AquaPIM membrane technology is well positioned to accelerate the introduction of flow batteries that use scalable, low-cost water-based chemical compositions," said Brett Helms, Principal Investigator at the Joint Center for Energy Storage Research. (JCESR). and scientist at the Molecular Foundry of Berkeley Lab who led the study. "By using our technology and the corresponding empirical models for battery performance and life, other researchers will be able to quickly assess the readiness of each component entering the battery, from the membrane to the battery. This should save time and resources for researchers and product developers.
Most of the chemical compositions of grid batteries have strongly alkaline (or basic) electrodes - a positively charged cathode on one side and a negatively charged anode on the other side. However, state-of-the-art membranes are designed for acidic chemicals, such as fluorinated membranes in fuel cells, but not for alkaline batteries. (In chemistry, pH is a measure of the hydrogen ion concentration of a solution.) The pure water has a pH of 7 and is considered neutral.The acidic solutions have a high concentration of hydrogen ions and are described as having a low pH or a pH below 7. On the other hand, the alkaline solutions have low concentrations of hydrogen ions and therefore have a high pH, or a pH greater than 7. In alkaline cells, the pH can reach 14 or 15.)
Fluoropolymer membranes are also expensive. According to Helms, they can represent between 15% and 20% of the cost of the battery, which can reach $ 300 / kWh.
One way to reduce the cost of discharge batteries is to completely eliminate fluoropolymer membranes and offer a high-performance but less expensive alternative, such as AquaPIM, said Miranda Baran, a research student graduate of the Helms research group. author. Baran is also a Ph.D. student in UC Berkeley's Department of Chemistry.
Getting back to basics
Helms and his co-authors have discovered AquaPIM technology - which means "water-compatible intrinsic microporosity polymers" - while developing polymer membranes for alkaline (or basic) aqueous systems as part of a collaboration with the company. - author Yet-Ming Chiang. , Principal Investigator at JCESR and Professor Kyocera of Materials Science and Engineering at the Massachusetts Institute of Technology (MIT).
Through these early experiments, researchers learned that membranes modified with an exotic chemical called "amidoxime" allowed ions to move rapidly between the anode and the cathode.
Later, while evaluating the performance of the AquaPIM membrane and its compatibility with different network battery chemistries (for example, an experimental setup used zinc as anode and an iron-based compound as a cathode), the researchers found that AquaPIM membranes led to remarkably stable alkaline cells. .
In addition, they found that the AquaPIM prototypes maintained the integrity of the materials storing the charges in the cathode as well as in the anode. When researchers characterized Berkeley Lab's Advanced Light Source (ALS) membranes, they found that these features were universal for all AquaPIM variants.
Baran et al. Then tested the operation of an AquaPIM membrane with an aqueous alkaline electrolyte. In this experiment, they found that under alkaline conditions, polymer-bound amidoximes are stable - a surprising result given that organic materials are generally not stable at high pH.
Such stability prevented the pores of the AquaPIM membrane from collapsing, allowing them to remain conductive without loss of performance over time, while the pores of a commercial fluoropolymer membrane collapsed as expected, at detriment of its ion transport properties, explained Helms.
This behavior was corroborated by theoretical studies conducted by Artem Baskin, a postdoctoral researcher working with David Prendergast, acting director of Molecular Foundry of Berkeley Lab, and senior researcher at JCESR alongside Chiang and Helms.
Baskin simulated the AquaPIM membrane structures using Berkeley Lab's National Energy Research Center (NERSC) computational resources and found that the structure of the polymers composing the membrane was significantly resistant to pore collapse. under very basic conditions of alkaline electrolytes.
A screen test for better batteries
While evaluating the performance of the AquaPIM membrane and the compatibility with different chemistries of grid batteries, the researchers have developed a model linking the performance of the battery performance of different membranes. This model could predict the life and efficiency of a flow battery without having to build a complete device. They have also shown that similar models could be applied to other battery chemistries and their membranes.
"As a general rule, it would take weeks, if not months, to determine how long a battery will last after assembling the whole cell.The use of a fast, easy-to-use membrane screen reduces processing time to a few hours or days, "Helms said.
The researchers then plan to apply AquaPIM membranes to a wider range of aqueous flow battery chemistries, from metals and inorganics to organic products and polymers. They also expect these membranes to be compatible with other aqueous zinc alkaline cells, including cells that use oxygen, manganese oxide or organic metal structures as the cathode.
Researchers from Berkeley Lab, University of California at Berkeley, Massachusetts Institute of Technology and Istituto Italiano di Tecnologia participated in the study.
Miranda J. Baran, Miles N. Braten, Swagat Sahu, Artem Baskin, Stephen M. Meckler, Longjun Li, Lorenzo Maserati, Mark E. Carrington, Yet-Ming Chiang, David Prendergast, Brett A. Helms. Design Rules for Membranes from Polymers of Intrinsic Microporosity for Crossover-free Aqueous Electrochemical Devices. Joule, 2019; DOI: 10.1016/j.joule.2019.08.025