New blueprint for affordable, sustainable ‘flow batteries’ developed at Berkeley Lab could accelerate an electrical grid powered by the sun and wind
By Theresa Duque
November 7, 2019 – 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, which store electricity in tanks of liquid
electrolyte – could be the answer, but so far utilities have yet to find a
cost-effective battery that can reliably power thousands of homes throughout a
lifecycle of 10 to 20 years.
Now, a battery membrane technology
developed by researchers at the U.S. Department of Energy’s Lawrence Berkeley
National Laboratory (Berkeley Lab) may point to a solution.
As reported in the journal 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, which 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 battery chemistries have
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 membranes are designed for acidic chemistries, such as the
fluorinated membranes found in fuel cells, but not for alkaline flow batteries.
(In chemistry, pH is a measure of the hydrogen ion concentration of a solution.
Pure water has a pH of 7 and is considered neutral. 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, alkaline solutions have low concentrations of
hydrogen ions and therefore have a high pH, or a pH above 7. In alkaline
batteries, the pH can be as high as 14 or 15.)
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 and the
study’s lead author. Baran is also a Ph.D. student in the Department of
Chemistry at UC Berkeley.
Getting back to basics
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 as part of a collaboration with co-author Yet-Ming Chiang, a
principal investigator in JCESR and Kyocera 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 quickly travel 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), the researchers found
that these characteristics 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 under alkaline conditions, polymer-bound
amidoximes are stable – 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, Helms explained.
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 were significantly resistant to pore
collapse under highly basic conditions in alkaline electrolytes.
A screen test for better batteries
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. 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.
Researchers from Berkeley Lab, UC
Berkeley, Massachusetts Institute of Technology, and Istituto Italiano di
Tecnologia participated in the study.
This work was supported by the Joint
Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by
the U.S. Department of Energy, Office of Science. Additional funding was provided by the Center for Gas Separations Relevant to Clean Energy
Technologies, a DOE Office of Science Energy Frontier Research Center.
Portions of the work, including polymer
synthesis and characterization, were carried out at Berkeley Lab’s Molecular
Foundry, a DOE Office of Science User Facility that specializes in nanoscale
science.
The study also used GIWAXS
(grazing-incidence wide angle X-ray scattering) instruments at the ALS to
characterize the AquaPIMs, and supercomputing resources at NERSC to model the
polymer. The ALS and NERSC are DOE Office of Science User Facilities.
This technology is available for
licensing and collaboration. If interested, please contact Berkeley Lab’s
Intellectual Property Office, ipo@lbl.gov.
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