A compound widely used in candles promises a much more modern energy challenge – storing large amounts of energy that need to be fed into the power grid when needed.
Scientists at the Department of Energy's Pacific Northwest National Laboratory have shown that inexpensive organic compounds hold promise for storing grid energy. Ordinary fluorenone, a bright yellow powder, was a reluctant participant at first, but with sufficient chemical persuasion it has proven to be an effective partner for energy storage in flow battery systems, large systems that store energy for the grid.
The development of such a memory is crucial. If the network goes offline due to storms, for example, the large batteries under development are activated, which increases network stability and minimizes disruptions. The batteries can also be used to store renewable energy from wind and sun when the wind is calm or the sun is not shining.
Details of the research supported by the DOE Office of Electricity will be published in the May 21st issue of Science journal.
“Flow-through battery technology is an important part of the Department of Energy's goal to reduce the cost of storing grid energy over the next decade,” said Imre Gyuk, director of energy storage at the DOE Office of Electricity. “Progress has been rapid and costs have come down significantly. However, more research is needed to make grid-scale energy storage widely available.”
Flow batteries for the power grid: Bio
Scientists are making tremendous strides in creating better batteries – they store more energy at less cost and last longer than ever before. The results touch many aspects of our lives and lead to a more resilient power grid, longer-lasting laptop batteries, more electric vehicles and greater use of renewable energies through wind, sunshine or running water.
With grid-scale batteries, identifying the right materials and combining them into a new recipe for energy storage is a critical step in the world's ability to use and store renewable energy. The most widely used grid-scale batteries use lithium-ion technology. However, it is difficult to tweak these from moment to moment so that they are most useful to the network, and there are security concerns. Redox flow batteries are a growing alternative. However, most use vanadium, which is expensive, not readily available, and prone to price fluctuations. These characteristics pose barriers to widespread grid-scale energy storage.
Alternative materials for flow batteries include organic molecules, which are far more available, environmentally friendly, and cheaper than vanadium. But organics have not withstood the demands of flow battery technology well and usually fade faster than necessary. The long-term stability of the molecules is important so that they can carry out chemical reactions over many years.
“These organic materials are made from the most common materials available – carbon, hydrogen and oxygen,” said Wei Wang, the PNNL scientist who leads the flow battery team. “They are readily available; they don't have to break down like substances like vanadium. This makes them very attractive for grid-scale energy storage.”
In the science paper, Wang's team showed that, surprisingly, inexpensive organic fluorenone is not only a viable candidate, but also an outstanding performer in terms of energy storage.
In laboratory tests that mimicked real-world conditions, the PNNL battery worked non-stop for 120 days, only ending when other non-battery devices wore out. The battery went through 1,111 full charge and discharge cycles – this corresponds to several years of operation under normal circumstances – and lost less than 3 percent of its energy capacity. Other organic-based flow batteries have worked much shorter.
The flow battery created by the team is only about 10 square centimeters, about the size of a large postage stamp, and has an output of about 500 milliwatts, which is not even enough to power a cell phone camera. The tiny structure holds promise, however: its energy density is more than twice that of the vanadium batteries used today, and its chemical components are inexpensive, durable, and widely available.
Fluorenone is reversed through molecular engineering
The development was made possible by a team of scientists including first author Ruozhu Feng, technical director Xin Zhang and others.
PNNL scientists played an important role in the development of the vanadium-based flow batteries used today. A few years ago, the team focused on organic molecules because of its wide availability and low cost. In 2018, Zhang joined the team to optimize the material for energy storage and gain deep knowledge of fluorenone from previous LED research.
Fluorenone is also used in solar panels, in medicines such as medicines used to treat malaria, and in candles to give them a pleasant scent. It is inexpensive and readily available as a waste product from coal tar and from the manufacture of benzoic acid, a common food additive.
Zhang focused his attention on fluorenone as the heart of an aqueous (water-based) flow battery, but there were barriers. For one thing, the molecule wasn't water soluble enough. And the molecule had shown no redox reversibility in aqueous solutions; That said, scientists hadn't proven that it could pick up and give up electrons easily, two complementary and mandatory steps for a flow-through battery.
Feng created a series of complex chemical steps – what Wang calls “molecular engineering” – to convert fluorenone into a redox-reversible, water-soluble compound. Part of the process has long been easy for fluorenone: gaining an electron in a process known as reduction. But it took Feng a tenacious chemical belief to bring about the other half of the process – oxidation, loss of an electron – to make the process reversible and suitable for energy storage.
Unexpectedly, Feng discovered that the ability of fluorenone to perform reversible reactions depends on its concentration – more of the substance dissolved in the water enables the reversibility. Scientists had not previously observed the phenomenon with organic molecules.
“This is a great demonstration of using molecular engineering to turn a material from what is generally considered impossible to something useful for energy storage,” said Wang. “This opens up important new chemical spaces for us to explore.”
The team also increased the solubility of fluorenone in water from almost zero with virgin fluorenone to 1.5 moles per liter, depending on the modifications of the compound. Solubility in a water-based flow cell battery is critical. The more the material dissolves in water, the more it is available as a chemical partner in the exchange of electrons in the heart of the battery.
PNNL promotes the marketing of aqueous redox flow batteries based on fluorenone and has applied for a patent for the innovation as a first step.
Work on flow batteries is part of an extensive program at PNNL to develop and test new technologies for grid-scale energy storage. PNNL was selected earlier this year as the home of the Grid Storage Launchpad, a facility established by the DOE Office of Electricity to accelerate the development and testing of large grid batteries. A major goal is to increase the use of readily available materials and reduce costs in order to enable renewable energy to be stored for longer periods of time.
In addition to Feng, Zhang and Wang, the authors also include PNNL scientists Vijayakumar Murugesan, Aaron Hollas, Ying Chen, Yuyan Shao, Eric Walter, Nadeesha Wellala, Litao Yan and Kevin Rosso. Several measurements using mass spectrometry and nuclear magnetic resonance were made at EMSL, the Environmental Molecular Sciences Laboratory, a user facility of the DOE Office of Science.