The intermittent nature of renewable energy sources renders their energy output difficult to store, but researchers believe they have the solution: the low-cost flow battery.
With potential shortages of fossil fuels and the threat of climate change caused by global warming, many people have their hopes pinned on renewable energy sources such as wind and sunlight.
Significant improvements have been made in these areas over recent years. However, they share a fundamental limitation: it is not always sunny and it is not always windy.
There has been considerable research into how electricity generated in these ways can be stored until it is needed.
"I believe that large-scale energy storage is the biggest obstacle to us getting a very large fraction of our energy from renewable sources," says Michael J Aziz, professor of materials and energy technologies at the Harvard School of Engineering and Applied Sciences.
To this end, Aziz's research group has, over recent years, focused on flow batteries. These batteries are based on tanks of electroactive compounds dissolved in liquids (electrolytes) that are separated by a membrane across which electrons and protons can transfer, reversibly storing and generating electricity.
The electrolytes are stored in tanks and pumped through the battery as needed, meaning that the batteries can be used for storing energy from intermittent electricity generation methods such as wind or solar. This also means that energy storage can be varied, depending on the size of the tanks.
"Unlike a normal battery, with a flow battery you decouple the peak power and the energy capacity. This means that the limit to the power is the size of the tanks and the electrodes," Aziz explains.
However, flow batteries have also had limitations to date. The most commercially-advanced flow batteries, which are developed and sold by several companies today, are vanadium redox flow batteries, which use vanadium ions in different oxidation states. However, these are expensive and their costs limit the realistic scale that these systems can be used at - and this scale falls well short of what would be of value to a nationwide electricity network, for example.
Other flow batteries contain precious metal electrocatalysts such as the platinum used in fuel cells but these metals are even more expensive.
Aziz says that his group has looked at cheaper inorganic chemicals but it was hard to maintain the performance seen with the vanadium redox system.
The team did not give up though. Sitting quietly in the background was an alternative candidate for flow batteries, one that was already known for its electrochemical properties and the role these play in nature. This was the group of organic compounds known as quinones, which are aromatic ring systems with carbonyl, or double-bonded carbon-oxygen, groups as substituents.
Many quinones are naturally occurring and abundant in plants and crude oil. Given the abundance of starting materials, they are inexpensive to make. They are also known for their electrochemical properties and many quinones are soluble in water.
Solubility is a key concern for a large flow battery. Being able to dissolve in water prevents the risk of fire as the size and power increases. A lithium ion battery, for example, would be very dangerous at a size of 1,000m3, according to Aziz, but a 1,000m3 flow battery with quinones dissolved in water should be fine because of the presence of the water.
The screening process
Quinones seemed a good candidate for the Harvard team. But things are never that simple. There are tens of thousands of potential quinones that the researchers could look at and only a small number will have the right set of properties to be effective in a flow battery. "There was a vast array; some are awful and some are great," Aziz says.
For the past two years or so, an interdisciplinary team at Harvard has been discussing this problem. Part of the solution lay with Alán Aspuru-Guzik, professor of chemistry and chemical biology, who is a theoretical chemist. He took on the task of carrying out high-throughput molecular screening on the vast array of quinones to help identify some suitable candidates for further research.
He screened over 10,000 possible compounds with one, two or three benzene rings based on three criteria for possible compounds: reduction potential, solubility and stability.
Reduction potential is key for successful flow batteries, according to Aziz. Unless the voltage across the battery is reasonably high, large tanks and more equipment would be needed. In addition, there would be higher ohmic losses in lower-voltage systems.
The next criterion is solubility in water. As Aziz observes, "if you double the solubility, you can halve the tank size".
The third key criterion, which is not easy to test theoretically and so was mostly done empirically, is stability. There is a limit to the potential voltage before the quinone begins to spit out oxygen, an undesired result. "We want the molecule to pick up two protons or two electrons and nothing else," Aziz says.
Aspuru-Guzik's screening resulted in a shortlist of possible candidates. "This project illustrates what the synergy of high-throughput quantum chemistry and experimental insight can do," he says. "In a very quick time period, our team honed into the right molecule. Computational screening, together with experimentation, can lead to discovery of new materials in many application domains."
At this stage the baton was passed on to Roy G Gordon, professor of chemistry and professor of materials science at Harvard, who led the work on the synthesis and chemical screening of molecules, while Brian Huskinson and Michael P Marshak designed and tested the battery, with direction from Aziz.
The interdisciplinary efforts proved productive and at the start of this year the team published a paper, 'A metal-free organic-inorganic aqueous flow battery', in the journal Nature.
This paper described a flow battery based on one of the quinones studied. In fact, the quinone chosen, 9,10-anthraquinone-2,7-disulphonic acid (AQDS), is chemically very similar to a compound found naturally in rhubarb - "rhubarb but with higher solubility", as Aziz describes it. According to the paper, "AQDS undergoes extremely rapid and reversible two-electron two-proton reduction on a glassy carbon electrode in sulphuric acid". This prototype aqueous flow battery, with an AQDS on the negative side and a standard bromine-based redox couple on the positive side, yielded a peak galvanic power density that exceeded 0.6W/cm2 at 1.3A/ cm2. Cycling of this quinone-bromide flow battery showed better than 99 per cent storage capacity retention per cycle, say the authors.
According to the Harvard team, the new flow battery already performs as well as vanadium redox flow batteries, with chemicals that are significantly less expensive, and with no precious metal electrocatalyst. The authors write in the Nature paper: "The use of p-aromatic redox-active organic molecules instead of redox-active metals represents a new and promising direction for realising massive electrical energy storage at greatly reduced cost."
This is not the first time that people have looked at organic compounds for this type of system. However, many other candidates have been not shown as good reversibility as quinones.
"The high degree of reversibility of quinones is known in nature," says Aziz. "We'think that a lot of other molecules essentially need to restructure between oxidation and reduction. In our molecule, the rings don't change; the only thing that'happens is that a carbonyl group [double'bond between carbon and oxygen] changes to an alcohol group [single bond between the carbon and oxygen and the addition of hydrogen on the oxygen]. We'think that's why they are highly reversible.
"We hope that this paper will inspire someone to support us in doing basic research into these systems," he adds.
Potential applications for quinone-based batteries
This development holds plenty of potential, according to Aziz. He described three key application areas at which such flow batteries could provide electricity storage to complement renewable energy systems.
The first of these is for a single building. For example, he says, a house may have an array of solar panels on the roof and then a flow-battery tank in the basement roughly the size of a home-heating oil system that can store enough electricity for a day.
On a much bigger scale, these systems could be positioned at various points on a regional or nation-wide electricity network. They could then be used to ensure that electricity generated through wind farms is fed onto the grid at the time of optimal need and electricity price.
"Market price fluctuates; in fact in the USA at windy times you can actually end up paying to put wind-generated electricity on the grid. Storage could be used to buffer the availability and therefore the price," Aziz explains. He adds that the team will be looking into these economic issues in more detail with another Harvard colleague, William Hogan, Raymond Plank professor of Global Energy Policy at Harvard Kennedy School, who is an expert on electricity markets.
The third possibility for using quinone-based flow batteries is at an intermediate scale, with an individual 1MW wind turbine to enable electricity supply when the wind is not blowing.
"When wind turbines are not working you are not using the transmission capacity but with these batteries you can optimise delivery," Aziz says. "We've studied how much storage you need between windy periods and you need to store roughly two days' worth."
Of course, such buffering could be done with conventional batteries too. However, because in solid-electrode batteries the energy storage is tied to their power capacity, you would need to buy much more power capacity than you actually need in order to get the required energy storage. The researchers have estimated that lead acid batteries could cost up to $100m to store the energy from a $2m wind turbine in order for it to release this energy over two days.
Towards the future
The Harvard team are excited about the possibilities of their quinone-base flow battery. However, there is still a long way to go before they are realised.
So far, the team is about a year into a Department of Energy (DoE) grant to prove the concept and is now negotiating a further three-year contract with the DoE to deal with the immediate challenges.
Among these challenges is the stability cycle. "In the paper we reported 15 cycles but so far we have cycled 100 times and seen no degradation. However, for commercial applications we would need to do thousands of cycles," explains Aziz.
In addition, the team's vision is for an all-quinone flow battery. "The quinone we have is on the negative terminal. We are trying to develop a flow battery with a quinone on the positive terminal too," Aziz says.
The system described in the Nature paper has bromine dissolved in hydrobromic acid on the positive terminal. "This is toxic in high concentrations and difficult to maintain," Aziz says. "We use it because it is inexpensive and also to have a known quantity on the positive side so that we could evaluate the effects on the negative."
In parallel, the group also hopes to gain funding to learn more about the basic chemistry of the systems.
Another challenge for anything intended for commercial applications is to manufacture it in an inexpensive and reliable way. For this part of the project, the university team has lined up a commercial partner, Connecticut-based Sustainable Innovation, which is said to already have an ultra-low-cost electrochemical cell design and system architecture under development for energy storage applications.
"In three years' time we expect to have a demo unit that is about the size of a small horse trailer that works well and could be driven to solar arrays to demonstrate the technology," says Aziz. The portable, scaled-up storage system could be hooked up to solar panels on the roof of a commercial building, and electricity from the solar panels could either directly supply the needs of the building or go into storage and come out of storage when there's a need.
And, if this goal is realised, it won't seem such a big step to begin to meet the team's aims of large-scale energy storage based on such systems to help make renewable energy generation a much bigger part of the energy story.