Heating accounts for nearly half of the global energy demand, and two-thirds of that is met by burning fossil fuels like natural gas, oil, and coal. Solar energy is a possible alternative, but while we have become reasonably good at storing solar electricity in lithium-ion batteries, we’re not nearly as good at storing heat.
To store heat for days, weeks, or months, you need to trap the energy in the bonds of a molecule that can later release heat on demand. The approach to this particular chemistry problem is called molecular solar thermal (MOST) energy storage. While it has been the next big thing for decades, it never really took off.
In a recent Science paper, a team of researchers from the University of California, Santa Barbara, and UCLA demonstrate a breakthrough that might finally make MOST energy storage effective.
The DNA connection
In the past, MOST energy storage solutions have been plagued by lackluster performance. The molecules either didn’t store enough energy, degraded too quickly, or required toxic solvents that made them impractical. To find a way around these issues, the team led by Han P. Nguyen, a chemist at the University of California, Santa Barbara, drew inspiration from the genetic damage caused by sunburn. The idea was to store energy using a reaction similar to the one that allows UV light to damage DNA.
When you stay out on the beach too long, high-energy ultraviolet light can cause adjacent bases in the DNA (thymine, the T in the genetic code) to link together. This forms a structure known as a (6-4) lesion. When that lesion is exposed to even more UV light, it twists into an even stranger shape called a “Dewar” isomer. In biology, this is rather bad news, as Dewar isomers cause kinks in the DNA’s double-helix spiral that disrupt copying the DNA and can lead to mutations or cancer.
To counter this effect, evolution shaped a specific enzyme called photolyase to hunt (6-4) lesions down and snap them back into their safe, stable forms. The researchers realized that the Dewar isomer is essentially a molecular battery. This snap-back effect was exactly what Nguyen’s team was looking for, since it releases a lot of heat.
Rechargeable fuel
Molecular batteries, in principle, are extremely good at storing energy. Heating oil, arguably the most popular molecular battery we use for heating, is essentially ancient solar energy stored in chemical bonds. Its energy density stands at around 40 Megajoules per kilo. To put that in perspective, Li-ion batteries usually pack less than one MJ/kg. One of the problems with heating oil, though, is that it is single-use only—it gets burnt when you use it. What Nguyen and her colleagues aimed to achieve with their DNA-inspired substance is essentially a reusable fuel.
To do that, researchers synthesized a derivative of 2-pyrimidone, a chemical cousin of the thymine found in DNA. They engineered this molecule to reliably fold into a Dewar isomer under sunlight and then unfold on command. The result was a rechargeable fuel that could absorb the energy when exposed to sunlight, release it when needed, and return to a “relaxed” state where it’s ready to be charged up again.
Previous attempts at MOST systems have struggled to compete with Li-ion batteries. Norbornadiene, one of the best-studied candidates, tops out at around 0.97 MJ/kg. Another contender, azaborinine, manages only 0.65 MJ/kg. They may be scientifically interesting, but they are not going to heat your house. Nguyen’s pyrimidone-based system blew those numbers out of the water. The researchers achieved an energy storage density of 1.65 MJ/kg—nearly double the capacity of Li-ion batteries and substantially higher than any previous MOST material.
arstechnica.com
