Sunlight to heat: New solid materials store and release solar energy by rotating bonds

Title: Light-Responsive Solid-Solid Phase Change Materials for Photon and Thermal Energy Storage
Authors: Xiang Li, Sungwon Cho, Grace G. D. Han.
Journal: ACS Materials Au
Year: 2022
Featured image by Li et al. ACS Mat. Au, 2022, ASAP.

The sun provides enough energy to the Earth in a single hour to power the entire world for a whole year. It’s our most abundant source of renewable energy. Storing solar energy for later use—during times when the supply of solar energy is low but demand is high (like at night, or on cloudy days)—remains the largest challenge to incorporating solar into a clean energy economy. Luckily, a class of chemicals known as photoswitches can be used for solar energy storage.

Photoswitches are molecules that can store energy from light. When photoswitches absorb light, they undergo isomerization, or rotation about a double bond. Figure 1 shows an example of the isomerization of azobenzene, a common photoswitch. The isomerization reaction converts the molecule from a stable, low-energy state to a less stable, high-energy state. Later, the molecule can relax back to the stable, low-energy state by releasing its pent-up energy as heat.

Figure 1. When azobenzene is exposed to light, it undergoes isomerization from the low energy E isomer to the high energy Z isomer. (Image credit: Skyler Ware for Chembites)

Not only does azobenzene isomerize to a higher energy state, it transitions from a low energy crystalline solid to a high energy liquid. This phase transition raises the overall amount of energy that can be stored by the photoswitch: in addition to the energy from transitioning between the high- and low-energy isomers, the photoswitch can also store the energy required to change from a solid to a liquid.

Unfortunately, there’s a critical challenge to using the stored energy in practice: azobenzene doesn’t isomerize at room temperature because it is crystalline. Inside the crystal, the azobenzene molecules are locked in place, like connected Legos. To rotate around the double bond and become the high-energy isomer when exposed to light, they need more space. The easiest way to give them that space is to melt the crystal, but doing so means the photoswitch can’t store the energy from the phase change.

A newly developed photoswitch addresses the challenge of solid-state isomerization. A team at Brandeis University developed a set of azobenzene-based photoswitches that are separated from each other by adamantane molecules, with carbon chains of different lengths anchoring the azobenzenes to the adamantane. Figure 2 shows the structure of the photoswitches. The resulting matrix is crystalline at room temperature, but spacing out the azobenzene groups gives them enough room to rotate around the double bond. Therefore, the crystals don’t have to be melted before the photoswitch can store energy.

Figure 2. Structures of the newly developed adamantane-separated photoswitches. The structure of adamantane is shown in the top left, where R represents a carbon chain separating the azobenzenes from the adamantane. Three such carbon chains are shown here. (Image credit: Li et al. ACS Mat. Au, 2022, ASAP.)

This new photoswitch still benefits from a phase change on top of molecular isomerization, just not from a solid to a liquid. The photoswitches instead undergo a different type of phase transition before they isomerize: a transition from an ordered, crystalline solid phase to a disordered, amorphous solid phase. That means that the photoswitch can still store energy from both the phase transition and the isomerization, and they can do so while remaining solids even after isomerization.

The team studied the performance of the new class of photoswitches in two ways. First, using UV-Visible spectroscopy, they measured the wavelengths of light that cause the photoswitch to isomerize. They found that under ultraviolet (UV) light, the low energy isomer goes through both a phase transition from crystalline to amorphous and isomerization to the high energyphase. Under visible light, the isomers revert back to the low energy, crystalline phase, releasing the energy difference as heat. Once they knew how to trigger the heat release, they measured exactly how much heat—and therefore energy—the material can store and release using differential scanning calorimetry (DSC). Between the phase transition and the isomerization, the photoswitches store quite a bit—roughly two kilograms of the photoswitch can store enough energy to boil a liter of water. Figure 3 shows the total energy storage capacity of the photoswitch (ΔHtotal) as a combination of the phase transition energy (ΔHc) and the isomerization energy (ΔHiso). Together, these measurements gave the researchers an idea of the material’s best energy storage capabilities.

Figure 3. The photoswitch can store and release energy from both the phase change (ΔHc) and the isomerization (ΔHiso). (Image credit: Li et al. ACS Mat. Au, 2022, ASAP.)

This new class of solid-state photoswitches offers a new way to approach storing solar energy in the form of heat, which can be used directly in industrial or residential heating or converted into another type of energy for later use. The material, as well as the new approach of separating the photoswitches to take advantage of both the phase change and isomerization energy in the solid state, can open doors to more efficient solar energy storage.

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