Shape-shifting chemical is key to new solar battery | Science News for Students

Shape-shifting chemical is key to new solar battery

It can absorb and hold energy until needed hours to months later
Mar 29, 2019 — 6:30 am EST
Kasper Moth-Poulsen shows off a solar collector

Kasper Moth-Poulsen shows off the solar collector his team used to test its liquid battery.

Johan Bodell/Chalmers University of Technology

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.

What powers the device you’re using? Electricity, obviously. But where did that come from? Two thirds of the electricity used in the United States comes from power plants fueled by fossil fuels — coal, oil or natural gas. Solar energy produces just 1.3 percent of the electricity. Yet energy from the sun could easily power our every need if it could be stored for use when the sun doesn’t shine (such as at night). Researchers in Sweden now think they might have a way to do just that.

As a chemical engineer, Kasper Moth-Poulsen uses chemistry and physics to design solutions to problems. He works at Chalmers University of Technology in Gothenburg, Sweden. He teamed up with other researchers in Sweden and Spain to tackle the problem of storing energy from the sun. Their solution: Store that energy inside the bonds of molecules that have been suspended in a liquid.

Molecules consist of two or more atoms. Those atoms share electrons through bonds that hold them together.

Different types of molecules have distinct 3-D shapes. For example, methane is shaped like a three-sided pyramid called a tetrahedron (Teh-tra-HE-drun). Other molecules have different shapes. Adding energy to a molecule can alter its shape. New bonds may now form between its atoms — ones that may hold different amounts of energy. When a molecule later absorbs energy, that energy can become trapped within those new bonds.

That’s the key to the new solar-energy battery.

Using bonds inside a molecule to store solar energy isn’t new. Moth-Poulsen’s group had been working on that for years. But the molecules it initially worked with contained a rare and pricey metal called ruthenium (Roo-THEE-nee-um). The researchers needed a less costly alternative.

For inspiration, they turned to work by other chemists. They soon found a promising candidate called norbornadiene (Nor-BORN-uh-DY-een). It is made mostly of carbon and hydrogen, atoms that are found in all living things. That means this molecule should be cheap and easy to make.

an illustration of the sunlight absorbing molecule's structure and how it traps energy
Researchers made a molecule (shown in blue) that absorbs sunlight. Its atoms trap the sun’s energy in chemical bonds. When the energy-storing version of this molecule (shown in red) flows over a catalyst, the bonds between atoms change and the molecule releases its energy as heat.
Z. Wang et al/Energy & Environmental Science 2019 (CC BY 3.0)

Yet there was still one problem. This chemical could absorb only ultraviolet (UV) light — a small part of the sun’s light. To make this molecule more useful, the researchers tweaked it in such a way that it would absorb more wavelengths (colors) of sunlight. That innovation sounds easy. In fact, it took them seven years to pull off. Now their molecule can absorb energy not only from UV, but also from blue and green light.

One end of the molecule reacts to this light. As it absorbs this energy, the molecule snaps into a new shape. New bonds between its atoms trap that energy. And they hold it tight, even after the molecule cools to room temperature.

But storing energy isn’t useful unless you can release that energy when you need it. So Moth-Poulsen’s team found a way to get its molecule to release the stored energy as heat. Researchers pass the liquid over a type of salt that acts as a catalyst. (Catalysts are materials that speed up chemical reactions. They’re not used up in the reactions, but nudge reactions along.) The salt causes the molecule to change back into its original shape. When it does so, the molecule releases the energy stored in its bonds. That raises the temperature of the liquid by 63.4 degrees Celsius (114 degrees Fahrenheit) — enough to heat a home.

The team published its findings in the January issue of Energy & Environmental Science.

Solar-powered future

A liquid battery made with these molecules can store solar energy for days, months or even years, Moth-Poulsen says. So energy absorbed during long summer days can be held for use at night or during the winter, when days are short.

The team has tested its system in a rooftop experiment at their lab in Sweden. The system works well — but not yet well enough to put in every home. First, the team needs to increase how much of the sun’s energy the molecule can absorb. “We are aiming at reaching 5 to 10 percent” of that energy, Moth-Poulsen says.

Storing more energy in the molecule’s bonds means these could later release more heat. And while the system doesn’t make electricity, the heat it releases could be used to drive a turbine that does, Moth-Poulsen says. One day, such a system might both heat and power buildings without any connection to outside sources of electric power. Those buildings also could stay warm without a need for energy from fossil fuels.

The liquid battery created by Moth-Poulsen’s team would become part of a system (illustrated) that the team designed to heat a home.
Z. Wang et al/Energy & Environmental Science 2019 (CC BY 3.0)

“We have discovered some new tricks recently,” Moth-Poulsen says. He hopes these will help the home-heating system work even better. That should increase its affordability and attractiveness.

Jeffrey Grossman finds the new data exciting. This study, he explains, “demonstrates real world use of this technology.” Grossman is a materials scientist at the Massachusetts Institute of Technology in Cambridge. He was not involved with the study.

Deepa Khushalani was not either. She’s a chemist and materials scientist at the Tata Institute of Fundamental Research in Mumbai, India. Right now, Khushalani is less excited about the new technology’s prospects for making electricity. To drive a turbine or other engine, she notes, the molecule must release enough heat to turn water into steam. That means the system would need to heat water to more than 100° Celsius (212° Fahrenheit). Batteries that store electricity are a more practical way to harness the sun’s energy, she suspects.

But Moth-Poulsen plans to get the extra heat needed from the new energy-storage molecule. His team is working to make it absorb energy from yellow and orange light, too. Heating and cooling accounts for almost half of the energy used in the European Union. “If [the new technology] can just make a small impact on this, it will make a huge difference,” he says.

Power Words

(more about Power Words)

3-D     Short for three-dimensional. This term is an adjective for something that has features that can be described in three dimensions — height, width and length. 

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

battery     A device that can convert chemical energy into electrical energy.

bond     (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

catalyst     A substance that helps a chemical reaction to proceed faster. Examples include enzymes and elements such as platinum and iridium.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemical engineer     A researcher who uses chemistry to solve problems related to the production of food, fuel, medicines and many other products.

chemical reaction     A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

chemistry     The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) Chemistry also is used as a term to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.

colleague     Someone who works with another; a co-worker or team member.

electricity     A flow of charge, usually from the movement of negatively charged particles, called electrons.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

engine     A machine designed to convert energy into useful mechanical motion. Sometimes an engine is called a motor. (in computer science) A computer program that performs a particular, narrow range of functions.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

environmental science     The study of ecosystems to help identify environmental problems and possible solutions. Environmental science can bring together many fields including physics, chemistry, biology and oceanography to understand how ecosystems function and how humans can coexist with them in harmony. People who work in this field are known as environmental scientists.

European Union     The confederation of 28 European countries that have agreed to work peacefully together. Residents of EU can move freely between its member countries and sell goods to them. Most members have also adopted the same currency, known as the Euro.

fossil fuels     Any fuels — such as coal, petroleum (crude oil) or natural gas — that have developed within the Earth over millions of years from the decayed remains of bacteria, plants or animals.

fundamental     Something that is basic or serves as the foundation for another thing or idea.

hydrogen     The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues. It’s made of a single proton (which serves as its nucleus) orbited by a single electron.

innovation     (v. to innovate; adj. innovative) An adaptation or improvement to an existing idea, process or product that is new, clever, more effective or more practical.

liquid     A material that flows freely but keeps a constant volume, like water or oil.

materials science  The study of how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application. A scientist who works in this field is known as a materials scientist.

methane     A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

physics     The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.

pyramid     A monumental structure with a square or triangular base and sloping sides that meet in a point at the top.

salt     A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called “sea salt.” Common table salt is a made of sodium and chlorine.

solar energy     The energy in sunlight that can be captured as heat or converted into heat or electrical energy. Some people refer to wind power as a form of solar energy. The reason: Winds are driven by the variations in temperatures and the density of the air, both of which are affected by the solar heating of the air, ground and surface waters.

suspension     (in chemistry) A mixture in which particles are dispersed (or suspended) throughout the bulk of a fluid.

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

turbine     A device with extended arm-like blades (often curved) to catch a moving fluid — anything from a gas or steam to water — and then convert the energy in that movement into rotary motion. Often that rotary motion will drive a system to generate electricity.

ultraviolet     A portion of the light spectrum that is close to violet but invisible to the human eye.

wavelength     The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.


Journal: Z. Wang et al. Macroscopic heat release in a molecular solar thermal energy storage system. Energy & Environmental Science. Vol. 12, January 2019, p. 187. doi:​ ​10.1039/C8EE01011K.