Scientists Develop A Device To Turn Car Exhaust Into Renewable Energy

As innovators seek out ways to make our society more energy efficient, one renewable resource has proved harder to “renew” than others: heat.

So-called “waste heat” is generated by everything from cars to appliances to small electronic devices. Transforming excess heat into usable electricity has been a hot topic among scientists for years, and new research out of Washington State University may represent a huge leap forward for the growing field of thermoelectrics.

A WSU team led by physicist Yi Gu has developed a device capable of converting heat to electricity three times more efficiently than common methods. Their innovation — called a van der Waals Schottky diode — relies on a unique composite material that converts heat into powerful, storeable electric current.

A diode is a specialized electronic device designed to conduct the flow of electric current in a single direction (similar to how a valve in a water main directs the flow of liquid). Every diode is polarized at its ends, so electric current flows “forward” from its positive end (or anode) to its negative end (cathode).

By design, that polarized structure blocks current from traveling the opposite direction. That’s partly why lightbulbs made with “light emitting diodes,” or LEDs, can produce so much more light than ordinary incandescent bulbs, which disseminate energy in a less efficient manner.

Second to LED, the best-known diode in the world of electronics is the Schottky diode, which is formed by joining a semiconductor material (such as silicon) with a metal (like aluminum or copper). This type of diode is aptly referred to as the “hot-carrier” diode because it can carry heat forward in a manner similar to the way that an LED carries light.

Schottky diodes have a low voltage drop (or the amount of voltage loss that occurs in a part of a circuit). This makes them useful for devices such as solar cells, which require a low voltage drop diodes to prevent loss of energy. Other common applications include radio frequency mixers and power rectifiers.

But traditional Schottky diodes have limitations for converting heat to electricity:

“When you attach a metal to a semiconductor material like silicon to form a Schottky diode, there are always some defects that form at the interface,” said physicist Matthew McCluskey, who co-authored the WSU study. “These imperfections trap electrons, impeding the flow of electricity.” This makes them inefficient devices for turning heat into electricity.

The team at WSU developed their new diode with the intent of improving on the the Schottky model. By reducing impediments to electricity flow, the researchers could produce diodes that were more capable of converting “bulk” levels of heat into electricity.

The van der Waals Schottky diode developed by Yi Gu and colleagues does not utilize metal or silicon; rather, their diode is a “multicomponent” material made from multiple microscopic layers of a crystalline compound called indium selenide (InSe).

The team used a simple heating process to modify these layers of InSe, making one layer behave like a metal and another layer behave like a semiconductor.

This means that rather than joining together two materials (a metal and a semiconductor) to create a typical Schottky diode, the van der Waals Schottky diode allows one material, InSe, to behave as two materials would.

In the van der Waals Schottky diode, InSe does not exhibit the impurities or defects that occur at the interface where, in a Schottky diode, metal and semiconductor materials are joined together.

Without these imperfections, electricity can flow through the WSU team’s device with near 100% efficiency.

The conductive efficiency of the van der Waals Schottky diode was measured using the confocal microscope pictured left. Researchers found that their innovation exhibited “ideal diode behaviors” (in terms of forward-current flow) as well as enhanced thermoelectric power, totaling “three orders of magnitude” higher than the typical Schottky diode.

That higher magnitude represents a greater ability to transform heat into electricity than most innovations to date — which could subsequently make the van der Waals model a standard-bearer for bulk “recycling” of waste heat.

Before that can happen however, science must make crystalline InSe into a more accessible material: at present, it remains too complex (and expensive) to be used in manufacturing electronics. Gu and colleagues say they are exploring ways to synthesize larger quantities of the material so that it can be developed into useful devices.

If they succeed, the new van der Waals Schottky diode could someday provide another source of renewable power.

“In the future, one layer could be attached to something hot like a car exhaust or a computer motor and another to a surface at room temperature,” said Yi Gu, who led the WSU study. “The diode would then use the heat differential between the two surfaces to create an electric current that could be stored in a battery and used when needed.”

The original research “Phase-Defined van der Waals Schottky Junctions with Significantly Enhanced Thermoelectric Properties” was published in the Journal Of Physical Chemistry Letters in June 2017. Full information is available here.