LONDON — Hydrogen has the potential to replace fossil fuels as an energy carrier, but it is highly flammable, so sensors to detect it are an absolute necessity if there’s to be a transition to a hydrogen economy.

However, the challenge to date is that most current hydrogen sensors require high temperatures in order to function and have low sensitivity and slow response times.

Researchers at TU Delft (The Delft University of Technology in the Netherlands), say they have cracked this challenge with a new sensor made of a thin layer of tungsten trioxide (WO3). Its high electrical resistance coupled with ability to sense hydrogen using a platinum catalyst means it can detect hydrogen concentrations down to 1 pppm near room temperature, with response times as low as 1 s when the concentration exceeds 100 pppm.

One of the properties of tungsten trioxide is that its crystal lattice structure contains a lot of open spaces. As a result, the material can easily be doped, which is the practice of changing its electronic properties by introducing other atoms.

“By itself, tungsten trioxide is an insulator," said Giordano Mattoni, the lead author of the paper published on their work. “But when you dope it, you add electronic charges which turn the material into a different colour and also gradually change it into a metal. We wanted to try to dope thin films of tungsten trioxide with hydrogen gas to see if it could function as a sensor.”

It turns out that it can. The researchers first created thin sheets of tungsten trioxide using a method called pulsed laser deposition. That way, they were able to deposit single layers of the material onto a substrate one by one. “Using this method, we created sheets of tungsten trioxide with a thickness of only nine nanometres," said Mattoni.

The researchers then put platinum droplets on top of the thin layers of tungsten trioxide. Platinum is well known to function as a catalyst which separates the hydrogen molecules into single hydrogen atoms. These atoms, the researchers observed, could then enter the lattice structure of tungsten trioxide, slowly turning it from an insulator into a metal.

Room temperature electrical and optical changes with hydrogen gas. The WO3 thin film is exposed to different environmental conditions: 1 bar of air, 1 bar of hydrogen mixture, and vacuum. Both electrical resistance and sample color (optical images) change upon exposure to H2 and recover their initial state once in air.
Room temperature electrical and optical changes with hydrogen gas. The WO3 thin film is exposed to different environmental conditions: 1 bar of air, 1 bar of hydrogen mixture, and vacuum. Both electrical resistance and sample color (optical images) change upon exposure to H2 and recover their initial state once in air.

“This means that, by measuring the resistance of the material, we can determine the amount of hydrogen present in the environment," Mattoni said.

What sets this new hydrogen sensor apart from most other sensors is that it can be used at room temperature. “It is also much more sensitive than commercially available products and it can be reused in a matter of minutes," Mattoni added. “Also, by increasing or decreasing the temperature of the sensor, the sensitivity range can be tuned for different applications.”

Finally, the thin film nature and the compatibility with current semiconductor technologies allow the sensor to be scaled up towards mass production. Mattoni and TU Delft have filed a patent application for this new sensing technology.

— Nitin Dahad is a European correspondent for EE Times.