Hydrogen is a promising potential fuel for cars, buses, and other vehicles, and can be converted into electricity in fuel cells. It already is used in medicine and space exploration, as well as in the production of industrial chemicals and food products.
Safety is an important issue when using hydrogen. An explosive mixture can form if hydrogen leaks into the air from a tank or valve, posing a hazard to drivers, equipment operators, or others nearby.
Commercially available sensors can detect the presence of hydrogen and then close valves, shut down equipment, or trigger alarms; however, current technologies typically have limitations related to cost, speed of operation, susceptibility to interference from other gases, and temperature range.
A tiny sensor was created that reliably overcomes the limitations associated with current hydrogen sensor designs. The key to its performance is its nanostructured, self-assembled, thin-film construction. First, a one-molecule-thick layer of siloxane is applied to a glass substrate. The “sticky foot” of the siloxane molecule binds it strongly to the glass, while the rest of the long-chain molecule remains slippery. Onto this slippery layer, researchers evaporate an extremely thin blanket of tiny (2- to 10-nanometer) palladium beads.
Palladium particles are chosen because, when exposed to hydrogen, they adsorb the gas and swell slightly to form palladium hydride. Some currently available thick film sensors rely on the different conductivity of palladium and palladium hydride to indicate hydrogen concentration. In contrast, the Argonne design employs an ultra-thin layer of palladium beads that allows both faster hydrogen adsorption and greater sensitivity due to the mobility of the palladium hydride beads. As the enlarged beads move into contact with each other, they create pathways for electrical current. When hydrogen concentration drops, the particles shrink quickly and revert to palladium. Electrical conductivity also drops.
Hydrogen can be burned directly in an engine, or electrochemically combined with oxygen in an onboard fuel cell with electric output powering the wheels and other vehicle systems. The sensor helps to ensure the safe and proper operation of subsystems aboard a vehicle, in component fabrication areas, and in facilities for vehicle repair or fueling.
The hydrogen sensor requires no warm-up and responds in less than 75 milliseconds in a 2% hydrogen atmosphere. Competing sensors typically take one second to tens of seconds. It detects hydrogen concentrations as low as 0.0025% (25 parts per million) without elaborate signal amplification.
Minor leaks can be found before concentration levels require full system shutdown. The sensor detects hydrogen reproducibly, even in the presence of oxygen, water vapor, and other gases. Unlike other sensors, it requires no heaters or other supplementary power.
Construction is based on scalable processes routinely used to make electronic components, resulting in minimizing the ultimate cost. Only $50 worth of palladium is needed for approximately 16,000 sensors.