(Nanowerk Spotlight) In the (hopefully not too distant) future hydrogen-based economy, hydrogen (H2) sensors will be a critical component for safety and widely needed. For example, H2 sensors will detect leaks from hydrogen-powered cars and fueling stations long before the gas becomes an explosive hazard. But even today a wide range of potential applications for H2 sensors exists, such as sensing H2 buildups in lead acid storage cells found in most vehicles; detecting H2 leaks during petrochemical applications where high pressure H2 is used; detecting impending transformer failure in electric power plants; or monitoring H2 buildup in radioactive waste tanks and in plutonium reprocessing.
Another example is the Space Shuttle which uses a combination of hydrogen and oxygen as fuel for its main engines. Any hydrogen leak could potentially result in a hydrogen fire, which is invisible to the naked eye. Today, the leakage of hydrogen caused by a tiny pinhole in the pipe of a Space Shuttle could not be easily detected by individual rigid detectors because the locations of pinholes are not predetermined.
The problem with most current hydrogen sensor designs is that they are built on rigid substrates, which cannot be bent, and therefore, their applications might be limited due to the mechanical rigidity. In addition, they use expensive, pure palladium. A new type of sensors is bendy and use single-walled carbon nanotubes (SWCNTs) to improve efficiency and reduce cost. In the example of the space shuttle, laminating a dense array of flexible sensors on the whole surface of a pipe can detect any leakage of hydrogen prior to diffusion and alert control units to remedy the malfunction. This use of large-area sensory skins would not significantly increase the overall weight of the Shuttle due to the lightweight nature associated with these flexible sensors. The development of these hydrogen sensors is just another step that will help us ensure economical, environmental and societal safety in using hydrogen as the main fuel source in tomorrow's society.
"The most exciting contribution of our research is the first-time fabrication of hydrogen sensors with mechanical bendability and superb sensing performance by using nanostructured materials, i.e., single-walled carbon nanotubes decorated with palladium nanoparticles" Dr. Yugang Sun tells Nanowerk. "In comparison to previously designed hydrogen sensors, which are rigid and rely on expensive, pure palladium as sensing components, our sensors are flexible and use single-walled carbon nanotubes to improve efficiency and reduce cost. The mechanical bendability of our hydrogen sensors is beneficial to application in many systems which require demanding low cost, large area, light weight, mechanical flexibility and mechanical shock resistance."
Images of the as-fabricated flexible hydrogen sensor (Image: Dr. Sun/Argonne)
Sun is a scientist at The Center for Nanoscale Materials at Argonne National Laboratory in Argonne, Illinois. Together with H. Hau Wang from Argonne's Materials Science Division he fabricated the new sensing devices using a two-step process separated by high and low temperatures. First, at around 900 degrees Celsius (°C), Sun and Hau grow SWCNTs on a silicon substrate using chemical vapor deposition. Then, they transfer the SWCNTs onto a plastic substrate at temperatures lower than 150°C using a technique called dry transfer printing.
It has been shown previously that nanotube networks grown through chemical vapor deposition had enhanced sensing capability for hydrogen when the SWCNTs were decorated with palladium nanoparticles via electron beam evaporation ("Functionalized Carbon Nanotubes for Molecular Hydrogen Sensors"). One problem with this method is the high temperature step (about 900°C) involved in the growth of SWCNTs, which is not compatible with plastic substrates that can only withstand much lower temperatures of below 300°C.
A solution became available with the dry transfer printing process for transferring CVD nanotubes onto plastic substrates where device fabrications can be processed at relatively low temperatures of below 100°C (see our related Spotlight about John A. Rogers' work: Gutenberg + nanotechnology = printable electronics).
With the separation of high-temperature steps and low-temperature steps the fabrication of flexible thin-film transistors on plastic sheets with the use of CVD SWCNTs became possible. Sun and Hau now have combined the dry transfer printing technique and modification of SWCNTs with palladium nanoparticles to prepare
high-performance hydrogen sensors with excellent mechanical flexibility on plastic substrates.
This precise process is what allows the film of nanotubes to form on the plastic, after which the palladium nanoparticles can be deposited on the SWCNTs to make the sensors. The palladium nanoparticles play an important role in increasing the interaction between hydrogen and the SWCNTs to enhance the change of resistance of the device when it is exposed to hydrogen molecules.
"The driving force for us to develop low-cost, high-performance hydrogen sensors was the Department of Energy’s (DOE) Energy Hydrogen Program which is making progress towards the goal of a 2015 commercialization decision" says Sun. "In the program, fast and precise detection of leakage of hydrogen is critical to ensure the safe use of hydrogen technologies. Therefore, hydrogen sensors represent one of the core technologies in the program. As one of the largest national laboratories of DOE, Argonne has the opportunity to propel the research pace in this field."
These new, flexible hydrogen sensors, although not optimized yet, can detect hydrogen with concentration of as low as 30 ppm in air at room temperature. The sensors show a change of 75% in their resistance when exposed to hydrogen at a concentration of 0.05% in air. The devices can detect the presence of 1% hydrogen at room temperature in 3 seconds. Even after bending – with a bending radius of approximately 7.5 mm – and relaxing 2,000 times, the devices still perform with as much effectiveness.
The sensors' performance did not significantly degrade even when they were laminated on curved surfaces with radius as small as 2 mm and/or after they underwent 1000 times of bending/relaxing cycles.
The technique described by the Argonne scientists provides the possibility of a versatile route to fabricate flexible sensors for various gases. According to Sun, next steps for the scientists will include scaling up the fabrication to produce large-area arrays of hydrogen sensors, i.e., sensory skins, as well as integrating the sensory skins with flexible electronic control circuits.