A new nano-sensor for hydrogen gas detection promises a cheap and safe method for identifying leaks to prevent tragedies such as the Hindenburg disaster.
After two and a half years under the supervision of professor Mo Mojahedi, director of the Emerging Communications Technology Institute, and professor Stewart Aitchison, the vice dean of research, PhD student Muhammad Alam has developed an integrated optical device to detect the presence of hydrogen in the atmosphere.
The sensor is made up of numerous waveguides, more commonly known as nano-wires, coated with palladium. Alam explains, “Palladium absorbs hydrogen like crazy, over 900 times its volume. When hydrogen is present [the coating] becomes palladium hydride.”
The chemical change from palladium to palladium hydroxide causes light to grow more intense. “You can then correlate the change in power to the amount of hydrogen present,” says Professor Mojahedi. For the process to work, the light field must be concentrated where the hydrogen is. “To do this you make narrow silicon waveguides of hundreds of nano-meters in size,” Mojahedi adds.
The more common optical fibre causes light to leak, thus it never actually passes through the palladium coating. The nano dimensions of the silicon waveguide, on the other hand, allow light to come through the palladium layer and a change in power can be detected if hydrogen is present.
Mojahedi clarifies that the small dimensions of the device increase its functionality since hundreds of waveguides can be placed next to each other. By applying the same architecture, the device can detect changes in temperature, humidity, and power input, thus eliminating experimental errors that can cause faulty results.
When divided into its basic elements, Mojahedi says the nano-sensor follows “a LEGO approach, [where] you have five fundamental pieces, but how you arrange them provides different outcomes. You can make a very expensive and sophisticated sensor with all the sensitivity, for people who are willing to pay thousands of dollars, or you can make cheap sensors [for commercial use].”
The current model of the nano-sensor is capable of detecting between one and three per cent volumetric hydrogen presence. This interval is within the range of safe atmospheric hydrogen levels.
Four per cent per metric cube is considered the explosive limit—if a flame is present the hydrogen will ignite.
Past hydrogen detectors use numerous methods. However, “people are using them because there is no alternative solution” says Alam. “The catalytic sensor is the most widely used in the industry, but [it detects] hydrogen even if it is present in other gases like methane.” Other techniques, like heat conduction detectors, are not only extremely bulky but also very expensive, costing over $3,000 to detect the presence of two to three per cent volumetric hydrogen.
“Like any other design project, you start by looking at what other people have done, and you try to see if it fits your requirements. You ask yourself how you can improve it,” says Mojahedi. Methods such as Micro Electric Mechanical Systems rely heavily on electronics, and introduce the possibility of an electric spark, which is dangerous for hydrogen since it ignites immediately. “You don’t want to detect hydrogen electronically, you want to do it optically. Optical waves do not have a spark. From the [perspective] of safety, optical sensors are far superior.”
When Ford, Boeing, and NASA expressed interest in the technology, the design team began to see the benefits of electronic integration of the optical nano-sensor. With increasing demand for eco-friendly cars, Ford is looking to develop hydrogen-dependent automotives, which means looking into reducing the risk of explosion. A sensor that when plugged in the electronic system of the car, effortlessly detects possible safety hazards, has become an obvious application for the device.
NASA uses liquid hydrogen in shuttles to fuel lift-off. “If a bulky hydrogen sensor were to be used it would not be easily integrated,” says Mojahedi. At the moment, when there is a hydrogen leak, NASA must go through a time-consuming process of dismantling the tanks and filling them with helium to determine the location of the leak. The optical nano-sensor would provide a fast, safe, and electronically integratable approach to the inconveniences of previous methods.
With regards to the cost of the device, Mojahedi acknowledges that it is hard to configure a price for a product in its research stage. However, he suspects that this technology will be much cheaper than the current methods. This is not only due to the sensor itself, but the laser light source and the power detector which are standard technologies in optics and telecommunication.
While the team tried many different waveguides and hydrogen-attracting substances, the current model of palladium-coated nano-wires remains the most promising. “We are currently in the testing phase. We have reason to think that it should work,” says Alam.
In the future, the project will be extended to other gases such as carbon monoxide and dioxide. “[This expansion] is not an easy thing to do, particularly if you want to make it very cheap,” says Mojahedi. The detection of such gases requires light sources in the far infrared spectrum which are expensive. Moreover, “finding a material that captures that substance and only that substance is difficult [because] a material may absorb gas A, but if it also absorbs gas B and C, it is thus not very selective,” Mojahedi adds.
The research conducted by Aitchison, Mojahedi, and Alam is applicable to today’s hydrogen production industry. “There are billions of dollars spent on hydrogen each year, [which poses] problems of safety, storage, and transmission. If there is success from [this] technology, there is a market.”
