Hydrogen is a promising fuel alternative for buses, cars and other automobiles as it can be converted into electricity in fuel cells. It is already employed for medical applications, space exploration and industrial applications such as production of food products and industrial chemicals.
However, safety is important while using hydrogen as the leakage of gas into air from a valve or tank results in the formation of an explosive mixture, thus posing a hazard to the equipment operators, drivers, and surrounding.
The presence of hydrogen can be detected by a gas detector known as hydrogen sensor. The hydrogen sensor is durable, compact, easy to maintain and inexpensive to use when compared to other conventional gas detectors.
Commercially available hydrogen sensors can detect hydrogen leakage and trigger alarms, shut down equipment or close the valves. However, current technologies have certain limitations related to temperature range, operation speed, and susceptibility to interference from other gases.
In 2004, Fawcett TJ et al from the University of South Florida fabricated and tested a hydrogen gas sensor having planar electrical NiCr contacts formed on a 3C-SiC epitaxial layer surface.
The 3C-SiC epitaxial layer having a thickness of 4 μm and doping density of 1018 cm-3 was formed under low-pressure conditions. An n-type Si sensor was also fabricated and tested under the same conditions. The stability and sensitivity of the 3CSiC sensor were found to be high when compared to that of the n-type Si sensor.
Also, the 3CSiC sensor was able to detect hydrogen at high concentrations ranging from 0.333% to 100% in Ar than the Si sensor that detects hydrogen at concentrations ranging from 2% to 100% in Ar.
It was concluded that 3C-SiC hydrogen sensors are suitable for harsh environmental applications owing to their higher sensitivity to hydrogen and larger dynamic range than Si sensors.
In 2006, GaN resistive gas sensors for detecting hydrogen was fabricated and tested by Yun F et al from the University of South Florida.
The Si-doped n-type GaN was formed on c-plane sapphire substrates by organometallic vapor phase epitaxy. The sensors were found to be sensitive to H2 gas at low concentration ~0.1% H2 in air. Signal saturation was not observed up to 100% flow.
However, a clear and sharp response was recorded in the continuous operation mode at varying H2 concentrations. Also, a change in sensor geometry was found to initiate change in current at a fixed voltage to hydrogen.
In April 2012, researchers from Hubei University carried out a detailed investigation on the hydrogen sensing properties of semiconductor oxide (SMO) nanostructures. They provided a comprehensive review of the progress of research on hydrogen gas sensors based on one-dimensional structures and SMO thin film.
The team also examined the noble metal-decoration, doping, size reduction and heterojunctions, and proved that these methods are effective for improving the sensing performance of one-dimensional structures and SMO thin film. In addition, sensor architecture such as electrode size and nanojunctions, and the effect of hydrogen response of one-dimensional structures and SMO thin film were studied.
Some of the current applications of hydrogen sensor include the following:
- Petroleum refining
- Stationary fuel cell installations
- Medical diagnostics
- Space shuttle and space station systems
- Nuclear reactors
- Battery charging
- Mine safety and ore reduction operations
- Hydrogenation of edible oils
- Ammonia and methanol production.
Conventional hydrogen sensors such as specific ionization gas pressure sensors, mass spectrometers and gas chromatographs are restricted by their slow response, high cost and large size.
Therefore, hydrogen sensors of low power consumption, faster response, low production cost and smaller size are needed for widespread use including in-situ monitoring.
At present, various kinds of hydrogen sensors such as metallic, optical, acoustic, thermoelectric, semiconductor and electrochemical sensors are commercially available. Among all these, semiconductor sensors have long-term stability, excellent hydrogen sensing performance and fast response.
However, this type of sensors is still limited by high operation temperatures resulting to potential safety hazards and high power consumption. Moreover, another critical issue of semiconductor sensor that needs to be resolved for improved sensing accuracy is the cross selectivity to other reducing or combustible gases.
Semiconductor nanostructures that include thin films and nanowires have been employed as sensing materials in recent years for developing high-performance hydrogen sensors.
These nanostructures have good electron transportation properties and high specific surface area. One-dimensional nanostructures, on the other hand, exhibit broader limit of detection, lower operation temperature, higher sensitivity and high response time than thin films. Also, nanoparticles-decoration semiconductor nanostructures have been widely examined for improved selectivity and sensitivity to hydrogen gas.
- Fawcett TJ, et al. Hydrogen gas sensors using 3C-SiC/Si epitaxial layers. Materials Science Forum. 2004;457-460:1499.
- Yun F, et al. GaN Resistive Gas Sensors for Hydrogen Detection. Materials Science Forum. 2006:527-529;1553.
- Hydrogen Gas Sensors Based on Semiconductor Oxide Nanostructures - NCBI
- Hydrogen Sensor – Argonne National Laboratory