Hydrogen sensors are of increasing importance in connection with the development and expanded use of hydrogen gas as an energy carrier and as a chemical reactant. There are an immense number of sensors reported in the literature for hydrogen detection and in this work these sensors are classified into eight different operating principles. Characteristic performance parameters of these sensor types, such as measuring range, sensitivity, selectivity and response time are reviewed and the latest technology developments are reported. Testing and validation of sensor performance are described in relation to standardisation and use in potentially explosive atmospheres so as to identify the requirements on hydrogen sensors for practical applications.

Hydrogen Sensors - A review

Since the late 1980s there have been spectacular developments in micromechanical or microelectro-mechanical (MEMS) systems which have enabled the exploration of transduction modes that involve mechanical energy and are based primarily on mechanical phenomena. As a result an innovative family of chemical and biological sensors has emerged. In this article, we discuss sensors with transducers in a form of cantilevers. While MEMS represents a diverse family of designs, devices with simple cantilever configurations are especially attractive as transducers for chemical and biological sensors. The review deals with four important aspects of cantilever transducers: (i) operation principles and models; (ii) microfabrication; (iii) figures of merit; and (iv) applications of cantilever sensors. We also provide a brief analysis of historical predecessors of the modern cantilever sensors.

Cantilever transducers as a platform for chemical and biological sensors

Wireless sensors are devices in which sensing electronic transducers are spatially and galvanically separated from their associated readout/display components. The main benefits of wireless sensors, as compared to traditional tethered sensors, include the non-obtrusive nature of their installations, higher nodal densities, and lower installation costs without the need for extensive wiring.1–3 These attractive features of wireless sensors facilitate their development toward measurements of a wide range of physical, chemical, and biological parameters of interest. Examples of available wireless sensors include devices for sensing of pH, pressure, and temperature in medical, pharmaceutical, animal health, livestock condition, automotive, and other applications.4–7

Some implementations of wireless gas sensors can be already found in monitoring of analyte gases (e.g. carbon dioxide, water vapor, oxygen, combustibles) in relatively interference-free industrial and indoor environments.8,9 However, unobtrusive wireless gas sensors are urgently needed for many more diverse applications ranging from wearable sensors at the workplace, urban environment, and battlefield, to monitoring of containers with toxic industrial chemicals while in transit, to medical monitoring of hospitalized and in-house patients, to detection of food freshness in individual packages, and to distributed networked sensors over large areas (also known as wireless sensor networks, WSNs). Unfortunately, in these and numerous other practical applications, the available wireless gas sensors fall short of meeting emerging measurement needs in complex environments. In particular, existing wireless gas sensors cannot perform highly selective gas detection in the presence of high levels of interferences and cannot quantitate several components in gas mixtures.

1.1. Diversity Of Monitoring Needs Of Volatiles
The monitoring of numerous gases of environmental, industrial, and homeland security concern is needed over the broad range of their regulated exposure concentrations. Figure 1 illustrates the relationships between several regulated exposure levels spanning several orders of magnitude of gas concentrations. Typical examples of concentrations of regulated exposure are presented in Table 110–14 for three groups of toxic volatiles such as volatile organic compounds (VOCs), toxic industrial chemicals (TICs), and chemical warfare agents (CWAs). These examples demonstrate the need for gas sensing capabilities with broad measurement dynamic ranges to cover 2 – 4 orders of magnitude in gas concentrations.



Figure 1

Examples of regulated vapor-exposure limits established by different organizations: 

GPL: General Population Limit, established by USACHPPM – U.S. Army Center for Health Promotion and Preventative Medicine;


PEL: Permissible Exposure Limit, established


...





Table 1

Examples of regulated concentration levels (in ppm by volume) from three representative classes of toxic gases: VOCs, TICs, and CWAs.10–14



Additional needs for detection of volatiles originate from medical diagnostics, food safety, process monitoring, and other areas.15–17 In those applications, the types and levels of detected volatiles can provide the needed information for further control actions.

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Materials and transducers toward selective wireless gas sensing.

A low-cost, low-power volatile organic compound (VOC) sensor has been constructed from an array of micromachined parallel-plate capacitors. The sensor has demonstrated detection of many VOCs well below the lower explosive limits and could be used in industrial leak monitoring applications or for homeland defense. In place of a standard dielectric, the individual capacitors were filled with selectively absorbing polymers. Absorption of a target vapor alters the permittivity of the polymers and thereby changes the capacitance of the elements in the array. A variety of polymers have been used, including polyethylene-co-vinylacetate, which was sensitive to nonpolar hydrocarbons, and siloxanefluoro alcohol, which was highly sensitive to polar VOCs and chemical warfare agent simulants. The response magnitude for each element depends on a combination of different phenomenon such as the dielectric constant of the analyte and polymer swelling. The measured sensitivity of the sensor to most VOCs was found to be in the low parts per million (ppm) range. The response magnitude from one capacitor to the next is reproducible to within 3.2% at 20 °C. The sensor typically responded within a second but frequently required 5–10 min to reach equilibrium. Response times could likely be substantially improved with an optimized capacitor structure that contains a decreased gap between the plates and provisions for more rapid vapor exchange.

Chemicapacitive microsensors for volatile organic compound detection

Hydrogen gas, within the concentration range of 100 ppm–4 vol.%, is successfully sensed at lower operating temperatures, 25 and 50 °C, using the Pt-sputtered sol–gel dip-coated nanocrystalline (6–7 nm) 6.5 mol% In 2 O 3 -doped SnO 2 semiconductor thin (100–150 nm) film sensor. Typically, for 1000 ppm of hydrogen, the maximum sensitivity values of 32 and 1600% are observed at 25 and 50 °C, respectively; while for 2 vol.% hydrogen, the maximum sensitivity values of 50 and 70,000% are recorded at 25 and 50 °C, respectively. At 25 °C, for 4 vol.% (explosive limit as set by NASA) hydrogen, the maximum hydrogen gas sensitivity values of 107,887 and 2083% are observed for the Pt-sputtered thin films calcined at 500 and 600 °C, respectively.

Nanocrystalline indium oxide-doped tin oxide thin film as low temperature hydrogen sensor

This paper describes the design of, and the effects of basic environmental parameters on, a microelectromechanical (MEMS) hydrogen sensor. The sensor contains an array of 10 micromachined cantilever beams. Each cantilever is 500 μm wide×267 μm long×2 μm thick and has a capacitance readout capable of measuring cantilever deflection to within 1 nm. A 20-nm-thick coating of 90% palladium–10% nickel bends some of the cantilevers in the presence of hydrogen. The palladium–nickel coatings are deposited in ultra-high-vacuum (UHV) to ensure freedom from a “relaxation” artifact apparently caused by oxidation of the coatings. The sensor consumes 84 mW of power in continuous operation, and can detect hydrogen concentrations between 0.1 and 100% with a roughly linear response between 10 and 90% hydrogen. The response magnitude decreases with increasing temperature, humidity, and oxygen concentration, and the response time decreases with increasing temperature and hydrogen concentration. The 0–90% response time of an unheated cantilever to 1% hydrogen in air is about 90 s at 25 °C and 0% humidity.

D. McCorkle

Papers

Design and performance of a microcantilever-based hydrogen sensor