The Environmental Sensors Team of the Synergy Materials Research Center, part of the National Institute of Advanced Industrial Science and Technology (AIST), has developed a novel gas sensor device combining a thermoelectric conversion material with a platinum catalyst. They confirmed the basic operation of the device which responds only to hydrogen gas and operates at room temperature. The research is expected to have applications in fuel cell-powered vehicles and other areas, as it features low power consumption and can be integrated with a silicon chip.

Scientists and engineers are working toward a hydrogen gas energy-driven society, with applications such as fuel cell-powered vehicles and dispersed power generation using fuel cells for residential use. Consequently, there is expected to be increased demand for hydrogen gas concentration sensors to accurately regulate hydrogen gas concentrations, and hydrogen gas leak alarms to ensure product safety.

The previous hydrogen gas sensors, semiconductor sensors, were mostly metal oxides, for example, stannic oxide, which detect gases via variations in their resistance. At high temperature, in air, oxygen chemisorbs onto the surface of stannic oxide, and removes electrons from the conduction band. When a combustible gas, including hydrogen, chemisorbs onto the surface, it is oxidized by the oxygen already there, so injecting the electrons and decreasing the resistance of the stannic oxide. The most semiconductor sensors have to be heated to around 400ºC in order to keep the oxygen adsorbates on the surface. This type of sensor has poor selectivity of the hydrogen gas and responses also to other combustible gases, such as methane and carbon monoxide.

The newly developed hydrogen gas sensor is fabricated with a film of thermoelectric material coated with platinum as the catalyst on half of its surface (see figure). When this sensor was exposed to air mixed with hydrogen gas, the catalytic reaction heated up the Pt-coated surface, and then thermoelectric voltage appeared across the hot and cold region of the oxide film. At room temperature, the platinum catalyst reacts only with hydrogen gas, so the sensor has high selectivity to hydrogen gas. Moreover, the sensor is energy efficient, as it operates at room temperature, and is suitable for integration into silicon substrates.

The sensor prototype used lithium-doped nickel oxide as the thermoelectric material. A thick film of nickel oxide was formed using a screen-printing method and the platinum catalyst thin film was deposited by r.f. sputtering method. The basic operation of the hydrogen gas sensor was confirmed at room temperature, with a 0.15 mV voltage signal for the 3% hydrogen/air mixed gas. The team also has confirmed the linearity of signal for gas concentration, and expect the sensor to have applications in controlling hydrogen gas concentration. Further research is planned to optimize the platinum catalyst and thermoelectric material.