# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 2 Catalyst Combustion Hydrogen Sensors
### Status of Research  
Catalyst combustion sensors have a long history. The first catalyst combustion sensor was proposed by [PERSON] in 1923 using a bare Pt wire and was first used for methane detection in mines [24]. The bare Pt wire sensing element is simple in structure, easy to make, and highly resistant to toxicity. However, due to the high operating temperature, the service life was greatly reduced. To further improve the performance of catalytic sensors, [PERSON] first introduced the concept of pollistor in 1959 using a carrier and catalyst coated into a Pt wire ring to prepare a catalytic sensor [25]. This catalytic element usually uses a metal Pt with a diameter of 10 to 50 \(\upmu\)m that is embedded into a metal Pd catalyst with a refractory material as a carrier. As catalytic combustion proceeds, the temperature rises, leading to an increase in the resistance of the Pt wire, which is then output as a signal. Although many researchers subsequently used research to improve the sensing performance, the structure and catalytic principle of catalytic combustion-type sensors have not changed significantly and have been applied until today. With the development of science and technology, researchers worked to improve the sensitivity, reduce power consumption, and miniaturize the catalytic element mainly through optimizing the sensor components and improving the preparation of catalysts. [PERSON] et al. [26] proposed a miniature thermoelectric catalytic hydrogen sensor, which consists of a thermopile of 18 thermocouples on an insulating thin film. The thermocouples convert the catalytic combustion heat into voltage and current signals output, with their output voltage being 11 times higher than that of a single thermocouple sensor, while the thermoelectric power is 15 times higher. [PERSON] et al. [27] fabricated an integrated catalytic combustion-type sensor for the detection of combustible gases using a thin film deposition technique. The sensor has very low power, typically only 100 mW, and an operating temperature of 400 \({}^{\circ}\)C. [PERSON] et al. [28] prepared thin-film micro hydrogen sensors via sol-gel synthesis of SnO\({}_{2}\)-Ag\({}_{2}\)O-PtOx composites loaded on alumina substrates, which exhibited high selectivity and fast response. [PERSON] et al. [29] synthesized hollow microrod-like Pt nanostructures on a small suspended microheater platform. As the catalytic layer of the low-power thermoelectric catalytic hydrogen sensor, the hydrogen collides locally on the surface of the Pt nanostructures during operation and transfers heat to the microheater, thus changing its resistance. The sensor exhibits high sensitivity, fast response, and low power consumption. [PERSON] et al. [30] studied several platinum group catalysts and discussed the sensitivity and temperature dependence of the sensors. It was found that the use of Iridium and Rhodium as catalysts could effectively improve the low-temperature performance of hydrogen sensors with Pd and Pt as catalysts. Some scholars also attempted to use nanostructures or new materials, such as graphene, for catalyst surface modification, which was performed to improve the sensitivity and detection range of thermoelectric catalytic hydrogen sensors [31; 32]. For example, [PERSON] et al. [33] developed a sensor based on functionalized thermoelectric silicon nanotubes with a detection limit of 250 ppm at room temperature, and the response signal and hydrogen concentration remained well linear over a wide range.  
Catalytic hydrogen sensors have the advantages of fast response times, long lives, and high accuracy. The technology is now mature and commercially available. However, the gas selectivity of these sensors is poor, and they must be used in the presence of oxygen to achieve measurements. In addition, the catalytic hydrogen sensor will release a lot of heat during operation, which may cause the risk of ignition or explosion.