# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 6 Comparison and Development Trend of Each Sensor  
The number of research papers published on different types of hydrogen sensors from 2000 to 2021 (data from Web of Science) is shown in Figure 13, which shows that the interest in and research on hydrogen sensors increased in the past 20 years. The number of articles on semiconductor and fiber-optic sensors is significantly higher than those on catalytic and thermal sensors, which shows that the future research direction and focus will be on semiconductor- and fiber-optic-type hydrogen sensors. Table 1 screens some of the better-performing hydrogen sensors mentioned in the literature. For different types of hydrogen sensors, the focus of current research is slightly different. For example, for catalytic sensors, the power consumption and operating temperature of the sensor are considered, while thermal conductivity sensors usually evaluate the signal-to-noise ratio, output current, and voltage, as well as the effect of temperature  
Figure 13: Various sensor research trends since 2000.  
and humidity; fiber-optic sensors focus more on sensitivity and signal demodulation. In terms of response time, the catalytic element- and micromirror-type sensors usually take less than 10 s to respond, while the response time of semiconductor-type sensors is usually less than 30 s, and the response times of grating-, interferometric-, and thermoelectric-type sensors are mostly around 1 min or even longer. Since different unit systems are used to measure the sensitivity of different types of sensors, the lower detection limit is used for comparison in this paper. The lower detection limits of semiconductor-type hydrogen sensors are significantly better than other types, reaching below 10 ppm; the lower detection limits of catalytic- and fiber-optic-type hydrogen sensors are mostly in the range of several hundred ppm. The sensitivity of the sensor is affected by the measurement range, ambient temperature, humidity, the type and thickness of the sensitive material, etc. Changes in temperature and humidity can affect the sensitivity of the sensor to varying degrees. The sensitivity of fiber-optic hydrogen sensors and interferometric sensors is relatively high, reaching more than 10 nm/%; however, these two sensors are susceptible to interference from the measurement environment. The sensitivity of the grating type is relatively low, being generally below 3 nm/%; however, it is easy to compensate for temperature and humidity and realize distributed measurements for industrial applications. Therefore, recent research on grating-type hydrogen sensors focused on developing new sensitive materials and grating structures to improve their sensitivity and avoid their shortcomings. Most of the micro-mirror sensors are scattered light intensity modulated, and most of the literature does not give specific sensitivity values; thus, the sensitivity values of these two sensors are not given in Table 1. The advantage of the micromirror sensor is its simple structure and fast response; however, its structural limitations prevent it from achieving distributed measurements. In summary, catalytic-type hydrogen sensors have simple structures and fast response times; however, they have high operating temperatures, mostly over 100 \({}^{\circ}\)C, which can be a safety hazard, and are cross-sensitive to some gases, such as methane, while oxygen must be present in the environment for use. Thermally conductive hydrogen sensors have long service lives and do not suffer from degradation of device performance due to contamination. However, their selectivity and sensitivity to hydrogen are lower than electrochemical- and semiconductor-type hydrogen sensors. Semiconductor-type hydrogen sensors are simple in structure, small in size, easy to integrate, low in cost, fast in response, and suitable for mass production and use; however, they are affected by electromagnetic interference and prone to signal drift, making them difficult to use in harsher environmental conditions.