# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 7 Summary and Prospect  
At present, the technology behind hydrogen sensors at room temperature is more mature, while the research on sensitivity, response time, safety, lifetime, and other performance factors is also sufficient to meet the needs of the industry. With the continuous development of hydrogen energy in fuel cells, distributed power generation, and human spaceflight, the research on hydrogen sensors, as the core components of the future hydrogen energy-based Internet of Things (IoT), will gradually move towards miniaturization, integration, and distributed sensing, while also focusing on the need to achieve stable measurement in harsh environments, such as high temperature and high humidity. Future research will focus on the following four points:  
1. Reducing the cross-sensitivity of temperature, humidity, and other interference factors, as well as achieving accurate measurement of hydrogen in different environments, is required. The response signal of most hydrogen sensors is affected by temperature and humidity, resulting in cross-sensitivity. Some reducing gases are also absorbed by the hydrogen-sensitive material, which affects hydrogen detection. The cross-sensitivity of the sensor can be reduced via temperature and humidity compensation and surface modification of the hydrogen-sensitive material. The temperature and humidity compensation of the sensor mainly uses hardware and software compensation methods. Hardware compensation includes the bridge arm parallel resistance method, the bridge arm thermistor compensation method, the series-parallel thermistor compensation method outside the bridge, etc. Hardware compensation is usually based on the test data of the sensor in high- and low-temperature environments, and the corresponding temperature compensation components are selected to compensate for the initial zero point and output sensitivity drift of the sensor. Common software compensation methods include interpolation, least squares fitting, BP neural network algorithm, etc. Software compensation is based on the real-time ambient temperature collected via the temperature measurement element, the sensor's zero point, sensitivity, etc., and the temperature change algorithm is integrated into the high-speed computing microcontroller to achieve real-time temperature compensation of the sensor. The use of hardware correction cannot be a complex algorithm compensation, and the compensation circuit is also susceptible to the influence of an electromagnetic environment. In contrast, the software compensation method has high accuracy, a wide range of applications, and simple debugging, and can, therefore, become a common method of sensor temperature compensation. 2. To develop new hydrogen-sensitive materials, the defects of common hydrogen-sensitive materials, such as Pd and WO\({}_{3}\), can be improved by introducing nanotechnology and adding new materials, such as graphene. The research on hydrogen-sensitive materials never stopped, and the selection of hydrogen-sensitive materials plays a decisive role in the performance of sensors in terms of sensitivity, measurement range, operating temperature, and lifetime. With the development of material science, more new materials will be applied to hydrogen-sensitive materials to improve sensor performance. New hydrogen-sensitive materials, including nanomaterials, hybrid materials, new carbon materials, two-dimensional materials, and metal-organic framework materials, are studied to improve their selectivity and shorten the response time; their sensing materials are gradually developed from traditional metal oxides, such as SnO\({}_{2}\), ZnO, and Fe\({}_{2}\)O\({}_{3}\), in the early stage to new metal oxides, such as WO\({}_{3}\), MoO\({}_{3}\), and In\({}_{2}\)O\({}_{3}\). Among them, WO\({}_{3}\), as an n-type semiconductor, has a wide range of applications in hydrogen sensing.