Chemiresistive Hydrogen Sensors: Fundamentals, Recent Advances, and Challenges

Hydrogen (H-2) is one of the next-generation energy sources because it is abundant in nature and has a high combustion efficiency that produces environmentally benign products (H2O). However, H-2/air mixtures are explosive at H-2 concentrations above 4%, thus any leakage of H-2 must be rapidly and reliably detected at much lower concentrations to ensure safety. Among the various types of H-2 sensors, chemiresistive sensors are one of the most promising sensing systems due to their simplicity and low cost. This review highlights the advances in H-2 chemiresistors, including metal-, semiconducting metal oxide-, carbon-based materials, and other materials. The underlying sensing mechanisms for different types of materials are discussed, and the correlation of sensing performances with nanostructures, surface chemistry, and electronic properties is presented. In addition, the discussion of each material emphasizes key advances and strategies to develop superior H-2 sensors. Furthermore, recent key advances in other types of H-2 sensors are briefly discussed. Finally, the review concludes with a brief outlook, perspective, and future directions.