Resistive hydrogen sensors based on carbon nanotubes: A review

The raise in environmental awareness led to the implementation of hydrogen as a viable energy vector towards achieving decarbonization. Europe, Japan and China are among the regions which have developed a roadmap to implement hydrogen in various forms, with "Green Hydrogen"being the leading technology. Hydrogen is odourless, colourless, has a very low relative molecular mass, and a lower flammability level of 4%. Spontaneous ignition and explosion present considerable risk, therefore, it is imperative to constantly monitor hydrogen levels. Developing low-cost, reliable and sensitive H2 sensors is urgent. Resistive sensors offer exclusive advantages and occupy the largest market share. In this article the promising studies employing carbon nanotubes and their unique properties as resistive gas sensors are reviewed, as well as the physico-chemical aspects behind the sensing mechanism. Despite the variety of CNT synthesis procedures, functionalization techniques and sensor designs, the most common practices fundamentally involve functionalization with either Palladium or Platinum. Response levels of up to 520% for 4% H2 concentration at room temperature were observed. Best response and recovery times exhibited by these devices were below 10 s, which meets the performance requirements established by the United States Department of Energy, which validates the argument that CNT based resistive H2 sensors can be a viable strategy towards solving control and monitoring necessities of energetic strategies involving hydrogen.