Low-cost Hydrogen Gas Sensors based on Solution-Synthesized Palladium Nanostructures

The growing use of hydrogen as an energy carrier, especially outside of its traditional industrial applications, requires low-cost hydrogen gas sensors with fast response. Ideally, these should also have wide dynamic range and high sensitivity. Over the past several years, our group has explored the use of various palladium nanostructures for producing chemiresistive hydrogen gas sensors, including nanowires and nanosheets of different sizes and morphologies prepared by solution-phase synthesis followed by simple drop-casting onto prefabricated interdigitated electrodes, or even hand-drawn electrodes on paper substrates. Despite the high cost of palladium, its cost per device can be negligible, as each device requires only a few micrograms of material. These sensors can operate via several mechanisms that all involve dissociation of hydrogen on the palladium surface followed by diffusion of atomic hydrogen into palladium to form palladium hydride. This increases the intrinsic electrical resistance, increases the specific volume, and decreases the work function of palladium. When the sensor resistance is dominated by the intrinsic resistance of palladium, this sensor resistance increases with hydrogen concentration. When sensor resistance is dominated by contact resistance between nanostructures, volume expansion leads to closing of gaps and decreasing resistance with increasing hydrogen concentration. When conduction is primarily through a semiconducting support material decorated with palladium nanostructures, the change in work function of palladium can result in charge transfer to that substrate, producing large changes in resistance and much higher responsivity than other mechanisms. This talk will provide an overview of our work on chemiresistive palladium-based hydrogen sensors based on each of these mechanisms, addressing their relative advantages and disadvantages.