(150h) Low-Cost Paper-Based Sensors for Ultra-Fast Room-Temperature Hydrogen Detection Using Palladium Alloy Nanowires

Use of hydrogen as a carbon-free energy carrier, particularly for use in fuel cell vehicles, is being widely explored and hydrogen-powered vehicles are beginning to enter the market. In this context, low-cost, low-power, widely-deployable hydrogen sensors are essential for safe operations, as H₂ forms explosive mixtures in air at concentrations as low as 4%. Paper-based sensors are of increasing interest for sensing applications due to their inherent advantages such as low-cost, portability, flexibility, and their environment-friendly degradable substrate. Paper-based sensors are effective for gas sensing applications due to their porous nature which enables more contact between the analyte and the sensing material, as well as their stability in the presence of most gaseous analytes. Penta-twinned palladium nanowires (PdNWs) are an excellent choice for sensitive H₂ gas-sensing applications due to their high aspect-ratio and electrical conductivity. Hydrogen adsorbs on Pd at ambient conditions, dissociates, and dissolves in the bulk to form palladium hydrides (PdH_x) with much lower conductivity than Pd. Thus, H₂ is detected based on a change in resistance. Here, we synthesized thin penta-twinned PdNWs in high yield using a hydrothermal method with polyvinylpyrrolidone (PVP) as a low-cost, high-molecular-weight surfactant to control growth and fabricated sensors by simple drop-casting. PdNWs drop-cast on an interdigitated electrode (IDE) on a silicon wafer gave a response of 0.3% to 1% H₂ in air, with response and recovery times of 12 s and 20 s, respectively. However, the PdNWs gave a negative response (decreased resistance) at lower concentrations due to the formation of new conduction channels between PdNWs upon volume expansion associated with hydrogen absorption and hydride formation. This has been widely observed in sensors based on Pd nanostructures, and results from poor contact between Pd nanostructures within the device. PdNWs on a paper substrate with hand-painted silver contacts had a more than 10-fold increase in response to 1% H₂, compared with those on an IDE, with improved response and recovery times of 10 s each. However, these sensors still had a negative response at lower concentrations of hydrogen. Such reverse sensing phenomenon is problematic for reliable hydrogen sensing over a broad range of concentrations. We observed that ozone treatment, by exposing the PdNWs-paper sensor to UV-O lights overnight, eliminates the PVP used in the synthesis of PdNWs and improves contact between the PdNWs, which eliminates the reverse sensing phenomenon. The UV-O-treated PdNWs sensor gave a response of 5% to 1% H₂ in air, with response and recovery times of 15 seconds. With elimination of the negative response at a lower concentration, we could easily detect as little as 100 ppm H₂ in air. Finally, upon coating the PdNWs with a small amount of Pt (<5% by weight), the response and recovery time improved dramatically to 5 s to 1% H₂. We note that the PdNW or PdPtNW loading per device is roughly 0.1 mg, corresponding to less than $0.01 of metal value. Hence, for the first time, we report a low-cost solution-processed palladium alloy (PdPt) nanowire-based paper sensor with ultra-fast response and recovery time of 5 seconds to 1% H₂ in air. These operate at room temperature, with minimal power requirements, and could therefore be widely deployed in support of safe use of H₂ as an energy carrier.