Light-assisted amperometric based hydrogen gas sensing at low concentrations
Creators: name: Alenezy, Ebtsam
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340108
subject: Sensor technology (incl. chemical aspects)
classificationCode: 340603
subject: Colloid and surface chemistry
classificationCode: 401605
subject: Functional materials

description: Hydrogen (H2) is widely used in large-scale fuel-cells and chemical refineries. However, its greatest impact may end up being as a clean energy source. From a safety aspect, detecting low concentrations of H2 gas at low temperatures is now gaining more attention as the use of H2 in consumer-based energy banks is gaining popularity. Additionally, due to its inflammable and explosive nature, safety issues during its production, transportation, and usage must be addressed. Thus, detection well below the lower explosive level (4%) down to ppm levels is imperative in enhancing end-user confidence for future uptake of H2-based fuel technologies. Furthermore, such sensors could also be used for other applications including in medical industry where it can replace the conventional yet expensive and labour-intensive breath analysers that can detect trace amounts of hydrogen gas to diagnose many gastrointestinal diseases (GIDs). Several attempts to develop semiconductor-based hydrogen sensors which can operate at low temperatures have typically resulted in trade-offs on critical performance aspects such as selectivity and sensitivity. Achieving an operating temperature close to room temperature is an important requirement when considering safety, reduced energy consumption, and compatibility etc. Consequently, the major aim of this research has been to develop a light assisted amperometric gas sensor (AGS) that is highly selective toward low concentrations (ppm levels) of H2 gas using transducer platforms that requires little to no external heating sources and operate at close to room temperatures. 

A critical literature review revealed that there were several major research questions, and thus knowledge gaps, that needed to be addressed prior to successfully developing an AGS for H2. Various material nanostructure designs were systematically developed to enhance sensor performance while operating at low temperatures. The materials that were developed in progressive sequence included TiO2/MoS2 core shell heterostructure, Pd NPs decorated on TiO2 based colloidal crystals (CCs), and soot templated TiO2 decorated with Pd NPs. Each AGS design and material type addressed a specific research question on H2 sensing performance. The feasibility of the developed sensors was tested under simulated industrial conditions which include carbon dioxide, methyl ethyl ketone, acetone, acetaldehyde, nitric oxide, and humidity. The AGS based hydrogen sensors were tested toward H2 gas concentrations of 50, 75, 100, 200, 300, 400 and 500 ppm with/without the presence of several foreign gas species. The tests involved applying different biases (0.1, 3, 6 and 9 V) with and without light excitation at an operating temperature of 33 ºC. These tests enabled detection of low concentrations at low operating temperatures with various degrees of freedom to control sensor performance, thus enabling tailor designed sensors for different applications. The data from each developed sensor was analysed to determine the effect of fabricated material on each sensor's performance in terms of response magnitudes, limits of detection, response time, recovery time, calibration curve trends, memory tests, sensitivity, and selectivity, all of which are critically relevant when using such devices in real-world industrial conditions. 

The analysis of the H2 sensing data revealed that material properties play an important role in sensing performance. For instance, replacing the MoS2 component of the composite with Pd resulted in excellent results with lower LoD, as low as 3.5 ppm under the highest applied potential of 9 V and under a light illumination of 365 nm with an intensity of 2024 micro watts per cm2. This was due to Pd nanoparticles being good H2 sorbents and catalysts for hydrogen sensing while the MoS2 photoactive semiconductor forms heterojunctions with the Pd NPs. This combination resulted in low detection limits and superior operation at low temperatures. By replacing the MoS2 CCs with TiO2 CCs, a significant improvement in selectivity as well as sensitivity was observed. Furthermore, it was postulated that an increase in the surface area of the TiO2-Pd composite, while controlling the uniformity of Pd NPs decoration, should enhance H2 gas sensitivity. This was achieved by using soot templated TiO2 decorated with Pd NPs which resulted in better detection limits while maintaining near room temperature (33 °C) operation.
  
The success of the data presented in this thesis has resulted in a PCT patent (PCT/AU2021/051274) of the developed AGS-based H2 sensor and is due to undergo preliminary testing for potential aerospace applications at Infinity Fuels (contractors to NASA). Furthermore, the low detection limit and high selectivity at room temperature is a significant technological breakthrough with potential applications in many sectors (aviation, transport, medical, manufacturing etc.) involving hydrogen gas sensing.