Improvement of the performance and the stability of hydrogen gas sensor with ligand-linked Pt nanoparticles catalyst

The increasing use of hydrogen as an alternative energy source demands a highly
reliable sensor for ensuring the safety issue in different applications. A newly established idea, ligand-linked nanoparticles catalyst, can comply with the growing need
for a sensor with high sensitivity and fast response. However, a fundamental challenge is the long-term stability of the sensor with ligand-linked nanopartcles catalyst.
In the context of this dissertation, a combustible hydrogen gas sensor is developed using ligand-linked Pt nanoparticles catalyst with improved sensitivity, selectivity and
stability.
One of the prime grounds for the deactivation of the ligand-linked nanoparticles
catalyst is the inhomogeneous heating. The ligands in the over-heating places can be
destroyed, as a result, the nanoparticles are sintered. Conversely, water accumulation
in the low heating places deactivates the catalyst. Therefore, the sensor is designed
through modelling based on physical constraints focusing on the homogeneous temperature distribution in the catalyst area. The design is optimized also to meet the
other requirements such as low power consumption, high sensitivity and thermal insulation, transparency in the catalyst area etc. Subsequently, the challenges of the microtechnological processes were confronted and the problems were solved through a series
of additional experiments in the chronological fabrication process. Finally, a highly sensitive thermoelectric sensor with a Seebeck coefficient of 273 µV/K, the main hub of the
thesis, is developed. An additional thermoresistive sensor is fabricated that gives the
privilege of a comparative performance analysis with the thermoelectric sensor.

Pt nanoparticles linked by five different bi-functional ligands containing the aromatic ring in the backbone is used as the catalyst in this work, namely Pt-PDA, PtDAN, Pt-DACH, Pt-BEN, Pt-DATER. The catalyst is characterized through morphological analysis (e.g. SEM) as well as gas sensing experiments. The performance of the sensor based on the ligand-linked Pt nanoparticles catalyst are executed in a chronological manner and the optimum ligand-linked Pt nanoparticles are selected, Pt-DATER, based on the sensitivity, selectivity and lower detection limits.

The main focus of this work is the stability improvement of the catalyst. Within the
scope of this work, a comprehensive analysis of the stability of different ligand-linked
Pt nanoparticles has proceeded through several consecutive long-term tests at different
operating temperatures. The best ligand-linked nanoparticles are selected from the test
results based on the stability, Pt-PDA and Pt-DATER. The stability of the ligand-linked
nanoparticles catalyst is improved through achieving a homogeneous temperature distribution on the membrane by optimizing the sensor design. Further improvement in
catalyst stability is earned by implementing a new idea. A rough porous gold layer is
created on the membrane through partial etching of gold thin film by iodine solution.
The porous layer creates a mechanical bonding between the catalyst and the membrane
substrate, hence, reduces the delamination of the catalyst during the combustion reaction.

The developed sensor shows very high average sensitivity, 394 mV/1% vol. H2 and
linearity with a lower detection limit of 0.001% vol. H2. The power consumption is
reduced to 23 mW at 110 ◦C while the response time is 420 ms. In addition to that, the
stability of the sensor is satisfactory, Pt-PDA and Pt-DATER catalyst survived in 1.5%
vol. H2 flow for 72 h. Attributable to the further stability improvement with iodine
etched gold layer on the membrane, almost all the catalyst remained stable after the 35
days test in the continuous H2 flow at 1% vol. concentration.