Electrochemical deposition and surface studies of Mo-oxide based thin films for CO2 sorption applications

• Main Author: Mohammad Tanhaei
• Other Authors: Sum Tze Chien
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Chemistry::Physical chemistry::Surface chemistry; Engineering::Materials::Microelectronics and semiconductor materials::Thin films; Engineering::Materials::Material testing and characterization
• Online Access: https://hdl.handle.net/10356/163000
• Institution: Nanyang Technological University

With about 40 gigatonnes of carbon emissions every year, global warming is an international concern. The most accessible scenario to mitigate climate change according to the Intergovernmental Panel on Climate Change (IPCC), is CO2 removal. Carbon capture is a viable technology that requires specific materials design to selectively and efficiently capture carbon dioxide from the fluent gas or air. This technology becomes even more important when considering negative emissions and direct air capture (DAC). Solid adsorbents remain a promising group of materials for carbon capture applications. The majority of available solid sorbents are studied in the form of powder, and the viability of thin-films is yet to be investigated. Thin-film sorption may have inherent advantages, such as overcoming diffusion limitations and low storage cost. It is necessary to understand the fundamentals of solid-gas interactions to capture one of the most stable molecules, like CO2, in ambient conditions. Therefore, this thesis investigates fundamental strategies in materials design to improve CO2 sorption performance. This is accomplished using a thin film approach that allows a detailed and controlled study of solid-gas interaction. The first part of the thesis aims to explore how defects affect the sorption performance of molybdenum (Mo) oxide. In controlling defects generation using electrodeposition, the thesis introduces a novel approach to directly incorporate defects in the coating without the need for post-processing methods. The produced film’s CO2 interaction was studied by examining its chemical state evolution. Together with sorption isotherms studies, supported by NAP-XPS and density functional theory calculations, it was found that Mo4+ defects favorably improve defect-film interaction and boost the sorption performance by about 50%. In the second part of the thesis, further modification of the thin film was explored by incorporating polyethyleneimine (PEI) with Mo-oxide to yield a hybrid film. In this approach, the thesis introduced a simple protocol to establish the use of the pulse-electrodeposition method to incorporate such insulating polymer that has not been shown before. The films showed that PEI incorporation provides a strong and active surfactant that improved the morphology and could further boost the hybrid film’s sorption performance. The enhancement in capture efficiency at ambient conditions is attributed to smaller grain size obtained during growth and provision of more porous diffusion pathways via the PEI. In summary, the current thesis first established that the supported substrate approach for studying the sorbent-gas interaction is a robust platform for investigating and improving adsorbent material performance. Novel electrodeposition methods are introduced, and the thesis proves that meta-stable films, such as those with intentionally introduced defects, are a practical way to increase sorption performance, while the incorporation of polymers such as PEI as a hybrid film can favorably improve the film’s microstructure and also provide better diffusion pathways.