(6jo) Solution-Processing Chemistry: Making New Adsorbents, Catalysts and Composite Materials for Clean Energy Applications

The pressing escalation of atmospheric CO₂ concentration has raised world-wide concerns nowadays because of its adverse consequences mainly in climate changes and ocean acidification. A common consensus that has been reached is to capture and utilize CO₂ as an immediate solution. Hydrogen has been recognized as the ultimate fuel to solve this issue because of its high energy density and clean water by-products after combustion, but its storage and transportation remain as a major obstacle to be tackled. Ammonia has been presented as an attractive alternative to hydrogen storage due to its high hydrogen content both volumetrically (81 g L^-1) and gravimetrically (17.6 wt.%) and a narrow flammable range (16 ~ 25 %). In this context, it has been regarded as an attractive alternative to the current lack of efficient and economically viable methods to store hydrogen in a compact, safe and cost-effective manner. According to the 2020 US Department of Energy report, the deployment of hydrogen as energy vector lies on achieving a high-density storage (9 wt.% gravimetrically and 75 g L^-1 volumetrically in capacity) and its on-demand supply at temperatures below 100 ^◦C for proton exchange membrane fuel cells (PEMFCs). The key for such a system being feasible is to develop a highly efficient and stable catalyst capable of decomposing ammonia gas to hydrogen under mild conditions.

Metal-organic frameworks (MOFs), constructed from coordination bonds between metal cations and organic ligands, have emerged as promising adsorbent and catalytic materials. The last two decades have witnessed ample studies of MOFs and their applications in gas storage and separation, because of their ultra-high porosities, tunable pore size and structures, and versatile chemical functionalities. However, most of the reported MOFs so far suffer from their weak hydrothermal stabilities that prevent their industrial applications. Their repeatable synthesis and scale-up remain as a big challenge for their mass and commercial production. I have developed a general, mild, green modulated hydrothermal (MHT) approach to synthesize a series of MOFs with repeatable quality and scaled-up production. This method can not only be applied for the synthesis of reported MOFs, but also suitable for the design of new MOFs with multifunctionalities, such as Lewis acid sites, hierarchal pore structures, topological heterogeneity, etc. It has led to two patents, a review invitation from Dalton Transactions journal (RSC publishing), a book invitation from Pan Stanford Publishing, and special interest from MOFapps and UOP (Universal Oil Products) company.

Column breakthrough experiment is a common technique to evaluate the gas separation performance of adsorbents in industry, however, normally without considering the pressure drops and variable flow rates at the exit of the column, leading to inaccurate results, especially in the case of nanosized MOF adsorbents. I have thus designed and built a lab-scale breakthrough setup to accurately evaluate the gas separation performance and adsorption dynamics of nanosized MOF adsorbents, by systematic considerations of velocity at the exit of the column (for mass balance accuracy), pressure drop, mean residence time and moisture interference. I have pioneeringly introduced an internal gas (Argon) reference at the exit of the column to measure the flow rates at the exit of the column. The research has set up a standard for understanding the breakthrough theories and conducting accurate breakthrough experiments in future, which is highly beneficial for industrial collaborators.

During the last few years, use of ammonia as a hydrogen vector has demonstrated the release of hydrogen at temperatures ~250 ℃ using ruthenium-based catalysts supported on carbon nanotubes (CNT). It is believed that this high activity is related to the ruthenium particle size (~3 - 5 nm) facilitated by the CNT support where the concentration of B₅ active sites is maximized. In addition, metal-support interactions have been reported of playing a vital role in enhancing the activity and stability of supported catalysts. However, there lack of efficient methods to enhance the metal-support interactions to improve the efficiency and stability of metal nanoparticle sites. I have thus reported a MOF-templated approach to synthesize ruthenium nanoparticle catalysts with cesium promoters supported on mesoporous crystalline zirconia for ammonia decomposition to produce hydrogen. In the presence of catalysts, the equilibrium yield for ammonia dissociation remains thermodynamically limited at low temperatures. I have thus developed a separative membrane reactor that integrates the reaction and separation steps, allowing in-situ selective removal of hydrogen, thus dramatically decreasing the total volume of the system and increasing the ammonia conversion higher than in a fixed-bed reactor at given reaction temperatures. This technology has possibly paved a way for ammonia as an energy vector to be utilized in PEMFCs, putting forward the vision of “ammonia economy”.

Research Interests:

My future research projects are applying advanced chemistry and engineering principles to develop advanced porous adsorbents, catalysts, and membranes as well as related devices for carbon capture and conversion, process optimization and integration, gas storage and separation, electrochemical nanofabrication, and water purification.

Teaching Interests:

I would like to apply my fundamental knowledge and research expertise to the core undergraduate and postgraduate courses in chemical engineering and polymer chemistry. I’d like to contribute more to the core modules like "Polymer Chemistry and Polymer Physics", “Separation Processes”, and “Reaction Engineering”. In these modules, I’d also like to incorporate the advanced metal-organic framework (MOF) and covalent-organic framework (COF) adsorbent materials into the lectures to strengthen the topics of diffusion and adsorption phenomena as well as crystallization kinetics and control mechanisms. In addition, I’d like to develop some specialized modules alone or with other colleagues. The first module is “Global Energy Solutions and Environmental Challenges”. In this module, the short-term solution of carbon capture to mitigate the environmental deterioration will be covered in the first section; the transitional solution of methane storage and delivery will be taught in the second section; the long-term solution of development of clean energy sources, such as hydrogen, solar, etc., will be covered in the last section. The second module is “Membrane Technologies and Applications”. This module will cover the fundamental theories of solution-diffusion mechanisms, inorganic and organic membranes, membrane fabrication techniques, gas separation and water treatment applications. I’d also like to incorporate the advanced MOF/COF/graphene membranes into this module. The last module is “Soft Materials”. Basic theories of polymer chemistry and physics will be covered in the first section; electro, photo, redox, temperature, and pH responsive as well as self-healing polymers will be covered in the second section; polymer blends and composites will be covered in the last section.