Optimising Mass Transfer in Synthetic Porous Materials

Abstract

The performance of materials is determined by their intricate structures. A primary example can be seen in hierarchically porous materials. Their characteristic multi-level networks facilitate efficient mass transfer through larger pores and increase the specific surface area via smaller branching pores. Optimising these porous structures for enhanced transfer performance has attracted considerable effort. In this regard, Murray’s law has been adapted to design and construct synthetic porous materials, inspired by the efficient flow observed in blood vessels. This biomechanic principle defines the most efficient hierarchically branched network for fluid transfer and is expected to herald an era of ‘nanostructuring by design’. However, the original Murray’s law, derived from biological circular tubes, cannot be directly applied to synthetic materials with diverse pore architectures. The geometric mismatch between the current theory and nanostructured materials significantly undermines the promised optimal transfer. It is imperative to expand the law to accommodate diverse scenarios in Murray materials. My PhD dissertation focuses on optimising the porous structure of synthetic materials. I first present the shaping techniques of graphene-based aerogels utilised to fabricate Murray materials, including 3D printing and freeze-casting. For printed aerogels, I reveal the fundamental principles that govern the surface morphology. This study enables the fine control of the printed architectures’ surface porosity by regulating the crosslinking agents and shear stress. The major contribution of my doctoral work is the development of Universal Murray’s law, which is applicable to optimising synthetic porous materials with generalised pore configurations and transfer processes. I explore the extensive applicability of this newly proposed law in hierarchically branched networks with non-circular cross-sections and planar structures. The experimental validation confirms my proposal by exhibiting superior transfer performance in freeze-cast aerogels following the law. Additionally, I show that structural optimisation guided by this theory can substantially improve the kinetic performance of aerogel-based gas sensors, where the response and recovery times are shortened by 8.6% to 18.2%. This work provides a solid theoretical framework for designing synthetic Murray materials, with promising implications in catalysis, sensing, and energy applications.