Asymmetric Porous Materials from Non-Equilibrium Block Copolymer Self-Assembly for Separation, Catalysis, and Energy Storage Applications
Hesse, Sarah Aline
Block copolymer (BCP) self-assembly (SA) thermodynamics have been used to structure-direct organic/inorganic materials into periodic mesoscopic structures with 5-100 nm length scales for applications in energy conversion and storage, catalysis, and separations. While these materials typically possess high surface areas, quick access to these surfaces remains challenging due to slow transport through the small pores. Nature has found a way to combine high surface area with fast transport by creating asymmetric structures as exemplified by the human lung. However, engineering such asymmetric structures remains difficult, as it typically requires working away from equilibrium. The SNIPS (SA+NIPS) process combines BCP SA with an industrially-proven membrane fabrication process called non-solvent induced phase separation (NIPS). The resulting asymmetric polymeric membranes have a periodically ordered mesoporous top surface layer with high pore density and narrow pore size distribution atop a graded porous support structure with low resistance to flow – combining high selectivity with high flux. The periodically ordered top surface layer is a direct result of BCP SA. The highly permeable graded porous support structure is a direct result of the NIPS-process. In this thesis, BCPs are used to structure-direct (in)organic nanoparticles (NPs) into asymmetric membrane structures via the SNIPS process. Subsequent high temperature processing results in asymmetric porous inorganic materials in the form of oxides, nitrides, and carbons. As in conventional SNIPS derived polymer membranes, the structures have a mesoporous top surface with high pore density which merges into a porous support structure with continuous graded porosity and large macrovoids at the bottom. Furthermore, the walls of the substructure are mesoporous, providing a hierarchical pore structure and contributing to the surface area. Chapters 2 and 3 of this dissertation explore under which conditions carbon NP precursors (phenol-formaldehyde derived resols) can successfully be structure directed via SNIPS and converted into graphitic carbons. Using scattering and imaging techniques, early formation stages of the resulting membranes are investigated allowing property tuning of the final asymmetric materials with respect to e.g. achievable periodic order of the pores in the top surface layer and porosity profiles of the substructure. In chapter 4, asymmetric carbon membranes together with inorganic sol NP derived asymmetric titanium nitride (TiN) membranes are synthesized, characterized, and tested as electrochemical double-layer capacitors (EDLCs). These tests demonstrate that the asymmetric materials exhibit state-of-the-art power densities and competitive energy densities – a result of the BCP- and SNIPS-derived asymmetric membrane structures combining high surface area with fast transport of ions. It is expected that the knowledge gained from studying these first asymmetric inorganic materials can be used to introduce other functional materials into the SNIPS process. Furthermore, first successful results of asymmetric carbon and nitride structures in an energy application suggest that asymmetry in the pore size distribution of a porous material may not only be attractive to separation applications, but may also be worth exploring in other energy storage and conversion as well as catalysis applications.
Polymers; Materials Science; membranes; Nitride; oxide; Nanoscience; carbon; Polymer chemistry; Self-assembly
Wiesner, Ulrich B.
Estroff, Lara A.; Disalvo, Francis J.
Chemistry and Chemical Biology
Ph.D., Chemistry and Chemical Biology
Doctor of Philosophy
dissertation or thesis