Synthesis of Porous Materials and Their Applications in Electrochemistry and Additive Manufacturing

Abstract

Open cellular porous materials, such as polyurethane foams, ceramic membranes, and silicon aerogels, are useful in many applications, such as gas membranes, seawater desalination, and heat insulation, because they often possess exceptionally high surface area per unit mass (>100 m2/g), high porosity (> 90%), and low mass density (< 100 mg/cm3). The simplest porous structures often consist of only a single solid material, which limits the ability to tune properties. To address this issue, fillers and other additives, such as polymers, metal nanoparticles or carbon-based substances, can be incorporated to synthesize composites with desirable properties. Polymer-carbon composites stand out from the rest, partially because the soft portions (polymers) and hard compounds (carbon) often possess distinctive yet synergistic properties. For example, incorporating a small amount (< 1 wt%) of electrically conductive graphene nanoflakes into polydimethylsiloxane (PDMS) elastomer makes the product both mechanically robust and electrically conductive, which are desirable for applications in contact sensors and flexible electronics. Pore size, morphology, isotropy, and porosity are some of the most important factors to consider when evaluating the inherent performance of porous materials. These parameters are largely determined by the processing conditions, such as temperature, concentration of porogen (a templating substance that can be easily removed during post-processing, such as water or salt, leaving behind the pores), and method of synthesis, in addition to the selection of parent solid materials. Templating is one of the many routes employed to synthesize porous structures, where a sacrificial porogen is used to first form a percolating network and is later replaced by air when removed, typically via sublimation or washing. Compared to other routes such as foaming, sol-gel transition, etching or lithography, templating enables the fabrication of complex pore shapes and geometries over large-scales with tunability in the pore size, morphology, and pore connectivity of the final product; therefore, templating is considered one of the most versatile approaches. This thesis outlines the synthesis of open cellular porous polymers and polymer composites using freezing templated methods. We first designed a carbon-polymer aerogel which is highly porous (99.6% porosity), has low density (~ 5 mg/cm3), and is electrically conductive (5.3 ± 3 × 10-2 S/cm), making it an ideal substitute for the metal current-collectors in lithium-ion batteries. Next, we explored strategies to prepare graphene oxide aerogels with aligned microstructures via bi-directional freezing. Simulations were conducted to predict the structure of the aligned aerogel, which agreed reasonably well with experimental results. Lastly, we explored camphene, a solid cyclic hydrocarbon at room temperature, as the solvent and templating agent for 3D printing porous polymers. Upon subliming camphene, the resulting porous network exhibited improved interlayer strength and reduced anisotropy, and the tensile properties were comparable to those of compression-molded samples. This new strategy to prepare porous polymer materials via direct ink writing could be further applied to other common polymers, such as polyethylene or polypropylene, two commercial-grade materials that are very challenging to print via conventional methods.