Evolution of Porosity and Nanostructure in Preceramic Polymer Derived Nanoporous Particulates

Abstract

Inert pyrolysis of Si-containing polymers affords multifunctional amorphous ceramics. Enhancing the textural properties of these materials, including porosity, surface area, and nanostructure, one can enable a wide range of applications in adsorption, gas storage, gas separation membranes, ultra-capacitors, bio-implants, and Li ion batteries. However, the control of pore size distribution and retention of the high surface area at higher temperatures are engineering challenges that need to be met by novel materials and processing methods. Polymer derived silicon particulate ceramics can be effectively used to meet the above challenges. Pyrolysis of the preceramic polymers leads to the escape of organic moieties followed by sintering, resulting in a very low specific surface area (SSA) of the final particulate ceramics produced. Current work explores a novel and versatile method for the fabrication of a high surface area ceramic hybrid with controlled porosity at higher temperatures. In a typical process, the hybrid ceramics are proposed to be fabricated by coating the liquid preceramic polymer solutions around particulate ceramics. The template limits shrinkage of the PDC coating leading to retention of SSA over 290 m2 g-1, and micro-mesoporous hierarchical pore size distribution (PSD). The first part of the work deals with crystalline oxide as templates. Nanoparticulate boehmite powders were used as templates around which preceramic polymer were coated yielding an alumina-SiOC core-shell structure stable at higher temperatures. The final microstructure of the SiOC produced depends on many factors such as the polymer composition and pyrolytic parameters such as final temperature, and heating rate. Thus, two kinds of polymers were used based on their carbon content and their architecture such as a silica rich polymethylsilsequioxane and a carbon rich polymethylphenylsilsequioxane. Additionally, the effect of various processing parameters including polymer ratio, heating rate, and pyrolysis temperature was studied. The evolution of the phase and porosity was extensively studied with varying parameters to find an adequate process to produce a micro-mesoporous nanostructure. Subsequently, amorphous nanocarbon particulates of similar particle size were chosen as templates owing to the versatility of the process. Similar studies were performed for the nanocarbon-SiOC hybrids. It was observed that the porosity evolution in SiOC depends on the process of its conversion from polymer to ceramic and not on the template chosen, making it generic and versatile. In the next part, nanostructured-PDC hybrid was further explored by selectively sacrificing the species present in the SiOC microstructure. The unique SiOC structure in the coating, comprising of SiO2 based nanodomains (1 nm–5 nm) and graphene type carbon layers provided opportunities to create porosity by eliminating SiO2 domains from the nanostructure. Such carbon hybrid materials, produced by HF etching the ceramic hybrids have shown SSA in excess of 900 m2 g-1 to 2000 m2 g-1 with a major part being microporous (<2 nm). High resolution transmission electron micrograph (HRTEM) imaging of the hybrids has shown observable microporosity. Interestingly, a wide range of pore sizes and their distribution can be tailored according to the desired need with subtle changes made in process parameters. The high SSA, optimal PSD, and the highly ordered carbon structures formed in the carbon hybrids made the material suitable for electrode materials in electrical double layer capacitors (EDLC). The results obtained gave the highest value achieved of 333 F g-1 in aqueous electrolyte. The nearly rectangular shapes obtained from the cyclic voltammetry graphs observed for the material made it ideal for a supercapacitor electrode. The effect of the microstructure, including the pore shape and volume in the carbon hybrids, was seen to affect their supercapacitive behavior.