Fabrication of Carbonized Structures with Controlled Pore Direction for Improved CO₂ Capturing Functionality Using Bio-Material Based Nanofibers

Porous architectures demonstrating high strength-to-weight ratio, large surface area and high resilience have been of recent interest for gas storage, battery electrode and fuel cell applications. Desired properties can be achieved in this class of materials by tuning the characteristics of the pores, including wall thickness, pore size and directionality of the pores, and by making use of different constituent materials. Another application of such materials is carbon dioxide capture, which can contribute to halting global warming caused by an increase in atmospheric concentration of CO₂. Early stage of CO₂ capturing research was mainly focused on materials based on chemical absorption, such as metal-organic framework (MOF) materials, which exhibit reduced cyclability due to the low physical strength and low chemical durability including their low resistance to moisture. These drawbacks can be overcome by exploiting physical absorption through surface activation of carbon-based materials. However, previous research has mainly focused on powders, which typically do not achieve high efficiency of CO₂ absorption due to the inefficient and random pathways of gas penetration.<br/>In this work, we fabricated free-standing carbonized structures with oriented pores using nanofiber-type cellulose, chitin, and chitosan bio-materials. Using directional freeze-casting, the directional growth of ice crystals according to an applied temperature gradient functions as a template for the formation of a structure with oriented pores and bio-nanofibers walls. The porous architecture of the materials was controlled by the concentration of bio-materials in the solution. After sublimation of the ice crystals, free-standing structures were obtained with xylem-like architecture, which were then pyrolized resulting in carbonized structures with nanoscale thickness of walls containing cellulose, chitin and chitosan nanofibers. In addition, activation through potassium hydroxide (KOH) was studied for enhancing CO₂ capturing. To characterize the CO₂ capturing functionality, the surface activation of the oriented carbon structures was studied using multiple cycles of CO₂ absorption-desorption by Thermogravimetric analysis (TGA) and Raman spectroscopy. This study will elucidate the correlations between the pore architecture, which is characterized by well-oriented walls of nanoscale thickness, and its effect on the CO₂ capturing properties.