Utilization of Highly Reactive Calcium Oxide Nanofibers in the Sequestration of CO2

Developing carbon sequestration methods is one of fourteen Grand Challenges to be faced by engineers this century. Evidence continues to mount implicating carbon dioxide (CO₂) emissions as a primary contributor to climate change, and if CO₂ levels continue to rise, further changes to the climate could have dire consequences, resulting in rising sea levels, agriculture disruptions, and more extreme weather striking more often. Therefore, it has become necessary for CO₂ to be captured and stored safely away from the atmosphere. Over the years, several studies have been conducted to identify and develop efficacious candidates for CO₂ sequestration and utilization. Calcium oxide (CaO) is an attractive material for capturing CO₂ from flue gas due to its energy efficiency, high reactivity, and low material cost; however, its capturing capacity and thus efficiency is strongly affected by the synthesis method.

The primary objective of this research was to synthesize CaO nanofibers via electrospinning and test their performance as sorbents in a multi-cycle process of carbonation and decarbonation. CaO nanoparticles were also synthesized via a hydrothermal approach to compare their performance to the nanofibers. Electrospinning was used because it is a contactless electrostatic synthesis technique that produces one-dimensional nanofibers that have high surface area-to-volume ratios, interconnected porous networks, and greater mechanical properties. It is theorized that compared to other synthesis methods, electrospinning results in the fabrication of nanomaterials that contain more accessible reactive sites.

In this work, the electrospinning solution was prepared by dissolving 1:1 weight ratio of polyvinylpyrrolidone (PVP) MW=1,300,000 g/mol and calcium nitrate tetrahydrate in a co-solvent system containing 70:30 volumetric ratio of ethanol to water. Electrospinning was conducted at 30 kV, 1 mL/h extrusion rate, and a 16-inch separation between the collector and the tip of the extrusion needle. PVP-Ca nanofibers were collected at room temperature on aluminum foil and heat-treated afterwards at 650°C (2°C/min ramp rate) for 8 hours to decompose the PVP chains and produce CaO nanofibers.

The results indicate a promising opportunity for the application of electrospun materials in CO₂ sequestration. CaO nanofibers and nanoparticles were characterized using X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, and scanning electron microscopy (SEM) to elucidate information about crystallite size, specific surface area, and nanofiber morphology, respectively. The reactivity of the electrospun nanofibers and hydrothermally synthesized nanoparticles were tested using CO₂ thermogravimetry at 550°C, 600°C, and 650°C. Sorbent degradation was also tested over the course of ten cycles of carbonation at 600°C and calcination. CaO nanofibers exhibited significantly higher rates and conversion (100% vs 78.4%) compared to traditionally synthesized CaO nanoparticles. Furthermore, the CaO nanofibers continued to produce higher or comparable conversions over repeated cycling. The increased rate and conversion of the CaO nanofibers result from their smaller, more reactive domains. Smaller, more reactive metal oxide domains allow for these materials to achieve stoichiometric CO₂ capacities. These improvements in CO₂ sequestration materials have the potential to reduce costs of CO₂ capture by offering greater cost advantage than current conventional technologies due to the high availability and low cost of CaO sorbents.