Gas adsorption properties of electrospun carbon nanofibers for carbon dioxide capture and utilization

• Gasadsorption auf elektrogesponnenen Kohlenstoffnanofasern für Kohlendioxidabscheidung und -nutzung

Kretzschmar, Ansgar Karl Georg; Eichel, Rüdiger-A. (Thesis advisor); Wessling, Matthias (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

Electrospun Polyacrylonitrile-based carbon fibers are a powerful and tunable material for CO2 capture applications. In this work, unmodified carbon nanofibers carbonized at various temperatures are presented. Surface chemistry and morphology of these fibers are characterized using scanning electron microscopy, elemental analysis and X-Ray photoelectron spectroscopy as well as argon and CO2 sorption measurements. It is shown that the surface chemistry as well as the formation of ultramicropores depend significantly on the applied carbonization temperature, enabling a tailoring of the material for specific gas separation applications. To evaluate some of these applications, additional adsorptives are measured and give evidence that a molecular sieve effect is the major driver of the gas adsorption properties. In these measurements, an unusually high affinity to CO2, water and ammonia is found in comparison to other carbon materials, which is attributed to the extremely narrow pore width. The tailorable ultramicropore structure inducing the molecular sieve effect allows to evaluate concepts for the determination of molecular dimensions that play an important role in the discussion of molecular sieves, for example the kinetic diameter. Furthermore, the molecular sieve effect in the carbon nanofibers could be tailored for any gas separation application, given that their difference in molecular size is sufficient. An excellent adsorption selectivity is predicted from static sorption isotherms by the ideal adsorbed solution theory for the separation of CO2 and N2 as well as CO2 and CH4. The separation performance of CO2 and N2 is analyzed in greater detail using dynamic sorption methods that confirm the suitability of the carbon nanofibers predicted by static results. Furthermore, the adsorption kinetics are analyzed to evaluate possible limitations of the CO2 sorption rate in narrow ultramicropores. To increase the adsorption capacity of the carbon nanofibers, various concepts to introduce additional porosity are evaluated. While the introduction of mesopores with polymer-blend spinning does not enhance the CO2 sorption capacity, physical activation with CO2 and chemical activation with KOH significantly improve the high-pressures sorption performance, however at the expense of sorption selectivity and low-pressure performance. Overall, PAN-based carbon nanofibers turn out to be a well-performing material for CO2 adsorption and separation applications. This thesis provides a versatile toolbox to tailor electrospun carbon nanofibers to specific gas separation applications and process conditions.