Structural Design And Fabrication Of Electrospun Nanofibrous Materials For CO₂ Capture

Carbon dioxide?CO₂?emissions are believed to be a major contributor to global warming and assumed to be a key cause behind the greenhouse effect and climate change.That is why it is highly required to take immediate efforts to reduce CO₂ concentration in the atmosphere and limit further emission of CO₂ in the atmosphere.Carbon capture and storage technology will be an effective approach to reducing excessive CO₂ emissions from fossil fuel combustion.Presently,there are two major types of materials available,i.e.,liquid-based and solid based.However,these available materials which are capable of CO₂ capture have either particle form or corrosive nature,are produced by complex processes and are very costly.Thus,there is a massive demand for certain materials which have fibrous and flexible nature and offer considerable CO₂ uptake capacity.In this regard,electrospun nanofibers are potentially ideal materials for developing CO₂ sorbents owing to their easy and relatively economical fabrication process,free-standing nature,tailorable porous structure,and feasibility for various treatments for surface modification.In the present thesis,at first,a detailed review of theoretical background and recent progress on CO₂ capture,emphasizing the need for CO₂ capture,technologies/techniques used for CO₂ capture,different materials,and their types used for CO₂ capture,etc.has been provided.Next,essential parameters that are needed to be regulated for designing a potential sorbent for CO₂ adsorption are comprehensively discussed.Subsequently,we report the fabrication of nanofiber-based sorbents with considerable CO₂ uptake capacity via electrospinning.As CO₂ uptake capacity of sorbents mainly depends on the combination of high surface area,good porous structure,and the presence of active sites.Therefore,several strategies were employed to generate/improve the surface area and pore volume of electrospun nanofibers and to generate active sites on the nanofiber surface,including reduced fiber diameter?nano-nets?,differential volatility of two solvents,and pore-forming templates following by carbonization.Our engineered sorbents displayed inter-connected nanofiber/net network,considerably high roughness and porosity,and porous sea rod-like fiber with multiple apertures throughout the fiber.All of our designed nanofibers not only showed improved N₂ adsorption/desorption isotherms and reasonably high surface area but also offered significant CO₂ uptake.The designed sorbents also offer excellent selectivity towards CO₂molecules,which is an essential characteristic of CO₂ sorbent and would facilitate the sorbent to capture CO₂ molecules selectively from mixed gas streams.Various factors,challenges,and perspectives influencing the sorbent synthesis or their performance have also been thoroughly discussed.A number of techniques including scanning electron microscope,transmission electron microscope,X-ray diffraction,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,energy dispersive spectroscopy,thermogravimetric analysis,etc.have been used to evaluate the different characteristics and CO₂ uptake performance of fabricated nanofiber-based sorbents.The major contents and findings of the thesis are discussed below.1)At first,we developed a free-standing composite nanofibrous membrane combining the nanofiber and nano-nets structure for CO₂ capture.The approach behind the development was to effectively use the high surface area and compact structure of nano-nets to develop an efficient CO₂ sorbent.For this purpose,polyamide?PA?,a well-known textile polymer was chosen as base materials for developing nanofiber/nets membrane,where as carbon nanotubes?CNT?and polyethyleneimine?PEI?were used as reinforcement and surface modifier,respectively,to tune the mechanical as well as surface characteristics of the fabricated nanofiber/net composite membranes.The addition of CNT improved the mechanical properties,making the synthesized nanofiber/nets free-standing and self-supportive to withstand applied stresses.Surface modification with PEI provided Fabricated PA/CNT nanofiber/nets not only offered decent CO₂uptake of 51 mg/g?i.e.,1.16 mmol/g?at room temperature but also exhibited stable performance for up to 12 CO₂ adsorption/desorption cycles,proving the polymeric nano-nets as the potential sorbent to be considered CO₂ capture applications.2)Next,we designed porous nanofiber-based membranes with strong mechanical characteristics and analyzed their performance as CO₂ sorbents.Polystyrene?PS?has opted as base material owing to its ability to develop highly porous nanofibers.Electrospun PS nanofibers have an excellent porous structure,however,they are not strong enough to tolerate applied mechanical stresses.Thus,polyurethane?PU?was used as reinforcement to not only enhance the mechanical properties of the resultant nanofiber membrane but also retain the fibrous structure unaffected.Although,PS/PU nanofiber had an excellent porous structure and reasonably high surface,but they lacked affinity towards targeted CO₂ molecules.Therefore,they were chemically treated with three multiple types of amines.It was found out that PS/PU nanofibers modified with low molecular weight PEI offered not only a considerable amount of CO₂ uptake?1.64 mmol/g?but also exhibited outstanding selectivity?S=20?towards CO₂ molecules for selective capture of CO₂ from a mixed gas stream.Additionally,synthesized PS/PU nanofiber membranes also displayed even better cyclic stability for up 19 CO₂ adsorption/desorption cycles.3)Lastly,to further improve CO₂ uptake capacity,we designed electrospun nanofiber-based sorbents with a hierarchical micro-porous structure via the carbonization technique.For this purpose,we electrospun a composite solution comprising of polyacrylonitrile?PAN?and polyvinyl pyrrolidone?PVP?polymers.We opted for PAN,a famous polymer well-known for its high yield when carbonized as a precursor for carbon because of its nitrogen-rich chemistry,whereas PVP was used as a pore-forming template.After electrospinning PAN/PVP solution,PVP was selectively removed via the intensive washing process from the fabricated composite nanofibers to generate highly rough and extremely porous PAN nanofibers.These highly rough and extremely porous nanofibers were then carbonized to develop PAN-based carbon nanofibers with a hierarchical porous structure.Interestingly,after carbonization,nanofibers exhibited randomly oriented and visibly apparent apertures/openings,highly resembling famous porous sea rods.These apertures/openings and microstructure of the carbonized nanofibers was further tuned by regulating the carbonization temperature.The average pore size of carbon nanofibers?0.71 nm?was almost twice the kinetic diameter of a CO₂ gas molecule?0.33 nm?and conformed to the optimal pore diameter range of the gas adsorption material calculated by Monte Carlo simulation.Subsequently,the resultant nanofibers not only exhibited finely tuned the hierarchical porous structure and numerous apertures/openings throughout the fiber but also displayed a significant amount of nitrogen sites throughout the fiber matrix,especially the fiber surface.Since high porosity and active sites are the key factors that determine the CO₂ uptake capacity of the sorbents.Therefore,when exposed to CO₂,the fabricated nanofiber membranes exhibited high CO₂ gas selectivity?S=20?and offered superior CO₂ adsorption performance of 3.11 mmol/g.Moreover,no apparent change in mass for up to 50 cycles of CO₂ adsorption/desorption unveiled the long-term stability of synthesized PAN-based carbon nanofibers,making them a potential candidate for CO₂ adsorption application.In conclusion,this thesis summarizes the strategies to regulate microstructure and surface characteristics of electrospun nanofibers to design and develop efficient sorbents for CO₂adsorption.This engineering of highly porous nanofiber membranes having numerous active sites on surface paves new insights into the development of potentially ideal materials for CO₂ capture applications.