Cellulose-derived conductive nanofibrous materials for energy storage and tissue engineering applications

Sammanfattning: There is no doubt that nanofibrous materials are among the most opportune materials used in advanced applications nowadays. To supply anticipated high demand for these materials sustainable resources such as plant derived polymers should be explored. In this thesis, I demonstrate that the most abundant natural polymer cellulose is an excellent raw material for synthesis of new nanofibrous materials with valuable combination of properties such as electrical conductivity, porosity and topography. These materials can contribute to the solution of two rather different but equally important problems faced by modern society: lack of high power energy storage devices able to keep up with the technical progress and increased rate of neurodegenerative diseases, which inevitably accompanies an ageing population.

In connection with the first problem, supercapacitors are considered to be devices of choice when high power energy supply is needed. However, the effectiveness of supercapacitors mostly depends on active materials traditionally made of porous carbons which are used for accumulation of electrostatic charges. At the moment, the production of carbon materials mostly relies on unsustainable fossil precursors. In the present work, I describe the fabrication of freestanding functional carbon nanofibrous (CNF) materials derived from cellulose via consecutive steps of cellulose acetate electrospinning, subsequent deacetylation to cellulose, and carbonization. I report innovative technologically simple and environmentally friendly method of CNF synthesis that significantly increases carbon yield (from 13% to 20%) and allows time reduction of the regeneration step. The obtained CNF materials are mechanically stable, have hydrophobic surface and consist of nitrogen-doped randomly oriented nanofibrous network.

Moreover, the prospect of effective using of various modified CNF-based materials as electrodes in supercapacitors is demonstrated. Nitrogen-doped CNF materials have about 2.5 times higher specific capacitance than non-doped CNF materials due the positive effect of pseudocapacitance. Incorporation of highly conductive carbon nanotubes (double-walled CNTs, multi-walled CNTs and chemical vapor deposited CNTs) and reduced graphene oxide into the CNF frameworks further improves electrical conductivity and increases the surface area of the produced composite materials, which leads to high specific capacitance values (up to 241 F/g), cyclic stability, and power density of these materials. These results show that cellulose is a relevant precursor for the synthesis of sustainable and efficient carbonaceous electrodes for supercapacitors. Functionalization methods used in this study proved to be effective in enhancing the electrochemical performance of carbonized cellulose materials.

In connection with the second problem, an emerging tissue engineering approach can help to cure neurodegenerative diseases of elderly population via development of healthy replacement neural tissues or in vitro models for drug testing. In this thesis, several cellulose-derived nanostructures, such as above-mentioned CNFs and fibrous electrospun cellulose incorporated with CNTs, are assessed as scaffolds for the growth of neural tissue. These scaffold materials are characterized with good biocompatibility, optimal nanosized topography and electrical conductivity to support adhesion, growth and differentiation of SH-SY5Y neuroblastoma cells. Possibility of using inks from nanofibrillated cellulose for 3D printing allows even more effective assembly of designed conductive patterns for cell guidance. The results show prolific cell attachment, proliferation and differentiation of neural cells along the guidelines.

In overall, the positive implementation of the cellulose-derived nanofibrous materials in the above mentioned applications suggest that the synthesis of sustainable and efficient materials based on renewable resources is a very prospective approach. Such materials should play a major role in our future effort to satisfy the increasing demand on functional high-tech products.