Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
The aim of this study has been to comprehensively investigate the potential use of cellulose-based materials in electronics, both in terms of usage in electrically insulating as well as electrically conducting applications.
The initial attempt was to shed light on the electrically insulating properties of cellulose by fabricating regular paper of various thicknesses and to test these as porous insulators in for example lithium-ion batteries. The benefit of this approach was to fabricate papers with known components without additional fillers or such that otherwise would affect the insulating properties, as would be the case if using regular retail paper. However, this attempt was dropped due to time restraints and a mutual desire from both Stanford and KTH to use this limited amount of time to achieve something more groundbreaking.
The next attempt was therefore to use cellulose-based materials as substrates for coating by various conductive materials as an expansion of the concept derived by fellow group members Hu et al(2009) coating regular Xerox paper with conductive carbon nanotubes(CNTs). From then on, the work was split into two main projects, one with the goal of creating a novel and conductive material for energy storage and the other to obtain a novel transparent electrode for the use in optoelectronics.
In the energy storage part, a novel aerogel consisting of mechanically rigid and flexible nanofibrillated cellulose(NFC) and conductive CNTs with an addition of polyvinylamine was fabricated and optimized through a vacuum filtration and freeze drying approach. The structure showed a low sheet resistance and high stability in water in addition to a high porosity thereby making it excellent for a high load of additional active materials.
The fabricated aerogel was then deposited with the high capacity material silicon and used as an anode in a lithium-ion battery. The structure would in this case help to release the strain created from lithium insertion into silicon which otherwise is a major problem. The device showed a specific capacity of 1500 mAh/g, which is more than three times the theoretical maximum of graphite, and a fairly good capacity retention during cycling. A supercapacitor device based upon the structure was also fabricated showing average results.
Moreover, a new transparent electrode material using the NFC combined with either indium tin oxide(ITO) or CNTs was fabricated. The current strive to develop novel transparent electrodes for optoelectronics, e.g. solar cells, has really made this field interesting and this work yielded flexible films with high specular transmittance and low sheet resistance using a vacuum filtration approach. The material would have the potential to replace the current prime choice ITO on glass and enable cost-efficient roll-to-roll processing due to its flexibility and thermal stability.
Finally, the novel transparent electrode material was tested in organic and dye-sensitized solar cells yielding the first ever working solar cell based upon a NFC substrate, although its power conversion efficiency was low.
2011. , 73 p.