Basic And Applied Research On The Cellulose Nanofibers-based Energy Storage Materials

The ever decreasing storage of non-regenerative fossil fuels and climate change hasforced human beings to pay much attention to sustainable and regenerative energies andtheir conversion and storage. All solid-state supercapacitors, as energy storage devices,have attracted much attention due to their high power density, low cost, high safety, lowenvironmental impact, and long cycle life, which are desired in many fields such aspersonal consumer electronics. One-dimensional cellulose nanofibers (CNFs) possess anappropriate geometric structure (at least a few microns long and3–4nm wide) andhydrophilic characteristics. Furthermore, CNFs also have some excellent properties, such aslow density, low cost, environmentally friendly nature, and so on. So, CNFs are moresuitable for use as the nano-spacers, aqueous electrolyte nano-reservoirs, hierarchicalnanostructure maker, and template of electrode material for all solid-state supercapacitors.Therefore, This thesis deals with utilization of cellulose nanofibers in the energy storagematerials as well as more in-depth understanding of the positive role of cellulose nanofibersin the energy storage materials. The main contents of this paper are as follows:(1) Carbon nanotubes (CNTs) have excellent electrochemical properties. Nevertheless,CNTs tend to aggregate into bundles due to the hydrophobic nature of their side walls. Thisis a major bottleneck in the exploitation of their excellent capacitance performance.Cellulose nanofibers are first selected as dispersant of carbon nanotubes and the dispersingability and stability are explored. And then, more in-depth understanding of the positiverole of cellulose nanofibers in full realizing the electrochemical properties of carbonnanotubes. Ultimately, All solid-state flexible supercapacitors are fabricated usingCNFs/CNTs hybrid materials film as the electrode material. The results show that1Dcellulose nanofibers are an efficient (about3.6mg mL-1), low cost, biocompatible, andenvironmentally friendly dispersant of carbon nanotubes in water. The uniform dispersionof cellulose nanofibers/carbon nanotubes exhibits good gelation and wet-spinnability.Therefore, CNFs/CNTs hybrid aerogel and CNF/CNTs hybrid non-woven macrofiber matcan be prepared very easily.1D CNFs can effectively prevent the aggregation of CNTs,significantly enhance the re-wettability, and improve the utilization efficiency of themesopores. Therefore, CNFs/CNTs hybrid materials-based all-solid-state flexiblesupercapacitors exhibit excellent electrochemical properties: the specific capacitance isabout178F g-1(aergel-based supercapacitor) and the areal capacitance is about5.99mFcm-2(non-woven macrofiber mat-based supercapacitor). Furthermore, CNFs/CNTs hybrid non-woven macrofiber mat based wearable supercapacitors exhibit excellentelectrochemical performance, stability, tailorability and reliability.(2)2D graphene is a new nanocarbon material that are only one atom thick, which hasmany excellent properties such as excellent electrical conductivity, high surface area,excellent electrochemical stability, and lightweight. Unfortunately, graphene-based bulkmaterials have the structural characteristics of graphite owing to the strong π-π stackinginteractions between graphene nanosheets. Therefore, the performance of graphene-basedsupercapacitors is seriously degraded. Cellulose nanofibers are first selected asnano-spacers of graphene nanosheets. And then, more in-depth understanding of thepositive role of cellulose nanofibers in full realizing the electrochemical properties ofgraphene and the barrier capability of cellulose nanofibers. Ultimately, All solid-stateflexible supercapacitors are fabricated using CNFs/RGO hybrid aerogel film as theelectrode material. The results show that1D CNFs is an effective nano-spacers and aqueouselectrolyte nanoreservoirs of graphene-based electrode. So, graphene-based electrodematerials in an assembled bulk form have the structural characteristics of graphene insteadof graphite, which can decrease the distance of ion diffusion and enhance the utilization ofmesopores. Therefore, CNFs/RGO hybrid aerogel film-based all-solid-state flexiblesupercapacitors exhibit excellent electrochemical properties: the areal capacitance,maximum areal power, and areal energy density are158mF cm-2(207F g-1),15.5mW cm-2,and20mW h cm-2, respectively.(3) The excellent hydrophilicity of flexible transparent substrate can significantly improvethe electrochemical properties of transparent flexible thin film supercapacitors. The flexibletransparent CNFs films are first selected as substrate of transparent flexible thin filmsupercapacitors due to their excellent hydrophilicity. And then, more in-depthunderstanding of the positive role of cellulose nanofibers film in improving theelectrochemical properties of transparent flexible thin film supercapacitors. In order to takefull advantage of the electrochemical properties of RGO active layer, the π-π stackinginteraction is suppressed by the self-anti-stacking of distorted RGO nansheets under someconstraintd (Cu2+as anchoring agents). The results show that the excellent hydrophilicityand the unique surface morphology of CNFs film can be used as internal electrolytenanoreservoirs, which can decrease the distance of ion diffusion. In addition, theself-anti-stacking of distorted RGO nansheets can significantly improve the electrochemicalproperties of RGO active layer. Therefore, RGO-based all-solid-state transparent flexible thin film supercapacitors exhibit excellent electrochemical properties: the areal capacitanceis up to1.73mF cm-2at a current density of0.012mA cm-2and the transmittance ofT-SC-10is about56.5%(at550nm)..(4) Electronically conducting polymers (ECPs) have a high energy density and largepseudocapacitance. However, Uncontrollable preparation of nanostructure for ECPs is amajor bottleneck in the exploitation of their electrochemical properties and application inthe energy storage device. Controllable preparation method is first introduced into thepreparation process of the ECPs-based electrode material. And then, more in-depthunderstanding of the regulatory capacity of CIT-Fe3+in polymerization of pyrrole monomer,microstructure of PPy/CNFs hybrid aerogel, microstructure of PPy active layer and evenelectrochemical properties of PPy/CNFs hybrid aerogel under pH stimulation. The resultsshow that the microstructure of PPy/CNFs hybrid aerogel and PPy active layer can beeffectively regulated by the precursor of Fe3+oxidant-CIT-Fe3+under pH stimulation. ThePPy/CNFs hybrid aerogel film-based all-solid-state supercapacitors exhibit excellentelectrochemical properties: the maximum specific electrode capacitances of F-SC-25%andF-SC-35%are215F g-1(0.19mA cm-2) and203F g-1(0.09mA cm-2) respectively. TheF-SC-25%device capacitance still remains at81.6%of the initial capacitance after5000cycles.