Cellulose nanofibrils (CNFs) with carboxylated surface ligands are a class of materials with tunable surface functionality, good mechanical properties, and bio-/environmental friendliness. They have been used in many applications as scaffold, reinforcing, or functional materials, where the interaction between adsorbed moisture and the CNF could lead to different properties and structures and become critical to the performance of the materials. In this work, we exploited multiple experimental methods to study the water movement in hydrated films made of carboxylated CNFs prepared by TEMPO oxidation with two different surface charges of 600 and 1550 μmol·g^–1. A combination of quartz crystal microbalance with dissipation (QCM-D) and small-angle X-ray scattering (SAXS) shows that both the surface charge of a single fibril and the films’ network structure contribute to the moisture uptake. The films with 1550 μmol·g^–1 surface charges take up twice the amount of moisture per unit mass, leading to the formation of nanostructures with an average radius of gyration of 2.1 nm. Via the nondestructive quasi-elastic neutron scattering (QENS), a faster motion is explained as a localized movement of water molecules inside confined spheres, and a slow diffusive motion is found with the diffusion coefficient close to bulk water at room temperature via a random jump diffusion model and regardless of the surface charge in films made from CNFs. 

The performance of thin carboxylated cellulose nanofiber-based (CNF) membranes as proton exchange membranes in fuel cells has been measured in situ as a function of CNF surface charge density (600 and 1550 mu mol g(-1)), counterion (H+ or Na+), membrane thickness and fuel cell relative humidity (RH 55 to 95%). The structural evolution of the membranes as a function of RH, as measured by Small Angle X-ray Scattering, shows that water channels are formed only above 75% RH. The amount of absorbed water was shown to depend on the membrane surface charge and counter ions (H+ or Na+). The high affinity of CNF for water and the high aspect ratio of the nanofibers, together with a well-defined and homogenous membrane structure, ensures a proton conductivity exceeding 1 mS cm(-1) at 30 degrees C between 65 and 95% RH. This is two orders of magnitude larger than previously reported values for cellulose materials and only one order of magnitude lower than Nafion 212. Moreover, the CNF membranes are characterized by a lower hydrogen crossover than Nafion, despite being approximate to 30% thinner. Thanks to their environmental compatibility and promising fuel cell performance the CNF membranes should be considered for new generation proton exchange membrane fuel cells.

Carboxylated cellulose nanofibers, prepared by TEMPO-mediated oxidation (TOCN), were processed into asymmetric mesoporous membranes using a facile paper-making approach and investigated as lithium ion battery separators. Membranes made of TOCN with sodium carboxylate groups (TOCN-COO-Na+) showed capacity fading after a few cycles of charging and discharging. On the other hand, its protonated counterpart (TOCN-COOH) showed highly improved electrochemical and cycling stability, displaying 94.5% of discharge capacity maintained after 100 cycles at 1 C rate of charging and discharging. The asymmetric surface porosity of the membranes must be considered when assembling a battery cell as it influences the rate capabilities of the battery. The wood-based TOCN-membranes have a good potential as an ecofriendly alternative to conventional fossil fuel-derived separators without adverse side effects.

Technologies based on renewable materials are required to decrease the environmental cost and promote the development of a sustainable society. In this regard, nanocellulose extracted from wood finds many applications thanks to its intrinsic mechanical and chemical properties as well as the versatility in its manufacturing processes. In this thesis, I present the results of the investigations on carboxylated cellulose nanofibres (CNF) as ionic conductive membranes and electrode component in fuel cells and lithium ion batteries. Moreover, I also show the results of the assembly of CNF suspension and cellulose nanocrystals (CNC) - lepidocrocite nanorods (LpN) hybrids.

The fuel cell performance of CNF-based proton conductive membranes was evaluated as a function of intrinsic material parameters such as membrane thickness and surface charge density as well as extrinsic parameters such as the relative humidity (RH). It was found that the proton conductivity is about 2 mS cm^-1 at 30 °C between 65 and 95 % RH. At the same time, the water uptake of the membrane was measured and correlated with the structural evolution of the membrane using small angle X-ray scattering.

The performance of the CNF-based separator in lithium ion batteries was investigated as a function of membrane porosity and protonation of the functional groups. The Li-ion battery assembled with the protonated separators showed stable and good rate performance.

The CNF was also tested as binder in lithium ion battery, showing that the morphology and mechanical properties of the cathode depend on the nanofibre surface charge and degree of defibrillation. In particular, high surface charge and medium degree of defibrillation give the best electrochemical performance.

Pyrolysed CNF (cCNF) improved the electrochemical performance of silicon nanoparticles-based anode thanks to the carbon network derived from the nanofibres. Si-cCNF has a capacity retention of 72.2 % after 500 cycles at 1 C and better performance rate than the pristine silicon nanoparticles.

Regarding the assembly of nanocellulose, the nematic order of CNF suspension at different nanofibre concentrations (0.5 – 4.9 wt%) was studied by small angle X-ray scattering, polarized optical microscopy and rheological measurements. The order parameter reaches a maximum value of 0.8 depending on the CNF concentration. Small angle neutron scattering with contrast matching experiments reveals that the natural alignment of CNC and LpN can be switched using a combination of magnetic fields of up to 6.8 T and varying the amount of LpN incorporated in the CNC.

Carbon coating is essential for active materials in electrochemical applications that are often insulators or poor conductors. A conventional conformal carbon coating can hinder the ion diffusion to and from the active material and form an isolated conducting network. Especially, active materials with very large volume expansion, e.g., silicon, can destroy the carbon coating during lithiation, which makes conformal carbon coating inappropriate. This paper presents a novel textile-like carbon wrapping that provides efficient electron and ion diffusion paths via a wide-range carbon network and pores. The textile-like carbon wrapping can reduce the electrical contact loss during cycling through the wide-range carbon network, which makes it a suitable carbon coating for materials that undergo volume expansion. A textile-like carbon-wrapped silicon is formed by pyrolysis of a dried suspension of silicon nanoparticles mixed with enzymatically hydrolyzed cellulose nanofibers. It shows excellent electrochemical performance compared to a conformal carbon-coated silicon. It exhibits a reversible specific capacity of 680mA h g(-1) at 8.0 A g(-1) and shows excellent cycling stability (capacity retention of 94.5% after 500 cycles at 2.0 A g(-1)) with high Si content (95.71 wt%). Therefore, this novel textile-like carbon wrapping can be utilized in many electrochemical applications instead of the conventional carbon coating, especially for active materials that undergo large volume expansion.

Assembly of bio-based nano-sized particles into complex architectures and morphologies is an area of fundamental interest and technical importance. We have investigated the assembly of sulfonated cellulose nanocrystals (CNC) dispersed in a shrinking levitating aqueous drop using time-resolved small angle X-ray scattering (SAXS). Analysis of the scaling of the particle separation distance (d) with particle concentration (c) was used to follow the transition of CNC dispersions from an isotropic state at 1-2 vol% to a compressed nematic state at particle concentrations above 30 vol%. Comparison with SAXS measurements on CNC dispersions at near equilibrium conditions shows that evaporation-induced assembly of CNC in large levitating drops is comparable to bulk systems. Colloidal states with d vs. c scalings intermediate between isotropic dispersions and unidirectional compression of the nematic structure could be related to the biphasic region and gelation of CNC. Nanoscale structural information of CNC assembly up to very high particle concentrations can help to fabricate nanocellulose-based materials by evaporative methods.

The formation of nematically-ordered cellulose nanofiber (CNF) suspensions with an order parameter f(max) approximate to 0.8 is studied by polarized optical microscopy, small-angle X-ray scattering (SAXS), and rheological measurements as a function of CNF concentration. The wide range of CNF concentrations, from 0.5 wt% to 4.9 wt%, is obtained using osmotic dehydration. The rheological measurements show a strong entangled network over all the concentration range whereas SAXS measurements indicate that at concentrations >1.05 wt% the CNF suspension crosses an isotropic-anisotropic transition that is accompanied by a dramatic increase of the optical birefringence. The resulting nanostructures are modelled as mass fractal structures that converge into co-existing nematically-ordered regions and network-like regions where the correlation distances decrease with concentration. The use of rapid, upscalable osmotic dehydration is an effective method to increase the concentration of CNF suspensions while partly circumventing the gel/glass formation. The facile formation of highly ordered fibers can result in materials with interesting macroscopic properties.

Carboxylated cellulose nanofibers (CNF) prepared using the TEMPO-route are good binders of electrode components in flexible lithium-ion batteries (LIB). However, the different parameters employed for the defibrillation of CNF such as charge density and degree of homogenization affect its properties when used as binder. This work presents a systematic study of CNF prepared with different surface charge densities and varying degrees of homogenization and their performance as binder for flexible LiFePO4 electrodes. The results show that the CNF with high charge density had shorter fiber lengths compared with those of CNF with low charge density, as observed with atomic force microscopy. Also, CNF processed with a large number of passes in the homogenizer showed a better fiber dispersibility, as observed from rheological measurements. The electrodes fabricated with highly charged CNF exhibited the best mechanical and electrochemical properties. The CNF at the highest charge density (ISSO mu mol g(-1)) and lowest degree of homogenization (3 + 3 passes in the homogenizer) achieved the overall best performance, including a high Young's modulus of approximately 311 MPa and a good rate capability with a stable specific capacity of 116 mAh g(-1) even up to 1 C. This work allows a better understanding of the influence of the processing parameters of CNF on their performance as binder for flexible electrodes. The results also contribute to the understanding of the optimal processing parameters of CNF to fabricate other materials, e.g., membranes or separators.

Silicon is a good alternative to conventional graphite anode but it has bad cycling and rate performance. To overcome these severe problems, extensively interconnected silicon nanoparticles using carbon network derived from ultrathin cellulose nanofibers were synthesized. Ultrathin cellulose nanofibers, an abundant and sustainable material, entangle each silicon nanoparticle and become extensively interconnected carbon network after pyrolysis. This wide range interconnection provides an efficient electron path by decreasing the likelihood that electrons experience contact resistivity and also suppresses the volume expansion of silicon during lithiation. In addition, Ultrathin cellulose nanofibers are carboxylated and therefore adhesive to silicon nanoparticles through hydrogen bonding. This property makes ultrathin cellulose the perfect carbon source when making silicon composites. As a consequence, it exhibits 808 mAh g(-1) of the reversible capacity after 500 cycles at high current density of 2 A g(-1) with a coulombic efficiency of 99.8%. Even at high current density of 8 A g(-1), it shows a high reversible discharge capacity of 464 mAh g(-1). Moreover, extensively interconnected carbon network prevents the formation of a brittle electrode with a water-based binder. Therefore, this remarkable material has a huge potential for LIBs applications.

Cellulose is difficult to solubilize and undergoes thermal decomposition prior to melting. In recent years ionic liquids have been evaluated as solvents of cellulose. In the regeneration process the non-solvent governs the resulting material's crystallinity. Water adsorbs to amorphous cellulose, acts as plasticizer and lowers the T-g, hence the degree of crystallinity will affect the potential strain induced reorientation. We prepared regenerated cellulose fibers form ionic liquid using different non-solvents. The influence of shear forces upon cellulose chain alignment during extrusion was simulated in silica based upon rheological measurements. The regenerated fibers had different physical, morphological and mechanical properties. Molecular re-orientation in fibers induced by mechanical strain, at humidities above the Tg, resulted in much improved mechanical properties with the Young's modulus reaching 23.4 +/- 0.8 GPa and the stress at break 504.6 +/- 51.9 MPa, which is comparable to commercially available cellulose fibers.