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.