This thesis describes the use of cellulose nanocrystals for the fabrication of porous nanocomposites with electronic conducting polymers for electrochemical supercapacitor applications. The exceptional strength and negatively charged surface functionalities on cellulose nanocrystals are utilised in these nanocomposites. The negatively charged surface functionalities on cellulose nanocrystals allow their simultaneous incorporation into electropolymerised, positively charged conducting polymer for charge balancing, i.e. co-electrodeposition occurs. The exception is the case of polyaniline-cellulose nanocomposites which formed with uncharged cellulose nanocrystals. As a result, the cellulose nanocrystals form the structural backbone of the nanocomposites in which the mechanical integrity of the nanocomposites becomes significantly improved. In Chapter 1, supercapacitors and the electrode materials are introduced. The equations relating to the characterisation of supercapacitor materials and devices are also introduced in Chapter 1. In the first half of Chapter 2, the basics of electrochemistry and electrochemical methods used in this work are discussed. In the second half, the preparation of the cellulose nanocrystals is reported. Chapters 3 and 4 report the fabrication and characterisation of polypyrrole-cellulose nanocrystal composites with respect to their capacitance, stability and charging characteristics. Chapter 5 discusses the making of the polypyrrole-cellulose nanocomposites at a practical scale for supercapacitors, and consequently reports the making and testing of a laboratory prototype supercapacitor. Chapter 6 extends the PPy work to other ECPs by the fabrication and characterisation of polyaniline and poly(3,4-ethylenedioxythiophene) nanocomposites with cellulose nanocrystals. Finally, Chapter 7 contains the closing conclusions that I have made for this thesis, and in Chapter 8, I have made some suggestions for future work in this area. In this project, the materials were characterised using mainly scanning electron microscopy and a range of electrochemical techniques. Specifically, the performance of the polypyrrole-cellulose nanocomposites was compared against that of polypyrrole-carbon nanotube nanocomposites, current state-of-the-art materials for supercapacitor. The performance of all the nanocomposites described in this thesis was also critically compared against that of the best available similar materials in literature, to assess the viability of these materials for applications in supercapacitor devices. Significantly, to the best of my knowledge, this is the first time that the nanocomposites of electronic conducting polymers with non-conducting rod-like nanoparticles fabricated using the co-electrodeposition method were described. The performance of conducting polymer composites was significantly enhanced by the presence of the cellulose nanocrystals as the backbone. This work also proves, for the first time, that conducting polymer composite containing non-conducting nanofillers can also achieve high performance. This is a very interesting finding, compared to previous work reported in the literature for similar materials, such as those developed using carbon nanotubes as the composite filler.