Electrochemical energy storage and conversion devices are among clean energy technologies that play an important role in overcoming global pollution. Supercapacitors (SCs) are an important class of energy storage devices owing to their fast charge-discharge rate, high power density, low maintenance, and long cycle life. They bridge the gap between conventional capacitors and rechargeable batteries and are most useful in applications that require a fast power boost and delivery. Polyaniline (PAni) is a promising conducting polymer with the advantages of high pseudocapacitance, environmental stability, low cost and ease of synthesis. However, there are few shortcomings that limit the practical application of PAni as an electrode material for SCs. PAni suffers from poor cycle life which considerably compromises the advantage of long cycle life of SCs over batteries. In addition, it is formidable to produce mechanically robust films from pristine PAni since it is a brittle material. To overcome the limitations of PAni, this thesis explores various approaches to construct a flexible PAni-based composite film by combining PAni with other nanomaterials. A low-cost PAni-based electrode for solid-state SCs is developed on the basis of PAni and acid-treated carbon particles. Later on, a new strategy to construct a flexible PAni-based electrode with an adjustable mass-loading of active materials is introduced. Cellulose is employed as a backbone for a composite film of PAni, reduced graphene oxide, and silver nanowires. In a similar approach, a flexible electrode for high-performance SCs is developed on basis of PAni, cellulose, graphite-based exfoliated graphite, and silver nano-particles. This strategy allows designing composite electrodes with large surface area and porosity, and adjustable energy and power densities. Another strategy to construct a flexible PAni-based electrode and address the poor cycle life of PAni is growing PAni on nanofibrous and cushiony substrates. A flexible electrode for SCs with a remarkable cycling stability is developed using electrospun polyurethane as a cushiony support for growing flower-like PAni. Moreover, a novel flexible electrode is constructed by growing PAni on graphene-coated cross-linked polyvinyl alcohol nanofibers. The as-prepared SC maintains its capacitive performance for 84k cycles of charge-discharge indicating an unprecedented cycling stability.