The synthesis of conducting polymers has been the object of increasing investigation since 1977, when the high conductivity of doped polyacetylene was discovered 1–7 and several diverse applications are being proposed, among which electrocatalysis 8–13, energy storage 14 and energy conversion 15.
Among other conducting polymers, such as poly(p-phenylene) 16, polythiophene 17,18, and polyaniline 19, polypyrrole (PPy) is considered the most promising one 13 due to the following appealing features: (i) high electrical conductivity and stability in the oxidized state 20, (ii) excellent support capability for metal oxides, functional anions/cations 21 and metal-based complexes 22,23, (iii) interesting redox properties 24 and (iv) easy chemical and electrochemical synthesis routes 25. Moreover, the monomer pyrrole, Py, is easily oxidized, water soluble and commercially available.In the last years the research activities dedicated to the synthesis of conducting polymers nanostructures have received progressively growing attention as a result of their high surface-to-volume ratio and unique catalytic, redox and electrochemical properties compared with their bulk form 26. Large surface areas with three-dimensional architectures, such as nanotubes (NTs), nanowires (NWs), nanoparticles (NPs), are preferred because the easy access of ion, gas and liquid allows high ion exchange capacity, sensing and catalytic activities.
Currently, conducting polymer nanostructures are synthesized by chemical and electrochemical methods 10,27–30 using template or template-free approaches 31–33.*Address correspondence to this author at the Dipartimento di Ingegneria dell’Innovazione, Università del Salento, via Monteroni, B.O.
73100 Lecce, Italy; E-mail: [email protected] The interest in electrochemical polymerization over the chemical routes reside in: (i) the possibility of growing electroactive conducting material directly attached to the electrode in a film shape rather than powder, as in the chemical polymerisation approach, allowing better morphological and chemical control, with the possibility of producing high-purity products, (ii) the excellent control of mass and thickness of the film thanks to a charge efficiency close to 100% (iii) the possibility of gaining fine control over the morpho-chemical properties of the PPy electrodeposits by regulating the physico-chemical conditions of the electrochemical bath (solvent 34–37, temperature 38, pH 39,40, Py concentration 41), the electrodeposition (ED) mode (potentiostatic, cyclic voltammetry, pulse plating), the applied potential or current density values 42,43, and the electrode surface conditions (chemical nature, morphology, chemical and physical treatments).However, the literature is still lacking in evaluating the performance of electrochemically polymerized PPy nanostructures as electrode material for technological energy devices. This paper reviews on the general aspects of PPy electropolymerization and PPy energy applications by paying attention to the literature approaches used for the synthesis of PPy nanostructures by electropolymerization.