template carbon nanoparticles, prepared by pyrolysis of polyfurfuryl alcohol inside the nanopores of Vycor glass, showed strongly enhanced electrochemical redox reactions, and enhanced chemical and physical stabilities [67].

Probably the main objective in creating new carbon– conducting polymer nanocomposites is their potential use in electrochemical energy storage systems like supercapacitors. When oxidized with strong oxidants, carbon electrodes take on a pseudocapacitance property due to electrochemical redox reactions, taking place at the expense of various surface functional groups [68]. The relative pseudocapacitance grows with increasing relative electrode surface area, attaining its highest values for nanosized carbon structures. Electroactive and conducting polymers, when deposited at carbon electrodes, enhance the electrode capacity due to fast Faradaic pseudocapacitance effects. For supercapacitor application, polypyrrole has been deposited by electropolymerization as a thin homogeneous film 5 nm in thickness on multiwalled carbon nanotubes, and the resulting composite material

low frequency specific capacitance per mass and geometric area, 192 F g−1 and 1 F cm−2, respectively, were reported for electrochemically grown composite films of multiwalled carbon nanotubes and polypyrrole [70]. As an alternative, chemical polymerization of pyrrole at multiwalled carbon nanotubes has been achieved, yielding a composite material with much higher specific capacitance than pure polypyrrole and as-grown carbon nanotube electrodes [71]. In addition to multiwalled carbon nanotubes, aligned arrays of carbon nanotubes have been coated with polypyrrole, and the electrochemical capacitance of the resulting nanocomposite material has been studied [72]. Polypyrrole-coated carbon nanotube composites have been reported to possess higher specific capacitances, reaching 170 F g−1, than oxygenated surface functionalized nanotubes (130 F g−1), making them potentially useful for application in electrochemical energy storage systems [73].

In addition to energy storage, a few other possible applications for nanocomposite carbon–conducting polymer materials have been analysed. By a combination of electrochemical polymerization of pyrrole and electrophoretic deposition of carbon nanotubes, a new composite material has been prepared and tested for triode-type field emission array application [74]. An ultramicroscale pH-sensitive electrode has been reported, prepared by electropolymerization of pyrrole on an ion-beam-etched carbon fibre with a tip diameter of 100–500 nm. The electrochemical potential of this sensor showed a linear dependence on the solution pH of −60 mV per pH unit within a relatively broad pH range of 2.0–12.5 [75]. A glucose biosensor consisting of glucose oxidase enzyme, electrochemically entrapped in an ultrathin polypyrrole layer deposited on aligned carbon nanotubes, has been reported to operate at a low anodic potential with a high sensitivity and selectivity [76].

5. Nanocomposites with inorganic materials

For this group of composite materials, most of the published works deal with metal oxide-based composites. Among

them, vanadium pentoxide (V2O5) is a very popular material because of its intrinsic electrochemical redox activity due to electrochemically induceable change in the oxidation degree of vanadium (V) species. Also, nanocomposites with TiO2, Fe2O3, and some other oxides have been studied, although to a lesser extent. A number of works deal also with clay-based nanocomposites.

5.1. Composites with vanadium oxide

Vanadium (V) oxide xerogel is capable of intercalating metal ions or organic molecules, forming nanostructurized composite materials with a wide spectrum of possible applications, especially electrochemical ones [77]. Among other species, aniline or pyrrole monomers can be intercalated into a nanostructurized V2O5 xerogel, where the polymerization of these monomers take place either at the expense of an intrinsic oxidation by the V(V) species present, or by molecular oxygen. V2O5 xerogel can be prepared following several known techniques. For example, PANIcontaining V2O5 xerogel has been prepared by dissolving V2O5 in a 10% solution of hydrogen peroxide, turning it into a wet gel by ageing for seven days, drying under vacuum at elevated temperature, intercalating aniline in a mixed aqueous– organic solution, and calcination of this precursor in an oxygen atmosphere at 460 ◦C for several hours [78]. As a result, a nanocomposite material, suitable for application as a cathode material for rechargeable lithium ions, could be obtained [78].

Intercalation of polyaniline into V2 O5 results in significant changes of the host matrix structure. It has been shown by means of different techniques that, due to its nanostructure, PANI–V2O5 composite shows specific properties associated with intermolecular interaction and charge transfer between intercalated polyaniline and V5+ ions of the host matrix [79]. An x-ray diffraction study showed the expansion of the V2O5 interlayer distance upon intercalation of polyaniline, by 0.48 nm [80].

Most of the works dealing with V2O5 nanocomposites with polyaniline or polypyrrole pursue rechargeable battery applications; thus, electrochemical properties of these materials are of primary interest. In a nonaqueous electrolyte, V2O5- based intercalation materials with polyaniline and poly(2,5- dimercapto-1,3,4-thiadiazole) exhibit an initial electric discharge capacity of 190 mA h g−1, that can be increased up to 220 mA h g−1 upon oxygen treatment of the material [80]. It has been shown by an electrochemical impedance technique that in nonaqueous Li+-ion-containing solutions the charge compensation during the redox cycling of a V2O5–polyaniline composite proceeds mainly by Li+ electromigration, associated with an enhanced diffusion coefficient of Li+, by one order of magnitude, compared to V2O5, and excellent stability of the composite material under repeated charge/discharge cycling has been noted [81]. Hydrothermal treatment of V2O5– polyaniline nanocomposite and hexadecylamine resulted in the formation of nanocomposite nanofibres, 1–10 µm in length and 15–400 nm in width, showing a specific capacity of 150 A h kg−1 during 10 initial cycles and presenting a material showing promise for application as a cathode material for Li+ ion batteries [82]. An enhancement of the charge storage capability for V2O5–polyaniline nanocomposites up

to 2.25 mC cm−2 compared to 1.86 mC cm−2 for the sum of the isolated contributions from polyaniline and V2O5 has been achieved by electrostatic layer-by-layer assembly of this composite with a thickness of 2.5 nm per bilayer [83]. Platinum nanoparticles 4 nm in size have been included into a V2O5– polyaniline nanocomposite, and an electrocatalytic effect of electrochemical oxidation of methanol has been demonstrated for this composite material [84].

In addition to the parent polyaniline, its sulfonated derivatives (‘self-doped polyanilines’) have been employed in the preparation of nanocomposites [85]. These sulfonated derivatives contain a sulfonate anion bound to the polymer backbone, and thus do not require an external anion doping in the course of electrochemical redox processes [86]. It has been shown with the use of an electrochemical quartz crystal microbalance (EQCM) that the charge compensation during charge/discharge cycling for V2O5 nanocomposite with poly(N-sulfopropylaniline) proceeds predominantly by Li+ ion transport [87, 88]. The cyclability of this composite material is higher than that of V2O5 due to the suppression of the swelling process [88]. Flexible nanocomposite films of V2O5 and poly(aniline-co-N-(4-sulfophenyl)aniline) were prepared, having a better electrochemical stability relative to the copolymer itself, and better mechanical properties than V2O5 xerogel [89].

In addition to polyaniline and its derivatives, polypyrrole has also been employed in the synthesis of nanocomposites. It has been shown by x-ray diffractometry that polypyrrole chains are intercalated within the interlayer region of V2O5, leading to an increase of the d-spacing of the host material from 1.185 to 1.38 nm [90]. The nanocomposite shows

containing alkyl chains, namely poly(3-decylpyrrole) and poly(3-hexadecylpyrrole), have been intercalated into a V2O5 host matrix, and the resulting nanocomposite material has been reported to be electrochemically more stable than V2O5– polypyrrole composite, and to have a higher specific capacity, of 106–115 A h kg−1, than V2O5–polypyrrole, 50 A h kg−1, after 50 electrochemical charging cycles [91].

Similarly, the oxidative intercalation of 2,5-dimercapto- 1,3,4-thiadiazole into the layered structure of V2O5 xerogel leads to spontaneous polymerization of this monomer with a characteristic expansion of the layer spacing from 1.155 to 1.35 nm [92]. It has been shown that V2O5 facilitates the redox reactions of the thiol–disulfide redox couple of the intercalated material [92]. An enhancement of the interlayer spacing has been found also for poly(3,4-ethylene dioxythiophene), intercalated into V2O5 [93]. An increase of the discharge capacity to 240 mA h g−1 (from 140 mA h g−1 for V2O5) has been found for this nanocomposite material [93].

5.2. Composites with other metal oxides and related materials

Titanium dioxide nanoparticles with a grain size of 25– 250 nm have been chemically covered with a thin (1– 2 nm) layer of polythiophene, and a resulting nanocomposite has been pressed and used as an electrode, showing the properties of a p/n junction [94]. In photocurrent spectra,

these electrodes show a characteristic anodic peak of TiO2

new feature of the p/n interface [94]. In the search for solar energy conversion applications, nanocomposite films of p-aminothiophenol/TiO2/polyaniline have been prepared electrochemically at the gold electrode surface, and it was shown that the wavelength region of the photocurrent generated covers violet and red light regions [95, 96].

For possible photovoltaic applications, nanocomposite material consisting of Fe2O3 nanoparticles 4–40 nm in size have been introduced into a polypyrrole layer deposited at a transparent ITO electrode, and new photovoltaic behaviour has been observed [97]. Maghemite (γ -Fe2O3) nanoparticles have been covered with a polypyrrole layer by chemical polymerization, and used as a cathode material for Li+ ion batteries, showing an enhanced cyclability [98]. Attempts have been made to use oxide–conducting polymer composites for corrosion protection. Fe3O4 particles have been embedded into polypyrrole film during its preparation in order to maintain the polymer in its oxidized state corresponding to the iron passive domain, and a significant improvement of corrosion protection efficiency of composite films has been reported [99].

A nanoparticulated spinel-type mixed valence oxide of copper and manganese Cu1.4Mn1.6O4 has been employed in a composition with polypyrrole as an electrode material for electrocatalytic oxygen reduction [100]. Similarly, nanocomposite material consisting of polypyrrole and mixed valence oxide Nix Co3−x O4 (where x = 0.3 or 1) nanoparticles has been prepared by electropolymerization, and good electrocatalytic properties of this material as regards oxygen reduction have been found [101]. Nanosized polyoxometallate clusters have been combined with a conducting polymer matrix, leading to composite material showing the combined activity of both its components, useful for solid state electrochemical supercapacitor applications [102]. Functionalized pyrroles, bearing complexing groups, have been electropolymerized at zirconium oxopolymer sol–gel coatings on ITO electrodes, and the electrochemical properties of the resulting modified electrodes have been studied [103, 104]. Polyaniline composite with sol–gel derived silica has been prepared, and improved electrochemical characteristics of polyaniline in this composite as regards oxidation and reduction charges and charge–discharge capacity have been reported [105]. Nanoclusters consisting mainly of Cu2O of mean diameter 160 nm have been incorporated into a polypyrrole matrix following electrochemical deposition of a polypyrrole layer, and subsequent pulsed potentiostatic deposition of copper species from CuCl2 solution [106].

in the preparation of nanocomposites with conducting polymers. Electrochemically layered and layer-by-layer chemically assembled nanostructures of either polypyrrole or poly(3-methylthiophene) and trioctylphosphine oxide capped n-type CdSe nanoparticles 20–40 nm in diameter have been prepared and characterized at conducting substrates, possessing rectifying behaviour in the forward and a Zener breakdown in the reverse direction [107]. Polyaniline composite films containing CdS nanoparticles 3 nm in diameter have been prepared and characterized at a fluorine-doped

tin-oxide-coated glass electrode by an electropolymerization procedure [108]. Strongly luminescent CdTe semiconductor nanocrystals have been incorporated into an electrochemically grown polypyrrole layer, and a significant increase of their electroluminescence quantum efficiency was found [109]. With the use of a layer-by-layer self-assembly technique, nanostructured multilayer films of ring-sulfonated polyaniline and CdS nanoparticles were prepared, showing photocathodic or photoanodic currents depending on the electrode potential applied [110]. Polypyrrole nanocomposite films consisting of multiple layers of CdSe and CdTe have been prepared by electrochemical deposition, and significantly improved photoluminescence performance of a nanocomposite film has been found [111].

Since a pioneering work of Ghosh and Bard [112], clay-modified electrodes have been attracting a great deal of interest. Clays are able to intercalate diverse chemical species. When deposited at an electrode surface, these claybound species show characteristic electrochemical properties that can be altered by means of some experimental variables [113]. Electrochemical study of polypyrrole nanocomposite with montmorillonite indicated a significant shift of redox processes on the electrode potential scale towards negative values, as compared to native polypyrrole [114]. The electrochemical behaviour of some common redox couples like ferri/ferrocyanide and benzoquinone/hydroquinone has been studied at electrodes, modified with clay–polyaniline nanocomposite [115]. Potential applications as electrode