(Photo)Electrochemically Prepared Organic/Inorganic Hybrid Assemblies for Energy Conversion and Storage

Hybrid materials based on organic conjugated polymers and inorganic nanostructures have been at the leading edge of research and development. Hybridization of metal oxide semiconductors with conducting polymers is particularly interesting, since a wide range of properties can be combined with the properties of the polymer. TiO₂, ZnO, WO₃, Fe₃O₄, V₂O₅, MoO₃ etc. are often combined with different polymers (most importantly with thiophene-derivatives, polypyrrole, and polyaniline). Such hybrid assemblies are promising candidates for utilization in various energy related applications such as solar cells, (photo)-electrocatalysts, supercapacitors and Li-ion batteries.

As summarized in our recent review article, photo(electrodeposition) can be a particularly powerful tool for the in situ polymerization on a nanostructured electrode.¹ In principle, the intrinsic electroactivity of the monomers can be exploited to electropolymerize them on an inorganic semiconductor as the working electrode substrate. In this paper we present various examples of hybrid materials based on TiO₂ nanotubes and nanoporous WO₃, in combination with conjugated polymers as PEDOT, polyaniline^2,3 and polypyrrole⁴. Galvanostatic, potentiostatic and potentiodynamic growth methods will be presented. The limitations of such procedures will be emphasized and generalized solutions will be given to overcome the difficulties arising from the limited conductivity of the nanostructured working electrodes. In the second part of the talk, selected examples from our laboratories will be presented, on how there hybrid assemblies can be used in (i) solar energy harvesting, (ii) electrochemical energy storage in supercapacitors.

In the first example, we show how ternary composite materials can be applied in solar energy conversion, in solid-state DSSCs. Through the careful control over the composition and morphology of these hybrid materials we aim to make improvements in: light absorption, interfaces (charge separation), and charge transport. Such architectures contain: (i) an oxide SC with organized nanoscale structure, (ii) a QD sensitizer (chalcogenide/perovskite quantum dots) anchored on the oxide surface and (iii) an organic CP. Electrochemical anodization was employed to synthesize interconnected oxide SCs, such as TiO₂ nanotube arrays, nanoporous WO₃, or Nb₂O₅ nanoveins. These architectures have several beneficial properties, most importantly the: (i) efficient pathway for electron transfer; (ii) optimized light propagation through the architecture; (iii) substantial surface area while maintaining structural order; and (iv) optimized charge carrier collection. As the second step, the sensitizer was anchored onto the oxide surface either through hydrothermal synthesis. As the last step, the CP component was in situ generated inside the decorated oxide nanostructure, using novel electrochemical and photoelectrochemical methods.⁵ These methods result in high pore-filling ratios, and in an intimate contact between the organic and inorganic components, both at the physical and electronic level which is likely to contribute to an improved solar cell performance.

In the second example we briefly present examples for the electrosynthesis SC/CP composite materials for supercapacitors. WO₃ / polyaniline nanohybrids were synthesized with various composition and morphology. Charge storage properties of these materials were subsequently evaluated by both cyclic voltammetry and galvanostatic charge discharge measurements. Structure-property relationships were established and will be discussed. 

References:

(1)           C. Janáky, K. Rajeshwar, Prog. Pol. Sci. in press, DOI: 10.1016/j.progpolymsci.2014.10.003

(2)           Janáky, C. ; Chanmanee, W.; de Tacconi, N. R.; Rajeshwar, K. J. Phys. Chem. C  2012, 116, 4234-4242.

(3)           Janáky, C. ; Chanmanee, W.; de Tacconi, N. R.;  Rajeshwar, K. J. Phys. Chem. C  2012, 116, 19145-19155.

(4)           Janáky, C. ; Chanmanee, W.; Rajeshwar, K. Electrochimica Acta  2014, 122, 323-329.

(5)           Samu, G. F. ; Visy, C.; Subramanian, RV; Sarker, S; Rajeshwar, K.; Janáky, C. Electrochimica Acta  2015, 151, 467-476.