Elsevier

Journal of Power Sources

Volume 161, Issue 1, 20 October 2006, Pages 580-586
Journal of Power Sources

Triple hybrid materials: A novel concept within the field of organic–inorganic hybrids

https://doi.org/10.1016/j.jpowsour.2006.04.138Get rights and content

Abstract

We explored a new approach within the field of hybrid materials, namely, an integration of an electroactive inorganic molecule into a conducting polymer that in turn is intercalated into an extended inorganic oxide. In particular we present the specific material formed by hexacyanoferrate-doped polypyrrole or polyaniline inserted in turn into layered V2O5. This novel kind of hybrid with three components interacting at a molecular level is what we have called, triple hybrid materials (THM). The synthetic approach was based on our earlier work on PAni/V2O5, PAni/HCF and PPy/HCF systems. The materials obtained were characterized by FTIR, XRD, TGA, elemental analyses, and ICP. The electrochemical properties of THMs as insertion cathodes in rechargeable Li cells were also explored. The initial specific charge was high for PPy/HCF/V2O5 system (160 Ah kg−1), giving a greater value than for their corresponding simple hybrids: PPy/HCF (69 Ah kg−1) and PPy/V2O5 (120 Ah kg−1). Repeated charge–discharge cycles showed a poor cyclability, which could be related to the voltage limit values during recharge, overoxidation of the polymer, or to the detrimental effect of structural water from THMs. Nevertheless, the present work showed a novel route towards a more complex and versatile electroactive hybrid design.

Introduction

Nowadays, hybrid materials based on organic and inorganic components represent a wide field of innovation within material science [1a]. Research on the topic of hybrid materials involves challenges and opportunities. The most important challenge is managing to synthesize hybrid combinations that keep or enhance the best properties of each of the components while eliminating or reducing their particular limitations. This challenge opens the opportunity to develop new materials with sinergistic behavior. Also, the hybrid approach is especially useful for combining organic and inorganic species with complementary properties and reactivities. This behavior can lead to improved performances or to the finding of new and useful properties [1].

The work in our group has been centered on a particularly fruitful and wide group of functional hybrid materials based on conducting organic polymers (COPs) as components. This category alone accounts for a large number of materials and applications [1b]. Their functionality, based on the electroactivity of their components, allows them to be used as electrodes for energy storage devices. These types of hybrid material have been classified into three groups depending on the type of matrix and guest phase: “OI” (organic–inorganic, molecular hybrids), “IO” (inorganic–organic, intercalation compounds), and “nanocomposite” materials [1b].

In this context we considered the possibility of triple hybrids with “nested” species inorganic (molecular)–organic (polymer)–inorganic (extended oxide) in which all the components would be electroactive, thus, aiming at higher energy storage capacities over wider potential ranges. In this paper we present the development of this idea in the form of a hexacyanoferrate (HCF)–COP–V2O5 combination. This triple hybrid material was constructed through the doping of a conducting polymer with [Fe(CN)6]3− hexacyanoferrate (HCF), followed in some cases by the intercalation of the resulting O–I hybrid in between the layers of solid V2O5. Leading to what we call a I–O–I hybrid. Fig. 1 shows a schematic view of the desired structure for this triple hybrid.

It should be noted that this idea was also conceived and followed by other groups in quite different ways. Thus, simultaneously with our work, described below, the synthesis of similar triple hybrids was attempted through different procedures. Da Silva et al. [2] used a surfactant that was inserted between the layers of V2O5, then, according to the authors, HCF was immobilized on the surface of the hybrid instead of being intercalated. This work was essentially synthetic and only cyclic voltammetry was reported. On the other hand, the work carried out by Pokhodenko et al. [3] a double doping of polyaniline with phosphomolybdic acid, and m-cresol was reported. In contrast, our triple hybrid approach aimed from the beginning at the integration of all three electroactive species into a single bulk material in order to be used as Li insertion electrodes.

Earlier work carried out in our group concerning the PAni/HCF (OI) hybrid system showed that a stable good specific charge value was obtained for this molecular hybrid when used as a cathode material in lithium rechargeable cells (137 Ah kg−1 at C/15, 110 Ah kg−1 at C/5 charge–discharge rates) [4]. In contrast, the PPy/HCF hybrid system was also studied, but resulted only in moderated specific charge values, due to the formation of oxygenated groups (carbonyl, hydroxyl) on the polypyrrole chain [5], [6]. With the triple hybrid approach we are trying to improve upon the performance of those binary hybrids (PAni/HCF, PPy/HCF).

We will describe here our syntheses of novel triple hybrid materials with polyaniline (PAni) or polypyrrole (PPy), doped with HCF and presumably its intercalation between the layers of V2O5. These syntheses were generally carried out in one-pot reactions and the resulting materials characterized by FTIR, XRD, TGA, elemental analyses, ICP, and finally tested in compact rechargeable lithium cells.

Section snippets

Reagents

Pyrrole and aniline were purchased from Aldrich, distilled under reduced pressure, and kept under nitrogen at 4 °C. HClO4 (70%) from Panreac and potassium ferricyanide K3Fe(CN)6·3H2O 99% from Aldrich were used according to product specifications. For the synthesis of vanadium pentoxide gel we used a Dowex 50wx2-100 resin and sodium metavanadate 90% from Aldrich without further purification.

For the assembly of rechargeable lithium cells, metallic lithium ribbon (99.9%, 0.38 mm thick) from Aldrich

Synthesis

The synthesis of PPy/HCF/V2O5 and PAni/HCF/V2O5 triple hybrids were carried out by means of an oxidative polymerization of the monomer (aniline or pyrrole), in the presence of H3Fe(CN)6 and V2O5 gel solutions. The two inorganic reagents oxidize aniline or pyrrole to get incorporated to the hybrid compensating at the same time the positive charge of the conducting polymer. Table 1 shows the molar ratio used for the synthesis of each polypyrrole (PPy) and polyaniline (PAni) triple hybrid

Conclusions

We carried out the syntheses of two series of PPy/HCF/V2O5 and PAni/HCF/V2O5 triple hybrids, confirming by FTIR and chemical analyses, the presence of all three components in the resulting materials. XRD patterns presented a shoulder in some samples at 2θ = 4–5°, which suggested a wider expansion between the layers of V2O5 (of an otherwise very poorly crystalline material) adequate to accommodate both HCF and the conducting polymers between the V2O5 layers. All these analyses indicated the

Acknowledgements

Partial financial support from the Ministry of Science and Technology (Spain) (Grant MAT2002-04529-C03) and from CONACYT (México) (fellowship to A.K. Cuentas-Gallegos) are gratefully acknowledged.

References (12)

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