Nanocomposites of Conducting Polymers and Polyoxometalates

Polyoxometalates (POMs) resemble clusters of metal oxides both from the structural and electronic point of view; they are formed by a small number of metal centers (typically 6-18 W or Mo) coordinated by bridging oxygen atoms, presenting well-known structures [36] and undergoing reversible multielectron reduction processes both electrochemically or photochemically similar to quantum-sized oxide particles [ 37] [ Nevertheless, their solubility derived from their molecular nature has caused them to be ignored as active compounds for electrodes or for any kind of application where collective properties were needed. POMs have been extensively studied from a chemical point of view and have been used in catalysis and photocatalysis, either as homogeneous catalyst or supported onto polymers. Some examples are known of POMs doping conducting organic polymers such as polyaniline for application in catalysis and in energy storage [15].

The similarities between POMs and oxides are not limited to their composition and topologies. Their electrochemical and photochemical behaviors are also parallel. Thus, POMs can be electrochemically or photochemically reduced to form blue species. These reduced species are chemically and spectroscopically equivalent to tungsten or molybdenum bronzes in the form of colloidal semiconducting quantum dots, with the added advantage for POMs of a well-known structure that is stable in solution [38] . One such structure is the Keggin structure, common to many heter-opolyacids. In addition to their reversible redox activity, these species present high proton conductivities in their solid (acid) form. Furthermore, they represent the ultimate limit for the dispersion of oxide species, since all metal centers can be considered to be "surface" centers, in contact with an electrolyte. This makes them good "a priori" candidates for electrode materials for electrochemical supercapacitors.

Conducting organic polymers (COPs) on the other hand have been extensively studied as promising novel materials, based on their possible use for rechargeable batteries [38-42] and electrochemical supercapacitors [43-45] . Yet, one of the frequent problems related to the application of COPs is a relatively low capacity to store charge in such devices.

The combination of conducting polymers and electroactive molecular, cluster or extended inorganic species, to form nanocomposite hybrid materials represents an opportunity for the design of novel concept materials with improved properties and enhanced energy storage capabilities, a line of work that we have recently developed in our laboratory [15, 46] . In particular, the anchoring of POMs within the network of COPs such as PAni leads to the fabrication of molecular hybrid materials in which the inorganic clusters keep their integrity and activity while benefiting from the conducting properties and polymeric nature of the hybrid structure. Some of these hybrid materials have been studied in nonaqueous solvents as lithium-inserting electrodes for the potential use in lithium batteries. However, under such conditions, the elec-troactivity of POMs could not be harnessed for too many charging-discharging cycles. Furthermore, the electroactivity of these inorganic clusters integrated in a hybrid material is heavily dependent of the electrolyte used. Thus, the use of aqueous acidic electrolytes facilitates the counterion flux and promotes the concomitant protonation of the cluster upon reduction, leading to quick and reversible redox chemistry [44] . All of these considerations, added to the fact mentioned above that POM clusters effectively constitute the ultimate degree of dispersion for an oxide phase, strongly suggest that POM species could act as ideal active materials for electrochemical supercapacitors when combined with acidic electrolytes [8, 47, 48].

Figure 7.2 shows the electrochemical supercapacitor behavior of a hybrid organic-inorganic material based on polyaniline and PMo12. The graph shows the variation in capacitance with successive charge/discharge cycles applying a current density of 400 mA/g. A stabilization period was observed during the initial cycles indicating an electroactivation process. The final capacitance value obtained was 120 F/g with good cyclability [8]. Our work constituted the first example of the use of hybrid molecular nanocomposite materials formed by polyaniline (PAni) and polyoxometalates, H4SiW12O40 (SiW12), H3PW12O40 (PW12) or H3PMo12O40 (PMo12) as electrodes in solid-state electrochemical supercapacitors.

Fig. 7.2 Electrochemical supercapacitor made of polyaniline and polyoxometalate PMo12O . The graph shows the capacitance values obtained for the first 1,000 cycles [8]

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