Theoretical Efforts on the Understanding of Solid-State Systems Based on Polyoxometales Grafted on Carbon Substrates for Energy Storage Applications

Submit Manuscript | http://medcraveonline.com the heteroatom and M, the addenda atom. Particularly, POMs of Keggin-symmetry are recognized to be used in devices of energy storage such as super capacitors and Li-ion batteries, catalysis and renewable energy applications [2]. POMs present in such systems are susceptible to transfer electronic charge and to anchor at organic or inorganic substrates. This opens a field of research to design novel materials to be implemented on electrodes of solid state. In this respect, the tailoring of POMs adsorbed on carbon substrates aids in the development of improved devices. A technique to achieve this goal is the chemisorption of POMs on carbon substrates [3,4]. This technique is developed through the oxidation of the carbon substrate, and it may be extended to multi-walled carbon nanotubes (MWCNT) [5], activated carbon [6], carbon nanofibers [7] or graphene. For instance, Zhou and Han [8] found a route to obtain a composite material with a strong interaction between a PMo12 polyoxometalate and a graphene sheet. This material reported a larger surface area than that found on graphene oxides (GO) and reduced-GOs (RGO). The increase in electron redistribution reported on this system was used to achieve controllable Hydrogen adsorption-desorption properties. On the other hand, the synthesis of PMo12, PW12 and PMo11V supported on RGO and alkyl-functionalized RGO sheets have also been reported [9]. It was evidenced that this interaction changes the electronic properties of POMs, such as a linear-shift reported on the metal-oxygen vibrational frequency, which depends on the loading of the POMs. Besides, as a consequence of the POMs dispersion on the graphene sheets, they resulted less oxidized, indicating that the POMs are sensitive to the environment, and particularly to the interaction with the support. Kume et al. [10] reported the synthesis of PMo12 adsorbed on an RGO surface. This ensemble showed a better performance as a cathode in a molecular cluster battery, than that of a POM adsorbed on a single walled carbon nanotube (SWCNT) or with a POM isolated system. This addresses a peculiar increasing on the capabilities of the RGO as a capacitor, which is directly induced by the presence of the adsorbed POM. Nevertheless, such enhancement could not be explained from experimental grounds. Additionally, Li et al. [11] synthesized RGO by using an UV-irradiated photo-reduction process, where PMo12 acted as a photocatalyst. This POM was also adsorbed on the RGO surface and yielded the graphene layers as water-dispersible.