Bond formation, electronic structure, and energy storage properties on polyoxometalate–carbon nanocomposites

• Published in: Theoretical chemistry accounts 2016-04, Vol.135 (4), Article 92
• Main Authors: Muñiz, Jesús, Cuentas-Gallegos, Ana Karina, Robles, Miguel, Valdéz, Maximiliano
• Format: Article
• Language: English
• Subjects: Atomic/Molecular Structure and Spectra; Chemistry; Chemistry and Materials Science; Inorganic Chemistry; Organic Chemistry; Physical Chemistry; Regular Article; Theoretical and Computational Chemistry
• ISSN: 1432-881X; 1432-2234
• DOI: 10.1007/s00214-016-1855-3

Keggin polyoxometalate structures are molecular clusters that, anchored to carbon matrices, have been used to form electrodes for energy storage devices, such as lithium batteries and supercapacitors. [ PMo 12 O 40 ] 3 - polyanions (PMo12) are examples of this kind of nanostructures that, grafted on amorphous carbon, have the capability to enhance the capacitive properties of these electrochemical ensembles. However, there is yet a poor understanding of the fundamental mechanisms for bond formation between them and carbon structures. It has been found experimentally that the presence of functional groups such as φ -NH 2 and φ -OH assists on the chemical absorption of PMo12, but there is not enough information on the actual mechanism of the process. In order to gather further knowledge on these issues, we have performed quantum mechanical calculations, based on the density functional theory of atomic arrangements using graphene as carbon structure model, different functional groups, and PMo12. Our aim was to look for the nature of bonding among them, and to dig into the charge properties to relate them with the experimental observation. From the computations performed with PMo12 polyanion near to a graphene sheet, with and without the presence of functional groups, we conclude that there is a non-covalent/electrostatic bonding, made of weak π – π stacking interactions between PMo12 and graphene. Calculations show that φ -NH 2 and φ -OH functional groups are able to form covalent bonds with PMo12 in top and side fashion arrangements, being the latter the most stable. This is a powerful argument to explain the empirical observation on these groups, enhancing the PMo12 adsorption over carbon structures. We also found that the presence of the functional groups together with PMo12 creates electronic states that may act as alternative pathways that ions can track within electrochemical cells. Our results offer first-principle information relevant to the understanding of these composite materials, and the methodology could be directly applied to other Keggin structures or different functional groups, attached to graphene, to find potential advantages for energy storage devices.