6 research outputs found
Using theoretical chemistry to understand the properties of Polyoxometalates and their potential as energy storage materials
Polyoxometalates (POMs) are very appealing compounds as these transition metal oxide nanoclusters exhibit the ability to store multiple electrons
in a reversible manner. Recently, POM-based energy storage devices, like redox flow batteries (RFBs) and alkali-ion batteries, have been extensively employed, since they meet equally the need of higher energy demand and low
impact on the environment. However, POM-based technologies are still at
early stage of development, mostly because of the difficulties to understand
POM electronic behaviour.
In this PhD thesis, it has been presented a theoretical study of POMs in
different environments in order to understand the basic mechanisms behind
their behaviour. In the first part, a general overview of electronic structures
of POMs is given. Furthermore, the advantages and limits of POM-based
batteries are discussed in detail. In Chapter 2 the state-of-the-art theoretical
approaches used to study POMs are discussed, in particular, strong emphasis is given to the density functional theory (DFT), and both classical and
quantum molecular dynamics (MD). In the third Chapter, the redox properties of POMs are investigated by means of simulations in an implicit and
explicit environment. The implicit solvation is semiquantitative, and uncertainties arise due to the limit of the model. Quantum MD simulations reveal
that an explicit environment can improve the calculated redox potentials of
POMs, providing useful insights into their molecular nature. A spectroscopic
study of x-ray absorption near-edge spectra (XANES) and extended x-ray absorption fine structure (EXAFS) is presented in Chapter 4. These techniques
alongside with first-principles calculations have shown to be powerful tools
to unveil the structure-property relationships of super reduced POMs. Chapter 5 is devoted to the study of self-assembly process of POMs. Classical
MD simulations show that a rich network of hydrogen bonds mediates the
POM-POM interaction, and their agglomeration strongly depends on total
charge. Furthermore, first-principles calculations illustrate the effect of POM
agglomeration on their redox potential, and catalytic efficiency towards the
hydrogen evolution reaction.
The results from this study show that it is now possible to adopt a range of
computational approaches to understand the properties of POMs in different
physical contexts. Specifically, the advantages and limits of DFT have been
highlighted when computing the redox potentials of POMs, showing that
further accuracy and insights into their electronic structure can be achieved
by explicitly including the solvent molecules. For instance, it has been shown
that the POM ability to undergo multiple redox reductions is due to possibility of delocalizing further electron density over all metallic atoms, regardless
of the POM type. This behaviour is linked to their molecular structure, which
undergoes an elongation of metal-oxygens bond lengths and formation of
metal-metal bonds when further electrons are added to POMs. Furthermore,
the inclusion of an explicit environment was shown to be an important factor
to understand other properties of POMs, like the profile of their x-ray spectra
or the self-assembly process. In the first case, QM/MM calculations shows
that the polariazation of POM electron density returns more realistic molecular structures with respect to static DFT calculations, thus influencing the
sensitivity of their simulated x-ray spectra. On the other hand, MD simulations revealed that the dynamical behaviour of POMs and the formation of
long-lived agglomerates depends on several factors, like the total charge of
POM, its counter ions, and solvent