Browsing by Author "Vilciauskas, Linas"

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Proton transport mechanisms of phosphoric acid and related phosphorus oxoacid systems : a first principles molecular dynamics study
(2012) Vilciauskas, Linas; Maier, Joachim (Prof. Dr.)
Fundamental understanding of proton transport in hydrogen bonded systems on the molecular level remains a key problem in many areas of science ranging from electrochemical energy conversion to biological systems. Despite the enormous advances in the research of these processes, the ostensibly simplest case, proton transport in homogeneous bulk media at thermodynamic equilibrium, proved to be one of the most challenging and elusive. It is only through enormous theoretical and experimental efforts that clear mechanistic pictures of the transport of excess protonic charge defects in water have emerged. However, water has negligible intrinsic proton conductivity. By contrast, the class of compounds known as phosphorus oxoacids have some of the highest reported proton conductivities. In this work, the molecular level proton transport mechanisms in this family of proton conductors (H3PO4, H3PO3 and H3PO2) and some closely related systems (H3PO4-H2O mixtures) are investigated with the help of ab initio molecular dynamics simulations. In fact, neat liquid phosphoric acid has the highest intrinsic proton conductivity of any known substance. Apart from playing a central role in the structure and function of biological systems, systems containing phosphates/phosphonates are attracting an increasing interest as high-temperature electrolytes for emerging fuel cell applications. The results show that strong, mutually polarizable hydrogen bonds give rise to coupled proton motion and a pronounced protic dielectric response of the medium. This allows for the formation of extended, polarized hydrogen bonded (Grotthuss) chains, never truly observed in bulk hydrogen bonded systems. The results show that, in phosphoric acid such chains containing up to five consecutive hydrogen bonds can form. It is the interplay between these chains and a frustrated (there are more proton donor than acceptor sites) hydrogen bond network, which is found to lead to extremely high proton conductivity in phosphoric acid. This strongly contrasts to water, wherein the anomalously high rate of excess charge transport occurs not through extended chains but rather through local hydrogen bond rearrangements that drive individual proton transfer reactions. The mechanism proposed in this work, suggests that strong hydrogen bonding does not necessarily lead to protonic ordering and slow dynamics of the system, demonstrating that weak solvent coupling and sufficient degree of configurational disorder can lead to fast proton transport. Although, phosphonic and phosphinic acids possess even stronger hydrogen bonds, the stronger dipolar and dynamic backgrounds tend to oppose the formation of extended Grotthuss chains. Moreover, these systems do not have the same intrinsically frustrated hydrogen bond network (there are more proton acceptor than donor sites), thus hindering the solvent reorganization (depolarization). Nevertheless, the results show that the weak hydrogen bonded configurations, although not an intrinsic property of the hydrogen bond network, are still forming in a dynamical sense due to liquid disorder. The latter, together with the formation of polarized chains explain the high charge carrier concentrations and conductivities reported in these materials, especially in H3PO3, where they are only slightly lower than in the case of H3PO4. Apparently, proton transport in phosphoric acid is extremely susceptible to nearly all types of chemical perturbations. Apart from the severe conductivity reduction caused by the addition of bases, even the addition of acids leads to some decrease in conductivity. The only dopant that increases the conductivity of H3PO4 is water which, together with some condensation products is already present even in a nominally dry acid under the conditions of thermodynamic equilibrium. In fact, the severe increase in the conductivity of phosphoric acid upon dilution cannot be explained by simple hydrodynamic diffusion of hydronium ion, indicating that proton structural diffusion plays a major role in these systems as well. The results show that very similar molecular mechanisms are at play in phosphoric acid - water system as in neat oxoacid systems. The properties of hydrogen bonds even in 1:1 H3PO4 - H2O mixture are virtually identical to those of pure H3PO4, generally, showing no resemblance to liquid H2O. It is due to the strong and polarizable acid-water hydrogen bonds, that some degree of cooperativity can still be observed in the proton transport mechanism, although the solvent coupling in this case is much stronger due to the significantly different dielectric nature of the water phase. In addition to some vehicular contribution to proton conductivity, water also has some plasticizing effect, increasing the configurational disorder in the hydrogen bond network, therefore resulting in significantly higher conductivities observed in these systems.