Issue 1, 2017

Proton conduction mechanisms in the phosphoric acid–water system (H4P2O7–H3PO4·2H2O): a 1H, 31P and 17O PFG-NMR and conductivity study

Abstract

Ionic charge carrier formation and mobility, including the underlying conduction mechanisms, are investigated for phosphoric acid at water contents relevant for the acid's application as electrolyte in fuel cells. The high conductivity contribution from structural diffusion involving intermolecular proton transfer (∼97%) in neat phosphoric acid (H3PO4) passes through a maximum at this composition. Hydrogen bond network frustration (imbalance of the number of proton donors and acceptors), which is closely related to the appearance of structural diffusion, decreases with both elimination and addition of water. Structural diffusion is virtually disappearing for H3PO4·2H2O, yet, the overall conductivity increases with increasing water content and reaches a maximum at a composition of H3PO4·5H2O. The conductivity increase is a consequence of the progressive de-coupling of the diffusion of aqueous species from that of phosphate species and the strongly enhanced acidity of phosphoric acid at low water contents. High concentrations of protonated aqueous species with high diffusivity then lead to high conductivity contributions from vehicular transport. The increased water transport associated with the change in transport mechanism is suggested to have severe implications for fuel cell applications. At low water contents the conductivity contribution of structural diffusion is also reduced, but it is accompanied by conductivity contributions from a high concentration of multiply charged condensation products (e.g. H2P2O72−, H3P3O102− and H2P3O103−). The results underline the singularity of structure diffusion in neat phosphoric acid (H3PO4) and its sensitivity against any perturbation.

Graphical abstract: Proton conduction mechanisms in the phosphoric acid–water system (H4P2O7–H3PO4·2H2O): a 1H, 31P and 17O PFG-NMR and conductivity study

Supplementary files

Article information

Article type
Paper
Submitted
12 Jul 2016
Accepted
12 Nov 2016
First published
05 Dec 2016

Phys. Chem. Chem. Phys., 2017,19, 587-600

Proton conduction mechanisms in the phosphoric acid–water system (H4P2O7–H3PO4·2H2O): a 1H, 31P and 17O PFG-NMR and conductivity study

J. Melchior, K. Kreuer and J. Maier, Phys. Chem. Chem. Phys., 2017, 19, 587 DOI: 10.1039/C6CP04855B

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements