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Mechanism of Efficient Proton Conduction in Diphosphoric Acid Elucidated via First-Principles Simulation and NMR

Krueger, Rachel A. and Vilčiauskas, Linas and Melchior, Jan-Patrick and Bester, Gabriel and Kreuer, Klaus-Dieter (2015) Mechanism of Efficient Proton Conduction in Diphosphoric Acid Elucidated via First-Principles Simulation and NMR. Journal of Physical Chemistry B, 119 (52). pp. 15866-15875. ISSN 1520-6106. https://resolver.caltech.edu/CaltechAUTHORS:20160104-125711332

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Abstract

Diphosphoric acid (H_4P_2O_7) is the first condensation product of phosphoric acid (H_3PO_4), the compound with the highest intrinsic proton conductivity in the liquid state. It exists at higher temperature (T > 200 °C) and lower relative humidity (RH ≈ 0.01%) and shows significant ionic conductivity under these conditions. In this work, ab initio molecular dynamics simulations of a pure H_4P_2O_7 model system and NMR spectroscopy on nominal H_4P_2O_7 (which contains significant amounts of ortho- and triphosphoric acid in thermodynamic equilibrium) were performed to reveal the nature and underlying mechanisms of the ionic conductivity. The central oxygen of the molecule is found to be excluded from any hydrogen bonding, which has two interesting consequences: (i) compared to H_3PO_4, the acidity of H_4P_2O_7 is severely increased, and (ii) the condensation reaction only leads to a minor decrease in hydrogen bond network frustration, which is thought to be one of the features enabling high proton conductivity. A topological analysis of diphosphoric acid’s hydrogen bond network shows remarkable similarities to that of phosphonic acid (H_3PO_3). The hydrogen bonding facilitates protonic polarization fluctuations (Zundel polarization) extending over several molecules (Grotthuss chains), the other important ingredient for efficient structural diffusion of protons. At T = 160 °C, this is estimated to make a conductivity contribution of about 0.1 S/cm, which accounts for half of the total ionic conductivity (σ ≈ 0.2 S/cm). The other half is suggested to result from diffusion of charged phosphate species (vehicle mechanism) that are present in high concentration, resembling conduction in ionic liquids.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/acs.jpcb.5b09684DOIArticle
http://pubs.acs.org/doi/abs/10.1021/acs.jpcb.5b09684PublisherArticle
ORCID:
AuthorORCID
Krueger, Rachel A.0000-0002-5457-2931
Additional Information:© 2015 American Chemical Society. Received: October 3, 2015. Revised: December 1, 2015. Published: December 3, 2015. R.A.K. acknowledges financial support from the Deutscher Akademischer Austausch Dienst (DAAD) Graduate Research fellowship program (Grant A/12/82840). L.V. acknowledges financial support from the DAAD Postdoctoral Research Fellowship program (Grant D/12/41214). We are grateful to R. Usiskin for proofreading the manuscript. We thank the Max Planck Computing and Data Facility for the use of computational resources. The authors declare no competing financial interest.
Funders:
Funding AgencyGrant Number
Deutscher Akademischer Austauschdienst (DAAD)A/12/82840
Deutscher Akademischer Austauschdienst (DAAD)D/12/4121
Issue or Number:52
Record Number:CaltechAUTHORS:20160104-125711332
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20160104-125711332
Official Citation:Mechanism of Efficient Proton Conduction in Diphosphoric Acid Elucidated via First-Principles Simulation and NMR Rachel A. Krueger, Linas Vilčiauskas, Jan-Patrick Melchior, Gabriel Bester, and Klaus-Dieter Kreuer The Journal of Physical Chemistry B 2015 119 (52), 15866-15875 DOI: 10.1021/acs.jpcb.5b09684
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:63330
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:04 Jan 2016 22:18
Last Modified:09 Mar 2020 13:18

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