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The mechanism of proton conduction in phosphoric acid

Nature Chemistry volume 4, pages 461–466 (2012)Cite this article

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Abstract

Neat liquid phosphoric acid (H3PO4) has the highest intrinsic proton conductivity of any known substance and is a useful model for understanding proton transport in other phosphate-based systems in biology and clean energy technologies. Here, we present an ab initio molecular dynamics study that reveals, for the first time, the microscopic mechanism of this high proton conductivity. Anomalously fast proton transport in hydrogen-bonded systems involves a structural diffusion mechanism in which intramolecular proton transfer is driven by specific hydrogen bond rearrangements in the surrounding environment. Aqueous media transport excess charge defects through local hydrogen bond rearrangements that drive individual proton transfer reactions. In contrast, strong, polarizable hydrogen bonds in phosphoric acid produce coupled proton motion and a pronounced protic dielectric response of the medium, leading to the formation of extended, polarized hydrogen-bonded chains. The interplay between these chains and a frustrated hydrogen-bond network gives rise to the high proton conductivity.

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Figure 1: Characterization of the proton–proton coupling in terms of the interproton force and radial distribution function.
Figure 2: Oxygen–oxygen radial distribution functions (gOO) and integrated coordination numbers (nOO) with respect to H4PO4+, H3PO4 and H2PO4− species.
Figure 3: Proton coupling correlation function Cpc(n) as a function of connectivity n.
Figure 4: Snapshots of the elementary steps of the proton conduction mechanism in H3PO4.

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Acknowledgements

L.V. acknowledges partial financial support from the EU FP6 Integrated Project ‘Autobrane’ and Stiftung Energieforschung Baden-Württemberg (FKZ A 23305). M.E.T. acknowledges support from the National Science Foundation (award no. CHE-1012545). The authors thank B. Frick (Institut Laue-Langevin, Grenoble) for valuable discussions and the Rechenzentrum Garching of the Max Planck Society for the use of computational resources.

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Authors and Affiliations

  1. Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, Stuttgart, D-70569, Germany

    Linas Vilčiauskas, Gabriel Bester & Klaus-Dieter Kreuer

  2. Department of Chemistry and Courant Institute of Mathematical Sciences, New York University, New York, 10003, New York, USA

    Mark E. Tuckerman

  3. Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, 37996, Tennessee, USA

    Stephen J. Paddison

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Contributions

L.V., M.E.T., G.B., S.J.P. and K.D.K. designed the research. L.V., M.E.T. and G.B. performed AIMD simulations. L.V., M.E.T., G.B., S.J.P. and K.D.K carried out analysis. All authors discussed the results and contributed to preparation of the manuscript.

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Correspondence to Mark E. Tuckerman or Klaus-Dieter Kreuer.

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Vilčiauskas, L., Tuckerman, M., Bester, G. et al. The mechanism of proton conduction in phosphoric acid. Nature Chem 4, 461–466 (2012). https://doi.org/10.1038/nchem.1329

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