Characterization of the dynamics in the protonic conductor CsH₂PO₄ by ¹⁷O solid-state NMR spectroscopy and first-principles calculations: correlating phosphate and protonic motion.

Gunwoo Kim,Frédéric Blanc,Sossina M Haile,Clare P Grey,John M Griffin

doi:10.1021/jacs.5b00280

Abstract

17O NMR spectroscopy combined with first-principles calculations was employed to understand the local structure and dynamics of the phosphate ions and protons in the paraelectric phase of the proton conductor CsH2PO4. For the room-temperature structure, the results confirm that one proton (H1) is localized in an asymmetric H-bond (between O1 donor and O2 acceptor oxygen atoms), whereas the H2 proton undergoes rapid exchange between two sites in a hydrogen bond with a symmetric double potential well at a rate ≥107 Hz. Variable-temperature 17O NMR spectra recorded from 22 to 214 °C were interpreted by considering different models for the rotation of the phosphate anions. At least two distinct rate constants for rotations about four pseudo C3 axes of the phosphate ion were required in order to achieve good agreement with the experimental data. An activation energy of 0.21 ± 0.06 eV was observed for rotation about the P–O1 axis, with a higher activation energy of 0.50 ± 0.07 eV being obtained for rotation about the P–O2, P–O3d, and P–O3a axes, with the superscripts denoting, respectively, dynamic donor and acceptor oxygen atoms of the H-bond. The higher activation energy of the second process is most likely associated with the cost of breaking an O1–H1 bond. The activation energy of this process is slightly lower than that obtained from the 1H exchange process (0.70 ± 0.07 eV) (Kim, G.; Blanc, F.; Hu, Y.-Y.; Grey, C. P. J. Phys. Chem. C2013, 117, 6504−6515) associated with the translational motion of the protons. The relationship between proton jumps and phosphate rotation was analyzed in detail by considering uncorrelated motion, motion of individual PO4 ions and the four connected/H-bonded protons, and concerted motions of adjacent phosphate units, mediated by proton hops. We conclude that, while phosphate rotations aid proton motion, not all phosphate rotations result in proton jumps.

Highlights

Solid inorganic acids are a promising class of proton-conducting solid electrolytes for use in fuel cells that operate in an intermediate-temperature range (200−600 °C).[1−4] In particular, CsH2PO4, has been intensively studied because of its desirable operating temperature (230−260 °C5) and high protonic conductivity (σ = 2.2 × 10−2 S cm−1 at 240 °C6) in the so-called superprotonic conducting phase.[7]
17O NMR spectroscopy combined with first-principles calculations has been applied to provide a detailed understanding of the local structure and dynamics of the phosphate ions and protons in CsH2PO4
A good match between the experimental and simulated 17O line shapes was achieved by the use of a dynamic model involving rapid exchange between the two H2 sites in the symmetrical double well

Summary

Introduction

Solid inorganic acids are a promising class of proton-conducting solid electrolytes for use in fuel cells that operate in an intermediate-temperature range (200−600 °C).[1−4] In particular, CsH2PO4, has been intensively studied because of its desirable operating temperature (230−260 °C5) and high protonic conductivity (σ = 2.2 × 10−2 S cm−1 at 240 °C6) in the so-called superprotonic conducting phase.[7] One of the key questions remaining for CsH2PO4 is the mechanism of proton conduction in the superprotonic phase, proton conductivity arising from structural and dynamic disorder of hydrogen bonds and phosphate anions.[6] Understanding this is key to engineering novel, improved electrolytic materials for these applications In this context, mixed compositions of CsH2PO4 with other oxyanions,[8,9] other cations,[10] inorganic/organic scaffolds,[11−13] and other known proton conductors[14] have been extensively investigated in the search to improve desired physical properties, such as higher protonic conductivities, better mechanical properties, and improved thermal stability at high temperatures. In a recent Car−Parrinello ab initio molecular dynamics study of the superprotonic phase,[21] it was suggested that proton hopping is faster than the phosphate

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