Proton Conductivity in Phosphoric Acid: The Role of Quantum Effects

Heres, Maximilian; Wang, Y.; Griffin, Philip J.; Gainaru, C.; Sokolov, Alexei P.

doi:10.1103/physrevlett.117.156001

Cited by 17 publications

(18 citation statements)

References 34 publications

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“…The contribution of vehicle conductivity increases with temperature for phosphoric acid which also explains the reported higher isotope (hydrogen/deuterium) effects on conductivity at very low temperatures. 75 Here it is shown how through the choice of measuring technique different aspects of the dynamic processes can also be identified on the nanosecond timescale. In detail, structural diffusion, i.e., fast intermolecular proton transfer, was investigated using 17 O NMR relaxation measurements at different water contents and using backscattering quasielastic neutron scattering at different levels of the benzimidazole content in phosphoric acid benzimidazole mixtures.…”

Section: Discussionmentioning

confidence: 99%

On the nanosecond proton dynamics in phosphoric acid–benzimidazole and phosphoric acid–water mixtures

Melchior

Frick

2017

Phys. Chem. Chem. Phys.

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The unique proton conduction mechanism of phosphoric acid is important for the functions of complex phosphate containing biological and technological systems (e.g. phospholipid membranes and polybenzimidazole phosphoric acid membranes for high-temperature PEM fuel cells). In neat phosphoric acid structural proton diffusion, i.e. proton hopping between phosphoric acid molecules, is superimposed onto hydrodynamic diffusion of the molecules in the viscous liquid. In this study we separate the two dynamic contributions on the nanosecond timescale for the model systems phosphoric acid-water and phosphoric acid-benzimidazole. We demonstrate that H NMR dipolar relaxation measurements are controlled by hydrodynamic diffusion for the investigated conditions, whileO NMR quadrupolar relaxation measurements reflect local proton displacement as part of structural diffusion. Quasielastic neutron scattering (QENS) applying high resolution backscattering spectroscopy (nBSS) confirms structural proton diffusion measurements using PFG-NMR in phosphoric acid-benzimidazole mixtures at different concentrations. With increasing benzimidazole content proton diffusion coefficients on the nanosecond scale decrease, thus following the trend of reduced hydrogen bond network frustration. The momentum transfer (Q) dependence of the width of the QENS spectra indicates the jump diffusion mechanism and can be scaled to a master plot both for different temperatures and different benzimidazole contents. This indicates a fundamentally unchanged structural proton diffusion process, however, with a lower probability of occurrence for successful intermolecular proton transfer with increasing benzimidazole content. Results of this work enable a better separation of different diffusion processes on short timescales also in more complex phosphoric acid containing systems.

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Section: Discussionmentioning

confidence: 99%

On the nanosecond proton dynamics in phosphoric acid–benzimidazole and phosphoric acid–water mixtures

Melchior

Frick

2017

Phys. Chem. Chem. Phys.

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“…Therefore, based on the operating current (200 mA cm −2 ), the phosphoric acid amount (assuming the~15 mg cm −2 of phosphoric acid is pure H 3 PO 4 ) in the MEA, and the efficiencies of the substitution for each step, it takes 6.75 min for all protons to be substituted by deuterons statistically (for steps 1, 2, and 3, the time is 1.23 min, 1.84 min, and 3.68 min respectively). However, due to the isotope effect, the activation energy of deuteron transport is higher than that of proton transport, which has been reported in [34][35][36][37][38]. Additionally, the lower conductivity of deuteron transport compared to proton transport has been proven in previous tests (Figure 3).…”

Section: Overall H/d Exchange Process In the Meamentioning

confidence: 59%

In Operando Neutron Radiography Analysis of a High-Temperature Polymer Electrolyte Fuel Cell Based on a Phosphoric Acid-Doped Polybenzimidazole Membrane Using the Hydrogen-Deuterium Contrast Method

et al. 2018

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In order to characterize high temperature polymer electrolyte fuel cells (HT-PEFCs) in operando, neutron radiography imaging, in combination with the deuterium contrast method, was used to analyze the hydrogen distribution and proton exchange processes in operando. These measurements were then combined with the electrochemical impedance spectroscopy measurements. The cell was operated under different current densities and stoichiometries. Neutron images of the active area of the cell were captured in order to study the changeover times when the fuel supply was switched between hydrogen and deuterium, as well as to analyze the cell during steady state conditions. This work demonstrates that the changeover from proton to deuteron (and vice versa) leads to local varying media distributions in the electrolyte, independent of the overall exchange dynamics. A faster proton-to-deuteron exchange was re-discovered when switching the gas supply from H2 to D2 than that from D2 to H2. Furthermore, the D2 uptake and discharge were faster at a higher current density. Specifically, the changeover from H to D takes 5–6 min at 200 mA cm−2, 2–3 min at 400 mA cm−2 and 1–2 min at 600 mA cm−2. An effect on the transmittance changes is apparent when the stoichiometry changes.

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“…The conductivity has shown a temperature dependent conduction up to 130 °C. However, above 130 °C the conductivity is constant despite the isotropic phase in which the molecules have extra kinetic energy . The conductivity parallel to the axis of molecular columns (ionic channels) increased considerably up to10 −2 S/cm above 80 °C (Figure b).…”

Section: Methodsmentioning

confidence: 99%

“…Furthermore, a linear conductivity as a function of temperature up to 120 °C indicated a similar proton conducting process known as Grotthuss mechanism. Further insight into the proton conduction mechanism can be interpreted as a hopping process through a network of hydrogen bonds …”

Section: Methodsmentioning

confidence: 99%

One Dimensional Enhanced Anhydrous Proton Conduction in Well Defined Molecular Columns Induced by Non‐Covalent Interactions

2019

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1D anhydrous proton conduction is enhanced significantly in ionic channels created by self‐assembly of functionalized organic phosphonic acid and aromatic heterocyclic 1,2,4‐triazole molecules. This study reveals high proton conduction in one dimension through a well‐defined supramolecular architecture in which two different molecules undergo host–guest synergy and self‐assemble to provide two‐fold advantages: 1) formation of the ionic channels and 2) higher proton conduction in the absence of water. A clear correlation is found between the phenomena of ionic channels and anhydrous conductivity in the absolute dry state and we demonstrate that the one‐dimensional conductivity can be as high as that recorded for 3D channels in, for instance, Nafion.

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Proton Conductivity in Phosphoric Acid: The Role of Quantum Effects

Cited by 17 publications

References 34 publications

On the nanosecond proton dynamics in phosphoric acid–benzimidazole and phosphoric acid–water mixtures

On the nanosecond proton dynamics in phosphoric acid–benzimidazole and phosphoric acid–water mixtures

In Operando Neutron Radiography Analysis of a High-Temperature Polymer Electrolyte Fuel Cell Based on a Phosphoric Acid-Doped Polybenzimidazole Membrane Using the Hydrogen-Deuterium Contrast Method

One Dimensional Enhanced Anhydrous Proton Conduction in Well Defined Molecular Columns Induced by Non‐Covalent Interactions

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