First principles molecular dynamics study of proton dynamics and transport in phosphoric acid/imidazole (2:1) system

Vilčiauskas, Linas; Tuckerman, Mark E.; Melchior, Jan-Patrick; Bester, Gabriel; Kreuer, Klaus‐Dieter

doi:10.1016/j.ssi.2013.07.003

Cited by 44 publications

(49 citation statements)

References 56 publications

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“…Phenomenologically, phosphoric acid shows the highest intrinsic proton conductivity, with a high contribution of up to 97% by structural proton diffusion to the acid's overall conductivity. 2,5 Remarkably, this conductivity does not increase with the addition of any base or acid 6,[21][22][23][24] as observed for water and heterocycles. In contrast, except for the addition of water, 5,[25][26][27][28] the addition of bases, and to a lesser extent acids, reduces the proton conductivity of phosphoric acid.…”

Section: Introductionmentioning

confidence: 69%

“…27,29 Both proton transfer and hydrogen bond formation reactions are a consequence of the acid's frustrated hydrogen bond network (there is a severe imbalance of potential proton donors and acceptors) and the strength of its highly polarizable hydrogen bonds. 4 The recently proposed mechanism explains in a natural way phosphoric acid's high conductivity, its reduction through additives, 5,6,23 and the striking observation that decreasing viscosity goes along with decreasing proton conductivity for the series of phosphorous oxoacids: phosphoric acid (H 3 PO 4 )-phosphonic acid (H 3 PO 3 )-phosphinic acid (H 3 PO 2 ). 30 This observation is at odds with the so-called Walden rule, 31 relating viscosity and equivalent conductivity.…”

Section: Introductionmentioning

confidence: 99%

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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: Introductionmentioning

confidence: 69%

Section: Introductionmentioning

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 existence of tightly bound, yet highly mobile protons offers experimental support to the Grotthuss-type mechanism for cooperative proton transport in Nafion membranes. [36] However, it is unclear whether this mechanism operates exclusively, or whether there is also a fraction of proton transport facilitated by water molecules (vehicle mechanism).…”

Section: Methodsmentioning

confidence: 99%

Nanometer‐Scale Water‐ and Proton‐Diffusion Heterogeneities across Water Channels in Polymer Electrolyte Membranes

Song

Han

2015

Angew Chem Int Ed

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“…The existence of tightly bound, yet highly mobile protons offer experimental support to the Grotthuss type mechanism for cooperative proton transport in Nafion membranes. [36] However, it is unclear whether this is the exclusive mechanism, or whether there is also a fraction of proton transport facilitated by water molecules (Vehicle mechanism).We conclude that distinctly heterogeneous water and proton diffusivity clearly correlates with spatial and chemical heterogeneities of the water channels in Nafion membranes, such as regions comprising near the sulfonic acid groups, the fluorocarbon surface and the water channel core, as pictorially summarized in Figure 3. Our findings suggest that the local water diffusivity, and hence the water transport, can be significantly enhanced by decreasing the charged sulfonic acid group density and accompanied increase of the accessible fluorocarbon surface area.…”

mentioning

confidence: 68%

Nanometer‐Scale Water‐ and Proton‐Diffusion Heterogeneities across Water Channels in Polymer Electrolyte Membranes

Song

Han

2015

Angewandte Chemie

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Nafion, the most widely used polymer for electrolyte membranes (PEM) in fuel cells, consists of fluorocarbon backbones and acidic groups that, upon hydration, swell to form percolated channels through which water and ions diffuse. While the effects of the channel structures and the acidic groups on water/ion transport have been studied before, the surface chemistry or the spatially heterogeneous diffusivity across water channels has never been shown to directly influence water/ion transport. Using molecular spin probes that selectively partition into heterogeneous regions of PEM and Overhauser dynamic nuclear polarization relaxometry, this study reveals that both water and proton diffusivity are significantly faster near the fluorocarbon and the acidic groups lining the water channels compared to within the water channels. The concept that surface chemistry at the (sub-)nanometer scale dictates water and proton diffusivity invokes a new design principle for PEM. Keywordselectron paramagnetic resonance spectroscopy; Overhauser dynamic nuclear polarization relaxometry; water and proton dynamics; polymer electrolyte membrane; heterogeneous structure A polymer electrolyte membrane (PEM) placed between a cathode and an anode in a fuel cell, which is a device that converts chemical energy of fuel into electricity through an electrochemical reaction, should be a good conductor for protons but not for electrons and the molecular constituents of the fuel. Nafion's success as PEM stems from the good ionic conductivity through water channels, [1] as well as high chemical and mechanical stability for a wide operating temperature range between 50°C and 100°C. [2] Nafion is composed of a Correspondence to: Oc Hee Han, ohhan@kbsi.re.kr; Songi Han, songi@chem.ucsb.edu. HHS Public Access Author ManuscriptAuthor Manuscript Author ManuscriptAuthor Manuscript poly(tetrafluoroethylene) backbone with side arms decorated with sulfonic acid groups, as illustrated in Figure S1 of Supporting Information (SI). Nafion is known, upon hydration, to swell to form percolated channels through which water, protons and ions diffuse. [3] Nafion's hydrophobic domains are known to have ordered helical fibers with crystallinity similar to that of pure poly(tetrafluoroethylene) at elevated temperatures. [4] However, the exact structure and ordering of the water channels lined by the sulfonic acid groups is still a subject of controversy. Crucially, the channel dimension was found to increase linearly with the water volume fraction [3] indicating locally flat structures of the channels, [5] while scattering patterns are consistent with rod-like shapes for hydrated Nafion membrane at water volume fraction < 50%. [6][7][8] The structure and connectivity of the water channels has been thought to play an important role in the proton and water diffusivity through Nafion membranes, although a firm relationship between structure and transport function has not been established. At a hydration level of ~20 water molecules per sulfonic acid in Nafion 11...

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First principles molecular dynamics study of proton dynamics and transport in phosphoric acid/imidazole (2:1) system

Cited by 44 publications

References 56 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

Nanometer‐Scale Water‐ and Proton‐Diffusion Heterogeneities across Water Channels in Polymer Electrolyte Membranes

Nanometer‐Scale Water‐ and Proton‐Diffusion Heterogeneities across Water Channels in Polymer Electrolyte Membranes

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