Giant Haven Ratio for Proton Transport in Sodium Hydroxide

Spaeth, M.; Kreuer, Klaus‐Dieter; Maier, Joachim; Cramer, Cornelia

doi:10.1006/jssc.1999.8495

Cited by 31 publications

(22 citation statements)

References 26 publications

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“…The latter is frequently observed for proton conductors with a high concentration of mobile protons, which may be translocated on closed trajectories (e.g. in ring exchange processes) as observed for proton diffusion in the plastic phases of alkaline metal hydroxides [33,34], in phosphoric acid [32] and in phosphonic acid [35]. In all cases, regions containing excess protons must be charge compensated by proton deficient regions with electrostatic attraction between these regions (defects) that depends on their mutual separation distance and the dielectric constant of the medium.…”

Section: Self-diffusion Coefficients Of Cyclic Oligomersmentioning

confidence: 97%

“…If the other proton is transferred back, the protons interchange their positions in the hydrogen-bond network, which generates diffusion but no conductivity since the transient charge separation is completely reversed. As illustrated for the most simple mechanism of this type in Figure 10, the sum of all proton translocation vectors form a closed trajectory reminiscent of cyclic intermolecular proton transfer reactions known to take place in hydroxides [33,34]. Presently, there is no direct proof for such a mechanism in imidazole based systems, but the observation that the ratio of the proton diffusion and conduction rates virtually coincide with the Boltzmann factor (9) neglecting any entropic effects as part of a pre-exponential factor…”

Section: Self-diffusion Coefficients Of Cyclic Oligomersmentioning

confidence: 99%

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Anhydrous Polymeric Proton Conductors Based on Imidazole Functionalized Polysiloxane

et al. 2006

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Intrinsically proton conducting polymers with imidazole as proton solvent tethered to a polysiloxane backbone via a flexible spacer have been synthesized. Apart from the standard characterization also their thermal properties and transport behavior have been investigated. The materials exhibit proton conductivity as a consequence of self‐dissociation of the imidazole moieties and “structure diffusion” of the resulting defects. In particular, no liquid phase such as water or monomeric imidazole is needed for the observed proton conductivities.To study the influence of the tether structure on the transport properties, cyclic oligomers and open chain polymers with different spacer lengths have been synthesized.The materials are thermally stable up to 200 °C and become soft around room temperature. The conductivity exhibits VTF and WLF behavior with maximum conductivities around σ = 1.5.10–3 S cm–1 at T = 160 °C. The activation volume of the conductivity as derived from pressure dependent measurements is found to be unusually high. The lowest activation volumes and the highest conductivities are observed for the materials with the highest segmental mobilities, i.e. the longest spacers.Proton self‐diffusion coefficients as obtained from PFG NMR diffusion measurements are significantly higher than expected from the proton conductivities obtained by dielectric spectroscopy. This corresponds to unusually high Haven ratios which have been interpreted by correlated proton transfers allowing for fast proton diffusion while minimizing the separation of ionic charge carriers.

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Section: Self-diffusion Coefficients Of Cyclic Oligomersmentioning

confidence: 97%

Section: Self-diffusion Coefficients Of Cyclic Oligomersmentioning

confidence: 99%

Anhydrous Polymeric Proton Conductors Based on Imidazole Functionalized Polysiloxane

et al. 2006

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“…The latter is frequently observed for proton conductors with a high concentration of mobile protons, which may be translocated on closed trajectories (e.g. in ring exchange processes) as observed for proton diffusion in the plastic phases of alkaline metal hydroxides [48,49] and phosphoric acid [50]. Such cooperativities allow for high proton diffusion coefficients while preventing electrostatically expensive charge separation.…”

Section: Transportmentioning

confidence: 99%

About the Choice of the Protogenic Group in PEM Separator Materials for Intermediate Temperature, Low Humidity Operation: A Critical Comparison of Sulfonic Acid, Phosphonic Acid and Imidazole Functionalized Model Compounds

et al. 2005

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Traditionally, sulfonated polymers are used as separator materials in PEM fuel cells. Based on recent experimental results on model compounds this paper critically discusses the potentials and limits of sulfonic acid and alternatively phosphonic acid and heterocycles (imidazole) as protogenic groups for PEM fuel cell electrolytes operating at intermediate temperatures (T > 100 °C) and low humidification. Apart from transport properties, the stability and reactivity of mono‐functionalized model compounds (1‐heptylsulfonic acid (S‐C7), 1‐heptylphosphonic acid (P‐C7) and 2‐heptyl‐imidazole (I‐C7)) and a few diphosphonic acids are examined under wet and dry conditions. These are characterized with respect to their proton conductivity (ac impedance spectroscopy), proton diffusion coefficient (pulsed‐field gradient NMR), thermo‐oxidative stability (TGA under air), electrochemical stability (cyclic voltammetry) and their hydration behavior (TGA under water vapor). The sulfonic acid functionalized compound shows reasonable properties only when a minimum hydration level is guaranteed, while phosphonic acid functionalized compounds combine satisfactory proton conductivity even in the water‐free state at intermediate temperatures (T < 200 °C), comparatively high thermo‐oxidative and electrochemical stability and electrochemical reactivity (hydrogen oxidation and oxygen reduction at platinum surfaces). The presence of water leads to moderate water uptake allowing for reasonable conductivities even at room temperature and prevents condensation reactions at higher temperature. The imidazole based system shows the largest electrochemical stability window, but its moderate proton conductivity and thermo‐oxidative stability and the very high overpotential for oxygen reduction on platinum turn out to be severe disadvantages for the envisaged application.

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“…This has been explained by the correlated motion of the oppositely charged defects (H 2 PO 4 -, H 4 PO 4 + ) when they are close to one another (i.e., the case just after their formation (by dissociation of H 3 PO 4 ) and before their neutralization). Correlation effects are actually quite common in proton conductors with high concentrations of charge carriers and they are even more pronounced in other systems (see Section 3.1.1.3 on heterocycles and proton transport in alkaline-metal hydroxides 157,158 ). Molecular details of the structure diffusion mechanism with the hydrogen-bond breaking and forming and the proton transfer between the different phos-…”

Section: Mobility Of Defect Protonsmentioning

confidence: 99%

“…As illustrated for the most simple mechanism of this type in Figure 4, the sum of all proton translocation vectors form a closed trajectory reminiscent of cyclic intermolecular proton-transfer reactions known to occur in certain organic pyrazole-containing complexes 175 and proton diffusion in hydroxides. 157,158 Presently, there is no direct proof for such a mechanism in pure imidazole (e.g., by 15 N NMR); however, the observation that the ratio of the proton diffusion and conduction rates virtually coincide with the Boltzmann factor (i.e., exp(-E Φ ( )/(kT)), where E Φ is the electrostatic separation energy of two unit charges in a continuum of dielectric constant ) is a strong indication.…”

mentioning

confidence: 99%

Transport in Proton Conductors for Fuel‐Cell Applications: Simulations, Elementary Reactions, and Phenomenology

Kreuer¹,

Paddison²,

Spohr³

et al. 2004

ChemInform

126

193

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Giant Haven Ratio for Proton Transport in Sodium Hydroxide

Cited by 31 publications

References 26 publications

Anhydrous Polymeric Proton Conductors Based on Imidazole Functionalized Polysiloxane

Anhydrous Polymeric Proton Conductors Based on Imidazole Functionalized Polysiloxane

About the Choice of the Protogenic Group in PEM Separator Materials for Intermediate Temperature, Low Humidity Operation: A Critical Comparison of Sulfonic Acid, Phosphonic Acid and Imidazole Functionalized Model Compounds

Transport in Proton Conductors for Fuel‐Cell Applications: Simulations, Elementary Reactions, and Phenomenology

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