Vehicle Mechanism, A New Model for the Interpretation of the Conductivity of Fast Proton Conductors
01 Mar 1982-Angewandte Chemie (John Wiley & Sons, Ltd)-Vol. 21, Iss: 3, pp 208-209
About: This article is published in Angewandte Chemie.The article was published on 1982-03-01. It has received 678 citations till now.
TL;DR: Light scattering experiments revealed that the radius of gyration had a linear dependence on the molar mass of the aggregates, which suggests that the particles are in the form of rods or ribbons, or at least some elongated structure.
Abstract: Equivalent weight (EW) is the number of grams of dry Nafion per mole of sulfonic acid groups when the material is in the acid form. This is an average EW in the sense that the comonomer sequence distribution (that is usually unknown to the investigator and largely unreported) gives a distribution in m in this formula. EW can be ascertained by acid-base titration, by analysis of atomic sulfur, and by FT-IR spectroscopy. The relationship between EW and m is EW ) 100m + 446 so that, for example, the side chains are separated by around 14 CF2 units in a membrane of 1100 EW. Common at the time of this writing are Nafion 117 films. The designation “117” refers to a film having 1100 EW and a nominal thickness of 0.007 in., although 115 and 112 films have also been available. Early-reported studies involved 1200 EW samples as well as special experimental varieties, some being rather thin. The equivalent weight is related to the property more often seen in the field of conventional ion exchange resins, namely the ion exchange capacity (IEC), by the equation IEC ) 1000/EW. The mention of the molecular weight of high equivalent weight (EW > 1000 g‚mol-1) Nafion is almost absent in the literature, although the range 105-106 Da has been mentioned. As this polymer does not form true solutions, the common methods of light scattering and gel permeation chromatography cannot be used to determine molecular weight as well as the size and shape of isolated, truly dissolved molecules. Studies of the structure of this polymer in solvent (albeit not a true solution) will be mentioned in the scattering section of this review. It should be noted that Curtin et al. performed size exclusion chromatography determinations of the molecular weight distribution in Nafion aqueous dispersions after they were heated to high temperatures (230, 250, and 270 °C).1 Before heating, there was a high molecular weight shoulder on a bimodal distribution, due to molecular aggregates, but this shoulder disappeared upon heating, which indicated that the aggregates were disrupted. The peaks for the monomodal distribution for the heated samples were all located at molecular weights slightly higher than 105 g‚mol-1. Also, light scattering experiments revealed that the radius of gyration had a linear dependence on the molar mass of the aggregates, which suggests that the particles are in the form of rods or ribbons, or at least some elongated structure. Nafion ionomers are usually derived from the thermoplastic -SO2F precursor form that can be extruded into sheets of required thickness. Strong interactions between the ionic groups are an obstacle to melt processing. This precursor does not possess the clustered morphology that will be of great concern in this article but does possess Teflon-like crystallinity which persists when the sulfonyl fluoride form is converted to, for example, the K+ form by reacting it with KOH in water and DMSO. Thereafter, the -SO3H form is achieved by soaking the film in a sufficiently concentrated aqueous acid solution. Extrusion of the sulfonyl fluoride precursor can cause microstructural orientation in the machine direction, * Address correspondence to either author. Phone: 601-266-5595/ 4480. Fax: 601-266-5635. E-mail: Kenneth.Mauritz@usm.edu; RBMoore@usm.edu. 4535 Chem. Rev. 2004, 104, 4535−4585
TL;DR: In this paper, the authors explain the transport properties and the swelling behaviour of NAFION and different sulfonated polyetherketones in terms of distinct differences on the microstructures and in the p K a of the acidic functional groups.
Abstract: The transport properties and the swelling behaviour of NAFION and different sulfonated polyetherketones are explained in terms of distinct differences on the microstructures and in the p K a of the acidic functional groups. The less pronounced hydrophobic/hydrophilic separation of sulfonated polyetherketones compared to NAFION corresponds to narrower, less connected hydrophilic channels and to larger separations between less acidic sulfonic acid functional groups. At high water contents, this is shown to significantly reduce electroosmotic drag and water permeation whilst maintaining high proton conductivity. Blending of sulfonated polyetherketones with other polyaryls even further reduces the solvent permeation (a factor of 20 compared to NAFION), increases the membrane flexibility in the dry state and leads to an improved swelling behaviour. Therefore, polymers based on sulfonated polyetherketones are not only interesting low-cost alternative membrane material for hydrogen fuel cell applications, they may also help to reduce the problems associated with high water drag and high methanol cross-over in direct liquid methanol fuel cells (DMFC). The relatively high conductivities observed for oligomers containing imidazole as functional groups may be exploited in fully polymeric proton conducting systems with no volatile proton solvent operating at temperatures significantly beyond 100°C, where methanol vapour may be used as a fuel in DMFCs.
TL;DR: In this article, the authors present the latest status of PEM fuel cell technology development and applications in the transportation, stationary, and portable/micro power generation sectors through an overview of the state-of-the-art and most recent technical progress.
Abstract: Polymer electrolyte membrane (PEM) fuel cells, which convert the chemical energy stored in hydrogen fuel directly and efficiently to electrical energy with water as the only byproduct, have the potential to reduce our energy use, pollutant emissions, and dependence on fossil fuels. Great deal of efforts has been made in the past, particularly during the last couple of decades or so, to advance the PEM fuel cell technology and fundamental research. Factors such as durability and cost still remain as the major barriers to fuel cell commercialization. In the past two years, more than 35% cost reduction has been achieved in fuel cell fabrication, the current status of $61/kW (2009) for transportation fuel cell is still over 50% higher than the target of the US Department of Energy (DOE), i.e. $30/kW by 2015, in order to compete with the conventional technology of internal-combustion engines. In addition, a lifetime of ∼2500 h (for transportation PEM fuel cells) was achieved in 2009, yet still needs to be doubled to meet the DOE’s target, i.e. 5000 h. Breakthroughs are urgently needed to overcome these barriers. In this regard, fundamental studies play an important and indeed critical role. Issues such as water and heat management, and new material development remain the focus of fuel-cell performance improvement and cost reduction. Previous reviews mostly focus on one aspect, either a specific fuel cell application or a particular area of fuel cell research. The objective of this review is three folds: (1) to present the latest status of PEM fuel cell technology development and applications in the transportation, stationary, and portable/micro power generation sectors through an overview of the state-of-the-art and most recent technical progress; (2) to describe the need for fundamental research in this field and fill the gap of addressing the role of fundamental research in fuel cell technology; and (3) to outline major challenges in fuel cell technology development and the needs for fundamental research for the near future and prior to fuel cell commercialization.
TL;DR: In this paper, a review of the proton conductivity in materials and the elements of proton conduction mechanisms are discussed with a special emphasis on proton chemistry, including structural reorganization and diffusional motion of extended moieties.
Abstract: In this review the phenomenon of proton conductivity in materials and the elements of proton conduction mechanismsproton transfer, structural reorganization and diffusional motion of extended moietiesare discussed with special emphasis on proton chemistry. This is characterized by a strong proton localization within the valence electron density of electronegative species (e.g., oxygen, nitrogen) and self-localization effects due to solvent interactions which allows for significant proton diffusivities only when assisted by the dynamics of the proton environment in Grotthuss and vehicle type mechanisms. In systems with high proton density, proton/proton interactions lead to proton ordering below first-order phase transition rather than to coherent proton transfers along extended hydrogen-bond chains as is frequently suggested in textbooks of physical chemistry. There is no indication for significant proton tunneling in fast proton conduction phenomena for which almost barrierless proton transfer is suggest...
TL;DR: Theoretical Methodologies and Simulation Tools, and Poisson−Boltzmann Theory, and Phenomenology of Transport inProton-Conducting Materials for Fuel-CellApplications46664.2.1.
Abstract: 1. Introduction 46372. Theoretical Methodologies and Simulation Tools 46402.1. Ab Initio Quantum Chemistry 46412.2. Molecular Dynamics 46422.2.1. Classical Molecular Dynamics and MonteCarlo Simulations46432.2.2. Empirical Valence Bond Models 46442.2.3. Ab Initio Molecular Dynamics (AIMD) 46452.3. Poisson−Boltzmann Theory 46452.4. Nonequilibrium Statistical Mechanical IonTransport Modeling46462.5. Dielectric Saturation 46473. Transport Mechanisms 46483.1. Proton Conduction Mechanisms 46483.1.1. Homogeneous Media 46483.1.2. Heterogeneous Systems (ConfinementEffects)46553.2. Mechanisms of Parasitic Transport 46613.2.1. Solvated Acidic Polymers 46613.2.2. Oxides 46654. Phenomenology of Transport inProton-Conducting Materials for Fuel-CellApplications46664.1. Hydrated Acidic Polymers 46664.2. PBI−H
TL;DR: In this paper, a Grotthus-type mechanism of conduction is proposed which involves intermolecular transfer steps (hopping) and inter-parallel transfer steps in comparable numbers, the former facilitated by the high concentration of H 3 O + ions in the structure, and the latter most likely facilitated by high H-bond vacancies.
Abstract: We have found that hydrogen uranyl phosphate tetrahydrate HUO 2 PO 4 ·4H 2 O has a high proton conductivity. The ac conductivity was 0.4 ohm −1 m −1 at 290°K measured parallel to the faces of sintered disks of the compound. The activation energy was found to be 31 ± 3 kJ mole −1 . The values of conductivity were between 3 and 10 times lower when measured perpendicular to the disk faces due to preferred orientation of the plate-like crystals. Both the powder and sintered disks are stable in air and insoluble in phosphoric acid solution of pH 2.5. Experiments are described which enable possible grain boundary contributions to the conductivity to be determined in such hydrates. The extrinsic grain boundary contribution to the conductivity was found to be small from experiments in which the pH in a solution cell was varied. The abnormally high bulk H + conductivity thus inferred is attributed primarily to the high concentration of H + , which exists as H 3 O + in the interlamellar hydrogen-bonded network. A Grotthus-type mechanism of conduction is proposed which involves intermolecular transfer steps (hopping) and intramolecular transfer steps, in comparable numbers, the former facilitated by the high concentration of H 3 O + ions in the structure, and the latter most likely facilitated by the high concentration of H-bond vacancies.
TL;DR: In this paper, single-crystal measurements on hydrogen uranyl phosphate tetrahydrate, HUO 2 PO 4 ·4H 2 O (HUP), have confirmed that the high proton conductivity is a bulk characteristic.
Abstract: Single-crystal measurements on hydrogen uranyl phosphate tetrahydrate, HUO 2 PO 4 ·4H 2 O (HUP), have confirmed that the high proton conductivity is a bulk characteristic. The conductivity values were in substantial agreement with those previously reported for polycrystalline disks. A conductivity of 0.6 ohm −1 m −1 at 290°K and an activation energy of 30 ± 1 kJ mole −1 were measured parallel to the structural layers of the crystal. The conductivity was at least 100 times lower when measured in the perpendicular direction. A reasonable attempt frequency ω 0 of approximately 10 15 Hz could be derived from the parallel conductivity on the assumption that the charge carrier concentration was equal to that of the H 3 O + ions. This implies a low proton mobility, of the order of 10 −9 m 2 V −1 sec −1 at 290°K, in support of previous estimates. We have also shown that polycrystalline hydrogen uranyl arsenate tetrahydrate, HUO 2 AsO 4 ·4H 2 O (HUAs), has a high conductivity of 0.6 ohm −1 cm −1 at 310°K, with an activation energy of 31 ± 2 kJ mole −1 . Below the respective dielectric ordering transition temperatures of HUP and HUAs of 274 and 301°K, the lower conductivity values show a marked frequency dependence, which may be due to dispersion effects caused by water reorientations.
TL;DR: In this article, LiN 2 H 5 SO 4 has been shown to have a conductivity of 2 × 10 −8 ω −1 cm −1 at 25°C and an activation enthalpy of 0.75 ± 0.07 eV.
Abstract: One-dimensional bulk proton conduction parallel to the c axis was observed in solid lithium hydrazinium sulfate, LiN 2 H 5 SO 4 . The conductivity in this direction is 2 × 10 −8 ω −1 cm −1 at 25°C and shows an activation enthalpy of 0.75 ± 0.07 eV. The two-dimensional conductors HUO 2 AsO 4 ·4H 2 O and HUO 2 PO 4 ·4H 2 O were studied as a function of their water content. The conductivities are 8 × 10 −6 and 3 × 10 −5 ω −1 cm −1 in the orthorhombic phase at −10°C, with activation enthalpies of 0.70 ± 0.05 and 0.57 ± 0.07 eV, respectively. Indications of peritectic transitions to the tetragonal phases were observed in the temperature ranges 15 to 47 and −5 to 10°C, respectively. The transition depends on the water content which appears to control the increase in conductivity in this material. The dependence upon various sample parameters is discussed. Fast proton transport in solids is proposed to occur by a “vehicle mechanism”, i.e. the motion of N 2 H + 5 , H 3 O + - or other proton-containing groups.
TL;DR: In this article, diffusion and conductivity mechanisms for DUO2AsO4·4D2O (DUAs) are considered in the light of neutron diffraction evidence for partial ordering and the presence of D5O+2 and D4O2 units in the structure.
Abstract: Diffusion and conductivity mechanisms for DUO2AsO4·4D2O (DUAs) are considered in the light of neutron diffraction evidence for partial ordering and the presence of D5O+2 and D4O2 units in the structure.
TL;DR: In this article, the temperature dependence of the rotating frame relaxation time T(1 rho) for protons in powdered lithium hydrazinium sulfate, Li(N2H5)SO4, has been determined from 140 to 495K.
Abstract: : The temperature dependence of the rotating frame relaxation time T(1 rho) for protons in powdered lithium hydrazinium sulfate, Li(N2H5)SO4, has been determined from 140 to 495K. These measurements indicate that the -NH2 part of the N2H5(+) ion executes 180 deg flips about the bisectrix of the H-N-H angle. Evidence for motion of the entire N2H5(+) ion was also obtained. Separate reorientation and diffusion motions of the N2H5(+) ion could not be distinguished, although evidence that the N2H5(+) motion detected in the T(1 rho) measurements includes diffusion is obtained by comparing the T(1 rho) results with the temperature dependence of the proton second moment. (Author)