Vehicular Mechanism Research Articles

Molecular Dynamics Study of Proton Transport in Modeled Water Cluster Structure of Polymer Electrolyte Membrane

The polymer electrolyte membrane (PEM) is one of the principal components for polymer electrolyte fuel cells. In the nanoscopic structure of the membrane, ion clusters are formed by water molecules gathered vicinity of sulfonate groups which are hydrophilic parts of Nafion membrane, and proton transport (PT) are largely attributed to the nanoscopic structure of polymer membranes and water aggregations. Thus, it is critical to understand the PT mechanisms through polymer membrane and clarify an important link between the membrane nanostructure and the PT properties. In the bulk aqueous solution, it is observed that proton mobility is roughly 5 times higher than other cations such as K+ which have an ionic radius similar to the hydronium ions. This property of PT is attributed to a combination of proton hopping, known as the Grotthuss mechanism, and vehicular mechanism where protons diffuse in solution as a hydrated form (e.g. hydronium ions). Many research efforts have been focused on understanding the PT mechanism using theoretical basis for the proton hopping models such as the empirical valence bond (EVB) approaches which allow PT phenomena to be simulated within a molecular dynamics (MD) framework. In PEM, it is considered that proton hopping transport still takes place although the water dynamic property in the bulk aqueous solution is inhibited by a lack of bulk-like water structure due to the phase separation in PEM. Since the phase separation is considered one of the primary factors affecting its performance, the morphological properties of PEM have been studied experimentally and been characterized by proposing the cluster models (e.g., the cylinder model, the lamellar model, and the cluster-rod model) which reasonably fit the experimental scattering spectra. However, a detailed relationship between the morphological features and PT is still under debate. In this study, atomistic MD simulations have been performed to study the effects of water cluster structure on PT properties. The systems were built to closely approximate the proposed hydrophilic cluster structures (the cylinder model and the lamellar model) that are the most major morphological models of Nafion. The PT properties were estimated in terms of diffusion coefficient and proton pathway using a modified EVB model developed based on the previous study of the two-state EVB model by Walbran et al. The diffusion coefficients in each dimension were calculated and correlated with the cluster size as well as the type of cluster models. It was found that proton diffusion is strongly affected by the cluster model and its size, which can be associated with the proton pathway in each cluster model. The influence of the sulfonate groups on the PT properties were also investigated. Our simulation results provide insight into quantitative information about PT properties in atomic level.

Effects of Water Structure on Proton Transport in Nafion Thin Films with Molecular Dynamics Simulations

Polymer electrolyte fuel cell (PEFC) is expected to play important roles for next generation power systems in terms of high efficiency and low environmental burden and capability for emergency use, but there are still many hurdles to be largely commercialized. One of them results from problems in catalyst layers (CL). What required for CL is, for example, reducing the amount of Pt particles with keeping high current density. To accomplish this, knowledge about optimum structures of CL based on material transport properties is needed because there is a strong correlation between current density and transport properties. Although there are many reports about material transport properties in the polymer electrolyte membrane (PEM), it is predicted that material transport properties in CL show some unique characteristics because of effects of interfaces in CL. It means that the knowledge obtained from PEM should not be applied to CL. In addition, the researches about CL have been tried by means of experiments and simulations, but especially the transport properties have not been clarified yet. There are nanoscopic structures in Nafion thin films and it is hard to capture the transport phenomena in such a small area by experiments and macroscopic simulations. In this study, we analyze proton transport properties in the cathode CL using molecular dynamics simulations, focusing on the dependences of the diffusion coefficient on water content. We consider the proton transport mechanism as a combination of the Vehicular mechanism and the Grotthuss mechanism. The Grotthuss mechanism is represented using empirical valence bond methods. It has been shown by previous researches in PEM that the Grotthuss mechanism is a dominant factor for proton diffusion in relatively high water contents. Thus, it is essential to consider the Grotthuss mechanism for analysis of proton transport in CL as well. The system consists of graphite surface and Nafion thin films. Pt particles are neglected because of a restriction of calculation cost. In PEM analysis, the diffusion coefficient of protons increases with increasing water contents. First, we report whether the same trend as observed in PEM can be seen in CL and the correlation between diffusibility and structures. Next, unique characteristics of proton transport properties in CL compared to the PEM analysis are reported. They are evaluated in terms of mass distribution of the thin films and solvent molecules. From this analysis, it is found that the diffusibility is not much affected by surface because of a lack of solvent molecules on surface, which is attributed to the hydrophobic properties of graphite.

Electrochemical and structural characterization of sulfonated polysulfone

We describe the synthesis, as well as the electrochemical and structural characterization, of sulfonated polysulfone intended for use in PEM fuel cells. Starting from a commercial polysulfone, we assessed the performance of these prepared ionomers using synthesis protocols compatible with industrial production. The efficiency of the trimethylsilyl chlorosulfonate and chlorosulfonic acid reagents in the sulfonation process was confirmed by 1H NMR, FTIR, elemental analysis, chemical titration and thermal analysis (DSC and TGA). Chlorosulfonic acid was the most effective sulfonation reagent. However, based on SEC-MALLS, this reagent induced degradation of the backbone that is detrimental to the thermomechanical stability and lifespan of the membranes. The electrical characterization of the membranes was undertaken using impedance spectroscopy in contact with different HCl aqueous solutions at various temperatures. The activation energies, which ranged from 8.2 to 11 kJ/mol, were in agreement with the prevailing proton vehicular mechanism.

Molecular Dynamics Simulation of Proton Transport in Polymer Electrolyte Membrane.

We have performed a detailed analysis of proton solvation and transport properties in hydrated Nafion using molecular dynamics simulation. The revised empirical valence bond (EVB) method was developed in order to treat the excess proton transport through the Grotthuss mechanism. The new EVB model predicts a significantly enhanced transport in comparison with previous hopping models as well as the classical hydronium diffusion, which largely improves the agreement with the available experimental data. Our results suggest that a proton hopping mechanism has a small effect on the proton dissociation from the first solvation shell of sulfonate groups, namely that protons are not enhanced to separate from the sulfonate groups by the hopping mechanisms. From diffusion comparison between the Grotthuss and vehicular mechanism, the Grotthuss mechanism dominates the proton diffusion at the studied hydration levels including a hydration level of 3. It was also found that the vehicular mechanism dominates the electroosmotic transport of water molecules at the studied hydration levels.

An Improved EVB Model for Proton Transport in Polymer Electrolyte Membrane

The heart of Polymer electrolyte fuel cells (PEFCs) is polymer electrolyte membrane (PEM) that separated the reactant gases and conducts protons. In the nanoscopic structure of the membrane, ion clusters are formed by water molecules gathered vicinity of sulfonate groups which are hydrophilic parts of Nafion membrane, and proton transport (PT) are largely attributed to the nanoscopic structure of polymer membranes and water aggregations. Thus, many macroscopic simulations that are based on continuum theory have found it difficult to discuss the relation between the structural and dynamical properties of protons within the membrane in previous experimental studies. It is critical to understand the PT mechanisms through polymer membrane and clarify an important link between the membrane nanostructure and the PT properties. In the bulk aqueous solution, it is observed that proton mobility is roughly 5 times higher than other cations such as K+ which have an ionic radius similar to the hydronium ions. This property of PT is attributed to a combination of proton hopping, known as the Grotthuss mechanism, and vehicular mechanism where protons diffuse in solution as a hydrated form (e.g. hydronium ions). Many research efforts have been focused on understanding the PT mechanism using theoretical basis for the proton hopping models such as the empirical valence bond (EVB) approaches which allow PT phenomena to be simulated accurately in MD simulations. In Nafion membranes, it is considered that proton hopping transport still takes place although the water dynamic property in the bulk aqueous solution is inhibited by a lack of bulklike water structure at low water contents. Although the PT properties including hopping mechanism in hydrated polymer membrane have been studied using the original two-state EVB method by Walbran et al., the diffusion of water was found to be significantly higher than that of protons as well as that of real water in their model. In this study, a revised EVB model has been developed based on the previous study of the two-state EVB model, the impact of the proton hopping mechanism on the structure and dynamics properties of protons in hydrated Nafion membrane has been investigated at different hydration levels. The new EVB model in bulk water predicts a significantly enhanced transport in comparison with previous hopping models, which largely improves the agreement with the available experimental data. We have determined the diffusion coefficients of protons and water molecules in hydrated Nafion membrane as a function of hydration level, and the results are in good agreement with experimental data for the studied hydration levels. Proton hopping mechanism is found to become more significant at higher hydration levels. The influence of the sulfonate groups on the PT properties have also been investigated in this study. Our simulation results provide insight into quantitative information about PT properties in atomic level.

Proton dynamics in sulfonated ionic salt composites: Alternative membrane materials for proton exchange membrane fuel cells

Hydrated Nafion, the most prevalent proton exchange membrane utilizes a vehicular mechanism for proton conduction. However, there is an increasing need for such membranes to perform under anhydrous conditions, at high temperatures, which would employ a structural transport mechanism for proton conductivity. Here, several solid-acids are characterized, both as pristine salts, and as polymer composites. Materials of interest include benzimidazolium methanesulfonate (BMSA), imidazolium methanesulfonate (IMSA), and imidazolium trifluoromethanesulfate (IFMS). The proton dynamics of these solid acids are characterized as pure salts, and as composites, embedded into porous Teflon, by solid state NMR. It was determined that spin lattice (T1) relaxation of the composites are systematically lower than that of the pure salt, indicating that local dynamics are enhanced in the composites. Spin–spin relaxation (T2∗) was measured as a function of temperature to determine the activation energy for local mobility for each salt and composite. The activation energy for local proton mobility in each salt decreased after being inserted into porous Teflon. Finally, the long-range ion transport was characterized using impedance spectroscopy. The IFMS–Teflon composite possessed the lowest activation energy for local proton mobility, the highest thermal stability, and the most favorable proton conductivity, among the investigated materials.

Proton Transport under External Applied Voltage

Proton transport through an electrolyte layer between platinum electrodes under a range of applied voltages is explored using reactive molecular dynamics simulation. The proton transport process is decomposed into vehicular and Grotthuss hopping components, and the two mechanisms and their correlation are investigated as a function of applied voltage. At higher applied voltages, the effect of the hopping mechanism is much larger as compared with the vehicular mechanism. As the voltage is increased, the net correlation between the two mechanisms goes from negative to positive, and both the hopping frequency as well as the number of consecutive forward hops increases. This behavior results in a larger total diffusion constant at higher values of the voltage. The behavior of the hydrated excess proton is therefore substantially different under an applied external voltage than in the normal bulk water environment.

Mechanism of ion transport in PEO/LiTFSI complexes: Effect of temperature, molecular weight and end groups

The conductivity and viscosity of PEO/LiTFSI complexes are determined as a function of temperature, molecular weight (Mn) and the end group nature in view of the design of future polymer electrolytes. The results show the crucial role of the end groups on the dynamics of polymers at low Mn. A new method is proposed to estimate the glass transition temperature variation as function of Mn and end groups using conductivity data. The conductivity and viscosity plotted at constant friction factor follow a master curve which suggests that the main impact of end groups is to modify the available free volume which governs in turn the segmental dynamics. The anion and cation conductivities are separated using the cationic transport number obtained by pfg-NMR. Finally, an empirical equation based on Rouse dynamics taking into account the effect of the end groups is proposed. It reproduces with a good degree of accuracy the conductivities over the whole temperature and Mn ranges. In agreement with molecular dynamic simulations, at high Mn the limiting step is the jump of the lithium ion from one coordination site to another and is not influenced by the dynamics of the PEO chain reptation, whereas at low Mn the transport is mainly ensured by a vehicular mechanism.

Proton diffusion in polyelectrolytes based on hydrogenated polynorbornenes with imide side groups in the repeat unit as determined by NMR and impedance spectroscopies

The conductivity of sulfonated membranes based on hydrogenated polynorbornene functionalized with dicarboximide side groups, specifically hydrogenated poly(exo,endo- N-phenyl-norbornene-5,6-dicarboximide), was determined by impedance spectroscopy in the temperature range 10–40 °C. Using Nernst–Planck transport equations, the diffusion coefficient of protons in the membranes was estimated. The diffusion of protons was also measured by pulsed field gradient (PFG) NMR techniques at several temperatures. The comparison of the temperature dependence of the diffusion coefficients of protons obtained by the two techniques suggests that a vehicular type mechanism governs the transport. Solid state 23Na magic angle spinning (MAS) NMR was used to determine the state of sodium counterions in membranes equilibrated with 1 M sodium chloride solutions. The spectra exhibit two peaks, one narrow and another broad, corresponding, respectively, to isolated hydrated sodium ions and clustered sodium ions.

Two new siloxanic proton conducting membranes: Part II. Proton conductivity mechanism and NMR study

The synthesis and structural characterization of two types of membranes with formulas {Si(CH 3) 3O[Si(CH 3)HO] 21.26-[Si(CH 3)((CH 2) 3SO 3H)O] 1.8-[Si(CH 3)((CH 2) 3Si(CH 3) 2O-)-O] 14-Si(CH 3) 3} n ( A) and {Si(CH 3) 3O[Si(CH 3)HO] 21.26-[Si(CH 3)((CH 2) 3SO 3H)O] 1.8-[Si(CH 3)((CH 2) 3(Si(CH 3) 2O- ) w )-O ] v [Si(CH 3)((CH 2) 3Si(CH 3) 2O-)-O ] 14 − v Si(CH 3) 3} n ( B), ( w = 20.31 ), were previously proposed. The ac electrical response of A and B was fully characterized in the 40 Hz–2 MHz frequency region by studying the impedance spectra in the medium and low frequency regions by equivalent circuits and complex dielectric spectra at high frequency in terms of dielectric relaxation modes. Results demonstrated that A and B conduct ionically by means of a proton exchange event which occurs via a vehicular mechanism between neighboring water clusters formed by water molecules aggregated around each sulfonic acid group of the siloxane side chains. The proton conductivities at 115 °C of ca. 1.9 × 10 −3 and 1.8 × 10 −4 S cm −1 of fully hydrated membranes A and B, respectively, classify these silicone networks as good proton conductors. Membrane B was chosen for a closer investigation using NMR spectroscopy. Solid state 29Si MAS NMR experiments gave further insight about the three-dimensional structure. Proton diffusion measurements provided some encouraging results about proton dynamics of this membrane signaling the great potential of siloxanic based proton conductors.

Dynamics of the hydrogen and phosphate ions in proton conducting gel/D3PO4 electrolytes: A H2 and P31 nuclear magnetic resonance study

The synthesis and characterization of protonic conducting polymeric gels is motivated by their possible application in various electrochemical devices. Nonaqueous proton-conducting gel electrolytes are being developed for use in various sensors and electrochromic devices operating at ambient temperatures. The electrolyte must have a relatively high conductivity and chemical and physical stability. One method of producing nonaqueous conducting polymeric gel electrolytes is to entrap organic solutions of a strong acid such as H3PO4 in a polymer matrix. Results are reported on the system consisting of phosphoric acid dissolved in N,N-dimethyl formamide (DMF) within a gel network formed with poly(glycidyl methacrylate) (PGMA). Using selective deuteration, the diffusion coefficients for both the deuteron and phosphorus from the phosphoric acid and the DMF are measured by field gradient nuclear magnetic resonance (NMR) techniques. Combining the diffusion with conductivity measurements in the Nernst–Einstein equation leads to a better understanding of the number of charge carriers in the mixture and the temperature dependence of this number. Nuclear spin-lattice relaxation is used as a tool to probe the ion dynamics in these materials. In addition to NMR measurements, the samples have been characterized by electrical conductivity, differential scanning calorimetry, and viscosity measurements. The results show that the Grotthus mechanism involving the hopping of the proton from one molecular site to another, as well as the vehicular mechanism due to the motion of the D2PO4− and D4PO4+ ions are most responsible for the motion of the proton in these electrolytes. The variety of protonation sites in the PGMA/DMF/H3PO4 system is much more diverse that in the previously studied PMMA/PC/D3PO4 system. In the PGMA/DMF/H3PO4 system there are sites on the polymer, DMF and H3PO4.

Proton conducting sulfon/sulfonamide functionalized materials based on inorganic–organic matrices

A new class of inorganic–organic protonic polymer electrolyte was developed recently by grafting sulfonic and sulfonamide groups to the inorganic network by the sol–gel route. It associates the mechanical and thermal resistance of the silica backbone to the chemical reactivity induced by the organic chains grafted to the silica network. The organic chains are slightly acidic proton conductors bearing sulfonic and sulfonamide groups. The polycondensation of alkoxysilanes provides the inorganic silica backbone whereas the organic network is formed from reactive functional groups R′ of alkoxysilanes of the type R′Si(OR) 3, or by copolymerization of reactive organic monomers with functionalized alkoxysilanes. The synthesis of the resins is completed by organic crosslinking reactions (thermal or UV-curing). The transport of the protons through the solid could be described as a mechanism in which the proton was transferred from a donor (sulfonic group) to a suitable placed acceptor (e.g. sulfonamide group) in the case of a dry material. The conductivity was also studied as a function of relative humidity (r.h.) (wet proton conductors). Here, the proton transport could be described as a vehicular mechanism where the proton rides on a carrier molecule (H 3O +). Furthermore the conductivity dependence on temperature follows a VTF behavior. By increasing the water content of the membranes up to 16 mass%, the conductivity increases from 10 −4 to 6×10 −2 S cm −1 at 70°C. These materials will be developed for thin film batteries. Their mechanical properties, thermal stability and glass transition temperature are discussed in connection with the conductivity results.

VEHICULAR TRAUMA TRIAGE BY MECHANISM

An instrument was developed using routinely available field data to identify the sizable subgroup of stable vehicular trauma victims initially triaged to the trauma center by mechanism indicators alone who are in reality at minimal risk for serious injury. The six most common vehicular mechanism indicators seen at a level I trauma center were evaluated: rollover, head-on greater than 30 mph, intrusion, prolonged extrication, other death in same vehicle, and ejection. Review of 1235 consecutive trauma team activations yielded 349 victims with a qualifying vehicular mechanism. Outcome indicators were used to classify patients into two groups: Minor Injury (MI) and Severe Injury (SI). Nineteen common field data elements routinely reported on arrival by the regional Emergency Medical Service (EMS) personnel were then reviewed. Data patterns associated only with the MI group were sought. A checklist was developed for Mechanism vehicular trauma utilizing physiologic, anatomic, and neurologic elements. A single positive element would define trauma team activations. Retrospectively, use of this instrument would have excluded 56% of the MI group from unproductive trauma team referral, but nearly none of the SI group. We conclude that an identifiable subset of trauma patients referred by vehicular mechanism criteria alone could be safely evaluated on arrival in the emergency department as a form of secondary triage rather than by referral to the trauma tea. The use of an appropriate exclusionary instrument can still preserve the sensitivity of trauma team activation for severely injured victims.

VEIDAJAR TRAUMA THIACE BY MECHANISM AVOIDANCE OF THE UNPROUCTIVE EVALUATION

An instrument was developed using routinely available field data to identify the sizable subgroup of stable vehicular trauma victims initially triaged to the trauma center by mechanism indicators alone who are in reality at minimal risk for serious injury. The six most common vehicular mechanism indicators seen at a level I trauma center were evaluated: rollover, head-on greater than 30 mph, intrusion, prolonged extrication, other death in same vehicle, and ejection. Review of 1235 consecutive trauma team activations yielded 349 victims with a qualifying vehicular mechanism. Outcome indicators were used to classify patients into two groups: Minor Injury (MI) and Severe Injury (SI). Nineteen common field data elements routinely reported on arrival by the regional Emergency Medical Service (EMS) personnel were then reviewed. Data patterns associated only with the MI group were sought. A checklist was developed for Mechanism vehicular trauma utilizing physiologic, anatomic, and neurologic elements. A single positive element would define trauma team activations. Retrospectively, use of this instrument would have excluded 56% of the MI group from unproductive trauma team referral, but nearly none of the SI group. We conclude that an identifiable subset of trauma patients referred by vehicular mechanism criteria alone could be safely evaluated on arrival in the emergency department as a form of secondary triage rather than by referral to the trauma team. The use of an appropriate exclusionary instrument can still preserve the sensitivity of trauma team activation for severely injured victims.

Zinc Conducting Polymer Electrolytes Based on a Vehicular Process

The transport of a metallic cation in a solid polymer electrolyte can be increased drastically by a vehicular mechanism. Results are presented for polyethylene oxide (PEO)‐based electrolyte on Zn2+ transport with various anionic ligand salts. Preliminary results have shown that chloride ion is the most suitable ligand for zinc ionic transport. Electrochemical investigations based on cyclic voltametry, ac impedance, and dc polarization have demonstrated a reversible zinc plating/stripping process with fair conductivities. Furthermore, zinc‐transport current densities of more than 1 mA/cm2 were obtained with a small polarization on a symmetrical vehicular electrolyte cell, without any appreciable transport limitation of the electrolyte.

Ionic conduction in (PEO) nKCu xI 1 + x complexes

We introduce a new family of solid polymer electrolytes where the Cu + cation is coordinated with an anionic ligand I −, resulting in a complex carrying at least one negative charge. In the PEO n KCu x I 1 + x electrolyte, potassium iodide acts as a supporting electrolyte where copper cations Cu + are carried as CuI − 2 complex anions. The transport of Cu + thus occurs via a vehicular mechanism. The conductivity of these electrolytes is around 5 × 10 −6 Ω −1 cm − at 40°C and the transference number of copper component has been found to be 0.4.