Hydration and Proton Transfer Processes in Sulfonated Nata De Coco Membrane with Density Functional Theory

Direct Methanol Fuel Cells (DMFCs) is one of the most promising alternative energy resources to meet human energy needs. DMFCs is fuel cells that use polymer membranes as the electrolytes to transfer the protons from anode to cathode. The characteristics of those two types of membranes in ion exchange capacity (IEC) and degree of swelling (swelling) have shown a very important role of water in the proton transfer. However, the mechanism of interaction between the repeating units of the polymer with water molecules has not been studied in depth. Computational methods can be used to study such interactions as well as the transfer of protons. To examine the transfer of protons in the membrane, studies of computing via electronic structure calculations, geometry optimization, interaction inter/intra molecular, as well as the hydration process and transfer of protons in the sulfonated nata-de-coco membranes (NDCS) has been conducted in this work. All calculations were performed using DFT with B3LYP functional and basis set 6-311G(d). The repeating units of the membranes were optimized (n=1,2,...,5), to obtain the structure with minimum energy. The optimized structure was then interacted with one water molecule in the same position to study the effect of chain length on its interaction strength with water molecules. The thermodynamic and proton dissociation parameters was calculated by adding n water molecules (n=1,2, …,10) to determine the hydration process and the proton transfer on the membranes. The calculations showed that for interactions with water, the polymer structure in NDCS can be represented/modeled by two repeating units. Therefore, the hydration process and transfer of protons in the membranes were studied by adding n water molecules gradually into the two repeating units. The results showed that the proton dissociation process in NDCS membrane started with the addition of two molecules of water. The presence of water molecules promoted the proton dissociation in the -SO3H groups to form SO3- and H3O+ ions, which further formed Zundel ions and Eigen ions. The energy profile of proton transfer showed that the barrier energy was 58.13 kcal/mol for NDCS-5(H2O). Its thermodynamic parameters, the calculation showed that the interaction energy (ΔE), the enthalpy change (ΔH) and the Gibbs free energy (ΔG) to its interaction with n water molecules (n=1,2,…,10) in NDCS are getting more negative. This indicated that the interaction with water molecule is stronger. So, based on these results, it can be concluded that the computational calculations using DFT method at B3LYP functional and 6-311G(d) basis set can be used to describe the process of hydration and proton transfer in the interactions in the polymer electrolyte membrane (NDCS membrane)

Download Full-text

Nanofludic Analysis on Methanol Crossover of Direct Methanol Fuel Cells

ASME 2008 First International Conference on Micro/Nanoscale Heat Transfer, Parts A and B ◽

10.1115/mnht2008-52095 ◽

2008 ◽

Author(s):

Peng-Yu Chen ◽

Wei-Hui Chen ◽

Che-Wun Hong

Keyword(s):

Fuel Cells ◽

Sulfonic Acid ◽

Direct Methanol Fuel Cells ◽

Simulation System ◽

Methanol Concentration ◽

Water Molecules ◽

Methanol Crossover ◽

Direct Methanol ◽

Methanol Fuel Cells ◽

Sulfonic Acid Groups

Direct methanol fuel cells (DMFCs) are considered as a competitive power source candidate for portable electronic devices. Nafion® has been widely used for the electrolyte of DMFCs because of its good proton conductivity and high chemical and mechanical stability. However, the major problem that must be solved before commercialization is the high methanol crossover through the membrane. There are a number of studies on experiments about the methanol crossover rate through the membrane but only few theoretical investigations have been presented [1–3]. In this paper, an atomistic model [4] is presented to analyze the molecular structure of the electrolyte and dynamic properties of nanofluids at different methanol concentration. In the same time, the nano-scopic phenomenon of methanol crossover through the membrane is observed. The simulation system consists of the Nafion fragments, hydronium ions, water clusters and methanol molecules. Fig. 1 shows the simplified Nafion fragment in our simulation. Both intra- and inter-molecular interactions were involved in this study. Intermolecular interactions include the van der Waals and the electrostatic potentials. Intramolecular interactions consist of bond, angle and dihedral potentials. The force constants used above were determined from the DREIDING force field. The SPC/E model was employed for water molecules. The three-site OPLS potential model was utilized for the intermolecular potential in methanol. Each proton which migrates inside the electrolyte is assumed to combine with one water molecule to form the hydronium (H3O+). The force parameters for the hydronium were taken from Burykin et al [5]. The atomistic simulation was carried out on the software DLPOLY. First, a 500 ps NPT ensemble was performed to make the system reach a proper configuration. This step was followed by another 500 ps NVT simulation. All molecular simulations were performed at a temperature of 323K with three-dimensional periodic boundary conditions. The intermolecular interactions were truncated at 10 Å and the equations of motion were solved using the Verlet scheme with a time step of 1 fs. Fig. 2 shows the calculated density of the simulation system for different methanol concentrations at 323K. It can be seen that the density decreases with the methanol uptakes. The volume of the system increases as the methanol concentration increases, which means that the membrane swelling with methanol uptakes. The radial distribution functions (RDFs) of the ether-like oxygen (O2) toward water and methanol molecules for different methanol concentrations at 323K are shown in Fig. 3. From this figure, we find that methanol molecules can reside in the vicinity of the hydrophobic part of the side chain while water can not. Fig. 4 shows the RDFs between the oxygen atom of the sulfonic acid groups (O3) and solvents for different methanol concentrations at 323K. As shown in Fig. 4, both water and methanol have a tendency to cluster near the sulfonic acid groups, but water molecules prefer to associate with the sulfonic acid groups in comparison with methanol molecules. The mean square displacements (MSDs) of water and methanol molecules for different methanol concentrations at 323K are displayed in Fig. 5. It is shown that MSD curves have a linear tendency, which means both water and methanol molecules are diffusing in the system during the simulation. As the methanol concentration increases, the slope of MSD curve increases for methanol and decreases for water. This indicates higher methanol content constrains the mobility of water molecules but enhances the mobility of methanol molecules that cross the electrolyte. In summary, molecular simulations of the Nafion membrane swollen in different methanol concentrations (0, 11.23, 21.40, 46.92 wt%) at 323K have been carried out. Both methanol migration mechanism and hydronium diffusion phenomenon have been visualized by monitoring the trajectories of the specific species in the system. MSDs are used to evaluate the mobility and shows that the higher the methanol concentration, the greater the tendency of methanol crossover.

Download Full-text

Hydration and Proton Transfer in 3M™ PEM Ionomers: An Ab Initio Study

MRS Proceedings ◽

10.1557/opl.2012.187 ◽

2012 ◽

Vol 1384 ◽

Cited By ~ 1

Author(s):

Jeffrey K. Clark ◽

Stephen J. Paddison

Keyword(s):

Hydrogen Bond ◽

Proton Transfer ◽

The State ◽

Side Chain ◽

Water Molecules ◽

Side Chains ◽

Hydrogen Bond Network ◽

Group Separation ◽

Proton Dissociation ◽

Bond Network

ABSTRACTElectronic structure calculations were performed to study the effects local hydration, neighboring side chain connectivity, and protogenic group separation have in facilitating proton dissociation and transfer in fragments of 3M ionomers under conditions of low hydration. Two different types of ionomers, each consisting of a poly(tetrafluoroethylene) (PTFE) backbone, were considered: (1) perfluorosulfonic acid (PFSA) ionomeric fragments containing two pendant side chains (–O(CF2)4SO3H) of distinct separation along the PTFE backbone to model different equivalent weight ionomers and (2) single side chain fragments of three bis(sulfonyl imide)- based fragments with multiple and distinct acid groups per side chain having structural and chemical differences mediating protogenic group separation (side chains: –O(CF2)4SO2(NH)- SO2C6H4SO3H) with the sulfonic acid group located in either the meta or the ortho position on the phenyl ring and –O(CF2)4SO2(NH)SO2(CF2)3SO3H). Fully optimized structures of these fragments with and without the addition of water molecules at the B3LYP/6-311G** level revealed that both side chain connectivity and protogenic group separation, along with local hydration, are key contributors to proton dissociation and the energetics of proton transfer in these materials. Specifically, cooperative interaction between protogenic groups through hydrogen bonding and electron withdrawing –CF2– groups are critical for first proton dissociation and the state of the dissociated proton at low levels of hydration. However, the close proximity of protogenic groups in the ortho bis acid precluded second proton dissociation at low hydration as the relatively fixed protogenic group separation promoted interactions between water molecules, while the labile side chains in the PFSA ionomers allowed for greater freedom in the hydrogen bond network formed. Potential energy profiles for proton transfer were determined at the B3LYP/6-31G** level. The energetic penalty associated with proton transfer was found to be strongly dependent on the surrounding hydrogen bond network and the state of the dissociated proton(s), as well as, the separation between protogenic groups.

Download Full-text

B3LYP Study on Reduction Mechanisms from to at the Catalytic Sites of Fully Reduced and Mixed-Valence Bovine Cytochrome Oxidases

Bioinorganic Chemistry and Applications ◽

10.1155/2010/182804 ◽

2010 ◽

Vol 2010 ◽

pp. 1-18 ◽

Cited By ~ 4

Author(s):

Yasunori Yoshioka ◽

Masaki Mitani

Keyword(s):

Water Molecule ◽

Proton Transfer ◽

Mixed Valence ◽

Water Molecules ◽

Cytochrome Oxidases ◽

Reduction Mechanisms ◽

Catalytic Sites ◽

Added Water ◽

B3lyp Calculations ◽

Network Of Hydrogen Bonds

Reduction mechanisms of oxygen molecule to water molecules in the fully reduced (FR) and mixed-valence (MV) bovine cytochrome oxidases () have been systematically examined based on the B3LYP calculations. The catalytic cycle using four electrons and four protons has been also shown consistently. The MV catalyses reduction to produce one water molecule, while the FR catalyses to produce two water molecules. One water molecule is added into vacant space between His240 and His290 in the catalytic site. This water molecule constructs the network of hydrogen bonds of Tyr244, farnesyl ethyl, and Thr316 that is a terminal residue of the K-pathway. It plays crucial roles for the proton transfer to the dioxygen to produce the water molecules in both MV and FR . Tyr244 functions as a relay of the proton transfer from the K-pathway to the added water molecule, not as donors of a proton and an electron to the dioxygen. The reduction mechanisms of MV and FR are strictly distinguished. In the FR , the Cu atom at the site maintains the reduced state Cu(I) during the process of formation of first water molecule and plays an electron storage. At the final stage of formation of first water molecule, the Cu(I) atom releases an electron to Fe-O. During the process of formation of second water molecule, the Cu atom maintains the oxidized state Cu(II). In contrast with experimental proposals, the K-pathway functions for formation of first water molecule, while the D-pathway functions for second water molecule. The intermediates, , , F, and O, obtained in this work are compared with those proposed experimentally.

Download Full-text