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

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Abstract
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)

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