AB-INITIO COMPUTATIONAL STUDY OF HYDRATION AND PROTON TRANSFER PROCESSES ON SULFONATED AND PHOSPHORYLATED NATA DE COCO AS POLYMER ELECTROLYTE MEMBRANE

• Main Author: RAHMAWATI (NIM:30513003), SITTI
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/31013
• Institution: Institut Teknologi Bandung

Polymer Electrolyte Membrane Fuel Cells (PEMFCs) and Direct Methanol Fuel Cells (DMFCs) are two of the most promising alternative energy resources to meet human energy needs. PEMFCs and DMFCs are fuel cells that use polymer membranes as the electrolytes to transfer the protons from anode to cathode. The polymer membranes used are commercial membranes, Nafion®, that have various advantages such as good ability to conduct proton transfer, but relatively expensive. Therefore, it is necessary to study some alternative materials. Several studies that have been done by previous groups used bacterial cellulose (nata de coco) as the base material for fuel cell membranes. Nata de coco membrane modification with sulphate and phosphate groups showed an increase in proton conductivity up to the order of 10-2 S/cm, which is slightly lower than Nafion®. 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. <br /> <br /> To examine the transfer of protons in the membrane, studies of computing via ab initio 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) and phosphorylated nata de coco membranes (NDCP) 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 both 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 were calculated by adding n water molecules (n=1,2, …,10) to determine the hydration process and the proton transfer on both membranes. Furthermore, NBO analysis was performed on both membranes, to determine the contribution of electrostatic forces, polarization and charge transfer to the strength of intra and intermolecular interactions. <br /> <br /> Computational calculations on the structure of the polymer requires structural modeling. The calculations showed that for interactions with water, the polymer structure in NDCS and NDCP can be represented/modeled by two repeating units. Therefore, the hydration process and transfer of protons in both 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, while in NDCP membrane four water molecules. 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 dissociation process indicated that proton conductivity in NDCS was better than in NDCP membrane. <br /> <br /> The energy profile of proton transfer showed that the barrier energy was 58.13 kcal/mol for NDCS-5(H2O) and 138.60 kcal /mol for NDCP-5 (H2O). Proton transfer in NDCS-5(H2O) produced a lower energy barrier than NDCP-5(H2O). Similar to its thermodynamic parameters, the calculation showed that the interaction energy (&#916;E), the enthalpy change (&#916;H) and the Gibbs free energy (&#916;G) to its interaction with n water molecules (n=1,2,…,10) in NDCS was more negative than in NDCP. This indicated that the ability of proton transfer in NDCS was better than in the NDCP membrane <br /> <br /> Based on the distance and angle between the donor and acceptor of protons, the hydrogen bonds on the NDCS-5(H2O) interaction was stronger than the NDCP-5(H2O) interaction. Similarly, the stabilization energy in NDCS-5(H2O) was much larger (100.92 kcal/mol) than that obtained in the interaction in NDCP-5(H2O), (44.66 kcal/mol). It showed that hydrogen bonds among the sulfonate groups and water molecules were stronger than the ones among the phosphonate groups and water. Consequently, to the transfer of protons were easier among the sulfonate groups. <br /> <br /> So, based on these results, it can be concluded that the computational ab initio 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. The results indicated that the proton conductivity of NDCS membrane was higher than of NDCP membrane.