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45666-AC5
Proton Transfer in Phosphoric Acid Based Fuel Cell Membranes

Joel Robert Fried, University of Cincinnati

The ultimate goal of this project is to investigate the mechanism of proton transport in a typical fuel cell proton exchange membrane by use of a hybrid simulation method using Atom-centered Density Matrix Propagation (ADMP) and molecular dynamics implemented in ONIOM. The focus of this work is phosphoric-acid (PA) doped polybenzimidazole (PBI). In this initial year of the project, the focus has been phosphoric acid. In year two, methodology developed in this initial aspect of the project will be used in the study of the PA-doped PBI system. The first effort focused on determining the appropriate DFT functional and basis set to use in ADMP calculation. Guiding this choice was the accuracy of predicting geometry, dipole moment, charge distribution, proton affinity, and the barrier for proton transfer in PA. Results indicated that X3LYP/6-311+G(d,p) provided the best trade-off between computational cost and its accuracy in modeling PT events in PA systems as compared to MP2/aug-cc-pVTZ calculations; however, X3LYP/6-31+G(d,p) was considered to be an adequate choice to reduce computational cost. At the MP2/aug-cc-pVTZ level of theory, the proton affinity of PA was determined to be 196 kcal/mol with a dipole moment of 0.461 D. The COMPASS force field was used for molecular dynamics simulation of PA. COMPASS provided very good results for density, diffusion coefficients, and radial distribution functions for PA and PA/water systems. These results will be compared to the use of the DREIDING and Universal force fields available for ONIOM calculations. Proton transfer in PA was investigated in a PA consisting of a five molecule cluster using X3LYP/6-31+G(d,p). Results at 450 K indicated that one proton transfer event occurred every 100 fs.

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