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Fuel cell membrane

Polybenzimidazole and phosphoric acid based membrane

Presently we are looking at Polybenzimidazole (PBI) and Phosphoric acid (PA) based fuel cell membrane in atomistic length scale. In this case we are trying to understand the mechanism of proton conductance and the participation of PBI in that from ab-initio quantum chemical calculation. Simultaneously we are also working on the classical molecular dynamics simulation of the same system to elucidate the formation of hydrogen bond network among PA and PBI molecules. A snapshot of such a system is depicted in the following figure. 

Polymer electrolyte membrane fuel cell (PEMFC) can generate power with high efficiency and minimal pollution and have various applications in aerospace, automobiles, portable devices, transportation etc. Phosphoric acid (PA) doped polybenzimidazole (PBI) system is one of the promising polyelectrolyte membrane for high temperature (>1000C) PEMFC. The main function of the membrane is to conduct proton from cathode to anode in a PEMFC. The efficiency of the fuel cell is governed by the proton conductivity of the membrane. PBI has been experimentaly observed to have high proton conductivity after doping with PA. To get the structural insight and dynamical properties of the membrane, we have performed classical molecular dynamics simulation of the monomer (BI) of the PBI mixed with varied amount of PA. From the structural analysis, the arrangement of PA near BI and different types of inter and intramolecular H-bonds in the system have been predicted. The arrangement of BI among themselves have been examined and how it changes with varying amount of PA which gives us the favourable arrangement for efficient proton conduction. As a dynamic quantity, imidazole ring flipping and diffusion of BI and PA have been calculated that shows initial drop and further increase in self-diffusion constant of both BI and PA with gradual increase in PA concentration. The origin of such anomalous diffusion has been identified. This is may be due to three different types of H-bonding arrangement of PA near BI and H-bonding between PA molecules. The stability of the H-bonding network consisting of these different types of H-bonds between BI and PA has been calculated. Dynamic heterogeneity has been observed in the BI-PA system. We characterized the microscopic picture of dynamic heterogeneity by examining whether there is any correlation between dynamical heterogeneity and structural arrangement of the components in BI-PA system. Fast and slow PA molecules in the system have been observed due to coexistance of different types of H-bonds.

For further details, please refer:
1.S. Pahari, C.K. Choudhury, P. R. Pandey,  M. More,  A. Venkatnathan,  S. Roy*, Molecular Dynamics Simulation of Phosphoric Acid Doped Monomer of Polybenzimidazole: A Potential Component of Polymer Electrolyte Membrane of fuel cell,  The Journal of Physical Chemistry B 2012 116 (24), 7357-7366

2. M. More, S. Pahari, S. Roy*,  A Venkatnathan,  Characterization of structure and dynamics of phosphoric acid doped benzimidazole mixtures: A molecular dynamics study

3. S. Pahari, and S. Roy*, Evidence and characterization of  binary mixtures of phosphoric acid and benzimidazole, J. Chem. Phys. 139, 154701 (2013)

Morphology of block co-polymer based membrane 

Self-assembly process, morphology of block co-polymers etc are large length (micrometer) and long time (microsecond) scale phenomena.  To simulate such properties we apply dissipative particle dynamics (DPD) simulation method. The interaction potentials we use for it is only repulsive and related to solubility between different blocks of the systems. The solubility constants are calculated from all atomistic molecular dynamics simulations. So we connect different methods in different scales via parameter transfer. In this way we have used multiscale molecular modeling method for the simulation of phosphonic acid based fuel cell polymer electrolyte membrane. Phosphonic acid based membranes are block co-polymers where phosphonic acid functionalized blocks are responsible for proton transport in fuel cell. We have calculated the mechanism of proton transport in such system and showed that protons are transferred via hydrogen bond network formed by the phosphonic acid blocks. The morphologies of such block copolymers, which are dependent on the composition and topology of different blocks of the copolymer, are calculated by DPD method. The morphology of grafted block copolymer consists of polystyrene backbone and heptylphosphonic acid side-chain looks like the following figure.