Molecular dynamics and kinetic Monte Carlo simulations of protonation dynamics in fuel cell membrane materials and membrane proteins

• Applicant: Professor Dr. Daniel Sebastiani
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation ; Theoretical Chemistry: Molecules, Materials, Surfaces
• Term: from 2009 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 122839674

• Final Report Year: 2019

Final Report Abstract

The central objective of this project was the development, implementation and benchmarking of a scale-bridging stochastic scheme for the efficient simulation of ion diffusion in condensed phase compounds. The natural subsequent objective was the application of this scheme to fuel cell membrane materials. The unique feature of this combined approach developed is that the combination of MD and kinetic Monte Carlo allows to calculate the proton diffusion on time scales only accessible to kinetic Monte Carlo while still taking into account the accurate temporal evolution of the proton pathways as determined by molecular dynamics. In the first part of the project, the focus lay on the development of the cMD/LMC scheme with the aim to simulate proton transfer on extended time scales while still maintaining structural and dynamical accuracy as provided by ab initio simulations. In order to capture the dynamical features of the proton hopping mechanism, a parameterized proton jump rate function was determined and fitted to ab initio data. In combination with trajectories of the molecular structure, a time-dependent proton transfer rate matrix could be determined, which was used as an input to the Lattice Monte Carlo scheme. The predictive accuracy of the cMD/LMC scheme with regard to proton dynamics was examined using the example of proton diffusion in polymers. The second part represents a refinement of the cMD/LMC method in view of enhancing the flexibility of the Monte-Carlo transfer rate topology in order to achieve a better transferability of the approach to different chemical systems. The improved generality and accuracy of the cMD/LMC scheme is demonstrated on the example of a liquid crystalline proton conductor (p-6PA-HPB ) and a crystalline solid acid compound (CsH2 PO4 ). The last part of the project constitutes a proof of concept for the applicability of the cMD/LMC in highly fluctuating systems. On the example of an excess proton in water, the cMD/LMC scheme is further enhanced to realistically describe the complex proton transfer mechanisms caused by the strongly fluctuating hydrogen bonds and the dielectrical relaxation effects in water. The enhancement consists of two parts: firstly, the LMC scheme needs to be able to mimic the shortening of the hydrogen bonds caused by the electrostatic effects between the positively charged H3 O+ and its first solvation shell. Secondly, the time scale of this dielectrical relaxation needs to be accounted for in order to realistically model the response of the hydrogen network after an excess charge transfer. The calculations here show that both effects are represented quantitatively. In conclusion, the cMD/LMC scheme has been shown to be an effective tool for the simulation of proton diffusion on mesoscopic time scales. The combination of molecular dynamics with a kinetic Monte Carlo scheme allows accurate calculations with low computational effort. This makes it an attractive tool for postprocessing of existing ab initiotrajectories in cases where the heavy atom structure has converged, but the motion of the light hydrogen atoms has not been sufficiently sampled yet.

Publications

• A Coupled Molecular Dynamics/Kinetic Monte Carlo Approach for Protonation Dynamics in Extended Systems. J. Chem. Theory. Comput. 2014, 10, 4221–4228
  Kabbe G., Wehmeyer C. and Sebastiani D.
  
• Insight from Atomistic Simulations of Protonation Dynamics at the Nanoscale. Fuel Cells 2016, 16, 682–694
  Dreßler C., Kabbe G. and Sebastiani, D.
  
• Proton Conductivity in Hydrogen Phosphate/Sulfates from a Coupled Molecular Dynamics/Lattice Monte Carlo (cMD/LMC) Approach. J. Phys. Chem. C 2016, 120, 19913–19922
  Dreßler C., Kabbe G. and Sebastiani, D.
  
• Toward Realistic Transfer Rates within the Coupled Molecular Dynamics/Lattice Monte Carlo Approach. J. Phys. Chem. C 2016, 120, 19905–19912
  Kabbe G., Dreßler C. and Sebastiani D.
  
• Proton Mobility in Aqueous Systems: Combining ab initio Accuracy with Millisecond Timescales. Phys Chem Chem Phys 2017, 19, 28604–28609
  Kabbe G., Dreßler C. and Sebastiani, D.
  
• Development of a Coupled Molecular Dynamics / Lattice Monte Carlo Scheme. PhD thesis, MLU Halle, 2018
  G. Kabbe