High-Resolution Coarse-Grained Model of Hydrated Anion-Exchange Membranes that Accounts for Hydrophobic and Ionic Interactions through Short-Ranged Potentials

Molecular
simulations provide a versatile tool to study the structure,
anion conductivity, and stability of anion-exchange membrane (AEM)
materials and can provide a fundamental understanding of the relation
between structure and property of membranes that is key for their
use in fuel cells and other applications. The quest for large spatial
and temporal scales required to model the multiscale structure and
transport processes in the polymer electrolyte membranes, however,
cannot be met with fully atomistic models, and the available coarse-grained
(CG) models suffer from several challenges associated with their low-resolution.
Here, we develop a high-resolution CG force field for hydrated polyphenylene
oxide/trimethylamine chloride (PPO/TMACl) membranes compatible with
the mW water model using a hierarchical parametrization approach based
on Uncertainty Quantification and reference atomistic simulations
modeled with the Generalized Amber Force Field (GAFF) and TIP4P/2005
water. The parametrization weighs multiple properties, including coordination
numbers, radial distribution functions (RDFs), self-diffusion coefficients
of water and ions, relative vapor pressure of water in the solution,
hydration enthalpy of the tetramethylammonium chloride (TMACl) salt,
and cohesive energy of its aqueous solutions. We analyze the interdependence
between properties and address how to compromise between the accuracies
of the properties to achieve an overall best representability. Our
optimized CG model FFcomp quantitatively reproduces the
diffusivities and RDFs of the reference atomistic model and qualitatively
reproduces the experimental relative vapor pressure of water in solutions
of tetramethylammonium chloride. These properties are of utmost relevance
for the design and operation of fuel cell membranes. To our knowledge,
this is the first CG model that includes explicitly each water and
ion and accounts for hydrophobic, ionic, and intramolecular interactions
explicitly parametrized to reproduce multiple properties of interest
for hydrated polyelectrolyte membranes. The CG model of hydrated PPO/TMACl
water is about 100 times faster than the reference atomistic GAFF-TIP4P/2005
model. The strategy implemented here can be used in the parametrization
of CG models for other substances, such as biomolecular systems and
membranes for desalination, water purification, and redox flow batteries.
We anticipate that the large spatial and temporal simulations made
possible by the CG model will advance the quest for anion-exchange
membranes with improved transport and mechanical properties.

分子模拟（Molecular simulations）是研究阴离子交换膜（anion-exchange membrane, AEM）材料的结构、阴离子电导率与稳定性的通用工具，可从基础层面阐明膜的构效关系，而该构效关系正是其应用于燃料电池及其他场景的核心依据。然而，针对聚合物电解质膜多尺度结构与输运过程建模所需的大空间与时间尺度，全原子模型无法满足；现有粗粒化（coarse-grained, CG）模型则受限于低分辨率，存在诸多挑战。本文开发了一种适配mW水模型的高分辨率粗粒化力场，用于水合聚苯醚/三甲基氯化铵（polyphenylene oxide/trimethylamine chloride, PPO/TMACl）膜的建模，其参数化流程基于不确定性量化（Uncertainty Quantification）与参考原子模拟，参考原子模拟采用通用AMBER力场（Generalized Amber Force Field, GAFF）与TIP4P/2005水模型构建。该参数化过程兼顾多项物性，包括配位数、径向分布函数（radial distribution functions, RDFs）、水与离子的自扩散系数、溶液中水的相对蒸气压、四甲基氯化铵（tetramethylammonium chloride, TMACl）盐的水化焓，以及其水溶液的内聚能。我们分析了各物性间的相互关联，并探讨了如何在不同物性的精度间进行权衡，以实现整体最优的模型表征能力。优化后的粗粒化力场FFcomp可定量复现参考原子模型的扩散系数与径向分布函数，并定性复现四甲基氯化铵溶液中水的实验相对蒸气压。上述物性对于燃料电池膜的设计与运行具有至关重要的意义。据我们所知，这是首个显式包含每个水分子与离子，并针对水合聚电解质膜的多项目标物性显式参数化疏水相互作用、离子相互作用与分子内相互作用的粗粒化模型。该水合PPO/TMACl粗粒化模型的计算速度约为参考原子级GAFF-TIP4P/2005模型的100倍。本文所采用的参数化策略，亦可推广至其他体系的粗粒化模型构建，例如生物分子系统、用于海水淡化、水净化及氧化还原液流电池的膜材料。我们预期，该粗粒化模型所支持的大空间与时间尺度模拟，将推动开发具备更优异输运与力学性能的阴离子交换膜的研究进程。