CDS&E: Organization and Dynamics of Charged Molecules in Heterogeneous Media

Northwestern University, Evanston IL

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports computational and theoretical research, and education on polymer electrolytes. Polymer electrolytes are long chain-like molecules that contain charged units. They are emerging materials for the production and the storage of clean energy in devices such as fuel cell electric vehicles that emit water as exhaust and vehicles powered by rechargeable lithium-ion batteries. Polymer electrolytes are ideal candidates because they are flexible, lightweight, recyclable, and inexpensive. However, polymer fuel cells and batteries have low efficiencies. In order to optimize their efficiency, it is crucial to understand their structure, and how electric charges, or ions, move across the polymer electrolyte material that lies between the terminals and inside these devices. In this project the PI aims to advance fundamental understanding of the complex interplay among different polymer electrolyte phases and the distribution of ions and their motion inside polymer electrolyte materials to enable the design of a nanoscale 'highway system' for ions to travel inside higher performance batteries and fuel cells. The formation of molecular-scale structures called nanostructures within the material guides how ions move. Predictive theoretical and computational tools that can describe and explain the complex behavior of polyelectrolytes could help optimize transport of ions through materials to increase efficiency of energy storage devices. The PI will build on a recently developed approach to describe the effect of inhomogeneities in the polymer electrolyte materials have on the ion motion and on the correlations in the motions of the ions as they move through the materials. The PI aims to develop a self-consistent description that takes into account how the complex distribution of ions and their motions affect the polymer electrolyte material and how the structure of the polymer electrolyte affects the distribution of ions and their motions. Of particular interest is the use of molecular dynamics and other computational methods to analyze ionic transport of charged units confined by interfaces with different materials and their associated electric charges, an important step toward a capability to design nanoscale 'highway systems' for ions to travel inside high performance batteries and fuel cells. Computational tools developed through this project to address problems that arise in materials design have more general application, for example in biological systems and in industrial processes. These tools will enable advances in understanding how molecular units that carry electric charge are confined in small regions as a consequence of an environment made of different materials with different electronic properties. This will help stimulate innovative solutions for manipulating and designing materials for energy storage, as well as nanofluidic devices. Among the impacts derived from this project will be a set of studies compiled into publications and open access programs to assist researchers working in related theoretical and computational soft matter problems. TECHNICAL SUMMARY This award supports computational and theoretical research, and education on polyelectrolyte blends and copolymers which have been identified as suitable candidate materials for use in high-density energy storage and generation devices. They combine the low volatility and high flexibility of polymers with ion-selective conductivity of the charge-carrying backbone. In polyelectrolyte blends and in neutral-charged copolymer melts, ionic correlations can significantly reduce miscibility, inducing phase separation into nanophases with different concentrations and ordering of ions given that the dielectric constant is relatively low in these materials. Using a hybrid of self-consistent field and liquid state theories, the PI will investigate the nanophase structures formed by copolymer electrolytes, their interfacial properties and ion conductivity. The effect of ionic correlations in polyelectrolyte copolymer melts determines the structure at multiple length scales. The ion distribution depends on the dielectric properties of the media and on the dielectric heterogeneities that developed due to ionic correlations and to the degree of miscibility of the components. The PI will develop models to account for these effects self-consistently. Nanophase segregated structures as well as nanochannels where one dimension is comparable to the Debye length, possess an electrostatic potential that can be significantly modulated by the soft ionic structure inside the channel and by the dielectric heterogeneity at the interface. The ionic concentrations and structure of the nanochannels affect the mechanical and transport properties dramatically. Present studies mainly deal with simple symmetric monovalent electrolytes in aqueous solutions because it is a simple physical system that can be understood by Poisson Boltzmann theory. However, real applications may involve multivalent ion species as well as dielectric interfaces of materials with low dielectric constants. It is therefore crucial to understand the effect of correlations in the structure of charged-neutral copolymer melts, to determine how ions transport through nanochannels. Important effects to determine include the nanostructure symmetry and periodicity which is strongly dependent on ion sizes, molecular weight, copolymer composition and charge distribution along the chains. The PI will implement MD simulations that include ion correlations, finite size of molecules and dielectric heterogeneities to determine the structure of polyelectrolyte copolymer melts and ion conductivities. These simulations, which are based on a true energy functional, are versatile enough to treat the case of multiple and curved interfaces, multivalent salts, and asymmetric ion sizes to study the dynamical evolution of the soft ionic structure. Computational tools developed through this project to address problems that arise in materials design have more general application, for example in biological systems and in industrial processes. These tools will enable advances in understanding of charged soft matter and polymeric materials and will help stimulate innovative solutions for manipulating and designing materials for energy storage, soft electronics, nanofluidic devices, and other application areas. Among the impacts derived from this project will be a set of studies compiled into publications and open access programs to assist researchers working in related theoretical and computational soft matter problems.