Li⁺ Transport in Ethylene Carbonate Based Comb-Branched Solid Polymer Electrolyte: A Molecular Dynamics Simulation Study

Solid polymer electrolytes (SPEs) have the potential to resolve safety issues, be compatible with high-voltage cathode materials, and allow flexible designs of Li-ion batteries. Due to the limited Li+ transference number, a high degree of crystallinity at room temperature, and instability toward oxidation, polyether-based SPEs have been limited in batteries with the high-voltage cathodes and Li-metal anodes. Low ionic conductivity remains one of the biggest challenges for all types of SPE. Furthermore, the understanding of Li+ transport mechanisms and the related correlations with polymer structure are limited. In this study, extensive atomistic molecular dynamics simulations employing polarizable force field were conducted for a series of poly­(alkyl ethylene carbonate) comb-branched architectures doped with lithium bis­(trifluoromethane)­sulfonimide salt. By studying systems with systematic variance in the polymer structure, the Li+ transport mechanisms have been investigated through structural and dynamical correlations of cation local environments. The molecular-scale insights into the Li+ transport allow proposing principles for the design of comb-branched SPEs with improved conductivity

Binding of Perfluorooctanoate to Poly(ethylene oxide)

To inform the design of polymer-based adsorbent materials for sequestration of per- and polyfluoroalkyl substances (PFAS) from aqueous solution, we report here on the critical aggregation concentration (CAC), shape, size, composition, and interactions of assemblies formed in water between perfluorooctanoic acid ammonium salt (PFOA) and the nonionic polymer poly­(ethylene oxide) (PEO), obtained from complementary experiments (conductivity, surface tension, pyrene fluorescence, viscosity, and small-angle neutron scattering (SANS)) and atomistic molecular dynamics (MD) simulations. PEO–PFAS binding commences at concentrations lower than the PFOA critical micelle concentration (CMC) and is driven by PEO localizing on the micelle surface and shielding the fluorocarbon parts of PFOA from contact with water. PFOA + PEO mixed micelles have a 10% higher association number and are 40% more elongated compared to polymer-free PFOA micelles. This is the first investigation on the structure of polymer + fluorocarbon surfactant mixed micelles and contributes fundamental insights into the association of water-soluble polymers with PFAS surfactants

Adsorption Mechanism of Perfluorooctanoate on Cyclodextrin-Based Polymers: Probing the Synergy of Electrostatic and Hydrophobic Interactions with Molecular Dynamics Simulations

Contamination of natural water resources by per- and polyfluorinated alkyl substances (PFAS) has affected millions of people around the world and emphasized the need for development of novel and effective adsorbent materials. We demonstrate how atomistic molecular dynamics (MD) simulations can be used to provide molecular scale insight into the role of electrostatic and hydrophobic interactions on the adsorption of the perfluorooctanoate (PFOA) surfactant, a prominent longer-chain PFAS, on a polymer-based network in water. Specifically, the adsorption of ammonium perfluorooctanoate salt has been investigated on the β-cyclodextrin (CD) network cross-linked with decafluorobiphenyl linkers as an example of an absorbent material that has already demonstrated efficient PFAS adsorption. Examination of pairwise interactions reveals the importance of the dual pronged adsorption mechanism involving both electrostatic and hydrophobic interactions. The adsorption of ammonium counterions on the CD segments facilitates attraction of the anionic headgroup of the PFOA surfactant, while fluorinated linkers provide an additional hydrophobic attraction for the PFOA tail as well as higher affinity of the network toward PFOA in comparison with hydrocarbons. These competing interactions result in PFOA adsorption primarily outside of the CD cavity with the PFOA tail mostly interacting with fluorinated linkers. We demonstrate that simulations using “what if” scenarios are a powerful approach to infer the role of different interactions in the adsorption of PFAS

Supramolecular Self-Assembly of Methylated Rotaxanes for Solid Polymer Electrolyte Application

Li⁺-conducting solid polymer electrolytes (SPEs) obtained from supramolecular self-assembly of trimethylated cyclodextrin (TMCD), poly­(ethylene oxide) (PEO), and lithium salt are investigated for application in lithium-metal batteries (LMBs) and lithium-ion batteries (LIBs). The considered electrolytes comprise nanochannels for fast lithium-ion transport formed by CD threaded on PEO chains. It is demonstrated that tailored modification of CD beneficially influences the structure and transport properties of solid polymer electrolytes, thereby enabling their application in LMBs. Molecular dynamics (MD) simulation and experimental data reveal that modification of CDs shifts the steady state between lithium ions inside and outside the channels, in this way improving the achievable ionic conductivity. Notably, the designed SPEs facilitated galvanostatic cycling in LMBs at fast charging and discharging rates for more than 200 cycles and high Coulombic efficiency

Molecular Design of Functional Polymers for Silica Scale Inhibition

Silica polymerization, which involves the condensation reaction of silicic acid, is a fundamental process with wide-ranging implications in biological systems, material synthesis, and scale formation. The formation of a silica-based scale poses significant technological challenges to energy-efficient operations in various industrial processes, including heat exchangers and water treatment membranes. Despite the common strategy of applying functional polymers for inhibiting silica polymerization, the underlying mechanisms of inhibition remain elusive. In this study, we synthesized a series of nitrogen-containing polymers as silica inhibitors and elucidated the role of their molecular structures in stabilizing silicic acids. Polymers with both charged amine and uncharged amide groups in their backbones exhibit superior inhibition performance, retaining up to 430 ppm of reactive silica intact for 8 h under neutral pH conditions. In contrast, monomers of these amine/amide-containing polymers as well as polymers containing only amine or amide functionalities present insignificant inhibition. Molecular dynamics simulations reveal strong binding between the deprotonated silicic acid and a polymer when the amine groups in the polymer are protonated. Notably, an extended chain conformation of the polymer is crucial to prevent proximity between the interacting monomeric silica species, thereby facilitating effective silica inhibition. Furthermore, the hydrophobic nature of alkyl segments in polymer chains disrupts the hydration shell around the polymer, resulting in enhanced binding with ionized silicic acid precursors compared to monomers. Our findings provide novel mechanistic insights into the stabilization of silicic acids with functional polymers, highlighting the molecular design principles of effective inhibitors for silica polymerization

Aqueous Electrolytes Reinforced by Mg and Ca Ions for Highly Reversible Fe Metal Batteries

Iron (Fe) metal batteries, such as Fe-ion batteries and all Fe flow batteries, are promising energy storage technologies for grid applications due to the extremely low cost of Fe and Fe salts. Nonetheless, the cycle life of Fe metal batteries is poor primarily due to the low Coulombic efficiency of the Fe deposition/stripping reaction. Current aqueous electrolytes based on Fe chloride or sulfate salts can only operate at a Coulombic efficiency of <91% under mild operation conditions (2), largely due to undesired hydrogen evolution reaction (HER). This work reports a series of novel Fe electrolytes, Fe electrolytes reinforced with Mg ions (FERMI) and Ca ions (FERCI), which have remarkably better Coulombic efficiency, higher conductivity, and faster deposition/stripping kinetics. By the addition of 4.5 M MgCl2 or CaCl2 into the baseline FeCl2 electrolyte, the Fe deposition/stripping efficiency can be significantly improved to 99.1%, which greatly boosts the cycling performance of Fe metal batteries in both half-cells and full-cells. Mechanistic studies reveal that the remarkably improved efficiency is due to a reduced amount of “dead Fe” as well as suppressed HER. By the combination of experiments and molecular dynamics and density functional theory computation, the electrolyte structure is revealed, and the mechanism for enhanced water reduction resistance is elucidated. These novel electrolytes not only enable a highly reversible Fe metal anode for low-cost energy storage technologies but also have the potential to address the HER side reaction problem in other electrochemical technologies based on aqueous electrolytes, such as CO2 reduction, NH3 synthesis, etc

Chemical Feedback in the Self-Assembly and Function of Air–Liquid Interfaces: Insight into the Bottlenecks of CO₂ Direct Air Capture

As fossil fuels remain a major source of energy throughout the world, developing efficient negative emission technologies, such as direct air capture (DAC), which remove carbon dioxide (CO2) from the air, becomes critical for mitigating climate change. Although all DAC processes involve CO2 transport from air into a sorbent/solvent, through an air–solid or air–liquid interface, the fundamental roles the interfaces play in DAC remain poorly understood. Herein, we study the interfacial behavior of amino acid (AA) solvents used in DAC through a combination of vibrational sum frequency generation spectroscopy and molecular dynamics simulations. This study revealed that the absorption of atmospheric CO2 has antagonistic effects on subsequent capture events that are driven by changes in bulk pH and specific ion effects that feedback on surface organization and interactions. Among the three AAs (leucine, valine, and phenylalanine) studied, we identify and separate behaviors from CO2 loading, chemical changes, variations in pH, and specific ion effects that tune structural and chemical degrees of freedom at the air–aqueous interface. The fundamental mechanistic findings described here are anticipated to enable new approaches to DAC based on exploiting interfaces as a tool to address climate change