Functional Group-Dependent Proton Conductivity of Phosphoric Acid-Doped Ion-Pair Coordinated Polymer Electrolytes: A Molecular Dynamics Study.

Toward deployment of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) in our daily lives, multiple research efforts have been dedicated to develop high-performance phosphate-doped polymer electrolytes. Recently, ion-pair coordinated polymers have garnered attention for their high stability and proton conductivity. However, a comprehensive understanding of how proton transport properties are modified by the functional groups present in these polymers is still lacking. In this study, we employ molecular dynamics (MD) simulations to investigate the impact of different functional group types and conversion ratios on conductivity. We find that Grotthuss-type hopping transport predominantly governs the overall conductivity, surpassing vehicular transport by factors of 100-1000. As conductivity scales with proton concentration, we observe that less-bulky functional groups offer advantages by minimizing the volume expansion associated with increased conversion ratios. Additionally, we show that a strong ion-pair interaction between the cationic functional group and the phosphate anion disrupts the suitable intermolecular orientations required for efficient proton hopping between phosphate and phosphoric acid molecules, thereby diminishing the proton conductivity. Our study underscores the importance of optimizing the strength of ion-pair interactions to balance stability and proton conductivity, thus paving the way for the development of ion-pair coordinated polymer electrolytes with improved performance.

Penetration of Hydrogen into Polymer Electrolyte Membrane for Fuel Cells by Quantum and Molecular Dynamics Simulations.

The advent of the Hydrogen Society created great interest around hydrogen-based energy a decade ago, with several types of vehicles based on hydrogen fuel cells already being produced in the automotive sector. For highly efficient fuel cell systems, the control of hydrogen inside a polymer-based electrolyte membrane is crucial. In this study, we investigated the molecular behavior of hydrogen inside a polymer-based proton-exchange membrane, using quantum and molecular dynamics simulations. In particular, this study focused on the structural difference of the pendent-like side chain polymer, resulting in the penetration ratio of hydrogen into the membrane deriving from the penetration depth of the membrane's thickness while keeping the simulation time constant. The results reveal that the penetration ratio of the polymer with a shorter side chain was higher than that with the longer side chain. This was justified via two perspectives; electrostatic and van der Waals molecular interactions, and the structural difference of the polymers resulting in the free volume and different behavior of the side chain. In conclusion, we found that a longer side chain is more trembling and acts as an obstruction, dominating the penetration of hydrogen inside the polymer membrane.

Morphological effect of side chain on H3O+ transfer inside polymer electrolyte membranes across polymeric chain via molecular dynamics simulation.

Performance and durability of polymer electrolyte membrane are critical to fuel cell quality. As fuel cell vehicles become increasingly popular, membrane fundamentals must be understood in detail. Here, this study used molecular dynamic simulations to explore the morphological effects of perfluorosulfonic acid (PFSA)-based membranes on ionic conductivity. In particular, I developed an intuitive quantitative approach focusing principally on hydronium adsorbing to, and desorbing from, negatively charged sulfonate groups, while conventional ionic conductivity calculations featured the use of mean square displacements that included natural atomic vibrations. The results revealed that shorter side-chains caused more hydroniums to enter the conductive state, associated with higher ion conductivity. In addition, the hydronium path tracking showed that shorter side-chains allowed hydroniums to move among host groups, facilitating chain adsorption, in agreement with a mechanism suggested in earlier studies.

Tailoring Graphene Nanosheets for Highly Improved Dispersion Stability and Quantitative Assessment in Nonaqueous Solvent.

Aggregation is a critical limitation for the practical application of graphene-based materials. Herein, we report that graphene oxide (GO) nanosheets chemically modified with ethanolamine (EA), ethylene glycol (EG), and sulfanilic acid (SA) demonstrate superior dispersion stability in organic solvents, specifically EG, based on the differences in their covalent chemistries. Functionalized GO was successfully dispersed in EG at a concentration of 9.0 mg mL(-1) (0.50 vol %), the highest dispersion concentration reported to date. Moreover, our study introduces a unique analytical method for the assessment of dispersion stability and successfully quantifies the instability index based on transmission profiles under centrifugation cycles. Interestingly, GO-EG and GO-EA exhibited highly improved dispersion stabilities approximately 96 and 48 times greater than that of GO in EG solvent, respectively. This finding highlights the critical role of surface functional groups in the enhancement of chemical affinity and miscibility in the surrounding media. We anticipate that the novel structural designs and unique tools presented in this study will further the understanding and application of chemically functionalized carbon materials.

Quantitative Evaluation of the Dispersion of Graphene Sheets With and Without Functional Groups Using Molecular Dynamics Simulations.

Nanofluids with enhanced thermal properties are candidates for thermal management in automotive systems, with scope for improving energy efficiency. In particular, many studies have reported on dispersions of nanoparticles with long-term stability in the base fluid, with qualitative evaluations of the dispersion stability via either the naked eye or optical instruments. Additives such as surfactants can be used to enhance the dispersion of nanoparticles; however, this may diminish their intrinsic thermal properties. Here, we describe molecular dynamics simulations of nanofluids containing graphene sheets dispersed in ethylene glycol and water. We go on to suggest a quantitative evaluation method for the degree of dispersion, based on the ratio of the total number of nanoparticles to the number of clustered nanoparticles. Moreover, we investigate the effects of functional groups on the surface of graphene, which are expected to improve the dispersion without requiring additives such as surfactants due to steric hindrance and chemical affinity for the surrounding fluid. We find that, for pure graphene, the degree of dispersion decreased as the quantity of graphene sheets increased, which is attributed to an increased probability of aggregation at higher loadings; however, the presence of functional groups inhibited the graphene sheets from forming aggregates.