Proton dissociation and transfer in proton exchange membrane ionomers with multiple and distinct pendant acid groups: An ab initio study

HBCalculator: A Tool for Hydrogen Bond Distribution Calculations in Molecular Dynamics Simulations

Hydrogen bonds, crucial noncovalent interactions in molecular systems, significantly impact biological, chemical, and energy-related processes; therefore, characterizing hydrogen bond information is of importance to fundamental studies. This work introduces the HBCalculator, a Tcl-based tool integrated with VMD for calculating 1D and 2D distributions of hydrogen bond density and strength. The tool facilitates spatial analysis, overcoming limitations in existing packages that lack direct spatial distribution output. By employing HBCalculator in MD simulations, three systems of cellulose/water and graphene/water interfaces, were tested to showcase its functionality. The 1D and 2D hydrogen bond distributions reveal insights into interfacial properties, reflecting the impact of material hydrophobicity. The simplicity of usage, along with its potential for diverse molecular systems, positions HBCalculator as a valuable tool for researchers exploring hydrogen bond networks.

The degradation effect on proton dissociation and transfer in perfluorosulfonic acid membranes

In the operation of proton exchange membrane fuel cells (PEMFCs), the ionomer-perfluorosulfonic acid (PSFA) membrane side chains are easily attacked by free radicals, resulting in membrane degradation. In this work, the chemical degradation effect of side chains in the PSFA membrane on proton dissociation and transfer behaviors is investigated by means of the quantum chemistry calculation. The rotation of the H atom in the acid group after the degradation is evaluated. The impact of the electrostatic potential (ESP) and electronegativity of the side chains is analyzed. The results demonstrate that the membrane degradation decreases the positive potential of the proton in the acid group, leading to the proton being less active so that more water molecules are required for the spontaneous proton dissociation. The rotation of the H atom in the acid group affects the proton dissociation mode owing to the change of the hydrogen bond network. It is found that the ESP of the acid group in two side chain fragments influences each other and the water molecules between two side chains can be shared to reduce the number of water molecules for the proton dissociation.

DFT-Machine Learning Approach for Accurate Prediction of pKa

In this study, we propose a novel method of pK_a prediction in a diverse set of acids, which combines density functional theory (DFT) method with machine learning (ML) methods. First, the DFT method with B3LYP/6-31++G**/SM8 is used to predict pK_a, yielding a mean absolute error of 1.85 pK_a units. Subsequently, such pK_a values predicted from the DFT method are employed as one of 10 molecular descriptors for developing ML models trained on experimental data. Kernel Ridge Regression (KRR), Gaussian Process Regression, and Artificial Neural Network are optimized using three Pipelines: Pipeline 1 involving only hyperparameter optimization (HPO), Pipeline 2 involving HPO followed by a relative contribution analysis (RCA) and recursive feature elimination (RFE), and Pipeline 3 involving HPO followed by RCA and RFE on an expanded set of composite features. Finally, it is demonstrated that KRR with Pipeline 3 yields optimal pK_a prediction at an MAE of 0.60 log units. This algorithm was then utilized to predict the pK_a of 37 novel acids. The two most important features were determined to be the number of hydrogen atoms in the molecule and the degree of oxidation of the acid. The predicted pK_a values were documented for future reference.

Water sub-diffusion in membranes for fuel cells

We investigate the dynamics of water confined in soft ionic nano-assemblies, an issue critical for a general understanding of the multi-scale structure-function interplay in advanced materials. We focus in particular on hydrated perfluoro-sulfonic acid compounds employed as electrolytes in fuel cells. These materials form phase-separated morphologies that show outstanding proton-conducting properties, directly related to the state and dynamics of the absorbed water. We have quantified water motion and ion transport by combining Quasi Elastic Neutron Scattering, Pulsed Field Gradient Nuclear Magnetic Resonance, and Molecular Dynamics computer simulation. Effective water and ion diffusion coefficients have been determined together with their variation upon hydration at the relevant atomic, nanoscopic and macroscopic scales, providing a complete picture of transport. We demonstrate that confinement at the nanoscale and direct interaction with the charged interfaces produce anomalous sub-diffusion, due to a heterogeneous space-dependent dynamics within the ionic nanochannels. This is irrespective of the details of the chemistry of the hydrophobic confining matrix, confirming the statistical significance of our conclusions. Our findings turn out to indicate interesting connections and possibilities of cross-fertilization with other domains, including biophysics. They also establish fruitful correspondences with advanced topics in statistical mechanics, resulting in new possibilities for the analysis of Neutron scattering data.

New Insights into Perfluorinated Sulfonic-Acid Ionomers

In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers' complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradation phenomena, and PFSA thin films are presented. Throughout, the impact of PFSA chemistry and side-chain is also discussed to present a broader perspective.

Infrared dynamics study of thermally treated perfluoroimide acid proton exchange membranes

In situinfrared investigation of the water hydrogen-bonded network inside PFIA accounts for improved proton conductivity under hotter and dryer conditions.

Moisture and thermal expansion properties and mechanism of interaction between ions of a Nafion-based membrane electrode assembly

The coefficient of moisture and thermal expansion for each layer of membrane electrode assembly.

Alkaline Anion-Exchange Membranes Containing Mobile Ion Shuttles

A new class of alkaline anion-exchange membranes containing mobile ion shuttles is developed. It is achieved by threading ionic linear guests into poly(crown ether) hosts via host-guest molecular interaction. The thermal- and pH-triggered shuttling of ionic linear guests remarkably increases the solvation-shell fluctuations in inactive hydrated hydroxide ion complexes (OH(-) (H2 O)4 ) and accelerates the OH(-) transport.

Ab initio molecular dynamics simulations of aqueous triflic acid confined in carbon nanotubes

Ab initio molecular dynamics simulations were performed to investigate the effects of nanoscale confinement on the structural and dynamical properties of aqueous triflic acid (CF3SO3H). Single-walled carbon nanotubes (CNTs) with diameters ranging from ∼11 to 14 Å were used as confinement vessels, and the inner surface of the CNT were either left bare or fluorinated to probe the influence of the confined environment on structural and dynamical properties of the water and triflic acidic. The systems were simulated at hydration levels of n = 1-3 H2O/CF3SO3H. Proton dissociation expectedly increased with increasing hydration. Along with the level of hydration, hydrogen bond connectivity between the triflic acid molecules, both directly and via a single water molecule, played a role on proton dissociation. Direct hydrogen bonding between the CF3SO3H molecules, most commonly found in the larger bare CNT, also promoted interactions between water molecules allowing for greater separation of the dissociated protons from the CF3SO3(-) as the hydration level was increased. However, this also resulted in a decrease in the overall proportion of dissociated protons. The confinement dimensions altered both the hydrogen bond network and the distribution of water molecules where the H2O in the fluorinated CNTs tended to form small clusters with less proton dissociation at n = 1 and 2 but the highest at n = 3. In the absence of nearby hydrogen bond accepting sites from H2O or triflic acid SO3H groups, the water molecules formed weak hydrogen bonds with the fluorine atoms. In the bare CNT systems, these involved the CF3 groups of triflic acid and were more frequently observed when direct hydrogen bonding between CF3SO3H hindered potential hydrogen bonding sites. In the fluorinated tubes, interactions with the covalently bound fluorine atoms of the CNT wall dominated which appear to stabilize the hydrogen bond network. Increasing the hydration level increased the frequency of the OH···F (CNT) hydrogen bonding which was highly pronounced in the smaller fluorinated CNT indicating an influence on the confinement dimensions on these interactions.

Novel sulfonated poly(arylene ether sulfone) containing hydroxyl groups for enhanced proton exchange membrane properties

We here report a new sulfonated poly(arylene ether sulfone) modified with hydroxyl side groups for a proton exchange membrane fuel cell (PEMFC).