Probing the Internal Microstructure of Polyamide Thin-Film Composite Membranes Using Resonant Soft X-ray Scattering

Unravel-engineer-design: a three-pronged approach to advance ionomer performance at interfaces in proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs), which use hydrogen as fuel, present an eco-friendly alternative to internal combustion engines (ICEs) for powering low-to-heavy-duty vehicles and various devices. Despite their promise, PEMFCs must meet strict cost, performance, and durability standards to reach their full potential. A key challenge lies in optimizing the electrode, where a thin ionomer layer is responsible for proton conduction and binding catalyst particles to the electrode. Enhancing ion transport within these sub-μm thick films is critical to improving the oxygen reduction reaction (ORR) at the cathodes of PEMFCs. For the past 15 years, our research has targeted this limitation through a comprehensive "Unravel - Engineer - Design" approach. We first unraveled the behavior of ionomers, gaining deeper insights into both the average and distributed proton conduction properties within sub-μm thick films and at interfaces that mimic catalyst binder layers. Next, we engineered ionomer-substrate interfaces to gain control over interfacial makeup and boost proton conductivity, essential for PEMFC efficiency. Finally, we designed novel nature-derived or nature-inspired, fluorine-free ionomers to tackle the ion transport limitations seen in state-of-the-art ionomers under thin-film confinement. Some of these ionomers even pave the way to address cost and sustainability challenges in PEMFC materials. This feature article highlights our contributions and their importance in advancing PEMFCs and other sustainable energy conversion and storage technologies.

Flexible Pectin Nanopatterning Drives Cell Wall Organization in Plants

Plant cell walls are abundant sources of materials and energy. Nevertheless, cell wall nanostructure, specifically how pectins interact with cellulose and hemicelluloses to construct a robust and flexible biomaterial, is poorly understood. X-ray scattering measurements are minimally invasive and can reveal ultrastructural, compositional, and physical properties of materials. Resonant X-ray scattering takes advantage of compositional differences by tuning the energy of the incident X-ray to absorption edges of specific elements in a material. Using Tender Resonant X-ray Scattering (TReXS) at the calcium K-edge to study hypocotyls of the model plant, Arabidopsis thaliana, we detected distinctive Ca features that we hypothesize correspond to previously unreported Ca-Homogalacturonan (Ca-HG) nanostructures. When Ca-HG structures were perturbed by chemical and enzymatic treatments, cellulose microfibrils were also rearranged. Moreover, Ca-HG nanostructure was altered in mutants with abnormal cellulose, pectin, or hemicellulose content. Our results indicate direct structural interlinks between components of the plant cell wall at the nanoscale and reveal mechanisms that underpin both the structural integrity of these components and the molecular architecture of the plant cell wall.

Unraveling Depth-Specific Ionic Conduction and Stiffness Behavior across Ionomer Thin Films and Bulk Membranes

Interfacial behavior of submicron thick polymer films critically controls the performance of electrochemical devices. We developed a robust, everyday-accessible, fluorescence confocal laser scanning microscopy (CLSM)-based strategy that can probe the distribution of mobility, ion conduction, and other properties across ionomer samples. When fluorescent photoacid probe 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) was incorporated into <1 μm thick Nafion films on substrates, the depth-profile images showed thickness- and interface-dependent proton conduction behavior. In these films, proton conduction was weak over a region next to substrate interface, then gradually increased until air interface at 88% RH. Conversely, consistently high proton conduction with no interface dependence was observed across 35-50 μm thick bulk, free-standing Nafion membranes. A hump-like mobility/stiffness distribution was observed across Nafion films containing mobility-sensitive probe (9-(2-carboxy-2-cyanovinyl)julolidine) (CCVJ). The proton conduction and mobility distribution were rationalized as a combinatorial effect of interfacial interaction, ionomer chain orientation, chain density, and ionic domain characteristics.

Photoacid-Modified Nafion Membrane Morphology Determined by Resonant X-ray Scattering and Spectroscopy

Covalent attachment of photoacid dye molecules to perfluorinated sulfonic acid membranes is a promising route to enable active light-driven ion pumps, but the complex relationship between chemical modification and morphology is not well understood in this class of functional materials. In this study we demonstrate the effect of bound photoacid dyes on phase-segregated membrane morphology. Resonant X-ray scattering near the sulfur K-edge reveals that introduction of photoacid dyes to the end of the ionomer side chains enhances phase segregation among ionomer domains, and the ionomer domain spacing increases with increasing amount of bound dye. Furthermore, relative crystallinity is marginally enhanced within semicrystalline domains composed of the perfluorinated backbone. X-ray absorption spectroscopy coupled with first-principles density functional theory calculations suggest that above a critical concentration, the multiple hydrophilic groups on the attached photoacid dye may help increase residual water content and promote hydration of adjacent sulfonic acid side chains under dry or ambient conditions.

Molecular Insights into the Composition-Structure-Property Relationships of Polyamide Thin Films for Reverse Osmosis Desalination

A molecular-level understanding of the structure-property relationship of polyamide (PA) active layers in thin-film-composite membranes remains unclear. We developed an approach to build and hydrate the PA layer in molecular dynamics simulations and reproduced realistic membrane properties, which enabled us to examine the composition-structure-permeability relationships at the molecular level. We discovered the variation of pore size distributions in the dry PA structures at different monomer compositions, leading to different water cluster distributions and wetting properties of hydrated PA films. Membrane swelling was linearly dependent on the degree of cross-linking (DC), and higher water flux was obtained in the more swelling-prone PA films because of the transition in water transport mechanisms. Continuum-like and jumping transport both occurred in PA films with smaller DC, where visible and more persistent channels existed. In the denser films, water molecules relied only on the on-and-off channels to jump from one cavity to another; however, jumping transport was more pronounced even in the less dense PA films, and all the PA structures exhibited oscillations, which provided evidence for the solution-diffusion model rather than the pore-flow model. The results not only contribute to fundamental understanding but also provide insights into the molecule-level design for next-generation membranes.

Molecular Structure of Aromatic Reverse Osmosis Polyamide Barrier Layers

The molecular structures of polyamide barrier layers in reverse osmosis membranes, made by interfacial polymerization of m-phenylenediamine and trimesoyl chloride under different reaction and post-treatment conditions, were characterized by grazing incidence wide-angle X-ray scattering (GIWAXS). The molecular backbone packing is consistent with two different aromatic molecular packing motifs (parallel and perpendicular) with preferential surface-induced orientation. The results suggest that the perpendicular, T-shaped, packing motif (5 Å spacing) might be associated with optimal membrane permeance, compared with the parallel packings (3.5-4.0 Å spacings).

Quantifying Carboxylic Acid Concentration in Model Polyamide Desalination Membranes via Fourier Transform Infrared Spectroscopy

Carboxylic acid groups impart hydrophilicity and ionizable moieties to polyamide membranes for desalination, hence influencing water and ion transport through the material. Model polyamide films were synthesized via molecular layer-by-layer deposition on planar substrates to study the formation process of these materials and overcome the chemical and topological inhomogeneity inherent to conventional interfacially polymerized polyamide membranes. The carboxylic acid content in these model films was characterized using Fourier transform infrared (FTIR) spectroscopy by quantifying the C=O band at 1718 cm-1. The concentration of carboxylic acid groups decreased as the thickness of the membrane increased, suggestive of an increase in crosslink density as the polyamide network develops. For the thinnest molecular layer-by-layer (mLbL) samples, the carboxylic acid concentration for films on gold was 0.35 mmol g-1, whereas analogous films on silicon had an acid content of 0.56 mmol g-1, indicating a clear influence of the substrate on the initial network formation. As the thickness of the membrane increased, the influence of the substrate and initial layer growth became less significant as the carboxylic acid concentration on both substrates reached a value of 0.12 mmol g-1. We demonstrate that FTIR spectroscopy is a practical and accessible way to quantify the carboxylic acid content in these types of extremely thin polyamide membranes to help quantify network formation in these materials.