Christopher G. Arges

Two-dimensional NMR spectroscopy reveals cation-triggered backbone degradation in polysulfone-based anion exchange membranes

Anion exchange membranes (AEMs) find widespread applications as an electrolyte and/or electrode binder in fuel cells, electrodialysis stacks, flow and metal-air batteries, and electrolyzers. AEMs exhibit poor stability in alkaline media; their degradation is induced by the hydroxide ion, a potent nucleophile. We have used 2D NMR techniques to investigate polymer backbone stability (as opposed to cation stability) of the AEM in alkaline media. We report the mechanism behind a peculiar, often-observed phenomenon, wherein a demonstrably stable polysulfone backbone degrades rapidly in alkaline solutions upon derivatization with alkaline stable fixed cation groups. Using COSY and heteronuclear multiple quantum correlation spectroscopy (2D NMR), we unequivocally demonstrate that the added cation group triggers degradation of the polymer backbone in alkaline via quaternary carbon hydrolysis and ether hydrolysis, leading to rapid failure. This finding challenges the existing perception that having a stable cation moiety is sufficient to yield a stable AEM and emphasizes the importance of the often ignored issue of backbone stability.

Assessing the influence of different cation chemistries on ionic conductivity and alkaline stability of anion exchange membranes

Polysulfone (PSF) backbones were functionalized with reactive chloromethyl groups for preparing thin film anion exchange membranes (AEMs) with fixed benzyl quaternary cations. Three different cation chemistries of varying basicity were evaluated: 1,4-dimethylpiperazinium (DMP+), trimethylammonium (TMA+), and trimethylphosphonium (TMP+). The water uptake, ionic conductivity, and stability in alkaline media of these AEMs were assessed with both chloride and hydroxide counteranions. The results obtained revealed that the basicity value of the free base conjugate of the functionalized quaternary cations correlated well with gains in ionic conductivity. Cation basicity also correlated well with the alkaline stability of cations with the same inorganic atom, but was not an appropriate heuristic for comparing alkaline stability across cations with different inorganic atoms. The alkaline stability studies indicated that the primal degradation pathway of the TMA+ cation differed from that of the TMP+ cation (direct nucleophilic attack versus ylide formation). PSF with TMA+ and DMP+ cations were demonstrated to show alkaline fuel cell performance that reflected their respective ionic conductivity values.

A perfluorinated anion exchange membrane with a 1,4-dimethylpiperazinium cation

A perfluorinated anion exchange membrane with a 1,4-dimethylpiperazinium cation demonstrated better hydroxide ion conductivity and three-fold reduction in water uptake compared to a hydrocarbon AEM with the same cation. The perfluorinated AEM was stable over 30 days in 2 M KOH at 60 °C and demonstrated good fuel cell performance.

Polysulfone-based anion exchange membranes demonstrate excellent chemical stability and performance for the all-vanadium redox flow battery

A polysulfone-based anion exchange membrane functionalized with quaternary benzyl trimethylammonium groups (PSF-TMA+) demonstrated a 40-fold reduction in vanadium(IV) permeability when compared to a Nafion® membrane. Comprehensive 2D NMR (COSY and heteronuclear quantum correlation spectroscopy) studies verified that PSF-TMA+ remained chemically stable even after exposure to a 1.5 M vanadium(V) solution for 90 days. Excellent energy efficiencies (85%) were attained and sustained over several charge–discharge cycles for a vanadium redox flow battery prepared using the PSF-TMA+ separator.

Investigation of polymer electrolyte membrane chemical degradation and degradation mitigation using in situ fluorescence spectroscopy

A fluorescent molecular probe, 6-carboxy fluorescein, was used in conjunction with in situ fluorescence spectroscopy to facilitate real-time monitoring of degradation inducing reactive oxygen species within the polymer electrolyte membrane (PEM) of an operating PEM fuel cell. The key requirements of suitable molecular probes for in situ monitoring of ROS are presented. The utility of using free radical scavengers such as CeO2 nanoparticles to mitigate reactive oxygen species induced PEM degradation was demonstrated. The addition of CeO2 to uncatalyzed membranes resulted in close to 100% capture of ROS generated in situ within the PEM for a period of about 7 h and the incorporation of CeO2 into the catalyzed membrane provided an eightfold reduction in ROS generation rate.

Degradation of anion exchange membranes used for hydrogen production by ultrapure water electrolysis

Solid-state alkaline water electrolysis using a pure water feed offers several distinct advantages over liquid alkaline electrolyte water electrolysis and proton exchange membrane water electrolysis. These advantages include a larger array of electrocatalyst available for oxygen evolution, no electrolyte management, and the ability to apply differential pressure. To date, there have been only a handful of reports on solid-state alkaline water electrolyzers using anion exchange membranes (AEMs), and there have been no reports that investigate loss in system performance over time. In this work, a solid-state alkaline water electrolyzer was successfully demonstrated with several types of polysulfone-based AEMs using a relatively expensive but highly active lead ruthenate pyrochlore electrocatalyst for the oxygen evolution reaction. The electrolysis of ultrapure water at 50 °C resulted in a current density of 400 mA cm−2 at 1.80 V. We demonstrated that the short-term degradation of water electrolyzer performance over time was largely a consequence of carbon dioxide intrusion into the system and could be easily remedied, while long-term deterioration was a consequence of irreversible AEM polymer degradation.

Quarternary Ammonium and Phosphonium Based Anion Exchange Membranes for Alkaline Fuel Cells

In this work, Udel® polysulfone was functionalized with quaternary ammonium and phosphonium based cations for thin-film anion exchange membranes (AEMs). At this time, discussion is limited to the chemical, mechanical, and thermal attributes of quaternary ammonium polysulfone (QAPSF). Key results indicate that the thermal properties of QAPSF are well suited for low temperature fuel cells, but improvement to QAPSF conductivity and mechanical properties is needed for durable and high performing alkaline fuel cells. It was observed that water uptake of nearly 15% of the normalized membrane weight is needed to facilitate ion conductivity in the QAPSF membrane. Finally, fuel cell performance is demonstrated with a QAPSF membrane electrode assembly.

Directed Self-Assembly of Polystyrene-b-poly(propylene carbonate) on Chemical Patterns via Thermal Annealing for Next-Generation Lithography.

Directed self-assembly (DSA) of block copolymers (BCPs) combines advantages of conventional photolithography and polymeric materials and shows competence in semiconductors and data storage applications. Driven by the more integrated, much smaller and higher performance of the electronics, however, the industry standard polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) in DSA strategy cannot meet the rapid development of lithography technology because its intrinsic limited Flory-Huggins interaction parameter (χ). Despite hundreds of block copolymers have been developed, these BCPs systems are usually subject to a trade-off between high χ and thermal treatment, resulting in incompatibility with the current nanomanufacturing fab processes. Here we discover that polystyrene-b-poly(propylene carbonate) (PS-b-PPC) is well qualified to fill key positions on DSA strategy for the next-generation lithography. The estimated χ-value for PS-b-PPC is 0.079, that is, two times greater than PS-b-PMMA (χ = 0.029 at 150 °C), while processing the ability to form perpendicular sub-10 nm morphologies (cylinder and lamellae) via the industry preferred thermal-treatment. DSA of lamellae forming PS-b-PPC on chemoepitaxial density multiplication demonstrates successful sub-10 nm long-range order features on large-area patterning for nanofabrication. Pattern transfer to the silicon substrate through industrial sequential infiltration synthesis is also implemented successfully. Compared with the previously reported methods to orientation control BCPs with high χ-value (including solvent annealing, neutral top-coats, and chemical modification), the easy preparation, high χ value, and etch selectivity while enduring thermal treatment demonstrates PS-b-PPC as a rare and valuable candidate for advancing the field of nanolithography.

Directed Self-Assembly of Colloidal Particles onto Nematic Liquid Crystalline Defects Engineered by Chemically Patterned Surfaces

In exploiting topological defects of liquid crystals as the targeting sites for trapping colloidal objects, previous work has relied on topographic features with uniform anchoring to create defects, achieving limited density and spacing of particles. We report a generalizable strategy to create topological defects on chemically patterned surfaces to assemble particles in precisely defined locations with a tunable interparticle distance at nanoscale dimensions. Informed by experimental observations and numerical simulations that indicate that liquid crystals, confined between a homeotropic-anchoring surface and a surface with lithographically defined planar-anchoring stripes in a homeotropic-anchoring background, display splay-bend deformation, we successfully create pairs of defects and subsequently trap particles with controlled spacing by designing patterns of intersecting stripes aligned at 45° with homeotropic-anchoring gaps at the intersections. Application of electric fields allows for dynamic control of trapped particles. The tunability, responsiveness, and adaptability of this platform provide the opportunities for assembly of colloidal structures toward functional materials.

Simple and facile synthesis of water-soluble poly(phosphazenium) polymer electrolytes

Water-soluble polymer electrolytes are important industrial materials used as absorbents, rheology modifiers, network formers, and colloidal stabilizers and destabilizers. Herein, the synthesis and alkaline stability of a new class of water-soluble polymer poly(phosphazenium) electrolytes is reported. The charge on the poly(phosphazenium) structure was controlled by the amount of methylating agent used and the resultant electrolytes were water-soluble with a relatively small amount of charge per polymer chain (one charged group per 10 repeat units). Despite phosphazenium salts having excellent alkaline stability, the poly(phosphazenium) polymers with N-methylcyclohexyamino substituents degraded rapidly in alkaline solutions at 60 °C. Two-dimensional NMR was used to characterize the poly(phosphazenium) polymer electrolytes and their hydroxide ion induced degradation products.