Zandrie Borneman

Formation of macrovoids in the membrane structure is a common occurrence in the process of nonsolvent-induced phase separation (NIPS), yet it is often undesired due to low structural homogeneity, poor performance, and introduction of weak mechanical points. Here, we introduce an ether-based solvent, 2,5,7,10-tetraoxaundecane (TOU), for fabrication of macrovoid-free polysulfone (PSf) ultrafiltration (UF) membranes. We investigate how strong polymer–solvent affinity, high chain entanglement, and high viscosity influence the phase-inversion process and promote the formation of homogeneous membrane structures during NIPS. Membranes prepared from PSf/TOU dope solutions exhibited stable sponge-like morphology, without macrovoids, in contrast to PSf membranes prepared from the more common solvent N-methyl-2-pyrrolidone (NMP), which typically display large fingerlike macrovoids. For PSf/TOU systems, mixing ethanol and water in the coagulation bath led to fabrication of porous, permeable membranes while maintaining the macrovoid-free morphology. Membrane performance was further optimized by changing the polymer concentration and the addition of a pore-forming agent in the dope solution. The best-performing membranes showed pure water permeance between 149 and 204 L·m–2·h–1·bar–1 and a bovine serum albumin rejection around 90%. This work establishes TOU as a previously unexplored yet effective solvent for NIPS fabrication of PSf UF membranes, enabling precise control over membrane structure and effective suppression of macrovoid formation and ultimately broadening the design space for PSf membranes.

Background sodium chloride in hydrochloric acid and sodium hydroxide solutions leads to large overpotentials when a bipolar membrane (BPM) is operated under forward bias (FB). Under FB polarization, the accumulation of salt ions at the junction hinders the transport of H⁺ and OH^– ions, thus increasing the mass transport resistance and lowering the water recombination rate. The “ionic blockade” phenomenon is mainly observed if the base is contaminated with Cl^– ions due to the poor OH^–/Cl^– selectivity of the BPM’s anion exchange layer (AEL). This shortcoming is successfully reduced by modifying the AEL with a sub-micrometer thick poly­(benzimidazole) (PBI) coating. Ionic crosslinking between the AEL and PBI leads to a denser interface that enhances the size exclusion of Cl^– ions. Furthermore, the negative charges of deprotonated benzimidazole units at the basic operating conditions contribute to the Donnan exclusion of Cl^– ions, while the OH^– ions can still hop between the alkaline-doped free volumes of the PBI film. The enhanced OH^–/Cl^– selectivity prevents the accumulation of Cl^– ions at the junction and leads to lower overpotentials during the forward bias operation of BPMs in salt-contaminated acid and base. As a result, the PBI-coated BPM has a peak power density 1.6 times higher than that of an uncoated BPM when harvesting electrical energy from a pH gradient. The BPM modification also benefits flow battery applications, as the calculated BPM voltaic efficiency at 100 A/m² (dis)­charge current density is increased from −3.7% to 57% with the addition of the PBI coating.

Porous electrodes govern the electrochemical performance and pumping requirements in redox flow batteries, yet conventional carbon‐fiber‐based porous electrodes have not been tailored to sustain the requirements of liquid‐phase electrochemistry. 3D printing is an effective approach to manufacturing deterministic architectures, enabling the tuning of electrochemical performance and pressure drop. In this work, model grid structures are manufactured with stereolithography 3D printing followed by carbonization and tested as flow battery electrode materials. Microscopy, tomography, spectroscopy, fluid dynamics, and electrochemical diagnostics are employed to investigate the resulting electrode properties, mass transport, and pressure drop of ordered lattice structures. The influence of the printing direction, pillar geometry, and flow field type on the cell performance is investigated and mass transfer vs. electrode structure correlations are elucidated. It is found that the printing direction impacts the electrode performance through a change in morphology, resulting in enhanced performance for diagonally printed electrodes. Furthermore, mass transfer rates within the electrode are improved by helical or triangular pillar shapes or by using interdigitated flow field designs. This study shows the potential of stereolithography 3D printing to manufacture customized electrode scaffolds, which could enable multiscale structures with superior electrochemical performance and low pumping losses.

Porous electrodes govern the electrochemical performance and pumping requirements in redox flow batteries, yet conventional carbon-fiber-based porous electrodes have not been tailored to sustain the requirements of liquid-phase electrochemistry. 3D printing is an effective approach to manufacture deterministic architectures, enabling the tuning of electrochemical performance and pressure drop. In this work, model grid structures are manufactured with stereolithography 3D printing followed by carbonization and tested as flow battery electrode materials. Microscopy, tomography, spectroscopy, fluid dynamics, and electrochemical diagnostics are employed to investigate the resulting electrode properties, mass transport, and pressure drop of ordered lattice structures. The influence of the printing direction, pillar geometry, and flow field type on the cell performance is investigated and mass transfer vs. electrode structure correlations are elucidated. It is found that the printing direction impacts the electrode performance through a change in morphology, resulting in enhanced performance for diagonally printed electrodes. Furthermore, mass transfer rates within the electrode are improved by helical or triangular pillar shapes or by using interdigitated flow field designs. This study shows the potential of stereolithography 3D printing to manufacture customized electrode scaffolds, which could enable multiscale structures with superior electrochemical performance and low pumping losses.

Motivated by the need for efficient purification methods for the recovery of valuable resources, we developed a wire-electrospun membrane adsorber without the need for post-modification. The relationship between the fiber structure, functional-group density, and performance of electrospun sulfonated poly(ether ether ketone) (sPEEK) membrane adsorbers was explored. The sulfonate groups enable selective binding of lysozyme at neutral pH through electrostatic interactions. Our results show a dynamic lysozyme adsorption capacity of 59.3 mg/g at 10% breakthrough, which is independent of the flow velocity confirming dominant convective mass transport. Membrane adsorbers with three different fiber diameters (measured by SEM) were fabricated by altering the concentration of the polymer solution. The specific surface area as measured with BET and the dynamic adsorption capacity were minimally affected by variations in fiber diameter, offering membrane adsorbers with consistent performance. To study the effect of functional-group density, membrane adsorbers from sPEEK with different sulfonation degrees (52%, 62%, and 72%) were fabricated. Despite the increased functional-group density, the dynamic adsorption capacity did not increase accordingly. However, in all presented cases, at least a monolayer coverage was obtained, demonstrating ample functional groups available within the area occupied by a lysozyme molecule. Our study showcases a ready-to-use membrane adsorber for the recovery of positively charged molecules, using lysozyme as a model protein, with potential applications in removing heavy metals, dyes, and pharmaceutical components from process streams. Furthermore, this study highlights factors, such as fiber diameter and functional-group density, for optimizing the membrane adsorber’s performance.