PROGRAM | Chemical Engineering

By: Laj Xiong Chair: Yushan Yan

ABSTRACT

Anion-exchange membranes (AEMs) are versatile tools whose charge selective behavior can be used in creative ways. Inspired by these features, this work utilizes the strengths of anion-exchange membranes to engineer products that can be immediately applied to solve major problems in two very important applications. In the first application, we detail the development of a high-performance anion-exchange membrane which can be competitive for carbon capture, showing great potential for cost savings and carbon footprint reduction.  In the second application, we utilize an anion-exchange membrane to design a new process for water remediation in cooling towers, which is safer and more energy efficient than the traditionally used approach.

In the design of a carbon capture membrane, the objective was to achieve performance above the Robeson Upper Bound, an empirical limit describing the inverse relationship between permeability and selectivity. By incorporating a carrier (hydroxide anion) which could reversibly react with CO₂ to form a charged carrier-complex, an ion-conductive pathway could be used to improve the bottleneck process that was diffusion within the membrane. To maximize gains from the ion-conductive pathway, trends of functional group basicity with ion conductivity were realized in order to select quaternary phosphonium (QPOH) as a fixed-site carrier for facilitated transport. Consequently, the resulting membrane exhibited CO₂ permeabilities as high as 1090 Barrer and CO₂/N₂ selectivities as high as 275, ultimately achieving separation performance surpassing the Robeson Upper Bound. Additional tests proved that a CO₂-selective reaction-diffusion mechanism supplemented the existing solution-diffusion transport pathway, effectively improving CO₂ uptake within the matrix.

Though the performance suggested a membrane structure favorable for ion transport, very little was known of the actual morphology for this particular AEM, or AEMs in general. Consequently, the second objective was to determine quantitative and qualitative understanding of the morphology within two similar AEMs, polysulfone (PSf-QPOH) and poly(phenylene oxide)-based quaternary phosphonium hydroxide (PPO-QPOH). By using small angle neutron scattering, a Teubner-Strey and fractal model was combined to produce an excellent fit for scattering data, suggesting the morphology to be representative of a bicontinuous, hydrophilic-hydrophobic system that exhibited fractal-like behavior. Repeat lengths were determined to be 5-8 nm for PSf-QPOH and 3.5-4.5 nm for PPO-QPOH. Whereas, the fractal dimension of 1 for both PSf-QPOH and PPO-QPOH described the bicontinuous phases as being largely smooth cylinders or rods that have extensive length. These morphological characteristics, combined with the highly basic quaternary phosphonium functional group, result in the largely effective anion transport observed in both PSf-QPOH and PPO-QPOH.

In the second application, the main objective was to show proof-of-concept operation of an electrolyzer that produced chlorine for water remediation at cell voltages far below that of the traditional chlor-alkali process (E°=2.19 V). The design of the electrolyzer was successfully realized for the cathodic redox pairs, Fe(III)/Fe(II) and p-benzoquinone/hydroquinone, where Fe(III) exhibited a very low cell potential of 0.76 V, and p-benzoquinone exhibited either 0.93 V or 1.55 V, depending on the presence of acid (current density 2 mA cm^-2). In addition, all cases achieved oxidation reduction potentials above 900 mV in less than 30 minutes of operation, showing excellent biocidal activity for water remediation. While the performance of these two redox pairs is promising, rising acidity in the anolyte was highlighted as a problem to be addressed for future applications using this electrolyzer.

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