Anion-exchange membranes for carbon capture and an in situ chlorine evolution electrolyzer
Date
2015
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
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 applied to solve major problems in two very important applications: carbon capture and water remediation. ā In the design of a carbon capture membrane, the objective was to achieve performance above the Robeson Upper Bound, an empirical separation limit describing the inverse relationship between gas permeability and selectivity. By incorporating a carrier (e.g. hydroxide) which could react with CO2 to form a charged carrier-complex, a new ion-conductive pathway was formed to aid membrane transport. To maximize gains from the ion-conductive pathway, trends of functional group basicity with ion conductivity were realized to select quaternary phosphonium (QPOH) as a fixed-site carrier for facilitated transport. Consequently, the resulting polysulfone-functionalized membrane (PSf-QPOH) exhibited CO2 permeabilities up to 1090 Barrer and CO2/N2 selectivities as high as 275, surpassing the Robeson Upper Bound. ā The second objective was to elucidate the morphology for the family of quaternary phosphonium membranes: PSf-QPOH and poly(phenylene oxide)-functionalized quaternary phosphonium (PPO-QPOH). Using small angle neutron scattering, a Teubner-Strey and fractal model was combined to produce an excellent fit of scattering data, suggesting the morphology to be bicontinuous, hydrophilic-hydrophobic scatterers 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. ā 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 traditional chlor-alkali processes (EĀ°=2.19 V). The electrolyzer was successfully realized by incorporating an AEM design and the cathodic redox pairs of either Fe(III)/Fe(II) or p-benzoquinone/hydroquinone. Fe(III) exhibited a cell potential of 0.76 V, while p-benzoquinone exhibited either 0.93 V or 1.55 V, depending on the presence of acid. In addition, all cases achieved anodic oxidation reduction potentials above 900 mV in less than 30 minutes of operation, showing excellent biocidal activity for water remediation.
Description
Keywords
Carbon, Electrolyzer, Gas separation, Membrane, Polymer, Small angle neutron scattering