UC Irvine

This thesis explores mechanisms of photovoltaic action, bulk current conduction and electric-field-enhanced and chemically catalyzed water dissociation in bipolar membranes, polymeric devices that, when exposed to water and when pretreated to have only H+ and OH‒ as the mobile species, abide by the physics and mimic the behavior of semiconductor pn-junctions. The first study entails the fabrication of a protonic solar cell consisting of a polymeric bipolar membrane sensitized to visible light with covalently bound photoacid dye molecules. Electrodes were applied to either face of the membrane and fed with humidified H2 to form RHEs as part of MEAs that allowed interconversion of electronic and protonic signals. Photon absorption by photoacids covalently bound within the bipolar MEA resulted in proton transfer to water and/or hydroxide. Classical solar cell current-voltage curves and Mott-Schottky analyses in the dark and under illumination showed “reverse” photovoltaic action, consistent with a light-induced loss of protonic mobile charge carriers or dynamic processes in these new polymeric materials. The second study concerns similar bipolar membrane-electrode assemblies (MEAs) but rather than containing photoacidic dyes, the junction is coated with an intermediate polymer that contains organic proton-transfer catalysts, specifically phosphonic acid groups, that are akin to Shockley‒Read‒Hall recombination centers in traditional semiconductors and can enhance the rate of water dissociation at small applied potentials. The report described herein is the first to utilize a polymer scaffold, poly(2,6-dimethyl-1,4-phenylene oxide), to anchor organic functional groups with controlled pKa values in the bipolar membrane. The champion bipolar membrane in the study yielded a two order of magnitude enhancement in the rate of water dissociation compared to the state-of-the-art commercial metal oxide-containing bipolar membrane. A thickness dependence study was performed by varying the dimensions of the phosphonic acid catalyst layer, and insight into the mechanism of water dissociation current and current conduction in the BPM was gained. A numerical model was also developed using the Law of Mass Balance and Poisson-Nernst-Planck transport physics to rationalize experimental trends. The third study utilizes finite-element modeling to identify design principles for a floating reactor capable of direct oceanic carbon capture. A thermodynamically rigorous model was developed to include a web of 13 chemical reactions at play in the system, namely proton-transfer processes between water, dissolved inorganic carbon species, and catalyst species that we propose to functionalize the reactor with to enhance the rate of CO2 capture. Analyses have identified the range of liquid flow rates and vacuum levels at which the system should operate to achieve substantial carbon capture and means to enhance performance using catalysts for specific reaction steps.