Abstract:

Bipolar membranes are 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. This talk explores mechanisms of photovoltaic action, bulk current conduction and electric field-enhanced and chemically catalyzed water dissociation as they relate to bipolar membrane systems. First, the fabrication of a protonic solar cell consisting of a bipolar membrane sensitized to visible light with covalently bound photoacid dye molecules will be discussed. 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. Bipolar membrane-electrode assemblies containing thin films of phosphonic acid proton-transfer catalysts will also be discussed. The phosphonic acid catalysts are akin to Shockley‒Read‒Hall recombination centers in traditional semiconductors and were observed to enhance the rate of water dissociation and formation in the bipolar membrane under small applied potentials. Lastly, the design and use of a finite-element model to identify design principles for a floating reactor capable of direct oceanic carbon capture will be described. The thermodynamic model considers a web of chemical reactions at play in the oceanic CO₂ system, namely proton-transfer processes between water, dissolved inorganic carbon species, and catalyst species that are being considered to functionalize the reactor with in order to enhance the rate of CO₂ capture. Liquid flow rates and vacuum levels that yield the most efficient reactor operation will be identified in addition to catalytic strategies that can be pursued to speed rate-limiting steps in the system.