Solar Generation of Ion Gradients Using Dye-Sensitized Ion-Exchange Membranes

Abstract

Most solar energy conversion technologies generate power through transport of energized electron particles; however, the physics that describes these technologies only requires that the particles be charged and not specifically that they are electrons. My research group studies solar energy conversion technologies that generate power from sunlight through ion transport. In my presentation I will report on my research group’s recent demonstration of ion transport against concentration gradients driven by solar illumination of dye-sensitized ion-exchange materials. Mechanistically, visible light was used to drive endergonic excited-state proton transfer from a covalent photoacid-functionalized polymer membrane. Photoacid molecules convert the energy in light into a change in the chemical potential of protons via a weakening of protic functional groups on the photoacid, i.e. a drop in its pK a. As a model system for ion-channels in polymer-electrolyte ion-exchange membranes, dye-sensitized conical nanopores in poly(ethylene terephthalate) (1 – 108 pores/cm2) were investigated. Remarkably, in a region occupied by ~20 zeptoliters (~2 x 10-20 L) of aqueous electrolyte, electrochemistry was used to determine the number of binding groups of the dyes and the ground-state pK a of the dyes, and fluorescence microscopy was used to determine the conditions where excited-state proton transfer occurred. These data were consistent with a hypothesis that pK a values of the photoacids in the ground-state and excited-state were significantly smaller than those measured for dye molecules in solution, likely due to incomplete screening of surface charges in the confined nanopores. Dye-sensitized Nafion monopolar ion-exchange membranes and bipolar ion-exchange membranes were also investigated. Under sunlight-simulated illumination these materials were found to exhibit photovoltaic action, i.e. generation of a photocurrent and a photovoltage. Bipolar membranes are a class of polymeric ion-exchange materials that consist of a monopolar cation-exchange membrane that is in intimate contact with a monopolar anion-exchange membrane. They are unique among the ion-exchange membranes in that they that separate and maintain pH differences across the membrane even during passage of ionic current. Moreover, the physics of ion equilibration processes within these membranes resembles that which occurs during equilibration of semiconductor pn-junctions. This body of work represents an underexplored solar energy conversion process that is being pioneered by my research group to operate via a mechanism similar to that in semiconductor pn-junctions. The applicability and practicality of these materials as standalone ionic photoelectrochemical devices will also be presented.