Ion-conducting polymers are passive. They respond to electric biases that are applied across them by transporting ions via migration. However, there is no reason these materials cannot be made active, such that upon absorption of light, ions are driven in directions pre-defined by built-in potential distributions in the materials and collected via selective contacts to generate power. This is exactly how traditional solid-state solar cells work and the physics that dictates this behavior only requires that the mobile carriers be charged, like ions, and not that they specifically be electrons and/or holes.

My research group studies solar energy conversion technologies that generate power from sunlight absorption through ion transport. In my presentation I will report on my research group’s recent demonstration of ion transport against concentration gradients that was driven by solar illumination of dye-sensitized ion-conducting materials. Mechanistically, visible light was used to drive endergonic excited-state proton transfer from a covalent photoacid-modified cation-conductive membrane. Photoacid molecules convert the energy in light into a change in the chemical potential of a proton via weakening of a protic functional group on the photoacid, i.e. a decrease in its pK_a. A cation-conductive membrane served as the selective contact for protons such that absorption of light resulted in photovoltaic action, i.e. a photocurrent and a photovoltage. Bipolar membranes are another class of ion-conductive polymers that were used to demonstrate similar photo-activity. They consist of a monopolar cation-conductive polymer that is in intimate contact with a monopolar anion-conductive polymer. The physics that describes the ion-equilibration processes within bipolar membranes resembles that which occurs during equilibration of semiconductor pn-junctions.

As a model system for ion-channels in phase-segregated ion-conducting polymers, 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 and fluorescence microscopy were used to determine the photoacidity of the surface-bound dye molecules, which changed from the values found in bulk solution, likely as a result of surface charges in the confined nanopores.

This body of work represents an underappreciated solar energy conversion process that is being pioneered by my research group to operate via a mechanism similar to that of semiconductor pn-junctions. The applicability and practicality of these materials as standalone devices for desalination of salt water will also be presented.