Most electrochemical technologies that operate under ambient conditions require ion-conducting polymer electrolytes. These polymers are passive in that electric bias drives ion migration in the thermodynamically favored direction. Recently, my group engineered two important features into passive ion-selective polymers to introduce the active function of photovoltaic action and demonstration of an ionic solar cell. These features were covalent bonding of photoacid dyes to the polymers such that absorption of visible light resulted in liberation of protons, and synthesis of polymer membranes with charge-selective contacts to facilitate separation and collection of H+ and OH–. Light excitation from either side of the polymer membranes resulted in H+ dissociation followed by directional charge collection. The charge collection direction was dictated by the electrostatic asymmetry in the polymers, which was formed due to an external pH difference setup across the membrane.

Joining a monopolar cation-selective polymer to a monopolar anion-selective polymer forms a bipolar membrane, which mimics a rectifying semiconductor pn-junction diode in form and function, and is able to maintain pH differences across it. Using a photoacid-dye-modified bipolar membrane, we measured a photovoltage of ~120 mV under conditions of solar-simulated excitation. In addition to more traditional electrochemical techniques, insights into materials function were obtained using finite-element numerical modeling of photoacid kinetics and membrane physics; beam-line x-ray scattering measurements; electrochemical impedance, solid-state NMR, and pulsed-laser spectroscopies; and fluorescence, electron, and force microscopies. Collectively, these photo-responsive polymers represent a new class of functional materials that use light to trigger changes in local ion concentration and electrostatic potential. These local changes can be used to affect macroscopic processes such as direct sunlight-driven redox chemistry or desalination of salt water, chemical catalysis, and triggering of cellular processes.

References
(1) Reiter, R. S.; White, W.; Ardo, S. Journal of The Electrochemical Society, 2016, 163, H3132–H3134. Electrochemical characterization of commercial bipolar membranes under electrolyte conditions relevant to solar fuels technologies.
(2) White, W.; Sanborn, C. D.; Reiter, R. S.; Fabian, D. M.; Ardo, S. Journal of the American Chemical Society, 2017, 139, 11726–11733. Observation of photovoltaic action from photoacid-modified Nafion due to light-driven ion transport.
(3) White, W.; Sanborn, C. D.; Fabian, D. M.; Ardo, S. Joule, 2018, 2, 94–109. Conversion of visible light into ionic power using photoacid-dye-sensitized bipolar ion-exchange membranes.












Shane Ardo (University of California Irvine)