Conversion of Visible Light into Ionic Power Using Photoacid-Dye-Sensitized Bipolar Ion-Exchange Membranes

For over half a century, it has been recognized that the classical physics that dictates the behavior of hydrated ion-exchange membranes, such as Nafion, and semiconductors, such as doped silicon, are similar. However, no demonstrations of photovoltaic action from ion-exchange materials existed. We recently reported a synthetic light-driven proton pump derived from an ion-exchange membrane. Absorption of light by covalently bound photoacid molecules resulted in photovoltaic action. This design lacked a second permselective membrane contact, which limited its performance. Here, we report a similar to 60-fold increase in the photovoltage through use of a bipolar membrane structure consisting of a cation-exchange membrane affixed to an anion-exchange membrane. The junction between the layers was characterized in detail using electrochemistry, scanning electron microscopy, spectroscopy, and thermal gravimetric analysis. Our results represent considerable progress toward a device that directly converts sunlight into ionic electricity, which has implications for direct solar desalination of salt water.