UC Irvine

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 ∼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. Access to clean and potable water is one of the top issues facing humanity and is expected to become even more pressing in the next several decades. While efficient and renewable energy technologies could supply the power needed to generate potable water, advances are slow and are in general incremental from prior demonstrations. Moreover, the developing world cannot afford current clean water solutions, yet the majority of the four billion people expected to be under water stress in approximately the next decade will live in the developing world. In the long term, it is expected that newly developed nations with large gross domestic product and an established grid or micro-grid infrastructure will utilize state-of-the-art cost-effective plant-scale technologies for clean water generation, such as those based on osmosis, distillation, or electrodialysis. In the immediate near term, an inexpensive technology to desalinate water could support economic development. A composite ion-exchange polymer membrane was shown to mimic a traditional solar cell and exhibit photovoltaic action. Visible-light excitation of the membrane produced ionic power when photoacid dyes were covalently bonded to the polymer and ion-selective contacts were used. The photogenerated ionic power can be used to decrease power demands in any electrochemical device and could be particularly useful for direct light-driven desalination of salt water.