Nanoporous Anion Exchange Membranes for Solar Fuel Generators

Solar photoelectrochemical cells utilize sunlight, water (and sometimes also CO2), and convert them into hydrogen or other hydrocarbons by splitting water into hydrogen and oxygen (and optionally reacting the hydrogen with CO2 to form synthesis gas with CO2). This method is one of the most attractive technologies to produce carbon neutral or carbon negative fuels for transportation. However, to make photoelectrochemical cells effective, a durable separator is needed. Specifically, there is a need for an anion exchange membrane (AEM) with excellent chemical stability in a strongly alkaline environment, good photo-stability, good hydroxide conductivity, and one that is mechanically robust to limit hydrogen and oxygen crossover. This project will develop an AEM made from a very thin self-assembled, ionically conductive, nanoporous, and crosslinked polymer layer that can be coated onto a microporous expanded PTFE support membrane (ePTFE). The nanoporous polymer will consist of both chemically stable and ionically conductive domains that self-assembles into complex nanostructures with interconnected ionic channels, allowing hydroxide ions to travel across the membrane. The polymer material will also be highly crosslinked to prevent membrane swelling and to resist hydrogen and oxygen crossover. The ion-conducting region will contain stabilized imidazolium functional groups, which can resist hydroxide and UV attack and, thus, can operate at pH 14 under sunlight exposure for extended periods, as required for solar photoelectrochemical cells. In Phase I, TDA prepared, characterized, and tested these nanoporous AEMs in photoelectrochemical cells (PECs). We measured the ion exchange capacity, hydroxide conductivity, photo-stability, and correlated them to the nanoporous structure. Process-structure-property relationships were used to guide our development of these new membrane materials as highly selective anion-exchange membranes. The new membranes had vastly improved stability and a reduced hydrogen crossover, while maintaining a high hydroxide conductivity. In Phase II, TDA will optimize the new membranes for use in PECs, scale up the production of all the needed precursors and scale up the membrane production to 1 square foot sheets. These scaled up membranes will be fully tested in a PEC, optimized for operating conditions (humidity) and demonstrated in multi-month tests in a benchtop PEC under 1-Sol light. Commercial applications include photoelectrochemical cells for long-duration hydrogen generation (to manufacture hydrogen for use in fuel cells, synthesis gas-to-transportation fuels, and back-up power systems).