Anion Exchange Membrane for High Efficiency Solar-to-Hydrogen Generation

The increasing availability of renewable energy sources, including solar, coupled with the desire to reduce greenhouse gas emissions, has led to an increased interest in using renewables to generate alternate energy sources. While the electricity produced by solar energy from photovoltaics can be used directly, intermittency andgeographic limitations make it unfeasible to completely replace fuel sources that can be used whenever and wherever they are needed. Therefore, the ability to convert solar energy directly into chemical fuels, such as hydrogen, has garnered increasing attention over the past several decades. Hydrogen is an ideal fossil fuel alternative because it is storable, transportable, and burns cleanly, producing power without polluting the environment. To improve utilization of solar energy and reduce fossil fuel consumption, we will build device to convert solar energy and water directly into storable hydrogen fuel. While significant progress has been made in the development of HER and OER catalysts for use in electrolyzers and fuel cells, and in photoanodes and photocathodes for photovoltaics, integration of these components with membranes remains a challenge, particularly in alkaline environments. Most work thus far has focused on proton exchange membrane PEM) systems due to the relative ease of proton transfer compared with hydroxide transfer and durability furthering device lifetime. However, alkaline environments favor the sluggish OER reaction, enabling the use of cheaper, more abundant non-precious metal group catalysts. Unfortunately, there are currently no commercially available anion exchange membranes that are stable for long-term use in a direct solar-to-hydrogen cell. This is partly due to the particular challenges associated with solar fuel formation, in particular the need for UV light stability, seasonal temperature variation, and on/off cycling with the day/night, as well as more stringent crossover limitations and integration with solar absorbers. In this project, we will build on the success of the PAP AEMs developed at the University of Delaware and the initial success demonstrated by hyperbranched polymer architecture applied to AEMs. We will synthesize hyperbranched piperidinium and imidazole functionalized polymers.The hyperbranched structures will then be tethered together by a hydrophobic matrix material. We will evaluate the durability of the membrane at appropriate reduction and oxidation conditions and exposure to UV radiation. The PAP AEM polymer was designed to be oxidatively stable by elimination of susceptible functional groups. These novel membranes will be used to form a membrane electrode assembly consisting of the membrane laminated with photoelectrodes decorated with HER and OER electrocatalysts. The MEAs will be used to build an alkaline solar cell to generate hydrogen and oxygen directly from water. In Phase II, optimization of the photoelectrochemical cell will be performed, including development of photoanodes, photocathodes, and catalysts that will be integrated directly with the membrane for improved efficiency and hydrogen production.