X-ray dual phase grating interferometer for quantitative dark-field imaging of fuel cell catalyst
The energy system of today’s world relies mainly on fossil fuels as the primary source of energy. The limited availability of fossil fuels coupled with concerns of global warming has made research into alternate fuels even more necessary. As many more population-rich developing countries become prosperous, the demand for energy would go up. Hence a carbon-free fuel economy would be essential to address these concerns. Polymer Electrolyte Fuel Cells (PEFC) are a key technology for achieving this goal. For the past few years, research activities are being carried out worldwide on PEFC as they can be used for both stationary (for storage of fluctuating renewable energy sources) and mobile applications (PEFC cars and buses, chargers for portable devices) with high conversion efficiency( 60 % and higher). To be widely used, PEFC technology would still need improvements in efficiency, durability, performance and cost. Cost reduction could be achieved by increasing the power density, as smaller and more powerful systems require less material and fewer components. At the required high power densities of 1 W/cm2 and higher, the water management in the cells becomes the performance limiting factor. Hence, there is a need to understand water flow through components of a fuel cell.
Catalyst layer is the smallest and the most critical component of a fuel cell. Understanding of the liquid water behavior in catalyst layers of PEFC bears a great potential for further improvement. However to date, structure optimization to improve water management has mostly relied on trial and error approaches, as the characterization of the layers under operando conditions is extremely difficult and therefore limited approaches have been performed. Conventional X-ray imaging fails for catalyst layers because of very small pores in the layer (mean pore size is 110 nm), coupled with this, the presence of platinum in this layer makes the layer opaque for low energy X-rays. High resolution imaging techniques like ptychography has smaller field of view (FOV) and needs to be operated in special operation conditions. Hence, rendering it unusable for in-operando studies.
X ray dark field imaging has shown promise for imaging such materials due to its unique ability of accessing unresolvable structural information, which is otherwise inaccessible to conventional absorption-based full field imaging. Especially using dual phase grating interferometry, the sensitivity to microstructure size can be changed easily which could potentially access information about water content in various pore sizes in catalyst layer. This approach could be applied to fuel cell in situ experiments to better understand water transport in pores of catalyst layer.