MOF-Based Architectures towards Higher Activity Oxygen Reduction Reaction Catalysts

The electrocatalyst responsible for carrying out the oxygen reduction reaction (ORR) at the hydrogen fuel cell (FC) cathode has a demanding task to perform in a highly acidic environment and under a corrosive potential. Despite its cost, platinum (Pt) has become the most common catalyst proven to have high activity and durability towards ORR catalysis. There is, therefore, a need to design and develop more cost-effective catalysts that can either replace or reduce Pt content. A key challenge in the design of ORR catalysts is maximizing the conduction and utilization of electrons, protons, and oxygen to the active sites on the electrode surface. For use in intermediate temperature hydrogen fuel cells, which operate above the boiling point of water, the ability to minimize transport barriers in the electrolyte material and optimizing the way it couples to the ORR catalyst is of great importance to maximize catalytic turnover and efficiency. To that end, we have set out to develop approaches toward using metal-organic frameworks (MOFs) as templates for high-surface-area platinum structures to develop high-performance ORR catalysts. As a proof-of-concept, we have chosen the highly robust and well-known UiO series, which among other benefits has the potential for pore-size tuning that should be conducive for effective gas transport. The MOF synthesis has been optimized to target well-defined, single crystalline MOF particles with shape control and narrow size distributions and scaled to multi-gram batches. Subsequently, we have explored different approaches to deposit Pt onto these MOF nanoparticles using methodologies that include colloid-derived methods, as well as solvothermal and atomic layer deposition (ALD) approaches. In this work we compare the different Pt deposition approaches by assessing the relative ORR activities and correlate the results to X-ray photoelectron spectroscopy (XPS) and the catalyst morphology as determined by high-resolution transmission electron microscopy (HR-TEM).