Benefits and Limitations of Encapsulating Platinum Electrocatalysts with Niobium Oxide

Abstract

The development of energy infrastructure based on renewable sources is vital to reduce our reliance on fossil fuels. Technologies such as hydrogen fuel cells and water electrolyzers are able to bridge the gap between the intermittency issues associated with green energy sources such as wind and solar, and the demand for reliable electricity by being able to convert between electrical and chemical energy. Nanostructured and mesoporous platinum (Pt) prepared by electrodeposition or sputter coating have been shown to have increased performance in hydrogen fuel cells compared to the industry standard of Pt nanoparticles supported on carbon.1,2 Over the lifetime of a fuel cell Pt nanostructures and nanoparticles are, however, known to restructure and lose their surface area by mechanisms such as Ostwald ripening, dissolution, and coalescence. Previously, thin film Pt catalysts have been shown to be stabilized by a nanoscale film of niobium oxide.3 Herein, nanoscale thin films of niobium oxide are investigated as a stabilizing material through use by encapsulating nanostructured Pt catalysts.Nanostructured Pt catalysts were electrodeposited onto Pt-coated Si wafer substrates. Subsequently, niobium oxide films with thicknesses of 0.5 nm, 3 nm, and 4.5 nm were deposited onto the Pt catalyst using atomic layer deposition. X-ray fluorescence and conductive atomic force microscopy techniques were used to confirm the presence and coverage of the niobium oxide layer on the Pt catalyst. The performance of the encapsulated and un-encapsulated Pt catalysts as oxygen reduction reaction catalysts over the course of up to a thousand degradation cycles were evaluated using electrochemical techniques. The morphology of the catalyst before and after the degradation testing was also assessed using scanning electron microscopy techniques. Electrochemical impedance spectroscopy indicated that increasing the thickness of the niobium oxide layer increased the resistance of the catalyst towards the oxygen reduction reaction. The combined results of the analyses performed by electrochemical techniques and electron microscopy measurements revealed a range of film thicknesses for the niobium oxide that successfully stabilized the electrochemically active surface area and surface morphology of the Pt catalyst. Paul, M. T. Y.; Gates, B. D. Mesoporous Platinum Prepared by Electrodeposition for Ultralow Loading Proton Exchange Membrane Fuel Cells. Sci Rep 2019, 9, 4161.Debe, M. K. Tutorial on the Fundamental Characteristics and Practical Properties of Nanostructured Thin Film (NSTF) Catalysts. Electrochem. Soc. 2013, 160, F522.Eastcott, J.; Gates, B. D. Nanoscale Thin Films of Niobium Oxide on Platinum Surfaces: Creating a Platform for Optimizing Material Composition and Electrochemical Stability. Can. J. Chem. 2017, 96, 260–266.