STUDY OF Ir_CORE-Pt_SHELL CATALYSTS FOR THE OXYGEN REDUCTION REACTION

Proton–Exchange–Membrane (PEM) fuel cells have received much attention in recent years thanks to their high theoretical thermodynamic efficiencies and low emissions. However, PEM–FCs cannot yet meet the requirements of cost and durability for successful large–scale commercialization. The most expensive components of PEM–FCs are the platinum-based cathode and anode catalysts. In order to meet the cost goal, it is essential to reduce platinum group metal loadings a PEM–FC to 0.125 mg cm^-2, with a durability target of 10% voltage degradation after 5000 hours.

We report here on our success in the development of core-shell oxygen-reduction-reaction (ORR) catalysts. By using a core-shell nano-structure concept, we were able to reduce the amount of platinum in the catalysts. Several nano-size (3-4nm) Core - Shell, XC72-supported catalysts were synthesized in a multi-step successive deposition process with NaBH₄ as the reducing agent. The catalysts were composed of an Ir core and a varying wt% of Pt (10-51%). Platinum-surface (shell) composition varied from 22 to 85 atom%. The structure and composition of these core-shell catalysts were determined by EDS, XPS, TEM and XRD. Electrochemical characterization was performed with the use of cyclic-voltammetry and rotating–disk–electrode (RDE) techniques at room temperature and compared to that of a commercially available Pt/Carbon catalyst.

A 44% improvement of catalytic activity (in terms of A g^-1 of platinum) and 87% improvement of catalytic activity with ~3.5 times higher durability over the commercial catalyst were achieved for fresh electrodes and after an accelerated stress test, respectively. These results demonstrate the potential of Ir_Core - Pt_Shell structure in reducing costs and increasing durability of catalysts for the PEM–FC cathode.