In a recent study, ultrathin oxide overcoat was proven to have significant impact on the ORR kinetics of nanoporous Pt catalyst on yttria-doped zirconia (YSZ).¹ In the report, a few nm thick ultrathin yttria-stabilized zirconia (uYSZ) was coated on the porous Pt cathode by atomic layer deposition (ALD). The coating was found not only to preserve the overall Pt morphology but also to improve the electrode reaction. The uniform and conformal geometry of the as-deposited overcoat became percolated at the operational temperature (~500°C), which is conjectured beneficial to the catalytic activity due to enlarged Pt/YSZ interfaces.

In this presentation, we present our revised understanding of how the ultrathin oxide overcoat improved the ORR catalysis through a parametric study. Geometrically well-defined Pt/uYSZ electrode structures were prepared by accumulating Pt nanoparticles through a colloidal process. We controlled the Pt nanoparticle size, dominant surface facets, overall electrode thickness, YSZ overcoat thickness and stoichiometry of overcoat. Pt nanocrystals with different sizes and controlled surface facets were prepared through wet chemical and electrochemical processes. We then performed electrochemical analysis at various temperature (400 – 700 °C) and oxygen partial pressure (0.01 – 1 atm). A series of characterization indicated that the enhanced ORR activity is most likely originated from the enhanced ‘chemical’ processes such as dissociation and transport. However, in samples with thin Pt electrode layer, the enhancement is largely ascribed to increased triple phase boundary areas.

This project is funded by the NASA MUREP Institutional Research Opportunity (MIRO) Program (Grant No. NNX15AQ01A).

¹ I. Chang, S. Ji, J. Park, M.H. Lee, and S.W. Cha, Adv. Energy Mater. 5, 1402251 (2015).