Heterogeneous NiO2-to-Pd Epitaxial Structure Performs Outstanding Oxygen Reduction Reaction Activity

A novel nanocatalyst (NC) with an epitaxial structure of Ni oxide adjacent to a metallic Pd nanocrystal is developed for oxygen reduction reaction (ORR). We demonstrate that, by formation of an ordered local structure both in Ni oxide and Pd regions, the ORR performance of such an NC can be substantially enhanced eightfold as compared to that of NiOx/Pd nanocomposites (control sample) without a local ordering structure. By cross-referencing results of physical and electrochemical inspections, we revealed that the control sample has a complex cluster-in-cluster structure of Ni oxides/NiPd alloy/Pd nanocrystals when Ni ions are deposited on the carbon support (acid-treated carbon nanotubes) at 25 degrees C and then turns to a highly mismatched Pd to NiO2 epitaxial structure by increasing the deposition temperature to 70 degrees C. In this event, a strong lattice mismatch and electronegative difference preserve the metallic characteristics of Pd. Such a scenario reduces the energy barrier and kinetics for O-2-splitting, therefore boosting the ORR activity. With such a unique structure, the mass activity (MA) is 231.2 mA mg(-1) and specific activity is 0.492 mA cm(-2) for the NiO2/Pd epitaxial structure. Those properties are 3.5- and 2-times improved as compared to that of commercial Johnson Matthey [J.M.-Pt/C (20 wt % Pt)], which shines light on Ni-based catalysts to be potential candidates against expensive and limited Pt-based catalysts in fuel-cell applications.