NiO2 base crystal serves as an electron donor source which injects electrons to Pd nanocrystals by strong electronegativity difference and lattice strain. It creates a high density of active sites on the NC surface and thus improves the HER kinetics.

Tetrahedral symmetric NiO2 and Pd respectively facilitate H2 splitting and CO2 to CO reduction and thus enable an ultra-high CH4 production yield performance in the epitaxial interfaces in the bimetallic NiO2@Pd NPs.

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 NiO x /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 °C and then turns to a highly mismatched Pd to NiO2 epitaxial structure by increasing the deposition temperature to 70 °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 O2-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.

The oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) both are key electrochemical reactions for enabling next generation alternative-power supply technologies. Despite great merits, both of these reactions require robust electrocatalysts for lowering the overpotential and promoting their practical applications in energy conversion and storage devices. Although, noble metal-based catalysts (especially Pt-based catalysts) are at the forefront in boosting the ORR and HER kinetics, high cost, limited availability, and poor stability in harsh redox conditions make them unfit for scalable use. To this end, various strategies including downsizing the catalyst size, reducing the noble metal, and increasing metal utilization have been adopted to appropriately balance the performance and economic issues. This mini-review presents an overview of the current state of the technological advancements in noble metal-based heterogeneous nanocatalysts (NCs) for both ORR and HER applications. More specifically, we focused on establishing the structure–performance correlation.

Carbon nanotube supported ternary metallic nanocatalysts (NCs) comprising Ni core –Pd shell structure and Pt atomic scale clusters in shell (namely, Ni@Pd/Pt) are synthesized by using wet chemical reduction method with reaction time control. Effects of Pt 4+ adsorption time and Pt/Pd composition ratios on atomic structure with respect to electrochemical performances of experimental NCs are systematically investigated. By cross-referencing results of high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption, density functional theoretical calculations, and electrochemical analysis, we demonstrate that oxygen reduction reaction (ORR) activity is dominated by depth and distribution of Pt clusters in a Ni@Pd/Pt NC. For the optimum case (Pt 4+ adsorption time = 2 h), specific activity of Ni@Pd/Pt is 0.732 mA cm –2 in ORR. Such a value is 2.8-fold higher as compared to that of commercial J.M.-Pt/C at 0.85 V (vs reversible hydrogen electrode). Such improvement is attributed to the protection of defect sites from oxide reaction in the presence of Pt clusters in NC surface. When adsorption time is 10 s, Pt clusters tends to adsorb in the Ni@Pd surface. A substantially increased galvanic replacement between Pt 4+ ion and Pd/Ni metal is found to result in the formation of Ni@Pd shell with Pt cluster in the interface when adsorption time is 24 h. Both structures increase the surface defect density and delocalize charge density around Pt clusters, thereby suppressing the ORR activity of Ni@Pd/Pt NCs.