Beyond Strain and Ligand Effects: Microstrain-Induced Enhancement of the Oxygen Reduction Reaction Kinetics on Various PtNi/C Nanostructures

High Resolution Image Download MS PowerPoint Slide The electrical performance of a proton exchange membrane fuel cell is limited by the slow oxygen reduction reaction (ORR) kinetics. Catalytic improvements for the ORR have been obtained on alloyed PtM/C or [email protected] /C catalysts (where M is an early or late transition metal) in comparison to pure Pt/C, due to a combination of strain and ligand effects. However, the effect of the fine nanostructure of the nanomaterials on the ORR kinetics remains underinvestigated. Here, nanometer-sized PtNi/C electrocatalysts with low Ni content (∼15 atom %) but different nanostructures and different densities of grain boundary were synthesized: solid, hollow, or “sea sponge” PtNi/C nanoalloys, and solid [email protected] /C nanoparticles. These nanostructures were characterized by transmission and scanning transmission electron microscopy, X-ray energy dispersive spectroscopy, synchrotron wide-angle X-ray scattering (WAXS), atomic absorption spectroscopy, and electrochemical techniques. Their electrocatalytic activities for the ORR were determined and structure–activity relationships established. The results showed the following: (i) The compression of the Pt lattice by ca. 15 atom % Ni provides mild ORR activity enhancement in comparison to pure Pt/C. (ii) Highly defective PtNi/C nanostructures feature up to 9.3-fold enhancement of the ORR specific activity over a commercial Pt/C material with similar crystallite size. (iii) The enhancement of the ORR kinetics can be ascribed to the presence of structural defects, as shown by two independent parameters: the microstrain determined from WAXS and the average CO ads electrooxidation potential (μ 1 CO ) determined from CO ads stripping measurements. This work indicates that, at fixed Ni content, ORR activity can be tuned by nanostructuring and suggests that targeting structural disorder is a promising approach to improve the electrocatalytic properties of mono- or bimetallic nanocatalysts.