Three-DimensionallyConnected Platinum–CobaltNanoparticles as Support-Free Electrocatalysts for Oxygen Reduction

We developed a unique nanostructured material that is composed of three-dimensionally (3D) connected Pt<i>_x</i>–Co_{100–<i>x</i>} alloy nanoparticles (NPs) with chemically ordered arrangements. This material consists of a conductive nanonetwork of connected metal NPs, which can be used as an electrocatalyst without a conductive support, and possesses a unique surface structure that is different from that of conventional nanomaterials. Support-free connected Pt<i>_x</i>–Co_{100–<i>x</i>} NPs with controlled metal compositions and arrangements were successfully developed under different synthesis conditions and then applied as an oxygen reduction reaction (ORR) catalyst for polymer electrolyte fuel cells. Systematic investigation of the structural effect on the ORR performance showed that the unique surface of the connected Pt₇₀–Co₃₀ NPs with an L1₂ ordered structure enhanced the ORR specific activity, which was approximately 10 times that of a conventional catalyst of platinum NPs on carbon black. In addition, the combination of 3D connected NPs and high chemical ordering retained the highly active surface even after the load cycling durability test. Therefore, the characteristic structure of the connected NPs and the surface formed by them make the developed material an effective fuel cell electrocatalyst.