Organic-inorganic hybrid PtCo nanoparticle with high electrocatalytic activity and durability for oxygen reduction

Abstract

In Pt-transition metal (TM) alloy catalysts, the electron transfer from the TM to Pt is retarded owing to the inevitable oxidation of the TM surface by oxygen. In addition, acidic electrolytes such as those employed in fuel cells accelerate the dissolution of the surface TM oxide, which leads to catalyst degradation. Herein, we propose a novel synthesis strategy that selectively modifies the electronic structure of surface Co atoms with N-containing polymers, resulting in highly active and durable PtCo nanoparticle catalysts useful for the oxygen reduction reaction (ORR). The polymer, which is functionalized on carbon black, selectively interacts with the Co precursor, resulting in Co–N bond formation on the PtCo nanoparticle surface. Electron transfer from Co to Pt in the PtCo nanoparticles modified by the polymer is enhanced by the increase in the difference in electronegativity between Pt and Co compared with that in bare PtCo nanoparticles with the TM surface oxides. In addition, the dissolution of Co and Pt is prevented by the selective passivation of surface Co atoms and the decrease in the O-binding energy of surface Pt atoms. As a result, the catalytic activity and durability of PtCo nanoparticles for the ORR are significantly improved by the electronic ensemble effects. The proposed organic/inorganic hybrid concept will provide new insights into the tuning of nanomaterials consisting of heterogeneous metallic elements for various electrochemical and chemical applications. Attaching bimetallic nanoparticles to a special, polymer-modified support improves the durability and activity of fuel-cell catalysts. Nanoscale alloys consisting of platinum and transition metals such as cobalt or iron have emerged as promising catalysts for fuel-cell cathodes because they can reduce costs and accelerate the oxygen reduction reaction through electron-transfer processes. Sung Jong Yoo and co-workers from Korea and India have developed a catalyst support that prevents surface oxides from attacking platinum-cobalt (PtCo) alloys and degrading their performance. The team reacted a typical carbon-black support with poly(N-isopropylacrylamide), a polymer possessing nitrogen atoms that bind to cobalt without physically blocking access to platinum catalytic sites. The hybrid organic-inorganic catalyst reduces premature alloy dissolution during fuel-cell operation and enhances electron transfer from Co to Pt atoms. Amide group-containing polymer functionalized on carbon surfaces selectively interacts with the Co precursor, resulting in Co–N bond formation on PtCo nanoparticle surfaces. The electron transfer from Co to Pt is enhanced due to increased electronegativity difference between Pt and Co. In addition, the dissolution of Co and Pt during the oxygen reduction reaction (ORR) is retarded by the selective passivation of surface Co atoms and the decrease in the O-binding energy of surface Pt atoms. As a result, the catalytic activity and durability of the hybrid PtCo nanoparticles for the ORR are significantly improved by the electronic ensemble effects.