Enhancement of catalytic activity of a programmed gold nanoparticle superstructure modulated by supramolecular protein assembly

Abstract

Improving the efficiency of the oxygen reduction reaction (ORR) for use in commercial fuel cells has been the subject of diverse synthetic research activities. However, due to their inferior catalytic performance and dispensability, there are still limitations to achieving breakthroughs regarding ORR catalytic activity using carbon nanomaterials, despite their high electron-transfer and mass-transport properties. In this study, we mimicked nature by using a biomineralization approach for controlling the growth of inorganic materials and demonstrated improved ORR values. The designed peptide, Hexcoil-Ala, is able to supramolecular assembly on single-walled carbon nanotubes (SWNTs), leading to SWNTs that are well dispersed in aqueous solution. In order to direct gold nanoparticle (AuNP) nucleation sites, we substitute two residues in Hexcoil-Ala with cysteine to provide the mutated peptide, HexCoil-Ala-2Cys. This peptide affords a sophisticated, size-controlled, and well-dispersed arrangement of AuNPs. High-resolution transmission electron microscopy studies confirmed the homogeneously well-aligned distribution of nanosized AuNPs on the HexCoil-Ala-2Cys structure, along the direction of SWNT axis. The (AuNPs/P-SWNT) composite in water provides dispersed and stable metallic nanoparticles of electrostatically modified Au through synergistic effects involving the peptide. Consequently, this catalyst exhibits improved ORR performance compared to bulk gold and even, in case of number of electrons (n) transferred, higher than the number of Pt/C. X-ray photoelectron and Raman spectroscopies reveal the details of the electronic interactions among the components of the AuNP/P-SWNT composite, and how they facilitate the four-electron reaction pathway. This study provides valuable information for the optimization of catalyst synthesis and precise particle-size control, leading to stable, water-dispersive composites, with improved electronic properties for enhanced ORR performance in fuel cells.