Oxygen Reduction on Gold Nanocrystal Surfaces in Alkaline Electrolyte: Effects of Surface Proton Transfer

Abstract

The surface atomic structures of metal catalysts play critical roles in determining the kinetics and mechanisms of catalytic reactions. Intrinsic correlations between surface structures and catalytic performances have been extensively studied for the bulk single-crystal surfaces. In oxygen reduction reaction (ORR), an essential reaction in energy conversion devices such as alkaline fuel cells, a complete four-electron (4e) ORR is preferred because energy conversion is twice as efficient compared with the partial two-electron (2e) ORR pathway. Adzic and co-workers pioneered the study of structural sensitivity of the ORR pathway on gold (Au) surfaces. They found that the ORR in alkaline electrolytes proceeds via a 2e reduction to hydroperoxide ions (O2H−) on the majority of facets including Au(111)[ 1 ], while the complete 4e reduction to hydroxide ions (OH−) occurs only in a limited potential region on Au(100)[ 2 ] and on the high-index Au(910) and Au(11,1,1) single crystal surfaces with a large fraction of the {100} sub-facet[ 3 ]. However, the reasons for such ORR behaviors on Au surfaces remain unclear. In this study, we synthesized three monofacet Au nanocrystals with well-defined shapes[ 4 ], including octahedra, cubes and truncated ditetragonal prisms (TDPs), which are enclosed by {111}, {100}, and {310} surfaces, respectively. The surface cleanness and high crystallinity of Au nanocrystals are confirmed by several structural characterization techniques, and by electrochemical characterizations including cyclic voltammetry (CV) and thallium underpotential deposition (Tl UPD). The Au octahedral and cubic nanocrystals demonstrate good resemblance of the ORR behaviors to those on their bulk single-crystal counterparts. Intriguingly, the TDP particles enclosed with twelve {310} facets catalyze the 4e ORR over the full potential range that has not been seen. Density functional theory (DFT) calculations were carried out for the ORR on the three Au surfaces without and with co-adsorbed water. The DFT-calculated free energy diagrams, in combination with experimental electrochemical results, elucidate how the surface proton transfers activate the facet- and potential-dependent 4e ORR on Au {100} and {310}. The new insight helps mechanism studies of facet-sensitive catalytic reactions, and provides guidance for designing nanocatalysts with optimal surface structures for various reactions. Acknowledgment The research was carried out at the Center for Functional Nanomaterials and the Chemistry Division of Brookhaven National Laboratory, supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. We acknowledge the computational resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.