Catalytic Activity Enhancement in Oxygen Evolution Reaction on LDH Nanosheets By Metal-Cluster Loading

Abstract

Electrochemical water splitting enables to store electricity produced from intermittent renewable solar and wind energy resources in the form of H2. Development of highly active electrocatalysts for kinetically sluggish anode reaction, i.e., oxygen evolution reaction (OER) remains a major challenge for several decades. Recently, metal hydroxide nanosheets exfoliated from layered double hydroxide composed of Ni2+ and Fe3+ (NiFe-LDH) were found to show excellent catalytic activities for OER. However, a high overpotential on LDH was still required to accelerate OER. Considering that Fe sites are regarded as a catalytic site and the overpotential for OER on the LDH nanosheet catalysts depends on a binding energy between intermediate molecules and Fe sites, modification of electronic structure of LDH probably controls the binding states and enhances catalytic activities on LDH. In this study, we prepare NiFe-LDH nanosheets (NiFe-NS) decorated with Au clusters and examine influence of the Au loading both on electronic states of Fe sites and catalytic activities of the NS. NiFe-LDH was synthesized by hydrothermal method and colloidal Au clusters were prepared in DMF as reported previously. Carbonate ions in NiFe-LDH were exchanged to perchlorate ions, and the perchlorate ion–intercalated sample was dispersed in formamide, resulting in formation of colloidal NiFe-NS solution. Au-cluster-loaded NiFe-NS (Au/NiFe-NS) was prepared by mixing colloidal solutions of the NiFe-NS and Au clusters. Scanning TEM measurements revealed that small Au clusters around 1.2 nm in diameter are homogeneously dispersed on LDH. Fig. 1 shows iR-corrected polarization curves of pristine NiFe-NS, Au clusters and Au/NiFe-NS electrocatalysts in 1 M KOH aqueous solution. The Au/NiFe-NS exhibited much larger oxidation current at a lower potential than the pristine NiFe-NS whereas Au clusters showed negligible activity, indicating that catalytic activity of NiFe-NS was significantly enhanced through the Au-cluster loading. XPS and XAFS measurements elucidated that Fe sites of Au/NiFe-NS are in more reduced states compared to those in the pristine NiFe-NS, resulting from charge transfer from Au clusters to Fe sites. Since bond breaking between the Fe site and intermediates is a rate determining step, we then conclude that reduction of Fe ions contributes to weaken binding energy and enhance catalytic activities. We achieved enhancement of catalytic activities of NiFe-NS via modification of electronic states of Fe sites using charge transfer interaction induced by Au cluster loading. Fig. 1 IR-corrected polarization curves for OER on pristine NiFe-NS, Au clusters and Au/NiFe-NS in 1 M KOH aqueous solution. Figure 1