Research Center for Synchrotron Light Applications, Kyushu University

Brief Report

Clarification of mechanism for activity enhancement of Au cluster-loaded layered double hydroxide nanosheet electrocatalysts using XAFS measurement

International Institute for Carbon-Neutral Energy Research (I2CNER)
Miho Yamauchi

Professor Miho Yamauchi

International Institute for Carbon-Neutral Energy Research (I2CNER)
Postdoctral Research Associate
Sho Kitano

Postdoctral Research Associate Sho Kitano


 Environmentally-friendly material synthesis without CO2 emission through electrochemical reactions to utilize water as proton and electron source requires highly efficient oxygen evolution reaction (OER, equation (1)), water oxidation by withdrawing electrons on an anode (Figure 1). However, large overpotential, which is additional voltage beyond theoretical potential of water oxidation, 1.23 V vs RHE, to initiate OER taking out four electrons from a water molecule is a significant problem.

2H2O → 4H+ + 4e- + O2 1.23 V vs RHE (1)

 Recently, layered double hydroxides (LDH), which is 2-dimentional layered materials composed of hydroxide nanosheets including divalent and trivalent metal ions and intercalated water molecules and anions (Figure 2), have attracted attention as highly active electrocatalysts and it is known that hydroxide nanosheets exfoliated from LDH composed of Ni2+ and Fe3+ (LDH-ns) show high activities for OER. We newly synthesized Au clusters (AuNC) loaded LDH-ns electrocatalysts (Au/LDH-ns) to enhance catalytic activities of LDH-ns. STEM observation revealed that AuNC with 1 nm diameter were homogeneously loaded on LDH-ns. The Au/LDH-ns showed drastically lower overpotential to initiate OER than that of the pristine LDH-ns (Figure 3).

 We examined XAFS measurements using synchrotron radiation to investigate change in electronic structure of LDH-ns by loading of AuNC and clarify a mechanism of the decrease in overpotential. Ni and Fe K-edge XAFS spectra revealed that the LDH-ns has a crystalline structure of hydroxide form of Ni2+ and Fe3+. Figure 4 (a) and (b) shows Ni and Fe K-edge XANES spectra of Au/LDH-ns and LDH-ns. Change in Ni K-edge XANES spectra before and after loading of AuNC was negligible (Figure 4 (a)) whereas shift of Fe K-edge spectrum to lower energy by loading of AuNC was observed (Figure 4(b)), indicating that Fe species of Au/LDH-ns have a more reduced state than that of the pristine LDH-ns. Figure 4(c) shows Au L3-edge XANES spectra of Au/LDH-ns and AuNC. The spectrum of AuNC showed a similar absorption edge energy to that of bulk Au foil, indicating that a valence state of AuNC is zero, metallic states. The Au/LDH-ns showed a different spectrum from that of AuNC and white line originated from oxides was observed, indicating that the chemical state of AuNC on LDH-ns is more oxidative than that of unloaded AuNC. The results of XAFS measurements indicate that charge transfer from AuNC to Fe3+ of the LDH-ns occurs in the Au/LDH-ns. Previous studies have reported that the Fe3+ in LDH-ns works as an active site for OER. Reduced chemical states of Fe3+ by loading of AuNC changes binding energy of reaction intermediates on Fe3+ and contributes to decrease of overpotential. In this study, we clarified the mechanism for promotion of OER on LDH-ns by loading of Au clusters using the XAFS measurement.


Fig.1 Electrochemical reaction of water


Fig.2 Structure (left) and TEM (right) image of LDH


Fig.3 Linear sweep voltamograms on the Au/LDH-ns (red) and pristine LDH-ns (blue) for OER

Fig. 1
Fig. 2
Fig. 3

Fig.4 Ni (a), Fe (b) K-edge and Au (c) L3-edge XANES spectra of the Au/LDH-ns, LDH-ns and AuNC.


S. Kitano, M. Yamauchi, 16th Korea-Japan Symposium on Catalysis, 2017