Gold Nanoclusters as Electrocatalysts for Energy Conversion

Abstract

Gold nanoclusters (Au_n NCs) exhibit a size-specific electronic structure unlike bulk gold and can therefore be used as catalysts in various reactions. Ligand-protected Au_n NCs can be synthesized with atomic precision, and the geometric structures of many Au_n NCs have been determined by single-crystal X-ray diffraction analysis. In addition, Au_n NCs can be doped with various types of elements. Clarification of the effects of changes to the chemical composition, geometric structure, and associated electronic state on catalytic activity would enable a deep understanding of the active sites and mechanisms in catalytic reactions as well as key factors for high activation. Furthermore, it may be possible to synthesize Au_n NCs with properties that surpass those of conventional catalysts using the obtained design guidelines. With these expectations, catalyst research using Au_n NCs as a model catalyst has been actively conducted in recent years. This review focuses on the application of Au_n NCs as an electrocatalyst and outlines recent research progress.

Keywords: alloy; catalyst; cluster; fuel cells; gold; hydrogen evolution reaction; ligand-protected; oxygen evolution reaction; oxygen reduction reaction; water splitting.

Figure 1

(A) Schematic illustration of gold nanoclusters (Au_n NCs) for an electrocatalytic reaction in water splitting (hydrogen evolution reaction (HER) and oxygen evolution reaction (OER)) and fuel cells (oxygen reduction reaction (ORR)). (B) Current–potential characteristics for (a) HER, (b) OER, and (c) ORR.

Figure 2

(A) HER polarization curves of Au₂₅(SC₆H₁₃)₁₈- or Au₂₄Pt(SC₆H₁₃)₁₈-adsorbed glassy carbon electrode (GCE), or GCE. (B) H₂ production rates per mass of metals in the catalyst of Au₂₄Pt(SC₆H₁₃)₁₈/C (blue circles) and Pt/C (black triangles) electrodes. (C) DFT calculation results for Au₂₄Pt(SCH₃)₁₈. Color code: golden = Au core; olive = Au shell; purple = Pt; green = adsorbed H from the liquid medium; grey = S. Panels (A–C) are reproduced with permission from reference [102]. Copyright Springer Nature, 2017.

Figure 3

(A,C,E) Schematic illustration of coordination of ligands: (A) porphyrin SC₁P, (C) porphyrin SC₂P, and (E) PET. (B,D,F) TEM images of Au NCs with a core size of approximately 1.3 nm protected by porphyrin SC₁P, porphyrin SC₂P, or PET, respectively. (G) Comparison of overpotential at −10 mA cm⁻² and (H) current density at −0.4 V of each size of Au NCs protected with each ligand. Panels (A–H) are reproduced with permission from reference [107]. Copyright Royal Society of Chemistry, 2018.

Figure 4

(A) Schematic illustration of proton relay mechanism of Au₂₄Pt(SR)₁₈ nanocluster for formation of H₂ and (B) ligand structures: SC₆H₁₃, MPA, and MPS. Color codes: blue = Pt; golden = core Au; red = shell Au; and green = S. (C) HER polarization curves in 0.1 M KCl aqueous solution containing 180 mM acetic acid for MPA-Au₂₅ (red) or MPS-Au₂₅ (blue). (D) turnover frequencies (TOFs) obtained at various potentials in water (3.0 M KCl) containing 180 mM HOAc for MPA-Au₂₅ (red), MPS-Au₂₅ (blue), or MPS-Au₂₄Pt (green). Panels (A–D) are reproduced with permission from reference [109]. Copyright Royal Society of Chemistry, 2018.

Figure 5

(A) High-angle annular dark-field scanning TEM (HAADF-STEM) images, (B) HER polarization curves, and (C) Mo 3d X-ray photoelectron spectroscopy (XPS) spectra of Au₂₅(PET)₁₈/MoS₂. (D) HER polarization curves of Au₂₅(SePh)₁₈/MoS₂. Panels (A–D) are reproduced with permission from reference [108]. Copyright Wiley-VCH, 2017.

Figure 6

(A,B) HAADF-STEM images of Au₂₅(PET)₁₈/CoSe₂ composite at different magnifications. (C,D) OER polarization curves of CoSe₂, Au₁₀(SPh-^tBu)₁₀/CoSe₂, Au₂₅(PET)₁₈/CoSe₂, Au₁₄₄(PET)₆₀/CoSe₂, Au₃₃₃(PET)₇₉/CoSe₂, and Pt_NP/CB (CB = carbon black). (E) Co 2p XPS spectra and (F) Raman spectra of CoSe₂ and Au₂₅(PET)₁₈/CoSe₂ composite. Panels (A–F) are reproduced with permission from reference [110]. Copyright American Chemical Society, 2017.

Figure 7

(A) Cyclic voltammograms of Au_n(SR)_m/GCE (n = 11, 25, 55, and 140) saturated with O₂ and Au₁₁(PPh₃)₈Cl₃/GCE saturated with N₂ (thin solid curve). (B) Current density and overpotential of ORR activity with each size of Au_n NCs. (C) Koutecky–Levich plots at different applied potentials of a GCE modified with Au₁₁(PPh₃)₈Cl₃. (D) Rotating-disk voltammograms (rotation rate: 3600 rpm) of various Au_n(SR)_m/GCE (n = 11, 25, 55, and 140). Panels (A–D) are reproduced with permission from reference [111]. Copyright Wiley-VCH, 2009.

Figure 8

(A) X-ray crystal structures of Au_n(TBBT)_m NCs (n = 28, 36, 133, and 279). (B) Rotating-disk voltammograms recorded for the ORR activity of Au₃₆(TBBT)₂₄/GCE at different rotation rates. (C) Reaction rate constant ln(k) vs. overpotential E plots with each size of Au_n(TBBT)_m (n = 28, 36, 133, and 279). Panels (A–C) are reproduced with permission from reference [113]. Copyright American Chemical Society, 2018.

Figure 9

(A) Cyclic voltammograms, (B) electron transfer number (n), and (C) percentage of H₂O₂ of the ORR on Au₂₅(SC₁₂H₂₅)₁₈ with different charge states ([Au₂₅(SC₁₂H₂₅)₁₈]⁻, [Au₂₅(SC₁₂H₂₅)₁₈]⁰, and [Au₂₅(SC₁₂H₂₅)₁₈]⁺) in 0.1 M KOH aq saturated with O₂. (D) Accelerated durability tests of [Au₂₅(SC₁₂H₂₅)₁₈]⁻ performed for 1000 cycles. Panels (A–D) are reproduced with permission from reference [115]. Copyright Royal Society of Chemistry, 2014.