Gold Nanoclusters as Electrocatalysts for Energy Conversion.

Gold nanoclusters (Aun NCs) exhibit a size-specific electronic structure unlike bulk gold and can therefore be used as catalysts in various reactions. Ligand-protected Aun NCs can be synthesized with atomic precision, and the geometric structures of many Aun NCs have been determined by single-crystal X-ray diffraction analysis. In addition, Aun 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 Aun NCs with properties that surpass those of conventional catalysts using the obtained design guidelines. With these expectations, catalyst research using Aun NCs as a model catalyst has been actively conducted in recent years. This review focuses on the application of Aun NCs as an electrocatalyst and outlines recent research progress.

1. Introduction

Gold nanoclusters (Au NCs) have physical/chemical properties that differ from those of bulk Au owing to their size-specific electrical/geometrical structure [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. Therefore, Au NCs have been actively studied since the 1960s from the viewpoints of both basic science and application. Since Brust et al., discovered a method for synthesizing Au NCs protected by thiolate (Au(SR)) in 1994 [1], researches on Au NCs in particular have grown [6]. Au(SR) NCs exhibit high stability both in solution and in the solid state because Au forms a strong bond with SR. In addition, Au(SR) NCs can be synthesized by simply mixing reagents under the ambient atmosphere. Au(SR) NCs with these unique characteristics have a low handling threshold even for researchers unfamiliar with the chemical synthesis of metal clusters. Au(SR) NCs are thus currently one of the most studied metal NCs [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. For these Au(SR) NCs, it became possible to synthesize a series of Au(SR) NCs with atomic precision in 2005 [19]. In addition, since 2007, the geometric structures of many Au(SR) NCs have been determined through single-crystal X-ray diffraction (SC-XRD) analysis [20]. Since 2009, partial replacement of the Au atoms of Au(SR) NCs with other elements such as silver (Ag), copper (Cu), platinum (Pt), palladium (Pd), cadmium (Cd), and mercury (Hg) has also been realized [3,4,5,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44].

In parallel to these synthesis and structural analysis studies, studies on the functions of Au NCs have also been actively conducted. Au NCs have been observed to possess catalytic activity for several reactions, including carbon monoxide oxidation [45,46,47,48,49,50,51,52,53,54,55], alcohol oxidation [56,57,58,59,60,61,62,63,64,65], styrene oxidation [66,67,68,69,70], aromatic compound oxidation [71,72], sulfide oxidation [73,74,75], and carbon dioxide reduction [76,77,78,79,80,81,82,83]. One of the reasons for these active studies on the catalysis of Au NCs is that their electronic and geometric structures are well understood. Thus, if the obtained catalytic properties are compared with the electronic/geometrical structures of Au(SR) NCs, information on active sites, mechanisms, and key factors for high activation in catalytic reactions can be obtained. With these expectations, Au(SR) NCs have received great attention as model catalysts [45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83].

In addition, several studies on Au(SR) NCs as electrocatalysts have also been performed recently. To prevent serious environmental issues including the depletion of fossil fuels and global warming, the establishment of a system in which hydrogen (H2) is generated from water and solar energy using a photocatalyst is desired, with the generated H2 used for the generation of electricity using fuel cells [84,85]. Once such an energy conversion system is established, it will be possible to circulate an energy medium (H2) in addition to obtaining electricity only from solar energy and abundant water resources. However, realization of such an ultimate energy conversion system requires further improvement of the reaction efficiency of each half reaction of water splitting and fuel cells, including the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and oxygen reduction reaction (ORR; Figure 1A).

Figure 1

(A) Schematic illustration of gold nanoclusters (Au 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.

To improve the reactivity per unit volume, it is necessary to increase the specific surface area of the active sites and increase the reaction rate at the active sites. For the former, size reduction of the catalyst is one effective method. However, the latter is strongly related to the adsorption energy of reactive molecules on the catalyst surface. The activity of the chemical reaction on the catalyst surface is the highest when the Gibbs energy of adsorption between the catalyst and reactant is moderate according to the Sabatier principle [86]. This is because the reaction does not occur without the adsorption of reactants but is inhibited by the strong adsorption of reactants. Therefore, the relationship between the reaction efficiency and the Gibbs energy for the adsorption of reactants follows a curved line called an activity volcano plot [87]. Fine nanoparticle catalysts suitable for the HER [88,89,90,91,92], OER [93,94,95], and ORR [96,97,98,99,100,101] have been developed based on theoretical predictions of activity volcano plots using various metals and alloy nanoparticles (NPs). Au NCs have recently been observed to possess catalytic activity for the HER, OER, and ORR [77,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116] (Figure 1). Therefore, Au NCs are expected to become a model catalyst even in such an energy conversion system. A better understanding of the correlation between electronic/geometrical structures and the catalytic activity of the HER, OER, and ORR in Au NCs might lead to the discovery of new key factors for achieving high activation. Furthermore, because Au NCs are composed of several tens of atoms or less, the use of fine Au NCs as a catalyst is also effective in reducing the consumption of expensive noble metals. Thus, it may be possible to create HER, OER, and ORR catalysts with properties that surpass those of conventional catalysts using these unique characteristics of Au NCs. With these expectations, several groups are conducting research on the application of Au NCs as electrocatalysts. This article reviews the basic theory of electrocatalysts and recent research on HER, OER, and ORR catalysts using Au NCs and their alloy NCs.

2. Electrocatalytic Reaction in Water Splitting

H2 is expected to be an important energy source to support a sustainable energy society. Currently, H2 is generated as a by-product during steam reforming or coke production. However, if a water-splitting reaction using an electrocatalyst can be applied for hydrogen production, the large-scale facility of the current system would not be required. In addition, it would be possible to produce H2 only with water and electricity using the surplus power from a power plant. Therefore, water electrolysis is considered one of the cleanest energy production reactions for a sustainable energy society.

The water-splitting reaction consists of two half reactions, the HER and OER. When a voltage is applied to the metal electrode, a reduction reaction proceeds at the cathode and an oxidation reaction proceeds at the anode, resulting in the decomposition of water molecules into H2 and O2 at each electrode. However, the reactions do not proceed even if a potential equal to or higher than both the oxidation and reduction potentials in each reaction (HER: 0 V vs. SHE, OER: 1.23 V vs. SHE; SHE = standard hydrogen electrode) is applied to the electrode. This is because the activation energy of each reaction is too high. Therefore, noble metal NPs are used as a catalyst to reduce the activation energy of the reaction.

2.1. Hydrogen Evolution Reaction

In the HER, metal surface atoms of the catalyst form bonding orbitals with protons (H+) through the Volmer–Heyrovsky or Volmer–Tafel mechanism, producing molecular hydrogen [117].

Under acidic conditions, the following reactions occur:Volmer reaction: M + H Heyrovsky reaction: M–H + H Tafel reaction: 2M–H → 2M + H

However, under alkaline conditions, the following reactions occur:Volmer reaction: 2M + 2H Heyrovsky reaction: M–H + H Tafel reaction: 2M–H → 2M + H

Bulk Au possesses almost no HER activity, whereas Au(SR) NCs possess HER activity. In addition, their activity can be further improved by doping Au(SR) NCs with appropriate heterogeneous elements. These effects were reported by Lee and Jiang et al., in 2017 [102]. They evaluated the HER activity using linear sweep voltammetry (LSV) in tetrahydrofuran (THF) solution with 1.0 M trifluoroacetic acid (TFA) and 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in the absence (black) and presence of Au25(SC6H13)18 or Au24Pt(SC6H13)18 (SC6H13 = 1-hexanethiolate) on a glassy carbon electrode (GCE). The onset potential of the HER (Figure 1B(a)) occurred at −1.25 V for the GCE blank (Figure 2A, black line), whereas it occurred at −1.1 V for the GCE with Au25(SC6H13)18 (Figure 2A, red line). In addition, for the GCE with Au24Pt(SC6H13)18, the onset potential of the HER was further reduced to −0.89 V (Figure 2A, blue line). These findings indicated that Au(SR) NCs has catalytic activity for the HER and that the HER activity can be further improved by substituting one Au atom of the Au(SR) NCs with a Pt atom (Table 1). They estimated the HER energies of Au25(SCH3)18 and Au24Pt(SCH3)18 (SCH3 = methanethiolate) using density functional theory (DFT) calculations to elucidate the reasons for this behavior (Figure 2C). In these DFT calculations, H+ solvated by two THF molecules was used as H+. The resulting energy change in the Volmer step was 0.539 eV for [Au25(SCH3)18]−, indicating that this reaction is endothermic. However, the energy change in the Volmer step was −0.059 eV for [Au24Pt(SCH3)18]2−, indicating that there is almost no energy change (Figure 2C, step 1). The higher HER activity of Au24Pt(SC6H13)18 was explained by these differences in the energy barriers in the reaction. In addition, Au24Pt(SC6H13)18 possessed higher HER activity even compared with Pt NPs, which are highly active materials for the HER (Figure 2B).

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.

Table 1

Representative references on HER activity of Au NCs and related alloy NCs.

Ligand	Support	Experimental Condition	Activity	Reference
SC6H13	−	1.0 M TFA and 0.1 M Bu4NPF6 in THF c	Au24Pt(SC6H13)18 > Au25(SC6H13)18	[102]
SC6H13	carbon black	1 M Britton–Robinson buffer solution in 2 M KCl aq (pH 3) c,d	Au24Pt(SC6H13)18 > Au24Pd(SC6H13)18 > Au25(SC6H13)18	[103]
SC6H13	carbon black	1 M Britton–Robinson buffer solution in 2 M KCl aq (pH 3) c,d	Au36Pt2(SC6H13)24 > Au36Pd2(SC6H13)24 > Au38(SC6H13)24	[103]
PPh3PPh2 aCl bPhF2S	MoS2	0.5 M phosphate buffer solution (pH 6.7) c,d	Au2Pd6(S4(PPh3)4(PhF2S)6)/MoS2 > Mixture of Au2Cl2C(PPh2)2 and Pd3(Cl(PPh2)2(PPh3)3)/MoS2 > Pd3(Cl(PPh2)2(PPh3)3)/MoS2 > Au2Cl2C(PPh2)2/MoS2 > MoS2	[104]
porphyrin SC1Pporphyrin SC2PPET	−	0.5 M H2SO4 aq e	Au(1.3 nm)(porphyrin SC1P) > Au(1.3 nm)(porphyrin SC2P) > Au(1.3 nm)(PET)	[107]
PETSePh	MoS2	0.5 M H2SO4 aq c,d	Au25(PET)18/MoS2 > Au25(SePh)18/MoS2 > MoS2	[108]
SC6H13MPAMPS	−	0.1 M KCl aq c	Au24Pt(MPS)18 > Au25(MPS)18 > Au25(MPA)18 > Au25(SC6H13)18	[109]

Diphenylphosphine. Chlorine. WE: Working electrode; GCE. WE: Containing Nafion. WE: Carbon tape.

Lee and Jiang et al., observed that a high HER activity and a high catalyst turnover frequency (TOF) can be achieved by doping Au25(SC6H13)18 with not only Pt but also Pd (Au24Pt(SC6H13)18 > Au24Pd(SC6H13)18 > Au25(SC6H13)18) [103]. They reported that TOF values of Au25(SC6H13)18, Au24Pd(SC6H13)18, and Au24Pt(SC6H13)18 were 8.2, 13.0, and 33.3 mol H2 (mol catalyst)−1 s−1 at −0.60 V vs. the reversible hydrogen electrode (RHE), respectively. In addition, it was revealed that the doping of Au38(SR)24 with different elements results in a similar activity enhancement effect with Au25(SC6H13)18 (Au36Pt2(SC6H13)24 > Au36Pd2(SC6H13)24 > Au38(SC6H13)24) [103]. These results are in good agreement with the DFT calculation results. In addition to these studies, Jiang et al., also investigated the doping effects of various elements (Pt, Pd, Ag, Cu, Hg, and Cd) in Au25(SCH3)18 using DFT calculations [105]. The results predicted that Au24Pt(SCH3)18, Au24Pd(SCH3)18, and Au24Cu(SCH3)18, in which the heteroatom (Pt, Pd, or Cu) is located at the center of the metal core, have a higher HER activity than Au25(SCH3)18. Zhu et al., reported that another fine alloy NC, Au2Pd6(S4(PPh3)4(PhF2S)6) (PPh3 = triphenylphosphine, PhF2S = 3,4-difluorobenzenethiolate), also exhibits HER activity (Table 1) [106]. These studies revealed that Au(SR) and their alloy NCs have HER activity and it can be improved by controlling the electronic structure of Au NCs through heteroatom doping.

The HER activity varies depending not only on the chemical composition of the metal core but also on the properties of the ligand. In 2018, Teranishi and Sakamoto et al., used Au NCs coordinated with SR-containing porphyrin (porphyrin SCP). They investigated the effects of the ligand structure on the HER activity of Au(SR) NCs [107]. In these clusters, the porphyrin ring coordinates horizontally to the gold core. Then, the distance between the porphyrin ring and the Au surface was controlled by changing the length of the alkyl chain between the porphyrin ring and the acetylthio group (Figure 3A,C) [118,119]. The alkyl chain is a methylene chain for porphyrin SC1P and an ethylene chain for porphyrin SC2P. The distance between the porphyrin ring and the acetylthio group was determined to be 3.4 Å for porphyrin SC1P and 4.9 Å for porphyrin SC2P by SC-XRD analysis. The researchers synthesized three sizes of Au NCs with a core size of approximately 1.3, 2.2, or 3.8 nm using porphyrin SC1P, porphyrin SC2P, or a common protective ligand, 2-phenylethanethiolate (PET). Transmission electron microscope (TEM) images of the synthesized Au(SR) NCs (SR = porphyrin SC1P, porphyrin SC2P, or PET) with a core size of approximately 1.3 nm are presented in Figure 3B,D,F, respectively. Among these products, matrix-assisted laser desorption/ionization mass spectrometry indicated that Au(porphyrin SC1P) NCs consisted of 77 Au atoms and 8 porphyrin SC1P molecules and Au(porphyrin SC2P) NCs consisted of 75 Au atoms and 11 porphyrin SC2P molecules. The effects of the ligand structure and Au core size on the HER activity of Au(SR) NCs were investigated using the obtained nine types of Au(SR) NCs. As a result, in Au(SR) NCs with a core size of approximately 1.3 nm, Au(porphyrin SC1P) and Au(porphyrin SC2P) NCs exhibited higher current densities of the HER than Au(PET) NCs (Table 1). For instance, Au(porphyrin SC1P) NCs resulted in a 4.6 times higher current density of the HER than Au(PET) NCs at −0.4 V vs. RHE. In addition, using Au(porphyrin SC1P) NCs, the HER occurred at a smaller overvoltage than using Au(porphyrin SC2P) NCs. These results indicate that the HER activity of Au NCs depends on the type of ligand and the distance between the ligand and the metal core in Au NCs [107]. In this work, the Au(SR) NCs with a core size of approximately 2.2 nm showed higher catalytic activity than those with a core size of approximately 1.3 nm (Figure 3G,H). This size dependence of the catalytic activity is a little strange considering the surface area of the metal core because a reduction of a core size of Au(SR) NCs typically leads to the increase in the surface area of Au metal core, which are active sites in HER. The authors have not discussed the details on this point in this paper probably due to the difficulty in precisely estimating the surface area of each Au(SR) NCs.

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.

The property of the ligand also strongly affects the interaction between Au(SR) NCs and the electrode as well as the affinity between Au(SR) NCs and water molecules. Lee and Jiang et al., synthesized Au(SR) NCs with SC6H13, 3-mercaptopropionic acid (MPA), or 3-mercapto-1-propanesulfonic acid (MPS; Figure 4B) as a ligand (Au25(SC6H13)18, Au25(MPA)18, and Au25(MPS)18) and used them to investigate the effect of ligand properties on the HER activity [109]. In the experiment, Au25(SC6H13)18, Au25(MPA)18, or Au25(MPS)18 was dissolved at a concentration of 1 mM in 0.1 M KCl aqueous solution, and LSV measurements were performed using a GCE (50 mV s−1). Although the blank current was 0.01 mA at −0.7 V vs. RHE (Figure 4C, black line), the HER current of the sample including Au25(MPA)18 increased up to 0.13 mA at −0.7 V vs. RHE (Figure 4C, red line). When Au25(MPS)18 was used, a higher HER current of 1.0 mA was observed at −0.7 V vs. RHE (Figure 4C, blue line). MPS and MPA have a hydrophilic functional group (sulfonic acid or carboxylic acid group, respectively) unlike SC6H13. These hydrophilic functional groups have the property of releasing H+ in an aqueous solution. In addition, the sulfonic acid group of MPS (pKa < 1) is expected to have higher H+ releasing ability than the carboxylic acid group of MPA (pKa = 3.7). For these reasons, it was interpreted that the difference in the HER activity described above is largely related to the difference in the H+ releasing ability of these ligands (Table 1). It was speculated that the energy barrier associated with the intermolecular and intramolecular H+ transfer steps is lowered by H+ relay in Au NCs with high HER activity (Figure 4A). In this paper, they also reported that the use of Au24Pt(MPS)18, in which Au25(MPS)18 is replaced with Pt, results in even higher HER activity than Au25(MPS)18 (Figure 4D and Table 1). They descried that the TOF value of Au24Pt(MPS)18 was 127 mol H2 (mol catalyst)−1 s−1, which was 4 times higher than that of Au25(MPS)18 at −0.7 V vs. RHE.

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.

An electronic interaction also occurs between the Au(SR) NCs and a catalytic support. This phenomenon was revealed by Jin et al., by measuring the HER activity of MoS2 nanosheets (catalytic support) carrying Au25(PET)18 (Au25(PET)18/MoS2) [108]. In this experiment, Au25(PET)18/MoS2 was prepared by mixing the MoS2 nanosheets synthesized by the hydrothermal method and Au25(PET)18 in dichloromethane for 1 h and drying the obtained products under nitrogen atmosphere. High-angle annular dark-field scanning TEM (HAADF-STEM) images confirmed that Au25(PET)18 was uniformly supported on MoS2 (Figure 5A). Au25(PET)18/MoS2 was then loaded on a GCE, and the HER polarization curve of Au25(PET)18/MoS2 was obtained by scanning the potential in a 0.5 M H2SO4 aqueous solution using the rotating disk electrode (RDE) method (Figure 5B,D). MoS2 without Au25(PET)18 exhibited a HER overvoltage of 0.33 V at a current density of 10 mA cm−2, whereas Au25(PET)18/MoS2 exhibited a smaller HER overvoltage of approximately −0.28 V at the same current density. In addition, Au25(PET)18/MoS2 (59.3 mA cm−2) exhibited a 1.79 times higher current density than that of MoS2 (33.2 mA cm−2) at an applied voltage of −0.4 V vs. RHE. Thus, the HER activity of the MoS2 nanosheets was greatly improved by carrying Au25(PET)18 (Table 1). This improvement of the HER activity was interpreted to be greatly related to the electronic interaction between Au25(PET)18 and MoS2. In fact, X-ray photoelectron spectroscopy (XPS) analysis confirmed that the binding energy of MoS2 in the Mo 3 d orbit was negatively shifted by 0.4 eV after Au25(PET)18 was loaded (Figure 5C). It was assumed that the charge transfer from Au25(PET)18 to MoS2 occurred in Au25(PET)18/MoS2, causing a high HER activity of Au25(PET)18/MoS2. In this study, the HER activity of MoS2 nanosheets carrying Au25(SePh)18 (SePh = phenylselenolate) (Au25(SePh)18/MoS2) was also investigated. Au25(SePh)18/MoS2 was shown to also exhibit higher HER activity than MoS2 nanosheets (Table 1). However, the improvement of the activity was smaller than that when carrying Au25(PET)18 (Figure 5D). This difference was attributed to the difference in the electron interaction and electron relay between Au cores of Au NCs and the MoS2 nanosheet depending on the ligands. In this way, the HER activity of the Au NCs-loaded catalyst was shown to depend on the electronic interaction between the Au NCs and the catalytic support.

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.

2.2. Oxygen Evolution Reaction

The OER is a multi-step four-electron reaction in which the reaction proceeds along different reaction paths depending on the binding energy between the metal and the OER intermediate (O, OH, and OOH).

Under acidic conditions, the following reactions occur:M + H M−OH → M−O + H 2(M−O) → 2M + O M−O + H M−OOH → M + O

However, under alkaline conditions, the following reactions occur:M + OH M−OH + OH 2(M−O) → 2M + O M−O + OH M−OOH + OH

As described above, because the reaction route of OER depends on the intermediates (O, OH, and OOH) on the surface of catalyst, the OER activity of the catalyst also depends on these intermediates. Catalysts that have neither too high nor too low binding energy with oxygen species are suitable for the OER. Previous studies have demonstrated that iridium oxide and ruthenium oxide have such desirable properties. Therefore, miniaturization of these metal oxides and prediction of their physical properties by theoretical calculation have been actively performed [120,121,122,123]. However, because these precious metals are expensive and have the problem of depletion, a search for low-cost catalysts is also being conducted. Related studies have shown that cobalt (Co)-based materials (oxides, hydroxides, selenides, and phosphides) can be used as good OER catalysts. Furthermore, it has been reported that when Au NPs are composited with such Co materials, the OER performance is greatly enhanced as a result of the improved electron conductivity and preferential formation of OOH intermediates on the surface of the catalyst [124,125,126].

Jin et al., have shown that these mixing effects also occur when Au NCs are used instead of Au NPs [110]. In this study, the Au25(PET)18-loaded CoSe2 nanosheet (Au25(PET)18/CoSe2) was prepared by stirring Au25(PET)18 and CoSe2 nanosheets in dichloromethane for 1 h. HAADF-STEM analysis confirmed that Au25(PET)18 was uniformly supported on the CoSe2 nanosheets (Figure 6A,B). Au25(PET)18/CoSe2 was loaded on the GCE, and their OER polarization curves were obtained by scanning the applied potential (5 mV s−1) in 0.1 M KOH aqueous solution. The CoSe2 nanosheets without Au25(PET)18 exhibited an OER overvoltage of 0.52 V at a current density of 10 mA cm−2 (Figure 1B(b)), whereas Au25(PET)18/CoSe2 exhibited a smaller OER overvoltage of 0.43 V at the same current density (Figure 6C). XPS (Figure 6E) and Raman spectroscopy (Figure 6F) analyses revealed that the electronic interaction occurred between the Au25(PET)18 and CoSe2 nanosheet even in such a composite catalyst. Furthermore, DFT calculation revealed that the formation of the intermediate via OH− is more advantageous by 0.21 eV mol−1 at the interface of Co–Au than at the surface of Co. It was thus interpreted that Au25(PET)18/CoSe2 exhibited higher OER activity than the CoSe2 nanosheets because Au25(PET)18/CoSe2 stabilized the generation of an OOH intermediate compared with only the CoSe2 nanosheet (Table 2). This study also revealed that the OER activity increases with the core size of Au(SR) NCs (Figure 6D).

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-Bu)10/CoSe2, Au25(PET)18/CoSe2, Au144(PET)60/CoSe2, Au333(PET)79/CoSe2, and PtNP/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.

Table 2

Representative reference on OER activity of Au(SR) NCs.

Ligand	Support	Experimental Condition	Activity	Reference
PET	CoSe2	0.l M KOH aq a, b	Au25(PET)18/CoSe2 > CoSe2	[110]

WE: GCE. WE: Containing Nafion.

3. Electrocatalytic Reactions in Fuel Cells

To establish a circulating energy system that does not use fossil fuels and only produces water and a small amount of carbon dioxide as waste, it is essential to further improve the functions of fuel cells. Fuel cells can be roughly classified into those using hydrogen and those using alcohol as a fuel. In fuel cells using hydrogen as a fuel, the HOR and ORR are involved in the system. The HOR is a one-electron reaction, and generally an HER-active catalyst is also useful for the HOR. However, the ORR is a four-electron reaction, and the reaction process is complicated. In addition, the OER is a reaction under oxidizing conditions, whereas the ORR is a reaction under reducing conditions. The surface state of the catalyst and the accompanying binding to the reactants also differ greatly between the OER and ORR. Therefore, catalysts that are active for OER are not necessarily useful for the ORR. Because the ORR is rate-limiting step in a fuel cell, controlling the ORR is important for further development of fuel cells. The ORR pathways under acidic and alkaline conditions are as follows [94].

Under acidic conditions:O O H

Under alkaline conditions:O O OOH

Equations (17) and (20) are four-electron reactions, and Equations (18), (19), (21), and (22) are two-electron reactions. For both sets of reactions, the reactions start with the breaking of the O−O bond. The theoretical redox potential is 1.23 V vs. SHE in the direct four-electron path and 0.68 V vs. SHE in the indirect two-electron path. Therefore, a higher energy conversion efficiency can be achieved using the direct four-electron path, and this reaction path is thus more desirable for fuel cells [81]. Although Pt is a useful catalyst for such a reaction pathway, it is expected to be replaced with another metal element because of the high cost of Pt and the resource depletion issue. In addition, synthesis methods of Pt NCs in ambient atmosphere with atomic precision are limited, and therefore, it is difficult to study the ORR mechanism using Pt NCs as model catalysts. However, for Au NCs, there are many examples of synthesis with atomic precision, and these catalysts are stable in ambient atmosphere. In addition, theoretical calculations [127,128] and experimental results [65,129] have predicted that O2 molecules can be highly activated on the surface of Au NCs. For these reasons, several studies have also been performed on the application of Au NCs as ORR catalysts.

In 2009, Chen et al., evaluated the ORR catalytic activity of Au11(PPh3)8Cl3, Au25(PET)18, Au55(PPh3)12Cl6, and Au140(SC6H13)53 (Cl = chlorine) [111]. In this experiment, after a series of Au NCs were loaded on the GCE, the ORR activity was measured by scanning the potential using the RDE method in a 0.1 M KOH aqueous solution filled with O2. When Au11(PPh3)8Cl3 was used as the Au NCs, the onset potential of the ORR (Figure 1B(c)) was about −0.08 V, and the peak current density was 2.4 mA cm−2 (Figure 7A). However, when Au140(SC6H13)53 was used as the Au NCs, the onset potential shifted to the more cathodic −0.22 V and the reduction peak current decreased to less than 1.0 mA cm−2. These results and those for the other two Au NCs indicated that the ORR activity increased with decreasing Au core size (Au11(PPh3)8Cl3 > Au25(PET)18 > Au55(PPh3)12Cl6 > Au140(SC6H13)53) (Figure 7A,B and Table 3). From estimation of the number of electrons for the ORR from a Koutecky–Levich plot [85], it was observed that the relatively small size of Au NCs (Au11(PPh3)8Cl3, Au25(PET)18, and Au55(PPh3)12Cl6) resulted in the occurrence of the four-electron reaction, whereas Au140(SC6H13)53 tended to follow the two-electron reaction pathway (Figure 7C,D). Later, these researchers also synthesized a series of Au(SR) NCs (Au25(PET)18, Au38(PET)24, and Au144(PET)60) with PET ligands and measured their ORR activities. The results revealed that a smaller core size was associated with higher ORR activity: Au25(PET)18 > Au38(PET)24 > Au144(PET)60 (Table 3) [112]. As the core size decreased, the ratio of low-coordinated surface atoms increased and the d-band center of the Fermi level changed. It was interpreted that smaller Au(SR) NCs exhibited higher ORR activity because the promotion of oxygen adsorption on the gold core surface was accelerated by miniaturization of the metal core.

Figure 7

(A) Cyclic voltammograms of Au(SR)/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 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(SR)/GCE (n = 11, 25, 55, and 140). Panels (A–D) are reproduced with permission from reference [111]. Copyright Wiley-VCH, 2009.

Table 3

Representative references on ORR activity of Au NCs.

Ligand	Support	Experimental Condition	Activity	Reference
PETSC6H13ClPPh3	−	0.1 M KOH aq a	Au11(PPh3)8Cl3 > Au25(PET)18 > Au55(PPh3)12Cl6 > Au140(SC6H13)53	[101]
PET	Reduced graphene oxide	0.1 M KOH aq a, b	Au25(PET)18 > Au38(PET)24 > Au144(PET)60	[112]
TBBT	SWNTs	0.1 M KOH aq a, b	Au36(TBBT)24 > Au133(TBBT)52 > Au279(TBBT)84 > Au28(TBBT)20	[113]
S-tBu	SWNTs	0.1 M KOH aq a, b	Au65(S-tBu)29 > Au46(S-tBu)24 > Au30(S-tBu)18 > Au23(S-tBu)16	[114]
SC12H25	−	0.1 M KOH aq a, b	[Au25(SC12H25)18]− > [Au25(SC12H25)18]0 > [Au25(SC12H25)18]+ c	[115]

WE: GCE. WE; Containing Nafion. Tow-electron reduction.

On the other hand, Dass et al., studied the dependence of the ORR activity on the core size using Au NCs protected by 4-tert-butylbenzenethiolate (TBBT), whose structure differs significantly from that of PET [113]. In this experiment, single-walled carbon nanotubes (SWNTs) carrying Au(TBBT) NCs (n = 28, 36, 133, and 279; Figure 8A; Au(TBBT) NCs/SWNTs) were loaded onto the GCE. The ORR actives were measured by scanning the potential using the RDE method in a 0.1 M KOH aqueous solution filled with O2 (Figure 8B). The overvoltage of the ORR was smaller in the order of Au36(TBBT)24 > Au133(TBBT)52 > Au279(TBBT)84 > Au28(TBBT)20. However, the selectivity of the four-electron reduction reaction was superior in the order of Au36(TBBT)24 ≈ Au133(TBBT)52 > Au279(TBBT)84 > Au28(TBBT)20 [113] (Figure 8C). Notably, this trend was similar to that of the size dependence of the stability of Au(TBBT) NCs itself. The same group performed similar studies using tert-butylthiolate (S-Bu) instead of TBBT as a ligand [114]. S-Bu has a bulky framework and when this ligand is used in the synthesis of Au(SR) NCs, the ratio of the metal atom and the ligand in the generated Au(SR) NCs is different from that in Au(SR) NCs synthesized using another ligand. Such Au(S-Bu) NCs exhibit a unique size dependency for ORR activity (Au65(S-Bu)29 > Au46(S-Bu)24 > Au30(S-Bu)18 > Au23(S-Bu)16) [114].

Figure 8

(A) X-ray crystal structures of Au(TBBT) 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(TBBT) (n = 28, 36, 133, and 279). Panels (A–C) are reproduced with permission from reference [113]. Copyright American Chemical Society, 2018.

In addition to these effects of core sizes and ligands, the ORR activity also depended on the charge state of Au(SR) NCs. Chen et al., carried [Au25(SC12H25)18]−, [Au25(SC12H25)18]0, and [Au25(SC12H25)18]+ (SC12H25 = 1-dodecanethiolate) on the GCE, and their ORR activities were evaluated by scanning the potential in a 0.1 M KOH aqueous solution using a rotating ring-disk electrode (RRDE) filled with O2 [115]. In addition, the generation of H2O2 was evaluated from the RRDE current at a fixed ring potential (0.5 V vs. saturated calomel electrode (SCE)). When [Au25(SC12H25)18]−, [Au25(SC12H25)18]0, and [Au25(SC12H25)18]+ were used, the efficiencies of H2O2 were 86%, 82%, and 72%, respectively. In addition, the number of electrons for the ORR was estimated to be 2.28 ([Au25(SC12H25)18]−), 2.35 ([Au25(SC12H25)18]0), and 2.56 ([Au25(SC12H25)18]+; Figure 9A–C). For [Au25(SC12H25)18]−, which showed the highest production rate of H2O2, the activity decreased only 9% even after 1000 cycles (Figure 9D). These results indicate that [Au25(SC12H25)18]− has high H2O2 generating ability (Table 3) [115]. Since H2O2 is a useful raw material for chemical products, the development of their highly selective production reactions is important. Jin et al., also studied the dependence of the ORR activity on the charge state of Au(SR) NCs using [Au25(PET)18]−, [Au25(PET)18]0, and [Au25(PET)18]+. They reported that too strong of an OH− adsorbing ability of [Au25(PET)18]+ reduces the ORR activity [77]. Thus, it has been clarified that the charge state of Au(SR) NCs also has a significant effect on the ORR activity of Au(SR) NCs.

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.

4. Conclusions

A system for the generation of a fuel such as hydrogen or methanol using natural energy (e.g., solar cells or photocatalytic water splitting) and the production of electricity by fuel cells using these fuels would be one of the ultimate energy conversion systems for our society. To realize such a system, high activation of the HER, OER, HOR, and ORR is indispensable. Recently, Au NCs have attracted considerable attention as model catalysts for these reactions. In this review, recent works on these materials were summarized. The overall characteristics of the HER, OER, and ORR can be summarized as follows.

1) Since the core size, doping metal, ligand structure, and charge state affect the electronic and geometrical structures of Au NCs, these parameters also have a great effect on the catalytic activity of Au NCs.

2) Although these three reactions proceed via different mechanisms, reducing the core size of Au NCs and improving the ligand conductivity tend to improve the activities.

3) When Au NCs are carried on a conventional catalytic support, their electronic structure changes and thus their catalytic activity also changes. Therefore, Au NCs are also useful for improving the catalytic activity of conventional catalytic materials.

5. Perspectives

Until recently, the materials with relatively high activity for all of HER, OER, and ORR are considered to be limited to Ir, Rh, Ru, and Pt [84,85]. However, the recent studies demonstrated that these properties could also be caused in Au by the discretization of the band structure (e.g., shift of d-band center [107,111]). For Au NCs, it is possible to precisely control the electronic/geometrical structures and thereby to elucidate the correlation between catalytic activity and electronic/geometrical structure. In addition, the use of fine Au NCs as a catalyst is effective in reducing the consumption of expensive noble metals. It is expected that the studies on the catalytic activities of Au NCs lead to solve the mechanism in catalytic reactions on the metal surface and create the amazing catalysts we have never seen.

However, to create such HER, OER, and ORR catalysts using Au NCs and their alloy NCs, further studies are required. Previous studies have shown that doping with Group 10 elements (Pt and Pd) induces high activation. Thus, a method for increasing the doping concentration of these elements is expected to be developed in the future. In addition, regarding the HER and OER, in spite of decomposing water, most studies thus far have used hydrophobic ligands that are not compatible with water. This may be related to the fact that the synthesis of hydrophobic Au NCs is easier than that of hydrophilic Au NCs. In particular, it is difficult to selectively synthesize a group-10-element-doped cluster using a hydrophilic ligand using the conventional synthesis method. However, as shown in this review, it is more appropriate to use hydrophilic Au NCs as HER and OER catalysts. Therefore, in the future, additional research on hydrophilic Au NCs is expected to increase the types of ligands and core sizes of hydrophilic Au NCs. Such studies are expected to lead to the creation of highly active HER, OER, and ORR catalysts and eventually to the development of design guidelines for establishing ultimate energy conversion systems.