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.


Introduction
Gold nanoclusters (Au n NCs) have physical/chemical properties that differ from those of bulk Au owing to their size-specific electrical/geometrical structure . Therefore, Au n 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 n NCs protected by thiolate (Au n (SR) m ) in 1994 [1], researches on Au n NCs in particular have grown [6]. Au n (SR) m NCs exhibit high stability both in solution and in the solid state because Au forms a strong bond with SR. In addition, Au n (SR) m NCs can be synthesized by simply mixing reagents under the ambient atmosphere. Au n (SR) m NCs with these unique characteristics have a low handling threshold even for researchers unfamiliar with the chemical synthesis of metal clusters. Au n (SR) m 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 n (SR) m NCs, it became possible to synthesize a series of Au n (SR) m NCs with atomic precision in 2005 [19]. In addition, since 2007, the geometric structures of many Au n (SR) m NCs have been determined through single-crystal X-ray diffraction (SC-XRD) analysis [20]. Since 2009, partial replacement of the Au atoms of Au n (SR) m 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].
In addition, several studies on Aun(SR)m 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). 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). Aun 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, Aun 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 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 n 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 n 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 n NCs might lead to the discovery of new key factors for achieving high activation. Furthermore, because Au n NCs are composed of several tens of atoms or less, the use of fine Au n 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 n NCs. With these expectations, several groups are conducting research on the application of Au n NCs as electrocatalysts. This article reviews the basic theory of electrocatalysts and recent research on HER, OER, and ORR catalysts using Au n NCs and their alloy NCs.

Electrocatalytic Reaction in Water Splitting
H 2 is expected to be an important energy source to support a sustainable energy society. Currently, H 2 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 H 2 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 H 2 and O 2 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.

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: However, under alkaline conditions, the following reactions occur: Heyrovsky reaction: Tafel reaction: Bulk Au possesses almost no HER activity, whereas Au n (SR) m NCs possess HER activity. In addition, their activity can be further improved by doping Au n (SR) m 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 (Bu 4 NPF 6 ) in the absence (black) and presence of Au 25 (SC 6 H 13 ) 18 or Au 24 Pt(SC 6 H 13 ) 18 (SC 6 H 13 = 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 Au 25 (SC 6 H 13 ) 18 (Figure 2A, red line). In addition, for the GCE with Au 24 Pt(SC 6 H 13 ) 18 , the onset potential of the HER was further reduced to −0.89 V (Figure 2A, blue line). These findings indicated that Au n (SR) m NCs has catalytic activity for the HER and that the HER activity can be further (Au 36 Pt 2 (SC 6 H 13 ) 24 > Au 36 Pd 2 (SC 6 H 13 ) 24 > Au 38 (SC 6 H 13 ) 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 Au 25 (SCH 3 ) 18 using DFT calculations [105]. The results predicted that Au 24 Pt(SCH 3 ) 18 , Au 24 Pd(SCH 3 ) 18 , and Au 24 Cu(SCH 3 ) 18 , in which the heteroatom (Pt, Pd, or Cu) is located at the center of the metal core, have a higher HER activity than Au 25 (SCH 3 ) 18 . Zhu et al., reported that another fine alloy NC, Au 2 Pd 6 (S 4 (PPh 3 ) 4 (PhF 2 S) 6 ) (PPh 3 = triphenylphosphine, PhF 2 S = 3,4-difluorobenzenethiolate), also exhibits HER activity (Table 1) [106]. These studies revealed that Au n (SR) m and their alloy NCs have HER activity and it can be improved by controlling the electronic structure of Au n 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 n NCs coordinated with SR-containing porphyrin (porphyrin SC x P). They investigated the effects of the ligand structure on the HER activity of Au n (SR) m 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 SC 1 P and an ethylene chain for porphyrin SC 2 P. The distance between the porphyrin ring and the acetylthio group was determined to be 3.4 Å for porphyrin SC 1 P and 4.9 Å for porphyrin SC 2 P by SC-XRD analysis. The researchers synthesized three sizes of Au n NCs with a core size of approximately 1.3, 2.2, or 3.8 nm using porphyrin SC 1 P, porphyrin SC 2 P, or a common protective ligand, 2-phenylethanethiolate (PET). Transmission electron microscope (TEM) images of the synthesized Au n (SR) m NCs (SR = porphyrin SC 1 P, porphyrin SC 2 P, 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 n (porphyrin SC 1 P) m NCs consisted of 77 Au atoms and 8 porphyrin SC 1 P molecules and Au n (porphyrin SC 2 P) m NCs consisted of 75 Au atoms and 11 porphyrin SC 2 P molecules. The effects of the ligand structure and Au core size on the HER activity of Au n (SR) m NCs were investigated using the obtained nine types of Au n (SR) m NCs. As a result, in Au n (SR) m NCs with a core size of approximately 1.3 nm, Au n (porphyrin SC 1 P) m and Au n (porphyrin SC 2 P) m NCs exhibited higher current densities of the HER than Au n (PET) m NCs (Table 1). For instance, Au n (porphyrin SC 1 P) m NCs resulted in a 4.6 times higher current density of the HER than Au n (PET) m NCs at −0.4 V vs. RHE. In addition, using Au n (porphyrin SC 1 P) m NCs, the HER occurred at a smaller overvoltage than using Au n (porphyrin SC 2 P) m NCs. These results indicate that the HER activity of Au n NCs depends on the type of ligand and the distance between the ligand and the metal core in Au n NCs [107]. In this work, the Au n (SR) m 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  The property of the ligand also strongly affects the interaction between Au n (SR) m NCs and the electrode as well as the affinity between Au n (SR) m NCs and water molecules. Lee and Jiang et al., synthesized Au n (SR) m NCs with SC 6 H 13 , 3-mercaptopropionic acid (MPA), or 3-mercapto-1-propanesulfonic acid (MPS; Figure 4B) as a ligand (Au 25 (SC 6 H 13 ) 18 , Au 25 (MPA) 18 , and Au 25 (MPS) 18 ) and used them to investigate the effect of ligand properties on the HER activity [109]. In the experiment, Au 25 (SC 6 H 13 ) 18 , Au 25 (MPA) 18 , or Au 25 (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 Au 25 (MPA) 18 increased up to 0.13 mA at −0.7 V vs. RHE ( Figure 4C, red line). When Au 25 (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 SC 6 H 13 . 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 n NCs with high HER activity ( Figure 4A). In this paper, they also reported that the use of Au 24 Pt(MPS) 18 , in which Au 25 (MPS) 18 is replaced with Pt, results in even higher HER activity than Au 25 (MPS) 18 ( Figure 4D and Table 1). They descried that the TOF value of Au 24 Pt(MPS) 18 was 127 mol H 2 (mol catalyst) −1 s −1 , which was 4 times higher than that of Au 25 (MPS) 18 ty. In addition, Au25(PET)18/MoS2 (59.3 mA cm ) exhibited a 1.79 times higher current de that of MoS2 (33.2 mA cm −2 ) at an applied voltage of −0.4 V vs. RHE. Thus, the HER activ oS2 nanosheets was greatly improved by carrying Au25(PET)18 (Table 1). This improveme ER activity was interpreted to be greatly related to the electronic interaction bet PET)18 and MoS2. In fact, X-ray photoelectron spectroscopy (XPS) analysis confirmed tha ng energy of MoS2 in the Mo 3 d orbit was negatively shifted by 0.4 eV after Au25(PET)1 d ( Figure 5C). It was assumed that the charge transfer from Au25(PET)18 to MoS2 occurr PET)18/MoS2, causing a high HER activity of Au25(PET)18/MoS2. In this study, the HER activ nanosheets carrying Au25(SePh)18 (SePh = phenylselenolate) (Au25(SePh)18/MoS2) was tigated. Au25(SePh)18/MoS2 was shown to also exhibit higher HER activity than MoS2 nanos e 1). However, the improvement of the activity was smaller than that when carrying Au25(P re 5D). This difference was attributed to the difference in the electron interaction and ele between Au cores of Aun NCs and the MoS2 nanosheet depending on the ligands. In this ER activity of the Aun NCs-loaded catalyst was shown to depend on the electronic intera een the Aun NCs and the catalytic support.   (Table 1). This improvement of the HER activity was interpreted to be greatly related to the electronic interaction between Au 25 (PET) 18 and MoS 2 . In fact, X-ray photoelectron spectroscopy (XPS) analysis confirmed that the binding energy of MoS 2 in the Mo 3 d orbit was negatively shifted by 0.4 eV after Au 25 (PET) 18 was loaded ( Figure 5C). It was assumed that the charge transfer from Au 25 (PET) 18 to MoS 2 occurred in Au 25 (PET) 18 /MoS 2 , causing a high HER activity of Au 25 (PET) 18 /MoS 2 . In this study, the HER activity of MoS 2 nanosheets carrying Au 25 (SePh) 18 (SePh = phenylselenolate) (Au 25 (SePh) 18 /MoS 2 ) was also investigated. Au 25 (SePh) 18 /MoS 2 was shown to also exhibit higher HER activity than MoS 2 nanosheets (Table 1). However, the improvement of the activity was smaller than that when carrying Au 25 (PET) 18 ( Figure 5D). This difference was attributed to the difference in the electron interaction and electron relay between Au cores of Au n NCs and the MoS 2 nanosheet depending on the ligands. In this way, the HER activity of the Au n NCs-loaded catalyst was shown to depend on the electronic interaction between the Au n NCs and the catalytic support.

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:

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: However, under alkaline conditions, the following reactions occur: 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 n NCs are used instead of Au NPs [110]. In this study, the Au 25 (PET) 18 -loaded CoSe 2 nanosheet (Au 25 (PET) 18 /CoSe 2 ) was prepared by stirring Au 25 (PET) 18 and CoSe 2 nanosheets in dichloromethane for 1 h. HAADF-STEM analysis confirmed that Au 25 (PET) 18 was uniformly supported on the CoSe 2 nanosheets ( Figure 6A,B). Au 25 (PET) 18 /CoSe 2 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 CoSe 2 nanosheets without Au 25 (PET) 18 exhibited an OER overvoltage of 0.52 V at a current density of 10 mA cm −2 ( Figure 1B(b)), whereas Au 25 (PET) 18 /CoSe 2 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 Au 25 (PET) 18 and CoSe 2 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 Au 25 (PET) 18 /CoSe 2 exhibited higher OER activity than the CoSe 2 nanosheets because Au 25 (PET) 18 (Table 2). This study also revealed that the OER activity increases with the core size of Au n (SR) m NCs ( Figure 6D).

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

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: Under alkaline conditions: 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 n NCs in ambient atmosphere with atomic precision are limited, and therefore, it is difficult to study the ORR mechanism using Pt n NCs as model catalysts. However, for Au n 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 O 2 molecules can be highly activated on the surface of Au n NCs. For these reasons, several studies have also been performed on the application of Au n NCs as ORR catalysts.
In 2009, Chen et al., evaluated the ORR catalytic activity of Au 11 (PPh 3 ) 8 Cl 3 , Au 25 (PET) 18 , Au 55 (PPh 3 ) 12 Cl 6 , and Au 140 (SC 6 H 13 ) 53 (Cl = chlorine) [111]. In this experiment, after a series of Au n 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 O 2 . When Au 11 (PPh 3 ) 8 Cl 3 was used as the Au n 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 Au 140 (SC 6 H 13 ) 53 was used as the Au n 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 n NCs indicated that the ORR activity increased with decreasing Au core size (Au 11 (PPh 3 ) 8 Cl 3 > Au 25 (PET) 18 > Au 55 (PPh 3 ) 12 Cl 6 > Au 140 (SC 6 H 13 ) 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 n NCs (Au 11 (PPh 3 ) 8 Cl 3 , Au 25 (PET) 18 , and Au 55 (PPh 3 ) 12 Cl 6 ) resulted in the occurrence of the four-electron reaction, whereas Au 140 (SC 6 H 13 ) 53 tended to follow the two-electron reaction pathway ( Figure 7C,D). Later, these researchers also synthesized a series of Au n (SR) m NCs (Au 25 (PET) 18 , Au 38 (PET) 24 , and Au 144 (PET) 60 ) with PET ligands and measured their ORR activities. The results revealed that a smaller core size was associated with higher ORR activity: Au 25 (PET) 18 > Au 38 (PET) 24 > Au 144 (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 n (SR) m NCs exhibited higher ORR activity because the promotion of oxygen adsorption on the gold core surface was accelerated by miniaturization of the metal core.
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 Aun 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 (Figures 7C,D). Later, these researchers also synthesized a series of Aun(SR)m 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 dband center of the Fermi level changed. It was interpreted that smaller Aun(SR)m NCs exhibited higher ORR activity because the promotion of oxygen adsorption on the gold core surface was accelerated by miniaturization of the metal core. On the other hand, Dass et al. studied the dependence of the ORR activity on the core size using Aun 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 Aun(TBBT)m NCs (n = 28, 36, 133, and 279; Figure 8A; Aun(TBBT)m 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 fourelectron 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 Aun(TBBT)m NCs itself. The same group performed similar studies using tert-butylthiolate  On the other hand, Dass et al., studied the dependence of the ORR activity on the core size using Au n 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 n (TBBT) m NCs (n = 28, 36, 133, and 279; Figure 8A; Au n (TBBT) m 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 O 2 ( Figure 8B). The overvoltage of the ORR was smaller in the order of Au 36 (TBBT) 24 > Au 133 (TBBT) 52 > Au 279 (TBBT) 84 > Au 28 (TBBT) 20 . However, the selectivity of the four-electron reduction reaction was superior in the order of Au 36 (TBBT) 24 ≈ Au 133 (TBBT) 52 > Au 279 (TBBT) 84 > Au 28 (TBBT) 20 [113] (Figure 8C). Notably, this trend was similar to that of the size dependence of the stability of Au n (TBBT) m NCs itself. The same group performed similar studies using tert-butylthiolate (S-t Bu) instead of TBBT as a ligand [114]. S-t Bu has a bulky framework and when this ligand is used in the synthesis of Au n (SR) m NCs, the ratio of the metal atom and the ligand in the generated Au n (SR) m NCs is different from that in Au n (SR) m NCs synthesized using another ligand. Such Au n (S-t Bu) m NCs exhibit a unique size dependency for ORR activity (Au 65 (S-t Bu) 29 > Au 46 (S-t Bu) 24 > Au 30 (S-t Bu) 18 > Au 23 (S-t Bu) 16 ) [114].
Nanomaterials 2020, 10, 238 13 of 21 stability of Aun(TBBT)m NCs itself. The same group performed similar studies using tert-butylthiolate (St Bu) instead of TBBT as a ligand [114]. St Bu has a bulky framework and when this ligand is used in the synthesis of Aun(SR)m NCs, the ratio of the metal atom and the ligand in the generated Aun(SR)m NCs is different from that in Aun(SR)m NCs synthesized using another ligand. Such Aun(St Bu)m NCs exhibit a unique size dependency for ORR activity (Au65(St Bu)29 > Au46(St Bu)24 > Au30(St Bu)18 > Au23(St Bu)16) [114].  [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.  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   In addition to these effects of core sizes and ligands, the ORR activity also depended on the charge state of Au n (SR) m NCs. Chen 18 ] − , which showed the highest production rate of H 2 O 2 , the activity decreased only 9% even after 1000 cycles ( Figure 9D). These results indicate that [Au 25 18 ] + reduces the ORR activity [77]. Thus, it has been clarified that the charge state of Au n (SR) m NCs also has a significant effect on the ORR activity of Au n (SR) m NCs.

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, Aun 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 Aun NCs, these parameters also have a great effect on the catalytic activity of Aun NCs.
2) Although these three reactions proceed via different mechanisms, reducing the core size of Aun NCs and improving the ligand conductivity tend to improve the activities.
3) When Aun NCs are carried on a conventional catalytic support, their electronic structure changes and thus their catalytic activity also changes. Therefore, Aun NCs are also useful for improving the catalytic activity of conventional catalytic materials.

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

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 n 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 n NCs, these parameters also have a great effect on the catalytic activity of Au n NCs.
2) Although these three reactions proceed via different mechanisms, reducing the core size of Au n NCs and improving the ligand conductivity tend to improve the activities.
3) When Au n NCs are carried on a conventional catalytic support, their electronic structure changes and thus their catalytic activity also changes. Therefore, Au n NCs are also useful for improving the catalytic activity of conventional catalytic materials.

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 n 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 n 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 n 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 n 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 n NCs is easier than that of hydrophilic Au n 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 n NCs as HER and OER catalysts. Therefore, in the future, additional research on hydrophilic Au n NCs is expected to increase the types of ligands and core sizes of hydrophilic Au n 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.
Author Contributions: T.K. and Y.N. developed the idea for this review article and wrote the paper. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
There are no conflicts to declare.