**1. Introduction**

Oxygen reduction reaction (ORR) plays a vital role in the working of fuel cells and metal-air batteries. Both of these technologies utilize oxygen from the air to generate electrical energy. The ORR mechanism in acidic environment is accompanied with the formation of water.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

$$2O\_2 + 4H^\* + 4e^- \rightarrow 2H\_2O\tag{1}$$

For alkaline medium, hydroxyl formation takes place as

$$2\text{O}\_2 + 2\text{H}\_2\text{O} + 4\text{e}^- \rightarrow 4\text{OH}^- \tag{2}$$

The current Li-ion battery technology is unable to offer the solutions for the long-range electric vehicles and energy storage grids. It is also postulated that the Li-ion battery system may reach its theoretical limit which will still be short of the demand for the long-range EVs. Metal air batteries such as Li-air (11,000 Wh/kg) and Zn–air systems (1,084 Wh/kg) offer much higher energy capacities [1–3]. A great deal of contemporary research is directed toward the realiza‐ tion of these high energy systems. Apart from being dense in energy, metal air batteries and fuel cells are green source of energy. The product of their working is free from toxic and harmful waste gases which damage the environment. These systems provide a valuable opportunity to cut the greenhouse gas emissions on a tremendous scale [4,5].

Electrocatalyst plays a crucial role in the working of metal air batteries and fuel cells. The ORR and OER mechanism are strongly related to the functionality of the electrocatalyst. Pt and Ptbased alloy catalysts are widely used for ORR, but prohibitive cost and catalytic poisoning are major drawbacks associated with Pt. Therefore, it is highly desirable to explore novel Pt-free cost-effective catalysts. Different non-platinum catalysts have been considered as a replace‐ ment of Pt. Silver being about 50 times cheaper than platinum is an attractive choice for catalyst in alkaline fuel cells. The pourbaix diagram reveals the superior stability of Ag over platinum in alkaline environment [6,7].

The ORR is accompanied with the formation of various adsorbed intermediates such as O, OH and OOH. Norkosov et al. evaluated the effect of potential on the free energy of various intermediates on Pt (111) by DFT calculations [8]. At high potential, the adsorbed oxygen was found to be stable. The ORR reaction was found to proceed only by lowering the potential, hence giving rise to overpotential. Bond energies of oxygen and hydroxyl on different metals were also calculated by DFT. The rate of ORR is limited by the removal of O, OH for metals which bind oxygen strongly where as in case of metals with poor oxygen binding, the rate is limited by the weak bonding of the adsorbed species. A volcano plot as shown in Figure 1 is obtained as a result of DFT calculations performed on various systems. Although platinum sits near the top of volcano plot developed by DFT calculations, alloying of metals can result in new materials with adsorption energies for the intermediates that are different from the constituent pure metals. Therefore, new generation of superior electrocatalysts can be devel‐ oped by alloying of metals to yield optimum binding of adsorbates onto alloy surface.

Tremendous amount of research has been performed in the past decade to enhance the activity of ORR catalyst. A myriad of bimetallic and multi metallic alloy compositions have been developed in a variety of structures such as core shell, skin alloys, thin films, ordered inter‐ metallics and solid solutions [9–12]. Skin alloys have been widely popular because of their

Ag-Cu Nanoalloys as Oxygen Reduction Electrocatalysts in Alkaline Media for Advanced... http://dx.doi.org/10.5772/62050 417

2 2 *O H e HO* 4 4 2 + - + +® (1)

2 2 *O H O e OH* 2 4 4 - - + +® (2)

The current Li-ion battery technology is unable to offer the solutions for the long-range electric vehicles and energy storage grids. It is also postulated that the Li-ion battery system may reach its theoretical limit which will still be short of the demand for the long-range EVs. Metal air batteries such as Li-air (11,000 Wh/kg) and Zn–air systems (1,084 Wh/kg) offer much higher energy capacities [1–3]. A great deal of contemporary research is directed toward the realiza‐ tion of these high energy systems. Apart from being dense in energy, metal air batteries and fuel cells are green source of energy. The product of their working is free from toxic and harmful waste gases which damage the environment. These systems provide a valuable

Electrocatalyst plays a crucial role in the working of metal air batteries and fuel cells. The ORR and OER mechanism are strongly related to the functionality of the electrocatalyst. Pt and Ptbased alloy catalysts are widely used for ORR, but prohibitive cost and catalytic poisoning are major drawbacks associated with Pt. Therefore, it is highly desirable to explore novel Pt-free cost-effective catalysts. Different non-platinum catalysts have been considered as a replace‐ ment of Pt. Silver being about 50 times cheaper than platinum is an attractive choice for catalyst in alkaline fuel cells. The pourbaix diagram reveals the superior stability of Ag over platinum

The ORR is accompanied with the formation of various adsorbed intermediates such as O, OH and OOH. Norkosov et al. evaluated the effect of potential on the free energy of various intermediates on Pt (111) by DFT calculations [8]. At high potential, the adsorbed oxygen was found to be stable. The ORR reaction was found to proceed only by lowering the potential, hence giving rise to overpotential. Bond energies of oxygen and hydroxyl on different metals were also calculated by DFT. The rate of ORR is limited by the removal of O, OH for metals which bind oxygen strongly where as in case of metals with poor oxygen binding, the rate is limited by the weak bonding of the adsorbed species. A volcano plot as shown in Figure 1 is obtained as a result of DFT calculations performed on various systems. Although platinum sits near the top of volcano plot developed by DFT calculations, alloying of metals can result in new materials with adsorption energies for the intermediates that are different from the constituent pure metals. Therefore, new generation of superior electrocatalysts can be devel‐ oped by alloying of metals to yield optimum binding of adsorbates onto alloy surface.

Tremendous amount of research has been performed in the past decade to enhance the activity of ORR catalyst. A myriad of bimetallic and multi metallic alloy compositions have been developed in a variety of structures such as core shell, skin alloys, thin films, ordered inter‐ metallics and solid solutions [9–12]. Skin alloys have been widely popular because of their

opportunity to cut the greenhouse gas emissions on a tremendous scale [4,5].

For alkaline medium, hydroxyl formation takes place as

416 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

in alkaline environment [6,7].

**Figure 1.** Variation of activity with oxygen adsorption energy for metals. Reproduced with permission from ref. 8. Copyright 2004, American Chemical Society.

superior activity as compared to the bulk alloys. This is accompanied with the reduction of cost of precious metal cost such as Pt in skin alloys by using a monolayer thick platinum on top of a non-precious metal/alloy core. As the skin is of Pt, therefore, stability in the corrosive chemical environment is still maintained in these alloys. The enhancement in the activity of these skin alloys is attributed to the geometric and ligand effect of the subsurface atoms on the skin. Recently, these structural form alloys have been reported to improve the ORR perform‐ ance of the silver-based alloys. Some possible modifications of the surface electronic structure by the ligand mechanism are revealed in Figure 2.

**Figure 2.** Modification of the surface geometric and electronic structure by subsurface configuration (a) skin on pure metal, (b) skin on alloy/compound, (c) subsurface ligand.

The rate-limiting step for ORR in case of Pt catalyst is the removal of adsorbed OH species. It is well established that the decrease in binding energy by 0.1–0.2 eV will optimize the per‐ formance of the Pt-based catalyst [13,14]. By using the Pt skin on a transition metal core, this reduction in binding energy can be achieved. This is attributed to the modification of the electronic structure of the surface atoms by the core. In case of transition metals such as Ti, Co, Ni, and Ru, charge transfers from the core to the dband of the surface atoms of platinum [15]. This occupancy of dband lowers the dband centre of the platinum atoms which in turns decreases the adsorption of the OH on the Pt skin. As a result of this mechanism, the activity of these alloy systems has been reported to be much superior than that of commercial Pt/C catalyst. The skin of platinum atoms can be either in compression or tension. This is because of the subsurface structure effect on the skin alloy. The compression of the Pt skin increases the overlap of the d orbitals which consequently increases the dband width. The result of this perturbation of the structure is the lowering of the dband center of the surface atoms [16, 17]. According to the dband theory, the lower the dband center of the surface, the lower its reactivity and vice versa. By the combination of geometric and ligand effects, the dband center of the surface can be tuned to the desired value so as to achieve the optimum adsorption of adsorbates such as O, OH and OOH on the surface of the metal. This is in accordance with the Sabatier principle which implies that for the catalytically induced chemical reactions catalyst for the reaction should have neither strong nor weak adsorption for the reaction species [18].

From Figure 1, it is evident that weak binding of oxygen onto silver is the cause of its shift from the volcano peak and weak ORR activity.

The oxygen reduction reaction for alkaline can occur by a direct four electron transfer method as in Eq. 2 or by indirect 2e transfers [19–21] as:

$$\rm O\_2 + H\_2O + 2e^- \rightarrow HO\_2^- + OH^- \tag{3}$$

$$\rm H\_2O + 2e^- + HO\_2^- \to \rm 3OH^- \tag{4}$$

In the indirect mechanism, second step, i.e., Eq. (4), is the rate-determining step. The oxygen reduction reaction by indirect mechanism seriously limits the performance of the cell. This is because of the fact that if the reaction 4 does not occur by direct mechanism, then the total electrons transferred during ORR reduce to two only. As the result, this lowers the total output voltage and energy density. Also the peroxide formed in Eq. (3) can undergo catalyst-induced conversion to O2 and OH– by Eq. (5).

$$2\text{ }HO\_2^- \rightarrow \text{ }O\_2 + 2\text{ }OH^- \tag{5}$$

Reaction 5 limits the catalytic activity of the catalyst and hence it is desirable that the ORR proceeds by direct four electron transfer mechanism. The effect of pH on the ORR mechanism has been studied by Blizanac et al. [22]. In case of ORR on Ag(111) in alkaline medium, four electron transfer was found to be the dominant mechanism at all overpotentials, but in case of low pH, i.e., in acidic solutions, 2 electron pathway was favored at low overpotentials. It was observed that the ORR on Ag (111) by 4 electron pathway could take place only at high overpotentials in the low pH electrolytes.

Owing to the stability and efficient ORR mechanism of silver-based catalysts in alkaline media, various research groups have focused on the synthesis of Ag-based catalysts. Holewinski et al. have investigated the effect of alloying on silver for the ORR performance [23]. Density functional theory (DFT) based calculations were performed on Ag alloy slabs with an Ag skin on top to investigate the effect of alloy core on the ORR performance. The 4 electrons transfer has been proposed by the following mechanism:

decreases the adsorption of the OH on the Pt skin. As a result of this mechanism, the activity of these alloy systems has been reported to be much superior than that of commercial Pt/C catalyst. The skin of platinum atoms can be either in compression or tension. This is because of the subsurface structure effect on the skin alloy. The compression of the Pt skin increases the overlap of the d orbitals which consequently increases the dband width. The result of this perturbation of the structure is the lowering of the dband center of the surface atoms [16, 17]. According to the dband theory, the lower the dband center of the surface, the lower its reactivity and vice versa. By the combination of geometric and ligand effects, the dband center of the surface can be tuned to the desired value so as to achieve the optimum adsorption of adsorbates such as O, OH and OOH on the surface of the metal. This is in accordance with the Sabatier principle which implies that for the catalytically induced chemical reactions catalyst for the reaction should have neither strong nor weak adsorption for the reaction species [18].

From Figure 1, it is evident that weak binding of oxygen onto silver is the cause of its shift from

The oxygen reduction reaction for alkaline can occur by a direct four electron transfer method

In the indirect mechanism, second step, i.e., Eq. (4), is the rate-determining step. The oxygen reduction reaction by indirect mechanism seriously limits the performance of the cell. This is because of the fact that if the reaction 4 does not occur by direct mechanism, then the total electrons transferred during ORR reduce to two only. As the result, this lowers the total output voltage and energy density. Also the peroxide formed in Eq. (3) can undergo catalyst-induced

Reaction 5 limits the catalytic activity of the catalyst and hence it is desirable that the ORR proceeds by direct four electron transfer mechanism. The effect of pH on the ORR mechanism has been studied by Blizanac et al. [22]. In case of ORR on Ag(111) in alkaline medium, four electron transfer was found to be the dominant mechanism at all overpotentials, but in case of low pH, i.e., in acidic solutions, 2 electron pathway was favored at low overpotentials. It was observed that the ORR on Ag (111) by 4 electron pathway could take place only at high

Owing to the stability and efficient ORR mechanism of silver-based catalysts in alkaline media, various research groups have focused on the synthesis of Ag-based catalysts. Holewinski et

2 2 <sup>2</sup> *O H O e HO OH* 2 - -- + +® + (3)

2 2 *H O e HO OH* 2 3 -- - ++ ® (4)

2 2 *HO O OH* 2 - - ® + (5)

the volcano peak and weak ORR activity.

conversion to O2 and OH–

overpotentials in the low pH electrolytes.

as in Eq. 2 or by indirect 2e transfers [19–21] as:

418 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

by Eq. (5).

$$\text{Ag} + \text{O}\_2 + \text{e}^{-1} \xrightarrow{H \cdot OH} \text{Ag} \cdots \text{OOH} + \text{OH}^{-1} \tag{6}$$

$$Ag\cdotsOOH + e^{-1} \stackrel{H\text{-}OH}{\rightarrow} Ag\cdots O + OH^{-1} \tag{7}$$

$$Ag\cdots \cdot \text{O} + e^{-1} \stackrel{H-\text{OH}}{\rightarrow} Ag\cdots \cdot \text{OH} + \text{OH}^{-1} \tag{8}$$

$$Ag \cdots OH + e^{-1} \stackrel{H-OH}{\rightarrow} Ag + \cdot OH^{-1} \tag{9}$$

A reaction coordinate diagram was developed for different alloys of silver on the basis of DFT calculation for the above mentioned reactions. From the theoretical calculations it was concluded that in case of silver-based catalysts, the rate-determining step for ORR is the initial adsorption of oxygen on the catalyst surface to form OOH adsorbate. In order to confirm the theoretical findings, the group synthesized Ag–Co alloys. After chemical etching of the alloy an Ag-skin with an Ag-Co core was developed. The resulting silver alloy showed phenomenal improvement in the ORR as compared to the unalloyed catalyst. Functionality tests on the Agskin /Ag-Co alloy core yielded an improvement in the area specific activity by a factor of 6 @ 0.8 VRHE. This significant enhancement was attributed to the perturbation of the electronic structure of the surface silver atoms which resulted in the lowering of the activation energy barriers for the ORR.

Composites of silver/graphene oxide and silver/graphene oxide/carbon were developed to investigate their ORR performance in alkaline environment [24]. The composites were facially synthesized by the reduction of AgNO3 with graphene oxide with or without the presence of Vulcan XC-72 carbon black. The average particle size of Ag/CO/C composites (ca d = 12.9 nm) was found to be almost twice of Ag/CO composite (ca d = 6.9). The composites were electro‐ chemically characterized which revealed the superior performance of Ag/CO/C for ORR as compared to Ag/CO composite. Rotating disc electrode (RDE) analysis revealed that the onset potential and the half wave potential shift positively for Ag/CO/C as compared to Ag/CO composite. This enhancement in ORR performance of Ag/CO/C composites as compared to Ag/CO composites was attributed to the 3D composite support which not only improves the electrical conductivity but also facilitated the mass transport in the catalyst layer. A similar beneficial effect of catalyst support was observed in the case of Ag/Mn3O4/C catalysts [25]. The catalyst performance for ORR in alkaline media was found to be superior to the simple Ag/C catalyst. This improvement was ascribed to the Mn3O4 support which perturbed the electronic structure of the silver particles. Charge was transferred to Mn3O4 support from Ag which was manifested by the lowering of the binding energy of the Ag 3d electrons in XPS measurements. This was accompanied with the rise of the d-band center of Ag in the Ag/Mn3O4/C catalyst as compared to the Ag/C catalyst. This was attributed to the tensile strain which results in less overlap of d orbitals and a corresponding rise of the dband center which in turn favors the kinetics of ORR by O—O bond breakage. Figure 3 reveals the oxygen reduction polarization curves for Ag/C, Ag/Mn3O4/C and Pt/C at 1600 rpm in O2-saturated 0.1 M KOH. The onset potential for Ag/Mn3O4/C was 0.92 V (vs. RHE) which is close to the onset potential for Pt/C catalyst. The limiting current incase of Ag/Mn3O4/C (c.a. Il = 5.5 mA/cm2 ) was also found to be very close to the limiting current of Pt/C (c.a. Il = 5.62 mA/cm2 ). Moreover, the electron transfer mechanism was found to proceed by four electrons from the Koutecky–Levich plots.

**Figure 3.** Oxygen reduction polarization curves for Ag/C, Ag/Mn3O4/C and Pt/C premetek at 1,600 rpm in O2-saturat‐ ed 0.1 M KOH at 10 mV s–1, and (inset) Koutecky–Levich plots for ORR in the presence of Ag/C, Ag/Mn3O4/C and Pt/C premetek at 0.32 V vs. RHE. Reproduced with permission from ref. 25. Copyright 2015, American Chemical Society.

The effect of morphology of the silver catalyst particles was studied by Ohyama et al. [26]. Silver particles with three distinct morphologies were investigated for their ORR performance in alkaline medium. Among the spherical, worm-like and the faceted particles, the maximum specific activity was observed in the worm-like particles with subsurface oxygen at surface defects. This was followed by the multifaceted particles with surface AgCO3 layer and defects. The smooth and spherical particles had the least specific activity of the three types. This increase in activity is justified by the large number of defects on these irregular-shaped particles which increase the reactivity of the silver catalyst toward the O2 during ORR.

## **2. Computational work**

The computational work involves initially the search for the structure with global minimum energy by genetic algorithm. The structure with minimum energy is further optimized by density functional theory (DFT) calculations. The most optimum structures are employed for simulations of the ORR reactions by density functional theory calculations.

### **2.1. Structural optimization**

structure of the silver particles. Charge was transferred to Mn3O4 support from Ag which was manifested by the lowering of the binding energy of the Ag 3d electrons in XPS measurements. This was accompanied with the rise of the d-band center of Ag in the Ag/Mn3O4/C catalyst as compared to the Ag/C catalyst. This was attributed to the tensile strain which results in less overlap of d orbitals and a corresponding rise of the dband center which in turn favors the kinetics of ORR by O—O bond breakage. Figure 3 reveals the oxygen reduction polarization curves for Ag/C, Ag/Mn3O4/C and Pt/C at 1600 rpm in O2-saturated 0.1 M KOH. The onset potential for Ag/Mn3O4/C was 0.92 V (vs. RHE) which is close to the onset potential for Pt/C

= 5.62 mA/cm2

) was also found to be

). Moreover, the electron transfer

catalyst. The limiting current incase of Ag/Mn3O4/C (c.a. Il = 5.5 mA/cm2

mechanism was found to proceed by four electrons from the Koutecky–Levich plots.

**Figure 3.** Oxygen reduction polarization curves for Ag/C, Ag/Mn3O4/C and Pt/C premetek at 1,600 rpm in O2-saturat‐ ed 0.1 M KOH at 10 mV s–1, and (inset) Koutecky–Levich plots for ORR in the presence of Ag/C, Ag/Mn3O4/C and Pt/C premetek at 0.32 V vs. RHE. Reproduced with permission from ref. 25. Copyright 2015, American Chemical Society.

The effect of morphology of the silver catalyst particles was studied by Ohyama et al. [26]. Silver particles with three distinct morphologies were investigated for their ORR performance in alkaline medium. Among the spherical, worm-like and the faceted particles, the maximum specific activity was observed in the worm-like particles with subsurface oxygen at surface defects. This was followed by the multifaceted particles with surface AgCO3 layer and defects. The smooth and spherical particles had the least specific activity of the three types. This increase in activity is justified by the large number of defects on these irregular-shaped particles which increase the reactivity of the silver catalyst toward the O2 during ORR.

The computational work involves initially the search for the structure with global minimum energy by genetic algorithm. The structure with minimum energy is further optimized by

very close to the limiting current of Pt/C (c.a. Il

420 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**2. Computational work**

The first step toward computational modeling of the nanoalloys is to search for the most stable geometry at absolute zero. This requires the modeling of the potential energy surface for the multi-element alloy. Gupta potential was used for the atomistic modeling of the nanoalloy [27, 28]. It is a semi-empirical method for the approximation of the potential energy surface. This method is based on the second moment approximation to the tight binding theory (SMATB). Total energy E for the nanoalloy A*x*B*<sup>N</sup>*–*<sup>x</sup>* is written as a sum of an attractive term Ej b and a repulsive term Ej r .

$$E = \sum\_{\langle \rangle} (E\_{\langle \rangle}^{\; b} + E\_{\langle \rangle}^{\; r}) \tag{10}$$

where Ej b and Ej r are further defined as:

$$E\_j^b = \sqrt{\sum\_{i \neq j}^N \xi^2 e^{-2q\left(t\_{ij} - r\_0 - 1\right)}} \tag{11}$$

and

$$E\_{\parallel}^{r} = \sum\_{i \neq j}^{N} A e^{-\rho \left(\frac{r\_{\parallel}}{r\_{\downarrow}} - 1\right)} \tag{12}$$

*N* is the number of atoms, *rij* is the distance between atoms *i* and *j* in the cluster and *ro* is the nearest-neighbor distance. The parameters A, *ro*, *ξ*, *p* and *q* for the pure species are fitted to several bulk experimental values, such as the cohesive energy, the lattice parameter and the elastic constants. The heteroatom interactions are fitted to the solubility energy of an impurity A into a B bulk. The Gupta potential parameters used for the Ag–Cu system are listed in Table 1 [29].


**Table 1.** Gupta potential parameters for Ag–Cu system

From the modeling of the potential energy surface the next step forward is to search for the global minimum (GM) by optimization. This is performed by the help of Birmingham cluster genetic algorithm (GA) [30]. From the initially randomly generated cluster population, the algorithm looks for the most stable cluster structure by mutation and cross over. For each generation, parent clusters are chosen with a probability depending on their fitness and offsprings are developed from parents by a cross-over scheme which is followed by a mutation step on the offspring to bring diversity in population. The whole population is ranked by fitness and the less fit, i.e. high-energy, clusters are replaced with more stable structures. The whole process is repeated till a predefined convergence criterion is met.

The 13 atom Ag–Cu bimetallic cluster was chosen for further geometric optimization by Dmol3 module available in the materials studio software package [31,32]. The 13 atom cluster is a magic size owing to special stability and relative abundance in case of Ag–Cu alloy clusters [33,34]. Spin-polarized DFT calculations are performed in real space within the framework of DFT-based semi core pseudo potentials (DSPPs) with the double numerical plus polarization (DNP) function. Grid integration is performed with a global cutoff of 5.0 °A. Self-consistent field procedures are performed with a convergence criterion of 10-6 Hartree on the total energy and the electron density. The Perdew et al. generalized gradient approximation (PBE/GGA) is used for the exchange–correlation functional during the geometry optimization for the Ag cluster. The ascending order of stability for the pure 13 atom Ag cluster is icosahedron (Ih) with binding energy Eb = –18.682 eV, decahedron (Dh) with Eb = –18.731 eV and cuboctahedron (COh) with Eb = –18.958 eV, where cuboctahedron (COh) structure was found to be the most stable configuration for the 13 atom Ag cluster. In case of the single Cu surface-doped Ag12Cu cluster, the increasing order of stability is also icosahedron (Ih) with Eb = –19.18727 eV, decahedron (Dh) with Eb = –19.40207 eV and cuboctahedron (COh) with Eb = –19.55135 eV. So for both pure Ag13 and Ag12Cusurface clusters, the most stable structural form was found to be cuboctahedron (COh).

## **2.2. ORR on 13 atom Ag-Cu clusters**

Ma et al. performed first principle calculations for the ORR process in alkaline media on the 13 atom pure Ag and Cu doped Ag clusters [35]. 13 atom Ag-Cu nanoalloy clusters have been previously identified as a potential candidate for ORR catalyst [36]. Pure Ag13 and Ag12Cusurface clusters with cuboctahedron (COh) symmetry were used for these calculations. The doping of copper significantly improves the ORR process. The ORR reaction was observed to occur by the efficient four electron transfer mechanism. Pure silver is a poor catalyst for the ORR because of its weak adsorption of oxygen. Doping of silver with copper atom on the surface improves the binding of the intermediates such as O, OH and OOH on the nanoalloy cluster. This optimum binding is critical for the efficient ORR. Binding energies of different adsorbates are described in Table 2.

From Table 2 it is evident that the binding energy of every adsorbate is more negative on the Ag12Cusurface cluster as compared to pure Ag13 cluster. This stronger binding facilitates the electron transfer reactions in the ORR process. A schematic of the ORR mechanism is provided in Figure 4.


**Table 2.** Binding energy of different adsorbates on nanoclusters

From the modeling of the potential energy surface the next step forward is to search for the global minimum (GM) by optimization. This is performed by the help of Birmingham cluster genetic algorithm (GA) [30]. From the initially randomly generated cluster population, the algorithm looks for the most stable cluster structure by mutation and cross over. For each generation, parent clusters are chosen with a probability depending on their fitness and offsprings are developed from parents by a cross-over scheme which is followed by a mutation step on the offspring to bring diversity in population. The whole population is ranked by fitness and the less fit, i.e. high-energy, clusters are replaced with more stable structures. The

The 13 atom Ag–Cu bimetallic cluster was chosen for further geometric optimization by Dmol3 module available in the materials studio software package [31,32]. The 13 atom cluster is a magic size owing to special stability and relative abundance in case of Ag–Cu alloy clusters [33,34]. Spin-polarized DFT calculations are performed in real space within the framework of DFT-based semi core pseudo potentials (DSPPs) with the double numerical plus polarization (DNP) function. Grid integration is performed with a global cutoff of 5.0 °A. Self-consistent field procedures are performed with a convergence criterion of 10-6 Hartree on the total energy and the electron density. The Perdew et al. generalized gradient approximation (PBE/GGA) is used for the exchange–correlation functional during the geometry optimization for the Ag cluster. The ascending order of stability for the pure 13 atom Ag cluster is icosahedron (Ih) with binding energy Eb = –18.682 eV, decahedron (Dh) with Eb = –18.731 eV and cuboctahedron (COh) with Eb = –18.958 eV, where cuboctahedron (COh) structure was found to be the most stable configuration for the 13 atom Ag cluster. In case of the single Cu surface-doped Ag12Cu cluster, the increasing order of stability is also icosahedron (Ih) with Eb = –19.18727 eV, decahedron (Dh) with Eb = –19.40207 eV and cuboctahedron (COh) with Eb = –19.55135 eV. So for both pure Ag13 and Ag12Cusurface clusters, the most stable structural form was found to be

Ma et al. performed first principle calculations for the ORR process in alkaline media on the 13 atom pure Ag and Cu doped Ag clusters [35]. 13 atom Ag-Cu nanoalloy clusters have been previously identified as a potential candidate for ORR catalyst [36]. Pure Ag13 and Ag12Cusurface clusters with cuboctahedron (COh) symmetry were used for these calculations. The doping of copper significantly improves the ORR process. The ORR reaction was observed to occur by the efficient four electron transfer mechanism. Pure silver is a poor catalyst for the ORR because of its weak adsorption of oxygen. Doping of silver with copper atom on the surface improves the binding of the intermediates such as O, OH and OOH on the nanoalloy cluster. This optimum binding is critical for the efficient ORR. Binding energies of different adsorbates are

From Table 2 it is evident that the binding energy of every adsorbate is more negative on the Ag12Cusurface cluster as compared to pure Ag13 cluster. This stronger binding facilitates the electron transfer reactions in the ORR process. A schematic of the ORR mechanism is provided

whole process is repeated till a predefined convergence criterion is met.

422 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

cuboctahedron (COh).

described in Table 2.

in Figure 4.

**2.2. ORR on 13 atom Ag-Cu clusters**

**Figure 4.** ORR pathway on Ag13(a,b,c,d) and Ag12Cu (a' , b' , c' ,d' ). Reprinted with permission from ref. 35. Copyright 2014, Springer.

The improvement of the ORR performance of the copper doped 13 atom nanocluster can be attributed to the modification of the dband. The dband center of the Ag13 cluster is at –3.078 eV. Doping with a copper atom at surface raises the dband center to –1.507eV. This can be explained by the dband theory of the reactivity of metal surfaces. According to that theory, the deeper the dband from the Fermi level, the lower is the surface reactivity. Alloying with copper raises the dband of the nanoalloy as compared to the pure metal cluster, which in turn raises the binding ability of the intermediates onto the nanoalloy cluster. The stronger binding of the intermediates is the reason behind the better ORR performance of Ag12Cu cluster as compared to the Ag13 cluster.

## **2.3. ORR on 38 atom Ag–Cu clusters**

The 38 atom Ag38–*x*Cu*x* cluster has been studied for ORR because of its relative stability by another group [37]. Truncated octahedron (TO) Ag32Cu6 alloy cluster was observed to perform as a better catalyst for ORR as compared to the TO Ag32Cu6 core-shell clusters. With the use of Gupta potential-based potential energy surface and genetic algorithm (GA) search for global minimum, polyicosahedron (PIh) Ag32Cu6 core-shell structure is found to be the most stable structure for Ag32Cu6 nanoalloy by Zhang et al.[38]. The stability of polyicosahedron (PIh) Ag32Cu6 core-shell structure exceeds that of truncated octahedron (TO) Ag32Cu6 core shell by 0.564 eV. For both pure Ag38 and Cu38 clusters truncated octahedron (TO) was the most stable geometry. Figure 5 shows that the minimum energy structure of the pure 38 atom silver took 35 iterations while more than 70 iterations were required for the Ag32Cu6 nanoalloy cluster.

**Figure 5.** Generations to reach global minimum by genetic algorithm. Adapted from ref. 38.

The polyicosahedron (PIh) Ag32Cu6 core-shell structure was further investigated for ORR because of its stability. ORR was found to proceed more favorably by the dissociation mech‐ anism as compared to the associative mechanism by 0.1 eV. Hence, dissociative ORR mecha‐ nism involving the scission of molecular oxygen to atomic form, i.e., *O*2→*O* + *O*, was considered for computational analysis. Four non-equivalent sites were identified on the Ag32Cu6 core-shell structure as B1 to B4. The ORR was then followed with the bond fracture and subsequent adsorption of atomic oxygen at hollow sites marked H1, H2 and H3 as shown in Figure 6.

Of the four adsorption configurations, we notice that the adsorption energy on B4 site has a highest value of –0.149 eV, and also the maximum value of 1.209 eV for the dissociation barrier, and an energy release of 0.259 eV, dissociating to H2 and H3 sites. The B1 site, which has similar adsorption energy to B4 site, –0.146 eV, further dissociates to two H2 sites with barrier of 0.993 eV and exothermicity of 0.259 eV. The O2 on B2 and B3 sites have bond-cleavage barriers of 0.715 and 1.134 eV and energy release of 1.088 and 0.368 eV, respectively. It is clear that the most favorable pathway for O2 dissociation is B2 sites with a minimum value of activation energy barrier.

**Figure 6.** Reaction coordinate diagram. Adapted from ref. 38.

structure for Ag32Cu6 nanoalloy by Zhang et al.[38]. The stability of polyicosahedron (PIh) Ag32Cu6 core-shell structure exceeds that of truncated octahedron (TO) Ag32Cu6 core shell by 0.564 eV. For both pure Ag38 and Cu38 clusters truncated octahedron (TO) was the most stable geometry. Figure 5 shows that the minimum energy structure of the pure 38 atom silver took 35 iterations while more than 70 iterations were required for the Ag32Cu6 nanoalloy cluster.

424 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 5.** Generations to reach global minimum by genetic algorithm. Adapted from ref. 38.

in Figure 6.

energy barrier.

The polyicosahedron (PIh) Ag32Cu6 core-shell structure was further investigated for ORR because of its stability. ORR was found to proceed more favorably by the dissociation mech‐ anism as compared to the associative mechanism by 0.1 eV. Hence, dissociative ORR mecha‐ nism involving the scission of molecular oxygen to atomic form, i.e., *O*2→*O* + *O*, was considered for computational analysis. Four non-equivalent sites were identified on the Ag32Cu6 core-shell structure as B1 to B4. The ORR was then followed with the bond fracture and subsequent adsorption of atomic oxygen at hollow sites marked H1, H2 and H3 as shown

Of the four adsorption configurations, we notice that the adsorption energy on B4 site has a highest value of –0.149 eV, and also the maximum value of 1.209 eV for the dissociation barrier, and an energy release of 0.259 eV, dissociating to H2 and H3 sites. The B1 site, which has similar adsorption energy to B4 site, –0.146 eV, further dissociates to two H2 sites with barrier of 0.993 eV and exothermicity of 0.259 eV. The O2 on B2 and B3 sites have bond-cleavage barriers of 0.715 and 1.134 eV and energy release of 1.088 and 0.368 eV, respectively. It is clear that the most favorable pathway for O2 dissociation is B2 sites with a minimum value of activation

The interaction strength of atoms and molecules with metal surface is defined by the *d*-band center of the metal. In order to explain further that B2 site is most favorable to display a good catalytic behavior, the electronic structure of these four adsorption configurations was addressed and the position of the *d*-band center relative to the Fermi energy for these different sites was calculated as shown in Figure 7. The *d*band center of B2 site is –3.395 eV, which is closest to the Fermi level. By having the dband center closest to the Fermi level as compared to the other adsorption sites on the polyicosahedron (PIh) Ag32Cu6 core-shell structure, B2 site is the most conducive for ORR as it enhances the otherwise weak affinity of silver alloys for the reaction intermediates.

**Figure 7.** Partial density of states (PDOS). Adapted from ref. 38.

Moreover, frontier orbital theory describes that the states of a metal which are involved in electron transfer with the adsorbates are closest to the Fermi level [39]. Therefore, the density of states (DOS) at the Fermi level is an indicator of the chemical activity. We notice that the density of states at the Fermi energy level is maximal for B2 site. This further endorses the superior activity of the B2 site for ORR on Ag32Cu6 core-shell nanoalloy.
