**3. Synthesis of Ag–Cu nanoalloy**

Bimetallic Ag–Cu nanoalloy particles were developed by both physical and chemical routes.

## **3.1. Pulse laser deposition**

Pulse laser deposition (PLD) is widely used in the field because the approach is a feasible way to control the thickness of catalyst layer [40]. As the utilization of catalyst in the fuel cell is determined mainly by the surface area of catalyst with electrolyte, the reduction of the thickness of catalytic layer can lead to improvement of the catalyst utilization and reduction of the fuel cell cost [41]. Moreover, compared to chemical preparation techniques such as electrochemical deposition, chemical vapor deposition, reduction of salts and facile hydro‐ thermal method, PLD method owns high repeatability and stability in process, making it to be a suitable route to obtain electrocatalyst with film state [42–44].

Ag–Cu alloyed catalyst was developed by PLD method in a vacuum chamber [45]. The target of Ag–Cu alloy with atomic ratio of 50:50 was irradiated with a nanosecond Q-switched Nd:YAG laser beam (EKSPLA, Lithuania). The wavelength was set to be 266 nm, and the pulse duration was ranging from 3 to 6 ns. The laser beam diameter was around 1 mm, with an energy density of 200 mJ/pulse. Both target and substrate (nickel foam) rotated at a speed of 5 rpm during deposition, and target was irradiated for 2 min at 10 Hz to clear away the oxide on the surface before deposition. The laser was operated at the frequency of 10 Hz. The deposition time is set as 90 min. The as-prepared product is Ag50Cu50 catalyst.

Figure 8 shows series of TEM analysis on Ag50Cu50 catalyst. According to Figure 8a, plenty of nanoparticles are distributed in a continuous film. The tiny nanoparticles with size under 5 nm dominate the film. Magnifying the blue rectangle area, the obtained HRTEM is shown in Figure 8b. It can be seen that they display two different states: few are amorphous, and the left are with crystallized state.

HAADF result shown in Figure 8c displays that contrast of the particles is brighter than the gap area between particles, demonstrating a higher atomic number *Z* for nanoparticles. The lower *Z* corresponding gap area then is attributed from Cu element. This is because *Z* of Cu (*Z* = 29) is smaller than Ag (*Z* = 47). Combining the amorphous state in gap area observed in Figure 8b, we can draw that Ag50Cu50 catalyst actually is Ag–Cu alloyed nanoparticles embedded in amorphous Cu film.

Electrochemical characterizations have been carried out on PLD synthesized Ag50Cu50 catalyst. Figure 9a shows RDE polarization curves of Ag50Cu50 catalyst with rotation rate 1,600 rpm in

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

Bimetallic Ag–Cu nanoalloy particles were developed by both physical and chemical routes.

Pulse laser deposition (PLD) is widely used in the field because the approach is a feasible way to control the thickness of catalyst layer [40]. As the utilization of catalyst in the fuel cell is determined mainly by the surface area of catalyst with electrolyte, the reduction of the thickness of catalytic layer can lead to improvement of the catalyst utilization and reduction of the fuel cell cost [41]. Moreover, compared to chemical preparation techniques such as electrochemical deposition, chemical vapor deposition, reduction of salts and facile hydro‐ thermal method, PLD method owns high repeatability and stability in process, making it to

Ag–Cu alloyed catalyst was developed by PLD method in a vacuum chamber [45]. The target of Ag–Cu alloy with atomic ratio of 50:50 was irradiated with a nanosecond Q-switched Nd:YAG laser beam (EKSPLA, Lithuania). The wavelength was set to be 266 nm, and the pulse duration was ranging from 3 to 6 ns. The laser beam diameter was around 1 mm, with an energy density of 200 mJ/pulse. Both target and substrate (nickel foam) rotated at a speed of 5 rpm during deposition, and target was irradiated for 2 min at 10 Hz to clear away the oxide on the surface before deposition. The laser was operated at the frequency of 10 Hz. The

Figure 8 shows series of TEM analysis on Ag50Cu50 catalyst. According to Figure 8a, plenty of nanoparticles are distributed in a continuous film. The tiny nanoparticles with size under 5 nm dominate the film. Magnifying the blue rectangle area, the obtained HRTEM is shown in Figure 8b. It can be seen that they display two different states: few are amorphous, and the left

HAADF result shown in Figure 8c displays that contrast of the particles is brighter than the gap area between particles, demonstrating a higher atomic number *Z* for nanoparticles. The lower *Z* corresponding gap area then is attributed from Cu element. This is because *Z* of Cu (*Z* = 29) is smaller than Ag (*Z* = 47). Combining the amorphous state in gap area observed in Figure 8b, we can draw that Ag50Cu50 catalyst actually is Ag–Cu alloyed nanoparticles

Electrochemical characterizations have been carried out on PLD synthesized Ag50Cu50 catalyst. Figure 9a shows RDE polarization curves of Ag50Cu50 catalyst with rotation rate 1,600 rpm in

superior activity of the B2 site for ORR on Ag32Cu6 core-shell nanoalloy.

be a suitable route to obtain electrocatalyst with film state [42–44].

deposition time is set as 90 min. The as-prepared product is Ag50Cu50 catalyst.

**3. Synthesis of Ag–Cu nanoalloy**

426 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**3.1. Pulse laser deposition**

are with crystallized state.

embedded in amorphous Cu film.

**Figure 8.** TEM and HAADF characterization of Ag50Cu50 catalysts. (a) bright field image, (b) HRTEM, (c) HAADF re‐ sult and (d) IFFT image. Adapted from ref. 45.

N2 and O2 saturated 0.1 M KOH solutions. It can be seen that there is reduction in current density in O2-saturated KOH solution, while that in N2-saturated solution is flat. This shows that the catalyst indeed works on O2. Figure 9b shows a set of RDE curves with rotation rates of 400, 800, 1,200 and 1,600 rpm. The Koutecky–Levich plots were then obtained from the limiting current density, as shown in Figure 9c. The plots show the inverse current density *J*−1 as a function of ω–1/2. From these plots the number of electrons transferred during ORR was found to be 3.76, 3.87, 3.85 and 3.97 when the potential was 0.5, 0.4, 0.3 and 0.2 V, respectively. Hence, four electrons route was found to be dominant for ORR in case of synthesized Ag50Cu50 catalyst. Finally, 9d gives a comparison of the performance of Ag, Ag50Cu50 and Pt/ C(20 wt%).

The catalytic layer was used to assemble a zinc-air battery and results showed open-circuit voltage (OCV) of the cell was around 1.48 V close to the theoretical value, and the maximum power density is 67 mW cm−2 at 100 mA cm−2. The resulting rechargeable zinc-air battery exhibits low charge–discharge voltage polarization of 1.1 V at 20 mAcm−2 and high durability over 100 cycles in natural air.

Wu et al. deposited Ag–Cu nanoalloys on nickel foam by pulse laser deposition. Several Ag– Cu alloys with Ag/Cu atom ratios of 90:10, 50:50 and 25:75 are used as the target material with the high-purity nickel foam (99.97%) as substrate [46]. Ag–Cu nanoalloys were used as the catalyst layer of the air cathode for a single zinc-air battery, and found to exhibit good

**Figure 9.** Electrochemical characterization of Ag50Cu50 catalyst. (a) The RDE curves of Ag50Cu50 catalyst in O2- and N2 saturated 0.1 M KOH solution; (b) the RDE curves at the rotation rates of 400, 800, 1,200 and 1,600 rpm; (c) the Kou‐ tecky–Levich plot of Ag50Cu50 catalyst; and (d) the ORR mass activity for Ag, Ag50Cu50 and Pt/C (20 wt%) catalysts. Adapted from ref. 45.

bifunctional catalytic performance. The effect of the Ag/Cu atom ratio on the average electron transfer numbers of the ORR was systematically investigated. This carbon-free binder-free bimetallic catalyst layer was found to possess both ORR and OER catalytic activity in the rechargeable zinc-air battery. Figure 10(a) shows representative microscopic images of the Ag50Cu50 alloys used in this work. The Ag–Cu nanoparticles were uniformly distributed in the substrate, and the electron diffraction pattern revealed the single phase and polycrystalline structure of the Ag–Cu alloy. Figure10 (b) shows a HRTEM image of more than 100 nanopar‐ ticles in the substrates. It is clear that the nanoparticles have an average size of 2.58 nm with a narrow size distribution between 1 and 5 nm. Figure 10(c,d) indicate that the nanoparticles are enriched with Ag atoms, with few doped copper atoms in them, and the films are enriched with Cu atoms.

Hence, it can be concluded that the nanocatalyst has crystalline Ag-enriched nanoparticles embedded in an amorphous Cu-enriched matrix. These copper-doped silver nanoparticles with composition Ag50Cu50 were observed to have superior catalytic performance for ORR as compared to pure silver as shown in Figure 11. The ORR was found to proceed via four electron transfer mechanism. It is for the first time the Ag-based electrocatalysts in amorphous films were created from the vapor phase under far-from-equilibrium condition by pulse laser ablation, previous works demonstrated that the face-centered cubic Ag–Cu solid solutions or completely amorphous Ag–Cu metal glasses were formed by rapid quenching from the liquid or vapor phase as corrosion-resistant non-equilibrium alloys and metastable phases.

**Figure 10.** (a)TEM bright-field images and SAED patterns (inset) of the Ag50Cu50 film prepared by PLD. (b) HRTEM images and the particle size distribution (inlet) of Ag–Cu nanoparticles in the film. (c) TEM element mapping for Ag, Cu and Ni on a Ag–Cu nanoparticle deposited on nickel grid. (d) EDS of the Ag50Cu50 film on nickel grid. Reprinted with permission from ref. 46. Copyright 2015, American Chemical Society.

bifunctional catalytic performance. The effect of the Ag/Cu atom ratio on the average electron transfer numbers of the ORR was systematically investigated. This carbon-free binder-free bimetallic catalyst layer was found to possess both ORR and OER catalytic activity in the rechargeable zinc-air battery. Figure 10(a) shows representative microscopic images of the Ag50Cu50 alloys used in this work. The Ag–Cu nanoparticles were uniformly distributed in the substrate, and the electron diffraction pattern revealed the single phase and polycrystalline structure of the Ag–Cu alloy. Figure10 (b) shows a HRTEM image of more than 100 nanopar‐ ticles in the substrates. It is clear that the nanoparticles have an average size of 2.58 nm with a narrow size distribution between 1 and 5 nm. Figure 10(c,d) indicate that the nanoparticles are enriched with Ag atoms, with few doped copper atoms in them, and the films are enriched

**Figure 9.** Electrochemical characterization of Ag50Cu50 catalyst. (a) The RDE curves of Ag50Cu50 catalyst in O2- and N2 saturated 0.1 M KOH solution; (b) the RDE curves at the rotation rates of 400, 800, 1,200 and 1,600 rpm; (c) the Kou‐ tecky–Levich plot of Ag50Cu50 catalyst; and (d) the ORR mass activity for Ag, Ag50Cu50 and Pt/C (20 wt%) catalysts.

Hence, it can be concluded that the nanocatalyst has crystalline Ag-enriched nanoparticles embedded in an amorphous Cu-enriched matrix. These copper-doped silver nanoparticles with composition Ag50Cu50 were observed to have superior catalytic performance for ORR as compared to pure silver as shown in Figure 11. The ORR was found to proceed via four electron transfer mechanism. It is for the first time the Ag-based electrocatalysts in amorphous films were created from the vapor phase under far-from-equilibrium condition by pulse laser ablation, previous works demonstrated that the face-centered cubic Ag–Cu solid solutions or completely amorphous Ag–Cu metal glasses were formed by rapid quenching from the liquid

or vapor phase as corrosion-resistant non-equilibrium alloys and metastable phases.

with Cu atoms.

Adapted from ref. 45.

428 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 11.** (a) RDE polarization curve of different compositions in O2-saturated 0.1 M KOH solution. (b) RDE polariza‐ tion curve for Ag50Cu50 same environment (c) Koutecky–Levich plots for (b). Reprinted with permission from ref. 46. Copyright 2015, American Chemical Society.

Zinc-air batteries were assembled using Ag90Cu10 and Ag50Cu50 catalysts in the air cathode, and the discharge performance is shown in Figure 12(a). The cell voltage decreases nearly with increasing current density, demonstrating that the cell performance shows a strong depend‐ ence on the resistance of the battery. For Ag50Cu50 and Ag90Cu10 catalysts, the open-circuit voltages of the single cell are about 1.42 V and 1.44 V, the peak power densities of the zinc-air batteries are 86.3 mW.cm–2 and 82.1 mW.cm–2, and the current densities at a voltage of 1 V are 60 mA cm–2 and 50 mA cm–2, which is higher than Ag/C, N-doped CNTs and silver-molybdate catalysts [47,48]. Figure 12 (b) records the change of cell voltage with time at a current density of 20 mA cm–2. It is clear that Ag90Cu10 catalysts have a higher initial discharge voltage of 1.15 V than Ag50Cu50 catalysts (1.0V); however, after 30 h discharging, the discharge voltage of Ag90Cu10 catalysts gradually reduces to 1.11 V, while Ag50Cu50 catalysts gradually increase to 1.18 V. The discharge curve of the Ag50Cu50 catalysts gradually rises to a stable cell voltage and decrease by 16% as compared with the open-circuit potential, but for Ag90Cu10 catalysts, the curve gradually reduces to a stable voltage after about 30 h of discharge, showing a decrease of 20%. It can be concluded that the Ag50Cu50 catalyst has higher discharge voltage stability, being more stable than the Ag90Cu10 catalysts for applications in zinc-air batteries.

**Figure 12.** (a) The discharge polarization and power density curves for Ag90Cu50 and Ag50Cu50 catalyst layer of air-cath‐ ode in the primary zinc-air battery. (b) The single cell voltage and time curves at 20 mA cm–2 in the primary zinc-air battery. (In 6M KOH solution) Reprinted with permission from ref. 46. Copyright 2015, American Chemical Society.

### **3.2. Galvanic displacement synthesis**

Ag–Cu catalysts were synthesized directly on Ni foams by galvanic displacement reaction, which is an environment-friendly and straightforward process [42]. In this method, the catalysts were directly grown on Ni foams, thereby freeing the catalytic layer from carbon and binder. The driving mechanism to grow the various catalysts in this work is the large difference of the redox potentials of Ni2+/Ni (–0.25 V vs. SHE), Cu2+/Cu (0.34 V vs. SHE) and Ag+/Ag (0.799 V vs. SHE). The galvanic displacement reaction can be described by the following equations:

$$\text{Ni}(s) + \text{Cu}^{\ast 2}(aq) \rightarrow \text{Cu}(s) + \text{Ni}^{\ast 2}(aq) \tag{13}$$

$$2\text{ Ni(s)} + 2\text{Ag}^+\text{(aq)} \rightarrow 2\text{Ag(s)} + \text{Ni}^{\ast 2}\text{(aq)}\tag{14}$$

$$2\operatorname{Cu}(s) + 2\operatorname{Ag}^{+}(aq) \to 2\operatorname{Ag}(s) + \operatorname{Cu}^{\ast 2}(aq) \tag{15}$$

Figure 13 shows the typical SEM images of (a) Ni foam, (b) Cu nanoparticles, (c) AgCu-10 and (d) Ag catalysts. Figure 13(a) shows the pure Ni foam smooth surface. Immersion into CuSO4 solution for 3 h makes the surface of the foam rough (Figure 13(b)) along with coverage of octahedral copper nanoparticles (Figure 13(b) inset). The SEM of AgCu-10 catalyst prepared by two-step galvanic displacement reaction is shown in Figure 13(c). The catalyst has dendritic morphology. The dendrites shown in Figure 13(c) have a dense and uniform distribution, and the shape is complete. On the other hand, similarly dendritic morphologies are obtained for pure Ag catalyst (Figure 13(d)) prepared by directly immersing the as-prepared Ni foam into AgNO3 solution. The dendrites of Ag catalyst are thinly distributed on the Ni foam compared to that in Figure 13(c). The difference of the dendrites between AgCu-10 and Ag catalysts can be because of the different sacrificial templates. For the AgCu-10 catalyst, the Ni foam was already covered by octahedral copper nanoparticles while the nickel surface was free from copper in case of pure Ag particles.

voltages of the single cell are about 1.42 V and 1.44 V, the peak power densities of the zinc-air batteries are 86.3 mW.cm–2 and 82.1 mW.cm–2, and the current densities at a voltage of 1 V are 60 mA cm–2 and 50 mA cm–2, which is higher than Ag/C, N-doped CNTs and silver-molybdate catalysts [47,48]. Figure 12 (b) records the change of cell voltage with time at a current density of 20 mA cm–2. It is clear that Ag90Cu10 catalysts have a higher initial discharge voltage of 1.15 V than Ag50Cu50 catalysts (1.0V); however, after 30 h discharging, the discharge voltage of Ag90Cu10 catalysts gradually reduces to 1.11 V, while Ag50Cu50 catalysts gradually increase to 1.18 V. The discharge curve of the Ag50Cu50 catalysts gradually rises to a stable cell voltage and decrease by 16% as compared with the open-circuit potential, but for Ag90Cu10 catalysts, the curve gradually reduces to a stable voltage after about 30 h of discharge, showing a decrease of 20%. It can be concluded that the Ag50Cu50 catalyst has higher discharge voltage stability,

430 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

being more stable than the Ag90Cu10 catalysts for applications in zinc-air batteries.

**Figure 12.** (a) The discharge polarization and power density curves for Ag90Cu50 and Ag50Cu50 catalyst layer of air-cath‐ ode in the primary zinc-air battery. (b) The single cell voltage and time curves at 20 mA cm–2 in the primary zinc-air battery. (In 6M KOH solution) Reprinted with permission from ref. 46. Copyright 2015, American Chemical Society.

Ag–Cu catalysts were synthesized directly on Ni foams by galvanic displacement reaction, which is an environment-friendly and straightforward process [42]. In this method, the catalysts were directly grown on Ni foams, thereby freeing the catalytic layer from carbon and binder. The driving mechanism to grow the various catalysts in this work is the large difference of the redox potentials of Ni2+/Ni (–0.25 V vs. SHE), Cu2+/Cu (0.34 V vs. SHE) and Ag+/Ag (0.799 V vs. SHE). The galvanic displacement reaction can be described by the following equations:

() ( ) () ( ) 2 2 *Ni s Cu aq Cu s Ni aq* + + + ®+ (13)

() ( ) () ( ) <sup>2</sup> *Ni s Ag aq Ag s Ni aq* 2 2 + + + ®+ (14)

() ( ) () ( ) <sup>2</sup> *Cu s Ag aq Ag s Cu aq* 2 2 + + + ®+ (15)

**3.2. Galvanic displacement synthesis**

**Figure 13.** The FE-SEM images of (a) Ni foam. (b) Cu nanoparticles supported on Ni foam. The inset in (b) shows the high-magnification image of Cu nanoparticles. (c) Ag–Cu dendrites in AgCu-10 catalyst and (d) Ag dendrites support‐ ed on Ni foam. Reprinted with permission from ref. 42. Copyright 2015, Elsevier.

As shown in Figure 13(c), the one-dimensional dendrites prefer to form in a relatively high AgNO3 concentration (10 mM). The AgCu-10 bimetallic catalyst possessed a hierarchical structure characteristic and highly rough surface which provided more catalytic active sites so it showed higher catalytic current during the RDE polarization measurements. The SEM elemental mapping analysis for AgCu-10 catalyst shown in Figure 14(a–d) clearly prove that Ag and Cu are uniformly distributed. HRTEM and SAED images are shown in Figure 14 (e, f). The clear lattice fringes are observed for AgCu-10 catalyst. The SAED pattern shows a typical polycrystalline structure with the sharp diffraction rings for AgCu-10 catalyst. The diffraction spots for Ag (111), (2 0 0), (2 2 0), (311) facets and Cu (111), (2 2 0) facets are observed. A fast Fourier transform (FFT) diffraction pattern as shown in Figure 14 (g) is obtained on the area marked by the red rectangle in Figure 14 (e), exhibiting the two phases characteristic of AgCu-10 catalyst. The FFT image also reveals that Ag and Cu crystallites are in an epitaxial relationship relative to each other in parallel orientation. The grown orientation schematic is shown in Figure 14 (h).

**Figure 14.** EDS elemental map of the AgCu-10 catalyst: (a) Overlay image. (b) Ag map. (c) Cu map. (d) Ni map. The TEM characteristic for AgCu-10 catalyst: (e) HRTEM image. (f) SAED pattern. (g) Fast Fourier transform (FFT) image corresponding to (e) and schematic drawing with index of reflections (h). Reprinted with permission from ref. 42. Copyright 2015, Elsevier.

Rotating disc measurements were performed on the catalysts made by galvanic displacement and comparison was made with pure silver. AgCu-10 performed efficiently as compared to the pure silver particles in terms of both onset potential and the limiting current. The Kou‐ tecky–Levich plots revealed the four electron transfer mechanism during ORR. Rechargeable zinc-air battery was also fabricated by as-prepared AgCu-10 catalyst-based air cathode, 6 M KOH solution with 0.2 M zinc acetate (zinc acetate was dissolved in KOH to form zincate to ensure reversible Zn electrochemical reactions at the anode) and pure zinc plate anode. A charge–discharge cycle experiment was performed with a short cycle period of 20 min and a long cycle period of 4 h at 20 mA cm−2. At the short cycle period, the initial charge and discharge potentials of AgCu-10 based rechargeable zinc-air battery are 2.04 and 1.1 V, respectively. The round-trip efficiency corresponding to the first cycle is 53.9%. There is almost no apparent fluctuation for the charge and discharge potentials of the rechargeable zinc-air battery through all the cyclic process. The round-trip efficiency after 100 cycles is 53.08% compared to the initial 53.9% with a little decline of 0.82%. A further cycle performance study with a long cycle period of 4 h was carried out with the same rechargeable zinc-air battery after replacing the zinc anode and the electrolyte. The rechargeable zinc-air battery also shows high cycling stability at the long cycle period. The increase in charge and discharge potentials difference from the first to tenth cycle is as little as 0.06 V, which is comparable to the tri-electrode rechargeable zinc-air battery [49]. The cycling stability obtained on AgCu-10 catalyst based zinc-air battery is certainly appealing and significant for the large-scale application of metal-air batteries and fuel cells.

## **3.3. Electro-deposition of Ag–Cu nanoalloys**

Fourier transform (FFT) diffraction pattern as shown in Figure 14 (g) is obtained on the area marked by the red rectangle in Figure 14 (e), exhibiting the two phases characteristic of AgCu-10 catalyst. The FFT image also reveals that Ag and Cu crystallites are in an epitaxial relationship relative to each other in parallel orientation. The grown orientation schematic is

**Figure 14.** EDS elemental map of the AgCu-10 catalyst: (a) Overlay image. (b) Ag map. (c) Cu map. (d) Ni map. The TEM characteristic for AgCu-10 catalyst: (e) HRTEM image. (f) SAED pattern. (g) Fast Fourier transform (FFT) image corresponding to (e) and schematic drawing with index of reflections (h). Reprinted with permission from ref. 42.

Rotating disc measurements were performed on the catalysts made by galvanic displacement and comparison was made with pure silver. AgCu-10 performed efficiently as compared to the pure silver particles in terms of both onset potential and the limiting current. The Kou‐ tecky–Levich plots revealed the four electron transfer mechanism during ORR. Rechargeable zinc-air battery was also fabricated by as-prepared AgCu-10 catalyst-based air cathode, 6 M KOH solution with 0.2 M zinc acetate (zinc acetate was dissolved in KOH to form zincate to

shown in Figure 14 (h).

432 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Copyright 2015, Elsevier.

Ag–Cu catalysts were synthesized by the electrodeposition method under a potential of −0.4 V for a period of 50 s by using the conventional three-electrode cell system [50]. For synthesis, AgNO3, Cu(NO3)2 3H2O and 3 mM sodium citrate (Na3C6H5O7) are mixed by deionized water with the formula of Ag*x*Cu100–*x* (labelled as Ag25Cu75, Ag50Cu50 and Ag75Cu25 for *x* = 25, 50 and 75 mM, respectively).

Figure 15(a) describes the TEM and SEM (the inset) images of the bimetallic Ag–Cu catalyst. The nanoplatelets have diameters of 40–50 nm. The HRTEM analysis in Figure 15(b) clearly shows lattice fringes, indicating good crystallinity. The lattice spacing obtained from the HRTEM image is 0.239 nm. The particle demonstrates a single crystal pattern in the SAED of Figure15(c), indicating that Ag–Cu catalysts grow larger by the oriented attachment from small Ag–Cu nanoparticles. The cell constant of the single crystal is 0.3986 nm, which is between the standard cell parameter of FCC-Ag (*a* = 0.4086 nm) and FCC-Cu (*a* = 0.3615 nm), suggesting that the Cu atoms are partially alloyed with the Ag atoms.

**Figure 15.** Images of the Ag–Cu catalyst (a) TEM (inset SEM). (b) HRTEM. (c) SAED. Reprinted with permission from ref. 50. Copyright 2015, John Wiley & Sons.

The EDX spectrum of the Ag–Cu nanoalloy in Figure 16(a) exhibits that the Ag–Cu deposits contain both Ag and Cu elements and the nominal atomic composition are 1.5:1, 5:1 and 10:1 for the Ag25Cu75, Ag50Cu50 and Ag75Cu25 samples, respectively.

The survey spectrum of XPS for the Ag–Cu catalyst is shown in Figure 16(b), which shows clear Ag and Cu peaks. The binding energies of the Ag3d3/2 and Ag3d5/2 orbits observed from the high-resolution spectrum (Figure 16(b) inset) are 374.4 and 368.4 eV. This result indicates that the Ag atoms are zero-valent (Ag0 ) metals [51,52]. The binding energies of the Cu2p1/2 and Cu2p3/2 orbits are 952.6 and 932.7 eV (Figure 16(f)). An analysis of the Auger electron spectrum for Cu LMM (Figure 16(f) inset) is also conducted to determine the specific valence of Cu; an apparent kinetic energy peak at 918.6 eV is seen, corresponding to the zero-valent Cu[53]. Thus, the Ag and Cu atoms exist as elemental metallic substances in Ag–Cu catalyst.

**Figure 16.** (a) EDX. (b) XPS survey spectrum, inset: high-solution spectrum of Ag3d. (c) High-solution spectrum of Cu2p, inset: Cu LMM. Reprinted with permission from ref. 50. Copyright 2015, John Wiley & Sons.

As shown in Figure 17(a), a reduction current peak is observed from the CV curves of the Ag– Cu catalyst in O2-saturated 0.1 M KOH at −0.3 V (vs. SCE) but not in N2-saturated solution, indicating that Ag–Cu catalyst has catalytic activity for ORR. To study the catalysis kinetics of Ag–Cu catalyst for ORR, an RDE experiment is performed in O2-saturated 0.1 M KOH solution (Figure 17(b)). The number (*n*) of electrons transferred on the Ag–Cu catalyst during ORR, which determines the catalytic efficiency, is calculated by the Koutecky–Levich plots (see inset). The result with *n* = 3.8 and 3.7 at −0.7 and −0.8 V, respectively, indicates that the ORR catalyzed by the Ag–Cu catalyst occurs through a four-electron pathway, which is more efficient than a two-electron pathway.

The performance of primary battery fabricated from Ag50Cu50-based air cathode was evaluated as shown in Figure 18(a). The open-circuit voltage (OCV) and maximum power density are 1.49 V and 87 mWcm−2, respectively, which have significant improvements [54,55]. After the primary zinc–air battery undergoes 10 discharging cycles, the OCV and power density decrease slightly. This result may be attributed to the polarization of the zinc anode caused by the zincate produced during the discharge process. Replacing the zinc anode and electrolyte revitalizes the battery performance. The battery has no obvious voltage loss compared with the first cycle, suggesting that the Ag50Cu50-based air cathode is stable in alkaline solution and

Ag-Cu Nanoalloys as Oxygen Reduction Electrocatalysts in Alkaline Media for Advanced... http://dx.doi.org/10.5772/62050 435

The EDX spectrum of the Ag–Cu nanoalloy in Figure 16(a) exhibits that the Ag–Cu deposits contain both Ag and Cu elements and the nominal atomic composition are 1.5:1, 5:1 and 10:1

The survey spectrum of XPS for the Ag–Cu catalyst is shown in Figure 16(b), which shows clear Ag and Cu peaks. The binding energies of the Ag3d3/2 and Ag3d5/2 orbits observed from the high-resolution spectrum (Figure 16(b) inset) are 374.4 and 368.4 eV. This result indicates

Cu2p3/2 orbits are 952.6 and 932.7 eV (Figure 16(f)). An analysis of the Auger electron spectrum for Cu LMM (Figure 16(f) inset) is also conducted to determine the specific valence of Cu; an apparent kinetic energy peak at 918.6 eV is seen, corresponding to the zero-valent Cu[53]. Thus,

**Figure 16.** (a) EDX. (b) XPS survey spectrum, inset: high-solution spectrum of Ag3d. (c) High-solution spectrum of

As shown in Figure 17(a), a reduction current peak is observed from the CV curves of the Ag– Cu catalyst in O2-saturated 0.1 M KOH at −0.3 V (vs. SCE) but not in N2-saturated solution, indicating that Ag–Cu catalyst has catalytic activity for ORR. To study the catalysis kinetics of Ag–Cu catalyst for ORR, an RDE experiment is performed in O2-saturated 0.1 M KOH solution (Figure 17(b)). The number (*n*) of electrons transferred on the Ag–Cu catalyst during ORR, which determines the catalytic efficiency, is calculated by the Koutecky–Levich plots (see inset). The result with *n* = 3.8 and 3.7 at −0.7 and −0.8 V, respectively, indicates that the ORR catalyzed by the Ag–Cu catalyst occurs through a four-electron pathway, which is more

The performance of primary battery fabricated from Ag50Cu50-based air cathode was evaluated as shown in Figure 18(a). The open-circuit voltage (OCV) and maximum power density are 1.49 V and 87 mWcm−2, respectively, which have significant improvements [54,55]. After the primary zinc–air battery undergoes 10 discharging cycles, the OCV and power density decrease slightly. This result may be attributed to the polarization of the zinc anode caused by the zincate produced during the discharge process. Replacing the zinc anode and electrolyte revitalizes the battery performance. The battery has no obvious voltage loss compared with the first cycle, suggesting that the Ag50Cu50-based air cathode is stable in alkaline solution and

Cu2p, inset: Cu LMM. Reprinted with permission from ref. 50. Copyright 2015, John Wiley & Sons.

the Ag and Cu atoms exist as elemental metallic substances in Ag–Cu catalyst.

) metals [51,52]. The binding energies of the Cu2p1/2 and

for the Ag25Cu75, Ag50Cu50 and Ag75Cu25 samples, respectively.

434 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

that the Ag atoms are zero-valent (Ag0

efficient than a two-electron pathway.

**Figure 17.** (a) CV curves for Ag–Cu catalyst in O2- and N2-saturated 0.1 M KOH solutions at a scan rate of 10 mV s−1. (b) RDE polarization curves at different rotation rates in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s−1, inset: Koutecky–Levich plots. Reprinted with permission from ref. 50. Copyright 2015, John Wiley & Sons.

can be a potential candidate for rechargeable zinc-air batteries. Further analysis of discharging performance of this primary zinc-air battery is conducted at different constant current densities as in Figure 18(b). The cell voltages are stable during the whole discharge period of 20 h, except for a slight drop of approximately 0.05 V at the current density of 50 mA cm−2. This voltage drop can be attributed to the electrode polarization under high current density.

**Figure 18.** Performance of the primary zinc-air battery fabricated with an Ag50Cu50 catalyst-based air cathode: a) Cell voltage and power density polarization curves of the battery over 10 cycles and the polarization curve after replacing Zn anode and electrolyte; b) discharge voltage curves at different current densities. Reprinted with permission from ref. 50. Copyright 2015, John Wiley & Sons.

Figure 19(a) shows the charge and discharge polarization curves of the secondary zinc-air battery. An abrupt polarization occurs when the current densities increase from 0 to 10 mA cm−2 because of the activation polarization and anode polarization [56]. However, once the zinc-air battery begins its function, the polarization exhibits a steady increase with varying current densities from 10 to 100 mA cm−2. The charge–discharge voltage gap (i.e., the overpo‐ tential) at 20 mA cm−2 is 0.9 V, which is lower than that of Co3O4-based rechargeable zinc-air batteries [57,58]. The cycle performance with different cycle periods is shown in Figure 19(b). As seen in the bottom of Figure 19(b), the secondary zinc-air battery undergoes 100 charge and discharge cycles at 20 mA cm−2 with 20 min per step. The difference between the charge and discharge potentials is 0.9 V, and the overpotential shows no apparent fluctuation through all 100 cycles. The round-trip efficiency is up to 56.4%, which is a considerable improvement. A more violent charge and discharge cycle experiment is conducted with a cycle period of 4 h for the same rechargeable zinc-air battery after replacing the zinc anode and electrolyte. As shown in the top of Figure 19(b), the charge and discharge voltages are still stable even with the long cycle period; this result is comparable to the result of the tri-electrode rechargeable zinc-air battery.

**Figure 19.** (a) Charge–discharge polarization curves for the rechargeable zinc air battery. b) Cycle performance for the rechargeable zinc-air battery at 20 mA cm–2 with a 20 min cycle period for 100 cycles and a 4 h cycle period for 40 h. Reprinted with permission from ref. 50. Copyright 2015, John Wiley & Sons.

## **4. Conclusions**

Silver copper Ag–Cu nanoalloy particles have been investigated for prospective application as an electrocatalyst for oxygen reduction reaction in alkaline fuel cells and metal air battery systems. A holistic approach has been adopted incorporating density functional theory simulations along with synthesis of potential candidate compositions of Ag–Cu nanoalloys. Following conclusions can be drawn from this work:

