**5. Future outlook**

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

436 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**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.

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.

Reprinted with permission from ref. 50. Copyright 2015, John Wiley & Sons.

Following conclusions can be drawn from this work:

zinc-air battery.

**4. Conclusions**

Ag–Cu nanoalloy catalysts offer an attractive alternative for the otherwise costly Pt-based alloys. The performance of these Ag–Cu nanoalloy catalysts was found to be reasonable while the stability in alkaline media is much superior compared to the Pt-based alloy catalysts. The realization of high energy density metal air batteries and alkaline fuel cells for stationary and mobile applications demand swift ORR kinetics at cathode which can be achieved by highly active silver-based nanoalloy electrocatalysts. Very few Ag-based Ag/transition metal nano‐ alloy electrocatalysts have been reported for ORR. Recently, the number of publications on silver-based nanoalloy electrocatalyst for ORR in alkaline media has been on the rise. DFTbased calculations show that the surface electronic structure of the nanoalloy catalyst is sensitive to doping. This opens the window for carrying out investigations on the ternary Ag– Cu–M catalyst systems with different morphologies to further improve the catalytic activity. Moreover, the effect of support such as CNT, MWCNT, RGO is still to be investigated for the Ag–Cu nanoalloys. These advance catalyst supports have been previously reported to be highly beneficial for Ag-based systems. The highest amount of catalytic activity in Ag-based systems is reported in the Ag composites owing to the ensemble effect. Incorporation of Ag– Cu nanoalloys into composite structures can provide a big leap forward toward improving the catalytic activity of these nanoalloys. At present, the activity of silver nanoalloys in alkaline media is limited because of weak adsorption of ORR intermediates. Highly active silver nanoalloy compositions can be identified by DFT calculations which when combined with active supports can rival or even surpass commercial Pt/C in alkaline media. A universal approach incorporating first principle simulations and experimental analysis is pivotal for the realization of highly active and stable silver-based nanoalloy electrocatalysts for ORR in alkaline media.
