## 1 Introduction
### Cell Configuration of Secondary ZAB (S-Zab)  
In secondary ZABs, the negative anode, positive cathode, electrolyte, and membrane separator are electrochemically coupled, as shown in Figure 3a,b. The anode for alkaline secondary ZABs is a zinc electrode in the form of zinc metal foil, zinc powder, or zinc fiber, and the air electrode has two layers, the gas diffusion, and catalytic layers. There is a three-phase (solid-liquid-gas) reaction at the cathode. Here, the diffusion layer provides the physical support for the catalyst and runway for the passage of electrons to be collected as the current; the hydrophobic section of the gas diffusion layer has to permit the liquid electrolyte as well as protect the electrolyte from leaking out. Thus, an optimal gas-electrolyte-catalyst three-phase interface is required [31; 32].  
In general, to carry out the ion transportation between the Zn anode and air cathode, alkaline electrolytes such as KOH or NaOH have been used widely in ZABs because of the better ionic conductivity and high Zn reaction kinetics. The acidic electrolyte reacts with the Zn anode and produces hydrogen; this leads to zinc corrosion, shape change, and production of heat, and results in poor battery efficiency. These reasons restrict the use of acidic electrolytes in ZABs. Neutral electrolytes are not commonly used because of their low conductivity and because they are less friendly toward oxygen electrocatalysis than alkaline electrolytes. [33; 34]. The separator, a selectively permeable membrane, plays a vital role in blocking unwanted materials from passing through, such as zinc dendrites between the anode and the cathode [35].  
As the battery discharges, in the anode, the zinc reacts with hydroxide and oxidizes to form zincate ion, and this process continues until the zincate ion is supersaturated; then, it decomposes to insoluble white zinc oxide powder and precipitates on the active zinc su  
Figure 3: Schematic of a zinc–air battery under (**a**) discharging and (**b**) charging conditions [36]. Copyright 2017, Elsevier.  
ZnO powder produced makes the active area less active. Simultaneously, in the cathode, the pressure gradient induces O\({}_{2}\) entry through the diffusion layer and reduction to hydroxyl ions at the catalytic layer by reacting with water and electrons. Hence, oxygen is reduced during battery discharging (ORR) and oxidized (OER) during charging. Along with O\({}_{2}\), CO\({}_{2}\) enters the cathode and forms carbonates by reacting with KOH. This formation clogs the pore and restricts the entry of air to the cathode. These mechanisms are shown in Equations (1)-(6) [38].  
Discharge condition  
Zinc electrode:  
\[\mathrm{Zn_{(s)}\to Zn^{2+}{}_{(aq)}+2e^{-}} \tag{1}\]  
\[\mathrm{Zn^{2+}{}_{(aq)}+4 OH^{-}\to Zn\;(OH)_{4}{}^{2-}{}_{(aq)}} \tag{2}\]  
\[\mathrm{Zn\;(OH)_{4}{}^{2-}{}_{(aq)}\to ZnO_{(s)}+H_{2}O_{(0)}+2 OH^{-}\;(E^{0}= -1.25\;V\;vs.\;\mathrm{SHE})} \tag{3}\]  
Air electrode:  
\[\mathrm{O_{2(g)}+2H_{2}O_{(aq)}+4e^{-}\to 4 OH^{-}{}_{(aq)}\;(E^{0}=0.401\;V\;vs.\; \mathrm{SHE})} \tag{4}\]  
Overall reaction:  
\[\mathrm{2 Zn_{(s)}+O_{2(g)}\to 2 ZnO_{(s)}\;(Cell\;potential:\;1.65\;V)} \tag{5}\]  
Parasitic carbonate formation:  
\[\mathrm{2 KOH_{(aq)}+CO_{2(g)}/K_{2}CO_{3(s)}+H_{2}O_{(aq)}} \tag{6}\]