= Solution
An alkaline \b[<zinc-air battery>] combines a porous Zn negative <electrode>, aqueous KOH, an ion-permeable separator and a gas-diffusion positive <electrode>. At discharge, oxygen is reduced at the air <electrode>:
$$
\mathrm O_2+2\mathrm H_2\mathrm O+4e^-\to4\mathrm{OH}^-.
$$
Zinc <oxidation> can be written $\mathrm{Zn}+4\mathrm{OH}^-\to\mathrm{Zn(OH)}_4^{2-}+2e^-$, followed by ZnO precipitation and hydroxide regeneration. The net reaction is $2\mathrm{Zn}+\mathrm O_2\to2\mathrm{ZnO}$.
High energy requires high accessible Zn loading and low inactive mass; high power requires short <ion> paths, good electronic conduction and a large catalytically active <gas>/<electrolyte>/<electron> interface. A porous hydrophobic gas-diffusion layer admits oxygen while controlling <electrolyte> flooding and drying. Zn morphology and <electrolyte> composition must avoid premature passivation, excessive corrosion and hydrogen evolution; atmospheric CO2 can carbonate the alkaline <electrolyte>. A very high loading can impair transport, so energy and power must be optimized together.
\b[Electrical recharge is possible, but is not guaranteed by a primary Zn-air design.] It requires reversible Zn deposition with controlled morphology and an oxygen <electrode> that can evolve oxygen efficiently without rapid catalyst or support corrosion. Dendrites, shape change, ZnO redistribution, <gas> evolution and large oxygen-reaction overpotentials limit cycling. Bifunctional oxygen catalysts, or separate oxygen-reduction and oxygen-evolution <electrodes>, help. Mechanical replacement of spent Zn is a different form of refuelling.<Phase-transition tailored nanoporous zinc metal electrodes by Liangyu Li and others>[Demonstration of rechargeable alkaline Zn-air <electrodes> and transport/passivation limitations]{ref}
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