Calculate the limiting current density for a fuel cell cathode running on dry air at STP$.$ Assume that

the diffusion layer is $500\backslash$mu m thick and has a porosity of $40\%$ and a tortuosity of $1\mathrm{.}5.$ Hint: find the

diffusivity of O$2$ in air from the literature.

final answer:$-[$~$3\mathrm{.}4$ A$/$cm$2]$