Calculate the limiting current density for a fuel cell cathode running on dry air at STP$.$ Assume thatthe diffusion layer is $500$ m thick and has a porosity of $40\%$ and a tortuosity of $1\mathrm{.}5.$ Kindly show detailed calculations and especially how to calculate the concentration at STP$.$

The final answer is $3\mathrm{.}4$ A$/$cm$^2$