Look at what you did.
First, maybe you think this creates a vector in a? Instead, it just sets a to 0. Then it dumps 5 into the bit bucket.
Again, I wonder if you understand that just dumps the second number after the semi-colon into the bit bucket?
iH2 = @(n)(i0H2 *( exp((a*F*n)/(R*T)) - exp((-a*F*n)/(R*T)))) ;
iZn = @(n) (i0Zn * (exp((a*F*n)/(R*T)) - exp((-a*F*n)/(R*T))));
Finally, you try to plot those numbers. Since they are infinities, what do you expect to be plotted?
What you really wanted to do here is a complete guess. I won't even try to make one.