Computing i0 in the Butler-Volmer equation: How to interpret Wikipedia and Wolfram Alpha
k0 has units of frequency (1/time)
All concentrations are surface concentrations (mol/area)

Sometimes people use k0 with units length/time and volume concentrations (mol/volume)

On Wikipedia: Coxy stands for Ox concentration * at equilibrium *. Cred stands for Red concentration * at equilibrium *. It's for 1 electron. Multiply by n for n electrons. 

On Wolfram: [Ox] stands for Ox concentration at electrode, [Red] stands for Red concentration at the electrode. These concentrations are not necessarily at equilibrium.