The graph of the logarithm of the current density j against the overpotential η in electrochemistry in the high overpotential limit. This limit can occur in electrolysis either when the overpotential is large and positive, the electrode being the anode, or when the overpotential is large and negative, the electrode being the cathode. In the case of positive overpotential, the second exponential in the Butler-Volmer equation can be ignored, as it is much smaller than the first exponential, and thus j=j0exp[(1 – α)fη] In the case of a negative overpotential, the first exponential can be ignored, which gives j=j0exp(–αfη), so that ln(–j)=lnj0 – αfη. Thus, in the Tafel plot the value of α can be obtained from the slope and the value of j0 can be obtained from the intercept at η=0.
j=j0exp[(1 – α)fη]