The value of the Tafel slope can provide information about the rate-determining step (RDS) of the reaction. For example:
A Tafel slope of 120 mV/decade often suggests that the Volmer step (adsorption of hydrogen atoms) is the RDS in HER. A Tafel slope of 40 mV/decade might indicate that the Heyrovsky step (electrochemical desorption) is the RDS. A slope of 30 mV/decade is associated with the Tafel step (recombination of adsorbed hydrogen atoms).