The value of Tafel slope is inherently related with the reaction kinetics, so the value will be change by which reaction pathway you will drive the reaction and the field is called electrocatalysis. As you can understand that CO2 reduction has a fixed standard reduction potential which will always be fixed but when you will perform the reaction with the help of a catalyst, the reaction pathway will be change and it will be going through low energy reaction coordinate and hence the Kinetics will be fast.  The Tafel equation for electrokinetic reaction is a simple quadratic equation : Overpotential =a+blogc, where b is the Tafel slope. Lower the value of b corresponds to fast kinetics which can be theoritically derived by solving the Tafel kinetics. To verify the effect one should perform the experiment with and without the catalyst and it will be a distinguishable change in Tafel slope.

Thanks anirban for reply.