You mix AgNO3 and KI in the following molar ratio
Fixed layer of the electrical double layer migrates to cathode in
Being in equal stoichiometric ratio, Agl is precipitated. It carries no charge and, hence no migration
KI is in excess hence Agl adsorbs common ion of KI (ie. I-)
Fixed layer Mobile layer
Fixed layer carries negative charge and moves to anode plate.
Agl is precipitated and AgNO3 being in excess, Ag+ is adsorbed on AgI.
Fixed layer carries positive charge and migrates to cathode (-ve) plate.