To use the Nikon Df on AC power, you will need two devices. The first is the EH-5b AC Adapter which supplies the necessary DC voltage for the Nikon Df. The second part is the EP-5A Power Supply Connector that fits in the battery compartment of the Nikon Df and connects to the EH-5b.
Required parts (2):
Nikon Product Number 27018 (EP-5a )
Nikon Product Number 27055 (EH-5b)