Not a fuel cell in the electrochemical sense, but one to store additional gasoline for extending the car's range.
Cell. From Latin cella (“chamber, small room, compartment”)
biological cell? chamber with biological goop jail cell? chamber with convict inside it fuel cell? chamber with fuel in it electrochemical cell? chamber with electrodes and an electrolyte in it
fuel cell (again)? What do we name an electrochemical cell whose reactants are fluids? fuel cell!
The fuel cell is a catalytic converter of chemical to electrical energy. The hydrogen for it comes from what is called -- even in an interplanetary spacecraft -- a tank.
Calling an auxiliary gas tank a fuel cell has to have originated as some kind of cheesy marketing ploy, to make it sound like a more sophisticated thing than a gas can with baffles, and get more money for it.
Definitions change according to the topic of the conversation. In this context the "cell" containing fuel is a very highly engineered containment cell to ensure fuel isn't spilled or explodes in case of a high energy collision.