Propane fuel cells already exist, and are in use for remote power. The problem with using them for cars is that they operate at high temperature, so you spend a lot of battery warming the cell for steady state, but most trips are short so you would be there before it got to temp. Might make sense as a phev sort of thing where you punch in a trip and it decides to warm it up or not. You'd always leave enough battery to warm the cell while driving so you never get stuck.