Comment Hard part still remains (Score 5, Insightful) 162
Temperature is actually more important than the energy density in this case. At 650C never mind 900C, you'll still have a lot of trouble with heat--material have an unfortunate tendency to expand and warp (or, worse, snap) at that kind of temperature. Thus, you may be able to turn your car on and off only dozens of times before the SOFC breaks down. This is the real reason why SOFC has never been seriously considered for cars--SOFC has always been relatively compact for the amount of energy they produce (except for the apparatus you'd need to get rid of the huge amounts of heat).
Now, 650C is easy, at least if you are using natural gas as feedstock. (Gasoline may be somewhat more difficult, but not impossible.) Other solid oxide fuel cells that are trying to enter the market operate at or near that temperature range. 350C, though--wow. That will be remarkable, and may indeed be able to brings in an era of fuel cell vehicles, but it'll involve whole new set of chemistry, and I won't believe it until I see it.