Note that sodium is a liquid above 208 degF, so the fuel cell will have to keep the sodium hot whether it is being used or not. So it will be discharging NaOH all the time.!!
As a reference, the net equivalent energy for avgas is about 4,000 whr per kg, when accounting for the 90% eff of electric drives vs 25% for ic engines. So, this isn’t a very practical energy source for aircraft.