This sounds like an example of a fuel cell hybrid electric vehicle
yes but not a only a hybrid fuel cell... other types of flow batteries
what does it mean when they say optimum power can be acheived without the need to mazimize energy storage density
In most batteries, the cells are storing the energy and as a result the energy is stored in the whole capacity of the battery. For instance in a D cell battery, you notice it is much larger than a AAA battery. The D cell puts out more energy because it is able to store more energy within its cells. With flow batteries it the power output does not necessarily have a direct relation to the size of the cell.
Another example would be rechargeable flow batteries explained as follow:
The battery uses electrodes that cannot and do not take part in the reactions but merely serve as substrates for the reactions. There is therefore no loss of performance, as in most rechargeable batteries, from repeated cycling causing electrode material deterioration. When the Zinc-Bromine battery is completely discharged, all the metal Zinc plated on the negative electrodes is dissolved in the electrolyte. The Zinc is deposited again when the battery is charged. In the fully discharged state the Zinc-Bromine battery can be left indefinitely.
Energy densities three times better than Lead Acid batteries are claimed however the Coulombic (round trip) efficiency is typically only between 60% and 75%.