Actually, flywheel and SMES (Superconducting Magnetic Energy Storage) have the same energy density, it is limited by the hoop stress of the materials from which they are made (unless you use for SMES the magnetic containment design of US Pat #5,227,756, then you could probably go much higher), and with "normal materials" it is around 10^8 Joule/meter^3. Lithium polymer batteries have potential energy densities of about 10^9 Joule/meter^3, while the energy density of gasoline is way up there above 10^10 Joule/meter^3. I don't have figures for Hydrogen, energy content, but my guess is that is half that of gasoline, making it only marginally better (as far as energy density is concerned) than Lithium Polymer batteries, and since you need the fuel cell itself, a relatively massive "container" for the energy conversion, and also you need a relatively massive container for hydrogen (either a hydride like titanium or nickel), or a massive pressure vessel with a lot of safety buffering materials, I am not sure if the lower energy density of batteries is really that much of a hindrance.
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