what? what do you mean?
a batteries power is restricted by it's size, while a hydrogen fuel sell is not. Especially when it's fuel that can be ultra compressed into plasma. There are so many more possibilities for hydrogen powered cells and weapons.In a plasma state Hydrogen can be manipulated with magnets giving millions of more possibilities.
You're wrong about a few important things here.
While you are somewhat correct in that the amount of
energy a fuel cell can produce is not dependent on size, the
rate at which it can produce energy (i.e. the amount of power it can produce) is dependent on size. For an energy weapon, the power that an energy source can generate is just as important, if not more so, than the amount of energy it can generate. Let's say it takes 1000 joules of energy to fire one laser shot. If your energy source only produces 1 watt of power, then it will take 1000 seconds for it to charge up for the next shot.
Now, a hydrogen fuel cell (or more specifically a PEM fuel cell) is a fairly thin plate that is maybe an inch or two on each side. Here's a picture of one.
The actual fuel cell part is the dark grey plate in between the red and black end plates. That fuel cell can produce 1 watt of power. That's all it can produce. You can't make it produce more by flowing more hydrogen through, that's all you get. Now one watt of power ain't much. So to make more power, you wire up several fuel cells either in series or parallel (or most likely a combination of the two to up both the voltage and amperage). Here's a picture of that.
That's basically just 10 fuel cells stacked up together, and it produces 10 watts. See what I'm getting at?
You're also wrong about storing hydrogen. Compressing it will not make it turn into a plasma. Compressing (and cooling) hydrogen will make it turn into a liquid, and eventually a metallic solid (though we don't have the ability to actually make metallic hydrogen). To make a plasma, you actually need to do the exact opposite, heat the material to an extremely high temperature (tens of thousands of degrees kelvin, minimum). And barring unusual circumstances (such as the cores of stars) plasmas usually have a very low density, so a plasma would be a terrible way to store hydrogen. You'd be much better off with just tanks of compressed or liquid hydrogen.
And while magnetic fields can indeed manipulate plasmas, it takes extremely powerful magnetic fields to do so, which require large electromagnets to produce, which in turn require a lot of power to operate.
So basically, as far as weapons go, at the moment, fuel cells would be quite inferior to batteries. Both batteries and fuel cells though are currently incapable of providing the kind of power that an energy weapon capable of killing a person would require. Too really make energy weapons a reality, we need devices with much higher electrical energy storage density than is currently available. It's being worked on though, since such a technology would have extremely wide ranging benefits. Imagine a cell phone battery that could power your cell phone for years!