I thought they had fuel cells down to manageable sizes now (brick sized last I read) but I dunno what size would be enough to power that suit... Certainly wouldnt need one as big as that backpack... car sized fuel cells arent that big.
I bet they probably have some plutonium powered toys at darpa anyway (we just wont hear about them)... Not that fuel cells are that weak. Its just that the military doesnt shy away from toxicity hhe... Now if we could know the power requirements of that exo-suit we could easily derive what sized fuel cell is likely powering it.