The volumetric energy density of hydrogen is very low, so either you need enormous tanks or very strong tanks plus extreme compression. One way of making enormo

The volumetric energy density of hydrogen is very low, so either you need enormous tanks or very strong tanks plus extreme compression. One way of making enormous "tanks" that can tolerate fairly high pressures is by solution-mining caverns out of natural underground salt formations. But this is geographically limited to where such salt formations occur, like parts of Texas and Louisiana. The other problem is that the round-trip energy efficiency isn't great. You might expect 75% efficiency for electricity-to-hydrogen from electrolysis and then 60% hydrogen-to-electricity if you burn it in an efficient combined cycle gas turbine for 45% efficiency overall. Oh, and you irreversibly lose energy as heat when you compress the hydrogen, for further losses. And it's pretty expensive to buy big industrial-scale electrolysis units but run them only during the few hours each day when there's excess renewable energy you want to store (though it becomes a more attractive idea as the penetration rate of renewables, and corresponding excess generation times, increase.)