The other person who replied to you is deeply wrong. See my reply to them here for complete calculations. You can’t simply ignore the mass of the gas with which you’re replacing the air, despite their suggestion. If the mass of what’s inside the balloon doesn’t matter, you might as well fill it with lead. Hydrogen is something like 7% More efficient than helium for balloons. You have a good idea, and hydrogen balloons don’t just explode for no reason. They’re dangerous, but saying that they’re necessarily going to flameout is ridiculous