Besides it is not trivial to repurpose natural gas storage for hydrogen - you need an entirely different level of tightness, you would be disappointed about the amount of energy you can store this way, as hydrogen is way less dense than natural gas, so the capacity of the storage would be poor.
For very large scale storage we are not talking about steel pressure vessels that are subject to hydrogen embrittlement, but depleted natural gas reservoirs which are just gas-tight geological formations and have no issues whatsoever with hydrogen embrittlement.
For small and medium scale storage you are right. Which is why hydrogen for cars and even trucks is a bad idea.
And you would get much less hydrogen stored at the same pressure than methane. To store reasonable amounts of hydrogen, you need very high pressures. Gas powered cars use like 60 bars of pressure, hydrogen 700.
Yes, that is correct. But the end to end efficiency of Electricity -> H2 -> Electricity is much better than Electricity -> H2 -> CH4 -> Electricity.
Hydrogen at 0°C and 100 Bar (~10MPa) has a density of 8.3447 kg/m^3. It has an energy density of 120 MJ/kg. So about 33 kWh/kg. So you end up with 278 kWh/m^3.
Now of course you have to multiply this by the H2->Electricity efficiency. Let's be very pessimistic and take 0.6 or 60%, which is what a gas turbine plant can achieve today.
You end up with 166 kWh/m^3 of usable electricity per cubic meter.
One of many german gas storage facilities https://www.nafta-speicher.de/en/company has a volume of 1.8e9 m^3. That translates into 300 Terawatt-Hours of storage capacity for just this one facility.
