Hydrogen would be a terrible approach due to the low round-trip efficiency and the need to store huge amounts of compressed/cryogenic gas, but iron air batteries seem like they could actually do this.

Most people have the wrong idea about hydrogen fuel cells because the media almost exclusively talks about them in the context of cars.

When your fuel cell can be stationary and heavy and large, you can hit efficiencies above 50% without cogeneration and above 80% with cogeneration.