Water and electricity. The electricity can be from any source, but preferably the sun, hydroelectric, or wind.
https://www.energy.gov/eere/fuelcells/hydrogen-production-processes
There’s also ongoing research on solar-direct-to-hydrogen, skipping the “electricity” middleman.
“Water and electricity. The electricity can be from any source, but preferably the sun, hydroelectric, or wind.”
how many tens of thousands of windmills and hundreds of square miles of solar panels do you think it would take to make enough hydrogen from water to power the entire U.S. transportation fleet including automobiles, trucks, locomotives and ships? you know, so fossil fuels wouldn’t have to be used?