Hydrogen can be used to create energy in vehicles through a chemical reaction, rather than through combustion which generates negative externalities. The only by-products of hydrogen being used as energy are heat and water. https://www.energy.gov/eere/fuelcells/hydrogen-production. Hydrogen can be used in fuel cells to power cars, houses and other appliances.

Various sources say that “clean hydrogen” will play a large roll in the green energy movement of the future

Hydrogen is the simplest element that exists, but it does not naturally occur by itself- it needs to be created through chemical reactions. Currently, 6% of global natural gas and 2% global coal goes into hydrogen production; this also means that a large amount of CO2 is put into the atmosphere annually (830 million tons). https://www.iea.org/reports/the-future-of-hydrogen. However, electrolytic hydrogen is an option for non-fossil fuel-generated hydrogen. http://www.xylenepower.com/Electrolytic%20Hydrogen.htm.

Excess energy from solar or wind power systems can also be used to produce hydrogen, which can be stored efficiently and used during low sun or wind seasons.