A cheap method of splitting Hydrogen out of water would provide an alternative fuel for ICEs:
" Photobiological production of hydrogen involves using sunlight, a biological component, catalysts and an engineered system. Specific organisms, algae and bacteria, produce hydrogen as a byproduct of their metabolic processes. These organisms generally live in aqueous environments and therefore are extracting the hydrogen from water using their biological functions.
Currently, this technology is still in the research and development stage and the theoretical sunlight conversion efficiencies have been estimated up to 24%. Over 400 strains of primitive plants capable of producing hydrogen have been identified, with 25 impressively achieving carbon monoxide to hydrogen conversion efficiencies of 100%.
In one example, researchers have discovered that the alga, Chlamydomonas reinhardtii, possesses an enzyme called hydrogenase that is capable of splitting water into its component parts of hydrogen and oxygen. The researchers have determined the mechanism for starting and stopping this process, which could lead to an almost limitless method for producing clean, renewable hydrogen.
The algae need sulfur to grow and photosynthesize. Scientists found that when they starved the algae of sulfur, in an oxygen-free environment, the algae reverted to a hydrogenase-utilizing mode. This mechanism of producing hydrogen from water has developed over millions of years of evolution for survival in oxygen-rich and oxygen-free environments. Once in this cycle, the algae released hydrogen, not oxygen. Further research is necessary to improve the efficiencies of the engineered plant systems, collection methods and the costs of hydrogen generation."
https://www.greenoptimistic.com/hydrogen-from-water/
Burning Hydrogen and air is not pollution free, as there is, in air, Nitrogen, but carbon emissions would be absent. If a cheap way of producing and storing Oxygen could also be found, a Hydrogen/Oxygen engine would only produce water.