I know Boeing proposed Methane turbofans, and they were estimated to have 60% less carbon emissions and cheaper than jet fuel - after eating the large development and infrastructure costs. You could take this even further, producing Hydrogen from clean electric energy is about three times more expensive than methane for the same energy content, leading to a moderate increase in the cost of even a low cost ticket.

But as with all radically new technology, there needs to be a strong industry, government and consumer push. The program that produced the Methane turbofan concept (SUGAR Freeze) has lost any political appeal.