Does the hydrogen hype that once surrounded cars have better prospects with planes? Yes, but probably not in time to meet the aviation industry’s emissions targets.

The past year has brought some vindication to those who see hydrogen as aviation’s passport to a cleaner future. Last fall, European plane maker Airbus unveiled three hydrogen-powered aircraft concepts for 2035. More recently, U.K. startup ZeroAvia got backing from British Airways as part of a $24 million funding round. Likewise, Universal Hydrogen, led by former Airbus executive Paul Eremenko, has just raised $21 million from heavyweights such as the venture-capital subsidiaries of JetBlue and Toyota.

For decades, hydrogen was a promising future power source for passenger cars. Now most light vehicle makers favor batteries, and hydrogen is looking for a better home in trains and trucks, as championed by startups such as Nikola.

The aviation industry has set itself a target of halving emissions by 2050, which would be roughly in line with the 2016 Paris Agreement to limit climate change. Only a third of the reduction is expected to come from improvements in turbofans and airframes. Sustainable fuels can play a role, but production capacity is limited, and the most affordable ones remain pollutive.

Aviation’s initial interest in the electric-vehicle revolution faded as executives realized that carrying heavy batteries more than very small ranges through the air is unfeasible. Rechargeable lithium-ion batteries only deliver 0.9 megajoules per kilogram of weight, compared with 40 MJ/Kg for jet fuel.