PARIS (Reuters) - Universal Hydrogen, a U.S.-based firm planning to provide hydrogen propulsion for regional aircraft, said on Thursday it would accelerate development after raising $20.5 million in funds amid a debate over carbon-free aviation. The second-stage funding was led by Silicon Valley incubator Playground Global with investors including venture capital arms of Airbus, Toyota and JetBlue. Hydrogen has become a hot topic as aviation faces pressure to slash carbon emissions. Europe’s Airbus has said it is working on a zero-emission airplane for 2035, expected to use hydrogen combustion. But Universal Hydrogen founder Paul Eremenko has held out the prospect of quicker progress towards “decarbonised” air travel by using electricity from hydrogen fuel cells onboard small regional planes, carrying 40-60 people. ADVERTISEMENT It aims to adapt planes to carry hydrogen in capsules that could be retrofitted. Universal Hydrogen said on Thursday it aimed to make commercial flights in 2025. Critics say a challenge will be getting the planes certified in such short time. The initiative comes as engine-makers study a less radical alternative: hybrid engines combining electricity with traditional gas turbines for best-selling 150-seaters. They say this is more realistic in the next decade, especially for anything larger than a regional jet or turboprop. In a recent interview, Eremenko said pressure to curb emissions favoured hydrogen. “There is a willingness and a need to start talking about such measures,” the former Airbus technology chief told Reuters. “You can no longer say in 2030-ish I will do a hybrid and then in 2050 do something more drastic. If we wait that long it might be too late. We have got to move in a more aggressive way.” A lack of infrastructure and a lack of urgency have hampered the switch, Eremenko said. “I think now the urgency is there. That is the tectonic shift in the industry.”