SupersonicScrub
SupersonicScrub on Hacker News
Maximum power requirements come from take-off and climb-out, so the engine size is designed around the max take-off power. This means that the engine is over-designed and less-efficient for the majority of the flight.
By implementing a hybrid engine, the gas-powered component and the electric powered component work together to provide take-off power requirements. By the time the aircraft reaches cruising altitude, the batteries are dead, and the gas-powered component works alone. This allows the engine to be designed for the cruise power requirements, which results in a much more efficient engine. UTC is currently experimenting with this concept.
https://www.flightglobal.com/news/articles/utc-to-test-hybri...
Yes it's not carbon neutral, but it's a step in the right direction.