Turbofans work by burning jet fuel to spin turbines, ultimately powering the massive fan you see in the front of the engine. Could you substitute the jet fuel with liquid hydrogen and, in theory, make a jet plane that produces no CO2

High temperature electrolysis(HTC) is a technology to producing hydrogen from water. High temperature electrolysis is more efficient than traditional room-temperature electrolysis. At 100 degree Celsius, 350 megajoules of thermal energy are required(41% efficient) and at 850 degree Celsius, 225 megajoules are required(64% efficient).

Possible supplies of cheap high temperature heat for HTE are all nonchemical, including high temperature nuclear reactors, concentrating solar thermal reactors, and geothermal sources.