A six-seater Piper M-Class fitted out at a research and development hub in the UK ran on hydrogen on its maiden flight. Whether hydrogen is used to power a fuel cell to generate electricity or directly combusted for motive power, the only waste product is clean water. Importantly in the context of flight, hydrogen packs a lot of energy per unit of mass – three times more than conventional jet fuel, and more than a hundred times that of lithium-ion batteries.
www.bbc.com/future/article/20210401-the-worlds-first-commercial-hydrogen-plane