One of the most promising emission-free plane start-ups – ZeroAvia – has received a boost in funding from major investors including Shell and Amazon. With plans to have the first commercial hydrogen-electric planes in our skies by 2023, the injection of $21.4 million – while small by the investor’s standards – certainly won’t hurt the company’s ambitions to decarbonize what is arguable the hardest-to-decarbonize sector of the economy. Series A funding was secured on Wednesday as part of a round led by Bill Gates’ Breakthrough Energy Ventures and the Ecosystem Integrity Fund. Investment came from Amazon’s $2 billion Climate Pledge Fund, Shell’s Shell Ventures unit, as well as others including Hong Kong-based venture capital firm Horizons Ventures and Summa Equity.…