A startup working to make hydrogen-powered airplanes raised $24.3 million from investors including funds backed by Bill Gates and Hong Kong billionaire Li Ka-shing.

ZeroAvia Inc. will use the cash to start scaling up its hydrogen-powered propulsion system to work with larger planes that could carry at least 50 passengers. Its latest and biggest model currently has around 19 seats.

Hydrogen is seen as a key fuel to cut aviation emissions. While batteries work well for cars and trucks, they’re too heavy for the energy-intensive and weight-sensitive airline industry. Hydrogen only emits water when it’s burned and if the gas is produced from renewable energy, its use has little polluting impact on the atmosphere.

The funding for ZeroAvia follows a round at the end of last year that drew nearly $38 million, including support from the U.K. government, to advance the company’s technology in a smaller aircraft.

“We a have a road map that goes into larger and larger airframes,” ZeroAvia Chief Executive Officer Val Miftakhov said in an interview. “We thought we’d go for the next size in a year or two, but there’s been so much interest, we decided to accelerate.”

This round of funding came from Li Ka-shing’s Horizons Ventures Ltd., International Consolidated Airlines Group SA’s British Airways Plc, the Gates-backed Breakthrough Energy Ventures, Ecosystem Integrity Fund, Royal Dutch Shell Plc, Summa Equity, and British-based consultant SYSTEMIQ Ltd.

By the end of 2021, Miftakhov expects ZeroAvia to raise at least $100 million more for designing and developing the larger hydrogen-powered plane and move it closer to commercialization. The total cost of the program to develop the larger aircraft will be about $250 million, he said. Some of the additional funds will likely come from government grants.

The company is currently working toward running its planes entirely on hydrogen rather than using its current hybrid of battery-power and hydrogen. It also plans to switch from condensed gas hydrogen to liquid hydrogen, because it’s more efficient to store.