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A United Airlines airplane shortly after take off from Madrid, Spain, on September 25, 2021.
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United Airlines has taken an equity stake in ZeroAvia, a firm focused on powering electric motors by utilizing hydrogen fuel cells.

Under the deal, United said it expects to purchase up to 100 of ZeroAvia’s ZA2000-RJ — an engine it described as zero-emission and 100% hydrogen-electric.

The airline said the engine was “expected to be used in pairs as a new power source for existing regional aircraft.”

United said it plans to pursue a conditional purchase agreement for 50 of the engines, with an option for 50 more. The tech could be retrofitted to aircraft from 2028, it added.

In a statement issued Monday, United CEO Scott Kirby said hydrogen-electric engines were “one of the most promising paths to zero-emission air travel for smaller aircraft.”

In a separate announcement, ZeroAvia said it had raised $35 million in funding. Alongside United, others taking part in the funding round include Alaska Air Group, whose investment was previously announced.

In total, ZeroAvia says it has attracted $115 million of investment from a range of stakeholders including Shell Ventures, Amazon’s Climate Pledge Fund and Breakthrough Energy Ventures.

As concerns about sustainability and the environment mount — the World Wildlife Fund describes air travel as “the most carbon intensive activity an individual can make” — discussions around aviation are increasingly focused on how new tech and ideas could cut its environmental footprint.

Over the last few years, a number of companies have sought to develop plans and concepts related to low and zero-emission aviation.

Earlier this year, Rolls-Royce’s first all-electric aircraft completed its maiden flight, taking to the skies in the U.K. for around 15 minutes.

Meanwhile, in Sept. 2020, a hydrogen fuel-cell plane from ZeroAvia undertook its first flight. The same month saw Airbus release details of three hydrogen-fueled concept planes, with the European aerospace giant claiming they could enter service by 2035.

While there is excitement in some quarters regarding the potential of new, lower-emission forms of aviation, some within the industry are circumspect about how such innovations will develop over the coming years.

Speaking to CNBC in October, for example, Ryanair CEO Michael O’Leary was cautious when it came to the outlook for new and emerging technologies in the sector.

“I think … we should be honest again,” he said. “Certainly, for the next decade … I don’t think you’re going to see any — there’s no technology out there that’s going to replace … carbon, jet aviation.”

“I don’t see the arrival of … hydrogen fuels, I don’t see the arrival of sustainable fuels, I don’t see the arrival of electric propulsion systems, certainly not before 2030,” he added.