Hydrogen's potential as a clean fuel may be limited by a chemical reaction in the lower atmosphere, according to research from Princeton University and NOAA.
This is because hydrogen gas reacts readily in the atmosphere with the same molecule that is primarily responsible for breaking down methane, a potent greenhouse gas. If hydrogen emissions exceed a certain threshold, that shared reaction will likely lead to methane accumulation in the atmosphere, with decades-long climate consequences.
"Hydrogen is theoretically the fuel of the future," Matteo Bertagni, a postdoctoral researcher at the High Meadows Environmental Institute working on the Carbon Mitigation Initiative, said in a statement. "In practice, however, it raises many environmental and technological concerns that still need to be addressed."
Bertagni is the first author of a research paper published in Nature Communications, in which researchers modeled the effect of hydrogen emissions on atmospheric methane. They found that above a certain threshold, even when replacing fossil fuel use, a leaky hydrogen economy could cause short-term environmental damage by increasing the amount of methane in the atmosphere. The risk of harm is compounded by hydrogen production methods that use methane as an input, highlighting the critical need to manage and minimize emissions from hydrogen production.
"We have a lot to learn about the consequences of hydrogen use, so switching to hydrogen, a seemingly clean fuel, does not create new environmental challenges," said Amilcare Porporato, professor of civil and environmental engineering at the High Meadows Environmental Institute.
The problem boils down to a small, hard-to-measure molecule known as the hydroxyl radical (OH). Often referred to as "the detergent of the troposphere," OH plays a key role in removing greenhouse gases such as methane and ozone from the atmosphere.
The hydroxyl radical also reacts with hydrogen gas in the atmosphere. And since a limited amount of OH is generated each day, any increase in hydrogen emissions means that more OH would be used to break down hydrogen, leaving less OH available to break down methane. As a consequence, methane would remain in the atmosphere longer, extending its warming impacts.
According to Bertagni, the effects of a hydrogen spike that could occur as government incentives for hydrogen production expand could have climate consequences for the planet for decades.
"If you emit some hydrogen into the atmosphere now, it will lead to a progressive buildup of methane over the next few years," Bertagni said. "Even though hydrogen only has a lifetime of about two years in the atmosphere, you'll still have methane feedback from that hydrogen 30 years from now."
In the study, the researchers identified the tipping point at which hydrogen emissions would lead to an increase in atmospheric methane and thus undermine some of the near-term benefits of hydrogen as a clean fuel. By identifying that threshold, the researchers set targets for managing hydrogen emissions.
"It is imperative that we are proactive in setting thresholds for hydrogen emissions so that they can be used to inform the design and implementation of future hydrogen infrastructure," Porporato said.
For hydrogen called green hydrogen, which is produced by splitting water into hydrogen and oxygen using electricity from renewable sources, Bertagni said the critical threshold for hydrogen emissions is around 9%. That means that if more than 9% of the green hydrogen produced leaks into the atmosphere, either at the point of production, at some point during transportation or elsewhere along the value chain, atmospheric methane would increase in the coming decades, canceling out some of the climate benefits of moving away from fossil fuels.
And for blue hydrogen, which refers to hydrogen produced through methane reforming with subsequent carbon capture and storage, the emissions threshold is even lower. Because methane itself is the primary input to the methane reforming process, blue hydrogen producers must consider direct methane leakage in addition to hydrogen leakage. For example, the researchers found that even with a methane leakage rate as low as 0.5%, hydrogen leakage would have to be kept below 4.5% to avoid increasing methane concentrations in the atmosphere.
"Managing hydrogen and methane leakage rates will be critical," Bertagni said. "If you have only a small amount of methane leakage and a little bit of hydrogen leakage, then the blue hydrogen you produce really might not be much better than using fossil fuels, at least for the next 20 to 30 years."