Despite all its advantages, hydrogen also has drawbacks. The most important one: it's very hard to produce hydrogen without creating greenhouse gases. Indeed, most hydrogen (around 95% as of 2020) is produced out of fossil fuels. The rest is obtained by water electrolysis using a lot of electric energy, which is also mostly produced by burning fossil fuels. So, yeah, either way, not quite green, unless the electricity is produced from renewable sources
But there’s another problem that scientists have overlooked. According to a new UK Government study, the hydrogen tanks all leak hydrogen, and the leaked gas is worse for the environment than the CO2. Over a 100-year time period, a ton of hydrogen released in the atmosphere will warm the Earth 11 times more than a ton of CO2. This is because hydrogen interacts with other gases and vapors in the air to produce powerful warming effects.
According to another study, compressed gas cylinders lose about 0.12-0.24% per day, while hydrogen transported as cryogenic liquid loses about 1% per day. This gets into the higher layers of the atmosphere, where it interacts with the same tropospheric oxidants that "clean up" methane emissions. This leads to increased concentrations of methane and the methane staying in the atmosphere longer. The presence of hydrogen also increases the concentration of both tropospheric ozone and stratospheric water vapor, pushing the temperatures even higher.
Despite the new findings, even in the worst-case scenario where the hydrogen leakage rate gets to 10%, using hydrogen will offset a 4% carbon emissions reduction. This is still an enormous improvement over using fossil fuels. But the study “clearly demonstrates the importance of controlling hydrogen leakage within a hydrogen economy.”