Hydrogen production from the air

Hydrogen is the ultimate clean energy. Despite being the most abundant element in the universe, hydrogen exists on the earth mainly in compounds like water. H₂ produced by water electrolysis using renewable energy, namely, the green hydrogen, represents the most promising energy carrier of the low-carbon economy^1,2,3. H₂ can also be used as a medium of energy storage for intermittent energies such as solar, wind, and tidal^4,5,6.

The deployment of water electrolyzer is geographically constrained by the availability of freshwater, which, however, can be a scarce commodity. More than one-third of the earth’s land surface is arid or semi-arid, supporting 20% of the world’s population, where freshwater is extremely difficult to access for daily life, let alone electrolysis^7,8. In the meanwhile, water scarcity has been exacerbated by pollution, industrial consumption, and global warming. Desalination may be used to facilitate water electrolysis in coastal areas, however, substantially increasing the cost and complexity of hydrogen production. On the other hand, areas rich in renewable energies are commonly short in water supply⁹. Figure 1a and 1b shows a distinctive geographic match between the shortage of freshwater and the potential of solar power and wind power, respectively, in the majority of the continents, such as North Africa, West, and Central Asia, Midwest Oceania, and southwest of North America.

Few studies have been trying to mitigate the water shortage for electrolysis. Direct saline splitting can produce hydrogen, which, however, faces a serious challenge of handling chlorine byproduct^10,11. Some proton/anion exchange membrane electrolyzers can use high humidity vapor feed to the anode; however, the cathode of all of these electrolyzers must operate in an air-free atmosphere^{12,13,14,15,16,17,18,19,20}, purged by an inert carrier gas such as nitrogen or argon, resulting in particularly low H₂ product purity of less than 2%. On another note, photocatalytic water splitting has a potential to use vapor feed²¹, but the biggest problem of this method is its low solar-to-hydrogen efficiency (around 1%) in real-world demonstrations^22,23 and to make it more complicated, the product is a mixture of H₂ and O₂ gases which require an extra separation process.

In this work, we corroborate that moisture in the air can directly be used for hydrogen production via electrolysis, owing to its universal availability and natural inexhaustibility^{24,25,26,27,28}—there are 12.9 trillion tons of water in air at any moment which is in a dynamic equilibrium with the aqua-sphere²⁹. For example, even in the Sahel desert, the average relative humidity (R.H.) is about 20%¹⁹, and the average daytime R.H. at Uluru (Ayers Rock) in the central desert of Australia is 21%³⁰. Considering deliquescent materials such as potassium hydroxide, sulfuric acid, propylene glycol^31,32 can absorb water vapor from a bone-dry air, here, we demonstrate a method to produce high purity hydrogen by electrolyzing in situ hygroscopic electrolyte exposed to air. The electrolyzer operates steadily under a wide range of R.H., as low as 4%, while producing high purity hydrogen with a Faradaic efficiency around 95% for more than 12 consecutive days, without any input of liquid water. A solar-driven prototype with five parallel electrolyzers has been devised to work in the open air, achieving an average hydrogen generation rate of 745 L H₂ day⁻¹ m⁻² cathode; and a wind-driven prototype has also been demonstrated for H₂ production from the air. This work opens up a sustainable pathway to produce green hydrogen without consuming liquid water.

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