Introduction

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 economy1,2,3. H2 can also be used as a medium of energy storage for intermittent energies such as solar, wind, and tidal4,5,6.

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 electrolysis7,8. 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.

Fig. 1: Superimposed atlas of water risk and the renewable energy.
figure 1

a water risk and solar energy potential; b water risk and wind energy potential (excluding coast areas). Separate maps are available in Supplementary Fig. 13. Source: World Resources Institute (WRI) Aqueductaqueduct.wri.org; World Bank Grouphttps://globalsolaratlas.info; Technical University of Denmarkhttps://globalwindatlas.info—Creative Commons Attribution International 4.0 License.

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 byproduct10,11. 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 atmosphere12,13,14,15,16,17,18,19,20, 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 demonstrations22,23 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 inexhaustibility24,25,26,27,28—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%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.

Results

Design of the Direct Air Electrolysis (DAE) module for hydrogen production

Hydrogen production from the air was realized through our DAE module. As shown in the sandwich structure in Fig.2a, b, this module consists of a water harvesting unit in the middle and electrodes on both sides paired with gas collectors. The module is integrated with a power supply, for example, a solar panel, a wind turbine, and any other renewable generators. Importantly, the water harvesting unit also serves as the reservoir to hold the electrolyte. Porous medium such as melamine sponge, sintered glass foam is soaked with deliquescent ionic substance to absorb moisture from the air via the exposed surfaces. The captured water in the liquid phase is transferred to the surfaces of the electrodes via diffusion and subsequently split into hydrogen and oxygen in situ which are collected separately as a pure gas, since both electrodes are isolated from air (Supplementary Figs. 46). The reservoir between the endplate and the porous foam (Supplementary Fig. 5b) works as an air barrier and a buffer for the volume of the ionic solution at excessive fluctuation of the air humidity. This reservoir avoids the overflow of the electrolyte from the DAE module or the dry-up of the wetted foam. When glass foam is chosen as the porous media, quartz wool is tightly packed in between the foam and the electrodes to ensure the connectivity of the aqueous phase (Supplementary Fig. 7). The porous media also ensure the free movement of the electrolyte in the capillary of the foam (Supplementary Fig. 8, Supplementary Movie 1). The foam filled with ionic solutions forms a physical barrier that effectively isolates hydrogen, oxygen, and air from any mixing.

Fig. 2: The concept of direct air electrolysis (DAE) for hydrogen production.
figure 2

a A schematic diagram of the DAE module with a water harvesting unit made of porous medium soaked with the hygroscopic ionic solution. b A schematic diagram of the cross-section of the DAE module, showing the electrodes are isolated from the air feed, and the absorbed water are transported to the electrode by capillaries of the sponge. c Equilibrium water uptakes of hygroscopic solutions at different air R.H.31,32d J–V curves for DAE modules using Pt or Ni electrodes sandwiched with KOH electrolyte (in equilibrium with 15% and 60% R.H. at 20 °C) soaked in a melamine sponge. e Effect of sponge materials on J-V performance of DAE modules using H2SO4 electrolyte in equilibrium with 30% R.H. at 25 °C. The inset shows the optical micro image for the glass foam. Source data are provided as a Source Data file.