Green hydrogen, produced via water electrolysis using renewable energy, is a crucial energy carrier for a low-carbon economy. However, the uneven distribution of renewable energy sources and freshwater resources poses a significant challenge to large-scale hydrogen production. Many regions rich in solar or wind energy experience water scarcity, hindering the deployment of conventional water electrolyzers. Desalination is a costly and complex alternative. This research aims to overcome this geographical limitation by developing a method for direct hydrogen production from atmospheric water vapor, a universally available and inexhaustible resource. The abundance of water in the air, even in arid environments, suggests the potential for a sustainable hydrogen production method that bypasses the need for liquid water sources. This approach holds significant promise for providing green hydrogen to remote, arid, and semi-arid regions.

Existing attempts to address water scarcity in electrolysis include direct saline splitting, which faces challenges with brine management, and phosphorus-based anion exchange membrane electrolyzers, which require high humidity and produce low-purity hydrogen (<2%). Photocatalytic water splitting, while potentially vapor-fed, suffers from low solar-to-hydrogen efficiency (around 1%) and necessitates gas separation. This study builds upon prior research demonstrating the water absorption capabilities of deliquescent materials like potassium hydroxide, sulfuric acid, and propylene glycol, which can extract moisture from air even at low relative humidity, making it feasible to directly use atmospheric moisture for hydrogen production.

The researchers developed a direct air electrolysis (DAE) module with a sandwich structure. The core of the module is a water-harvesting unit, which also serves as an electrolyte reservoir. This unit consists of a porous medium (e.g., melamine sponge or sintered glass foam) soaked with a hygroscopic ionic solution (e.g., CH3COOK, KOH, H2SO4). The porous medium absorbs moisture from the air, and the captured water is transferred to the electrodes for in situ electrolysis. Electrodes on both sides of the water-harvesting unit are paired with gas collectors, ensuring the separation of hydrogen and oxygen. The module is integrated with a renewable energy source, such as a solar panel or wind turbine. Various hygroscopic materials were tested, with sulfuric acid showing superior performance in terms of water absorption and current density. The researchers investigated the influence of factors such as relative humidity, temperature, porous medium type (pore size and thickness), and electrolyte concentration on the DAE module's performance. Current density-voltage (J-V) curves were obtained, and the long-term stability of the DAE module was evaluated through extended operation tests (up to 288 hours). Gas chromatography (GC) was used to analyze the purity of the produced hydrogen and oxygen. The DAE module was also tested in an open-air environment using a solar-powered prototype with five parallel electrolyzers and a wind-powered prototype. Detailed experimental setups are described, including materials, characterization techniques, and data collection methods. Calculations for Faradaic efficiency are also presented.

The DAE module successfully produced high-purity hydrogen (≥99%) from ambient air with a Faradaic efficiency of around 95%. The module operated stably under a wide range of relative humidity, as low as 4%. Using a Ni electrode and KOH electrolyte with 60% RH, a high current density of 574 mA cm⁻² at 4.0 V was achieved. However, KOH's conversion to K2CO3 and KHCO3 limited its long-term stability. Sulfuric acid proved to be a superior electrolyte due to its high hygroscopic nature, non-toxicity, and ability to operate at low temperatures. A melamine sponge showed higher current density than glass foam due to lower resistance. Using Si-doped glass foam (1.5 cm thick) and H2SO4, a current density of 37.8 mA cm⁻² at 3.0 V was achieved, increasing to 44.8 mA cm⁻² at 45°C. A solar-driven prototype with five parallel electrolyzers produced an average hydrogen generation rate of 745 L·h⁻¹·m⁻² of cathode. A wind-driven prototype also successfully produced hydrogen. The outdoor test showed a hydrogen production rate of 93.1 m³ h⁻¹. The study demonstrated the long-term stability of the DAE module for 12 consecutive days under constant current density, operating at a stable current density. The DAE module exceeded the U.S. Department of Energy's target of 20% solar-to-hydrogen efficiency. The system's modular design allows for easy scalability by stacking additional units.

The successful demonstration of direct hydrogen production from ambient air using the DAE module addresses a critical challenge in green hydrogen production: the geographical mismatch between renewable energy sources and freshwater availability. The technology's ability to operate at low relative humidity makes it particularly suitable for arid and semi-arid regions, opening up new possibilities for widespread hydrogen production. The high purity of the produced hydrogen, along with the relatively high efficiency and scalability of the system, positions the DAE technology as a viable and sustainable alternative to conventional water electrolysis. The use of readily available and inexpensive materials further enhances the potential for economic viability and global deployment. The successful demonstration with both solar and wind power confirms the versatility and adaptability of the technology to various renewable energy sources.

This research successfully demonstrated a novel method for sustainable green hydrogen production directly from atmospheric water vapor, solving the crucial issue of water scarcity in conventional hydrogen production. The direct air electrolysis (DAE) module exhibits exceptional performance in terms of efficiency, stability, and adaptability to various environmental conditions. Its scalability and compatibility with diverse renewable energy sources make it a promising technology for decentralized hydrogen generation in water-scarce regions. Future research could focus on optimizing the electrolyte selection, improving the module's design for even higher efficiency, and exploring different configurations for various applications.

While the DAE module demonstrated impressive performance, several limitations should be noted. The long-term effects of electrolyte degradation on module performance require further investigation. The optimal design of the porous medium and the balance between water absorption and conductivity remain areas for improvement. The economic feasibility of large-scale deployment needs further assessment, considering the cost of materials and manufacturing. Finally, the impact of environmental factors like dust and pollutants on the long-term performance of the system requires further study.