Scientists at the University of Sydney have developed a novel photothermal method for producing clean hydrogen from sunlight and water at low temperatures, achieving an overall efficiency of 12.9%. This process does not require an electrolyser or a photocatalytic reactor.

Process and Mechanism

The method utilizes liquid gallium, a soft metal with a melting point of 29.8°C. Particles of this metal are suspended in water within a sealed glass column. When a light source is focused on the mixture, it triggers a photothermal reaction, heating the water to between 48 and 93°C. This heat causes the gallium to oxidize, forming gallium oxyhydroxide on its surface and releasing hydrogen bubbles into the water. The light exposure also breaks an oxide layer on the gallium, enabling a continuous reaction. The generated hydrogen is pure, as confirmed by gas chromatography.

Efficiency and Scalability

The overall 12.9% efficiency is derived from a 57.5% chemical reaction efficiency and a 22.5% photothermal efficiency. Researchers consider this initial proof-of-concept highly competitive, drawing a parallel to the historical development of silicon solar cells. The process is described as efficient and easy to scale up, with ongoing work aimed at increasing efficiency and developing a mid-scale reactor for hydrogen extraction.

Key Advantages and Characteristics

The method offers several distinct advantages:

• Circular Process: The gallium oxyhydroxide byproduct can be reduced back to gallium and reused for subsequent hydrogen production.
• Safety: Unlike electrochemical reactions, the process does not produce oxygen, potentially avoiding explosion risks associated with separating hydrogen and oxygen.
• Water Source Flexibility: It works with equal efficiency using either fresh water or filtered seawater.
• Light Source Flexibility: It functions efficiently with both artificial light (e.g., a 50-watt halogen lamp) and natural sunlight, though solar performance depends on conditions.

Distinctions and Performance Notes

This photothermal process is distinct from photocatalysis, as gallium is a reactant that is chemically changed, not a catalyst. Testing revealed that a 100 W sunlight simulator was inefficient, taking nearly four times longer to produce a comparable hydrogen yield. Using heat alone without light was also inefficient, producing only 21% of the yield even with triple the reaction time.