For decades, scientists have looked for an inexpensive, efficient and safe way to produce hydrogen. One of the most interesting methods to achieve this is through the use of solar energy: light is used to accelerate the reaction that causes water molecules to divide into oxygen and hydrogen. In this method, the most popular catalyst is titanium dioxide, but new research has found a substitute that is not only cheaper but also up to 25 times more efficient: rust.

The discovery was made by a group of Japanese scientists led by Prof Ken-ichi Katsumata of the University of Tokyo. The experiment, conducted by Katsumata and his colleagues, aimed to address the main challenges encountered in the use of semiconductor catalysts for the production of solar fuels. The study, published in Chemistry, describes three obstacles:
firstly, the catalyst needs to be activated by light. Secondly, current photocatalysts require the use of rare or “noble” metals as co-catalysts; materials that are expensive and difficult to obtain. The last proble.m stems from the effective production of hydrogen and oxygen. If not separated immediately, the mixture of these two gases can at best greatly reduce the production of hydrogen and, in the worst case, cause an explosion.

The team experimented with α-FeOOH (rust) to evaluate its photocatalytic efficiency, starting from a mixture of water and methanol irradiated by an Hg-X lamp.

“We were really surprised at the generation of hydrogen using this catalyst,” states Professor Katsumata, “because most of the iron oxides are not known to reduce to hydrogen. Subsequently, we searched for the condition for activating α-FeOOH and found that oxygen was an indispensable factor, which was the second surprise because many studies showed that oxygen suppresses hydrogen production by capturing the excited electrons.”

The team confirmed the mechanism using “gas-chromatography-mass spectrometry”, and demonstrated that α-FeOOH is 25 times more active than titanium dioxide and provided a stable hydrogen production for over 400 hours.

Scientists explain that more research will be needed to optimize the process.

“Although we could quickly elucidate the generation of fuel (hydrogen), it took about three years to investigate the role of oxygen (why is it consumed?),” Professor Katsumata explained, “at this stage, the reaction occurs only with ultraviolet light, and its quantum yield is not high. We need to continue our research to improve the efficiency of the reaction.”

For now, these findings represent promising new advancements in the production of a clean, zero-emissions energy source that may be central to the sustainable societies of the future.