Yet another discovery has been made in hydrogen production, as researchers and scientists around the world are working to find solutions to the global climate crisis by tapping into clean renewable hydrogen energy. Scientists from the Tokyo Institute of Technology (Tokyo Tech) have developed an affordable way to produce hydrogen, using cobalt and visible light to split water molecules.
Using cobalt, an earth-abundant material, the team was able to demonstrate the first visible-light photoelectrochemical system for water splitting using TiO2, which was recently published in ACS Applied Materials & Interfaces.
Photoelectrochemical water splitting is accomplished when light energy is used to split water molecules into oxygen (O2) and hydrogen (H2). Electrochemical cells contain an external circuit that connects an anode and cathode and is submerged in water.
Water oxidation takes place at the anode where oxygen is produced by capturing energy from the light waves. Energy is transferred through the waves to the electrons of the anode, enabling their movement through the external circuit to reach the cathode. Once the electrons reach the cathode, hydrogen forms.
Finding photoelectrochemical systems, that effectively pull this off, is not easy. One widely used photoanode material is titanium dioxide (TiO2), unfortunately, it can only absorb high energy light made up of shorter wavelengths in the ultraviolet region. It’s possible to mix TiO2 with noble metals (like silver or gold) to create longer wavelengths of light, but this is an expensive process.
In looking for a solution the team from Tokyo Tech experimented with cobalt and created the “first visible-light photoanode made of TiO2.” It turns out the process was quite simple, they used photoanode fabrication. As the study points out; “thin TiO2 films are grown onto a substrate through a standard procedure and then cobalt is introduced by immersing them into an aqueous cobalt nitrate solution.”
The research was led by Professor Kazuhiko Maeda, who talks about the team’s findings:
This study demonstrates that a visible-light-driven photoelectrochemical cell for water oxidation can be constructed through the use of earth-abundant metals without the need for complicated preparation procedures.
After accomplishing this, the researchers set out to identify the specific composition and structure of the cobalt modified surface of the TiO2 photoanode. They needed to understand how cobalt enabled the material to absorb more light, mobilizing the electrons to cause water oxidation. The team used many different types of spectrometry analysis and scanning electron microscopy to find their answers.
What they found was cobalt domains do more than just capture visible light and transfer electron charges, they also act as catalytic sites facilitating water oxidation. The team also discovered that performance was affected when the structure of the base TiO2 thin film was changed, enabling for the accumulation of more or less cobalt atoms. This structure is something that can easily be adjusted, which the team will keep testing to find optimal results, and achieve higher water oxidation rates.
Professor Maeda speaks further about his team’s findings:
So far, cobalt-sensitized water photooxidation systems had been comprised of powder-based photocatalysis, which works only in the presence of a sacrificial electron acceptor. Therefore, the present study also demonstrates sacrificial reagent-free visible-light water splitting using a cobalt-sensitized semiconductor material (TiO2).
Producing hydrogen using cobalt and visible light, abundant resources, is like having a ‘boon of the gods’ in terms of sustainability. The researchers are hopeful this discovery can lead to further advancements for affordable water splitting, by using earth-rich materials in the quest to create clean renewable hydrogen.
