Photocurrent spectroscopy uncovers hidden energy losses in water splitting

Hydrogen fuel is emerging as a clean energy source that could replace fossil fuels. One way to produce hydrogen sustainably is through photoelectrochemical (PEC) water splitting, where a photoanode such as titanium dioxide (TiO₂) absorbs sunlight and facilitates oxygen generation, while hydrogen is produced at the cathode.

However, the process in the photoanode suffers from inefficiencies due to electrons and holes recombining before they can complete the reaction. Understanding these losses is essential to improving the technology.

A study published in the Journal of the American Chemical Society on 22 February 2025 offers new insights into this challenge. In this study, Dr. Yohei Cho at Japan Advanced Institute of Science and Technology (JAIST) and Prof. Fumiaki Amano at Tokyo Metropolitan University, in collaboration with researchers from Institute of Science Tokyo, Imperial College London, and Swansea University, used an advanced technique to track electron movement in real-time.

By combining intensity-modulated photocurrent spectroscopy (IMPS) with distribution of relaxation times (DRT) analysis, the researchers identified charge transport behaviors that were previously inseparable. Unlike traditional methods, this approach does not rely on predefined circuit models, allowing for clearer and more direct analysis.

“Our methodology enables us to see electron movement in detail, revealing previously inseparable processes. This not only improves our fundamental understanding of charge transport but also offers direct pathways for enhancing material performance,” says Dr. Cho.

Until now, energy losses in PEC water splitting could not be quantitatively differentiated. This study revealed that recombination occurs through three distinct mechanisms.

At higher voltages, inefficiency arises when light penetrates too deeply into the material, leading to over-penetration induced recombination (OPR). At medium voltages, an excessive build-up of photogenerated holes causes a second type of recombination, named as excess hole induced recombination (EHR). At lower voltages, back electron-hole recombination (BER) occurs when holes recombine with returning electrons before they can contribute to the reaction.

The study also showed that these recombination effects shift depending on light intensity, revealing that material performance is highly dependent on external conditions.

One of the most exciting discoveries of the study was the detection of a previously unknown slow reaction, which the researchers call the “satellite peak.” Dr. Cho says, “The discovery of the satellite peak is crucial because it helps us pinpoint the rate-limiting step in water splitting. By addressing this, we can significantly enhance the efficiency of PEC systems.”

Beyond hydrogen production, this breakthrough has broader applications, from carbon dioxide reduction and wastewater treatment to self-cleaning and antibacterial surfaces. “Our approach is widely applicable across various photocatalytic systems. By understanding and mitigating recombination losses, we can optimize materials for a range of clean energy and environmental applications,” says Prof. Amano.

Looking ahead, this research could help pave the way for major advances in clean energy over the next five to 10 years. By providing a precise tool for diagnosing and reducing energy losses, scientists could develop new materials that significantly increase hydrogen production efficiency. This would make solar-powered hydrogen a more viable and affordable energy source, helping to reduce dependence on fossil fuels and accelerate the transition to a greener world.

“While further research is necessary to fully assess the long-term impacts, this work lays a solid foundation for potential advancements in semiconductor technology,” Dr. Cho says.