Photocurrent spectroscopy uncovers hidden energy losses in water splitting, helping to make hydrogen production more efficient

A new study has revealed key factors limiting the efficiency of photoelectrochemical water splitting using a titanium dioxide photoanode for clean hydrogen production. Researchers combined intensity-modulated photocurrent spectroscopy with the distribution of relaxation times analysis to analyze charge carrier dynamics. They identified distinct behaviors related to light intensity and recombination at different applied potentials and discovered a previously unreported "satellite peak," offering new insights for improving material design and hydrogen production efficiency.

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 photoanode such as a titanium dioxide (TiO₂) absorbs sunlight and facilitates the oxygen generation, while hydrogen is produced at the cathode. However, the process in 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 recent 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 were thought to 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 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." "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," highlights Dr. Cho.

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," comments Prof. Amano.

Looking ahead, this research could help pave the way for major advances in clean energy over the next five to ten 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 believes.

Image title: Recombination mechanisms in PEC water splitting at different potential regions
Image caption: Distribution of relaxation times analysis (top) reveals a newly observed satellite peak. The diagrams (bottom) illustrate how recombination mechanisms, such as over-penetration induced recombination (OPR), excess hole induced recombination (EHR), and back electron-hole recombination (BER) shift with potential and light intensity.
Image credit: Dr. Yohei Cho from JAIST.
Image license: Original Content
Usage restrictions: Cannot be reused without permission.

Reference

Funding information

The researchers received financial support from the Tokyo Tech Academy for Convergence of Materials and Informatics (TAC-MI), Tokyo Metropolitan University, JSPS Kakenhi (No. 20H02525, 21J21388, 22KJ1272, 23K26735, 23K17953, 24KJ1201, and 24H00463), the UK Engineering and Physical Sciences Research Council (EPSRC) via grant EP/S030727/1, and scholarships from the Department of Chemical Engineering at Imperial College London.

February 25, 2025