An explanation for a long-standing mystery in hydrogen production

Researchers from the Technion and Ben-Gurion University found a way to improve the use of a material called hematite to split water using solar energy

Newspaper Energy & Environmental Science reports a scientific breakthrough in the study of theThe hematite - An important and promising material in the field of converting solar energy into hydrogen by photoelectrochemical splitting of water. The research was led Prof. Avner Rothschild from the Faculty of Materials Science and Engineering at the Technion and Yifat Pickner, PhD student in the Grand Technion Energy Program (GTEP).

The importance of solar energy to our lives is clear to everyone. The sun sends endless energy to the earth, if we know how to harness it properly for our needs we can give up the use of polluting fossil fuels such as oil and gas. The main challenge in switching to solar energy is related in the variable availability of solar radiation During the hours of the day and throughout the changing seasons. Every place on Earth receives solar radiation for a limited time during the day, and at night that radiation disappears. Since the electricity grid needs constant and stable current at all hours of the day, the use of solar energy is dependent on our ability stock up it in a way that will allow us to use it at night and on cloudy days. The problem is that the form we are familiar with for storing electricity - batteries and accumulators - is not applicable when it comes to supplying electricity to a city, a neighborhood, a factory, etc. Furthermore, energy storage in batteries and accumulators is suitable for a range of hours and cannot provide a solution for long-term storage between seasons.

A possible solution to the storage problem is the conversion of solar energy into hydrogen using Photoelectrochemical solar cells. These cells are similar to photovoltaic solar cells that convert the sun's energy into electricity, but instead of electricity they produce hydrogen with the help of the electric current created in them. This current is channeled to the photoelectrochemical splitting of water molecules into hydrogen and oxygen.

The advantage of hydrogen over electricity is that it is cheap stock up and use it when needed for electricity production or for other needs such as driving electric vehicles with a fuel cell that replaces the heavy and expensive batteries in Tesla's electric vehicles and similar ones, domestic and industrial heating, production of ammonia and other raw materials. The advantage of hydrogen as a fuel is that its production and use are not accompanied by the emission of greenhouse gases or anything other than oxygen and water.  

One of the main challenges in photoelectrochemical cells is the development of efficient and stable photoelectrodes in a basic or acidic electrolyte, which is the chemical environment in which water can be split into hydrogen and oxygen with high efficiency. The photoelectrodes absorb the photons coming from the sun, and with the energy they receive from them, they produce a stream of electrical charges (called electrons and holes) that are used to create hydrogen and oxygen, respectively. Silicon, the material used in photovoltaic solar cells, cannot be used as such a photoelectrode since it is unstable in an electrolyte.

This is the background for the development of photoelectrochemical cells based on hematite photoelectrodes - an iron oxide with a chemical composition similar to rust. Hematite is a cheap, stable and non-toxic material with properties suitable for splitting water. However, hematite is not without its drawbacks either. One of them is the gap between the energetic survivor the theoretical his and the survivor The practical achieved in actual devices. For reasons that have not been revealed to date, despite decades of research, the conversion efficiency of the photons to hydrogen in hematite-based devices does not even reach half of the theoretical limit for this material. By comparison, the conversion efficiency of the photons in silicon solar cells is very close to the theoretical limit.

In the current study, which continues and expands findings recently published in the journal  Nature Materials, the research group headed by Prof. Rothschild presents an explanation for the mystery: it turns out that a significant portion of the photons absorbed by the hematite create electronic transitions that are "bound" to a specific atomic site in the crystal and are unable to progress in a way that creates an electric current that is used to split the water into hydrogen and oxygen.

And now, for the good news: using a new analysis method developed by Yifat Pickner with the help of her research colleagues Dr. David Ellis of the Technion and Dr. Daniel Groh, a senior lecturer at Ben-Gurion University of the Negev, the following data were measured for the first time:

• The quantum efficiency for creating mobile electric charges in matter as a result of absorbing photons at different wavelengths,
• The efficiency of separating the opposite electric charges, electrons and holes, and turning them into an electric current that splits the water molecules into hydrogen and oxygen.

This is the first time that these two properties, the first essentially optical and the second electrical, have been measured separately from each other, as previous studies measured the total effect of both together. The separation between them makes it possible to better trace the factors that affect the energy efficiency of materials for converting solar energy into hydrogen or electricity.

Beyond the practical achievement, the article is a scientific breakthrough that paves a new way for the study of the interaction between light and matter in materials with correlated electrons (Correlated Electron Materials).

The study was supported by the Research Focus on Photocatalysts and Photoelectrodes for Hydrogen Production in the National Science Foundation's (ISF) Oil Substitutes for Transportation Program, the Grand Batech Technion Energy Center (GTEP) and the Russell Berry Research Institute in Nanotechnology (RBNI).

for the article in the journal  Energy & Environmental Science