A step forward towards green hydrogen

How can we produce clean hydrogen without burning fossil hydrocarbons or other non-renewable energy sources? Through photoelectrochemistry, or artificial photosynthesis. A method that, just like photosynthesis, uses sunlight and water, as with electrolysis, to obtain hydrogen, without generating harmful emissions. The group of researchers from the Department of Physics of the University of Trento focused precisely on this approach.

One of the most innovative aspects of their research project is the use of photocatalysts (semiconductor materials) based on two-dimensional materials and, in particular, on graphitic carbon nitride (g-C3N4). This material is lightweight and sustainable and is used to break the chemical bond of the water molecule to produce hydrogen. The research has shown that, when used in the form of a single atomic layer, these photocatalysts offer superior performance compared to the thicker and less orderly structures previously tested. This discovery could open the way to a more efficient use of these materials in the production of green hydrogen. The results have been published in Carbon.

About the study. Hydrogen is considered one of the most promising solutions for the energy transition. But most hydrogen produced today is made via the "steam reforming" method, where methane (a fossil fuel) is heated to high temperatures. A process that is not a fully sustainable. The Trento-based research team instead focuses on the production of hydrogen through photoelectrochemical cells.

This is a clean process that does not use hydrocarbons or other non-renewable energy sources to break the chemical bond of the water molecule to produce hydrogen.

"The graphitic compound based on graphitic carbon nitride has been suggested as a possible photocatalyst. In contact with water, this semiconductor absorbs visible sunlight and transforms it into chemical energy to allow the movement of electrons within matter. Before our work, little was known about these mechanisms," explains Francesca Martini, lead author of the study. She continues: "By studying the formation and propagation of excitons (a bound electron-hole pair), particles produced by sunlight in carbon nitride formed by a single layer of atoms, we realized that they have a very low speed and move in the photocatalyst thanks to a combined motion that includes the vibrations of the atoms."

The authors of the study are surprised by this result. The electrons are more than two thousand times smaller than the atoms of the photocatalyst. Therefore, they move faster, just as a swarm of insects (the electrons) moves around a person (the atom). This, however, does not happen in carbon nitride. It is as if the swarm of insects agrees with the person to walk arm in arm like a couple, until they meet a hydrogen ion together. "When this happens," Matteo Calandra, study coordinator simplifies, "the atom bows and lets the electron that binds to the hydrogen ion pass through. Just as the father (the atom) of the bride (the electron) does when he takes her to the altar (hydrogen ion)."

The work of researchers will continue as they will perform numerical simulations on a database of over five thousand materials, they have access to, to perform a computational screening and identify better catalysts than the current ones.

"We hope that this research will lead to a strong innovation for the production of hydrogen from photoelectrolytic cells. Thanks to this methodology, we can now systematically identify better-performing materials and accelerate progress in the production of green hydrogen," Pietro Brangi, co-author of the study concludes.

This project represents a significant step towards energy sustainability and confirms the role of UniTrento as a centre of excellence in renewable energy research.

The work is part of H2@Tn, a research project that UniTrento started over a year ago with the support of the Province of Trento to focus on renewable energy and the production of green hydrogen. The project also includes Fondazione Bruno Kessler and received support from the European Union through Next Generation EU.

The authors of the study "Ultraflat excitonic dispersion in single layer g-C3N4" are Francesca Martini, Pietro Nicolò Brangi, Pierluigi Cudazzo and Matteo Calandra, all from the Physics Department of the University of Trento.

You can read the paper in Carbon magazine at: https://doi.org/10.1016/j.carbon.2024.119951

The article will be available in paper format from 5 March.