A new family of materials for the solar production of renewable hydrogen

The use of hydrogen as an energy carrier to produce electricity and heat on demand is an almost ideal energy storage solution in the context of combating global warming and sustainable development, for domestic needs, in transportation or on a large scale. in energy production. floors.

In fact, combined with the oxygen in the air, hydrogen makes it possible to produce thermal or electrical energy without emitting polluting emissions (mainly water). This is the case, for example, with fuel cells used in hydrogen-powered vehicles, which combine hydrogen and oxygen to produce electrical current and power an electric motor.

However, the hydrogen used today is mainly produced from fossil fuels, so other low-carbon production methods need to be found. One of the possibilities is to use solar energy directly to produce hydrogen from water in photoelectrochemical cells. These cells are made up of photoelectrodes, a kind of solar cells immersed directly in water, which allow solar energy to be collected and used to break water molecules to form hydrogen and oxygen molecules.

A new approach

This is the approach chosen by our consortium made up of scientists from Rennes, with Nicolas Bertru and Yoan Léger (Institut FOTON-CNRS, INSA Rennes) and Bruno Fabre (Institute of Chemical Sciences of Rennes–CNRS, University of Rennes 1), and in collaboration with members of the Physics Institute of Rennes–CNRS of the University of Rennes 1.

In the work that has just been published in the magazine advanced science, we propose to use a new family of materials with quite amazing photoelectric properties to produce solar hydrogen efficiently, at low cost and low environmental impact. This proposal is accompanied by several demonstrations of photoelectrodes that work under solar illumination.

Semiconductors are materials with intermediate properties between electrical conductors (most often metals) and insulators. These properties can be used, for example, to let or not pass electrical current on demand, as is the case with silicon, an abundant and cheap material, which forms the basis of all current electronic chips.

But they can also be used for the emission or absorption of light, as is the case with the so-called “III-V” semiconductors that are used in a wide range of applications, ranging from laser emitters or LEDs and other optical sensors, to cells. solar photovoltaic for the aerospace industry. They are called “III-V” because they consist of one or more elements from column III and column V of Mendeleev’s periodic table.

If these “III-V” materials are very efficient, they are also more expensive. It is in this context that many researchers have been trying since the 1980s to deposit very thin layers of these materials on silicon substrates in order to obtain high optical performance, necessary to guarantee, for example, good absorption of radiation in a solar cell, or to ensure efficient light emission in a laser, thus drastically reducing the cost of manufacturing and the environmental footprint of the components developed.

One of the main problems with this approach was related to the appearance of crystalline defects in the semiconductor material, that is, the presence of one or more atoms mispositioned with respect to the perfectly regular arrangement that the crystal atoms should ideally have. . This has the consequence of degrading the performance of the lasers or of the solar cells thus developed, for which reason research efforts were mainly focused on the reduction or elimination of these defects.

On the contrary, our team showed that these irregularities in the glass, generally considered as defects, had very original physical properties (inclusions with a metallic character), which could be used effectively for the production of solar hydrogen and for other photoelectric applications.

amazing properties

Our work therefore shows that the presence of antiphase walls (the acronym “APB” is used in the illustration), which are very specific crystalline defects that locally invert the arrangement of atoms, in III-V materials deposited on silicon, gives them a rather remarkable character. and unprecedented physical properties. In particular, we show that these walls behave locally (at the atomic scale) as metallic inclusions, in a material that is itself a semiconductor.

This allows the material to be both photoactive (absorbing light and converting into electrical charges) and locally metallic (transporting electrical charges). Even more surprising, the material can conduct both positive and negative charges (ambipolar character). In this work, a proof of concept is presented through the realization of several III-V/Si photoelectrodes (see photos of the attached figure) for the production of solar hydrogen, with performance comparable to the best conventional III-V photoelectrodes. electrodes, but with a much lower production cost and environmental impact due to the use of the silicon substrate.

For the time being, these samples have made it possible to produce hydrogen on a laboratory cell scale, but it seems possible to imagine that if the stability of these materials is improved, they could, in the future, be used as a substrate for converting solar energy into hydrogen. on a larger scale.

New properties for new applications

In this study, the demonstration of photoelectrodes for solar hydrogen production allows, on the one hand, to better understand the properties of the material and, on the other hand, to validate its application in a functional system. But, beyond this proven application, the intrinsic properties of this new family of materials, which can be developed in a very simple way, also allow many other applications to be contemplated. The material’s ability to efficiently convert light into electrical charges makes it, for example, a candidate of choice for photovoltaic solar cells or optical sensors. Its electrical charge transport and anisotropic conduction properties could be used for electronics and quantum computing. Finally, the physical phenomena related to light and electric current that take place at the nanometer scale, this material could also be considered to consider new integrated photonic architectures.