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

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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 fighting global warming and sustainable development, for household needs, in transport or on a large scale in manufacturing. of energy plants.

In fact, combined with oxygen in the air, hydrogen makes it possible to produce thermal or electrical energy without releasing 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 electric current and power an electric motor.

However, the hydrogen currently used is mainly produced from fossil fuels, and it is therefore necessary to find other low-carbon production methods. One possibility is to use solar energy directly to produce hydrogen from water in photoelectrochemical cells. These cells are composed of photoelectrodes, a kind of solar cells immersed directly in water, which allow to collect solar energy, and to use this energy to break water molecules to form hydrogen and oxygen molecules.

A new approach

This is the approach chosen by our consortium composed 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 the members of the Physics Institute of Rennes – CNRS at the University of Rennes 1.

In the work just published in the magazine Advanced science, we propose to use a new family of materials with rather surprising photoelectric properties to produce solar hydrogen efficiently, at low cost and environmental impact. This proposal is accompanied by several demonstrations of photoelectrodes working with solar lighting.

Semiconductors are materials with intermediate properties between electrical conductors (most often metals) and insulators. These properties can be used, for example, to let the electric current pass or not on demand, as in the case of 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 in the case of the so-called “III-V” semiconductors which are used in a wide range of applications, ranging from laser or LED emitters and other optical sensors, with photovoltaic solar cells for the aerospace sector. They are called “III-V” because they consist of one or more elements of column III and column V of Mendeleev’s periodic table.

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

One of the main problems of this approach was related to the appearance of crystalline defects in the semiconductor material, that is the presence of one or more atoms badly positioned with respect to the perfectly regular arrangement that ideally the atoms of the crystal should have. This has the consequence of degrading the performance of the lasers or solar cells thus developed, which is why research efforts have essentially focused on reducing or eliminating these defects.

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

Amazing properties

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

(Left): Schematic representation of a photoelectrode combining a thin layer (typically 1µm) of semiconductor III-V (pink) and a substrate of Si (purple), which can be used as an anode or cathode. (Right): The produced samples (top) have a surface area of ​​approximately 20 cm² and are used to produce photoelectrodes (bottom), which are used for photoelectrochemistry.
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This allows the material to be both photoactive (absorption of light and conversion into electrical charges) and locally metallic (transport of electrical charges). Even more surprisingly, the material can carry both positive and negative charges (ambipolar character). In this work a proof of concept is presented through the realization of numerous III-V / Si photoelectrodes (see photo of the attached figure) for the production of solar hydrogen, with performances comparable to the best photo-conventional III-V electrodes, but with a much lower production cost and environmental impact thanks to the use of the silicon substrate.

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

New properties for new applications

In this study, the demonstration of photoelectrodes for the production of solar hydrogen 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 us to predict many other applications. The material’s ability to efficiently convert light into electrical charges makes it, for example, an ideal candidate for photovoltaic solar cells or optical sensors. Its electric 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 occur at the nanometer scale, this material could also be considered to consider new integrated photonic architectures.

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