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 the fight once morest global warming and sustainable development, for domestic needs, in transport, or on a large scale in energy production plants.
In fact, combined with the oxygen in the air, hydrogen makes it possible to produce thermal or electrical energy without releasing any polluting emissions (mainly water). This is the case, for example, in the 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 of the possibilities is to use solar energy directly to produce hydrogen from water in photo-electro-chemical cells. These cells are composed of photo-electrodes, sorts of solar cells immersed directly in water, which make it possible to collect solar energy, and to use this energy to break water molecules to form molecules of hydrogen and d. ‘oxygen.
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 Institute of Physics of Rennes–CNRS at the University of Rennes 1.
In the work just published in the review 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 environmental impact. This proposal is accompanied by several demonstrations of photo-electrodes operating 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 the electric current on demand, as in the case of silicon, an abundant and inexpensive material, forming 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 so-called “III-V” semiconductors which are used in a wide range of applications, ranging from laser emitters or LEDs and other optical sensors, to photovoltaic solar cells for aerospace. 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 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, thereby drastically reducing the manufacturing cost and environmental footprint of the components developed.
One of the main problems of this approach was related to the appearance of crystalline defects in the semiconductor material, that is to say the presence of one or more atoms badly positioned with respect to the arrangement perfectly regular that the atoms of the crystal should ideally have. This has the consequence of degrading the performance of the lasers or of the solar cells thus developed, and this is why the research efforts focused essentially on the reduction or elimination of these defects.
Conversely, our team demonstrated that these irregularities in the crystal, usually considered as defects, had very original physical properties (inclusions with a metallic character), which might be used effectively for the production of solar hydrogen, and for other photoelectric applications.
Surprising 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 locally inverting the arrangement of atoms, in the III-V materials deposited on silicon, gives them quite remarkable and unprecedented physical properties. In particular, we show that these walls behave locally (at the atomic scale) like metallic inclusions, in a material which is itself a semiconductor.
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This allows the material to be both photo-active (absorption of light and conversion into electrical charges), and locally metallic (transport of electrical charges). Even more surprisingly, 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 photo-electrodes (see photos 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 due 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 is improved, they might, in the future, be used of substrate for a conversion of solar energy into hydrogen on a larger scale.
New properties for new applications
In this study, the demonstration of photo-electrodes 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 demonstrated application, the intrinsic properties of this new family of materials, which can be developed quite simply, also make it possible to envisage many other applications. The ability of the material to efficiently convert light into electrical charges makes it, for example, a candidate of choice for photovoltaic solar cells, or optical sensors. Its properties of electric charge transport and anisotropic conduction might be used for electronics and quantum computing. Finally, the physical phenomena related to light and electric current taking place at the nanometric scale, this material might also be considered to consider new integrated photonic architectures.
