The use of hydrogen as an energy carrier to produce electricity and heat on demand is an almost ideal solution for energy storage in the context of combating global warming and sustainable development, for domestic needs, in transportation, or on a large scale in energy production plants.
In fact, 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 fuel cells used in hydrogen-powered vehicles, which combine hydrogen and oxygen to produce an electric current and power an electric motor.
However, the hydrogen in use today is mainly produced from fossil fuels, and therefore it is necessary to find other low-carbon production methods. One possibility is to use solar energy directly to produce hydrogen from water in chemical photovoltaics. These cells consist of photoelectrodes, which are types of solar cells immersed directly in water, which makes it possible to collect solar energy, and use this energy to break down water molecules to form hydrogen and oxygen molecules d.
This is the approach chosen by our consortium of scientists from Rennes, with Nicolas Piertro and Yuan Leger (FOTON-CNRS Institute, INSA Rennes) and Bruno Fabre (Institute for 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 magazine advanced scienceWe propose to use a new family of materials with amazing photovoltaic properties to efficiently produce solar hydrogen at low cost and environmental impact. This proposal is accompanied by several demonstrations of solar photovoltaic electrodes.
Semiconductors are materials with properties intermediate between electrical conductors (often metals) and insulators. These properties can be used, for example, to allow or not to pass electric current on demand, as in the case of silicon, an abundant and inexpensive material that forms the basis of all current electronic chips.
But they can also be used to emit or absorb light, as in the case of so-called “III-V” semiconductors that are used in a wide range of applications, from laser emitters or LEDs and other optical sensors, to solar photovoltaic cells for space. 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 effective, they are also more expensive. In this context, many researchers have tried since the 1980s to deposit very thin layers of these materials on silicon substrates to obtain high optical performance, which is necessary to ensure, for example, good radiation absorption in a solar cell, or to ensure efficient optical emission. in lasers, which greatly reduces the manufacturing cost and environmental footprint of the developed components.
One of the main problems of this approach was related to the emergence of crystal defects in the semiconductor material, that is, the presence of one or more atoms in a poor position with respect to the perfectly regular arrangement that the atoms of the crystal should ideally contain. . This has degraded the performance of lasers or solar cells so developed, which is why research efforts have mainly focused on reducing or eliminating these drawbacks.
On the contrary, our team showed that these irregularities in the crystal, which are usually considered defects, have very original physical properties (impurities with a metallic character), which can be used effectively for solar hydrogen production, and for other photovoltaic applications.
Our work therefore demonstrates that the presence of antiphase walls (the abbreviation “APB” used in the illustration), which are very specific crystal defects that fluctuate the arrangement of atoms locally, in III-V materials deposited on silicon, gives them very remarkable and unprecedented physical properties. In particular, we show that these walls behave locally (at the atomic level) like a metallic inclusion, in a material that is itself a semiconductor.
This allows the material to be both photoactive (absorbing light and converting it into electrical charges), and locally metallic (transmitting electrical charges). Most surprisingly, the material can conduct both positive and negative charges (the dipole character). In this work, proof of concept is presented by realizing multiple photo-III-V/Si electrodes (see attached figure images) for solar hydrogen production, with performance comparable to the best conventional photo-III-V electrodes, but at a much lower production cost and environmental impact. Due to the use of a silicone substrate.
Currently, these samples have made it possible to produce hydrogen on a laboratory cell scale, but it seems conceivable that if the stability of these materials is improved, they could, in the future, be used from the substrate to convert solar energy into hydrogen on a larger scale.
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In this study, the presentation of the photovoltaic electrodes for solar hydrogen production makes it possible 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, in addition to this demonstrated application, the intrinsic properties of this new group of materials, which can be developed quite simply, also make it possible to envisage many other applications. The material’s ability to efficiently convert light into electrical charges makes it, for example, a preferred candidate for solar photovoltaic cells, or optical sensors. Its properties can be used for electric charge transfer and anisotropic conduction in electronics and quantum computing. Finally, physical phenomena related to light and electric current that occur on the nanometer scale, this material can also be considered for consideration of new integrated photonic architectures.
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