The use of hydrogen as an energy carrier to generate electricity and heat on demand is an almost ideal energy storage solution in the context of the fight against global warming and sustainable development, for domestic use, in transport or on a large scale in power plants.
In fact, when combined with the oxygen in the air, hydrogen allows the production of thermal or electrical energy without releasing polluting emissions (mainly water). This is the case, for example, with fuel cells in hydrogen-powered vehicles, which generate electricity from hydrogen and oxygen and drive an electric motor.
Nevertheless, the hydrogen currently used is essentially produced from fossil fuels and other low-carbon production processes must therefore be found. One of the possibilities is the direct use of solar energy to produce hydrogen from water in photoelectrochemical cells. These cells are made up of photoelectrodes, a type of solar cells that are immersed directly in water and allow solar energy to be collected and used to split water molecules to form hydrogen and oxygen molecules.
A new approach
This approach was chosen by our consortium 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 journal Advanced Science, we propose to use a new family of materials with amazing photoelectric properties to produce solar hydrogen in an efficient, inexpensive, and environmentally friendly way. This proposal is accompanied by several demonstrations of photoelectrodes working under sunlight.
Semiconductors are materials with intermediate properties between electrical conductors (mostly metals) and insulators. These properties can be used, for example, to allow the electric current to pass or not as required, 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, used in a wide variety of applications ranging from laser emitters or LEDs and other optical sensors to photovoltaic solar cells for the 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. In this context, since the 1980s, many researchers have tried to deposit very thin layers of these materials on silicon substrates in order to obtain high optical performance, necessary to ensure good radiation absorption in a solar cell, for example, or to ensure efficient light emission in a laser , which drastically reduces the manufacturing costs and the ecological footprint of the developed components.
One of the main problems of this approach related to the appearance of crystalline defects in the semiconductor material, i.e. the presence of one or more atoms that are ill-positioned with respect to the perfectly regular arrangement that the crystal’s atoms should ideally have. This results in a deterioration in the performance of the lasers and the solar cells developed in this way, which is why research efforts are essentially concentrated on reducing or eliminating these defects.
Conversely, our team showed that these crystal irregularities, which are usually considered defects, have very original physical properties (inclusions with a metallic character) that could be effectively exploited for solar hydrogen production and other photoelectric applications.
Surprising properties
Therefore, our work shows that the presence of antiphase walls (the acronym “APB” is used in the figure), which are very specific crystalline defects that locally reverse the arrangement of atoms, in the III-V materials deposited on silicon, they quite remarkable power and unprecedented physical properties. In particular, we show that locally (at the atomic level) these walls behave like metallic inclusions in a material that is itself a semiconductor.
(Left): Schematic of a photoelectrode combining a thin layer (typically 1 µm) of a III-V semiconductor (pink) and a Si substrate (purple), which can be used as an anode or cathode. (right): The samples produced (above) have a surface area of approx. 20 cm² and are used to produce photoelectrodes (below) for photoelectrochemistry. author provided
As a result, the material can be both photoactive (absorption of light and conversion into electrical charges) and locally metallic (transport of electrical charges). Even more surprising is that 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 performances comparable to the best conventional III-V photoelectrodes, but with much lower production costs and environmental impact due to the use of the silicon substrate.
With these samples it is currently possible to produce hydrogen on a laboratory cell scale, but it seems conceivable that with improved stability these materials could be used in the future as a substrate for a larger-scale conversion of solar energy into hydrogen.
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 working system. But beyond this demonstrated application, the intrinsic properties of this new family of materials, which can be developed quite easily, also make many other applications envisaged. The material’s ability to efficiently convert light into electrical charge makes it a prime candidate for photovoltaic solar cells or optical sensors, for example. Its properties of electric charge transport and anisotropic conduction could be used for electronics and quantum computers. Due to the physical phenomena related to light and electric current that take place at the nanometer scale, this material could also be considered as new integrated photonic architectures.