Extract water from the air by solar energy, a device that can extract water from the air and produce hydrogen, powered by solar energy, has been a dream of researchers for decades. Today, EPFL chemical engineer Kevin Sivula and his team have taken important steps to bring this vision closer to reality. They developed an amazing and simple system that combines technology based on semiconductors and new electrodes with two main characteristics: they run, to increase contact with water and air; and of course, to increase the sunlight exposure of the semiconductor coating. When the device is exposed only to sunlight, it removes water from the air and produces hydrogen gas. The results were published on January 4, 2023 in Advanced Materials.
What’s new?
It is their new media electrodes, transparent, porous and conductive, that allow the technology of the sun to change water – in the atmosphere of the atmosphere – into hydrogen.
“To achieve a sustainable society, we need a way to store renewable energy in the form of chemicals that can be used as fuel in infrastructure and industry. Solar energy is the largest form of renewable energy, and we are working to develop economically competitive ways to produce fuel,” said Sivula of EPFL’s Laboratory for Molecular Engineering of Optoelectronic Nanomaterials and Principal Investigator of the study.
The spirit comes from the leaves of the tree
In their research on renewable fuels, EPFL engineers, in collaboration with Toyota Motor Europe, were inspired by how plants can convert sunlight into chemical energy using carbon dioxide dioxide. Plants often extract carbon dioxide and water from their environment and, by adding the energy of sunlight, can turn these molecules into sugar and starch, a process known as photosynthesis. Solar energy is stored as chemical bonds in sugars and starches.
The transparent gas diffusion electrodes created by Sivula and his team, when covered with light-harvesting semiconductor material, so to speak act as artificial leaves, collecting water from the air and sunlight to produce hydrogen gas. Solar energy is stored in the form of hydrogen bonds.
Instead of producing electrodes with traditional layers that are opaque to sunlight, their substrate is actually a three-dimensional mesh of colored glass fibers.
Marina Caretti, the author of the work, says: “Creating our prototype device was a challenge because transparent gas-distributing electrodes had not been demonstrated before, we had to develop a new method for every step. However, since each step is simple and flexible, I believe that our approach will open new horizons for many applications, starting from the installation of gaseous media for the production of hydrogen that provides energy the sun.
From moisture and humidity in the air
Sivula and a group of other researchers have already shown that it is possible to perform artificial photosynthesis by producing hydrogen from water and sunlight using a device called a cell. photoelectrochemical (PEC). A PEC cell is often referred to as a device that uses incident light to stimulate a photosensitive material, such as a semiconductor, immersed in a liquid solution causing a chemical reaction. But for practical purposes, this method has its drawbacks, e.g. it is complicated to create a large PEC device using liquid.
Sivula wanted to demonstrate that PEC technology could be adapted to extract water from the air, which led to the development of their new diffusion electrode. Electrochemical cells (e.g. fuel cells) have already been demonstrated to work with gas instead of liquid, but gas-dispersing electrodes have been used poorly and are incompatible with solar-powered PEC technology. .
Now researchers are focusing their efforts on improving the process. What is the right belt size? What is the best pore size? What are the best semiconductors and leather? These are the questions being answered under the EU Sun-to-X project, which is dedicated to advancing this technology and developing new ways to convert hydrogen into fuel.
Understanding of transparent gas diffusion electrodes
In order to make air-dispersing air, the researchers start with a type of glass wool, which is basically quartz fibers (also called silicon oxide) and turn it into a sensitive material by and bonding the fibers together at high temperatures. Next, the wafer is covered with a bright film of fluorine-doped tin oxide, which is known for its good quality, resistance, and ease of cleaning. These first steps result in a transparent, porous and conductive wafer, which is necessary to increase the contact with water in the air and allow photons to pass through. The wafer is coated again, this time with a thin semiconductor film that absorbs sunlight.
This second thin layer still allows light to pass through, but appears opaque because of the large surface area of the porous substrate. Meanwhile, this coated wafer can produce flammable hydrogen when exposed to sunlight. The scientists constructed a small chamber with a wafer coating, and a membrane to separate the hydrogen gas produced for measurement. When their room is exposed to sunlight and humidity, hydrogen gas is produced, achieving what scientists have planned to do, showing that the concept of air circulation for the production of hydrogen gas has ability to achieve.
Although the scientists did not study well the conversion of the sun-to-hydrogen in their demonstration, they believe that it is modest for this example, and the current is lower than what can be achieved in the cells PEC is based on the sun. Depending on the materials used, the best solar-to-hydrogen conversion efficiency of the coated wafer is 12%, while water cells have been shown to work up to 19%.
