Chemical engineers at EPFL have invented a photovoltaic artificial leaf built on a new transparent, porous electrode that can harvest water from the air to convert it into hydrogen fuel. Semiconductor-based technology is scalable and easy to prepare.
A device capable of harvesting water from the air and supplying it with hydrogen fuel (powered entirely by solar energy) has been a dream of researchers for decades. Now EPFL chemical engineer Kevin Sivula and his team have taken an important step towards bringing this vision closer to reality. They have developed an ingenious yet simple system that combines semiconductor-based technology with a novel electrode that has two key characteristics of his. It is transparent, maximizing the exposure of the semiconductor coating to sunlight. When the device is simply exposed to sunlight, it takes in moisture from the air and produces hydrogen gas. The results will be published in Advanced Materials on January 4, 2023.
what’s new? It is a novel gas diffusion electrode that is transparent, porous and conductive, enabling this photovoltaic technology that converts gaseous water in the air into hydrogen fuel.
“To achieve a sustainable society, we need a way to store renewable energy as chemicals that can be used as fuels and feedstocks for industry. Solar energy is the most abundant form of renewable energy and We are working to develop economically competitive methods for producing solar fuels,” said Shivra of EPFL’s Molecular Engineering Laboratory for Optoelectronic Nanomaterials.
Inspiration from plant leaves
In researching renewable, fossil-free fuels, EPFL engineers, in collaboration with Toyota Motor Europe, were inspired by how plants can use carbon dioxide from the air to convert sunlight into chemical energy. . Plants essentially harvest carbon dioxide and water from their environment, and an additional boost of energy from sunlight allows them to convert these molecules into sugars and starches. This is the process known as photosynthesis. The energy of sunlight is stored in the form of chemical bonds inside sugars and starches.
The transparent gas diffusion electrodes developed by Sivula and his team, when coated with a light-harvesting semiconductor material, actually act like artificial leaves, harvesting water from the air and sunlight to produce hydrogen gas. generate. The energy of sunlight is stored in the form of hydrogen bonds.
Instead of constructing the electrodes with conventional layers that are opaque to sunlight, their substrate is actually a three-dimensional mesh of felted fiberglass.
Marina Caretti, lead author of the work, said: However, because each step is relatively simple and scalable, we believe that our approach opens up new horizons for a wide range of applications starting with gas diffusion substrates for photovoltaic hydrogen production. ”
From liquid water to humidity in the air
Shivra and other research groups have previously shown that artificial photosynthesis can be performed by producing hydrogen fuel from liquid water and sunlight using a device called a photoelectrochemical (PEC) cell. A PEC cell is commonly known as a device in which incident light stimulates a photosensitive material, such as a semiconductor, immersed in a solution, causing a chemical reaction. However, in practice, this process has its drawbacks. For example, creating a large area his PEC device that uses liquids is complicated.
Sivula hoped to show that PEC technology could be adapted to collect moisture from the air instead, leading to the development of new gas diffusion electrodes. Electrochemical cells (such as fuel cells) have already been shown to work with gases rather than liquids, but previously used gas diffusion electrodes are opaque and compatible with his PEC technology for photovoltaics. is not.
Researchers are now focused on optimizing the system. What is the ideal fiber size? What is the ideal pore size? The ideal semiconductor and membrane materials? These are the questions pursued by the EU project ‘Sun-to-X’, which is dedicated to advancing this technology and developing new ways to convert hydrogen into liquid fuels. am.
Making a transparent gas diffusion electrode
To make a transparent gas diffusion electrode, the research team starts with a kind of glass wool. It is essentially quartz (also known as silicon oxide) fibers that are felted by fusing the fibers at high temperatures into wafers. The wafer is then coated with a transparent thin film of fluorine-doped tin oxide. It is known for its excellent conductivity, robustness, and ease of scale-up. These first steps yield a transparent, porous, conducting wafer that is essential for maximizing contact with airborne water molecules and allowing photons to pass through. The wafer is then coated again with a thin film of semiconducting material, this time absorbing sunlight. This second thin coating of his still transmits light, but appears opaque due to the large surface area of the porous substrate. This coated wafer can already produce hydrogen fuel when exposed to sunlight.
Scientists continued to build a small chamber containing the coated wafers and a membrane to separate the hydrogen gas produced for the measurements. When their chamber was exposed to sunlight under humid conditions, hydrogen gas was produced, accomplishing what the scientists were trying to do, and developing a transparent gas diffusion electrode for solar-powered hydrogen gas production. It shows what the concept can achieve.
The scientists have not formally studied solar-to-hydrogen conversion efficiencies in demonstrations, but they were modest in this prototype, admitting they are less than can currently be achieved with liquid-based PEC cells. . Based on the materials used, coated wafers have a theoretical maximum conversion efficiency of 12% from the sun to hydrogen, while efficiencies of up to 19% have been demonstrated in liquid cells.
Original: A step from thin air to solar fuel
Than: Lausanne Federal Institute of Technology
