One step closer to solar fuels generated… from the air

EPFL chemists have created an artificial leaf and a porous electrode that can collect water vapor and turn it into hydrogen

Solar fuels: the chemical engineers of the Federal Polytechnic of Lausanne have invented an artificial solar-powered leaf, built on a new transparent and porous electrode, capable of collecting water from the air to convert it into hydrogen; this semiconductor-based technology is scalable and easy to prepare
Chemical engineers from the Federal Institute of Technology in Lausanne have invented a solar-powered artificial leaf, built on a new transparent and porous electrode, capable of collecting water from the air to convert it into hydrogen: this semiconductor-based technology is scalable and easy to prepare
(Photo: EPFL)

A device that can harvest water from the air and deliver hydrogen fuel, powered entirely by solar energy, has been a dream of researchers for decades.
Now, chemical engineer Kevin Sivula and his team at ETH Lausanne have taken a significant step to bring this vision closer to reality.
They have developed an ingenious and simple system that combines semiconductor-based technology with innovative electrodes that have two key characteristics.
They are porous, to maximize contact with water in the air, and transparent, to maximize sunlight exposure of the semiconductor coating.
When the device is simply exposed to sunlight, it takes water from the air and produces hydrogen gas. The results were published on January 4, 2023 in the journal Advanced Materials.

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Solar fuels: Kevin Sivula is an associate professor at the Molecular Engineering Laboratory of Optoelectronic Nanomaterials at the ETH Lausanne
Kevin Sivula is an associate professor of the Molecular Engineering Laboratory of Optoelectronic Nanomaterials of the Federal Institute of Technology in Lausanne
(Photo: Alain Herzog/EPFL)

Kevin Sivula: “Storing renewable energy in the form of chemicals”

What's new? Their new gas diffusion electrodes, transparent, porous and conductive, which enable this solar energy technology capable of transforming water, in a gaseous state in the air, into hydrogen fuel.
“To achieve a sustainable society, we need ways to store renewable energy in the form of chemicals that can be used as fuels and raw materials in industry. Solar energy is the most abundant form of renewable energy and we are looking to develop cost-effective ways to produce solar fuels”, explains Kevin Sivula of EPFL's Laboratory of Molecular Engineering of Optoelectronic Nanomaterials and principal investigator of the study.

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Solar fuels: the chemical engineers of the Federal Polytechnic of Lausanne have invented an artificial solar-powered leaf, built on a new transparent and porous electrode, capable of collecting water from the air to convert it into hydrogen; this semiconductor-based technology is scalable and easy to prepare
Chemical engineers from the Federal Institute of Technology in Lausanne have invented a solar-powered artificial leaf, built on a new transparent and porous electrode, capable of collecting water from the air to convert it into hydrogen: this semiconductor-based technology is scalable and easy to prepare
(Photo: EPFL)

Inspiration from a plant leaf, in collaboration with Toyota Motor Europe

In their research on fossil-free renewable fuels, engineers from the Federal Institute of Technology in Lausanne, in collaboration with Toyota Motor Europe, were inspired by the way plants are able to convert sunlight into chemical energy, using the carbon dioxide present in the air.
A plant essentially collects carbon dioxide and water from its surroundings and, thanks to the extra energy of sunlight, can transform these molecules into sugars and starches, a process that is already known to most people as photosynthesis.
The energy from sunlight is stored in the form of chemical bonds within sugars and starches.
The transparent gas diffusion electrodes developed by Dr. Sivula and his team, when coated with a light-trapping semiconductor material, effectively act like an artificial leaf, harvesting water from the air and sunlight to produce hydrogen gaseous.
The energy of sunlight is stored in the form of hydrogen bonds.
Instead of building electrodes with traditional layers that are opaque to sunlight, their substrate is actually a three-dimensional network of felted glass fibers.

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Solar fuels: Marina Caroline Michèle Caretti is a researcher at the Molecular Engineering Laboratory of Optoelectronic Nanomaterials at the Federal Polytechnic University of Lausanne
Marina Caroline Michèle Caretti is a researcher at the Molecular Engineering Laboratory of Optoelectronic Nanomaterials at the Federal Polytechnic University of Lausanne

Marina Caretti: “New procedures for transparent gas diffusion electrodes”

Main author of the work, Marina Caroline Michèle Caretti says: “Developing our prototype device was challenging, as transparent gas diffusion electrodes have not been scientifically proven previously, and we had to develop new procedures for each step. However, since each stage is relatively simple and scalable, I think our approach will break new ground for a wide range of applications, starting from gas diffusion substrates to solar-powered hydrogen production."

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Solar fuels: Kevin Sivula and his team at the ETH Lausanne have invented a kind of solar-powered artificial leaf to harvest hydrogen from water vapor in the air
Kevin Sivula and his team at the ETH Lausanne have invented a kind of solar-powered artificial leaf to harvest hydrogen from water vapor in the air
(Photo: Alain Herzog/EPFL)

From water to air humidity? Here is the “Sun-to-X” project of the European Union

Kevin Sivula and other research teams have previously shown that artificial photosynthesis can be performed by generating fuel hydrogen from liquid water and sunlight using a device called a photoelectrochemical cell (PEC).
A PEC cell is generally known as a device that uses incident light to stimulate a light-sensitive material, such as a semiconductor, immersed in a liquid solution to cause a chemical reaction.
However, for practical purposes, this process has disadvantages, for example it is difficult to make large PEC devices using liquids.
The ETH Lausanne team wanted to demonstrate that photoelectrochemical cell technology can be adapted to collect moisture from the air, leading to the development of the new gas diffusion electrode.
PEC cells (e.g. fuel cells) have already been shown to work with gases rather than liquids, but the previously used gas diffusion electrodes are opaque and incompatible with solar-powered photoelectrochemical technology.
Now the researchers are focusing their efforts on optimizing the system. What is the ideal fiber size? The ideal pore size? The ideal semiconductors and membrane materials?
These are the questions being asked within the framework of the European Union project “Sun-to-X”, dedicated to the advancement of this technology and the development of new ways to convert hydrogen into liquid fuels.

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Solar fuels: the chemical engineers of the Federal Polytechnic of Lausanne have invented an artificial solar-powered leaf, built on a new transparent and porous electrode, capable of collecting water from the air to convert it into hydrogen; this semiconductor-based technology is scalable and easy to prepare
Chemical engineers from the Federal Institute of Technology in Lausanne have invented a solar-powered artificial leaf, built on a new transparent and porous electrode, capable of collecting water from the air to convert it into hydrogen: this semiconductor-based technology is scalable and easy to prepare
(Photo: EPFL)

The maximum theoretical conversion efficiency is “only” 12 percent, but… promising

To make transparent gas diffusion electrodes, researchers start from a type of glass wool, which is essentially made of quartz fibers (also known as silicon dioxide), and turn it into felt wafers by fusing the fibers together at high temperature.
Subsequently, the wafer is coated with a thin transparent film of fluorine-doped tin oxide, known for its excellent conductivity, robustness, and ease of scalability.
These first steps give life to a transparent, porous and conductive wafer, essential for maximizing contact with the water molecules present in the air and passing photons.
The wafer is then coated again, this time with a thin film of semiconductor materials that absorb sunlight.
This second thin coating still lets light through, but appears opaque due to the large surface area of ​​the porous substrate.
As it is, this coated wafer can already produce fuel hydrogen when exposed to sunlight.
The scientists then built a small chamber containing the coated wafer and a membrane to separate the hydrogen gas produced for measurement.
When the chamber was exposed to sunlight under humid conditions, hydrogen gas was produced, achieving the scientists' goal of demonstrating that the transparent gas diffusion electrode concept for producing hydrogen gas at solar can be achieved.
While the scientists did not formally study the solar-hydrogen conversion efficiency in their demonstration, they acknowledge that it is modest for this prototype and currently lower than that achievable in liquid PEC cells.
Based on the materials used, the theoretical maximum solar-hydrogen conversion efficiency of the coated wafer is 12 percent, while liquid cells have demonstrated an efficiency of up to 19 percent.

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Solar fuels: Kevin Sivula and his team at the ETH Lausanne have invented a kind of solar-powered artificial leaf to harvest hydrogen from water vapor in the air
Kevin Sivula and his team at the ETH in Lausanne have invented a kind of solar-powered artificial leaf to harvest hydrogen from water vapor in the air (Photo: Alain Herzog/EPFL)