New porous electrodes can magic fuel out of thin air

A device that can harvest water from the air and produce hydrogen fuel –entirely powered by solar energy – has been a dream of researchers for decades. Now, Kevin Sivula, a chemical engineer at the Ecole Polytechnique Fédérale de Lausanne…

A device that can harvest water from the air and produce hydrogen fuel –entirely powered by solar energy – has been a dream of researchers for decades. Now, Kevin Sivula, a chemical engineer at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, and his team have made a significant step towards bringing this vision closer to reality.

They have developed an ingenious yet simple device that combines semiconductor-based technology with novel electrodes that possess two key characteristics: they are porous, to maximize contact with water in the air; and transparent, to allow sunlight to illuminate their semiconductor coating. When this device is exposed to sunlight, it takes water from the air and produces hydrogen gas. The researchers report their work in a paper in Advanced Materials. “To realize a sustainable society, we need ways to store renewable energy as chemicals that can be used as fuels and feedstocks in industry,” says Sivula of EPFL’s Laboratory for Molecular Engineering of Optoelectronic Nanomaterials and principal investigator of the study. “Solar energy is the most abundant form of renewable energy, and we are striving to develop economically competitive ways to produce solar fuels.” In their research, the EPFL engineers, in collaboration with colleagues at Toyota Motor Europe, took inspiration from the way plants are able to convert sunlight into chemical energy using carbon dioxide from the air. A plant essentially harvests carbon dioxide and water from its environment, and then with the extra boost of energy provided by sunlight, transforms these molecules into sugars and starches, a process known as photosynthesis. The sunlight’s energy is stored in the form of chemical bonds inside the sugars and starches. When coated with a light-harvesting semiconductor material, the transparent gas-diffusion electrodes developed by Sivula and his team act like an artificial leaf, harvesting water from the air and sunlight to produce hydrogen gas. The sunlight’s energy is stored in the form of hydrogen bonds. Instead of fabricating traditional electrodes with layers that are opaque to sunlight, they utilized a three-dimensional mesh of felted glass fibers. “Developing our prototype device was challenging since transparent gas-diffusion electrodes have not been previously demonstrated, and we had to develop new procedures for each step,” said EPFL’s Marina Caretti, lead author of the paper. “However, since each step is relatively simple and scalable, I think that our approach will open new horizons for a wide range of applications starting from gas-diffusion substrates for solar-driven hydrogen production.” Sivula and other research groups have previously shown that it is possible to perform artificial photosynthesis by generating hydrogen fuel from liquid water and sunlight using a device called a photoelectrochemical (PEC) cell. A PEC cell uses incident light to stimulate a photosensitive material, like a semiconductor, immersed in a liquid solution, thereby inducing a chemical reaction. But for practical purposes, this process has several disadvantages, such as the difficulty of making large-area PEC devices that use liquid. Sivula wanted to show that the PEC technology can be adapted for harvesting humidity from the air, leading to the development of their new gas-diffusion electrodes. Scientists have already shown that electrochemical cells (e.g. fuel cells) can work with gases instead of liquids, but the gas-diffusion electrodes used previously are opaque and incompatible with the solar-powered PEC technology. To make transparent gas-diffusion electrodes, the researchers started with a type of glass wool, which essentially comprises quartz (also known as silicon oxide) fibers, and processed it into a felt wafer by fusing the fibers together at high temperatures. Next, they coated the wafer with a transparent thin film of fluorine-doped tin oxide, known for its excellent conductivity, robustness and ease of scale-up. These first steps produced a transparent, porous and conducting wafer, essential for maximizing contact with the water molecules in the air and letting photons through. They then coated the wafer again, this time with a thin film of sunlight-absorbing semiconductor materials. This second thin coating still lets light through, but appears opaque due to the large surface area of the porous substrate. As is, this coated wafer can already produce hydrogen fuel once exposed to sunlight. The researchers went on to build a small chamber containing the coated wafer, as well as a membrane for separating the produced hydrogen gas for measurement. When their chamber is exposed to sunlight under humid conditions, hydrogen gas is produced, showing that the concept of a transparent gas-diffusion electrode for solar-powered hydrogen gas production can be achieved. While the scientists did not formally study the solar-to-hydrogen conversion efficiency in their demonstration, they acknowledge that the efficiency is modest for this prototype, and currently less than can be achieved in liquid-based PEC cells. Based on the materials used, the maximum theoretical solar-to-hydrogen conversion efficiency of the coated wafer is 12%, whereas liquid cells have been demonstrated up to 19% efficient. The researchers are therefore now focusing their efforts on optimizing the system, by determining the ideal fiber size, the ideal pore size, and the ideal semiconductors and membrane materials. These questions are being pursued in the EU Project ‘Sun-to-X’, which is dedicated to advancing this technology and developing new ways to convert hydrogen into liquid fuels. This story is adapted from material from EPFL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. 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