Switzerland – EPFL chemical researchers created a solar-powered artificial leaf based on a unique transparent and porous electrode capable of capturing water from the air and converting it into hydrogen fuel.
For decades, researchers have wished for a gadget that can gather water from the air and produce hydrogen fuel while being totally driven by solar energy. Kevin Sivula, an EPFL chemical engineer, and his team have taken an important step in making this vision a reality. They have created an inventive yet simple system that combines semiconductor-based technology with unique electrodes that have two crucial characteristics: they are porous, allowing for maximum contact with water in the air, and transparent, allowing the semiconductor coating to be exposed to sunlight. Simply exposing the device to sunlight causes it to absorb water from the air and make hydrogen gas.
Plants provide inspiration
The EPFL engineers, working with Toyota Motor Europe, were inspired by the way plants transform sunlight into chemical energy by using carbon dioxide from the air in their research for renewable fossil-free fuels. A plant takes carbon dioxide and water from its surroundings and, with the help of sunlight, converts these molecules into sugars and starches, a process known as photosynthesis. The energy of the sun is stored in the sugars and starches as chemical bonds.
When coated with a light harvesting semiconductor material, the transparent gas diffusion electrodes developed by Sivula and his team operate like an artificial leaf, harvesting water from the air and sunlight to produce hydrogen gas. The energy of the sun is stored in the form of hydrogen bonds.
Instead of using standard layers that are opaque to sunlight to construct electrodes, they use a 3-dimensional mesh of felted glass fibers.
Liquid solution
Sivula and other research groups have already demonstrated that artificial photosynthesis is possible by creating hydrogen fuel from liquid water and sunlight using a device known as a photoelectrochemical (PEC) cell. A PEC cell is a device that uses incident light to excite a photosensitive material, such as a semiconductor, immersed in liquid solution, resulting in a chemical reaction. However, for practical purposes, this approach has drawbacks, such as the difficulty of producing large-area PEC devices that employ liquid.
Sivula intended to demonstrate that PEC technology could be extended to collect humidity from the air instead, which led to the creation of their new gas diffusion electrode. Although electrochemical cells (such as fuel cells) have been demonstrated to work with gases rather than liquids, the gas diffusion electrodes used before are opaque and incompatible with solar-powered PEC technology.
Electrodes for gas diffusion
The researchers begin with a sort of glass wool that is essentially quartz (also known as silicon oxide) fibers and turn it into feeling wafers by fusing the fibers together at high temperatures to create transparent gas diffusion electrodes. The wafer is then covered with a transparent thin coating of fluorine-doped tin oxide, which is known for its outstanding conductivity, resilience, and simplicity of scaling-up.
These initial procedures produce a translucent, porous, and conducting wafer, which is critical for maximizing interaction with water molecules in the air and allowing photons to pass through. The wafer is then coated once more, this time with a thin sheet of semiconductor compounds that absorb sunlight. This second thin covering still allows light through but seems opaque due to the porous substrate’s huge surface area. When exposed to sunlight, this covered wafer can already manufacture hydrogen fuel.
The scientists then constructed a small chamber to house the covered wafer, as well as a membrane to separate the created hydrogen gas for measurement. When their chamber is exposed to sunlight under humid conditions, hydrogen gas is created, proving the scientists’ goal of developing a transparent gas- diffusion electrode for solar-powered hydrogen gas production.
While the scientists did not conduct a thorough analysis of the solar-to-hydrogen conversion efficiency in their presentation, they concede that it is modest for this prototype and is now less than that of liquid-based PEC cells. Based on the materials utilized, the coated wafer’s maximum theoretical solar-to-hydrogen conversion efficiency is 12%, although liquid cells have been shown to be up to 19% efficient.
