Swiss scientists create device that makes green fuel from water vapor and sunshine.

Researchers from the Swiss Federal Institute of Technology Lausanne have created a device that can harvest water from the atmosphere and turn it into hydrogen fuel – a process powered entirely with solar energy. While inspired by the natural mechanisms of photosynthesis, the scientists devised something more akin to an electrochemical sponge.

Such a device that can extract water from the air and provide hydrogen fuel through the most abundant source of renewable energy, the Sun, has been a dream of researchers for decades. Now, EPFL chemical engineer Kevin Sivula and his team have made a significant step toward bringing this vision closer to reality.

Their leaf-inspired hydrogen fuel generator is an ingenious yet simple system that combines semiconductor-based technology with novel electrodes that are porous to maximize contact with water in the air and transparent to maximize sunlight exposure of the semiconductor coating.

Powered by sunlight, this device draws in moisture from the air and produces hydrogen gas, much like the leaves on a tree.

"To realize a sustainable society, we need ways to store renewable energy as chemicals that can be used as fuels and feedstocks in industry," said Sivula. "Solar energy is the most abundant form of renewable energy, and we are striving to develop economically-competitive ways to produce solar fuels."

Inspiration from nature

In their research for renewable fossil-free fuels, EPFL engineers collaborated with Toyota Motor Europe and took inspiration from the way plants are able to convert sunlight into chemical energy using carbon dioxide from the air.

Essentially, all plants absorb carbon dioxide, water, and sunlight from the atmosphere through a process called photosynthesis.

While many have learned of this process in school, at the peak of academia, this simple yet complex mechanism is something that researchers still struggle to replicate into useful means – useful for humans, at least.

Converting carbon dioxide, water, and sunlight into sugars and starches is neat, but for human needs, researchers are looking for a method by which we can use the same process to gather and store energy in more economical ways.

Thus, the transparent gas diffusion electrodes developed by Sivula and his team, when coated with a light-harvesting semiconductive material, acts like an artificial leaf that uses the energy from the sun to transform water absorbed from the air to produce hydrogen.

What is truly extraordinary is that instead of building electrodes with traditional layers that typically end up opaque to sunlight, their substrate is actually a 3-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 Marina Caretti, lead author of the work published in the journal Advanced Materials. "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."

Swiss artificial leaf

EPFL is not the first to explore creating hydrogen fuel using artificial photosynthesis.

Work being done at the University of Cambridge in the United Kingdom has also devised a method, but for more targeted work, to eventually replace the high energy costs in the production of gasoline or petrol, as it is called over the pond, through its interpretation of an artificial leaf.

You can read about the University of Cambridge's artificial leaf at An artificial leaf to fuel world trade in the August 24, 2022 edition of Metal Tech News.

Both leaf-inspired hydrogen-producing devices are based on photoelectrochemical (PEC) cell technology.

A PEC cell is generally known as a device that uses incident light to stimulate a photosensitive material, such as a semiconductor, immersed in liquid solution to cause a chemical reaction.

While both research teams started with the same premise, Cambridge decided to make their leaves more marine-based – because where can one find enough water if not the ocean – and EPFL wanted to show that their PEC technology could be adapted for harvesting humidity from the air instead, leading to the development of their new gas diffusion electrode.

Already proven to work with gases instead of liquids, electrochemical cells in previous attempts were opaque, inefficient and generally incompatible when paired with solar-powered PEC technology.

How was it done?

In order to make transparent gas diffusion electrodes, the EPFL researchers started with a type of glass wool, which is essentially quartz or silicon oxide fibers and processed it into felt wafers by fusing the fibers together at high temperatures.

Next, the wafer is coated with a transparent thin film of fluorine-doped tin oxide, which is known for its excellent conductivity, robustness, and ease to scale-up.

These initial steps result in a transparent, porous, and conducting wafer, essential for maximizing contact with the water molecules in the air as well as letting photons pass through.

Next, the wafer is coated again, this time with a thin film of sunlight-absorbing semiconductive 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 could already begin producing hydrogen fuel once exposed to sunlight; however, the scientists wanted to create an environment to fully measure their device's capabilities.

Building a small chamber containing the coated wafer, as well as a membrane for separating the produced hydrogen gas for measurement, when the chamber was exposed to sunlight under humid conditions, hydrogen gas was indeed produced, achieving what the team had set out to do – their concept of a transparent gas-diffusion electrode for solar-powered hydrogen gas was successful.

While the team did not formally study the solar-to-hydrogen conversion efficiency in their demonstration, it was acknowledged that it was fairly modest for a prototype but still 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 was 12%, whereas liquid cells have been demonstrated to be up to 19% efficient. Nevertheless, not having all the eggs in one basket provides a competitive venture to perhaps discover the most efficient way to produce the fuel of the future.

Now, the researchers are focusing their efforts on optimizing the system. Asking questions such as, what is the ideal fiber size? The ideal pore size? The ideal semiconductors and membrane materials?

Now that a successful working prototype has proven the technology feasible, it's only a matter of time before these questions find their answers.