Producing Hydrogen Fuel from Air and Sunlight

by Ed Burke and Kelly Burke, Dennis K. Burke Inc.

New technology offers a preview of scalable, storable solar energy

Advancements in green hydrogen seem to be jumping ahead by leaps and bounds.

It seems far-fetched, but scientists have recently created a device that, when exposed to sunlight, takes water from the air and generates hydrogen gas. It’s a big step towards bringing this concept closer to reality.

In collaboration with Toyota Motor Europe, EPFL chemical engineer Kevin Sivula and his team have developed a semiconductor-based technology with new electrodes that are both porous to maximize contact with water in the air, and transparent to maximize sunlight exposure of the semiconductor coating.

What’s New?

It’s their innovative gas diffusion electrodes, which are transparent, porous, and conductive, enabling this solar-powered technology for turning water (in its gaseous state from the air) into hydrogen fuel.

“To realize a sustainable society, we need ways to store renewable energy as chemicals that can be used as fuels and feedstocks in industry. Solar energy is the most abundant form of renewable energy, and we are striving to develop economically competitive ways to produce solar fuels,” says Sivula of EPFL’s Laboratory for Molecular Engineering of Optoelectronic Nanomaterials and principal investigator of the study.

How It Works

In their research, the EPFL engineers looked at the way plants convert sunlight into chemical energy using carbon dioxide from the air. A plant essentially harvests carbon dioxide and water from its environment along with the energy from sunlight and transforms these molecules into sugars and starches in a process known as photosynthesis. The sunlight’s energy is stored in the form of chemical bonds inside of the sugars and starches.

The transparent gas diffusion electrodes developed by Sivula and his team, when coated with a light-harvesting semiconductor material, act like an artificial leaf, collecting water from the air and sunlight to produce hydrogen gas. The sunlight’s energy 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 3-dimensional mesh of felted glass fibers.

Marina Caretti, lead author of the work, says, “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. 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.”

Harvesting Water From Air

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 is generally a device that uses light to stimulate a photosensitive material like a semiconductor, which is immersed in a liquid solution to cause a chemical reaction. But there are issues with using this process. For practical purposes, it’s complicated to make large-area PEC devices that use liquid.

Instead, Sivula wanted to show that the PEC technology could be adapted for collecting water from the air. That led to the development of their new gas diffusion electrode.

Electrochemical cells (fuel cells) have already been shown to work with gases instead of liquids, but the gas diffusion electrodes used previously are opaque and incompatible with the solar-powered PEC technology. The researchers are now focusing their efforts on optimizing the system.

Transparent Gas-Diffusion Electrodes

In order to make transparent gas diffusion electrodes, the researchers start with a type of glass wool made of quartz fibers and then process it into felt wafers by fusing the fibers together at high temperature.

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

These first steps result in a transparent, porous, conductive wafer that maximizes contact with the water molecules in the air and lets photons through.

The wafer is then coated again, this time with a thin film of sunlight-absorbing semiconductor materials. This coating still lets light through the porous substrate. Once exposed to sunlight, the coated wafer produces hydrogen fuel.

The team then built 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, achieving what the scientists set out to do: showing that the concept of a transparent gas-diffusion electrode for solar-powered hydrogen gas production can be achieved.

While the scientists did not study the solar-to-hydrogen conversion efficiency in their demonstration, they acknowledge that it is modest for this prototype and currently less than what can be achieved in liquid-based PEC cells.

Is Green Hydrogen Competitive?

Green hydrogen is produced by using renewable energy to power the electrolysis of water. The high  cost of production  is the main factor behind the low use of green hydrogen. Nonetheless, the  United States Department of Energy  forecasts that the hydrogen market is expected to grow with the cost of hydrogen production falling significantly. The lower price should make green hydrogen competitive against other fuel sources.

Green hydrogen could be a more economical means of long-term renewable energy storage in terms of capital expenditures  than pumped-storage hydroelectricity or batteries.

Ed and Kelly Burke are respectively Chairman of the Board and Senior Marketing Manager at fuel distributor Dennis K. Burke Inc. They can be reached at 617-884-7800 or ed.burke@burkeoil.com and kelly.burke@burkeoil.com.

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