Green hydrogen from the air

Hydrogen produced in a climate-friendly manner is intended to replace oil as the universal fuel for industry. But in areas where the sun shines a lot, there is usually little water. What to do?

There is 12.9 trillion tons of water in the air – you just have to make it available.

Hydrogen is regarded as the beacon of hope for the energy transition – it is to be used as a fuel for aircraft, ships, and trucks, as a raw material for industry, or as a fuel for heating. If it is produced from water using renewable energy, the production is even almost climate-neutral. However, there is often not enough water for electrolysis in particularly sunny or windy regions. A research group led by chemical engineer Gang Kevin Li from the University of Melbourne has therefore developed a prototype that can absorb moist air, extract the water, and split it directly into the two gases oxygen and hydrogen. She presents the principle in the current issue of »Nature Communications«.

In order to reduce carbon dioxide emissions in industry, hydrogen (H₂) is extremely important. If it is burned, only water is produced. One day, hydrogen is set to replace coal in steel production, which is particularly energy-intensive. Certain processes in the chemical industry can only be made climate-friendly with green hydrogen. And gas-fired power plants are also to be operated with H ₂ in the future. Hydrogen is the most common chemical element in the universe. On earth, however, it occurs almost exclusively bound in the form of water.

Existing large-scale electrolysis plants often require complex material components, rare metals, and access to pure fresh water. This can lead to competition for drinking water supplies, which are already limited in some parts of the world. These factors drive up the cost of hydrogen production and limit its widespread use to date.

Gang Kevin Li and his colleagues want to circumvent this problem and access water supplies that can be found in even the driest regions of the world, such as the Sahara desert or the Australian outback. “At any point in time, there are 12.9 trillion tons of water in the air,” write the scientists, “universally available and inexhaustible.” All that is needed is efficient systems to be able to access it. They have now shown that this is possible. Hydrogen can still be generated down to a minimum relative humidity of four percent with this so-called »direct air electrolysis« (DAE).

The heart of their electrolytic cell is a porous substance soaked in sulfuric acid, which serves as an electrolyte on the one hand and removes moisture from the ambient air on the other. The researchers used platinum as the electrode material. They operated their module with solar energy and tested it for twelve days at a time for eight hours each. They caught the hydrogen and let the oxygen escape. They came up with an average production volume of 93 liters of pure hydrogen per hour and a square meter of cathode material.

While the authors write that their electrolyzers are scalable, they do not comment on the costs or the potential environmental impact of their technology.