Friday, June 24, 2022

New aerogel could produce 70 times more hydrogen than other methods

Hydrogen generation using abundant solar energy together with semiconductor photocatalysts holds significant potential to produce clean and sustainable energy carriers.

ETH Zurich researchers have developed a new photocatalyst made from an aerogel that could enable more efficient hydrogen production. The aerogel increases the efficiency of converting light into hydrogen energy, producing up to 70 times more hydrogen than rival methods.

Aerogels are extraordinary materials that have set Guinness World Records more than a dozen times, including the honorary position of becoming one of the world’s lightest solids. Professor Markus Niederberger from the Laboratory for Multifunctional Materials at ETH Zurich has been working with these special materials for some time.

Aerogels based on nanoparticles can be used as photocatalysts. These are employed whenever a chemical reaction needs to be enabled or accelerated with the aid of sunlight to produce extremely useful products in the modern world, including hydrogen.

The material of choice for photocatalysts is titanium dioxide (TiO2), which is also a semiconductor. But TiO2 has a major disadvantage – it can absorb only the UV spectrum of sunlight, which is just about 5% of the total shine of the sun. If photocatalysis is to be efficient and industrially useful, the catalyst must be able to utilize a broader range of wavelengths.

Niederberger’s doctoral student, Junggou Kwon, has been looking for a new and alternative way to optimize an aerogel made of TiO2 nanoparticles. She discovered that if the TiO2 nanoparticle aerogel is “doped” with nitrogen, such that nitrogen atoms replace individual oxygen atoms in the material, the aerogel can then absorb further visible portions of the sun’s spectrum. The doping process leaves the aerogel’s porous structure intact.

At first, Kwon produced the aerogel using TiO2 nanoparticles and small amounts of the noble metal palladium, which plays a critical role in the photocatalytic production of hydrogen. She then placed the aerogel in a reactor and infused it with ammonia gas. This caused individual nitrogen atoms to embed themselves in the crystal structure of the TiO2 nanoparticles.

In order to test whether an aerogel modified in this way actually increases the efficiency of the desired chemical reaction – in this case, the production of hydrogen from methanol and water – Kwon developed a special reactor into which she directly placed the aerogel monolith. She then introduced water vapor and methanol to the aerogel in the reactor before irradiating it with two LED lights.

Kwon stopped the experiment after five days, but up to that point, the reaction was stable and proceeded continuously in the test system. “The process would probably have been stable for longer,” Niederberger says. “Especially with regard to industrial applications, it’s important for it to be stable for as long as possible.” The researchers were satisfied with the reaction’s results as well. Adding the noble metal palladium significantly increased the conversion efficiency: using aerogels with palladium produced up to 70 times more hydrogen than using those without.

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