Nekoreguojami

Nekoreguojami

Nekoreguojami

2024. 04. 11 -

Green hydrogen and solar energy: Lithuanian scientists pave the way for future technologies

Dr. Milda Petrulevičienė. Photo: FTMC

Hydrogen is a chemical element essential for life, and is a component of organic compounds and water. Under natural conditions, hydrogen is a gas that exists as a diatomic molecule. Importantly, it is seen as the fuel of the future, which is expected to partially replace petrol and diesel.

Lithuanian scientists are trying to bring this future closer by finding new, environmentally friendly ways to extract hydrogen.

Currently, most of the world's hydrogen is produced industrially by steam reforming using fossil fuels, i.e. natural gas. This poses environmental problems as the process releases greenhouse gases into the environment.

To reduce climate warming, Europe and the rest of the world are taking steps to decarbonise by developing and deploying renewable energy technologies to move away from fossil fuels.

The European Green Deal (a strategy developed by the European Commission) foresees a major expansion of green hydrogen production across Europe. The aim is to achieve zero net greenhouse gas emissions and decouple economic growth from the use of natural resources by 2050. So energy, transport and other industries will face huge changes as we move away from fossil fuels in less than 30 years.

This is why particular attention is being paid to developing these technologies to increase their efficiency and reduce their cost.

"Green hydrogen is one of the most potential renewable energy storage options," says Dr. Milda Petrulevičienė from the FTMC Department of Chemical Engineering and Technology. She and her colleagues are studying photoelectrochemical processes that convert light energy into chemical energy, and the application of such technologies to the production of green hydrogen and other valuable chemical compounds.

(When will hydrogen become a widely available fuel? Scientists are working to bring that day closer. Photo: Unsplash.com)

The greenest hydrogen

First of all, what is green hydrogen?

Hydrogen is divided into grey, blue, green and geological hydrogen, also known as white or gold. Grey hydrogen, as mentioned above, is produced from methane gas by the process of reforming, which releases carbon dioxide - CO₂. Blue hydrogen is produced in the same way as grey hydrogen, but the CO₂ released during production is captured by dedicated filters and is not released into the environment. Green hydrogen is produced using renewable energy sources, while white (gold) hydrogen is extracted from the Earth's interior. This is hydrogen gas that is produced in the Earth's mantle and is released to the surface through various geological processes.

The most technologically demanding of these is green hydrogen, the production of which is completely pollution-free. It is produced by electrolysis of water, which involves the splitting of water into hydrogen and oxygen using an electric current. The electricity needed for this process is produced using solar or wind power, which are inexhaustible and renewable sources of energy.

According to chemist M. Petrulevičienė, electrolysis of water requires electrolysers - devices whose main component is electrodes of different poles (anode and cathode), on which the water splitting reactions take place.

Industrial green hydrogen plants are already operating in the US and Europe. However, this method of producing H₂ is still more expensive than producing grey hydrogen. So ways are being sought to increase the efficiency of the technology and reduce the cost of the final product.

The hydrogen produced can be used in a variety of applications. One possibility is fuel cells, which can convert the energy stored as hydrogen into electricity for transport, heating etc. Another important area is the chemical industry, which makes extensive use of hydrogen as a raw material.

(Dr. Milda Petrulevičienė. Photo: FTMC)

Search for alternative technologies

The Department of Chemical Engineering and Technology has an Electrochemical Energy Conversion Laboratory - where, among other research, green hydrogen and other valuable compounds are extracted. Lithuanian chemists are looking for and researching technologies for this.

Here, hydrogen is produced by photoelectrochemical processes known as artificial photosynthesis. As many of us know, normal photosynthesis involves plants taking CO₂ and water from the environment and absorbing sunlight to make the carbohydrates they need and releasing oxygen. Something similar happens in the FTMC laboratory: from one chemical we get another, in this case light strikes the electrodes (made of semiconductor materials), water is splitting, and other reactions take place on the surface.

This technology differs from conventional water electrolysis in that in a photoelectrochemical system, light interacts directly with the electrode materials. In other words, there is no need for solar cells - the electricity comes from the sun, "without intermediaries"; in this case, the electrodes act as solar cells, as they convert light into electricity, which in turn triggers electrochemical reactions. This is the goal of the researchers - and for now, everything is done in the laboratory, where sunlight is replaced by special simulators.

If you visit the Electrochemical Energy Conversion Laboratory at FTMC, you'll see an interesting thing - a "mini-reactor" that produces green hydrogen. The table-top device consists of a two-part cell (the equivalent of an electrolyser) and a solar simulator, whose light emission matches the spectrum and intensity of natural sunlight.

(A "mini reactor" is a solar simulator and cell for extracting hydrogen and other chemical compounds. Photo by FTMC)

How does it all work? First, a cell is constructed with electrodes synthesised and manufactured by the researchers themselves, then the cell is filled with aqueous electrolyte (a salt solution designed to increase electrical conductivity), connected to a potentiostat (a device that allows you to monitor the current flowing through the system, amongst other parameters), a hydrogen sensor is switched on and the experiment begins. As long as there is no illumination, the electric current flowing through the system is very weak, in the order of a few microamperes. At the moment the light source is switched on, the current increases hundreds of times as electrochemical reactions start to take place on the surface of the illuminated electrode.

Hydrogen is formed on one of the electrodes, the cathode. The hydrogen content is measured by a special sensor. In parallel, the reactions that take place on the other electrode, the photoanode, which is illuminated by the "artificial sun" in the "mini-reactor" are studied.

Recently, M. Petrulevičienė and her colleagues have been working mainly on tungsten oxide and bismuth vanadate photoanodes and their heterosanders. At FTMC, she is studying the processes that occur on the surface of the photoanode, modifying the system parameters, test conditions, etc., in order to refine the system and increase its performance.

The laboratory also carries out studies on the photoelectrochemical splitting of seawater, which will be useful in another field - not only energy. "As seawater contains a large amount of the dissolved salt sodium chloride, it is possible to produce hypochlorite, a strong oxidising agent used for disinfection, on a photoanode.

So artificial photosynthesis technology could have the dual benefit of producing strong oxidants and hydrogen, while at the same time making use of the virtually inexhaustible resources of seawater and sunlight," says chemist Milda.

M. Petrulevičienė carried out the photoelectrochemical disinfection experiments together with scientists from the Vilnius University Life Sciences Center.

"The tests were successful, producing strong oxidants (hypochlorite, persulphate) that damage the cell walls of the bacteria in the water and kill them. We can imagine the use of this technology in hot countries, for example, where swimming pool tiles are coated with our materials, the sun shines and the production of strong oxidants on the surface of the tiles, self-disinfecting, takes place," says the FTMC researcher. She points out that research is already being developed around the world to coat surfaces, door handles, etc., with special materials that have an antimicrobial effect when exposed to light.

(Photo: Unsplash.com)

Degradate what you don't need and merge what you do

Another important and promising application of the technology is the degradation of organic pollutants.

"One example would be pharmaceutical factories that recycle, incinerate or otherwise manage their waste. However, some of the pharmaceutical compounds (hormonal drugs, antibiotics, etc.) still end up in open water, to the detriment of wildlife.

Our photoelectrochemical systems help to break down pharmaceutical pollutants to less harmful derivatives or simply to CO₂ and water by means of strong oxidants produced by light. At the same time, hydrogen can be emitted on the cathode, which can also help factories, for example, to have their own energy source," M. Petrulevičienė reflects.

She and her colleagues are currently developing the technology and looking for solutions to increase its efficiency, says Milda:

"In our experiments, we estimate the yields of product formation in terms of photocurrent, i.e. we calculate the ratio between the experimentally determined and the theoretically estimated product yields. We can estimate the energy losses in the energy conversion processes, how much energy has been lost. Disinfectant oxidants such as hypochlorite and persulphate have yields of 85-95 % on a photocurrent basis. In our studies, the degradation efficiency of some compounds is as high as 100 %.

It is important to note that this depends not only on the nature and molecular structure of these compounds, but also on the salts dissolved in the electrolyte. So we are working hard in this area to understand the factors that influence the degradation efficiencies and what system parameters are needed to achieve the final degradation of the compounds."

M. Petrulevičienė and her colleagues are planning to develop a new topic - the "reverse" method, where organic compounds are not degraded, but synthesised and created. Only this time, these compounds would be useful, applicable in various fields such as the chemical industry or pharmaceuticals.

Written by Simonas Bendžius

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