2024-04-09 00:17:57
Lithuanian scientists are also trying to bring this future closer, looking for new, ecological ways to extract hydrogen.
Currently, most of the hydrogen in the world is produced industrially – steam reforming – using fossil fuel, i.e. natural gas. Ecological problems arise here, as greenhouse gases are released into the environment during the process.
In order to reduce climate warming, Europe and the whole world are taking so-called decarbonization measures – the aim is to expand and develop technologies for the use of renewable energy sources, which would allow us to abandon fossil fuels.
The European Green Deal (a strategy developed by the European Commission) envisages a major expansion of green hydrogen production across Europe. It is aimed that by 2050 net greenhouse gas emissions would be zero and economic growth would be decoupled from the use of natural resources. So, energy, transportation, and other industries will see huge changes as we will have to phase out fossil fuels completely in less than 30 years.
Therefore, special attention is paid to the development of these technologies in order to increase their efficiency and reduce the price.
“Green hydrogen is one of the most promising forms of renewable energy storage,” says Dr. Milda Petrulevičienė from the Department of Chemical Engineering and Technology of FTMC. She and her colleagues are researching photoelectrochemical processes, in which light energy is converted into chemical energy, and the application of such technologies to the production of green hydrogen and other valuable chemical compounds.
The most ecological hydrogen
First, what is “green hydrogen”?
According to the methods of obtaining, hydrogen is divided into gray, blue, green and geological, also called white or golden. Gray hydrogen, as mentioned, is produced from methane gas in the reforming process, when carbon dioxide – CO is released into the environment2. Blue hydrogen is produced in the same way as gray hydrogen, but with CO released during production2 is captured using dedicated filters and does not enter the environment. At that time, green hydrogen is the one that is produced from renewable energy sources, while white (golden) hydrogen is extracted from the depths of the Earth. It is hydrogen gas that forms in the Earth’s mantle and is released to the surface due to various geological processes.
Of all these types, the most technologically demanding is green hydrogen, the production of which does not pollute the environment at all. It is obtained by electrolysis of water, that is, when water is split into hydrogen and oxygen using an electric current. The electricity required for this process is produced using solar or wind energy, which are inexhaustible and renewable energy sources.
According to chemist M. Petrulevičienė, water electrolysis requires electrolyzers – devices whose main component is electrodes of different poles (anode and cathode); water splitting reactions take place on them.
Industrial green hydrogen plants are already operating in the US and Europe. However, for now, this method of producing H2 is still more expensive than producing gray hydrogen. Thus, ways are being sought to increase the efficiency of the technology and reduce the cost of the final product.
The produced hydrogen can be used in various fields. One possibility is fuel cells, which can convert energy stored in the form of hydrogen into electricity for transport, heating, etc. Another important area is the chemical industry, which widely uses hydrogen as a raw material.
Searches for alternative technologies
The Chemical Engineering and Technology Department of FTMC operates the Energy Electrochemical Conversion Laboratory – where, among other research, raw hydrogen and other valuable compounds are extracted. Lithuanian chemists are looking for technologies and researching them.
Here, hydrogen is produced by a photoelectrochemical process, also known as artificial photosynthesis. As many of us know, normal photosynthesis is this: plants take in CO from the environment2 and water and absorb sunlight, thereby producing the carbohydrates they need and releasing oxygen. Something similar happens in the FTMC laboratory: from one chemical substance we get another, in this case – light affects the electrodes (made of semiconductor materials), and water splitting and other reactions take place on their surface.
This technology differs from conventional water electrolysis in that, in the photoelectrochemical system, light interacts directly with electrode materials. In other words, solar cells are no longer needed here – electricity is obtained from the sun, “without intermediaries”; in this case, the electrodes act like solar cells because they convert light into electricity, which in turn causes electrochemical reactions. This is the goal set by scientists – and for now, everything happens in laboratories, where sunlight is replaced by special simulators.
If you visit the FTMC Energy Electrochemical Conversion Laboratory, you will see an interesting device – a “mini-reactor” that produces green hydrogen. The device that fits on the table consists of a two-part cell (equivalent to an electrolyzer) and a solar simulator, the light emitted by which corresponds to the spectrum and intensity of natural sunlight.
How does it all work? First, a cell is constructed, where the electrodes synthesized and manufactured by the scientists themselves are installed; the cell is filled with an aqueous electrolyte (a salt solution to increase electrical conductivity), connected to a potentiostat (a device that allows you to monitor the current flowing through the system and other parameters), and the hydrogen sensor is turned on, and the test begins. As long as there is no light, the current flowing in the system is very weak – a few microamperes. At the moment when the light source is turned on, the current increases hundreds of times, because electrochemical reactions begin to take place on the surface of the illuminated electrode.
Hydrogen is formed on one of the electrodes, the cathode. Its amount is measured by a special sensor. In parallel, the reactions that take place on another electrode – the photoanode – which is illuminated by the “artificial sun” in the aforementioned “mini reactor” are being studied.
Recently, M. Petrulevičienė, together with her colleagues, mainly works with tungsten oxide, bismuth vanadate photoanodes and their heterojunctions. The FTMC scientist examines the processes that take place on the surface of the photoanode, changes the system parameters, test conditions, etc., in order to improve the system and increase its performance.
The laboratory also conducts research on the photoelectrochemical decomposition of seawater, which will be useful in another area – no longer energy. “Since seawater contains a lot of dissolved salt sodium chloride, hypochlorite, a strong oxidizer that is used for disinfection, can be produced on the photoanode in this way.
Thus, artificial photosynthesis technology could provide a double benefit – the production of strong oxidants and hydrogen, at the same time using practically inexhaustible resources – sea water and sunlight,” says chemist Milda.
Photoelectrochemical disinfection experiments were performed by M. Petrulevičienė together with scientists from the Life Sciences Center of Vilnius University.
“The research was successful, strong oxidants (hypochlorite, persulfate) were formed, which damage the walls of the bacteria in the water, and they die. We can imagine the use of such technology in warm countries, for example, swimming pool tiles are covered with materials created by us, the sun shines, and the production of strong oxidants and self-disinfection takes place on the surface of the tiles,” says the FTMC scientist. She points out that such research is already being developed in the world, when surfaces, door handles, etc. are coated with special substances, which, in the presence of light, have an antimicrobial effect.
Destroy what is not needed and combine what is needed
Another important and promising area of application of the technology is the decomposition of organic pollutants.
“One example would be pharmaceutical factories that recycle, incinerate or otherwise manage their waste. However, some pharmaceutical compounds (hormonal drugs, antibiotics, etc.) still end up in open waters, harming wildlife.
Our developed photoelectrochemical systems help to break down pharmaceutical pollutants into less harmful derivatives or simply CO2 and water. At the same time, hydrogen can be released on the cathode, which can also help factories, for example, to have their own energy source”, the interviewer thinks.
According to scientist Milda, she and her colleagues are currently improving this technology and looking for solutions to increase its efficiency:
“In our experiments, we estimate the yields of product formation according to the photocurrent, that is, we calculate the ratio of the experimentally determined and theoretically estimated amount of products. We can estimate the losses of energy conversion processes, how much energy has been lost. Disinfecting oxidants such as hypochlorite and persulfate have 85-95 percent stream yield. At that time, in our research, the degradation efficiency of some compounds even reaches 100%.
It is important to mention that it depends not only on the nature of these compounds, the structure of molecules, but also on the salts dissolved in the electrolyte. So we are working intensively in this area to find out the factors that affect the degradation efficiencies and what system parameters are needed for the compounds to be finally degraded.”
M. Petrulevičienė and her colleagues plan to develop a new topic – the “reverse” method, when organic compounds are not decomposed, but synthesized and created. Only this time, those compounds would be useful, applied in various fields, such as the chemical industry or pharmaceuticals.
2024-04-09 00:17:57