RESEARCHERS AT KHALIFA UNIVERSITY ARE ON THE WAY TO DISCOVERING THE BEST PHOTOCATALYST FOR GREEN HYDROGEN PRODUCTION

Prof. Lourdes Vega

Director of the Research and Innovation Center on CO2 and H2 and Professor of Chemical Engineering

Khalifa University

Consumption of fossil fuels for energy purposes increases the amount of greenhouse gasses emitted, having a negative impact on sustainability. As such governments are looking towards green hydrogen which is considered an ideal and sustainable energy carrier able to deliver or store a tremendous amount of energy.

This green hydrogen can be produced, among other methods, from water and/or hydrogen sulfide photocatalysis utilizing solar energy without any greenhouse gas emissions.

Producing green hydrogen efficiently

The challenge is that while hydrogen is abundant on Earth, it is bound in water, hydrocarbons, and other organic matter, making the efficient extraction of that hydrogen difficult. That is why the research carried out by Khalifa University’s

Research and Innovation Center on CO2 and Hydrogen (RICH Center) developed a novel approach for efficient photocatalysts for hydrogen production.

The research entitled, “Identifying the Best Photocatalysts for Green Hydrogen Production Using Computational Screening” with Prof. Lourdes Vega, Director of RICH, Dr. Mutasem Sinnokrot, Dr. Daniel Bahamon, and Ph.D. student Yuting Li published in npj Computational Materials (a Nature journal) identified four promising novel co-catalysts with very good performance to be further explored in experimental studies.

In summary, the RICH team focused on green hydrogen production, using renewable energy sources to split water or hydrogen sulfide, which is abundant in the UAE from natural gas processing. The splitting process used solar energy and semiconductors for photocatalysis and operated at room temperature and ambient pressure, simplifying the equipment needs.

As per Prof Lourdes Vega, low carbon hydrogen production and utilization has been identified by experts and intergovernmental agencies such as the International Energy Agency (IEA) and the International Renewable Energy Agency (IRENA) as one of the key technologies to achieve net zero emissions, because of its potential contributions to clean energy and sustainable fuels.

She adds that “Hydrogen is not new, most of this hydrogen is produced today by methane (the main component of natural gas) and water vapor, in what it is known as the Steam Methane Reforming (SMR) process, generating grey hydrogen. This process is very efficient and it produces high-purity hydrogen, however the problem is that the production of hydrogen from this procedure generates a huge amount of CO2, emitted to the atmosphere, contributing to climate change.”

Some have tried to solve this by producing the same SMR but adding a CO2 capture unit to avoid emissions into the atmosphere, creating blue hydrogen, a low carbon hydrogen, or by water splitting to obtain green hydrogen. Yet the process is very energy demanding, and hence, not as industrially advanced as SMR.

Hydrogen as an energy vector for renewable energy

However, Prof. Lourdes explains, “While Hydrogen might not be a source of energy, it is an energy vector that can be produced, stored, and utilized as a clean fuel or for other applications. Green hydrogen can be stored and used to produce power when needed by using a fuel cell allowing the best use of intermittent renewable energy sources.”

Hydrogen can be used to decarbonize the industrial sector which emits large amounts of CO2. It can also be used as fuel for heavy transportation such as trucks, trains, ships, and airplanes. It can also be used to decarbonize the cement and steel sectors. In all these cases, the use of hydrogen allows replacing fossil fuels avoiding the emissions of CO2, hence decarbonizing these sectors.

Semiconductors utilized in H2O and H2S photocatalysis have attained considerable scientific attention for their potential to use solar energy for the hydrogen evolution reaction. As a result, cadmium sulfide (CdS) has been extensively studied as a visible light-active semiconductor by virtue of its low cost, proper band edges for visible light response, and proton reduction. However, the facile recombination of electron-hole pairs and the poor photo-stability of bare CdS make its wide application in various photocatalysis fields very challenging.

While photocatalyst is still less efficient than electrolysis and is still in the developmental stage, Prof. Lourdes states, “ The efficiency of photocatalyst depends on factors like the type of photocatalyst used, the wavelength and intensity of the light source, and the environmental conditions. Hence, improving efficiency remains a focus of research and development in this field before it can be implemented at a large scale.

Up to now, TiO2 is one of the most widely studied photocatalysts for hydrogen production. It is abundant, stable, and relatively inexpensive. However, it has limitations, such as a wide bandgap, which means it can only utilize ultraviolet (UV) light and relatively low quantum efficiency.

Procedure to screen potential photocatalysts

The researchers at the Research and Innovation Center on CO2 and Hydrogen (RICH Center) at Khalifa University devised a procedure to screen and prioritize potential photocatalysts for water and H2S splitting by calculating some of the key properties in the wish list using computational chemistry and machine learning.

Using advanced computational techniques and machine learning tools, researchers were able to model hypothetical photocatalytic materials and predict their properties. As shown in the study, after the first screening, computational tools allow for predicting in detail the photocatalytic activity of materials based on their electronic structure and surface properties, understanding the charge transfer processes and reaction mechanisms happening in photocatalysts at the atomic and molecular level, all of them relevant to optimize the performance of these materials.

performance the pristine CdS surface.

The results from the study show that most transition metals doped CdS photocatalysts improve the hydrogen evolution reaction (HER) performance compared to the pristine CdS surface (without dopants) by providing additional pathways for the flow of electric charge carriers. From all studied materials researchers were able to rank the performance of the different dopants, with Pt, Rh, and Pd coming out on top.

Moreover, the analysis indicated that dual-dopant materials, such as Co-Pt, Pd-Pt, and Co-Rh, have potentially even better HER performance due to synergistic effects. Compared with the existing experimental work, the study predicted that a new type of catalyst (Co@CdS), not experimentally explored yet, showed promising excellent performance in water and H2S photocatalytic splitting.

Prof. Lourdes believes this is a very important step towards finding the most efficient photocatalysts for hydrogen generation, and researchers were able to fine-tune the best ones.

She explains that this is still far from commercialization, as they have first to be synthesized and tested at the RICH labs, stating: “The good news is that we are on the correct path towards achieving efficient photocatalytic water and H2S splitting.”

She emphasizes, “The process can be sped up with the right resources, investments are always needed. The hope is that the push for producing green hydrogen will accelerate this promising technology while putting in the needed resources.

Future of GCC countries and green hydrogen

In conclusion, the UAE and GCC countries have a large amount of hydrogen sulfide available from natural gas processing, and given that from a thermodynamics point of view, and comparing Gibbs free energies, the direct decomposition of H2S requiring much less energy than the energy required for water splitting, hydrogen sulfide decomposition, is a more favorable route for hydrogen generation. Therefore, converting H2S into hydrogen and sulfur in an efficient manner will not only help to solve the issue of handling this toxic compound but also generate hydrogen in a less energy-demanding manner.