- How catalysts become more active
Jülich–A layer as thin as a single atom makes a huge difference: On the surface of an electrode it doubles the amount of water that is split in an electrolysis system – without increasing the energy requirement.
The ultra-thin layer also doubles the amount of hydrogen produced without increasing costs. This is what researchers from Jülich, Aachen, Stanford and Berkeley report in the current issue of the journal “Nature Materials”.
“By examining a model material, we have succeeded in gaining a more detailed understanding of how the properties of a catalytically active electrode result from its structure,” says Christoph Bäumer, first author of the study. Funded by a “Global Fellowship” from the Marie Skłodowska-Curie Measures, he advanced research in Jülich and Aachen as well as in the USA. The materials scientist continues: “From this expanded understanding, we hope that better catalysts can be developed in the future that produce green hydrogen in a more energy-efficient and thus more cost-effective manner than before.”
The colorless hydrogen gas is referred to as green if it is obtained through the electrolysis of water in a climate-neutral manner with electricity from renewable sources. Hydrogen is an essential component of the energy transition, among other things because it can store wind and solar energy in times of oversupply and release it again later. The electrolytic hydrogen production at the negatively charged electrode (cathode) cannot take place without the development of oxygen at the positively charged electrode (anode). Catalysts that promote this development of oxygen also make the overall process more energy-efficient. The high energy demand has so far been one of the main hurdles for the widespread use of hydrogen.
Lanthanum nickelate with the empirical formula LaNiO3, belonging to the perovskite material class, is such a catalyst. Nickel oxide and lanthanum oxide layers alternate in its crystal structure. “We produced lanthanum nickelate catalysts more precisely and examined them more closely than other scientists before,” says Felix Gunkel from the Peter Grünberg Institute, who headed research activities in Jülich. The researchers produced two different types of high-purity LaNiO3 crystals: In one type, the crystals end on a surface that only contains lanthanum and oxygen atoms. Experts speak of a lanthanum termination. In the other type, nickel and oxygen atoms form the surface (nickel termination).
It was found that a nickel-terminated anode produces twice as much oxygen in the same time as a lanthanum-terminated electrode of the same size. “Surprisingly, a single layer of nickel and oxygen atoms is responsible for a very considerable increase in the catalytic activity of the material,” says Bäumer. The team of scientists was also able to find a reason for this: During the electrolysis, a disordered, catalytically very active layer of nickel dioxide is formed on the nickel-terminated crystal, which cannot form with lanthanum termination. Compared to the original structure, this newly formed nickel oxide layer has ideal bonding states between nickel ions and oxygen or hydroxide ions, which increases the activity. “Our results indicate
In addition, the research results show the levers for setting this termination in perovskite materials: One of them is the temperature at which they are manufactured. With lanthanum nickel perovskite, high temperatures favor lanthanum termination, the scientists found. “In order to produce nickel-terminated crystals, we used a process with which an atomically thin layer of nickel atoms can be applied to the surface of a lanthanum-terminated crystal,” explains Bäumer.
The researchers were only able to understand their results because, for the first time in electrocatalyst research, they used a method in which the composition of the crystal surface is analyzed using standing waves of synchrotron X-rays. These standing waves can be generated by interference between the incoming and outgoing X-rays. The prerequisite for this was the atomically precise production of a LaNiO3 X-ray mirror from 40 alternating layers on which the researchers then applied the layer to be examined. The scientists used the Advanced Light Source in Berkeley, USA, as the X-ray source.
Original publication : ‘Tuning electrochemially driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis’
C. Baeumer, J. Li, Q. Lu, A. Liang, L. Jin, H. Martins, T. Duchoň, M. Glöß , SM Gericke, MA Wohlgemuth, M. Giesen, EE Penn, R. Dittmann, F. Gunkel, R. Waser, M. Bajdich, S. Nemšák, JT Mefford, WC Chueh
Nature Materials, 11 January 2021, DOI: 10.1038 / s41563-020-00877-1