Green hydrogen: faster progress with modern X-ray sources
Manganese oxides come in many different structural variants. They are an exciting class of materials for electrocatalysts. © M. Risch/HZB
In order to produce green hydrogen, water can be split up via electrocatalysis, powered by renewable sources such as sun or wind. A review article in the journal Angewandte Chemie Int. Ed. shows how modern X-ray sources such as BESSY II can advance the development of suitable electrocatalysts. In particular, X-ray absorption spectroscopy can be used to determine the active states of catalytically active materials for the oxygen evolution reaction. This is an important contribution to developing efficient catalysts from inexpensive and widely available elements.
Green hydrogen is an energy carrier with a future. It is obtained by electrolytically splitting water with energy from wind or sun and stores this energy in chemical form. To make the splitting of water molecules easier (and to reduce the energy input), the electrodes are coated with catalytically active materials. Dr. Marcel Risch and his Young Investigator Group Oxygen Evolution Mechanism Engineering are investigating oxygen evolution in the electrocatalysis of water. This is because oxygen evolution in particular must run more efficiently for economical hydrogen production.
Exciting class of materials
An exciting class of materials for electrocatalysts are manganese oxides, which occur in many different structural variants. "A decisive criterion for suitability as an electrocatalyst is the oxidation number of the material and how it changes in the course of the reaction," explains Risch. In the case of manganese oxides, there is also a great diversity in possible oxidation states. X-ray absorption spectroscopy (XAS) provides information about the oxidation states: X-ray quanta with suitable energy excite electrons on the innermost shells, which absorb these quanta. Depending on the oxidation number, this absorption can be observed at different excitation energies. Risch's team has constructed an electrolysis cell that enables XAS measurements during electrolysis.
X-ray absorption spectroscopy
"With X-ray absorption spectroscopy, we can not only determine the oxidation numbers, but also observe corrosion processes or phase changes in the material," says Risch. Combined with electrochemical measurements, the measurement data thus provide a much better understanding of the material during electrocatalysis. However, the required high intensity of the X-rays is only available at modern synchrotron light sources. In Berlin, HZB operates BESSY II for this purpose. There are about 50 such light sources for research worldwide.
Time scales from short to long
Risch still sees great potential for the application of X-ray absorption spectroscopy, especially with regard to the time scales of observation. This is because typical measurement times are a few minutes per measurement. Electrocatalytic reactions, however, take place on shorter time scales. "If we could watch electrocatalysis as it happens, we could better understand important details," says Risch. With this knowledge, cheap and environmentally friendly catalysts could be developed more quickly. On the other hand, many "ageing" processes take place within weeks or months. "We could, for example, examine the same sample again and again at regular intervals to understand these processes," Risch advises. This would also make it possible to develop electrocatalysts with long term stability.
- Iridium-free catalysts for acid water electrolysis investigatedHydrogen will play an important role, both as a fuel and as a raw material for industry. However, in order to produce relevant quantities of hydrogen, water electrolysis must become feasible on a multi-gigawatt scale. One bottleneck is the catalysts required, with iridium in particular being an extremely rare element. An international collaboration has therefore investigated iridium-free catalysts for acidic water electrolysis based on the element cobalt. Through investigations with various methods, among them experiments at the LiXEdrom at the BESSY II X-ray source in Berlin, they were able to elucidate processes that take place during water electrolysis in a cobalt-iron-lead oxide material as the anode. The study is published in Nature Energy.
- Lithium-sulphur batteries with lean electrolyte: problem areas clarifiedUsing a non-destructive method, a team at HZB investigated practical lithium-sulphur pouch cells with lean electrolyte for the first time. With operando neutron tomography, they could visualise in real-time how the liquid electrolyte distributes and wets the electrodes across multilayers during charging and discharging. These findings offer valuable insights into the cell failure mechanisms and are helpful to design compact Li-S batteries with a high energy density in formats relevant to industrial applications.
- Self assembling monolayer can improve lead-free perovskite solar cells tooTin perovskite solar cells are not only non-toxic, but also potentially more stable than lead-containing perovskite solar cells. However, they are also significantly less efficient. Now, an international team has succeeded in reducing losses in the lower contact layer of tin perovskite solar cells: The scienstists identified chemical compounds that self-assemble into a molecular layer that fits very well with the lattice structure of tin perovskites. On this monolayer, tin perovskite with excellent optoelectronic quality can be grown, which increases the performance of the solar cell.