Exsolved Nanocatalysts for Efficient and Robust Green Hydrogen Production

A significant decrease in greenhouse gas emissions is needed to reduce the consequences of climate change and slow down global warming. Therefore, a global strategy has been to increase the share of renewable energy through solar and wind power. However, their intermittency makes it difficult to predict their power output, and it is, therefore, difficult to match the energy production to the current energy demand. Therefore, large-scale and long-term energy storage solutions are needed to overcome this energy production and demand mismatch. Hydrogen can be used as a storage medium and has consequently been declared an essential strategy by policymakers globally. The International Energy Agency projects a rising demand for hydrogen produced by water splitting with an estimated electrolyzer capacity of 850 GW by 2030.

Amongst the different electrolysis technologies, alkaline electrolysis is considered the most mature technology. However, efficiency is still considered the biggest challenge, as the overall cell efficiency is dominated by the sluggish kinetics of the oxygen evolution reaction. Therefore, research has focused on finding highly active catalysts for the oxygen evolution reaction to minimize its overpotential losses. The catalyst’s stability is often neglected when searching for highly active materials. Therefore, this work focused on bridging the gap between catalyst research and validation under industrial operating conditions.

The work carried out in the context of this thesis can be divided into three parts: In the first part, a 3D-printed polyether ether ketone (PEEK) holder for catalyst and electrode testing was developed. The holder allows for testing of electrodes under more realistic conditions (zero-gap) than typical beaker-type setups. NiOOH was tested in Fe-free electrolyte to verify the reproducibility of the test setup. Furthermore, the test setup was used during long-term stability testing at 60°C for more than 250h applying a fixed current density of 500 mA cm⁻² , showcasing its versatility.

In the second part, La-based perovskites are tested as oxygen evolution reaction catalysts under industrial conditions. In the first step, La_0.75Sr_0.25Mn_O3–δ (LSMO), LaFeO_3–δ (LFO), (La_0.6Sr_0.4)_0.98FeO_3–δ (LSFO), La_0.6Sr_0.4Fe_0.8Co_0.2O_3–δ (LSCFO) and (La_0.6Sr_0.4)_0.99CoO_3–δ (LSCO) are all tested in an accelerated corrosion test to identify chemically stable compositions. Afterward, LSMO, LFO, LSFO, and LSCFO were electrochemically tested at industrial conditions. The most stable and active compositions, LFO and LSCFO with a low overpotential of 300mV at 10mA cm⁻² at 75 °C and 50 bar in 10 M KOH, were tested for 200 h at a constant current density of 10 mA cm⁻² at 75 °C and 50 bar in 10 M KOH. Post-mortem analysis revealed the breakdown of the perovskite surface structure of LSCFO and strong Fe-leaching from LFO, rendering none of the tested materials a stable catalyst under industrial conditions.

In the third part, Sr_0.98 Ti_0.7 Fe_0.25 Ni_0.05 O₃ (STFNO), a critical raw material lean material was tested as oxygen evolution reaction catalysts under industrial conditions. STFNO showed high stability at 100 °C and 50 bar in 10 M KOH and only showed signs of degradation when tested beyond commercial conditions (150 °C and 50bar in 10 M KOH). Furthermore, exsolution was used to enhance the catalytic activity of STFNO by exsolving mixed NiFeO nanoparticles on the surface of the stable STFNO host. Resulting in a low overpotential of 199mV at 75 °C and 50bar in 10M KOH. However, instability of the nanoparticles was observed when tested at 100 °C. STFNO arises as a promising catalyst for industrial applications.