Electrochemical and Spectroelectrochemical Characterization of Electrocatalysts for the Alkaline Water Electrolysis

Green hydrogen will be a key component for the decarbonization of Europe. The combination of renewable energies with water electrolysis offers a sustainable source of hydrogen. Alkaline water electrolysis is the most mature technology, despite having the lowest production rate amongst competing technologies.

One key component to improve it to a competitive level lies within the optimization of electrocatalysts. This Ph.D. project therefore investigated high-performing nickel-molybdenum electrocatalysts for the hydrogen evolution reaction and nickel-iron electrocatalysts for the oxygen evolution reaction for the alkaline water electrolysis.

Since focused work on the origin of the high performance and the stability of these electrocatalysts under technologically relevant conditions is of major interest for the industrial application, the Ph.D. project combined electrocatalyst studies with stability testing under technologically relevant conditions.

The electrochemical tests were furthermore extended with spectroelectrochemical Raman and X-ray diffraction studies, which were conducted with a self-developed setup called FeliS. The electrochemical active surface area of nickel and nickel-molybdenum HER electrocatalysts was studied to understand performance descriptors.

It was seen that an in situ impedance method can be applied. Based on this method, it could be hypothesized that nickel is the active site in nickel molybdenum electrocatalysts. This hypothesis is based on the congruent trends in decreasing the overpotential at 10 mA/cm2 with surface roughness between nickel-molybdenum and active nickel electrocatalysts.

Furthermore, it guides to additional studies, which have to address the chemical oxidation state of nickel in nickelmolybdenum. Stability studies were conducted to address the necessity for durable electrocatalysts under technologically relevant conditions. The long term investigated molybdenum oxide supported Ni4Mo electrocatalyst displayed stable operation during uninterrupted polarization.

However, it was sensitive to intermittent operation. Spectroelectrochemical Raman indicated that between 0.1-0.55 V vs RHE irreducible NiO forms, which decreases the apparent kinetics. At potentials greater than 0.55 V vs RHE, the molybdenum oxide supporting structure oxidizes to permolybdates, which irreversibly degrades the electrocatalyst.

For a bulk Ni8Fe2 layered double hydroxide electrocatalyst, stability testing revealed that it is sensitive to the entire operational window. It coagulated and depleted iron irrespective of its potential during uninterrupted polarization. It is therefore expected to degrade to a form close to a surface specific electrocatalyst.

Spectroelectrochemical X-ray diffraction showed that an exfoliated Ni8Fe2 layered double hydroxide electrocatalyst crystallizes with potential and time under technologically relevant conditions. In contrast to the bulk electrocatalyst, it was found that the exfoliated electrocatalyst is stable. The observed apparent performance loss of the exfoliated electrocatalyst was due to a reduced electrochemical active surface area as a consequence of the crystallization process.