Electrode process of alkaline and AEM water electrolysis

Anion-based water electrolysis, such as alkaline (AWE) and anion-exchange membrane-based (AEMWE) water electrolysis, are important electrochemical technologies for the production of green hydrogen. It is essential to have a very good understanding of the processes taking place at the electrodes – the cathode for hydrogen evolution and the anode for oxygen evolution. Fraunhofer IKTS is constantly building up and developing know-how in this area.

Spatially resolved analytics

The influences of charge transfer and ion conductivity are analyzed in operando, supported by reference electrodes. In addition to the analysis of spatially resolved pressure distribution in cells and stacks, spatially resolved current density and temperature distributions are also determined. For example, it is possible to visualize particularly active or inactive areas of a membrane electrode assembly (MEA) or an electrochemical cell. In the illustrated case of an AEMWE MEA, a high current density at the edge of the active area (blue) indicates an unoptimized flowfield design (Fig. 1).

Effects of gas bubbles

Another example stems from the cooperation within a Fraunhofer International Mobility Program with Prof. de Grootof TU Eindhoven, which investigated bubble behavior in alkaline electrolyzers. The gas bubbles play an important role in the efficiency of an electrolyzer, as they influence both the active surface area of the electrodes and the conductivity of the electrolyte.

Typical industrial nickel perforated plates with different hole diameters were used as electrodes in the experiments. With the help of a transparent cell and a high-speed camera, it is possible to observe the formation of bubbles of hydrogen and oxygen directly at the electrodes. These videos were supplemented by electrochemical analysis methods (e.g. impedance spectroscopy) in order to understand the relationships between bubble formation and electrochemical processes.

The nickel electrodes with 2 mm hole diameter show the lowest activity compared with electrodes with smaller holes (1 mm and 0.5 mm). The observation shows that the gas bubbles become larger as the hole diameter increases. The gas bubble size has a negative effect on electrolyte conductivity and the effective electrode surface area, which can be determined by impedance spectroscopy.

These findings will help to increase the efficiency of the electrolyzers and thus further optimize the technology of anionbased water electrolysis.