Operando Neutron Radiography/Tomography Studies to Characterize Gas Evolution and Two-Phase Flow System in Alkaline Water Electrolyzer
Authors: Liebert Michael , Arlt Tobias , Markötter Henning , Kardjilov Nikolay , Tengattini Alessandro , Manke Ingo , Joerissen Ludwig 
Published: December 2024

Energy storage is becoming increasingly urgent as the share of fluctuating renewable energies increases in order to utilize the full potential of power plants and stabilize the grid in times of lower yields. Water electrolysis is therefore a promising solution for converting energy into hydrogen, which offers the opportunity of long term, even seasonal energy storage, albeit with lower round-trip efficiency compared to established battery technology.

In a comparative technology assessment among AEM, SOEC and PEM, AEL is currently still the most cost-effective, as no precious metals have to be used, and the most reliable in terms of operating hours, although it has disadvantages in terms of power density and partial load stability.

In this work we investigated the formation and transport of product gas through a porous electrode in a small sample cell of an alkaline membrane electrolyzer and studied the impact of gas accumulation on the electrolyte conductivity in the cell. X-ray as well as neutron radiography and tomography are the most suitable non-destructive methods to examine porous structures for media transport on a microscopic level and to capture the different constituents of the cell, namely electrode, electrolyte and product gas in different time and voxel resolutions.

With our tomographic measurements, we show that it is possible to resolve multiple liquid/gas/solid phase boundaries after the application of current pulses, and thus determine the locations where gas bubbles form and track the gas volumes in the electrode.By superimposing the reconstructed tomographic images taken between the applied current pulses, the evolution of the gases can also be visualized as shown in Figure 1. Knowing the origin and temporal evolution of gas formation can be a valuable input for validation of various models and simulation efforts eventually leading to a reliable digital twin for pore size engineering.

Since tomography measurements are taking long time and should be performed on static samples to resolve phase boundaries, they are not ideal for dynamic gas evolution at practically relevant current densities. Studies under high current density are better carried out using operando radiographic methods. The much higher time resolution allows to investigate media transport in two directions in actual load cycles.

This study shows clear differences in the way gas is evolved from the anode and the cathode. Furthermore, clear differences in the amount of gas transported through the volume of the porous electrodes at the anode and the cathode were observed leading to significant differences in the degrees of filling in the anode and the cathode chamber. Frequently, eruptive gas release was observed.

Resolved volume fractions of gas/electrolyte can also be used to validate models and simulations based on the Euler-Euler approach, e.g. to fit transport parameters based on real data measured in such way. Since this type of simulation can be more easily scaled up to full electrode sizes, especially for industrial alkaline electrolysis, this approach is preferable for application research and development.

Figure 1