Low temperature alkaline water electrolysis is a well-known technology for hydrogen production and advantageous because less noble metals are needed compared to Proton Exchange Membrane (PEM) electrolysis. However, the two-phase flow behavior and especially the gas bubble distribution and flow during the operation in a zero-gap alkaline electrolysis cell have not been investigated in sufficient detail. Previous studies used e.g. transparent model electrolysis cells or focused on single flow phenomena e.g. gas bubble nucleation, growth or detachment.

Recent publications show evidence that the ohmic resistance of zero-gap alkaline electrolysis cells is greater than the ohmic resistance of the used diaphragm, which has been believed the only term contributing to the ohmic resistance in zero-gap cells [1]. This suggests that the evolving gas bubbles might have a relation to the total ohmic resistance of the cell. As possible reasons nano bubbles in the pores of the diaphragm and a possible finite gap between electrodes and diaphragm is mentioned [2]. Haverkort et al. assume that the frontal area of the electrodes might be inactive [3].

To prove the hypothesis and to deepen the general understanding of the two-phase flow inside an alkaline electrolysis cell we conducted operando cell measurements and visualized the electrolyte-gas bubble flow in an alkaline electrolysis cell by using neutron radiography. Thanks to the powerful neutron source at Institute Laue-Langevin in Grenoble, it was possible to observe the whole flow field with all flow channels at once with a sufficient temporal resolution to observe the movement of single gas bubbles, from nucleation to the moment of exiting the channel. The temporal resolution was 50 fps, which makes these measurement, to the best authors’ knowledge, the first to show the transient effects inside of a zero gap alkaline electrolysis cell. For the measurements, the NeXT (Neutron and X-Ray Tomograph) instrument was used which is the imaging station with one of the highest neutron fluxes of the world (1.4 E10 n/cm²/s at the end of the guide) [4]. The neutron beam was used to carry out through-plane measurements of the electrolysis cell.

The measurements were conducted at a variety of operation conditions, varying the current density, the electrolyte volume flow and the operation temperature. At different operation conditions Electrical Impedance Spectroscopy (EIS) measurements have been conducted to measure the ohmic resistance of the cell. The used cell is an alkaline single cell out of a Nickel alloy with vertical parallel flow channel and a pocket milled to the backside of the cell in order to increase the transmission through the cell material.

From the results of the measurements we observe a relation between current density (forming gas amount) and the ohmic resistance and thus establish a relation to the gas bubble behavior inside the electrolysis cell. Furthermore, we analyzed the velocity of the gas bubbles inside the channel depending on the different operation conditions, as well as the agglomeration behavior of the bubbles and their flow regime.

[1] Phillips et al.,Minimising the ohmic resistance of an alkaline electrolysis cell through effective cell design, 2017

[2] de Groot et al., Ohmic resistance in zero gap alkaline electrolysis with a Zirfon diaphragm,2021

[3] Haverkort et al., Voltage losses in zero-gap alkaline water electrolysis,2021

[4] Tengattini et al., NeXT-Grenoble, the Neutron and X-ray tomograph in Grenoble, 2020