Heat production in Alkaline Capillary-Fed Water Electrolysis

Abstract

This master thesis introduces a recently surfaced method to produce hydrogen and oxygen using alkaline water electrolysis. A hydrophilic porous diaphragm is used that can feed electrolyte laterally to two porous electrodes that are placed against it. Consequently, hydrogen and oxygen are produced directly at the electrodes without any bubble formation. More specifically, this master thesis aims to investigate the thermal behaviour of such a capillary-fed electrolyzer and the limits to which capillary-fed electrolysis can be sustained.
A small scale capillary-fed cell and a larger segmented cell are manufactured during the project. Both cells are used to perform experiments at constant current. Capillary rise in diaphragms with and without compression deviate from the Lucas-Washburn model for capillary rise. Particularly the polyethersulfone material outperforms the diaphragms that were studied. It follows the Lucas-Washburn equation relatively well and exhibits two times deeper penetration under compression. Furthermore, experiments have been performed studying the relation between electrolytic concentration and capillary wicking. Although more highly concentrated KOH solutions for electrolytes increase conductivity over time, it has shown that for the utilised diaphragm a KOH solution surpassing 3 moles per litre shows problematic capillarity. Contact angle experiments with 6M KOH have shown a hydrophobic contact angle of ̄Θ=78.14° compared to complete imbibition, i.e Θ = 0°, for 1,2 and 3M KOH solutions. The origin of these extremely large contact angles on the polyethersulfone substrate is not fully understood. 
The study has culminated in performing experiments at constant current and low concentration KOH utilising flushing and dilution techniques to prevent precipitation at the electrodes. The small cell reached an overall heat transfer coefficient of h = 121 W/m²K, showing very large heat production for a relatively small surface area. For the segmented cell, electrolyte supply with a bottom-up feed resulted in h = 19.13 W/m²K in which precipitation occurred. The top-down feed showed that the top segment has the largest areal heat dissipation. The heat transfer coefficient equates to h = 25.73 W/m²K. A final enhancement has been made by introducing Nickel Felt Fiber conducting layers into the cell to improve electronic conductivity. This resulted in an increased range of current densities that could be reached at lower overpotentials compared to absence of these conductive layers. This resulted in the bottom most segment to exhibit the largest areal heat dissipation. The heat transfer coefficient here equates to h = 43.86 W/m²K.
This study takes another important step into the regime of capillary-fed electrolysis and alternative electrolysis methods in general. Introducing an alternative design, assessing this design from experimentation and reporting on subsequent steps that can be taken.