Alkaline Water Electrolysis – Investigation of the Charge Transport Limitations across the Separator Under Low KOH Concentration
Authors: Denk Karel , Kodym Roman , Hnat Jaromir , Paidar Martin , Žitka Jan , Bouzek Karel 
Published: September 2023

Currently, the water electrolysis is employed when high purity hydrogen is required, or excess of cheap electricity is available. With a broader future perspective, an important benefit of this technology represents fact, that it can be powered from renewable energy sources. The hydrogen produced from renewables is, thus, “green” CO2 neutral, playing an important role as energy vector in the so called “hydrogen economy”. Nowadays, the hydrogen economy represents strategy for transition of our economy towards the renewable energy sources, with water electrolysis (denoted as Power-to-X technology) being a crucial part of this scheme.
Nowadays, the alkaline water electrolysis technology (AWE) operating at elevated temperatures (70 – 100 °C) with KOH electrolyte (25 – 30 wt.% KOH) dominates in the field of industrial hydrogen production by the water electrolysis. The alkaline environment is advantageous since it does not require platinum-group catalysts. On the other hand, highly concentrated caustic solution at elevated temperatures used as a liquid electrolyte limits the flexibility of the process operation. Currently several alternative technologies offering higher production intensity (proton-exchange membrane water electrolysis) and higher efficiency (high temperature solid-oxides steam-electrolysis) are developed and approach industrial application state. There are also promising innovations of the AWE process developed, e.g. application of an anion-selective membrane (AM) as the separator alternative to the diaphragm used in the conventional AWE. The AMs of interest exhibit improvement, contrary to diaphragm, of anionic conductivity with decreasing KOH concentration and operating temperature. Therefore, application of such AM allows to reduce KOH concentration in a liquid electrolyte (down to 5 – 15 wt.%) and operating temperature, thus, solve the above-mentioned problems of conventional AWE in connection with renewables. Moreover, the AM is dense, therefore might represent more effective barrier for produced gasses than diaphragm. However, one has to keep on mind increasing solubility of gasses with decreasing KOH concentration followed by potentially enhanced H2 cross-over. Another non-negligible benefit of lower KOH concentration and operating temperature is lower electrolyte conductivity in the manifolds contributing to reduced parasitic (bypass) current. However, systematic research aimed at evaluation of real application potential and searching for optimal conditions of the AM at diluted KOH concentrations is so far missing.
In the presented contribution, study on a behaviour of two types of the separators in the laboratory scale AWE stack in bipolar zero-gap configuration is presented. The separators used are: a) a homogeneous AM based on chlormethylated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene copolymer functionalized by 1,4-diazabicyclo-[2.2.2]octane (PSEBS-CM-DABCO) and b) a commercial composite diaphragm Zirfon™ Perl. The load curves were recorded to assess optimal performance of the stack with both separators at KOH concentration range of 1 – 15 wt.% and operating temperature range of 25 – 40 °C. The effect of different electrolyte flow rate through the Ni-foam electrodes was investigated as well. Additionally, 1-dimensional stationary mathematical model of single AWE (both with diaphragm and AM) cell corresponding to the studied laboratory set-up was developed to clarify the differences in the stack load curves recorded for the two studied separators. The Nernst-Planck equation is used to describe migration and diffusion transport in the cell, while convective flow of electrolyte in the porous electrode is considered in material balance. The gas evolution is neglected in the present model. A macrohomogeneous continuum theory is employed for description of the porous electrodes. The AM-solution interface is characterized by Donnan equilibrium. At all studied experimental conditions, the stack with AM outperforms the one with diaphragm. This fact is even more pronounced at KOH concentration below 5 wt.% due to occurring transport limitation, which is more significant in the case of the diaphragm, as demonstrated both by modelling and experimentally. The presented work clearly identifies interval of operating conditions, in which AM represents an important advantage over porous separator and opposite. Thus, it gives important hints for further development of this technology.
Project has received funding from the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 862509).