Characterisation of the Operational Parameters of the Laboratory-Scale Membrane Alkaline Water Electrolysis Stack

Abstract

Alkaline water electrolysis utilising porous diaphragm type separator of the electrode compartments represents industrially well-established technology of hydrogen production. This is due to the durable and stable operation utilizing abundant materials as iron, nickel, or stainless steel. However, many of the designs, construction solutions and approaches originate in the historical demands on the process (like mentioned durability rather than flexibility) and do not reflect up to date demands. Current demands on water electrolysis are given by the expected connection of water electrolysis technology with renewable sources of energy. The role of the water electrolysis is to use the surplus electrical energy in periods of the high output and/or low need for production of hydrogen. Hydrogen in such scheme represents an energy vector, which can be stored and used for electricity production in periods of the insufficient output of the renewable sources and/or high demand. Alternatively, hydrogen can be used as a fuel in fuel cell-based cars or as feedstock in industry. Due to connection with unpredictable renewable source, it is necessary for water electrolysis technologies to be flexible, highly efficient and be able to produce hydrogen of high purity. Current alkaline water electrolysis failed in several of these demands.Nowadays demands on water electrolysis are met by process utilizing proton exchange membrane (PEM) as separator of the electrode compartments. Unfortunately, PEM water electrolysis is limited by the need of the materials like titanium, platinum and iridium, which are rare, to be involved as electrodes, or catalysts of electrode reactions. The significant effort is paid to the possibility to combine advantages of both, alkaline and PEM processes by replacing porous diaphragm by dense anion-selective polymer membrane (ASM). Such change would allow to reduce the interelectrode distance to the thickness of the ASM used as separator of the electrode compartments, which would increase the performance of the membrane alkaline water electrolysis process. Utilisation of the ASM would also allow to decrease concentration of circulating electrolyte from commonly used 25 – 30 wt.% KOH to 1 – 5 wt.% KOH, increasing thus safety and flexibility. Another important beneficial impacts of ASM application are related to the improved current efficiency at low current densities and purity of the produced gasses, again especially at low current densities.Significant development was achieved in ASM synthesis in last decade and first promising materials are being commercially available. Despite the fact that many of the novel materials are tested in single cells under conditions of the membrane alkaline water electrolysis, the information on the stack behaviour are rare.In our work, we focused on the comparison of the porous diaphragm type and ASM separators in short stack comprising of 3 cells and geometrical area of the separator 78.5 cm2 under different operational conditions in terms of performance achieved. Besides the performance characterisation by mean of the load curve measurement, the complementary information on the gas purity, current efficiency and long-term stability were gathered for ASM separator.Zirfon Perl UTP 500 (Agfa, Belgium) was used as diaphragm type separator, meanwhile chloromethylated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene backbone functionalized by DABCO groups was used as ASM (TailorMem s.r.o., Czech Republic). Different concentrations of the liquid electrolyte represented by KOH solution were used in the range 1 – 15 wt.%. The temperature effect was verified at the same time by using the temperature of 25 and 40 °C. Load curves were measured galvanostically up to current density of 600 mA cm-2. The current efficiency was calculated based on the Faradays law by measuring the real oxygen production and comparing with theoretical value. The gas purity was measured for oxygen stream as the penetration of hydrogen into the oxygen stream is more important. Gas chromatography was used to measure the oxygen stream composition. The long-term stability was measured in 10 wt.% KOH solution at 40 °C at 240 mA cm-2.The results obtained indicate that at low KOH concentrations the performance of membrane alkaline water electrolysis is significantly better when compare to application of the diaphragm type separator. The differences in performance mitigated at increasing concentration. The characterisation of the membrane alkaline water electrolysis short-stack showed possibility to achieve high purity of the produced gasses and current efficiency even at low current densities. The long-term test showed excellent stability of the used ASM.Acknowledgement:This project has received funding from the European Union‘s Horizon 2020 Research and innovation action under grant agreement No 862509. Financial support by the Technology agency of the Czech Republic within the framework of the project No. TK02030103 is gratefully acknowledged.