Development of Membranes for Alkaline Water Electrolysis

Abstract

Alkaline water electrolysis is one of the important methods for generating high purity hydrogen. Alkaline electrolysis, because of the possibility of using relatively inexpensive electrode materials, is more economical than hydrolysis in acidic medium; the medium is also less corrosive. The challenges in water electrolysis in general are to reduce the cost, energy consumption and maintenance, hence increase the efficiency of the overall process. An important consideration in the above is the separator between the two electrode compartments, which has to be such that it minimizes (or eliminates) the possibility of mixing of gases evolved at the two electrodes, while not adding much to the overall voltage drop in the electrolyzer. The objective of this study is to develop a membrane separator with low ohmic resistance and hydrogen permeability, essential for the alkaline electrolysis operation. A thorough analysis of the membrane’s resistance is done based on a study of how synthesis parameters, and (through them) the structure, determine the physical and electrochemical properties and influence the electrolysis. Three categories of separators have been reported for alkaline water electrolysis: porous separators, anion exchange membranes and ion-solvating membranes. Anion exchange membranes have a functionalized polymeric base with cationic groups linked to the polymer backbone, for anion conduction across the membrane separator. An ion-solvating membrane is a dense polymeric material containing a polymer backbone with affinity to aqueous KOH (the normally used electrolyte) to achieve ionic conductivity. Porous separators are diaphragms which physically separate the anodic and cathodic chambers, although allowing some ion conduction by virtue of their ability to sorb the electrolyte. Our focus in this study is on the last type of separators. We report here our studies on a porous composite Zirfon membrane separator made up of polysulfone polymer and zirconium dioxide, with 80wt% loading of zirconium dioxide and a nylon mesh support. The membranes were made in-house and had an average thickness and ohmic area-resistance of 320μm and 4.5Ω-cm2 (measured in 0.5M NaOH via Electrochemical Impedance Spectroscopy), respectively. A lab-scale electrolyzer cell was installed with the composite Zirfon membrane and commercial Zirfon membrane and was tested for alkaline electrolysis; a quantitative relationship between the cell voltage, current and impedance results is established and analyzed by calculating the expected ohmic losses, Tafel kinetics and purity of hydrogen gas evolved. The electrolyzer achieved a cell voltage of 2.4V with the in-house membrane and 2V with the commercial membrane at a current density of 0.06Acm-2 in 0.5M NaOH solution with Nickel foam as electrodes. The Tafel slopes obtained for both hydrogen and oxygen evolution reactions for Nickel foam electrodes with commercial Zirfon and in-house composite Zirfon separator was 200 mV/dec and 220 mV/dec, respectively. The results obtained for the in-house membrane are benchmarked against the commercial Zirfon membrane. Figure 1