Hydrogen Diffusivity and Electrolyte Permeability of the Zirfon PERL Separator for Alkaline Water Electrolysis

Abstract

The hydrogen and oxygen evolved during alkaline water electrolysis with liquid KOH electrolytes are typically separated using porous separators such as Zirfon PERL (Agfa), a commercially available composite of zirconium oxide and polysulfone. In this study, the hydrogen diffusivity (driven by concentration differences) and electrolyte permeability (driven by differential pressures) of the Zirfon PERL separator were characterized as a function of the temperature and molarity of the KOH filling. The diffusivity of hydrogen in the separator was found to be approximately 16% of that of the electrolyte filling inside its pores. With respect to water electrolysis conditions, the extent of hydrogen cross-permeation caused by the convection of the cross-permeating electrolyte was estimated and compared to that caused by diffusion. On the basis of the physically characterized mechanisms, smaller pores were predicted to reduce the differential pressure driven gas cross-permeation. In a water electrolyzer, hydrogen is evolved at the cathode (the negative pole) while oxygen is evolved at the anode (the positive pole). When both electrodes are connected by an alkaline electrolyte, hydroxide ions must permeate from the cathode to the anode in order to bring about the electrochemical reactions at the electrodes. In such alkaline water electrolyzers, typically aqueous KOH electrolytes (which provide higher conductivity than other lyes 1) and porous sep-arators between both electrodes are used to provide the ionic conductivity between the electrodes and to separate the evolved gases. 2,3 Alternatively, alkaline polymer electrolyte membranes can be used as the electrolyte. 4-6 However, these membranes typically show low durability. 7,8 The thickness of the separator, its ionic conductivity and its gas permeability constitute a decisive property for the efficiency of water electrolysis, as these properties determine the ohmic drop and cross-permeation of the produced gases. 9-11 A recent comparison of acidic and alkaline water electrolysis showed, that the lower hydrogen and oxygen diffusivities in alkaline electrolytes than in acidic electrolytes may be a decisive advantage of alkaline electrolyzers. 9 Higher pressures at the gas outlets during water electrolysis means reducing the mass fraction of water in the produced gases and thus lower thermoneutral voltages. 9,12 In addition, applied pressures during water electrolysis enable efficient isothermal compression. 13 However, higher pressures increase the amount of hydrogen and oxygen diffusing through the separator, 9,10 so that porous separators that enable low gas crossover are required for efficient alkaline water electrolyzers. Composite materials of Zirconium oxide (zirconia) and polysul-fone are high-performing and stable separators for alkaline water electrolysis. 14,15 This type of separator combines the flexibility of the polymer with the stiffness and wettability of the ceramic zirconia. 16 Agfa provides a commercially available product of this type of sep-arator, denoted as 'Zirfon PERL UTP 500'. In the datasheet of the manufacturer 17 a thickness of approximately 500 ± 50 μm, a porosity of 50 ± 10% and pores of approximately 0.15 ± 0.05 μm diameter were reported. The manufacturing procedure and resulting physical properties, such as the wettability and porosity, are reported in the literature. 14,16,18 However, a detailed quantitative study of the hydrogen diffusivity and electrolyte permeability of the Zirfon PERL sep-arator or other types of separators has thus far not been presented in the literature, despite these properties have a decisive impact on the overall efficiency. 9 Given this lack of knowledge, in this study these properties are characterized as a function of pressure, temperature and the composition of aqueous KOH filling. With reference to the measured diffusivities, the solubilities of hydrogen in KOH as a function c Present address: of temperature are estimated. On the basis of these data, the amount of hydrogen that can be carried by the electrolyte permeation through the separator during water electrolysis are estimated. The hydrogen permeation fluxes through the separator during water electrolysis are estimated, providing the data to model the gas purities and efficiencies of electrolyzers. 9 The results obtained for the hydrogen diffusivity are transferred to that of oxygen on the basis of the presented measurements and literature data. Finally, pore size reduction of the separator is suggested in order to reduce the differential pressure driven elec-trolyte permeation through the separator during water electrolysis. Methods In the following, the experimental setups used to measure the properties of Zirfon PERL samples are briefly described. The thickness of the examined samples was d = 462 ± 10 μm. Measurement of the hydrogen diffusivity.-Figure 1 shows the experimental setup employed to measure the hydrogen diffusivity in the Zirfon PERL separator with the electrochemical monitoring technique. A general description of the electrochemical monitoring technique to measure gas crossover and the physical theory of gas diffusion through aqueous solutions was provided in a previous study. 19 In the setup used, a sample of the Zirfon PERL separator was in direct contact with two electrolyte chambers that were filled with an aqueous KOH solution. As illustrated in Figure 1, hydrogen is bubbled through the electrolyte chamber left to the sample, which is in the following denoted as 'left electrolyte chamber'. In the left electrolyte chamber a Titanium plate of 8 mm thickness with a window of 10.9 cm 2 was pressed onto the Zirfon PERL sample, which allowed the KOH solution to penetrate into the sample. In the electrolyte chamber on the other side of the Zirfon PERL sample (with reference to the illustration in Figure 1 denoted as the 'right electrolyte chamber'), a carbon fleece (Freudenberg, H23) that was coated via sputtering with 2 mg Platinum per cm 2 of geometric area was used. This catalyst coated carbon fleece is in the following denoted as 'working electrode' and is filled with the same KOH electrolyte as the Zirfon PERL sample. At the platinum catalyst, hydrogen diffusing through the Zirfon PERL sample was electrochemically oxidized: H 2 + 2OH − → 2H 2 O + 2e − [1] In order to enable this electrochemical oxidation reaction, the reverse reaction direction was provided at another electrode which is defined as the 'counter electrode'. This electrode is to identify with the Nickel plate of the cell that touched the electrolyte in the right chamber. Accordingly, the hydrogen at the working electrode is transported by an electrochemical pump to the counter electrode. Nitrogen) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 134.130.185.197 Downloaded on 2017-12-22 to IP