Asymmetric Porous Membranes with Improving Mass Transfer for Proton Exchange Membrane Water Electrolysis (PEMWE)

Abstract

Hydrogen is a promising and efficient energy source that can replace fossil fuels. Hydrogen can be obtained from various raw materials, including water. Recently, among various hydrogen production methods, high purity of hydrogen gas can be obtained by hydroelectricity. It also focused on water electrolysis, where efficient and continuous production of hydrogen is possible. Moreover, in terms of efficient and continuous production of hydrogen and environmental effects, PEMWE (Proton Exchange Membrane Water Electrolysis) has been evaluated as the most inspiring technology for producing high purity hydrogen of renewable energy and has the strength to discharge only oxygen by-products without hazards. These days, PEMWE research is surging as demand for hydrogen-based energy increases. Therefore, considerable research has recently been completed on membrane development for PEMWE. Currently, the PFSA (Perfluorosulfonic acid) polymer is used in PEMWE systems. A previous study reported that the thinner the PFSA membrane used for PEMWE, the lower the resistance of the MEA (Membrane Electrode Assembly), resulting in improved single-cell performance. Nevertheless, the single cell operates using a thick electrolyte membrane considering the safety aspects of the PEMWE system. Applying thick electrolytic membranes to PEMWE systems improves physical and chemical durability, but increasing the thickness of the MEA increases the resistance and mass transfer resistance, which degrades the performance of PEMWE. To compensate for these limitations, Previous studies have been conducted to reduce the resistance by modifying the catalyst structure constituting the MEA, but any case hasn't been reported to modify the structure of the membrane for PEMWE.Therefore, in this study, an asymmetric membrane was prepared to improve the performance of the PEMWE single cell by applying a porous structure into a thick membrane to ensure high durability. Research on asymmetric membranes has been conducted in energy fields such as vanadium redox flow batteries, fuel cells, and lithium-ion batteries. As a result of applying an asymmetric membrane to each energy research, it has been reported that the doping amount of the electrolyte increases due to the strengthening of the capillary force according to the porous structure, and accordingly, the ionic conductivity increases, thereby improving the performance of the cell. Besides, studies are showing that the diffusion rate of water and ions increases, and the internal resistance decreases due to the structure of the dense layer and the porous layer.In this study, to apply the performance improvement effect of the previous study to the electrolyte membrane for electrolysis, we referred the porous structure to confirm the performance of the PEMWE single cell. The porous layer of the asymmetric membrane was prepared using a solvent/non-solvent evaporation method. The solvent of the Nafion dispersion solution is alcohol, and the non-solvent is ortho-dichlorobenzene (ODB). The porous surface was prepared using the difference in boiling point and density between the solvent/non-solvent. Through the SEM analysis, the porous layer on the surface of the asymmetric membrane and dense layer on the bottom was observed. The Uniform round micropores distributed on the entire surface of the porous layer, which was below 10 μm. A cross-sectional view showed that the thickness of the asymmetric membrane is 175 ~ 180 μm. As a result of an EIS analysis, the asymmetric membrane showed a tendency to have lower ohmic and mass transfer resistance than the Nafion 117 membrane of the same thickness. Through the evaluation of a single cell performed under 80 ℃ temperature conditions, Nafion 117 was 1.5 A/cm2 and the asymmetric membrane was 2.5 A/cm2, about 1.67 times higher than Nafion117. The results of PEMWE single cell measurement confirmed that the porous structure of the asymmetric membrane improved the material transfer efficiency of the membrane, thus lowering the material transfer resistance, thereby improving the PEMWE performance.