In-Situ Analysis of Gas Crossover in PEM Water Electrolysis - the Effect of Membrane Variation

Abstract

Polymer electrolyte membrane (PEM) water electrolysis is well identified today as a sustainable solution for the production of hydrogen, and of paramount importance when coupled to renewable but intermittent power sources such as wind and solar. PEM water electrolysis is based on a solid polymer electrolyte concept first realized by Grubb in 1960s, where a thin solid sulfonated polystyrene membrane is used as an electrolyte. The polymer electrolyte membrane is responsible for providing high proton conductivity, compact system design, and low gas crossover. However, depending on the power load range, operating temperature, pressure range, membrane thickness, and a number of other parameters diffusion processes of the product gases through the membrane occur. The diffusion of hydrogen into the oxygen compartment for example reduces the efficiency of the electrolyzer, and allows the extensive mixing of gases that must be avoided in order to preserve efficiency and safety. This is particularly severe at a low power load – important when coupling to intermittent power sources – where the oxygen production rate decreases, thus drastically increasing the hydrogen concentration to unwanted and dangerous levels (lower explosion limit > 4 mol-% H2). Nevertheless, PEM water electrolysis can operate at high current densities, capable of achieving values above 4 Acm-2, provided by very low ohmic losses. Though, very high current densities (over 6 Acm-2) can only be achieved if thin membranes (20 – 50 µm) are used which lead to lower cell resistances. In this case unfortunately, the crossover of gases across the membrane is drastically enlarged, if compared to the use of thicker membranes (100 – 200 µm). In any case, PEM electrolysis is recognized for practically covering a wide power density range. However, the practical power operation range of PEM water electrolysis has not been consistently evaluated yet, especially when considering the variation of perfluorosulfonic acid (PFSA) membrane, thickness, and operation pressure. In this study, the in-situ crossover aspect of a series of PFSA Nafion membranes at different operation conditions will be evaluated and compared to a simulation study of the gas permeation pathways. The main message is to clearly define the power load and pressure range for the PEM water electrolysis technology when using different membrane alternatives, providing a road map in order to aid scientists with regards to the R&D of PFSA membranes for PEM water electrolysis.