Electrode Separators for the Next-Generation Alkaline Water Electrolyzers

Abstract

Multi-gigawatt-scale hydrogen production by water electrolysis is central in the green transition when it comes to storage of energy and forming the basis for sustainable fuels and materials. Alkaline water electrolysis plays a key role in this context, as the scale of implementation is not limited by the availability of scarce and expensive raw materials. Even though it is a mature technology, the new technological context of the renewable energy system demands more from the systems in terms of higher energy efficiency, enhanced rate capability, as well as dynamic, part-load, and differential pressure operation capability. New electrode separators that can support high currents at small ohmic losses, while effectively suppressing gas crossover, are essential to achieving this. This Focus Review compares the three main development paths that are currently being pursued in the field with the aim to identify the advantages and drawbacks of the different approaches in order to illuminate rational ways forward.

Figure 1

Schematic illustration of the different electrolyte concepts in pure water and combined with aqueous KOH based on (a) porous diaphragms, (b) AEM, and (c) ion-solvating membranes. The dimensions of the electrolyte filled pores in the bulk of SoA porous diaphragms (a) is typically in the micrometer range or slightly smaller. A general understanding of the nanomorphology of AEMs (b) has not been established, but a phase separated morphology at the nanometer length scale has been confirmed experimentally for some materials. The ion-solvating membranes (c) are generally understood as more homogeneous systems.

Figure 2

Examples of SoA polymer chemistries used for (a) porous diaphragms, (b) AEM in pure water and combined with an aqueous KOH support electrolyte, and (c) ion-solvating membranes.

Figure 3

Comparison of polarization data for current SoA cells equipped with noble-metal free electrodes and a porous diaphragm, an AEM in pure water, an AEM combined with KOH support electrolyte, and an ion-solvating membrane. The polarization curves have been reconstructed based on data points extracted from the original references. The reader is referred to the original references for further details about cell components, hardware, and operating parameters.