Traditionally, alkaline water electrolysis (AWE) uses diaphragms to separate anode and cathode and is operated with 5-7 M KOH feed solutions. The ban of asbestos diaphragms led to the development of polymeric diaphragms, which are now the state of the art material. A promising alternative is the ion solvating membrane. Recent developments show that high conductivities can also be obtained in 1 M KOH. A third technology is based on anion exchange membranes (AEM); because these systems use 0-1 M KOH feed solutions to balance the trade-off between conductivity and the AEM's lifetime in alkaline environment, it makes sense to treat them separately as AEM WE. However, the lifetime of AEM increased strongly over the last 10 years, and some electrode-related issues like oxidation of the ionomer binder at the anode can be mitigated by using KOH feed solutions. Therefore, AWE and AEM WE may get more similar in the future, and this review focuses on the developments in polymeric diaphragms, ion solvating membranes, and AEM.

The intermittent and volatile nature of renewable energy sources threatens the stable operation of power grids, necessitating dynamically operated energy storage. Power-to-gas technology is a promising method for managing electricity variations on a large gigawatt (GW) scale. The electrolyzer is a key component that can convert excess electricity into hydrogen with high flexibility. Recently, organic/inorganic composite separators have been widely used as diaphragm membranes; however, they are prone to increase ohmic resistance and gas crossover, which inhibit electrolyzer efficiency. Here, we show that the ceria nanoparticle and polysulfone composite separator exhibits a low area resistance of 0.16 Ω cm² and a hydrogen permeability of 1.2 × 10^–12 mol cm^–1 s^–1 bar^–1 in 30 wt% potassium hydroxide (KOH) electrolyte, which outperformed the commercial separator, the Zirfon PERL separator. The cell using a 100 nm ceria nanoparticle/polysulfone separator and advanced catalysts has a remarkable capability of 1.84 V at 800 mA cm⁻² at 30 wt% and 80 °C. The decrease in the average pore size of 77 nm and high wettability (contact angle 75°) contributed to the reduced ohmic resistance and low gas crossover. These results demonstrate that the use of ceria nanoparticle-based separators can achieve high performance compared to commercial zirconia-based separators.

Hierarchical porous carbon (HPC) materials have found advanced applications in energy storage, adsorption, and catalysis in recent years. The HPC can be synthesized from a vast range of inexpensive carbon precursors, and contain unique structural features, such as nano-scale dimension, high porosity, high surface area, and tunable pore surfaces. These materials hold immense potential for removing contaminants from water and wastewater. However, this area is severely under-explored yet. In this review, we have discussed the recent advances of synthesis, modification, and application of HPC for the removal of pollutants from water, especially focusing on organic pollutants. Owing to their intrinsic hydrophobic nature and unique interconnected porous structure, HPC demonstrates a high affinity to hydrophobic organic contaminants, which can be enhanced many folds by target-specific chemical activation. Successful high-performance removal of contaminants by pristine and modified HPC includes plastic-derived (e.g. bisphenol A), pharmaceutical (e.g. antibiotics), dye (e.g. methylene blue) and pesticide micro-pollutants. Future research is warranted to find optimal and effective HPC synthesis and modification methods for further improving their ability to remove aqueous organic contaminants as a low-cost and energy-inexpensive remediation technology.