A commercial porous polyvinylidene fluoride membrane (pore size 0.65 μm, nominally 125 µm thick) is spray coated with 1.2 -4 µm thick layers of polybenzimidazole. The area resistance of the porous support is 36.4 mΩ cm 2 in 2M sulfuric acid, in comparison to 540 mΩ cm 2 for a 27 µm thick acid doped polybenzimidazole membrane, and 124 mΩ cm 2 for PVDF-P20 (4 µm thick blocking layer). Addition of vanadium ions to the supporting electrolyte increases the resistance, but less than for Nafion. The expected reason is a change in the osmotic pressure when the ionic strength of the electrolyte is increased, reducing the water contents in the membrane. The orientation of the composite membranes has a strong impact. Lower permeability values are found when the blocking layer is oriented towards the vanadium-lean side in ex-situ measurements. Cells with the blocking layer on the positive side have significantly lower capacity fade, also much lower than cells using Nafion 212. The coulombic efficiency of cells with PVDF-PBI membranes (98.4%) is higher than that of cells using Nafion 212 (93.6%), whereas the voltage efficiency is just slightly lower, resulting in energy efficiencies of 85.1 and 83.3%, respectively, at 80 mA/cm 2 .

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.

The gem‐difluorocycloalkane family was extended to all possible regioisomers of the gem‐difluorocycloheptane, monofunctionalized by carboxilic‐, amino‐ or keto‐ group, that were synthesized on a multigram scale. The preparation of the corresponding building blocks was achieved from readily accessible starting materials either via six‐membered ring homologation or deoxofluorination of the appropriate seven‐membered cyclic ketones.

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