Layered composite membranes based on porous PVDF coated with a thin, dense PBI layer for vanadium redox flow batteries

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 cm2 in 2 M sulfuric acid, in comparison to 540 mΩ cm2 for a 27 μm thick acid doped polybenzimidazole membrane, and 124 mΩ cm2 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/cm2

Macromolecular reinforcement of alkaline ion-solvating polymer electrolytes

Alkaline ion-solvating polymer electrolyte membranes derived from poly(isatin biphenyl) and polybenzimidazole (m-PBI) blends with different ratios were prepared, characterized, and evaluated as electrode separators in alkaline water electrolysis. The physicochemical properties could be tuned by varying the composition of the blend, and the membrane with a poly(isatin biphenyl) content of 25% showed a suitable balance between conductivity and mechanical robustness. The polarization behavior was comparable to that of state-of-the-art separators, with significantly lower H2 crossover. No signs of degradation could be observed after 70 h of electrolysis testing in 30% aqueous KOH at 80 °C, supporting that macromolecular reinforcement is a promising way forward in the development of high-performing and durable alkaline ion-solvating membranes for water electrolysis

Carboxylated Polystyrene-<i>b</i>-poly(ethylene-<i>ran</i>-butylene)-<i>b</i>-polystyrene Membranes for Alkaline Water Electrolysis

A series of membranes based on carboxylated polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene (SEBS) were synthesized via Friedel-Crafts bromoalkylation, substitution with malonate esters, and heterogeneous ester hydrolysis. The physicochemical properties of the obtained membranes, including electrolyte uptake, swelling ratio, and ionic conductivity, were highly affected by the degree of functionalization and the concentration of the aqueous KOH solution. Membranes with degrees of functionalization higher than 100% suffered from excessive swelling and poor mechanical stability. However, membranes with degrees of functionalization in the range of 50-75% combined high electrolyte uptake and ion conductivity with mechanical robustness. The area specific resistance during electrolysis testing in 15 wt % aqueous KOH at 80 °C was 0.18 Ω cm2 after 1 week, with no signs of degradation. The H2 in the O2 level at 50 mA cm-2 was about 0.5%, which was considerably lower than that of Zirfon despite being four times thinner

Electrode Separators for the Next-Generation Alkaline Water Electrolyzers

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