Abstract
Anion exchange membranes (AEM) are key components in anion exchange membrane water electrolysers. Recently developed materials are less susceptible to the alkaline degradation of the polymer backbone and quaternary ammonium groups. A remaining challenge is the mechanical stability in contact with hot water and dimensional stability when the temperature of the feed solution changes. One solution is to reinforce membranes with a porous support. Since support materials like PEEK or PTFE have a different swelling behavior than the matrix and no strong interactions with the matrix, voids can form, and gas crossover increases. In this work, we approach this issue by pore filling polybenzimidazole nanofiber mats with the bromomethylated precursor of mTPN, an ultra-stable AEM material. During drying, a covalent interaction between support (PBI amine groups) and matrix (-CH2Br) is established. After quaternization, the optimized PBI/mTPN-50.120 composite membrane still shows a high conductivity of 62 mS cm−1, but 37% reduced length swelling in comparison to the non-reinforced membrane. Tensile strength and Young's modulus increase 17% and 56% to 49 MPa and 680 MPa, respectively. In an electrolyser, a stable voltage of 1.98V at 0.25 A cm−2 was achieved, and no change in membrane resistance was observed over the test time of 200 h (50 °C, 1 M KOH, catalysts based on Ni/Fe and Mo).
Highlights
Worldwide, societies aim to increase the use of renewable energy, mainly by increasing the use of intermittent energy sources like solar and wind power
We suggest the use of PBI nanofiber mats as a porous support for anion exchange membranes
MTPBr is casted on a glass plate, a PBI nanofiber mat is deposited on the wet film, and after a short drying time, a second mTPBr layer is cast on top
Summary
Societies aim to increase the use of renewable energy, mainly by increasing the use of intermittent energy sources like solar and wind power. Industrial alkaline water electrol ysis uses a porous diaphragm as a separator of the gas products, which can cause gas crossover and a hydrogen purity issue, especially at differential pressure [3] Another shortcoming in connection to the diaphragm is the low partial load range (operational at 20–40% full load) and poor dynamics of operation (startup/shutdown and load cycling) of the technology [4]. A learning from the fuel cell field, where PTFE-supported Nafion membranes are used, is that repeated water absorption and desorption delaminates the Nafion matrix from the rigid Teflon support, resulting in voids along the support’s surface, and increasing gas crossover To tackle this issue, Yu et al [11] and Deborah Jones and colleagues recently developed new support materials based on electrospun PBI nanofiber mats [12,13,14]. The effect of the porous support is investigated by measuring swelling behavior, conductivity, mechanical properties and the performance in the water electrolyser
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