Are complete metal-organic frameworks really responsible for improving the performance of high-temperature proton exchange membranes?

Constructing continuous proton transfer channels used metal-organic frameworks (MOFs), which can effectively improve proton conductivity of proton exchange membrane, have recently attracted a lot of attentions. MOFs have relatively harsh operating environment in phosphoric acid-doped (PA-doped) high-temperature proton exchange membranes (HTPEMs). However, there are few reports on the stability and state of MOFs in HTPEMs after PA doping. In this work, a series of MOFs (UIO-66, UIO-66-COOH, UIO-66-NH2, UIO-66-SO3H, MIL-101(Cr), and MIL-53(Al)) are selected to investigate their stability via simulating the operating environment for the first time. Composite membranes based on the MOFs are prepared to explore the influence of the stability and state of MOFs on HTPEMs properties. These results indicate that proton transfer channels are constructed in two different styles. After soaking in PA of UIO-66, UIO-66-COOH, MIL-101(Cr), and MIL-53(Al) at 160 °C, metal ions leave the ligands and dissolve, while the ligands are kept in the membranes. These ligands can provide proton transport sites in the membranes and help to construct proton transfer channels. UIO-66-NH2 and UIO-66-SO3H are dissolved completely in PA, leading to continuous nanopores. The proton transfer channels are constructed using the nanopores. From the results, we can infer that constructing proton transfer channels is an effectively method to improve the membranes performance, but the transmission mechanism needs to be revealed carefully.