Superprotonic Conductivity of MOFs and Other Crystalline Platforms Beyond 10<sup>−1</sup> S cm<sup>−1</sup>
Shyam Chand Pal, Madhab C. Das
Abstract
Abstract The proton‐exchange membrane (PEM) is a fundamental module of proton‐exchange membrane fuel cells (PEMFCs), permitting proton passage and thus governing the overall performance of PEMFCs. Till now, Nafion has been the extensively used marketable PEM material due to its high protonic conductivity of 10 −2 –10 −1 S cm −1 under high relative humidity and 80–85 °C. On the other hand, crystalline materials such as metal‐organic frameworks (MOFs), coordination polymers (CPs), covalent organic frameworks (COFs), hydrogen‐bonded organic framework (HOFs), metalo hydrogen‐bonded organic framework (MHOFs), and polyoxometalates (POMs) are emerging as potential PEM materials, where crystallinity has paved the way to study the conduction pathway and associated mechanisms to understand structure‐function relationships. However, to date, ultrahigh superprotonic conductivity to the level of 10 −1 S cm −1 , close to Nafion, is relatively scarce for the crystalline proton conductors. In this review, the discussion is focused on materials that demonstrate a conductivity order of 10 −1 S cm −1 and higher for those individual crystalline platforms (to be on the equal footing and superior to nafion, respectively) based on their synthesis approach while highlighting the design norms and key features for attaining such ultrahigh conductivity. While a critical analysis is made, the key issues and future prospects are also addressed.