Surface Enriched Sulfonic Acid Ionic Clusters of Nafion Nanofibers as Long‐Range Interconnected Ionic Nanochannels for Anisotropic Proton Transportation: Phenomenon and Molecular Mechanism

Abstract

Construction of long‐range interconnected ionic channels in proton exchange membranes (PEMs) is the key to realize anisotropic proton transportation. But, how to effectively modulate ionic channels at the molecular level still remains a great challenge for modern nanoenergy technology. Here, surface enriched sulfonic acid ionic clusters in Nafion (NF)/ZnO nanofibers are successfully designed and fabricated as long‐range interconnected ionic channels, in which the ZnO nanofiller (nano‐ZnO) act as an inducer to enable the spatial deformation and inversion of sulfonic acid groups in NF molecule from the inside to the surface of the NF/ZnO nanofibers, due to the strong intermolecular electrostatic interactions between nano‐ZnO and the CF2 of NF backbones. The phenomenon and molecular mechanism are visually confirmed by physicochemical analysis. Ultimately, the NF/ZnO nanofibers demonstrate the relative higher of proton conductivity up to 253.2 mS cm‐1 and power density of 115.72 mW cm‐2 than that of reported NF‐based PEMs. This study, for the first time, reveals that the surface enrichment of sulfonic acid ionic clusters is an efficient strategy to construct long‐range interconnected ionic channels in the NF‐based PEMs, proves that the formation of the ionic channels could be efficiently modulated by controlling the molecular activity and morphology of nanofiller.

Ionic Covalent Organic Frameworks for Energy Devices

“…[204] However, for example, Nafion membranes cannot meet the demands for energy conversion/ storage devices under extreme operating conditions, such as at high temperatures (>120 °C) and low RHs (<50%), [205,206] as their ionic conductivities decrease by a few orders of magnitude under such conditions. [207,208] In addition, hydrated Nafion membranes suffer from degradation during the repeated freeze-thaw cycles that occur due to external temperature variations. [209,210] Furthermore, the high cost, explosibility, and poor water/thermal management systems of Nafion membrane electrolytes are yet to be overcome.…”

From Chongqing to Beijing

“…Based on the above clarification, it can be determined that at low temperature, the water media mainly adopt Fick diffusion into membrane, whose surface and thickness can strongly inhibit water diffusion towards bulk polymer [26,45,46] . In the meanwhile, the membrane surface is enrich of sulfonic acid ionic clusters, leading to strong surface conductivity [23,26,[46][47][48] .…”

“…Nevertheless, there are many problems to obtain reliable in-plane conductivity for Nafion membrane due to following restrictions. Firstly, the in-plane conductivity is mainly affected by many internal properties of membrane, including membrane thickness, ion exchange capacity, water uptake and channels, gas/liquid permeability and chemical/physical stability [20][21][22][23] . Past studied tried to measure conductivity under humidified gas condition, but they usually confront complex water-membrane balance.…”