Recently, the team of Professor Zhang Gen from the School of Chemical Engineering, Nanjing University of Science and Technology has made the latest research progress in proton exchange membrane materials. The related research results are titled "PErfluoroalkyl-functionalized Covalent Organic frameworks with Superhydrophobicity for Anhydrous Proton Conduction" and published in the top international chemistry journal "J. Am. Chem. Soc.". Nanjing University of Science and Technology is the first to complete the work and the communication unit. Young teacher Wu Xiaowei is the first author of the paper, Professor Zhang Gen is the corresponding author, and Satoshi Horike of Kyoto University is the co-corresponding author of the paper.
At present, the environmental pollution and energy crisis caused by fossil fuels are becoming increasingly severe, and the development and utilization of new energy is accelerated. Fuel cell, as an energy conversion device that converts "chemical energy into electrical energy", has become one of the ten new technologies that change human life due to its high energy conversion efficiency, high energy density, no noise and pollution. Proton exchange membranes with excellent performance are the core technology in the development of fuel cells. In the past decades, the development of proton conducting materials has produced various perfluorinated polyelectrolytes, such as Nafion. However, due to the narrow applicable temperature range (<80 oC), high cost, and insufficient durability of this type of material, its application in high temperature resistant and high energy density fuel cells is limited.
Covalent organic framework (COF) is a new type of crystalline organic porous polymer, which is an ordered framework structure formed by covalently connecting organic structural units. One of their distinguishing features is the controllability of structure and performance. The uniform 1D channel formed by the two-dimensional COF is similar to the channel in the Nafion structure, which makes them a potential material for proton transport. However, traditional COF materials have poor chemical stability, which limits their application in acidic proton exchange membranes.
In view of this, Zhang Gen’s team developed a bottom-up self-assembly strategy, constructed a perfluoroalkyl functionalized two-dimensional COF, and systematically studied the COF crystal state and proton conductivity properties of fluorine chains of different lengths. Impact. Compared with fluorine-free COF, fluorinated COF has superior structural stability to strong acids due to its enhanced hydrophobicity (water contact angle is 144 o). It has strong structural stability in concentrated phosphoric acid (85%), concentrated nitric acid (65%) and concentrated It can exist stably in hydrochloric acid (38%). The characterization results found that after phosphoric acid doping modification, the proton transport conductivity of the fluorinated COF material under anhydrous conditions reached 4.2×10-2 S m-1 (140 ℃), which is the current anhydrous proton transport of organic porous materials One of the highest performance examples, and the ion transport performance is 10,000 times that of fluorine-free COF. The solid-state NMR characterization results show that phosphoric acid is connected to each other through hydrogen bonds in the COF channel. Most phosphoric acid has strong mobility, and the COF frame structure is rigid, which has the performance of rapid proton conduction.
This article provides a successful example of COF functional modification through the design of COF pore structure. This research paved the way for the pre-design and functionalization of the pore surface to achieve the target performance of COF, and highlights the great potential of the COF nanochannel as a fast ion transport platform.