Design of high-performance high temperature proton Exchange Membrane -Poly (triphenyl piperidinium) with long side chain alkyl quaternization

Proton exchange membrane fuel cell (PEMFC) is regarded as one of the most promising energy conversion systems, which has high social and economic benefits and good environmental protection advantages and has broad application prospects. As one of the key components of PEMFC, the proton exchange membrane (PEM) plays an irreplaceable role in transferring protons and isolating gases. Up to now, both the commercial perfluorosulfonic acid membranes (i.e. Nafion), and other pendant sulfonate group grafted polymer membranes show excellent fuel cell performance under full humidification below 80 °C. However, the relatively low operating temperature of PEMFC brings the following technical problems, such as low resistance to CO poisoning, complex hydrothermal management, and low electrode dynamics. Therefore, it is of great significance to develop non-fluorinated high temperature proton exchange membrane (HT-PEM) that can operate between 100 °C and 200 °C. Recently, a series of poly (triphenyl piperidinium) based HT-PEMs were synthesized by the team of Professor Jingshuai Yang of Northeastern University. Their work was published in the journal Industrial Chemistry & Materials on August 23, 2023.

"Our goal is to build and design a series of high-performance high temperature proton exchange membranes for fuel cell applications. The basic idea is to quaternize poly(triphenyl piperidine) (PTP) with alkyl side chains of different lengths to optimize the physicochemical properties of HT-PEMs" explained Jingshuai Yang, a professor at Northeastern University, China. The presence of N-methyl piperidine groups in the PTP repeat structural unit makes the membranes prone to adsorb PA by acid-base and hydrogen bonding. PTP was chosen as the matrix structure for adsorbing PA. The team grafted five side-chain alkyl groups of different lengths, including methyl, propyl, pentyl, heptyl and decyl groups, into the PTP polymer backbone through the Menshutkin reaction. The acid doping content (ADC%) and acid doping level (ADL) of the pure PTP membrane reach about 120% and 4.0. However, the grafted side-chain alkyl groups have a great influence on the acid doping of the PTP-Cx membranes.

The membranes were prepared by a simple one-step solution casting method. It can be seen from the photos and SEM images of membranes with different structures that the prepared membranes are uniformly transparent and dense without micropores, which is conducive to blocking the feed gas in HT-PEMFC.

The ADC% of HT-PEM will directly affect the conductivity and mechanical stability of the membrane. It can be seen from the obtained data that the introduction of side chain alkyl can significantly affect the PA doping content of the membranes. Specifically, with the increase of the number of side chain alkyl carbon atoms, the PA doping content of the membrane shows a law of first increasing and then decreasing. This may be the result of the free volume and hydrophilicity of the membrane changed by side chain alkyl. When the pentyl group was grafted, PTP-C5 membrane obtained the highest PA doping content of 202%.

PA doped HT-PEM relies on the hydrogen bond network between the PA and the polymer to achieve proton transfer. Therefore, the higher PA doping content of HT-PEM, the higher proton conductivity of the membranes. The PTP-C5 membrane has the highest conductivity (96 mS cm^-1) at 180 °C, which is obviously due to its high PA doping level (ADL= 8.0). However, PA molecules not only play a role in proton conduction but also bring about plasticizing effects and reduce interactions between polymer chains. Therefore, for PA doped HT-PEM, the trade-off between electrical conductivity and mechanical properties needs to be considered. The PTP-C5 membrane with the highest acid doping level showed the lowest tensile strength (4.6 MPa) at room temperature. However, the mechanical strength still meets the battery assembly and testing.

Considering the mechanical strength and electrical conductivity of the membrane, the PTP-C5/202%PA membrane was selected to assemble the cell, and the performance of the H₂-air fuel cell was tested under the condition without humidification and backpressure. The cell has a high open circuit voltage (OCV) of about 0.9 V at 160 -210 °C, revealing the low gas permeability of the PTP-C5/202% PA membrane. At the same time, the performance of the cell gradually improved with the increase of temperature, and the highest peak power density was obtained at 210 °C: 676 mW cm⁻². Therefore, this work provides a simple design method to fabricate HT-PEM with excellent performance and provides a new choice for other energy devices.

Looking ahead, the team hopes their work will provide insights into the development and design of high-performance HT-PEMs for use in fuel cells. "Our next step is to further improve the properties of membranes and determine the durability, in order to achieve the ultimate goal of commercialization and industrialization. The series of HT-PEMs we have developed may also be expected to be applied to various types of batteries, such as vanadium redox flow batteries, water zinc batteries, and so on," said Yang.

The research team included Jingshuai Yang, Xuefu Che, Lele Wang, Ting Wang and Jianhao Dong from Northeastern University.

This work was supported by the National Natural Science Foundation of China (51603031) and the Basic Research Foundation of the Central Universities of China (N2005026). Special thanks to the Professor Qingfeng Li of the Technical University of Denmark for helping to test the performance of the fuel cell.

Industrial Chemistry & Materials is a peer-reviewed interdisciplinary academic journal published by Royal Society of Chemistry (RSC) with APCs currently waived. Icm publishes significant innovative research and major technological breakthroughs in all aspects of industrial chemistry and materials, especially the important innovation of the low-carbon chemical industry, energy, and functional materials.