On October 14, 2022, Prof. Bo Wang's team from Beijing Institute of Technology published the article "Covalent organic framework-based porous ionomers for high-performance fuel cells" inScience. In terms of gas and water transport and electrocatalysis in the oxygen-water-catalyst three-phase interfaces of the core membrane electrode (MEA) of fuel cells, they firstly proposed and constructed covalent organic framework (COF) porous ionomers for fuel cells. The concept of porous framework ionomer breaks through the traditional chain ionomer and can significantly improve the mass transfer efficiency of the catalytic layer and significantly increase the power density of the fuel cell. The mass activity of Pt and the peak power density of the fuel cell with Pt/Vulcan both reached 1.6 times those values without the COF.

The first author of this work is Qingnuan Zhang, a postdoctoral fellow in the School of Chemistry and Chemical Engineering of Beijing Institute of Technology, and the corresponding authors are Bo Wang and Xiao Feng, and Beijing Institute of Technology is the first and only corresponding author of this work.

With the goal of carbon neutrality, the development of hydrogen energy technology has become an inevitable trend. Proton exchange membrane fuel cells, as a port for the large-scale utilization of hydrogen energy, is one of the most promising ways to utilize hydrogen energy. The key to realize the development and large-scale application of proton exchange membrane fuel cells technology lies in the development of high-performance and low-cost membrane electrode materials. The catalytic layer, which is composed of Pt/C catalyst and ionomer, serves as the core of the membrane electrode and is the site where electrochemical reactions occur in the fuel cell. To ensure the efficiency of the electrochemical reaction, the catalytic layer needs to simultaneously provide channels for the protons required for the reaction, the reaction gases that can reach the catalyst, and the ability to transport the water molecules generated by the reaction. Currently, perfluorosulfonic acid polymer (PFSA, or Nafion) is the most widely used ionomer, which enables fast proton conduction. However, at the same time, Nafion can cause excessive wrapping around the Pt catalyst, resulting in large gas resistance and low catalytic active site utilization, thus preventing the catalyst performance from being fully exploited.

To address the above problems, the authors of the paper proposed a strategy to construct covalent organic framework-based porous ionomers to optimize the three-phase interface on the catalyst surface, enabling the development of a high-performance low-platinum content proton exchange membrane fuel cell system. Guided by framework chemistry, the authors precisely tailored the synthesis of porous framework two-dimensional polymers with good chemical stability, thermal stability and swelling resistance by linking organic radicals through covalent bonds. The resulting two-dimensional polymer planar structure, consisting of a hexagonal skeleton extended infinitely periodically in the two-dimensional direction, utilizes sulfonic acid-based cantilevers inside the hexagonal skeleton to provide high proton conduction, with the remaining space capable of providing sufficient pathways for oxygen and water to facilitate transport. Using commercial Pt/C as the cathode catalyst with a catalyst content of only 0.07 mgPt cm^-2, the use of porous COF ionomers increased by 1.6 times in both the mass activity of the Pt/C catalyst and the peak power density of the fuel cell, with a peak H₂-Air power density of 1.08 W cm^-2.

Fig. 1. Overview of Pt/C@COF-Nafion catalytic layer and its mechanism of action in fuel cells.

In the paper, the mechanism of porous COF ionomers in the catalytic layer of fuel cells is discussed in detail, and the effects of mesoporous COF nanosheets on gas diffusion, proton transport and water management in the catalytic layer are analyzed. Compared with conventional chain ionomers, porous COF ionomers have the following advantages.

1) Conducive to gas mass transfer. The mass transfer resistance of O₂at the limiting current density in H₂-air cells was reduced by 40% after the addition of porous COF ionomers. Oxygen permeability test showed that the mixed COF and Nafion matrix membrane had significantly higher oxygen permeability than the pure Nafion membrane, and could maintain gas permeability under humidity.

2) High proton conductivity, optimized water management. The proton conductivity of porous COF ionomer is slightly higher than that of Nafion. The pore structure and water absorption and release ability of porous COF ionomer are not only conducive to inhibit the occurrence of high power density flooding, but also can help fuel cells to show better performance under low humidity.

3) Mitigation of catalyst poisoning. The spatial site blocking effect generated by the porous COF nanosheets significantly reduced the overwrapping of the ionomer on the Pt/C catalyst, alleviated the direct contact between the sulfonic acid group and Pt, helped to expose more Pt active sites, increased the electrochemically active surface area, and improved the mass activity of the catalyst.

Fig. 2. Comparing the cathode Pt loading and peak power density for MEAs based on commercial Pt/C catalyst in this study and previous reports.

By introducing COFs, i.e., the introduction of rigidly developed framework nanosheets with abundant mesopores without sacrificing plasmonic conductivity, the fuel cell performance can be significantly improved. With ultra-low platinum content, a new record power density has been achieved, which means that the cost of generating 1kW of electricity is expected to be reduced by about one-third. In addition, the DOE 2025 target is to reduce the Pt group metal total content to 0.1 g kW⁻¹. This goal can be expected to come true with commercial catalysts using porous COF ionomers to optimize the structure of the catalytic layer.

The good thermal stability, acid-base stability and structural designability of COF also make it promising for high-temperature fuel cells or alkaline fuel cells. The design strategy of porous COF ionomer is a milestone for optimizing the ORR three-phase microenvironment of fuel cell catalytic layer.

Note: This study was also supported by the Key Research and Development Program of the Ministry of Science and Technology, the Surface Project of the National Natural Science Foundation of China, the Excellent Youth Program, the Beijing Science and Technology Program, and the China Postdoctoral Science Foundation.

About the Author

Bo Wang

Professor of School of Chemistry and Chemical Engineering, Beijing Institute of Technology

Professor, doctoral advisor, member of the Standing Committee of the University Party Committee, Vice president; 10th Standing Committee Member of China Association for Science and Technology, Director of Frontier Science Center of High Energy Matter. He received his bachelor's degree from Peking University, Master's degree from University of Michigan, and Doctor's degree from University of California, Los Angeles. Winner of the National Science Fund for Outstanding Young Scholars, and selected as the leading young scientific and technological innovation talents in the National Talent Program and the National Innovation Talent Promotion Program. He is engaged in the research of novel nanoporous materials and molecular capture, separation and their applications in energy storage, transformation and biomedical applications. He undertakes many key projects including the Ministry of Science and Technology, National Natural Science Foundation of China and Beijing. He has published more than 100 papers in academic journals such asNature,Science,Nature Materials,JACS,Angew, with more than 17,000 SCI citations. He has been granted 6 US patents and 8 Chinese invention patents.

Xiao Feng

Professor of School of Chemistry and Chemical Engineering, Beijing Institute of Technology

Professor and doctoral supervisor, received his PhD degree from the School of Materials Science, Beijing Institute of Technology (supervisor, Dong Yoping), and studied in the Institute of Molecular Science, Japan (supervisor, Jiang Donglin) during his PhD. He is mainly engaged in the research of mass transfer and separation in porous materials such as covalent organic frame materials. He has hosted the National Natural Science Foundation of China Outstanding Youth Science Foundation projects, general projects, etc., published more than 40 papers as corresponding author on journals includingScience,Nat. Mater.,J. Am. Chem. Soc.,Angew. Chem. Int. Ed.,Nat. Commun. He has been awarded the "Jingqing Chemistry Emerging Prize" by the Chinese Chemical Society, "Widely Concerned Academic Papers in Beijing" and other awards. He serves as a youth editorial board member ofScience China: Chemistry,Sci. China Chem.andChin. Chem. Lett.

Qingnuan Zhang

Postdoctoral Fellow of School of Chemistry and Chemical Engineering, Beijing Institute of Technology

She graduated from Beijing Institute of Technology with Professor Zhang Yunhong as her supervisor, and then did post-doctoral research in Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences with Professor Yan Xingbin as her co-supervisor and Professor Wang Bo as his co-supervisor. She is engaged in the research of electrochemical energy storage devices, including fuel cells, supercapacitors, etc. She is also the director of the National Natural Science Foundation of China (NSFC) Youth Fund Project, the Young Backbone Individual Project supported by Beijing Excellent Talents Training Fund, the China Postdoctoral Special Fund (Station Center) Project and the China Postdoctoral General Fund Project. She has published 15 papers and 1 invention patent.