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Nitrogen-coordinated monoatomic cobalt electrocatalysts, especially electrocatalysts derived from high-temperature pyrolysis of cobalt based zeolite imidazole skeletons (ZIFs), have become a new frontier in the design of oxygen reduction positive electrodes for proton exchange membrane fuel power cells (PEMFCs) because they have higher durability and less Fenton effect related to membrane and ionomer degradation compared to prominent iron based electrocatalysts. However, pyrolysis techniques can lead to ambiguous active site configurations, unsatisfactory porosity and morphology, and fewer exposed active sites.

Recently, Professor Xiang Zhonghua from Beijing University of Chemical Technology and Professor Wu Gang from the State University of New York at Buffalo have directly prepared highly stable crosslinked nanofiber electrodes using a liquid processable cobalt based covalent organic polymer (Co-COP) obtained through a non pyrolysis strategy and electrospinning. The resulting fibers can easily be organized into a self supporting large area membrane with a uniform layered porous structure and complete dispersion of Co active sites on the catalyst surface. Focused ion beam field emission scanning electron microscopy and computational fluid dynamics experiments have confirmed that the relative diffusion coefficient has increased by 3.5 times, which can provide an effective way for reactants to enter the active site, and can effectively discharge emerging water. As a result, compared to traditional spraying methods, the peak power density of the integrated Co-COP nanofiber electrode was significantly increased by 1.72 times, and the durability was improved. It is worth noting that this nano fabrication technology also maintains good scalability and uniformity, which are necessary characteristics to assist in the preparation of PEMFCs membrane electrode assemblies.

Key points of the article:

This work directly prepared a highly stable cross-linked nanofiber electrode based on liquid processable CoCOP obtained using a non pyrolysis strategy through electrospinning technology.

2. Liquid processability Co-COP contains powerful conjugation systems and abundant ordered N-coordinated Co monoatomic centers. The layered porous structure obtained by electrospinning fully exposes the active points of Co-N4. Compared with the traditional spray electrode morphology, the large pores of a single nanofiber in the electrospun film facilitate the diffusion of gas to the active site. The secondary micron sized pores between the filaments can cause a large amount of gas to diffuse to the catalyst layer through a low pressure drop, forming a high-speed electron transfer network.

3. FIB-FESEM and CFD experiments further confirmed the lower oxygen diffusion resistance. According to Fick's first law, the relative diffusion coefficient is 46.9%, which can provide a good channel for the reactant to enter the active site and remove the water present, thereby not inhibiting the reaction. The results show that the power density of the integrated nanofiber electrode is 1.72 times that of the traditional spray electrode. It is worth noting that the crosslinked nanofiber electrode can still maintain 85% of the original voltage after 150 hours of durability testing.

This nano fabrication technology has good scalability. For example, the performance of a 200 square centimeter film obtained can maintain good consistency at randomly selected locations on the film. By designing a specific electrode structure to maximize the transmission efficiency of the overall electrode and improve the availability of active sites, this work overcomes the transmission limitations of traditional spray coated electrodes, and provides a new method for the future processing of PEMFC film electrodes.

Figure 1 Preparation of cross-linked nanofiber electrocatalyst

Figure 2 Structural Characterization

Figure 3 Performance and Durability Test of PEMFC

Figure 4 Computational Fluid Dynamics Simulation and Oxygen Diffusion

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