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Macro/Microporous Covalent Organic Frameworks for Efficient Electrocatalysis

Zhao, Xiaojia, Pachfule, Pradip, Li, Shuang, Langenhahn, Thomas, Ye, Mengyang, Schlesiger, Christopher, Praetz, Sebastian, Schmidt, Johannes, Thomas, Arne
Journal of the American Chemical Society 2019 v.141 no.16 pp. 6623-6630
cobalt, crystal structure, macropores, mass transfer, micropores, moieties, oxygen production, porous media, surface area
Covalent organic frameworks (COFs) are of interest for many applications originating from their mechanically robust architectures, low density, and high accessible surface area. Depending on their linkers and binding patterns, COFs mainly exhibit microporosity, even though COFs with small mesopores have been reported using extended linkers. For some applications, especially when fast mass transport is desired, hierarchical pore structures are an ideal solution, e.g., with small micropores providing large surface areas and larger macropores providing unhindered transport to and from the materials surface. Herein, we have developed a facile strategy for the fabrication of crystalline COFs with inherent microporosity and template-induced, homogeneously distributed, yet tunable, macroporous structures. This method has been successfully applied to obtain various β-ketoenamine-based COFs with interconnected macro–microporous structures. The as-synthesized macroporous COFs preserve high crystallinity with high specific surface area. When bipyridine moieties are introduced into the COF backbone, metals such as Co²⁺ can be coordinated within the hierarchical pore structure (macro-TpBpy-Co). The resulting macro-TpBpy-Co exhibits a high oxygen evolution reaction (OER) activity, which is much improved compared to the purely microporous COF with a competitive overpotential of 380 mV at 10 mA/cm². This can be attributed to the improved mass diffusion properties in the hierarchically porous COF structures, together with the easily accessible active Co²⁺-bipyridine sites.