Abstract
Catalysts derived from pyrolysis of metal organic frameworks (MOFs) are promising candidates to replace expensive and scarce platinum-based electrocatalysts commonly used in polymer electrolyte membrane fuel cells. MOFs contain ordered connections between metal centers and organic ligands. They can be pyrolyzed into metal- and nitrogen-doped carbons, which show electrocatalytic activity toward the oxygen reduction reaction (ORR). Furthermore, metal-free heteroatom-doped carbons, such as N-F-Cs, are known for being active as well. Thus, a carbon material with Co-N-F doping could possibly be even more promising as ORR electrocatalyst. Herein, we report the mechanochemical synthesis of two polymorphs of a zeolitic imidazole framework, Co-doped zinc 2-trifluoromethyl-1H-imidazolate (Zn0.9Co0.1(CF3-Im)2). Time-resolved in situ X-ray diffraction studies of the mechanochemical formation revealed a direct conversion of starting materials to the products. Both polymorphs of Zn0.9Co0.1(CF3-Im)2 were pyrolyzed, yielding Co-N-F containing carbons, which are active toward electrochemical ORR.
Highlights
Polymer membrane fuel cells (PMFCs) are of significant interest as a device for clean energy conversion
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author
All authors have contributed to the work and approved the final version of the manuscript
Summary
Polymer membrane fuel cells (PMFCs) are of significant interest as a device for clean energy conversion. Using zinc-based ZIFs with low amounts of cobalt results in a porous carbon material with uniformly distributed Co-N4 sites and no metallic cobalt particles, showing excellent ORR activity (Wang et al, 2016). For Co-doping, the zinc acetate dihydrate was replaced by cobalt acetate tetrahydrate (10 mol% of total metal content, 0.037 mmol, 9.2 mg), which was added to the milling jar (PMMA, 5 mL), along with ZnO (0.333 mmol, 27.1 mg), 2methylimidazole (0.740 mmol, 60.7 mg), NH4NO3 (0.037 mmol, 3.0 mg), methanol (15 μL), and a stainless-steel grinding ball (7mm diameter). Zn0.9Co0.1(CF3-Im) can be obtained when Zn5(CO3)2(OH) (0.046 mmol, 25.0 mg), cobalt acetate tetrahydrate (0.025 mmol, 6.3 mg), and 2-trifluoromethyl-1H-imidazol (0.505 mmol, 68.8 mg) are placed alongside a single grinding ball (5-mm diameter, stainless steel) into a custom-made milling jar (PMMA, 5 mL). Nitrogen gas sorption at 77 K was performed on an ASAP 2020 (Micrometrics) and was used to calculate the specific surface area from a multipoint adsorption isotherm with the BET (Brunauer–Emmit–Teller) calculation model (relative pressure range, 0.0012–0.0298) according to DIN ISO 9277:2014 (Brunauer et al, 1938)
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