Hierarchically porous Fe-N-C microspheres via pyrolyzing hypercrosslinked polypyrrole synthesized by Fe3+-catalysis for efficient oxygen reduction in Zn-air battery

Fe-N-C catalysts have developed into the most promising substitute for Pt-based precious metal catalysts toward cathodic oxygen reduction reaction (ORR) of fuel cells and Zn-air batteries. Hypercrosslinked polypyrrole microspheres coordinated with Fe (HCP-Py-Fe) were first prepared by condensation of pyrrole (Py) and dimethoxymethane (FDA) in the presence of FeCl3 as both catalyst and Fe source, and then thermally activated with KOH during pyrolysis followed by acid leaching and heat-treatment, thus constructing hierarchically porous Fe-N-C microspheres (denoted as PPy/FDA-Fe-y-HT2-x). The morphology and structural properties of the final synthesized PPy/FDA-Fe-y-HT2-x could be tailored to a certain extent by controlling the adding quantities of KOH (x) in the initial solid mixture before pyrolysis and FeCl3 (y) during condensation of Py and FDA. As a result, the PPy/FDA-Fe-0.4-HT2-0.25 demonstrated the highest half-wave potential of 0.87 V vs. RHE, with a desired 4-electron transfer for ORR in alkaline medium, which is attributed to the hierarchically micro/mesoporous structure, high surface area and extremely low content of magnetic iron species. Remarkable methanol resistance and durability with only a loss of similar to 4 mV in half-wave potential after 10000 potential cycles were obtained as well. The aqueous Zn-air battery (ZAB) fabricated with the PPy/FDA-Fe-0.4-HT2-0.25 as cathode catalyst delivered a comparable discharge performance to that of the Pt/C-based ZAB but far superior cycling stability. This facile route is promising for large-scale production of Fe-N-C materials as cathodic electrocatalyst applied in practical energy conversion devices.