Iron-Nitrogen co-Doped Porous Carbon Derived from ZIFs as an Effective ORR Electrocatalysts

Oxygen reduction reaction has been one the challenging reactions in the energy conversion and storage systems [1]. The reason behind this matter is ORR sluggish reaction and its urgent need to the high cost Pt catalyst. Hence, the widespread application of such devices is limited primarily because of the high price of Pt beside its scarce reserve, poor durability, and low poison resistance [2]. These issues have encouraged researchers to substitute Pt-based catalysts with nonprecious metal catalysts (NPMCs) especially metal-nitrogen-carbon. These catalysts have promising properties such as low cost, good activity and stability comparable or even better than those of Pt-based catalysts in both alkaline and acidic electrolyte [3]. Recently, Fe-based catalysts have emerged as one of the most promising NPMCs [4]. Many reports have been published in the domain of pyrolyzed carbon materials obtained of subclass of MOFs named zeolitic imidazolate framework (ZIF) especially ZIF-8 nanocrystals, composed of imidazole linkers containing carbon and nitrogen atoms and Zn+2 ions. A feature that makes these templates more applicable for fabrication of different catalysts with unique and special properties is their ability to mix with other precursors contain different metals (e.g., Co, Fe, Ni) [5]. Because the boiling point of Zn atoms is low (mp 420 ̊C, bp 907 ̊C), they can evaporate away at high temperatures over 800 ̊C, and second added metal (Fe) nodes is reduced in situ by carbonization of the organic linker. When the molar ratio between two metals was adjusted precisely, the resulted catalyst after high thermal treatment 69 can have single metal atoms anchored on N-doped porous carbon. Such single-atom catalysts (SACs) represent the lowest size limit to obtain the maximum atom efficiency and expose the most active sites in catalysts [6]. On the other hand, adjusting the selected thermal treatment is crucial to maintain this single atom dispersion on the N-doped carbon matrix. Highly dispersed FeNx active sites on the carbon matrix make excellent ORR performance in both acidic and alkaline media. In this report, the obtained Fe-Nx-900 single sites exhibited superior ORR performance with onset potentials (0.887 VRHE, 1.017 VRHE) in acidic and alkaline environments, respectively that the obtained results for commercial Pt/C were 0.937 VRHE and 0.987 VRHE, respectively. As can be seen, in the acidic media, the performance of SAC was appropriate rather than Pt/C but in the alkaline media, a better performance was obtained for SAC. On the other hand, when the temperature of heat treatment process was increased to 1000 ̊C, Fe NPs prevailed. In this temperature, likely the Fe SAs would sinter and Fe-N bonds braked for facile formation of Fe NPs. This conclusion was based on the current density drop than that of Fe-Nx-900 in which is because of decrease effective active sites.