Controlled Chelation between Tannic Acid and Fe Precursors to Obtain N,S Co-doped Carbon with High Density Fe-Single Atom-Nanoclusters for Highly Efficient Oxygen Reduction Reaction in Zn-Air Battery

In this work, three-dimensional N, S co-doped carbon with high density Fe-single atom-nanoclusters of homogenous dispersion (N, S co-doped CPANI-TA-Fe Fe-SA-NC catalysts) is successfully prepared by the optimal pyrolysis of freeze-dried polyaniline (PANI) hydrogels containing homogenously distributed Fe precursors due to the controlled chelation between tannic acid (TA) and ferric ions, and the interaction between Fe precursors and PANI for the first time, to the best of our knowledge. More importantly, active species (such as Fe single atoms, Fe-SA-NCs and Fe NPs) in the 3D N, S co-doped carbon materials can be controlled by judiciously adjusting the Fe-to-TA molar ratios. High density Fe-SA-NCs as active species (up to 5.6 wt %) in the N, S co-doped carbon are realized by controlling the Fe-to-TA molar ratio as 10. In addition, these Fe-SA-NCs are composed of isolated Fe-SA, which are separated by the coordination with four N atoms and two O atoms. The resulting N, S co-doped CPANI-TA-Fe Fe-SA-NC catalysts for oxygen reduction reaction (ORR) bear a higher half-wave potential (0.923 V vs 0.88 V of Pt/C), a higher onset potential (1.09 V vs 0.99 V of Pt/C), a higher kinetic-limiting current density (12 mA cm−2 at 0.9 V) and better long-term stability in alkaline media. Their ORR activity in acidic media is comparable to those recently reported in literature. Additionally, the suitability and durability of N, S co-doped CPANI-TA-Fe Fe-SA-NC catalysts as the air electrode of zinc–air battery were investigated. The fabricated Zn–air batteries exhibited a higher power density of 136.4 mW cm−2 and a higher specific capacity of 795.1 mAh g−1, which is better than those of state-of-the-art Commercial Pt/C catalyst. Our synthetic strategy can extend to prepare other metal nanoclusters-based porous carbon materials as electrocatalysts in electrochemical energy devices, as TA can be chelated with diverse metal ions.