Single atomic Fe-pyridine N catalyst with dense active sites improve bifunctional electrocatalyst activity for rechargeable and flexible Zn-air batteries

Abstract

The iron-pyridinic-N moiety is considered more active for the oxygen reduction reaction (ORR). However, low active site density and oxygen evolution reaction (OER) activity hinder the further improvement of Zn-air batteries (ZABs). Herein, the macromolecules of the different nitrogen sites have been rationally designed and fabricated to create a carbon material with a high content of pyridine N. On this basis, the activated carbon matrix allows the halfway reduction of Fe³⁺ to Fe^x+, which is coordinated with pyridinic N to form a high-loading Fe-pyridinic N bonding structure. It also stimulates the complete reduction of Fe³⁺ to a single Fe atom on a 3D sponge carbon structure (named Fe/Fe-NC-3). Benefiting from pyridine N, the Fe-pyridinic N-bonding structure, single atom dispersion, and hierarchical pore channels, the Fe/Fe-NC-3 shows superior ORR and OER electrochemical activity, approaching that of the commercial Pt/C and IrO₂. In addition, Fe/Fe-NC-3 as a ZAB air-electrode exhibits outstanding stability for over 500 h and a remarkable power density of 304.4 mW cm⁻² in reversible ZABs. Moreover, the flexible ZABs deliver a high open-circuit voltage of 1.35 V, stable voltage gap under flat/bent states, and discharge voltage at 2 mA cm⁻². This study provides a new perspective for preparing bifunctional catalysts with enriched active sites.