Bifunctional electrocatalysts based on hierarchical graphene/iron hybrid architectures branched by N-doped CNT

A noble metal-free, highly active and stable bifunctional electrocatalyst with high activity and stability is needed to replace existing precious ones for electrochemical energy conversion and storage systems, such as fuel cells and metal-air batteries. Herein, we report hierarchical steam-activated reduced graphene oxide (srGO)/Fe hybrid architectures branched by nitrogen-doped carbon nanotube (N-CNT) for bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts. Through the adequate choice of ferrocene precursor and microwave method, the CNT branches, the active iron phase, and N-doping are accomplished simultaneously to construct a bifunctional hybrid electrocatalyst. The Fe₂O₃/N-CNT@srGO hybrid architecture achieves an ORR onset potential of 0.72 V, a mass activity of 605.30 mA·mg_active^⁻1, an OER onset potential of 1.63 V, and a Tafel slope of 61 mV·dec^⁻1, which are much higher compared to those of the Fe₂O₃/CNT@srGO, Fe₂O₃@N-srGO, and Fe₂O₃@N-CNT catalysts. These results indicate the enhancement of bifunctional catalytic activity due to hierarchical porous structure, CNT branches, and N-doping.