Dendritic hollow nitrogen-doped carbon nanospheres for oxygen reduction at primary zinc–air batteries

Abstract

The cost and scarcity of platinum-based electrocatalysts are the major drawbacks for the large-scale commercialization of low-temperature fuel cells. Transition metal or metal-free heteroatom-doped carbon catalysts are promising alternative electrocatalysts for the oxygen reduction reaction in alkaline fuel cells. Dendritic hollow N-doped carbon spheres with a large inner accessible surface area are synthesized from the polyindole precursor on a dendritic fibrous nano-silica template. The thus-synthesized metal-free N-doped carbon material exhibits the highest onset (∼1.0 V vs. RHE) and half-wave potential (0.84 V vs. RHE), surpassing the ORR activity of the state-of-the-art Pt/C catalysts. The pore size plays an important role in the activity and in controlling the reaction pathway, wherein mesopores catalyze the partial reduction of O₂ to H₂O₂, and micropores facilitate the H₂O₂ reduction to H₂O. A zinc–air battery using the synthesised catalyst as the ORR catalyst exhibits the highest power density of 175 mW cm⁻² with excellent specific capacity (951 mA h g_zn⁻¹ @ 20 mA cm⁻²), depending on the surfactant used and synthesis method. The ORR mechanism on the dendritic hollow N-doped carbon, based on the synthesis method and surfactants, is discussed.