Co-doping regulation on Ni-based electrocatalysts to adjust the selectivity of oxygen reduction reaction for Zn–air batteries and H2O2 production

Abstract

Although Ni-based materials are widely used as electrocatalysts, it remains necessary to further explore their selectivity towards the four- or two-electron oxygen reduction reaction (ORR). Herein, it is proposed to synthesize NiO@NCNTs (NCNTs = N-doped carbon nanotubes) using a metal–organic framework (MOF), [Ni(BZIDA)(H₂O)]_n (NiMOF, BZIDA = benzimidazole-5,6-dicarboxylic acid), as a precursor after calcination with dicyandiamide (DCDA). Regarding NiO@NCNTs, small NiO particles are distributed in NCNTs derived from DCDA homogeneously. NiO@NCNTs act as a typical two-electron electrocatalyst. The H₂O₂ production rate of NiO@NCNTs reaches 0.5 mol g⁻¹ h⁻¹ at 0.46 V (vs. RHE). After the doping of Co²⁺ in NiMOF, Co/NiO@NCNTs were synthesized using a similar method, with the four-electron character shown in ORR. A Zn–air battery was assembled by applying Co/NiO@NCNTs as the cathode material. When discharge occurs at 5 and 10 mA cm⁻², its specific capacitance reaches 779.3 and 832.2 mA h g⁻¹ with an energy density of 928.6 and 948.5 W h kg⁻¹, respectively. Theoretical calculations suggest a variation in ORR selectivity between NiO@NCNTs and Co/NiO@NCNTs, which results from their different interactions with OOH*. This study demonstrates the effect of the structure on ORR selectivity for Ni-based electrocatalysts.