Waste to wealth: spent catalyst as an efficient and stable bifunctional oxygen electrocatalyst for zinc–air batteries

Abstract

The spent catalysts obtained from catalytic decomposition of methane are often considered waste and typically subjected to energy intensive processes such as high-temperature combustion for recycling or chemical treatment for the reclamation of metal constituents. For the first time, we show that the spent catalyst can be used as a bifunctional electrocatalyst for possible applications in the zinc–air battery because of its suitable structural and electronic properties. Detailed spectroscopic and microscopy characterization has revealed that it contains Ni nanoparticles along with multiwalled carbon nanotubes supported on porous alumina (NAlCNTs-750). The NAlCNTs-750 exhibits a low overpotential value of 370 mV at 10 mA cm⁻² with a Tafel slope of 119 mV dec⁻¹ towards the oxygen evolution reaction (OER). For the oxygen reduction reaction (ORR), the onset potential is found to be 0.82 V with a Tafel slope of 88 mV dec⁻¹. The chronoamperometry of NAlCNTs-750 exhibits good stability for 20 and 8 h towards the OER and ORR, respectively. Consequently, the NAlCNTs-750 employed in Zn–air batteries has displayed commendable charge–discharge performance up to 45 h with high reversibility. A careful electrochemical analysis of individual components reveals that the bifunctional nature of NAlCNTs-750 arises from Ni nanoparticles serving as active metal centers towards the OER and CNTs contributing to the ORR as well as facilitating low charge transfer resistance, whereas Al₂O₃ provides the required porous support. The spent catalysts from industry may realize the dream of achieving “today's waste is tomorrow's energy”.