Maximizing the utilization of Fe–NxC/CNx centres for an air-cathode material and practical demonstration of metal–air batteries

Abstract

Noble-metal free carbon-based electrocatalysts for oxygen reduction/evolution reaction (ORR/OER) in an alkaline medium have drawn significant attention in the search of suitable/potential replacement of commercial expensive Pt/Ru/Ir-based electrocatalysts. In this regard, nitrogen-doped carbon nanotubes (NCNTs) have emerged as potential candidates for ORR/OER and they emulate Pt/Ru-based catalysts. However, the unexposed/unused active sites rendered by N-doping, responsible for bamboo-shaped compartment formation, in NCNTs are hardly accessible to the reactant species and thus barely contribute to ORR/OER. To maximize the utilization of the active centres, Fe–Fe₃C nanoparticles entrapped in the graphitic layers and hollow structures of NCNTs are fabricated by pyrolyzing melamine with excess of ferrocene. Annealing at 375 °C in an oxygen environment removes the protective graphitic layers around Fe–Fe₃C, and acid-treatment provides an opening to the bamboo compartments of NCNTs (ANCNTs); this offers maximum accessibility of the active sites via the creation of more edges and roughness that enhances the ORR/OER activity. Interestingly, more positive ORR onset and half-wave potentials with excellent accelerated stability (∼98% current retention) and fuel tolerance along with lower onset and E_j=10(OER) potential with high OER current density as compared to those of commercial state-of-the-art electrocatalyst suggest the superior bifunctional behaviour of ANCNTs. The poisoning study with NaSCN on ANCNTs shows the direct involvement of Fe-based centres in oxygen electrochemistry. The overall oxygen electroactivity ΔE = E_j(OER)=10 − E_1/2(ORR) = 0.72 V for ANCNTs is much lower than that of the commercial and recently reported various state-of-the-art bifunctional catalysts. Finally, a prototype battery arrangement using NCNTs as a cathode electrode for powering a light-emitting diode has been demonstrated. Overall, NCNTs can serve as non-precious electrocatalysts for electrochemical energy devices.