A comprehensive analysis and rational designing of efficient Fe-based oxygen electrocatalysts for metal–air batteries

Abstract

Precious metal based electrocatalysts are considered as the most efficient ones to drive kinetically sluggish oxygen evolution/reduction reactions (OER/ORR) for metal–air batteries and fuel-cells. However, their monofunctionality in addition to their exorbitant cost has stimulated the quest for economically viable bifunctional electrocatalysts for use in next generation electrochemical energy devices. Here, we report Fe–Fe₃C encapsulated in Fe–Nx enriched spheres of N-doped carbon nanotubes (FCMR, M = 3, 10, 25, 40 represents the ramping rate of temperature during synthesis) as a potentially enduring, cost effective, highly efficient bifunctional electrocatalyst for total oxygen electrochemistry (ORR and OER) and a comprehensive study to elucidate the role of various Fe moieties. In addition to the improved OER/ORR activities as evident from the better onset potential, lower Tafel slopes and high current densities over commercially available RuO₂/Pt–C electrocatalysts and several recently reported state-of-the-art bi-functional electrocatalysts, FC10R shows a current retention value of ∼93 and ∼98% after the accelerated cyclic stability test for the OER and ORR, respectively. The preferable 4e⁻ pathways and suppressed peroxide generation in the ORR by FC10R further ensure maximum electrochemical energy harvesting. Remarkably, the complete oxygen electrochemistry of FC10R in alkaline medium as evaluated from ΔE (=E_{j(OER) = 10} − E_1/2(ORR) = 0.758 V) is significantly lower than that of commercially available/recently reported electrocatalysts and advocates the minimum cyclic loss. The overall study elucidates the synergistic effect of Fe–Nx coordination and Fe₃C moieties on oxygen electrochemistry, and FC10R has shown its potential to serve as a non-precious metal based bifunctional electrocatalyst for next generation electrochemical energy conversion and storage devices. Finally, a prototype Al–air battery arrangement using FC10R as an air-cathode for powering a green light emitting diode has been demonstrated.