Oxygen Electrocatalyst Based on Non-Precious Bifunctional Hybrid Material
Authors: Samanta Arpan , Raj C. Retna 
Published: February 2020

Synthesis of new functional materials for the development of energy conversion and storage devices like fuel cells, metal-air batteries received significant interest in the last decade. The limited availability of traditional fossil fuel demands renewable energy resources. The electrochemical oxygen reduction and oxygen evolution reaction are the key processes in energy conversion and storage devices like metal-air batteries. The development of rechargeable metal-air batteries requires efficient bifunctional electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Traditionally, Pt-, Ru- and Ir-based materials have been used for ORR and OER.1 The large overpotential due to the slow electron transfer kinetics, high cost and the electrochemical instability of these traditional precious metal-based electrocatalysts demands for active non-precious bifunctional catalysts. Moreover, the use of these traditional catalysts as an oxygen electrode is limited as they are not able to catalyse both the ORR and OER simultaneously. In the recent past, the oxides of transition metals such as Co, Mn, etc. including mixed valence spinel and perovskites have been proposed as alternate catalyst for these reactions.2 However, the poor electronic conductivity and the recyclability of these oxides limits their potential application.3 Recently, the hybrid electrocatalyst based on carbon and transition metal catalyst is emergingfor ORR and OER reactions.4

Our group is interested in the development of new functional materials for fuel cell and supercapacitor applications.2a,5 In continuation of our earlier efforts, herein we demonstrate the synthesis of two types of low-cost transition metal (Co, Fe) based hybrid bifunctional electrocatalyst. The two catalysts cobalt and nitrogen dual-doped carbon (Co-N-C) and reduced graphene oxide-CoFe (rGO-CoFe) were synthesized by chemical method followed by thermal annealing at an optimized temperature. The Co-N-C catalyst was obtained from cobalt tetra-aza macrocycle (Co-TET-A) whereas the rGO-CoFe catalyst was synthesized from multimetal complex, cobalt hexacyanoferrate. Both the catalysts were characterized by analytical techniques including XPS, TEM and XRD. The electrocatalytic response towards ORR and OER was evaluated by hydrodynamic voltammetry. The Co-N-C and rGO-CoFe catalysts show the limiting current density of -4.37 mA/cm2 and -5.58 mA/cm2, respectively and both the catalysts promote the 4-electron pathway for the reduction of oxygen. The OER activity was evaluated by measuring the overpotential at which 10 mA/cm2 current density was observed. Interestingly both Co-N-C and rGO-CoFe catalysts show the benchmark current density of 10 mA/cm2 at a low overpotential of 0.28 V and 0.22 V, respectively. The overall oxygen electrode activities were evaluated by calculating the difference in potential (ΔE) between the OER current density at 10 mA/cm2 and the ORR current density at −3 mA/cm2. We could achieve a ΔE of 0.92 V and 0.82 V on Co-N-C and rGO-CoFe catalyst respectively, confirming the efficient bifunctional activity of these catalysts. It is demonstrated that the synergistic effect between the metal and carbon and the porous nature of the catalysts plays an important role in the bifunctional activity. The detailed synthetic and bifunctional activity will be discussed.

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Figure 1