Efficient Co–N/PC@CNT bifunctional electrocatalytic materials for oxygen reduction and oxygen evolution reactions based on metal–organic frameworks

Abstract

Cobalt-based, nitrogen-doped porous carbon materials with in situ grown carbon nanotubes (CNTs) were synthesized by the facile carbonization of porous 3D Bio-MOF-11 [Co₂(ad)₂(CH₃COO)₂]·2DMF·0.5H₂O (ad = adenine). Co–N/PC@CNT-Ts inherit the octahedral shape from the precursor, and have a porous structure with in situ grown CNTs catalyzed by Co particles. Co–N/PC@CNT-T materials have excellent activities as bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in 0.1 M KOH electrolyte. Among the Co–N/PC@CNT-Ts, Co–N/PC@CNT-700 has the highest electrocatalytic activity. For ORR, Co–N/PC@CNT-700 has a higher onset potential of 0.92 V vs. reversible hydrogen electrode (RHE), high stability and methanol tolerance, which are even better than that of Pt/C. For OER, it has a low potential of 1.63 V at a current density of 10 mA cm⁻². In addition, Co–N/PC@CNT-700 affords a low reversible overvoltage (bifunctional performance parameter) of 0.862 V between ORR and OER compared to the current advancing bifunctional catalysts. The superb bifunctional activity can be attributed to uniform CoN_x active sites embedded in graphitized carbon, unique in situ grown CNT structure and ordered mesoporous structure. The synergistic effect enlarged the contact surface, exposed more active centers and provided many pathways, thereby boosting the electrocatalytic performance. In conclusion, this study provides a novel avenue for the application of stable transition metal-based, nitrogen-doped carbon materials as extremely efficient electrocatalysts for ORR and OER.