Monodisperse and Tiny Co₂N_0.67 Nanocrystals Uniformly Embedded over Two Curving Surfaces of Hollow Carbon Microfibers as Efficient Electrocatalyst for Oxygen Evolution Reaction

Rational design of highly efficient electrocatalysts for oxygen evolution reaction (OER) is critical for both water splitting and rechargeable metal–air batteries. Especially, developing excellent earth-abundant catalysts for OER is an ongoing challenge. We report a series of cobalt nitride (Co2N0.67)–carbon composites with different textural properties as high-activity OER materials herein. Experimental results demonstrate that the specific morphologies of different carbon matrices (such as carbon nanotubes (CNTs), graphene (GN), macroporous carbons (MPCs), and hollow carbon microfibers (CMFs)) play crucial roles in determining the overall OER catalytic efficacies of corresponding composites. Both SEM and TEM characterization results revealed that monodisperse and tiny Co2N0.67 nanoparticles (NPs) indeed grew uniformly over both the inner and outer walls of CMFs because of the widespread edges and defect sites. This is beneficial to fully expose and utilize active sites on each Co2N0.67 NP. In addition, carbon matrices can also enhance the N content in CoN NPs (from Co5.47N to Co2N0.67) and then promote the catalytic ability of active centers. Benefiting from the unique open hollow architecture, abundant out-of-plane pores, and vast exposed three-phase boundaries dispersed along both inner and outer curve surfaces of Co2N0.67-CMFs; the most notable feature of optimal Co2N0.67-CMFs can provide much better transport pathways that can promote O2(g) bubbles fast release from active site surfaces. Combined with the advantages described above, Co2N0.67-CMFs indeed possess high efficiency for OER catalysis. They exhibit only a small overpotential of 0.26 V to achieve the current density of 10 mA cm–2 for OER, which surpasses the state-of-the-art RuO2 OER catalyst (0.273 V). This result is also one of the best values reported up to now. Furthermore, this hybrid catalyst also displays outstanding long-term stability for OER. All results highlight the overwhelming superiority of CMFs as the carbon matrix compared to other reference carbon matrices. Thus, the relatively mild synthesis processes, low-cost precursors, and highly efficient OER catalytic activity of the Co2N0.67-CMFs hybrid catalyst make it promising for industrial water-splitting or rechargeable metal–air batteries.