Dual-template synthesis of CoNi alloy nanoparticles anchored on N-doped carbon nanotubes for efficient oxygen reduction reaction

Abstract

The fabrication of one-dimensional metal/N-doped carbon materials has shown a promising prospect as efficient electrocatalysts for oxygen reduction reaction (ORR). Herein, CoNi alloy nanoparticles anchored on N-doped carbon nanotubes (CoNi@NCNT) are prepared by a dual-template strategy, using polypyrrole (PPy) tubes and CoNi-based metal–organic framework as the precursors. The as-formed CoNi@NCNT catalyst displays a half-wave potential (0.83 V) as well as good durability under alkaline medium. The excellent electrocatalytic performance is ascribed to a synergistic coupling of hierarchically tubular structure, highly electronic conductivity, and abundantly alloy-type active sites. When the CoNi@NCNT catalyst is applied in zinc–air battery (ZAB), the device displays a stable charge–discharge cycling performance. The present work affords a useful approach to constructing alloy/nitrogen-incorporated carbonaceous materials as bifunctional electrocatalysts for high-performance ZABs.

1. Introduction

Zinc–air batteries (ZABs) have attracted much attention in past decades because of high energy density, low cost, environmental friendliness, and superior safety [1–2]. Unfortunately, the ZAB performance is significantly impeded by the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at air electrode [3–5]. Currently, precious-metal materials (Pt/C, Ru/IrO2, etc.) are the most effective electrocatalysts for ORR and OER, respectively [6–7]. However, the high cost, poor stability, incompatibility, and low reserves of noble-metals severely limit their wide application [7]. To solve these issues, developing effective and inexpensive bifunctional oxygen electrocatalysts is urgently required nowadays [8–10].

Among these advanced electrocatalysts [11], metal–nitrogen–carbon (M–N–C) especially alloy–N–C has shown intriguing activities towards both ORR and OER, benefiting from their unique electronic structure and rich electroactive sites [12–15]. In this regard, bimetallic–organic-frameworks (BMOFs) are regarded as ideal templates to fabricate alloy–N–C structures due to their high surface area, various active centers, and tunable porous structure [16–18]. Nevertheless, the pyrolysis of BMOFs inevitably lead to some issues problems (aggregation, morphology shrinkage/collapse, low conductivity, etc.), thereby reducing the catalytic activity and durability [19–21]. Among the reported synthesis strategies, hybridizing BMOFs derivatives with nitrogen-doped carbon supports has been a feasible route to prepare high-efficient alloy–N–C catalytic materials [22].

One-dimensional (1D) carbon materials (e.g., carbon nanotubes, carbon fibers, carbon rods, etc.) are usually utilized as ideal substrates for anchoring catalytic active species owing to their large surface-to-volume ratio, high charge transport capabilities, and rapid detachment of gas bubbles from the catalyst surface [23–26]. For instance, the N-doped carbon nanotubes (NCNT) derived from polypyrrole (PPy) tubes containing rich carbon/nitrogen groups can effectively accelerate the electrolyte/O2 transport as well as maximize the exposure of catalytic sites [27–29]. Accordingly, it is highly desirable and challenging for developing effective strategies to achieve the maximized exposure of accessible metal–Nx sites and accelerate the mass transport [30–31]. Nowadays, the single-template synthesis with Si is a promising method to tailor hierarchically porous structures [32]. Unfortunately, the removal of Si is complicated and in an unfriendly environment in a corrosive solution. Furthermore, the single-template generally led to specific pore structures rather than hierarchical pores [33]. Based on above considerations, a multiple-template strategy is undoubtedly attractive to not only fabricate the pre-designed architecture and hierarchical pores, but also increase the number of exposed active sites [34–35].