ZIF(Fe/Zn)-derived Hierarchically Porous Carbon Nanotubes as Oxygen Reduction Reaction Electrocatalysts

In fuel cells as energy conversion device, the oxygen reduction reaction (ORR) as a cathodic reaction is one of the most challenging sections in the researches and commercialization processes [1]. Although Pt-based materials have been extensively used to overcome the sluggish kinetic of ORR, problems such as the scarce reserve, high cost, poor durability, and low poison resistance of Pt significantly prohibit their large-scale commercial application [3]. Vast variety of novel carbon catalysts as substitute to Pt have been developed in the last few decades [2]. Nonporous carbons with various dopant species (e.g., N, S) as supporting materials can effectively improve the ORR catalytic activity owing to high surface area, highly porous structure, excellent electrical conductivity, and increased catalytic active sites [4]. Specifically, it has been illustrated that the carbon catalysts coordinated with transition metals (Co, Fe) and nitrogen (N) are emerging as the most promising substitute for Pt toward the ORR [5]. However, an aggregation of loaded metal nanoparticles is often observed following in high thermal treatment can be attributed to the lack of rational nanostructure engineering of the carbon catalysts and it usually leads to a decrease in the number of catalytically active sites. Therefore, improved catalytic activity of such catalysts depends on abundant and uniformly distribution of metal and N dopant on the carbon matrix [6]. Recently, metal organic frameworks (MOFs) derived carbon materials have attracted a lot of interests. The first interesting point is existence of an inherent coordination between metals, heteroatoms and carbon into one framework. Moreover, a homogeneous dispersion of metal-heteroatom doping throughout 71 framework can serve as active sites and increase ORR performance. It has been reported that a special class of MOFs named zeolitic imidazolate framework (ZIF) can be directly converted to carbon framework homogeneously doped with a transition metal (e.g., Co, Zn, Fe) and nitrogen [7]. In this study, a new ZIF-composite assembly strategy for achieve a new nonprecious metal catalyst based on nanoparticles encapsulated in porous pod-like N-doped carbon nanotubes is reported. This metal-N-doped carbon was prepared by mixing the as prepared Fe-doped ZIF-8 with pyrrole and FeCl3 precursors, followed by pyrolysis at 1000°C for 3h. The main aim to surrounding nanoparticles inside carbonaceous structures was the increasing of the metal NPs protection against corrosion especially in acidic media. It has been demonstrated that this active cites encapsulating strategy make indirectly facilitate the ORR. Because a charge transfer from the metal NPs clusters to the carbon nanotubes takes places, this modification is expected to decrease the local work function and increase the chemical reactivity of the functionalized regions of the CNTs exterior to augment the ORR. In summary, the synthesized electrocatalyst exhibited good and comparable ORR activity to that of the commercial Pt/C in both acidic (0.81 VRHE 0.95 VRHE) and alkaline (0.92 VRHE 1.00 VRHE) solution, respectively.