Critical role of iron carbide nanodots on 3D graphene based nonprecious metal catalysts for enhancing oxygen reduction reaction

Abstract

Iron-nitrogen-carbon materials (Fe-N-C) are considered as one of the most promising none precious candidates for oxygen reduction reaction. However, the role of iron carbides, one of the important accompanied species in Fe-N-C catalyst for oxygen reduction reaction, remains controversial. Herein, two different types of 3D graphene-based Fe-N-C catalysts with exposed and encapsulated Fe carbides were synthesized via pyrolysis of self-polymerization compounds of 2,6-Diaminopyridine using Fe(OH)₃ and FeCl₃ as both initiators and Fe sources, respectively. Interestingly, Mössbauer spectroscopy and X-ray photoelectron spectroscopy measurements show that both catalysts are composed of same Fe species with Fe^IIN₄/C, iron carbide, iron nitride, metallic iron where the proportion of each corresponding component is similar. However, there is a big gap between the two catalysts in catalytic performance for oxygen reduction reaction. The kinetic current densities of the former catalyst (3.565 mA cm⁻²) at 0.9 V vs.RHE is 4.68 times and 1.50 times higher than that of the latter (0.7617 mA cm⁻²) and Pt/C catalyst (2.376 mA cm⁻²), respectively. This sharp contrast directly identifies the critical role of exposed Fe carbides nanodots on Fe-N-C in promoting Fe-N-C activity for oxygen reduction reaction. The new discovery brings a new insight into developing advanced none precious metal catalysts.

Introduction

Polymer electrolyte membrane fuel cell (PEMFC) has been considered as the most promising alternative green energy conversion and storage systems [1]. To date, Pt-based catalysts are still widely employed for oxygen reduction reaction (ORR) due to their high efficient catalytic activity and full four-electron process [2]. However, their own limitations of prohibitive cost and shortage reserve impede the commercialization of PEMFC [3,4]. Therefore, seeking for alternative non-noble catalysts for ORR to replace to Pt-based catalysts is of prime importance due to their abundant reserves and low cost.

Transition metal-based nitrogen doped catalysts (M-N-C) have attracted considerable attention among various catalysts for ORR due to their high activity [5,6]. Numerous efforts have been carried out to investigate systematically into catalytic active sites for ORR in M-N-C catalysts. Several active sites have been proposed, taking Fe-N-C as an example, which mainly include: FeN₄/C moieties, Fe carbides, Fe nitrides, etc. [7,8] Among all these studied active sites, FeN₄/C moiety is the most generally accepted active site for ORR [9,10]. On the other hand, the role of Fe carbides is attracting greatly interests because they usually coexist with FeN₄/C in Fe-N-C catalysts [11]. So far, the widely reported Fe carbides in the Fe-N-C catalyst were often encased by carbon shell. However, these coating layers hindered the illumination of Fe carbides' specific catalytic mechanism for ORR. It is a hot topic whether Fe carbides themselves directly involved and what role they play in catalytic process for ORR [12,13]. Some researchers claimed that encapsulated Fe carbides particles act as important components in M-N-C catalysts via activating surrounding carbon layers and breed catalytic activity towards ORR [14,15]. Actually, other researchers argued that Fe carbide nanocrystals themselves can deliver high activity for ORR only if there is a coexistence of high content of Fe-Nx and sufficient metallic iron nanoparticles [16,17]. Even some other researches indicated that iron nitrides and iron carbides are considered to be inactive or non-contributing in the ORR [18]. These questions are still being debated due to the complicated electronic transmission process between Fe carbides and outer carbon layers. Therefore, designing a kind of Fe-N-C catalyst with exposed Fe carbides species is highly desirable to clarify the role of Fe carbides in the catalytic process for ORR.

In this work, two different types of Fe-N-C catalysts with 3D Graphene structure were synthesized via pyrolysis of self-polymerization compounds of 2,6-Diaminopyridine using Fe(OH)₃ and FeCl₃ as both initiators and Fe sources respectively. The Fe-N-C catalyst with Fe(OH)₃ features ultrafine Fe carbides and nitrides quantum dots (<2 nm) uniformly embedded onto a 3D graphene carbon matrix. In contrast, the Fe-N-C catalyst synthesized by FeCl₃ as precursor was characterized by crumpled graphene structure decorated with encapsulated Fe carbides nanoparticles (5–20 nm). Through comprehensive analysis and comparison, it is directly demonstrated that exposed Fe carbides nanodots greatly enhance Fe-N-C ′s catalytic activity for ORR.