Regular Article
Controllable synthesis of nitrogen-doped carbon containing Co and Co3Fe7 nanoparticles as effective catalysts for electrochemical oxygen conversion

https://doi.org/10.1016/j.jcis.2021.01.097Get rights and content

Abstract

Sufficient and well-distributed active sites and highly conductive carbon matrix are two important factors to achieve highly efficient electrocatalysts. In this study, we report an adjusted metal-organic frameworks (MOF)-based route for the preparation of nitrogen-doped Fe/Co bimetallic electrocatalysts. With suitable Fe/Co molar ratio (Fe/Co = 1/4.15), Co nanoparticles (NPs) with mild oxidation state and Co3Fe7 alloys wrapped with thin graphene layers are embedded in an integrated and continuous carbon network. The corresponding [email protected] catalyst exhibits excellent oxygen reduction reaction (ORR) activity (onset potential (Eonset) of 0.94 V and half-wave potential (E1/2) of 0.84 V vs RHE) in alkaline medium, close to commercial Pt/C and superior to the other two FC@NCs. The desirable ORR performance results from the uniform distribution Co3Fe7 active sites, electron density modification from Co NPs to surrounding carbon layers, hierarchical pore structure with large surface area, low carbon content, high pyridinic and graphitic N components. The [email protected] also displays satisfactory methanol crossover tolerance and durability.

Introduction

Metal-air batteries have emerged as a new technology with developing potential to satisfy the needs of sustainable and clean energy because of their preponderant theoretical energy density, rechargeable ability, superior safety and environmental harmlessness [1], [2]. However, the construction of stable and efficient electrode materials for oxygen conversion is still a critical choke point for their large-scale application. As cathode reaction, oxygen reduction reaction (ORR) is mainly responsible for restrictions of the popularization of metal-air batteries due to its intrinsically sluggish kinetics [3], [4], [5], [6], [7]. Although the commercial precious metals and metal oxides are currently foremost electrocatalysts, they still suffer from certain unexpected problems (e.g. limited reserves, scarcity and high price) [8], [9], [10]. Based on this, extensive researches have been made for the development of economic, effective and stable materials (e.g. non-precious metal based materials and metal-free heteroatoms doped carbon nanomaterials) [11], [12], [13], [14], [15], [16], [17], [18]. Among these electrocatalysts, transition metal-nitrogen-carbon materials (Msingle bondNsingle bondC, e.g. Co/Fesingle bondNsingle bondC) and the bimetal alloys (such as FeCo [19], FeNi [20] and NiCo [21]) as highly efficient active sites for ORR have aroused increasing interest currently.

For most electrocatalysts, carbon materials as substrates for supporting and dispersing active sites are indispensable to improve electron transfer rate in the electrochemical reactions process. Among them, graphene-based materials have been widely studied due to its unique nanosheet framework, remarkable chemical stability, high electroconductivity and flexibility for surface adjustment [22], [23], [24]. Compared with simple 2D layers, the 3D framework consisting of interconnected graphene-like carbon nanosheets is considered as an advantageous material in term of shorter diffusion paths for charge carriers, sufficient active sites on surface and approachable diffusion channels [25]. Furthermore, the doping with electron-rich nitrogen into carbon skeletons can not only bring more active sites for O2 absorption, but also enhance the electrical conductivity by optimizing conjoint carbon electronic structure [26].

Recently, metal–organic frameworks (MOFs) with versatile morphology and porosity have been broadly utilized as a platform to construct carbon-based composites, in which metal precursors are “pre-sealed” in carbon skeleton and transformed into active sites for electrochemical reaction. Zeolitic imidazolate frameworks (ZIFs, e.g. ZIF-8, ZIF-67) as a subclass of MOFs are frequently applied to prepare electrocatalysts for ORR. For instance, Song et al. synthesized a Cosingle bondNsingle bondC catalyst derived from pyrolysis of Zn/Co-ZIF, and this catalyst gave promoted performance due to the formation of Co-Nx moiety within the conductive carbon network [27]. The synergistic effect of multiple transition metals, heteroatomic species and conductive carbonaceous matrix can modify greatly electronic configuration to create more active sites and reduce the kinetic energy barriers, thereby accelerating electrochemical reactions [28], [29].

Here, we introduce N-doped graphene-like nanosheets, which were obtained from carbonizing cheap materials (natural soybean oil and melamine) at high temperature, into the synthesis process of bimetal FeCo-ZIF. During such a process, FeCo-ZIF nanoparticles (NPs) grow in situ on the surface of these nanosheets. Subsequently, the metal species in turn affect the morphology of final carbon network during the second pyrolysis of the products. The additive Fe/Co molar ratio has great effect on the morphology and microstructure of the products. With suitable Fe/Co molar ratio, the obtained product demonstrated an approving electrocatalytic performance for ORR, close to the commercial Pt/C catalyst.

Section snippets

Experimental section

Typically, 5.0 g melamine was grounded into 2.8 mL soya-bean oil for 20 min, and the homogenous slurry was poured into a quartz boat for the further heat treatment. After carbonization under 900 °C (heating rate 2 °C min−1) for 2 h in a tube furnace with N2 protection, the product was obtained and defined as N@Cs. Then 0.14 g N@Cs was ultrasonically dispersed into 50 mL distilled water, where 5.6 g 2-methylimidazole was dissolved firstly. Subsequently, 1.35 g C4H6CoO4·4H2O and 0.545 g Fe(NO3)3

Results and discussion

The formation of FC@NCs catalysts involves two main steps as demonstrated in Fig. 1. N-doped graphene-like carbon nanosheets (N@Cs) was firstly synthesized according to our previous report, [22] where soybean oil was used as carbon source and melamine was applied to form the layered sacrificial graphitic carbon nitride (g-C3N4) template during carbonization process (about 500–600 °C) [30], [31]. Subsequently, the raw materials (metal salt and dimethylimidazole) for the formation of ZIFs were

Conclusions

In short, N-doped graphene-like carbon nanosheets were introduced into the synthesis process of the FeCo-ZIF and the obtained products underwent a high-temperature carbonization to prepare a series of FC@NCs catalysts. The microstructure (e.g. active sites and carbon matrix) of the final materials can be adjusted by the initial Fe/Co mole ratio. With an appropriate molar ratio (Fe/Co = 1/4.15), the carbon network of the catalyst is integrated and continuous and the ORR active metal NPs wrapped

CRediT authorship contribution statement

Xiaodong Luo: Methodology, Investigation, Writing - original draft. Hui Ma: Investigation. Hang Ren: Data curation. Xuhui Zou: Visualization. Yuan Wang: Formal analysis, Validation. Xi Li: Funding acquisition. Zhangfeng Shen: Conceptualization, Writing - review & editing. Yangang Wang: Funding acquisition, Supervision. Lifeng Cui: Resources, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research get financial support from National Key Research and Development Program of China (Grant No. 2018YFB1502903), Zhejiang Provincial Natural Science Foundation of China (LQ20E030016 and LY19B060006), National Natural Science Foundation of China (Grant No. 21103024, 21603072 and 61171008), Technology Development Project of Jiaxing University and the fund from G60 STI Valley Industry & Innovation Institute, Jiaxing University.

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