Dual-template strategy for electrocatalyst of cobalt nanoparticles encapsulated in nitrogen-doped carbon nanotubes for oxygen reduction reaction

https://doi.org/10.1016/j.jcis.2020.07.008Get rights and content

Abstract

Development of non-precious metal catalysts (NPMCs) with high performance and stability has important value for oxygen reduction reaction (ORR) of fuel cells. In this paper, a novel structure of nitrogen-doped carbon nanotubes-encapsulated cobalt nanoparticles (Co@NCNTs) is synthesized by a simple dual-template strategy using silica colloid and tri-block copolymer (polyethylene oxide-polypropylene oxide-polyethylene oxide, PEO-PPO-PEO, F127) as hard and soft templates, respectively. The Co@NCNTs-800 synthesized at 800 °C shows an excellent ORR performance, which can be attributed to the desirable combination of their unique one-dimensional carbon nanotube structure, the adequate nitrogen doping level, the large surface area created by the dual-template strategy, and the synergistic effect between graphitic carbon layer and cobalt nanoparticles. The doped N atoms can provide coordination sites for cobalt nanoparticles and form Nsingle bondC moieties as dominant active sites, which provide positive effect on catalytic ORR activity. The graphitic carbon layers can protect cobalt nanoparticles against agglomeration and electrolyte corrosion, while cobalt nanoparticles can activate the bordering graphitic carbon layers and further increase ORR activities. This dual-template synthetic strategy provides an opportunity to promote the catalytic performance of NPMCs for application of polymer electrolyte membrane fuel cells.

Introduction

As a new generation of energy conversion devices, fuel cells (FCs) have been considered to be substitutes for traditional fossil fuels due to their high energy density, conversion efficiency and pollution-free emissions [1], [2]. However, sluggish kinetics of cathodic oxygen reduction reaction (ORR) process has limited the performance of fuel cells. To date, platinum-based catalysts are still the most practical catalysts for ORR [3], [4], [5]. Although Pt-based electrocatalysts can exhibit good electrochemical catalytic performance for ORR, their disadvantages of high cost and insufficient stability are compromised the large-scale commercialization of FCs. Thus, the development of low-cost ORR catalysts with high efficiency and long durability is of significance for the application of fuel cells [6], [7], [8].

In response, many researchers are committed to developing non-precious metal catalysts (NPMCs) towards ORR by replacing precious metals with earth abundant and inexpensive elements [8], [9], [10], [11]. Since cobalt phthalocyanine was observed to be catalytic active for ORR in 1964 [12], NPMCs with M-Nx/C (M = Fe, Co, etc) coordinated structures as active sites for ORR have been extensively explored in recent decades [13]. Despite the high catalytic activity, M-Nx/C catalysts show several serious challenges in terms of their catalytic ORR activity and stability. First, the activity and stability can be decreased due to the metal dissolution in harsh environments [14]. Second, the direct pyrolysis strategy of M-Nx/C catalysts from the mixture of metal, nitrogen and carbon sources suffers from the disadvantages of ununiformed structure, easy agglomeration of metal nanoparticles and uncontrolled distribution of active sites [15], which restrict the further enhancement of catalytic performance.

To overcome these challenges, a new type of nano-constrained structure with metal nanoparticles encapsulated in carbon layers has been developed to synthesize the effective and stable ORR electrocatalysts [16], [17], [18]. Due to the spatial constraints, surface effects and synergistic effects between the metal nanoparticles and the outer carbon layers, the nano-constrained structure can display the improved catalytic activity and enhanced durability [19], [20]. The outer graphitic carbon layers can protect the inner metal nanoparticles and avoid the metal dissolution problem even in acidic environments [21]. Furthermore, the encapsulated metal nanoparticles can lead to a unique host-guest electronic interaction [22] and activate the surrounding graphitic carbon layers [19]. In addition, the presence of metal nanoparticles can change the electronic structure and reduce the surface work function of the carbon walls, which can promote the electron transfer from metal to carbon walls [23], [24], [25] and consequently enhance the catalytic activity towards ORR. For example, Yang et al. [22] prepared uniform Fe3C nanoparticles encased in bamboo-like carbon nanotubes. The obtained Fe3C nanoparticles could form a unique host-guest local electronic interaction and then activate the outer surface of the graphitic carbon layers towards ORR process. Deng et al. [23] experimentally and computationally demonstrated that the coordinatively unsaturated metal nanostructures could change the charge-density distribution on outer N-doped carbon layers. The encapsulated metal nanoparticles were found to interact with the outer CNT shell where O2 was activated and reduced. Besides, the doping nitrogen atoms also play an important role in achieving enhanced ORR performance. Regarding this, the pyridinic and pyrrolic nitrogen atoms with long-pair electrons can be used as coordinating sites to bind metal atoms on carbon to form M-Nx/C active centers [25], further enhancing electrocatalytic activity. Meanwhile, the doping of N can introduce more defects and expose more active sites. Therefore, the combination of nano-constrain structure and N-doping strategy is a promising strategy for developing non-precious metal catalysts with high catalytic activity and durability [10], [26], [27], [28].

In our previous work, the synthesis of Fe3C nanoparticles encapsulated in bamboo-like N-doped carbon nanotubes (Fe3C/b-NCNTs) was preceded by a pre-synthesized melamine-formaldehyde resin (MF) prepolymer [29]. The synthesis process includes several steps such as pre-polymerization, physical mixing, carbonization and chemical etching, which seem too complicated to be suitable for mass production application. In this work, cobalt nanoparticles encapsulated in N-doped carbon nanotubes (Co@NCNTs-800) are successfully prepared through a simpler synthesis procedure by directly using melamine as the precursor with assist of Pluronic® F127 and SiO2 as soft and hard templates, respectively. By the dual-template strategy, specific surface area can be effectively increased to expose more active sites, which are essential to improve the electrochemical catalytic performance. Besides, the outer carbon nanotubes can not only prevent the agglomeration and collapse of cobalt nanoparticles, but also provide electronic channels during ORR processes. Furthermore, the charge transfer from the cobalt nanoparticles to the walls of carbon nanotubes can activate the surrounding carbon layers and enhance ORR activities. As a result, the obtained Co@NCNTs-800 exhibits both high catalytic activities toward ORR in alkaline solution with super methanol tolerance.

Section snippets

Reagents

Melamine, cobalt chloride hexahydrate (CoCl2·6H2O), potassium hydroxide (KOH), hydrofluoric acid (HF, 40 wt%) were analytical grade and purchased from Aladdin Co. Ltd. Nafion® solution (5 wt%), Pluronic® F127 (polyethylene oxide-polypropylene oxide-polyethylene oxide, PEO-PPO-PEO) and LUDOX® TM-50 silica colloidal suspension liquid (50 wt%) were purchased from Sigma-Aldrich. All reagents were not further purified. Deionized water was used to prepare all aqueous solutions.

Preparation of Co@NCNTs

The preparation

Results and discussion

Compared with our previous work, Co@NCNT samples were synthesized by a simper dual-template strategy by directly using melamine (N-rich organic compound) as precursor, as illustrated in Scheme 1. Each melamine molecule has three primary amine groups (–NH2) which can serve as effectively adsorption sites to bind Co2+ ions. Carbon nanotubes embedded metal nanoparticles can be obtained after pyrolysis of the mixture of melamine, F127, SiO2 and CoCl2, followed by a chemical etching. As hard

Conclusion

In summary, a facile dual-template synthetic strategy for controlled synthesis of cobalt nanoparticles encapsulated in the N-doped carbon nanotubes (Co@NCNTs-800) was well developed by simply annealing a mixture of melamine, F127, SiO2 and CoCl2. The dual-template strategy can create hierarchical porous structures to increase surface areas. The obtained Co@NCNTs-800 shows both higher activity and better stability than those of samples obtained via none-template or single-template strategy. The

CRediT authorship contribution statement

Fang Liu: Methodology. Xinquan Zhang: Writing - original draft. Xiaolong Zhang: Software. Minmin Liu: Supervision, Writing - review & editing.

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

The authors acknowledge the support of the Shanghai Sailing Program (18YF1408600); National Natural Science Foundation of China (21601122); National Key Research and Development Program of China (2017YFB0102900).

References (52)

  • F. Wang et al.

    N-doped honeycomb-like porous carbon towards high-performance supercapacitor

    Chin. Chem. Lett.

    (2020)
  • Y. Liu et al.

    Conformal coating of heterogeneous CoO/Co nanocomposites on carbon nanotubes as efficient bifunctional electrocatalyst for Li-Air batteries

    Electrochim. Acta

    (2016)
  • M. Liu et al.

    Flash nanoprecipitation of poly(styrene-co-acrylonitrile) colloids in the presence of hydrophobic organoplatinum and their derived Pt-carbon nanocomposites for oxygen reduction reaction

    Colloids Surf. A

    (2018)
  • X. Zhang et al.

    Probing the electro-catalytic ORR activity of cobalt-incorporated nitrogen-doped CNTs

    J. Catal.

    (2016)
  • M. Liu et al.

    Atomically dispersed metal catalysts for the oxygen reduction reaction: synthesis, characterization, reaction mechanisms and electrochemical energy applications

    Energy Environ. Sci.

    (2019)
  • M. Liu et al.

    Graphene-supported nanoelectrocatalysts for fuel cells: synthesis, properties, and applications

    Chem. Rev.

    (2014)
  • Y. Nie et al.

    Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction

    Chem. Soc. Rev.

    (2015)
  • Y. He et al.

    Metal-nitrogen-carbon catalysts for oxygen reduction in PEM fuel cells: self-template synthesis approach to enhancing catalytic activity and stability

    Electrochem. Energy Rev.

    (2019)
  • Y. Li et al.

    Recent progresses in oxygen reduction reaction electrocatalysts for electrochemical energy applications

    Electrochem. Energy Rev.

    (2019)
  • J. Guo et al.

    Nitrogen-doped porous carbon supported nonprecious metal single-atom electrocatalysts: from synthesis to application

    Small Methods

    (2019)
  • F. Meng et al.

    In Situ coupling of strung Co4N and intertwined N-C fibers toward free-standing bifunctional cathode for robust, efficient, and flexible Zn–Air batteries

    J. Am. Chem. Soc.

    (2016)
  • R. Jiang et al.

    Edge-site engineering of atomically dispersed Fe–N4 by selective C-N bond cleavage for enhanced oxygen reduction reaction activities

    J. Am. Chem. Soc.

    (2018)
  • L. Lin et al.

    Noble-metal-free Fe–N/C catalyst for highly efficient oxygen reduction reaction under both alkaline and acidic conditions

    J. Am. Chem. Soc.

    (2014)
  • R. Jasinski

    A new fuel cell cathode catalyst

    Nature

    (1964)
  • J. Du et al.

    M(Salen)-derived nitrogen-doped M/C (M = Fe Co, Ni) porous nanocomposites for electrocatalytic oxygen reduction

    Sci. Rep.

    (2014)
  • L. An et al.

    A highly active and durable iron/cobalt alloy catalyst encapsulated in N-doped graphitic carbon nanotubes for oxygen reduction reaction by a nanofibrous dicyandiamide template

    J. Mater. Chem. A

    (2018)
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