Metal-organic frameworks derived platinum-cobalt bimetallic nanoparticles in nitrogen-doped hollow porous carbon capsules as a highly active and durable catalyst for oxygen reduction reaction

https://doi.org/10.1016/j.apcatb.2017.11.077Get rights and content

Highlights

  • A new efficient method utilizing MOFs is developed to synthesize PtCo alloys.

  • Fine PtCo alloys within nitrogen-doped hollow porous carbon capsules are obtained.

  • The sample displays outstanding catalytic activity in oxygen reduction reaction.

  • The sample exhibits excellent catalytic durability and stability.

Abstract

Pt-based nanomaterials are regarded as the most efficient electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). However, widespread adoption of PEMFCs requires solutions to major challenges encountered with ORR catalysts, namely high cost, sluggish kinetics, and low durability. Herein, a new efficient method utilizing Co-based metal-organic frameworks is developed to produce PtCo bimetallic nanoparticles embedded in unique nitrogen-doped hollow porous carbon capsules. The obtained catalyst demonstrates an outstanding ORR performance, with a mass activity that is 5.5 and 13.5 times greater than that of commercial Pt/C and Pt black, respectively. Most importantly, the product exhibits dramatically improved durability in terms of both electrochemically active surface area (ECAS) and mass activity compared to commercial Pt/C and Pt black catalysts. The remarkable ORR performance demonstrated here can be attributed to the structural features of the catalyst (its alloy structure, high dispersion and fine particle size) and the carbon support (its nitrogen dopant, large surface area and hollow porous structure).

Introduction

The proton exchange membrane fuel cell (PEMFC) has long been regarded as one of the most promising clean and efficient energy conversion devices for a wide variety of applications [[1], [2], [3]]. Its ability to provide on-demand power from hydrogen, which importantly can be stored on a seasonal basis, makes it a vital component of future zero-carbon energy grids [[4], [5]]. However, the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode is currently preventing extensive usage of PEMFCs due to the consequential reduction in energy efficiency [[6], [7], [8]]. Existing carbon-supported Pt-based electrocatalysts can efficiently catalyze the ORR [[9], [10], [11], [12], [13]], but the scarcity and high cost of Pt as well as its poor stability still limit the practical applications of PEMFCs [[1], [2], [14]]. To tackle these challenges, the ORR catalyst community has traditionally focused on (i) engineering of the morphology, structure and component of Pt-based catalysts and (ii) optimization of the catalyst supports, for the purpose of maximizing both activity and durability.

Regarding the first strategy, an effective method of indirectly reducing the Pt mass requirement is to improve the ORR activity and stability of Pt-based catalysts via advanced morphologies and structures [[15], [16], [17], [18]]. Meanwhile, alloying of Pt with a secondary metal can further enhance the performance of Pt-based catalysts and concurrently reduce the usage of Pt [[19], [20]]. These bimetallic nanostructured Pt-based materials can exhibit a superior activity and stability with an optimized oxygen absorption energy [21]. Among all Pt-based bimetallic nanomaterials, alloys of Pt and transition metals, in particular PtCo and PtNi, have been identified as the most active and stable catalysts for ORR by numerous studies [[22], [23], [24], [25], [26], [27]]. The second strategy involves rational design the catalyst supports [28]. One effective method is to introduce heteroatom dopants such as nitrogen into the carbon support, which can not only increase chemical binding or “tethering” between the catalyst and support, but also largely facilitate interfacial electron transfer and adsorption of reactants (such as O2) by modifying the charge of adjacent C atoms [[29], [30]]. Moreover, supports with well-designed nanostructures such as carbon nanotubes [[31], [32]], hollow carbon spheres [[33], [34]], and hollow porous carbons (HPCs) [[35], [36], [37], [38]] further improve the ORR activity and stability for Pt-based catalysts. Particularly, when HPCs encapsulate metal nanocrystals, the hybrid catalysts often exhibit remarkable catalytic activity and stability due to the high surface area, efficient mass transport, excellent conductivity and high electrochemical stability of HPCs along with the shell protection of the metal nanocrystals against aggregation/sintering [[36], [37], [38], [39]].

Ideally, one should combine the above strategies such as high catalyst dispersion, transition metal alloying of Pt, heteroatom-doping of carbon support, and creation of a HPC structure to produce a top-performing Pt-based catalyst. More specifically, we envision that PtCo nanoparticles encapsulated in nitrogen-doped HPC would meet the exceptional ORR activity and durability requirements for commercial PEMFCs. However, it remains a great challenge to obtain this model catalyst owing to tedious and complex synthesis procedures currently described in the literature. Therefore, a procedure that can effectively and consistently produce the aforementioned hybrid material is highly desired.

In this study, we report for the first time an efficient method for synthesizing PtCo bimetallic nanoparticles mixed with Co nanoparticles encapsulated in nitrogen-doped hollow porous carbon capsules (denoted as PtCo/Co@NHPCC). It is derived from metal-organic frameworks (MOFs) via three steps, including introduction of Pt within the MOFs by a hydrophobic/hydrophilic approach, coating with a polymer shell, and finally a thermal treatment. The prepared products possess many desirable features such as well-dispersed nanoparticles, embedded alloys, hollow porous structures, capsule-like morphology, and nitrogen dopants. The obtained PtCo/Co@NHPCC displays an excellent catalytic activity for ORR in terms of mass activity and specific activity (0.566 A mgPt−1 and 0.876 mA cm−2), which are much better than those of the commercial Pt/C catalysts (0.102 A mgPt−1 and 0.177 mA cm−2) and commercial Pt black (0.042 A mgPt−1 and 0.221 mA cm−2). More notably, PtCo/Co@NHPCC exhibits outstanding structural stability and catalytic durability, as it shows no obvious change in its nanostructure and only a slight ORR activity change after 5000 potential sweeps. This work demonstrates that PtCo/Co@NHPCC, which owns the advantages of both Pt alloys and advanced supports, are indeed a promising ORR electrocatalyst with improved activity, durability, and utilization efficiency of Pt.

Section snippets

Preparation of ZIF-67

ZIF-67 materials were synthesized according to the published literature with a slight modification [40]. In a typical synthesis of ZIF-67, 0.718 g of Co(NO3)2‧6H2O and 1.622 g of 2-methylimidazole were respectively dissolved in 50 mL of methanol at room temperature, and then mixed under vigorous stirring. After 20 min, the stirring was stopped and the mixture was kept in the static state for 20 h. The products were collected by centrifugation and washed with methanol several times, followed by

Results and discussion

The overall synthesis procedure of the hybrid catalyst PtCo/Co@NHPCC is illustrated in Fig. 1. Pt nanoparticles are firstly encapsulated and dispersed into MOFs via the following hydrophobic/hydrophilic method: (i) synthesis of ZIF-67 [[40], [41]], a Co-based highly porous MOF with high nitrogen content and a hydrophilic nature, as the starting materials (Fig. 1a); (ii) dispersion of ZIF-67 in n-hexane, a hydrophobic solvent that cannot enter into the pores of ZIF-67 due to the high

Conclusions

In summary, we have presented an efficient and novel strategy to rationally design and synthesize PtCo bimetallic nanoparticles embedded in unique nitrogen-doped hollow porous carbon capsules. PtCo/Co@NHPCC shows much superior ORR activity and durability in comparison to commercial Pt/C and Pt black. Notably, both the mass activity (0.566 A mgPt−1) and specific activity (0.876 mA cm−2) of PtCo/Co@NHPCC are beyond the U.S. DOE recommended 2017 target of 0.440 A mgPt−1 and 0.720 mA cm−2,

Acknowledgments

This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the University of Waterloo, and the Waterloo Institute for Nanotechnology. The authors greatly acknowledge the Catalysis Research for Polymer Electrolyte Fuel Cells (CaRPE FC) Network administered from Simon Fraser Grant No. APCPJ 417858-11 through NSERC. TEM imaging was performed at the Canadian Center for Electron Microscopy (CCEM) located at McMaster University. Part of EM work was performed

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