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

TLDR: In this paper, 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.

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).

Figures

Fig. 5. CV curves of (a) PtCo/Co@NHPCC, (b) Pt/C, and (c) Pt black, ORR polarization curves of (d) PtCo/Co@NHPCC, (e) Pt/C, and (f) Pt black before and after ADT, and corresponding summary of (g) ECSA, (h) mass activity, and (i) specific activity for PtCo/Co@NHPCC, Pt/C, and Pt black before and after ADT, respectively.

Fig. 1. Schematic representation of the preparation of PtCo/Co@NHPCC.

Full text

Accepted Manuscript
Title: 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
Authors: Jie Ying, Jing Li, Gaopeng Jiang, Zachary Paul
Cano, Zhong Ma, Cheng Zhong, Dong Su, Zhongwei Chen
PII: S0926-3373(17)31143-8
DOI: https://doi.org/10.1016/j.apcatb.2017.11.077
Reference: APCATB 16229
To appear in: Applied Catalysis B: Environmental
Received date: 13-8-2017
Revised date: 17-10-2017
Accepted date: 28-11-2017
Please cite this article as: Jie Ying, Jing Li, Gaopeng Jiang, Zachary Paul Cano,
Zhong Ma, Cheng Zhong, Dong Su, Zhongwei Chen, 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,
Applied Catalysis B, Environmental https://doi.org/10.1016/j.apcatb.2017.11.077
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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
Jie Ying
a
, Jing Li
b
, Gaopeng Jiang
a
, Zachary Paul Cano
a
, Zhong Ma
a
, Cheng Zhong
c
, Dong
Su
b
, Zhongwei Chen
a,
*
a
Department of Chemical Engineering, University of Waterloo, Ontario N2L 3G1, Canada
b
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973,
USA
c
Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Education),
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
*Corresponding authors.
E-mail addresses: zhwchen@uwaterloo.ca (Z.Chen).
Graphical Abstract
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
ACCEPTED MANUSCRIPT

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).
Keywords: PtCo bimetallic nanoparticles; Metal-organic frameworks; Nitrogen-doping;
Hollow porous capsules; Oxygen reduction reaction
ACCEPTED MANUSCRIPT

1. 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 [1-3]. 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 [6-8]. Existing carbon-supported Pt-based electrocatalysts can
efficiently catalyze the ORR [9-13], 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 [15-18]. 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 [22-27]. 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 O
2
) 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) [35-38] further improve the ORR activity and stability for Pt-based catalysts.
Particularly, when HPCs encapsulate metal nanocrystals, the hybrid catalysts often exhibit
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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 [36-39].
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 mg
Pt
-1
and 0.876 mA cm
-2
),
which are much better than those of the commercial Pt/C catalysts (0.102 A mg
Pt
-1
and 0.177
mA cm
-2
) and commercial Pt black (0.042 A mg
Pt
-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.
2. Experimental section
2.1 Preparation of ZIF-67
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Frequently asked questions

Q1. What are the characteristics of the prepared products?

The prepared products possess many desirable features such as well-dispersed nanoparticles, embedded alloys, hollow porous structures, capsule-like morphology, and nitrogen dopants. 

Q2. What is the way to reduce the energy requirement of Pt-based catalysts?

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]. 

Q3. What is the effect of PtCo/Co@NHPCC on the ORR?

In virtue of their unique hollow porous nanostructures with the beneficial features ofembedded small alloyed particles, nitrogen-doped carbon, and high surface area, PtCo/Co@NHPCC is expected to exhibit significantly superior catalytic performance to traditional Pt-based catalysts, particularly in electrocatalysis. 

Q4. What is the way to improve the ORR activity of Pt-based catalysts?

supports with well-designed nanostructures such as carbon nanotubes [31,32], hollow carbon spheres [33,34], and hollow porous carbons (HPCs) [35-38] further improve the ORR activity and stability for Pt-based catalysts. 

Q5. What is the way to develop advanced ORRs?

Their investigation shows that advanced ORR electrocatalysts can be developed by combining the advantages of superior Pt-based nanostructured catalysts and novel support materials, which ultimately supports the widespread commercial penetration of PEMFCs. 

Q6. What is the way to reduce the usage of Pt?

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]. 

Q7. What is the way to reduce the mass requirement of Pt-based catalysts?

Regarding the first strategy, an effective method of indirectly reducing the Pt massrequirement is to improve the ORR activity and stability of Pt-based catalysts via advanced morphologies and structures [15-18]. 

Q8. What is the effective strategy for reducing the energy requirement of Pt-based catalysts?

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 [22-27]. 

Q9. How long did the solution remain stirring?

After 2 h stirring, 0.4 mL of 30 mM aqueous H2PtCl6·6H2O solution was added slowly and the solution was kept stirring for another 2 h. 

Q10. What is the simplest method of obtaining Pt nanoparticles?

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 ofAC CEPT EDM ANUS CIP TZIF-67 due to the high hydrophilicity of ZIF-67 (Fig. 1b); (iii) absorption of Pt precursor into the pores of ZIF-67 due to its hydrophilic affinity to ZIF-67 (Fig. 1c); (iv) removal of all of the solvents via evaporation (Fig. 1d) and (v) formation of Pt nanoparticles in the pores of ZIF-67 (Pt@ZIF-67) by hydrogen reduction (Fig. 1e).