Graphene oxide-polydopamine derived N, S-codoped carbon nanosheets as superior bifunctional electrocatalysts for oxygen reduction and evolution

doi:10.1016/j.nanoen.2015.11.027

Nano Energy

Volume 19, January 2016, Pages 373-381

https://doi.org/10.1016/j.nanoen.2015.11.027 Get rights and content

Highlights

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New synthesis method for high efficiency codoping.
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Fascinating nanostructure to facilitate electrocatalytical activities.
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Efficient bifunctional performance for ORR and OER.
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Reasonable mechanism analysis from the structural and component advantages.
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Highly potential significance for design of new advanced composite catalysts.

Abstract

Engineering carbon materials as the bifunctional catalysts for both electrocatalytic oxygen reduction/evolution reactions (ORR/OER) is highly promising for the large-scale commercialization of regenerative fuel cells and rechargeable metal-air batteries. Codoping carbons with heteroatoms can achieve unique electronic structures and show tailored electrocatalytic capabilities by rationally regulating their dopants. Sulfur is one of the most important dopants from both experimental and theoretical perspectives. In this work, a novel, highly efficient and environmentally benign method for sulfur incorporation into carbon framework has been developed facilely on the basis of graphene oxide-polydopamine (GD) hybrids to derive the N, S-codoped mesoporous carbon nanosheets. 16.7 at% S can be conjugated to the GD hybrids associated with the S doping efficiency up to 6.1% after 800 °C pyrolysis, which is higher than most previous S doping approaches. The resultant N, S-codoped mesoporous carbon nanosheets exhibit superior performance with favorable kinetics and excellent durability as a bifunctional ORR and OER catalyst, which is much better than that of most reported metal-free doped carbon, even transition metal and noble metal catalysts. The high concentrations of multiple dopants, abundant porous architecture and good electron transfer ability are believed to significantly expedite the ORR and OER catalytic processes. In the light of physicochemical versatility and structural tunability of polydopamine (PDA), this work provides a universal platform towards further development of PDA-based carbon materials with heteroatom dopants as the highly efficient electrocatalysts.

Graphical abstract

N, S-codoped mesoporous carbon nanosheets are facilely derived from the graphene oxide-polydopamine hybrids through a novel, highly efficient and environmentally benign method. The optimized material can act as the superior bifunctional electrocatalysts for oxygen reduction and oxygen evolution, and its performances are even better than transition-metal and noble metal catalysts because of its high concentration of multiple dopants, abundant porous architecture and excellent charge-transfer ability.

Introduction

Regenerative fuel cells and rechargeable metal-air batteries have drawn intensive attention in energy storage and conversion applications due to their theoretically high energy densities [1]. The reaction rates of cathodic oxygen reduction reaction (ORR) and anodic oxygen evolution reaction (OER) as half-reactions play key roles in the output performance of these devices. However, the sluggish kinetics of both ORR and OER have posed many scientific challenges due to their complicated multi-electron transfer processes resulting relative high overpotentials [1], [2]. The common electrocatalysts based on noble metals like Pt, Ir, and Ru, etc. are usually effective to boost the ORR or OER rates, but their practical applications are severely hindered by the prohibitive cost, scarce resource, and poor durability [3], [4], [5], [6]. Therefore, extensive studies have been dedicated to searching for alternative catalysts to replace noble metals with comparable electrocatalytic activity yet acceptable cost [7], [8], [9], [10]. Recently, the state-of-the-art non-metallic heteroatom-doped carbon materials have aroused tremendous interests because of their competitive activity, low cost and significantly enhanced stability [11], [12], [13], [14]. Among these materials, the approach of codoping two or more selected heteroatoms into the designed sites of carbon matrix is becoming one of the major trends nowadays, because it can create a unique electronic structure with synergistic coupling effect among heteroatom dopants [15], [16], [17]. These codoped carbon materials not only are more catalytically active than most single doped carbon, but also show tailored catalytic capabilities for different electrocatalytic reactions by altering doping types, sites and levels. For example, N, B or N, S-codoped graphene reveals much better ORR performance [16], [18], [19], while N, O-codoped carbon hydrogels show noticeable catalytic activity for the OER [20]. Therefore, the possibility of codoped carbon materials as the bifunctional catalysts for both ORR and OER is highly promising by rationally regulating their dopants.

Sulfur is one of the most important dopants to tailor the electrocatalytic activities of carbons, from both experimental and theoretical perspectives [16], [21], [22], [23], [24]. The conventional strategies reported for preparing codoped carbons with S (mostly N, S-codoped carbons) involve a thermal evaporation/vaporization process, i.e. heating carbon materials (graphene, carbon nanotube, etc) in the gas atmosphere containing N and S (NH₃, pyridine, H₂S, SO₂, and thiophene) [23], [24] or with some precursors (melamine, benzyl disulfide) [16] that can be pyrolyzed into gases at high temperature. However, there are some limitations arising from these post-treatment doping approaches. For example, although excessive N and S sources (e.g. melamine and benzyl disulfide) and high temperature (>900 °C) are used, the doping efficiency is still very low (<5 at%). More seriously, vast majority of raw materials are discharged in the form of N or S-bearing waste gases, which is apparently not environmentally benign [16], [23]. On the other hand, pyrolysis of rich N-containing precursors has been widely used for the fabrication of N-doped carbon materials which can greatly improve the doping efficiency of nitrogen [13], [17], [25]. However, this method is not practically applicable for the preparation of N, S-codoped materials in view of the high cost and the scarcity of N, S-containing precursors [26].

Recently, we used polydopamine (PDA) as the N-containing precursors to derive mesoporous carbon nanosheets as an efficient ORR catalyst [27]. Compared with other commonly used N-containing precursors such as melamine [13], polypyrrole [17], and polyaniline [25], [28], PDA displays many incomparable features. For example, PDA is nontoxic, extrmely soluble and has a high carbon yield [29]; PDA also has excellent structural tunability and strong chelation capability to metal ions, implying the possibility of fabricating many desired nanostructures with adjustable components [29], [30], [31], [32], [33]. The most important is the facile post-modification of PDA; PDA is particularly reactive to amine or thiol groups via Schiff base or Michael addition reaction [34], [35], [36], [37]. Especilly, the thiol addition reaction proceeds extremely fast with its rate constants ranging from 4×10⁵ to 3×10⁷ M⁻¹ s⁻¹ (in the case of cysteine at pH=7) [38]. These reactions proceed efficiently at room temperature without need of any harsh reaction condition. Therefore, it is particularly advantageous to prepare codoping carbon materials using the PDA as the N-containing precursor since a variety of heteroatoms including nitrogen and sulfur can be easily introduced via the post-modification of PDA. The above unique and remarkable features make PDA highly promising as a simple and effective candidate for the preparation of N, S-codoped functional carbon materials. Although PDA has been extensively serviced as the building block to construct metallic or non-metallic composites based on its remarkable physicochemical versatility [39], [40], few relevant eletrocatalytical works have been reported [41], [42].

Herein, we use graphene oxide and PDA to derive the N, S-codoped carbon sheets as the highly efficient bifunctional electrocatalysts for ORR and OER. Graphene oxide (GO) is used as the substrate to synthesize sulfur modified GO-PDA (GDS) hybrids, where dopamine (DA) polymerizes on the surface of GO to produce a uniform PDA layer, and 2-mercaptoethanol is then conjugated to the PDA through Schiff base or Michael addition reactions. The N, S-codoped mesoporous carbon nanosheets obtained from the pyrolysis of GDS hybrids possess a much higher S-doping efficiency with the assistance of PDA than most reported methods and exhibit excellent ORR/OER bifunctional activity and durability, even better than that of transition-metal and noble metal catalysts. This proof-of-concept study would lay a solid foundation for the further exploration and development of nanostructural PDA-based carbon materials for energy-relevant applications.

Section snippets

Materials

Natural graphite flakes, sulfuric acid (H₂SO₄, 95–98%), potassium permanganate (KMnO₄, 99%), phosphorous acid (H₃PO₄, 85%), hydrogen peroxide (30%), dopamine hydrochloride, 2-mercaptoethanol and disodium hydrogen phosphate (Na₂HPO₄) were purchased from Sigma-Aldrich and directly used without further purification. Milli-Q water (18.2 MΩ) was used throughout all experiments.

Materials characterization

Fourier transform infrared (FTIR) spectra were collected on the transmission module of a Thermo Nicolet 6700 FTIR

Results and discussion

As illustrated in Scheme 1, GD hybrids were first synthesized by mixing a given amount of DA with GO in PBS buffer (pH=8.5). DA polymerized to form a PDA thin film directly onto the surface of GO. 2-mercaptoethanol was then reacted with the PDA via the Schiff base or Michael addition reaction to produce the GDS hybrids. PBS buffer was used other than Tris buffer because the primary amine groups of Tris can interact covalently with the PDA, which might influence the deposition of PDA thin films

Conclusion

In summary, a robust, highly efficient and an environmentally benign method has been developed to introduce S to the GO-PDA hybrids to produce N, S-codoped mesoporous carbon nanosheets. As a result, the fabricated mesoporous carbon nanosheets have exhibited much better performances than most of other benchmarked bifunctional ORR and OER catalysts, which are attributed to their multiple doping, unique porous architecture and excellent charge-transfer ability. Due to the versatile physicochemical

Acknowledgments

This work was financially supported by the Australian Research Council (ARC) through the Discovery Projects of DP110102877, DP130104459 and DP140104062.

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CommunicationGraphene oxide-polydopamine derived N, S-codoped carbon nanosheets as superior bifunctional electrocatalysts for oxygen reduction and evolution

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Materials characterization

Results and discussion

Conclusion

Acknowledgments

J. Power Sources

Nature

Nature

J. Am. Chem. Soc.

ACS Catal.

Angew. Chem. Int. Ed.

Int. J. Hydrog. Energy

Nat. Chem.

Adv. Funct. Mater.

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Energy Environ. Sci.

Science

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Communication
Graphene oxide-polydopamine derived N, S-codoped carbon nanosheets as superior bifunctional electrocatalysts for oxygen reduction and evolution