Elsevier

Nano Energy

Volume 54, December 2018, Pages 192-199
Nano Energy

Full paper
Nitrogen and sulfur co-doped porous carbon sheets for energy storage and pH-universal oxygen reduction reaction

https://doi.org/10.1016/j.nanoen.2018.10.005Get rights and content

Highlights

  • Metal-free nitrogen and sulfur co-doped porous carbon sheet (NSPCS) was rationally designed.

  • The as prepared catalyst shows pH-universal activity towards oxygen reduction reaction.

  • The as prepared NSPCS also provides better methanol tolerance, longer time durability, and excellent capacitance in supercapacitors.

Abstract

Developing efficient electrocatalysts for energy storage and oxygen reduction reaction (ORR) is of great significance for the utilization of renewable energy. In particular, designing catalysts with both promising activity and long stability for ORR in pH-universal electrolytes still remain as a tremendous challenge. To tackle such a problem, metal-free nitrogen and sulfur co-doped porous carbon sheet (NSPCS) was rationally designed in this work in order to integrate the two reported routes of enhancing the electrocatalytic activity of graphene. The as-prepared NSPCS has an onset potential of 0.89 V vs. RHE, and half-wave potential E1/2 ≈ 0.75 V during ORR in acidic solution, making it as the most active ORR catalyst. Moreover, the resulting NSPCS also shows a 0.03 V positive shift of half-wave potential than commercial Pt/C for ORR and excellent charge capacitive performance in alkaline media. Electron microscopy revealed high degree of defects on NSPCS surface. This, coupled with synergistic doping effects of nitrogen and sulfur, optimized the active sites and charge transfer, rationalized the outstanding performance in both oxygen reduction reactions and supercapacitors.

Introduction

High cost, together with the drawback of methanol crossover and carbon mono-oxide poisoning effects of platinum [1], have prompted intensive research efforts on developing more durable, efficient, and less expensive alternative catalysts for the oxygen reduction reaction (ORR) in fuel cells. The report of cobalt based ORR catalysts [2] has sparked a tremendous interests in the study of nonprecious metal catalysts (NPMCs) [3], [4], [5], [6]. Problems associated with existing NPMCs include the fast degradation of metal components by peroxide intermediates, leading to low stability and insufficient activity [7]. Therefore, identifying the true catalytic sites in atomic level and guiding the design of new types of catalysts with outstanding activity and stability for ORR become quite urgent.

After extensive and in-depth searching, heteroatom (e.g. N, P, S, B) doped carbon materials [8], [9], [10], [11], [12], [13], [14], [15], [16], [17] have been identified as a promising new class of metal-free catalysts for ORR, where excellent electrocatalytic activity and stability have been achieved. A representative example is Dai's work, in which nitrogen doped carbon nanotube arrays were successfully designed, exhibiting a much better ORR performance than Pt/C catalysts in alkaline media [18]. Coupled with improved electron transportation along the aligned carbon nanotubes, the strong electronic affinity of N atoms is counterbalanced by the adjacent C atoms, favoring the adsorption of O2 and effectively weakening O-O bonds for efficient ORR performance via a four electron transfer pathway [19], [20], [21].

Recent worldwide research in this exciting field does not only confirm that electrocatalytic activities of heteroatom-doped carbon materials originate from the changes in charge and spin densities of carbon atoms adjacent to these doping heteroatoms [22], [23], but also reveals that the doping level as well as the type of bonds formed between dopants and carbon atoms have great impacts on oxygen charge transfer [24], [25], [26], [27]. Many studies have followed the above guidelines to carefully tune the active sites via controlling the heteroatom doping types and balancing the doping contents, such as N/S, N/P and N/S/P doped CNTs [28], [29], [30], [31], graphene [11], [32], [33], [34], [35], and graphite [36] for fuel cells and other applications. Even better ORR performance than commercial Pt/C catalysts, from excellent durability to methanol crossover effects, have been demonstrated by those co-/tri-doped carbon based metal-free ORR catalysts in alkaline electrolytes [37], [38], [39], [40].

However, there are only limited success in the design of heteroatom-doped metal-free catalysts in practical acidic polymer electrolyte membrane (PEM) fuel cells, suffering from their fast degradation and low ORR performance in acidic media [41], [42], [43], [44]. This is indicative that the optimal active sites for the ORR in alkaline and acidic environments are likely dissimilar. Therefore, it would be advantageous to design a class of materials which contain different types of ORR catalytic sites, while retaining the favorable 3-dimensional structure for electrolyte transportation and high electrical conductivity for electron transfer. In this work, we developed a new doping strategy for incorporating both nitrogen and sulfur doping types and contents, and rationally designed N/S co-doped porous carbon sheet (NSPCS) cathodes that may potentially work in both acidic and alkaline ORR electrolytes. Electrochemical experiments demonstrate that the as-prepared NSPCS has extraordinarily high onset and half-wave potential of ORR. To the best of our knowledge, its ORR activity in acidic media outperformed all the existing ORR catalysts. The anti-corrosive property of carbon-based NSPCS also provide better methanol tolerance, longer time durability, and excellent capacitance in supercapacitors, making such carbon-based metal-free catalysts as great potential candidates for replacing the commercial platinum catalysts in practical PEM fuel cells.

Section snippets

Results and discussion

The morphology and composites of N,S1,S2-CM1000-b were characterized by SEM, TEM and elemental mapping in Fig. 1. SEM image in Fig. 1a shows a carbon sheet structure, while N,S1,S2-CM1000 without ball milling exhibits a nearly spherical structure in Fig. S1c, illustrating that ball milling has destroyed the spherical structure. Elemental mapping confirms uniform distribution of S, N and O elements on the surface of porous carbon sheet, implying that the active sites arising from nitrogen and

Conclusions

We successfully synthesized a series of N,S-codoped porous carbon materials by a new doping strategy. The as-prepared N,S1,S2-CM1000-b materials composes of porous carbon sheets with high density of active sites on the surface and abundant structural defects. The synergistic effect of doped S and N results in excellent electrochemical performance. It exhibits a high onset and half-wave potential in acidic system that outperform all existing heteroatom-doped carbon based electrode materials.

Preparation of N,S1,S2-CM1000 materials

The preparation of N,S1,S2-CM1000 is accomplished via the pyrolysis of a homogeneous mixture consisting of polymeric materials and thiourea (see Fig. S1). To synthesize the polymer, 0.5 g thiocyanuric acid, 28.0 ml pyridine and 2.0 ml hexachlorobutadiene were mixed in a 50 ml stainless steel autoclave with a Teflon liner. The polymerization took place in an oven at 200 ℃ for 4 h, followed by naturally cooling down to the room temperature. Solid products were collected through centrifugation at

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51772219, 21471116, and 21628102), the Zhejiang Provincial Natural Science Foundation of China (LZ17E020002 and LZ15E020002). Jun Lu gratefully acknowledges support from the U. S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office. Argonne National Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357.

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