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Facile Synthesis of Fe2O3 [email protected] Graphene for Supercapacitor Electrode with Ultralong Cycle Life in KOH Electrolyte Li Liu,†,§ Junwei Lang,† Peng Zhang,†,§ Bin Hu,‡ and Xingbin Yan*,† †

Laboratory of Clean Energy Chemistry and Materials, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese of Academy of Sciences, Lanzhou, 730000, P. R. China ‡ State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China § Graduate University of Chinese Academy of Sciences, Beijing 100080, P. R. China S Supporting Information *

ABSTRACT: Fe2O3 nanodots supported on nitrogen-doped graphene sheets (denoted as Fe2O3 [email protected]) with different loading masses are prepared through a facile one-pot solvothermal method. The resulting Fe2O3 [email protected] composites exhibit outstanding electrochemical properties in aqueous KOH electrolyte. Among them, with the optimal loading mass of Fe2O3 NDs, the corresponding Fe2O3 [email protected] sample is able to deliver a high specific capacitance of 274 F g−1 at 1 A g−1 and the capacitance is still as high as 140 F g−1 even at a ultrahigh current density of 50 A g−1, indicating excellent rate capability. More remarkably, it displays superior capacitance retention after 100 000 cycles (about 75.3% at 5 A g−1), providing the best reported long-term cycling stability for iron oxides in alkaline electrolytes to date. Such excellent electrochemical performance is attributed to the right combination of highly dispersed Fe2O3 NDs and appropriately nitrogen-doped graphene sheets, which enable the Fe2O3 [email protected] to offer plenty of accessible redox active sites, facilitate the electron transfer and electrolyte diffusion, as well as effectively alleviate the volume change of Fe2O3 NDs during the charge−discharge process. KEYWORDS: iron oxide nanodots, nitrogen-doped graphene, supercapacitor, cycling stability, rate capability

1. INTRODUCTION

In addition to carbon materials, transition metal oxides and conductive polymers are typical pseudocapacitive electrode materials for supercapacitors, and they can offer higher specific capacitance due to the fast and reversible redox reactions on the surface and subsurface and thus provide higher energy density compared with carbon materials. However, the high electrical resistance of transition metal oxides leads to poor rate capability, and the pseudocapacitive materials commonly exhibit poor cyclic stability during the long-term charge− discharge process owing to their relatively poor structural stability, active materials loss, or overoxidative decomposition.11−14 Therefore, in order to make up the drawbacks of the pseudocapacitive electrode materials especially for transition metal oxides, two effective approaches have been explored: (i) reducing the particle size to nanoscale to increase the effective surface area and shorten the diffusion paths of the ion/electron. For instance, Zhao et al. reported the synthesis of tungsten oxide quantum dots (QDs) to endue the QDs with excellent electrical conductivity and greatly improved electrochemical kinetics.15 (ii) Combining conductive carbon materials with

Nowadays, the consumption of fossil fuels and the raise of environment pollution become more and more rigorous, and it is urgent to need more effective, clean, and renewable energy sources.1 The common sustainable energy sources (wind, solar, tidal power, and geothermal energy) exhibit a seasonal characteristic, so they need special devices for energy conversion and storage to improve efficiency and sustainability of energy systems.2,3 In the fields of many applications, the most effective and practical devices for energy conversion and storage are batteries, supercapacitors, solar cells, fuel cells, and so on. Among these, supercapacitors as a new type of energy storage devices have a strong appeal to the researchers, which is mainly owing to their high power density, superior rate capability for quick charge−discharge (within several seconds), and ultralong cycle life (>100 000 cycles).1,4,5 However, these advantages are based on the surface charge adsorption and desorption on carbon electrode materials (such as activated carbon, mesoporous carbon, carbon nanofibers, graphene) of the electrical double-layer capacitors (EDLCs).6−8 Meanwhile, these carbon materials are often exposed to low specific capacitance and low energy density due to the limited surface storage-energy mechanism.9,10 © 2016 American Chemical Society

Received: January 7, 2016 Accepted: March 23, 2016 Published: March 23, 2016 9335

DOI: 10.1021/acsami.6b00225 ACS Appl. Mater. Interfaces 2016, 8, 9335−9344

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustrations of the preparation for Fe2O3 [email protected]

composite with the best loading mass displays excellent electrochemical characteristics in 2 M KOH electrolyte, including high specific capacitance (274 F g−1 at 1 A g−1), outstanding rate capability (140 F g−1 at 50 A g−1), and superior long-term cycling stability (75.3% after 100 000 cycles at 5 A g−1), showing great potential as anode material for highperformance aqueous asymmetric supercapacitors.

transition metal oxides to enhance the electrochemical performance. In this aspect, the most common strategy is that the metal oxides are supported on carbon materials or wrapped with carbon layers, thus providing the enhanced rate capability and cycling performance.16−19 Iron oxides electrode materials have drawn greatly attention thanks to its abundance, high thermal stability, low-toxicity, as well as low manufacturing cost.20,21 Also because iron oxides have a suitable voltage window and high theoretical specific capacitance in negative potential, they are promising candidates for anode materials of asymmetric supercapacitors.22 Accordingly, based on the above-mentioned strategies, Fe2O3 and FeOOH QDs loaded on graphene sheets have been recently prepared for supercapacitor electrodes and demonstrated remarkably improved rate performance and cycling stability in aqueous neutral electrolytes.23,24 Nevertheless, as we know that alkaline electrolytes are the most suitable electrolytes for major metal oxides/hydroxides cathodes (such as NiO, Co3O4, Ni(OH)2) of asymmetric supercapacitors to fully exhibit their capacitances.25−27 The iron oxides provide pseudocapacitance in alkaline electrolyte with the capacitance of

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