Nitrogen and oxygen co-doped graphene quantum dots with high capacitance performance for micro-supercapacitors
Graphical abstract
Introduction
The rapid development of portable electronic equipment, implantable medical devices, wireless sensor networks and other micro-electromechanical systems has stimulated an increasing demand for micro-energy storage devices [[1], [2], [3], [4]]. Micro-supercapacitors (MSCs) with electrode sizes of tens to hundreds of micrometers are promising energy storage systems for miniaturized devices due to their excellent rate capability, high power density, and long cycle life [[5], [6], [7], [8], [9]]. Usually, carbon-based MSCs are fabricated based on the electrical double layer capacitance (EDLC) storage mechanism using large-size carbon materials as electrode materials, including carbon fibers (CF) [9], activated carbon (AC) [10], carbide derived carbon (CDC) [2,11], carbon nanotubes (CNT) [7], and graphene sheets [[4], [5], [6],8]. These carbon-based MSCs possess light weight, high power density, and excellent cycle life, but their practical applications are limited by low volumetric energy densities (0.6–9 mWh cm−3) due to the EDLC storage limitations and the low packing density of these carbon materials with large specific area. In this context, metal oxides (RuO2 [12] and MnO2 [13]) and conductive polymers (polyaniline [14] and polypyrrole [15]) are extensively explored as pseudocapacitve materials to enhance the energy densities of MSCs. For example, MSCs based on polyaniline networks and nanofibers were reported to show volumetric energy densities of 5.83 and 16.4 mWh cm−3, respectively [16,17]. However, the miniaturization applications of pseudocapacitve MSCs are limited by poor cycle lifetimes and low power densities owing to the low electrical conductivity and sluggish Faradic reaction dynamics of pseudocapacitve materials. The limited enhancement in volumetric energy densities is compromised by poor cycle lifetimes and low power densities, presenting a great challenge in the fabrication of MSCs with comprehensive superior electrochemical performances.
In recent years, ultrasmall nanographenes called graphene quantum dots (GQDs) with lateral sizes of several nanometers have attracted increasing attention in electrochemical, photoelectronic, biomedical, and other fields owing to their unique structure and superior properties [[18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]]. With rapid development towards device miniaturization, GQDs are more suitable than large carbon materials for the design and fabrication of micro-devices and micro-powers because of their ultrasmall sizes. Moreover, ultrasmall GQDs have much higher edge/core atomic ratios than usual carbon materials, which could greatly enhance electrochemical activities of carbon electrodes, given that edge carbon atoms are typical electrochemically active defects in sp2 hybrid carbon materials. Many studies have also revealed that the electrochemical activities of carbon materials can be further increased by introducing N or/and O heteroatoms [[32], [33], [34], [35], [36], [37]]. For example, doping carbon materials with N or O heteroatoms could induce additional pseudocapacitive activities in carbon-based supercapacitors and thus greatly increase specific capacitance. Moreover, N or O doping is favorable for improving the interface wettability of the carbon electrode for the aqueous electrolyte and thus enhancing Faradic reaction dynamics. Therefore, N/O co-doped GQDs (N-O-GQDs) with a ultrahigh overall doping concentration are expected to offer ultrahigh specific capacitances and energy storage densities for MSCs owing to their more pronounced edge and doping effects compared with usual carbon materials. Because of these advantages, GQD-based electrodes have recently been assembled for fabrication of symmetric GQD//GQD MSCs and asymmetric (GQD//MnO2 and GQD//PANI) MSCs. However, reported GQD-based MSCs in aqueous electrolyte showed rather low areal capacitance (0.5–3.0 mF cm−2) and energy densities (0.07–0.41 μWh cm−2) [22,24,25], and their all-solid-state MSCs (GQD//PANI) exhibited poorer performances (0.2 mF cm−2; 0.03 μWh cm−2) [25]. These unexpected performances are largely ascribed to the low doping level of GQDs and their poor loading ability on micro-electrodes.
Here, we report the first application of heavily co-doped N-O-GQDs in MSCs. N-O-GQDs containing 17.8 at% N and 21.3 at% O were synthesized by a modified molecular fusion method and their loading on interdigital finger gold electrodes was made by electrophoresis. Through the N/O enhanced interaction between the N-O-GQDs and the Au surface of electrodes, the GQDs can be facilely deposited on the micro-electrodes to form dense carbon films (3.57 g cm−3) containing self-assembled microtubes. Symmetric MSCs based on the N-O-GQD material offer superior performances in both aqueous and solid electrolytes. Their all-solid-state MSCs in PVA/H3PO4 show extremely high volumetric capacitance (56.1 F cm−3) and volumetric energy density (7.8 mWh cm−3), while their areal capacitance is also high (9.99 mF cm−2) and high cycling stability is maintained. The encouraging results make the N-O-GQD material promising for the next generation of high-performance MSCs.
Section snippets
Experimental
Materials: gold interdigital finger electrodes (20*10*0.635 mm) were purchased from Changchun Megaborui Technology Co., Ltd., China. All other chemicals were of analytical grade and commercially available from Shanghai Chemical Reagent Co. Ltd. and used as received without any further purification.
Fabrication of N-O-GQDs: In a typical procedure for synthesis of N-O-GQDs, pyrene (4 g) was nitrated into trinitropyrene in hot HNO3 (320 ml) at 80 °C under refluxing and stirring for 48 h. After
Results and discussion
N-O-GQD/Au microelectrodes were fabricated as followed. Firstly, colloidal N-O-GQDs were synthesized through a modified hydrothermal molecular fusion process [19,20] in the presence of hydrazine hydrate using 1,3,6-trinitropyrene as the molecular precursor. To prepare N and O co-doped carbon thin films on interdigital finger Au electrodes, an efficient and rapid electrophoresis process was employed using as-prepared N-O-GQD colloidal solution (Fig. 1a) as an electrolyte without adding other
Conclusions
In summary, we fabricated N-O-GQD MSCs with a facile electro-deposition method using N/O co-doped GQDs as the active material and H2SO4, PVA/H3PO4 as the electrolyte respectively. The N-O-GQD MSCs show extremely high volumetric capacitance (325 F cm−3 in H2SO4, 56.1 F cm−3 in PVA/H3PO4) due to enhanced electrolyte-electrode interaction, additional pseudocapacitance, modification polarity of carbon matrices to improve electrolyte wettability through appropriate nitrogen and oxygen doping. The
Acknowledgements
This work has been supported by National Natural Science Foundation of China (No. 11774216, 21471098, 11575106), the Innovation Program of Shanghai Municipal Education Commission (No. 13YZ017), and Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13078).
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