Date Published: March 06, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Henan Jia, Yifei Cai, Jinghuang Lin, Haoyan Liang, Junlei Qi, Jian Cao, Jicai Feng, WeiDong Fei.
The potential window of aqueous supercapacitors is limited by the theoretical value (≈1.23 V) and is usually lower than ≈1 V, which hinders further improvements for energy density. Here, a simple and scalable method is developed to fabricate unique graphene quantum dot (GQD)/MnO2 heterostructural electrodes to extend the potential window to 0–1.3 V for high‐performance aqueous supercapacitor. The GQD/MnO2 heterostructural electrode is fabricated by GQDs in situ formed on the surface of MnO2 nanosheet arrays with good interface bonding by the formation of Mn—O—C bonds. Further, it is interesting to find that the potential window can be extended to 1.3 V by a potential drop in the built‐in electric field of the GQD/MnO2 heterostructural region. Additionally, the specific capacitance up to 1170 F g−1 at a scan rate of 5 mV s−1 (1094 F g−1 at 0–1 V) and cycle performance (92.7%@10 000 cycles) between 0 and 1.3 V are observed. A 2.3 V aqueous GQD/MnO2‐3//nitrogen‐doped graphene ASC is assembled, which exhibits the high energy density of 118 Wh kg−1 at the power density of 923 W kg−1. This work opens new opportunities for developing high‐voltage aqueous supercapacitors using in situ formed heterostructures to further increase energy density.
With the tremendous growth in renewable power tools, it is imperative to meet the increasing demand for high‐performance electrochemical energy sources. Supercapacitors which pose faster charge/discharge rates and higher power density are becoming hot topics of current research worldwide.1, 2, 3, 4 However, the practical application of supercapacitors has long been a challenge because of their limited energy density. Thus, a great deal of research effort has been devoted on high energy density supercapacitors. According to the equation of energy density E = 1/2 CV2, the energy density (E) of supercapacitors can be enhanced by increasing either voltage window (V) or specific capacitance (C).5
In this paper, the GQDs/MnO2 heterostructural materials on Ni foam substrate were synthesized through a facile and effective method. Figure1 illustrates the two‐step synthesis of GQDs/MnO2 heterostructural electrodes using hydrothermal and PECVD synthesis. In the first step, a facile hydrothermal method followed by vacuum drying made MnO2 nanosheet arrays grown vertically on clean Ni foam substrate (step 1). In the second step, the PECVD process resulted in the in situ formation of GQDs on the surface of MnO2 nanosheets (step 2). In this step, we creatively use CO2 to replace common hydrocarbons (such as CH4) as carbon source to form GQDs/MnO2 heterostructures at low temperatures (350 °C). In our previous report,26, 27 MnO2 was synthesized through the same hydrothermal process reported in this paper, and we chose CH4 as carbon source in PECVD process. However, the results show that there is no GQDs formation, and we fabricated nanosized core–shell graphene–MnO2 nanosheet arrays. The pure MnO2 nanosheets are transformed into core–shell graphene–MnO2 nanoparticles by using CH4, which reveals that introducing CH4 in PECVD is beneficial for the growth of graphene. Moreover, Chen and co‐workers reported that H2 gas, which decomposes from CH4, can change the morphology of manganese oxide.28 Lu and co‐workers reported that hydrogen, which can decompose from common hydrocarbons, can partly reduce MnO2 nanosheets.29 This may cause the phase instability of pure MnO2 nanosheets. Furthermore, as previously reported, H2 can promote the subsequent nucleation and growth of graphene without GQDs.30 Therefore, in order to realize the GQDs growth in PECVD process, we creatively use CO2 (does not contain hydrogen atoms) as the carbon source to replace common hydrocarbons, and the obtained samples labeled as GQDs/MnO2‐X, where X means deposition time (1–10 min). Specific details and the calculation of mass loading are provided in the Experimental Section (Supporting Information).
In summary, a simple strategy has been demonstrated for fabricating GQDs/MnO2 heterostructural electrodes for ultrahigh energy density 1.3 V aqueous supercapacitors. The reliable heterostructures between GQDs and MnO2 nanosheet were in situ formed with good interface bonding by the formation of Mn—O—C bond in PECVD process. Furthermore, the GQDs/MnO2 heterostructural electrodes not only can enlarge the operating potential window to 0–1.3 V but also can improve the specific capacitance to 1170 F g−1. The extended potential window and improved specific capacitance of GQDs/MnO2 heterostructural electrodes can be attributed to the built‐in electric field in heterostructures. To construct ASC, NG was used as anode which is stable in the negative potential window of −1 to 0 V. The 2.3 V aqueous GQDs/MnO2‐3//NG ASC exhibited superior electrochemical performance, including high energy density (118 Wh kg−1) and high power density (12 351 W kg−1). Moreover, this rational design concept of in situ formed heterostructures between GQDs and TMO can be a general strategy for enhancing capacitance and extending voltage window for high energy density aqueous supercapacitors.
The authors declare no conflict of interest.