Graphitic Carbon Quantum Dots Modified Nickel Cobalt Sulfide as Cathode Materials for Alkaline Aqueous Batteries

The scalable graphitic carbon quantum dots (CQDs) are obtained with a high yield of more than 50%. The CQDs are utilized to synthesize novel NS-CQDs/NiCo2S4 composite which shows enhanced electrochemical properties. The assembled novel alkaline aqueous battery delivers superior electrochemical performances. The scalable graphitic carbon quantum dots (CQDs) are obtained with a high yield of more than 50%. The CQDs are utilized to synthesize novel NS-CQDs/NiCo2S4 composite which shows enhanced electrochemical properties. The assembled novel alkaline aqueous battery delivers superior electrochemical performances. Carbon quantum dots (CQDs) as a new class of emerging materials have gradually drawn researchers’ concern in recent years. In this work, the graphitic CQDs are prepared through a scalable approach, achieving a high yield with more than 50%. The obtained CQDs are further used as structure-directing and conductive agents to synthesize novel N,S-CQDs/NiCo2S4 composite cathode materials, manifesting the enhanced electrochemical properties resulted from the synergistic effect of highly conductive N,S-codoped CQDs offering fast electronic transport and unique micro-/nanostructured NiCo2S4 microspheres with Faradaic redox characteristic contributing large capacity. Moreover, the nitrogen-doped reduced graphene oxide (N-rGO)/Fe2O3 composite anode materials exhibit ultrahigh specific capacity as well as significantly improved rate property and cycle performance originating from the high-capacity prism-like Fe2O3 hexahedrons tightly wrapped by highly conductive N-rGO. A novel alkaline aqueous battery assembled by these materials displays a specific energy (50.2 Wh kg−1), ultrahigh specific power (9.7 kW kg−1) and excellent cycling performance with 91.5% of capacity retention at 3 A g−1 for 5000 cycles. The present research offers a valuable guidance for the exploitation of advanced energy storage devices by the rational design and selection of battery/capacitive composite materials.


Introduction
Carbon quantum dots (CQDs), as a novel type of zerodimensional (0D) carbon materials with ultrasmall sizes less than 10 nm, have been extensively applied in fluorescence, bioimaging, biosensing, photocatalysis and photovoltaics owing to their unique advantages such as quantum confinement and edge effects [1][2][3][4]. In particular, the application of CQDs in energy storage has gradually attracted researchers' attention in recent years [5][6][7]. In the early research, the reduced carbon quantum dots (rCQDs)/RuO 2 composites have been obtained as ultrafast supercapacitor electrode materials for the first time, where rCQDs are utilized as conductive agents to boost the electrical conductivity of electrode materials, enhancing the supercapacitive properties, especially rate capability [8]. Since this reported pioneering work, more researches about such CQDs-based energy storage materials as supercapacitors, lithium/sodium ion batteries and fuel cell have been explored [9][10][11][12][13][14], which indicates that these upcoming CQDs-based electrode materials present a great prospect as superior energy storage materials. Nevertheless, the utilization of CQDs as electrode materials is much less reported compared to the conventional carbon materials; hence, it shows a great potential to further explore them.
Currently, the methods for the synthesis of CQDs are usually classified into "top-down" and "bottom-up" types. The former is usually breaking down large-size carbon materials into CQDs, including arc discharge, electrochemical exfoliation, hydrothermal technique and chemical oxidation synthesis [15][16][17][18], whereas the latter is usually preparing CQDs from molecular precursors, such as combustion/ thermal decomposition method, supported synthesis and microwave pyrolysis synthetic route [19][20][21]. However, these approaches for the synthesis of CQDs still face major challenges, such as a low yield (usually lower than 10%), complicated processes, poor reproducibility and electronic conductivity [1]. In our previous study, graphitic CQDs were obtained via chemical oxidation synthesis route utilizing nano-graphite powders (~30 nm) as precursors, and a relatively low yield of less than 20% was acquired [8,10]. Hou et al. obtained non-graphitic CQDs through a facile and scalable solution growth method by mixing NaOH and acetone together, and an ultrahigh yield with gram level was achieved. However, the resulted CQDs presented low electronic conductivity, leading to the poor electrochemical properties [12]. From the perspective of practical application for energy storage electrode materials, it is crucially important to develop a scalable approach for synthesizing CQDs with a high yield, low cost and high electrical conductivity.
Besides, another major challenge for CQDs is that they are apt to aggregate and stack together, which results in a rapid decrease in their specific surface areas and kinetic behaviors, thus worsening the electrochemical properties of CQDs [22]. These obstacles above prevent their utilization as excellent energy storage electrode materials. The most effective method to solve the aggregation is to employ them as composite materials. Recently, ternary metal oxides/ sulfides have attracted extensive interests because of their higher electrical conductivity and richer electrochemical activity than those of the corresponding binary metal oxides/sulfides, which have been explored as potential cathode materials for alkaline aqueous batteries [23][24][25][26][27][28]. Among them, NiCo 2 S 4 is seen as the most prospective candidate for future utilization in such energy storage as supercapacitors and lithium ion batteries. To further enhance its comprehensive electrochemical performances, an effective way is to design various unique micro-/nanostructured NiCo 2 S 4 , and the enhanced electrochemical properties have been reported by researchers in the literature [29][30][31][32][33]. Another good way is to introduce carbon materials to prepare the composite electrode materials, leading to the enhanced electrochemical properties to some extent. Currently, the most reported carbon materials are graphene and carbon nanotubes, demonstrating decent electrochemical properties, but their high cost seriously prevents the practical application [34,35]. Therefore, it is very emergent to exploit novel and cheap carbon materials for the fabrication of superior composite electrode materials.
As discussed above, the scalable graphitic CQDs were first synthesized with a high yield of more than 50% utilizing highly conductive Super P powders as precursors. Then, the CQDs were employed as both structure-directing and conductive agents to prepare new N,S-codoped CQDs-decorated urchin-like NiCo 2 S 4 microspheres for the first time. In comparison with the pure NiCo 2 S 4 electrode, the N,S-CQDs/ NiCo 2 S 4 composite electrode, when for the first time used as cathode for alkaline aqueous batteries, exhibits enhanced electrochemical properties with a high specific capacity of 124.4 mAh g −1 at 2 A g −1 , exceptional capacity retention of 77.3% at 50 A g −1 in comparison with 2 A g −1 and excellent 1 3 cycling property with 97.9% of initial capacity retention at 3 A g −1 for 5000 cycles. Meanwhile, the N-rGO-wrapped prism-like Fe 2 O 3 hexahedrons composite anode materials are obtained, which manifest substantially enhanced electrochemical performances with an ultrahigh specific capacity (249.6 mAh g −1 at 1 A g −1 ) as well as greatly enhanced rate property and cycling life compared with those of the pristine Fe 2 O 3 electrode. A novel N,S-CQDs/NiCo 2 S 4 //N-rGO/ Fe 2 O 3 alkaline aqueous battery fabricated by these materials achieves a specific energy (50.2 Wh kg −1 ) and ultrahigh specific power (9,700 W kg −1 ) with excellent cycle property (91.5% of capacity retention for 5000 cycles). The encouraging results display the enormous potential of the new alkaline aqueous battery as highly promising energy storage system for potentially practical applications.

Synthesis of CQDs
CQDs were obtained by a scalable chemical oxidation approach. Super P powders (1 g, purchased from Sigma-Aldrich) were first added into concentrated H 2 SO 4 /HNO 3 mixed solution (80 mL, v/v, 3:1) and then sonicated for 1 h. The resulting mixture was refluxed for 24 h. Subsequently, the mixture was further neutralized with Na 2 CO 3 ·10H 2 O, and the pH was controlled at 7.0. After further dialysis and vacuum-drying, the final CQDs powders were obtained.

Synthesis of N,S-CQDs/NiCo 2 S 4 Composite Materials
The N,S-CQDs/NiCo 2 S 4 composite materials were synthesized by a one-step hydrothermal approach.

Materials Characterization
X-ray diffraction (XRD) analysis was implemented on an X-ray diffractometer (Rigaku D/max 2550 VB + ). Raman spectra were executed on a Raman spectrometer (Labram-010). X-ray photoelectron spectroscope (XPS) analysis was conducted on a photoelectron spectrometer (K-Alpha). FESEM images, EDS and elemental mappings were captured on a field emission scanning electron microscopy (FESEM, MIRA3). Transmission electron microscope (TEM) images, HRTEM images and SAED patterns were obtained on a transmission electron microscopy (JEM-2100F). TG curves were recorded from a thermal analysis instrument (NETZSCH STA449C). N 2 adsorption/desorption isotherms were acquired on a BET specific surface area analyzer (BELSORP-miniII).

Electrochemical Measurements
To fabricate the working electrodes for electrochemical tests, 70 wt% of electroactive material, 20 wt% of Super P and 10 wt% of PVDF with NMP as solvent were adequately mixed to obtain an uniform slurry. Afterward, the resulting slurry was dipped into Ni foam current collector, which was pretreated based on our previous work [10], and dried in a vacuum oven at 70 °C overnight. Finally, the as-fabricated electrodes were further pressed at 10 MPa. The electrochemical properties of the single electrode were conducted by employing a typical three-electrode cell with platinum foil, Hg/HgO and 2 M KOH solution as counter electrode, reference electrode and electrolyte, respectively. The alkaline aqueous coin battery (CR 2016) was further assembled by utilizing N,S-CQDs/NiCo 2 S 4 as cathode, N-rGO/Fe 2 O 3 as anode, glassy fibrous material as separator and 2 M KOH solution as electrolyte. The electrochemical tests such as CV, GCD and EIS for both single electrode and alkaline aqueous device were executed on an electrochemical workstation (CHI 760E).

Preparation Processes of the CQDs and the Composite Cathode and Anode
The synthetic processes of the graphitic CQDs, the N,S-CQDs/NiCo 2 S 4 composite cathode and N-rGO/Fe 2 O 3 composite anode are demonstrated in Fig. 1. First, the graphitic CQDs are prepared through a scalable mixedacid chemical oxidation approach using the industrialized, cheap and highly conductive Super P powders as precursors. The obtained CQDs achieve a high yield of more than 50%, which is far higher than that prepared by most of previous work reported in the literature [1,8,10,36,37], demonstrating the great prospect for their potentially practical application. Afterward, the resulting CQDs are further employed as structure-directing and conductive agents to prepare novel N,S-codoped CQDsdecorated urchin-like NiCo 2 S 4 microspheres composite cathode materials via a one-step hydrothermal approach, in which thioacetamide is used as precipitant to generate the NiCo 2 S 4 and codoped agent to achieve nitrogen and sulfur codoping into the CQDs. In this process, the CQDs are not only utilized as structure-directing agents to induce the formation of nanorods-assembled urchin-like composite microsphere structure, but also served as conductive agents to improve the electrical conductivity of the composite cathode materials. Meanwhile, the N-doped rGOwrapped prism-like Fe 2 O 3 hexahedrons composite anode materials are prepared by a facile hydrothermal synthetic route, where urea serves as precipitant to generate the Fe 2 O 3 and acts as doped agent to achieve nitrogen doping into the GO. The Fe 2 O 3 hexahedrons are tightly wrapped by the N-rGO sheets, and the close contact between the Fe 2 O 3 hexahedrons and the N-rGO sheets can improve the electrochemical kinetic behavior and structural stability of the composite anode materials. is the digital photograph of the final obtained CQDs powders, and the yield is up to 51%. Thus, the scalable synthesis of CQDs with a high yield, together with the advantages of employing a cheap and industrial carbon source, renders them being potential candidate electrode materials for large-scale storage application. Figure 2d illustrates the XRD pattern of CQDs, and the broad peak of CQDs at around 25° corresponds to (002) plane of the disordered graphite carbon resulting from the strong acid etching [38]. Also, no other peak such as inorganic salts is observed, indicating the high purity of CQDs. FTIR spectrum (Fig. 2e) reveals that the main peaks at 3445, 1720, 1592, 1370 and 1252 cm −1 are well fitted to the C-OH, C = O, C = C, C-H and C-O-C bonds [8,39], respectively, and the existence of these bonds shows that the CQDs are functionalized with the abundant oxygencontaining surface groups, which is particular in favor of the construction of composite materials by the utilization of the functionalized CQDs. Raman spectroscopy (Fig. 2f) was used to further characterize the structure of CQDs. The two obvious peaks at 1346 and 1581 cm −1 belong to the D-band and G-band of CQDs, which are attributed to the disordered carbon and ordered graphitic structure, respectively [40]. XPS was undertaken to characterize the elemental composition and carbon bonds of CQDs, as presented in Fig. 2g, h. The four peaks at 284.8, 286.5, 288.4 and 289.2 eV in the spectrum of C1s (Fig. 2h) are, respectively, ascribed to the C-C, C-O, C = O and O-C = O bonds, which accord well with FTIR analysis results [12]. Figure 2i, j exhibits the TEM images of the as-obtained CQDs, and it is clear that the as-prepared CQDs are monodisperse with the uniform distribution in the range of 5-8 nm. As exhibited in the inset of Fig. 2j, the HRTEM image manifests the lattice fringe of 0.215 nm, which accords well with the (100) plane of graphitic carbon, proving the formation of graphitic CQDs.

Physicochemical and Electrochemical Characterizations of the Cathode Materials
The morphology and microstructure of the as-resulted N,S-CQDs/NiCo 2 S 4 composite were examined by FESEM, TEM, HRTEM, EDS and elemental mappings. As demonstrated in Fig. 3a-c, the FESEM images show that the composite presents the urchin-like microsphere structure with a size of about 3 µm, and the microspheres are composed of numerous nanorods. The superior micro-/nanostructured morphology of the composite is achieved when compared to that of the pristine NiCo 2 S 4 where only the microspheres with irregular and larger sizes are obtained as exhibited in Fig.   (  S3, which is ascribed to the induced role of CQDs during the formation process of the composite. The micro-/nanostructured morphology of the composite was further surveyed by TEM and HRTEM. The TEM images (Fig. 3d, e) reveal the nanorods-assembled urchin-like microsphere structure with a size of approximately 3 µm, which is well matched with the FESEM images. As displayed in Fig. 3f, the HRTEM image of the composite shows that the CQDs are well combined with NiCo 2 S 4 to obtain the uniform composite structure. Moreover, the HRTEM image exhibits the lattice fringes of 0.28 and 0.24 nm, which, respectively, accord well with the (311) and (400) planes of NiCo 2 S 4 . In addition, the EDS analysis (Fig. 3g) and corresponding elemental mappings ( Fig. 3h) show the existence of Ni, Co, S, C and N, suggesting the formation of N,S-CQDs/NiCo 2 S 4 composite.
XRD was employed to examine the phase and structure of the pristine NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 composite, and the results are exhibited in Fig. 4a. The main diffraction peaks of the pristine NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 composite can be clearly observed from the XRD patterns, which all well correspond to the NiCo 2 S 4 (JCPDS Card No. 20-0782), suggesting that the structure of the composite is not changed in the combination of N,S-CQDs and NiCo 2 S 4 . However, it is hard to notice the peak of N,S-CQDs due to the relatively low content and weak diffraction peak intensity. Raman spectra were used to further characterize the structure of the pristine NiCo 2 S 4 and N,S-CQDs/  (Fig. 4b). Four peaks all can be seen for the pristine NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 composite, which correspond to two F 2g (191 and 524 cm −1 ), one E g (472 cm −1 ) and one A 1g (675 cm −1 ) Raman modes of NiCo 2 S 4 [29]. Moreover, the D-band (disordered carbon) and G-band (ordered graphitic carbon) situated at 1356 and 1590 cm −1 are characteristic Raman modes of carbon, which are seen from the Raman spectroscopy of the N,S-CQDs/ NiCo 2 S 4 composite. XPS was further undertaken to study the elemental composition and valence state of the as-synthesized N,S-CQDs/ NiCo 2 S 4 composite, as presented in Fig. 4c-g. The survey spectrum (Fig. S4) demonstrates the coexistence of Ni, Co, S, C, O and N. Figure 4c, d, respectively, exhibits the spectra of Ni 2p and Co 2p. The two peaks situated at 853.04 and 870.44 eV are indexed to Ni 2+ , and another two peaks situated at 854.24 and 876.04 eV are indexed to Ni 3+ , respectively [41]. Similarly, the two peaks situated at 780.54 and 797.24 eV can be ascribed to Co 2+ , and another two peaks situated at 778.74 and 793.79 eV can be attributed to Co 3+ , respectively [41]. The S 2p spectrum presented in Fig. 4e represents two main peaks situated at 161.9 and 163.0 eV, which are, respectively, ascribed to the S 2− in low coordination at the surface and the metal-sulfur bonds [34]. For the C 1 s spectrum (Fig. 4f), four main peaks situated at 284.64, 285.49, 286.70 and 289.04 eV are, respectively, assigned to the C-C, C-S, C-N and C-O bonds [24], and the as-formed C-S and C-N bonds indicate that the nitrogen and sulfur elements are successfully codoped into CQDs. Furthermore, Fig. 4g displays the N 1 s spectrum with two main peaks situated at 400.3 and 401.9 eV, which correspond to the pyrrolic N and graphitic N [42]. Based on these results above, the XPS characterization exhibits the coexistence of Ni, Co, S, C and N elements in the N,S-CQDs/NiCo 2 S 4 composite.
To further determine the composition of the prepared N,S-CQDs/NiCo 2 S 4 composite, the thermogravimetric analysis (TGA) was executed in air atmosphere from ambient temperature to 900 °C as demonstrated in Fig. 4h. The TG curves of the pristine NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 composite both show two apparent weight losses and one obvious weight gain. For the N,S-CQDs/NiCo 2 S 4 composite, the first weight loss before 500 °C is more obvious than that of the pristine NiCo 2 S 4 , which results from the burning of CQDs and the loss of physically adsorbed and crystalline water. The second apparent weight gain between 500 and 650 °C is attributed to the oxidation of NiCo 2 S 4 to NiSO 4 and CoSO 4 , and the third evident weight loss in the temperature range of 650 and 800 °C is ascribed to the further burning of CQDs as well as the decomposition of NiSO 4 and CoSO 4 to stable NiO and CoO as reported in the literature [43][44][45]. From the different weight losses of the pristine NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 composite, it is deduced that the content of CQDs in composite is about 15.9 wt%. N 2 adsorption/desorption test was employed to research the specific surface area of the pristine NiCo 2 S 4 and N,S-CQDs/ NiCo 2 S 4 composite, as presented in Fig. 4i. The N,S-CQDs/ NiCo 2 S 4 composite manifests a greater specific surface area (27.2 m 2 g −1 ) than that of the pristine NiCo 2 S 4 (14.9 m 2 g −1 ), which contributes to the enhancement of electrochemical properties.
The electrochemical performances of the NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 electrodes were investigated in a threeelectrode configuration with 2 M KOH solution as electrolyte. Figure 5a shows the comparison of the CV curves of NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 electrodes at 20 mV s −1 in the potential range of 0 to 0.6 V. The two electrodes both manifest strong and broad redox peaks in the CV curves, implying the combination of Faradaic battery and doublelayer capacitive behavior. The Faradaic battery behavior is derived from reversible redox reactions of Ni 2+ /Ni 3+ and Co 2+ /Co 3+ /Co 4+ according to Eqs. 1-3 [34]: Ex situ XRD measurement was utilized to characterize the phase change of the N,S-CQDs/NiCo 2 S 4 composite during different electrochemical steps, as exhibited in Fig. 5d. The alteration in reflections indicates the phase change in the charge-discharge process and verifies that Faradaic redox reactions occur in bulk, demonstrating the feature of Faradaic battery. Furthermore, it is clear that the area of CV curve of the N,S-CQDs/NiCo 2 S 4 electrode is greater than that of the NiCo 2 S 4 electrode, revealing that the N,S-CQDs/ NiCo 2 S 4 electrode shows higher specific capacity. Figure 5b, c displays the CV curves of the NiCo 2 S 4 and N,S-CQDs/ NiCo 2 S 4 electrodes at different scan rates, respectively. The N,S-CQDs/NiCo 2 S 4 electrode manifests a smaller peak position shift than that of the NiCo 2 S 4 electrode as the scanning speed increases from 5 to 100 mV s −1 , suggesting that the composite electrode has lower polarization, better conductivity and faster electrochemical kinetic. To further explore the difference of the electrochemical behaviors of the NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 electrodes, the quantitative analysis was conducted based on the mechanism of charge storage. The relationship of the peak currents (i p ) and scan rates is expressed as I = av b shown in Fig. 5e [26], where the b-value of 0.5 and 1 suggests the  diffusion-controlled and surface capacitive processes of the electrochemical reaction, respectively. The calculated b-value of the NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 electrodes is, respectively, 0.80 and 0.89, indicating a mixed behavior of diffusion-controlled and surface capacitive processes. The higher b-value of the N,S-CQDs/NiCo 2 S 4 electrode signifies more surface capacitive behaviors. Furthermore, the result is further verified by the quantitative calculation of the contribution of the diffusion-controlled and surface capacitive charge, which is obtained from the following equation [26]: i(V) = k 1 v + k 2 v 1/2 , in which k 1 v represents the surface capacitive current and k 2 v 1/2 denotes the diffusion-controlled current. The fitted lines of the i/v 1/2 to v 1/2 of the two electrodes are presented in Fig. 5f, and the values of the slope and intercepts, respectively, represent the constants of k 1 and k 2 . The charge contributions of the NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 electrodes are manifested in Fig. 5g, and it is visible that the N,S-CQDs/NiCo 2 S 4 electrode displays larger capacitive contribution ratio than that of the NiCo 2 S 4 electrode at all scan rates, which reasonably elucidates why the N,S-CQDs/ NiCo 2 S 4 electrode possesses faster electrochemical kinetic than that of the NiCo 2 S 4 electrode. Figure 5h, i exhibits the GCD curves of the NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 electrodes at different current densities in the potential window of 0-0.5 V, respectively. The GCD curves of the N,S-CQDs/NiCo 2 S 4 electrode present a better symmetry than that of the pristine NiCo 2 S 4 electrode, signifying that the composite electrode has lower resistance and higher rate performance. The specific capacities of the two electrodes are calculated according to the GCD curves, as exhibited in Fig. 5j. The specific capacity of the N,S-CQDs/ NiCo 2 S 4 electrode is, respectively, 124.4, 122.4, 118.6, 112.8, 106.4 and 96.1 mAh g −1 at 2, 5, 10, 20, 30 and 50 A g −1 , which is better than that of the NiCo 2 S 4 electrode (from 92.8 mAh g −1 at 2 A g −1 to 59.1 mAh g −1 at 50 A g −1 ). Correspondingly, the N,S-CQDs/NiCo 2 S 4 electrode manifests a greater capacity retention of 77.3% at 50 A g −1 than that of the NiCo 2 S 4 electrode (63.7% of capacity retention at 50 A g −1 ). The improved specific capacity and rate performance of the N,S-CQDs/NiCo 2 S 4 electrode originate from the positive synergistic effect between highly conductive N,S-CQDs with the capacitive feature and high-capacity NiCo 2 S 4 with Faradaic characteristic. Note that the specific capacity and rate performance of the as-obtained N,S-CQDs/NiCo 2 S 4 composite are comparable or even higher than those of nickel cobalt sulfide-based composite as reported in the literature [46][47][48][49][50][51][52][53]. The cycling property of the NiCo 2 S 4 and N,S-CQDs/NiCo 2 S 4 electrodes was surveyed by the repeated GCD tests at 5 A g −1 in the potential range of 0-0.5 V over 5000 cycles, as exhibited in Fig. 5k. The specific capacity of the two electrodes increases in the first hundreds of cycles due to the gradually sufficient activation process and then reduces with a gradual decline as the increasing cycles [23,26]. After 5000 cycles, the N,S-CQDs/NiCo 2 S 4 electrode manifests a better cycle property with 97.9% of the initial capacity retention than that of the NiCo 2 S 4 electrode (90.3%), revealing the excellent cycling stability of the asresulted composite electrode. Additionally, the EIS test was utilized to illustrate the superior electrochemical behaviors, which was implemented in the frequency range of 0.01 Hz to 100 kHz at the open-circuit potential (Fig. 5l). The Nyquist plot consists of the intercept and depressed semicircle in the high-frequency region as well as the slope line in the lowfrequency region [54,55]. The N,S-CQDs/NiCo 2 S 4 electrode displays smaller intercept, lower semicircle diameter and higher slope value than those of the NiCo 2 S 4 electrode, indicating the lower resistance and better electrical conductivity of the composite electrode, leading to the improved electrochemical properties.

Physicochemical and Electrochemical
Characterizations of the Anode Materials The EDS spectrum (Fig. 6d) verifies the existence of Fe, O, C, and N in the composite, and elemental mappings (Fig. 6e) reveal that these elements are uniformly distributed throughout the composite. XRD was utilized to further characterize the structure and phase of the pristine Fe 2 O 3 and N-rGO/Fe 2 O 3 composite (Fig. 6f). The XRD patterns show the strong diffraction peaks of the pristine Fe 2 O 3 and N-rGO/Fe 2 O 3 composite, which are both well fitted to the Fe 2 O 3 (JCPDS Card No. . Note that the characteristic peaks of rGO cannot be observed in the XRD pattern of the composite, which results from the weak and broad peak and low rGO content in composite. Raman spectra were used to further examine the structure of the pristine Fe 2 O 3 and N-rGO/Fe 2 O 3 composite (Fig. 6g). Five peaks all can be seen from the Raman spectra of the pristine Fe 2 O 3 and N-rGO/Fe 2 O 3 composite, which are attributed to two A 1g (218 and 483 cm −1 ) and three E g (283, 399, and 601 cm −1 ) Raman modes of Fe 2 O 3 [56]. Besides, two typical peaks situated at 1320 and 1596 cm −1 are assigned to the D-band and G-band, respectively, further suggesting that the Fe 2 O 3 is successfully combined with the N-rGO to form the composite.
XPS was further applied to explore the elemental composition and valence state of the obtained N-rGO/Fe 2 O 3 composite, as presented in Fig. 6h-j. The survey spectrum (Fig. S9) demonstrates the existence of Fe, O, C and N in the N-rGO/Fe 2 O 3 composite. For the Fe 2p spectrum (Fig. 6h), the peaks situated at 710.5 and 724.1 eV are assigned to Fe 2+ , and the peaks situated at 712.8 and 726.8 eV are well fitted to Fe 3+ [57]. As for the C 1 s spectrum shown in Fig. 6i, the peaks located at 284.7, 285.4, 286.3 and 288.1 eV correspond to the C-C, C-N, C-O and C = O bonds [58], and the as-formed C-N bond implies that the nitrogen element is well doped into rGO. Furthermore, the N 1 s spectrum manifested in Fig. 6j reveals that the peaks located at 398.5, 399.9 and 401.6 eV correspond well with the pyridinic N, pyrrolic N and graphitic N [59]. Thus, the XPS analysis further proves the formation of the N-rGO/ Fe 2 O 3 composite.
The content of GO in N-rGO/Fe 2 O 3 composite was determined by TGA test, which is executed in air atmosphere from room temperature to 800 °C as demonstrated in Fig. 6k. From the TGA curves, almost no weight loss of the pristine  (Fig. 7d) was also used to investigate the phase change of the N-rGO/Fe 2 O 3 composite during different electrochemical steps. The observed change in reflections reveals the phase change in the charge/discharge process and confirms the occurring of Faradaic redox reactions, further demonstrating the typical battery characteristic. In comparison with the Fe 2 O 3 electrode, the N-rGO/Fe 2 O 3 electrode shows a smaller potential difference of the anodic and cathodic peak (∆E P ), implying the lower resistance and better kinetic behavior. Figure 7b, c displays the CV curves of the Fe 2 O 3 and N-rGO/Fe 2 O 3 electrodes, respectively, and no distinct distortion can be noticed with the increasing scanning speed from 5 to 50 mV s −1 . Figure 7e, f exhibits the GCD curves of the Fe 2 O 3 and N-rGO/Fe 2 O 3 electrodes at various current densities in the potential range between− 1 and − 0.2 V, respectively. The calculated specific capacity of the N-rGO/Fe 2 O 3 electrode (Fig. 7g) is, respectively, 249. 6, 228.4, 206.7, 174.2, 130.4 and 96.2 mAh g −1 at 1, 2, 3, 5, 10 and 20 A g −1 , which is much better than that of the Fe 2 O 3 electrode (from 109.1 at 1 A g −1 to 21.8 mAh g −1 at 20 A g −1 ). Moreover, the specific capacity of the N-rGO/Fe 2 O 3 electrode is not only far higher than that of carbon-based anode materials, but also comparable to or even larger than that of most Fe 2 O 3 -based anode materials reported in the literature [56][57][58][60][61][62][63][64]. The cycle property of the Fe 2 O 3 and N-rGO/Fe 2 O 3 electrodes was further compared by the repeated GCD tests at 3 A g −1 over 5000 cycles, as manifested in Fig. 7h. The N-rGO/Fe 2 O 3 electrode manifests a better cycle stability with 92.1% capacity retention than that of the Fe 2 O 3 electrode (71.8%) after 5000 cycles, suggesting that the unique N-rGO-tightly-wrapped Fe 2 O 3 composite structure can prevent the agglomeration of Fe 2 O 3 and N-rGO and alleviate the volume change in the repeated charging-discharging processes, leading to the improvement of cycle performance. Moreover, the EIS measurement was taken, and the Nyquist plots are demonstrated in Fig. 7i. The N-rGO/Fe 2 O 3 electrode shows smaller intercept and semicircle diameter in the high-frequency region as well as higher slope value in the low-frequency region than those of the Fe 2 O 3 electrode. Therefore, the results well demonstrate why the N-rGO/Fe 2 O 3 electrode presents improved electrochemical performances in comparison with the Fe 2 O 3 electrode.

Electrochemical Characterizations of the N,S-CQDs/NiCo 2 S 4 //N-rGO/Fe 2 O 3 Alkaline Aqueous Battery
To further clarify the superior electrochemical properties of the N,S-CQDs/NiCo 2 S 4 and N-rGO/Fe 2 O 3 composites, a novel alkaline aqueous battery was assembled by, respectively, employing the two composites as the cathode and  Figure 8a displays the CV curves of the as-obtained alkaline aqueous battery at scan rates ranging from 5 to 100 mV s −1 , and the shape of CV curves indicates that the capacity originates from the combined contribution of both Faradaic redox reaction and capacitive EDLC at all scan rates. The GCD curves of the alkaline aqueous battery at current densities of 1-20 A g −1 are presented in Fig. 8b, and the as-obtained result is further employed to investigate the specific energy (E) and specific power (P) of the alkaline aqueous battery according to the following two formulas [10]: E = 0.5C(∆V) 2 /3.6 and P = E/t, where the C, ∆V and t are, respectively, the specific capacity, voltage difference and discharge time of the alkaline aqueous battery. As demonstrated in Ragone plot of Fig. 8c, it is found that the alkaline aqueous battery delivers a superior specific energy of 50.2 Wh kg −1 at a specific power of 485 W kg −1 and still maintains a decent specific energy of 16.1 Wh kg −1 even at an ultrahigh specific power of 9.7 kW kg −1 , implying the excellent power capability. Moreover, the highest specific energy of the assembled alkaline aqueous battery in our work is comparable to or even higher than that of alkaline aqueous devices reported in the literature, including  [69]. Additionally, the cycling property of the as-obtained device demonstrated in Fig. 8d was estimated by the GCD measurements at 3 A g −1 over 5000 cycles, and 91.5% of capacity retention is maintained, suggesting the superior cycle stability. The superior electrochemical properties of the N,S-CQDs/ NiCo 2 S 4 //N-rGO/Fe 2 O 3 device are attributed to the following factors: (1) The utilization of CQDs as structure-directing agents induces the formation of unique N,S-codoped CQDs-decorated urchin-like nanorods-assembled NiCo 2 S 4 microspheres composite. The nanosized building blocks of the composite with a large surface area afford more efficient electroactive sites for charge storage, contributing to the enhanced specific capacity, and the micrometer-sized assemblies of the composite possess desirable and mechanical properties and superior structure stability, leading to the improved cycling life. Meanwhile, the utilization of CQDs as conductive agents, together with the N,S-codoped modification, greatly enhances the electronic conductivity of the composite, leading to the improvement of rate property; (2) the unique prism-like nanostructured Fe 2 O 3 hexahedrons can provide sufficient surface active sites for redox reaction, giving rise to the enhanced specific capacity, and the conductive N-rGO-decorated Fe 2 O 3 can significantly boost the electrical conductivity of the composite anode materials, facilitating the fast electron transport kinetic and resulting in the enhanced rate property. Moreover, the Fe 2 O 3 hexahedrons are tightly wrapped by the N-rGO layers, which effectively prevents the agglomeration and buffer against the volume change during the charging/discharging cycle process, substantially improving the cycling performance; (3) the ultrahigh-capacity N-rGO/Fe 2 O 3 composite anode materials are utilized to couple with the high-performance N,S-CQDs/NiCo 2 S 4 composite cathode materials to fabricate the alkaline aqueous battery, and the rational design and selection of cathode and anode materials render the resulting device achieve the enhanced specific energy, ultrahigh specific power and superior cycling life.

Conclusions
In a word, the graphitic CQDs are prepared via a scalable mixed-acid chemical oxidation method employing highly conductive and low-cost Super P powders as precursors, and the high yield of more than 50% is successfully acquired. Then, the resulting CQDs are further used as both structuredirecting and conductive agents to controllably synthesize the N,S-codoped CQDs-decorated urchin-like NiCo 2 S 4 composite microsphere cathode materials for the first time, which display evidently improved electrochemical properties including specific capacity, rate performance and cycle life benefited from the synergistic effect of highly conductive N, S-codoped CQDs and unique micro-/nanostructured NiCo 2 S 4 with Faradaic redox feature. Meanwhile, the N-rGO-wrapped prism-like Fe 2 O 3 hexahedrons composite anode materials are prepared, which exhibit an ultrahigh specific capacity, substantially enhanced rate performance and cycle property due to the synergistic effect between highly conductive N-rGO and high-capacity nanostructured Fe 2 O 3 hexahedrons. In view of their superior electrochemical properties and rational coupling of cathode and anode materials, a novel alkaline aqueous battery is assembled by employing N,S-CQDs/NiCo 2 S 4 as cathode and N-rGO/ Fe 2 O 3 as anode in 2 M KOH aqueous electrolyte. The fabricated N,S-CQDs/NiCo 2 S 4 //N-rGO/Fe 2 O 3 device delivers a high specific energy of 50.2 Wh kg −1 at a specific power of 485 W kg −1 or an ultrahigh specific power of 9.7 kW kg −1 at a decent specific energy of 16.1 Wh kg −1 and good cycling life with 91.5% of capacity retention at 3 A g −1 over 5,000 cycles. The present research gives a valuable reference for the design and exploitation of superior energy storage devices using battery/capacitive composite materials simultaneously as both cathode and anode in aqueous electrolyte.