Nitrogen-doped carbon quantum dots (N-CQDs)/Co₃O₄ nanocomposite for high performance supercapacitor

2.1 Synthesis of nitrogen doped carbon quantum dost (N-CQDs)

To synthesize N-CQDs, melamine and sucrose were used as the source of Nitrogen and carbon respectively, 14 wt% (0.1130 g) of melamine (C₃H₆N₆) was added in 60 mL of DI H₂O and mixed through continuous stirring for 70 min. Subsequently, 1.5 g of sucrose was mixed to the previous transparent mixed solution at 55 °C with continuous stirring for 50 min. Later, this was poured to the Teflon autoclave reactor and heated at 170 °C for 6 hrs. Produced material was washed twice with help of ethanol/water and dried in vacuum at 65 °C for 5 hrs. Obtained powder referred to as N-CQDs.

2.2 Fabrication of N-CQDs/Co₃O₄ nanocomposite

In 100 mL of distilled water, 6 wt% of N-CQDs and 200 mg of Co₃O₄ nanoparticles were dispersed with the help of stirring for 45  min and ultrasonic treatment of 10 min. The obtained sample was centrifuged and dried at 50 °C for 15 hrs and further heated at 300 °C in Argon gas flow. A dark reddish black powder was obtained denoted as N-CQDs/Co₃O₄ nanocomposite. To characterize the N-CQDs/Co₃O₄ nanocomposite main analytical methods were used and elaborated in electronic supplementary information s-1. Additionally, electrochemical performance was checked on CHI-electrochemical work station by conducting cyclic volumetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS) tests in 6 M KOH with 3-electrode systems. The fabrication of working electrode detailed in S-2.

3.1 Physicochemical Properties:

XRD analysis established the purity and phase of N-CQDs/Co₃O₄ nanocomposite as shown in Fig. 1a. The broad diffraction peaks at 2θ of 33.23°, 37.13°, 38.43°, 45.17°, 56.21°, 59.78°, 66.36° and 78.37° ascribed to 220, 311, 222, 400, 422, 511, 440 and 533 planes of cubic Co₃O₄ structure with JCPDS card no: 073-1701. The characteristic peak of carbon was indexed at 2θ of 19.38°, which certifies the presence of CQDs in the materials. The Raman spectra (Fig. 1b) showed two significant bands at 1352 and 1590 cm⁻¹ belong to the D and G bands. The representative bands of Co₃O₄ at 475, 520 and 690 cm⁻¹ ascribed to E_g, F²_2g and A_1g modes, which confirms the formation of N-CQDs/Co₃O₄ nanocomposite. To assess the chemical bonds present in the materials, FTIR analysis was conducted, which distinctly verifies the existence of Cobalt-Nitrogen band at 578 cm⁻¹ (Yuan et al., 2018). Moreover, the bands marked at 671, 1050, 1556, 2285 and 3366 cm⁻¹ (Fig. 1c) ascribed to νC-N, νC-O-C, νCOO⁻_as and broad band at 3366 cm⁻¹ belongs to stretching vibrations of –OH bond due to the atmospheric moisture (Yang et al., 2020).

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Fig. 1

(a) PXRD diffraction (b) Raman spectra (c) FTIR and (d) N₂ isotherm of N-CQDs/Co₃O₄ nanocomposite.

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N₂ A-D isotherm (Fig. 1d) was conducted at 77 K in liquid Nitrogen. Moreover, the nitrogen adsorption desorption showed type IV isotherm with H2 hysteresis. Which shows the mono-multilayer adsorption. Excellent surface area (BET) of 880 m² g⁻¹ with pore size and volume of 21 nm, 0.81945 cm³ g⁻¹ was analyzed in P/P₀ 0.05–0.35 through multipoint BET equation (Al-Hartomy et al., 2013). The mesoporous nature of N-CQDs/Co₃O₄ nanocomposite was established by BJH pore size studies, which demonstrate a pore size of ∼21 nm.

3.2 Structural elucidation

FE-SEM studies show the aggregated spherical particles with an average particle size ∼28 nm as depicted in Fig. 2a.

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Fig. 2

(a) SEM micrograph (b) TEM micrograph (c) HR-TEM micrograph and (d) SAED patterns of N-CQDs/Co₃O₄ nanocomposite.

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Transmission electron microscopic studies (Fig. 2b) also validate the agglomeration of Co₃O₄ nanoparticles anchored with carbon quantum dots. The average particle size of N-CQDs/Co₃O₄ nanocomposite estimated to ∼30 nm. HR-TEM analysis (Fig. 2c) displays the clear lattice fringes (Fig. 2c) with d-spacing of 0.280 nm, matching to 220 crystal planes of Co₃O₄ (cubic). Furthermore, Fig. 2d demonstrate the SAED patterns of CQDs/Co₃O₄ nanocomposite, indexed with 220, 311, 222, 400, 422, 511 and 440 planes from inward to outward direction which corroborated well with the XRD studies and confirmed the formation of N-CQDs/Co₃O₄ nanocomposite.

3.3 Electron spectroscopy for chemical analysis (ESCA or XPS)

The chemical composition/chemical state on the surface was estimated through XPS as shown in Fig. 3a–d. Fig. 3a demonstrate the survey spectrum of N-CQDs/Co₃O₄ nanocomposite consists of Carbon (C), Nitrogen (N), Oxygen (O) and Cobalt (Co) elements. Fig. 3b exhibits the HR- spectrum of Co2P shows 2 prime spin orbit peaks of 2P_3/2, 2P_1/2 at 781.18 eV, 597 eV and two satellite peaks. Moreover, the deconvolution of high-resolution spectrum of carbon peak (Fig. 3c) exhibits four peaks with the binding energy value 284.21, 285.28, 285.78 and ∼288.07 eV, resembles to the C—C/C⚌C, C—N/C⚌N, C—O and C⚌O, respectively (Dou et al., 2016). HR-XPS of N exhibited in Fig. 3d as N1s, which is deconvoluted into 3 peaks with the binding energy of 398.24 eV, 400.14 eV and 401.46 eV corresponds to pyridinic-N, graphitic-N and pyrrolic-N (Alhokbany et al., 2020). The O 1s spectrum in S-3 exhibits the displays at 528.19, 531.15, and 535.18 eV, which belongs to Co—O, C—O, and O—H respectively. Such confirmation further endorses the formation of N-CQDs/Co₃O₄ nanocomposite

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Fig. 3

XPS analysis of N-CQDs/Co3O4 nanocomposite. (a) survey spectrum (b) high resolution spectrum of Co 2P (c) high resolution spectrum of C1s and (d) high resolution spectrum of N1s.

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3.4 Electrochemical performance of N-CQDs/Co₃O₄ nanocomposite

The electrochemical behaviour of N-CQDs/Co₃O₄ nanocomposite were examined in a 3-electrode setup with 6 M KOH electrolyte through CV, GCD and EIS examination. CV experiments were conducted in a potential window of −0.4 to +0.6 V with scan rates starting from 5 mV s⁻¹ to 100 mV s⁻¹ as depicted in Fig. 4a. Apparent reduction–oxidation curves can be attributed to the reversible redox actions amid the changed valence states of cobalt. The electrochemical reaction mechanism is elaborated in S-4. The redox peaks of CV in Fig. 4a clearly indicate the charge storage mechanism is pseudocapacitive owing to Faradaic processes. An exceptional specific capacitance of 1782 F g⁻¹ at 05 mV s⁻¹ scan rate was recorded in 6 M KOH electrolyte. Moreover, galvanostatic charge–discharge (GCD) analysis revealed a better capacitive performance with a Cs of 1867 F g⁻¹ at a current density of 1 A g⁻¹. The energy density was calculated 36.9 Wh kg⁻¹ at the power density of 480 W kg⁻¹_. The capacitance retention graph exhibited 96% stability by examining upto 500 GCD cycles at a current density of 1 A g⁻¹. It is widely described that the carbon content can enhance the electrical conductivity and graphite N play an extremely vibrant part in supporting electron passage so that promotes the transportation of ions from electrolyte solution to interface (Karuppasamy et al., 2020).

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Fig. 4

Electrochemical analysis of N-CQDs/Co3O4 nanocomposite (a) cyclic voltammograms (b) Cs by CV (c) galvanostatic charge discharge curves (d) Cs by GCD (e) capacitance retention graph (f) Nyquist plot.

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To further explain the capacitance properties, the impedance studies of N-CQDs/Co₃O₄ nanocomposite in 100 kHz to 0.01 Hz was carried out at 5 mV. In Nyquist plot the high-frequency region a small semicircle supports the lower inner resistance and faster charge transfer at the electrode–electrolyte interface as shown Fig. 4f.