Binder-Free and Flexible Carbon-Encapsulated Oxygen-Vacancy Cerium Dioxide Electrode for High-Performance Supercapacitor

Materials

Preparation of CeO₂ and CeO₂@C

Characterization

The morphology of the as-prepared samples was analyzed by scanning electron microscopy (SEM, JSM-7500F, JEOL, Japan) and high resolution transmission electron microscopy (HRTEM, Tecnai-G20, FEI, America). X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Fisher Scientific, America) was conducted to investigate valence states of elements. X-ray diffraction (XRD, Bruker D8, Bruker, Germany) with Cu Kα (λ = 0.154056 nm) was carried out to check crystal structure. The electrochemical performance of electrodes was tested at room temperature using CHI660E electrochemical analyzer (Shanghai, China), containing cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), cycle stability and electrochemical impedance spectroscopy (EIS). The platinum electrode was used as the counter electrode and saturated calomel electrode (SCE) was used as reference electrode. The Na₂SO₄ solution (1 M) and H₂SO₄ solution (1 M) were used as the electrolyte in 3-electrode and 2-electrode system, respectively. The specific capacitances (C_S) of the electrode are calculated according to the CV and GCD curves by following formula:

$$\begin{eqnarray}&&{C}_{{\rm{S}}}=\displaystyle \frac{\displaystyle {\int }_{{E}_{1}}^{{E}_{2}}{\rm{i}}\left({\rm{E}}\right)d({\rm{E}})}{{\rm{2mv}}\left({{\rm{E}}}_{2}-{{\rm{E}}}_{1}\right)}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{C}_{{\rm{S}}}=\displaystyle \frac{{\rm{I}}{\rm{\Delta }}{\rm{t}}}{{\rm{\Delta }}{\rm{Vm}}}\end{eqnarray} \tag{ 2 }$$

Where "E₁ " and "E₂ " are the starting and ending potentials in CV, respectively. "i(E)" represents the instantaneous current, and "i(E)d(E)" represents the area enclosed by the CV curve gained by integration of the positive and negative sweeps in the CV, "v" represents scan rate (V s⁻¹), and "m" represents mass (g) of the electrode. "I" represents charge and discharge current (A), and "Δt" represents the discharge time (s), "ΔV" represents the voltage window (V) during discharge. ³² Energy density (E, Wh kg⁻¹) and power density (P, W kg⁻¹) are calculated by following equations: ³³

$$\begin{eqnarray}&&{C}_{{\rm{s}}}=\displaystyle \frac{{\rm{4I}}{\rm{\Delta }}{\rm{t}}}{{\rm{\Delta }}{\rm{Vm}}}\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&E=\displaystyle \frac{1}{8}\times \displaystyle \frac{1}{{\rm{3.6}}}\times {C}_{{\rm{s}}}\times {\rm{\Delta }}{{\rm{V}}}^{2}\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&P=\displaystyle \frac{3600\times {\rm{E}}}{{\rm{t}}}\end{eqnarray} \tag{ 5 }$$

Where C_s represents the specific capacitance of single electrode for device and "m" represents total mass (g) of the electrodes of the ASC device.

The preparation of CeO₂@C is briefly described in Fig. 1, and the prepared CeO₂@C (about 2 mg cm⁻¹) can directly be anchored on the CC substrate, forming a flexible and robust electrode. The CeO₂@C/CC electrode can be bend to any angle. From the TEM and HRTEM images, as-prepared CeO₂ nanoparticles have uniform size of about 10 nm diameter (Fig. 2a), and there are no other substances on the surface of CeO₂ nanoparticles (Fig. 2b). The CeO₂@PVP composites are easily formed when the CeO₂ nanoparticles and PVP are mixed, and CeO₂@PVP composites can be well coated on the CC substrate. After calcining 700 °C at N₂ atmosphere, the color of CeO₂@PVP turned from pale yellow to black. In Fig. 2c, the CeO₂@C composites are uniformly arranged on the CC substrate. Fig. 2d shows the core–shell structure of CeO₂@C with core diameter of about 10 nm and the carbon layer shell diameter of about 1.5 nm. It can be calculated that the lattice space is 0.308 nm corresponding to the (111) plane. The selected area electron diffraction (SAED) pattern of CeO₂@C exhibits several distinct diffraction rings, from inside to outside assigned to the (111), (200), (220) and (311) planes of CeO₂ (Fig. 2e). At the same time, the well-resolved intense rings indicate the polycrystalline nature. In the XRD pattern (Fig. 2f), the CeO₂@C composites show characteristic diffraction peaks at 28.52°, 33.13°, 47.54°, 56.38°, 59.21°, 69.48°, 76.83°, 79.15° and 88.52° with corresponding phasic indexes of (111), (200), (220), (311), (222), (400), (311), (420) and (422) (JCPDS PDF#75-0120).

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As shown in Fig. 3, the chemical composition of as-synthesized CeO₂@C was investigated by XPS. In Fig. 3a, the XPS survey spectrum shows that CeO₂@C is composed of Ce, C and O. In Fig. 3b, the Ce 3d XPS spectrum consists of eight sub-peaks, two of which are located at 903.6 eV and 907.2 eV, representing the 3d¹⁰4f¹ electronic configuration related to Ce³⁺ oxidation state. ^34,35 The other six sub-peaks at 881.7 eV, 882.8 eV, 888.5 eV, 898.2 eV, 900.8 eV and 916.5 eV correspond to the 3d¹⁰4f⁰ electron configuration of the Ce⁴⁺ oxidation state. ^36,37 The results indicate that there are both Ce³⁺ and Ce⁴⁺ valence states in the sample. In Fig. 3c, the C 1s consists of three sub-peaks: C–C (284.5 eV), C–O (285.25 eV) and C=O (286.34 eV), which are attributed to carbon derived from PVP. As depicted in Fig. 3d, the O 1s spectrum is mainly divided into three sub-peaks, which are chemisorbed water (O_W) located at 532.4 eV, oxygen atoms in the vicinity of an oxygen vacancy (O_V) located at 531.2 eV and lattice oxygen (O_L) located at 529.6 eV, ³⁸ respectively. By using the ratio of the sum of the integrated peak intensity regions related to Ce³⁺ and Ce⁴⁺, the relative content of Ce³⁺ and Ce⁴⁺ of sample can be calculated. ³⁹ Ce³⁺ triggers the creation of oxygen vacancy defects in CeO₂ lattice. So the larger ratio of Ce³⁺/Ce⁴⁺, the higher percentage of oxygen vacancies. The relative concentration of Ce³⁺ valence state of CeO₂@C is calculated to be 14.86% according to the peak area percentages of different element components. The more Ce³⁺ ions, the more oxygen vacancies are formed on the CeO₂ surface. ^40–42 More importantly, oxygen vacancies are proven to accelerate the ion diffusion and charge mobility because the oxygen vacancies migration in CeO₂ occur through a jumping mechanism, ^43,44 Similar to the calculation method of cerium valence state, the O_V peak area percentage is calculated to be 11.23%. The presence of oxygen vacancies can enhance electronic conductivity and promote the kinetics of surface reactions, ²² thereby enhancing electrochemical reactions. ⁴⁵ In addition, oxygen vacancies can also be used as active sites for redox reactions to increase the capacitance. ^46–49

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Fig. 4 shows the electrochemical performance of CeO₂@C. For comparison, CV of the pure CeO₂ treated under 700 °C for 30 min is tested. The area of the CV curve is in proportion to capacitance. In Fig. 4a, the CV curve integrated area of the CeO₂@C electrode is far larger than that of pure CeO₂, which indicates that the CeO₂@C composites exhibit much higher capacitance than pure CeO₂. According to the results, it can be judged that the utilization rate of pure CeO₂ nanoparticles is poor, which may be due to their low specific surface area and poor conductivity. However, the CeO₂@C nanocomposites have a much higher conductivity due to the existence of carbon layer, which is conducive to the rapid transmission of electrons, thus greatly improving the utilization of the active materials. More importantly, the CeO₂@C shows strong electrochemical activity in a very wide potential range of −1 ∼ 0.8 V. The CV curves of CeO₂@C are also investigated at the different scan rates (Fig. 4b). In the CV curves, the broad redox peaks at about 0.23/0.35 V and −0.71/−0.63 V are attributed to the conversion of Ce³⁺/Ce⁴⁺ related to the redox reaction of CeO₂, ^50–52 as shown in the Eqs. 6, 7.

$$\begin{eqnarray}&&{{\rm{CeO}}}_{2}+{{\rm{Na}}}^{+}+{{\rm{e}}}^{-}\leftrightharpoons {\rm{CeOONa}}\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&2{\rm{CeOONa}}+{{\rm{H}}}_{2}{\rm{O}}\to 2{\rm{NaOH}}+{{\rm{Ce}}}_{2}{{\rm{O}}}_{3}\end{eqnarray} \tag{ 7 }$$

It can be observed that the CV curves of CeO₂@C have no obvious distortion from 5 mV s⁻¹ to 100 mV s⁻¹, implying excellent stability and capacitance performance. Based on Eq. 1, the CeO₂@C exhibits 72.67 F g⁻¹ at scan rate of 100 mV s⁻¹, only 26.80% decrease when the scan rate increased up to 100 mV s⁻¹ (Fig. 4c), indicating outstanding rate capability. The nonlinear GCD curves of CeO₂@C also prove the occurrence of redox reaction (Fig. 4d). Based on Eq. 2, the specific capacitance of CeO₂@C reaches 141.56 Fg⁻¹ at 1 Ag⁻¹ (Fig. 4e). As the current density increased, the specific capacitance gradually decreased. This is because the electrolyte ions cannot diffuse into the bulk phase of active material in time at high current density. Fig. 4f displays that the CeO₂@C electrode has a good cycling stability after 3000 cycles.

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Impedance spectrum curve demonstrates the response of the electrode and electrolyte to frequency, which is divided into a semicircle in high frequency region and an inclined line in low frequency region. The semicircle diameter corresponds to charge transfer resistance (R_ct) between electrode and electrolyte, and inclined line reflects the ion diffusion of the electrode. And the intercept of curve on real axis corresponds to equivalent series resistance (ESR, R_s). ⁵³ In Fig. 5, the CeO₂@C electrode exhibits big slope in low frequency region, very small semicircle (R_ct, 0.5 Ω) and low ESR (R_s, 2.7 Ω) in high frequency region.

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The CV curves in Fig. 6a display that the CeO₂@C and poly(3,4-ethylene dioxythiophene) (PEDOT) electrodes have stable electrochemical activity in negative polarization and positive polarization in three electrode system, respectively. It is expected that an asymmetric supercapacitor (ASC) can be assembled with CeO₂@C as negative electrode and PEDOT as positive electrode. Fig. 6b displays the CV curves of the ASC at different scan rates and demonstrates that the operational voltage window reaches 2.0 V. In Fig. 6c, based on Eq. 3, the specific capacitance of ASC is calculated to be about 80.0 F g⁻¹ at 2 Ag⁻¹, and at 5 Ag⁻¹, the specific capacitance is about 57.6 F g⁻¹. Fig. 6f represents Ragone plot, and the ASC exhibits a maximal energy density of 11.12 Wh kg⁻¹ and a power density of 2000 W kg⁻¹, which is higher than those values recently reported for CeO₂ and its carbon-based composites supercapacitor (Table I). The EIS of the ASC is further tested. In Fig. 7a, it can be seen that the R_s is about 2.8 Ω and R_ct value is about 27.2 Ω. The cycle stability of ASC has also been tested (Fig. 7b). There is still a capacitance retention of 71.43% after 5000 cycles, presenting good cycling stability.

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In summary, for the first time, we synthesized and fabricated a binder-free and flexible CeO₂@C composite electrode on CC via a hydrothermal method combined with a subsequent annealing process. As-prepared CeO₂@C exhibits CeO₂ core diameter of 10 nm and carbon shell thickness of 1.5 nm. The CeO₂@C has an electrochemical active window of 1.8 V and delivers a specific capacitance of 141.56 F g⁻¹ at 1 A g⁻¹. And the ASC is fabricated by using CeO₂@C as negative electrode and PEDOT as positive electrode, which can be operated in a 2.0 V voltage window. The energy density of ASC reaches 11.12 Wh kg⁻¹ at power density of 2000 W kg⁻¹. Hence, these results indicate that CeO₂@C is a promising electrode material for electrochemical energy storage application.

This work was supported by the National Natural Science Foundation of China (grant nos. 51862011, 51662012, 51863009), Jiangxi Outstanding Young Talent Fund Projects (20171BCB23076), the Natural Science Foundation of Jiangxi Province (20171BAB206013), Jiangxi Provincial Department of Education (GJJ201108) and Scientific Research Projects (YC2019-X14) of Jiangxi Science and Technology Normal University.