Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Rational Design of Co(II) Dominant and Oxygen Vacancy Defective CuCo2O4@CQDs Hollow Spheres for Enhanced Overall Water Splitting and Supercapacitor Performance Guijuan Wei,†,§ Jia He,‡ Weiqing Zhang,‡ Xixia Zhao,§ Shujun Qiu,† and Changhua An*,†,‡,§ †
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering and Tianjin Key Laboratory of Advanced Functional Porous Materials, Institute of New Energy Materials & Low-Carbon Technologies, Tianjin University of Technology, Tianjin 300384, PR China § College of Science, China University of Petroleum, Qingdao 266580, PR China ‡
S Supporting Information *
ABSTRACT: The hierarchical CuCo2O4@carbon quantum dots (CQDs) hollow microspheres constructed by 1D porous nanowires have been successfully prepared through a simple CQDs-induced hydrothermal self-assembly technique. XPS analysis shows the CuCo2O4@CQDs possesses the Co(II)-rich surface associated with the oxygen vacancies, which can effectively boost the Faradaic reactions and oxygen evolution reaction (OER) activity. For example, the as-synthesized 3D porous CuCo2O4@CQDs electrode exhibits high activity toward overall electrochemical water splitting, for example, an overpotential of 290 mV for OER and 331 mV for hydrogen evolution reaction (HER) in alkaline media have been achieved at 10 mA cm−2, respectively. Furthermore, an asymmetric supercapacitor (ASC) (CuCo2O4@CQDs// CNTs) delivers a high energy density of 45.9 Wh kg−1 at 763.4 W kg−1, as well as good cycling ability. The synergy of Co(II)-rich surface, oxygen vacancies, and well-defined 3D hollow structures facilitates the subsequent surface electrochemical reactions. This work presents a facile method to fabricate energetic nanocomposites with highly reactive, durable, and universal functionalities.
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endow them with promise in the field of supercapacitors and water splitting. Meanwhile, increasing the number of active sites is another important factor to boost the electrochemical performance. Structural defects often occur in the surface of the nanomaterials, the introduction of which has considered as a valid method to increase the active sites.17,18 It is noted that defects on the metal oxides have a significant effect on electrocatalytic process. The chemical bond between metal and adsorbed O2 on oxygen defects sites will lead to the elongation of O−O bond, which promotes the activation and partial decomposition of O2.19,20 Recent theoretical studies also demonstrate that introducing surface defects opens up a new avenue to enhance the specific capacitance, which could boost active sites, reversibility and charge transfer.21 Thus, it is desirable to rationally synthesize defects-enriched electrode materials with 3D porous hollow spheres, concurrently maximizing the electrochemically active surface area and offering abundant active sites. Mixed-valent transition metal oxides are a class of electrochemical active materials due to their numerous valence states compared to single metal oxides. Their electrochemical
INTRODUCTION With the increasing global crisis of environmental pollution and energetic problem, the development of alternative clean fuel and energy storage technologies has attracted considerable attention.1−4 The electrochemical reactions in these systems including supercapacitors and electrocatalytic water splitting are top concerns to be tackled, which usually suffer a multi-ion and multielectron transfer process, resulting in the low efficiency of the corresponding devices.5−9 To solve the bottleneck issues, the key is to design and develop novel electrode materials exposing fruitful surface active sites. Hierarchical materials with mesoporous properties and hollow structures are widely investigated due to their unique structural merits of large surface areas and short ion diffusion length for improving the mass and charge transport, as well as releasing the strain during the electrochemical reactions.10−12 Over recent decades, various methods including hard−soft templates and freezedrying have proved its efficacy in synthesizing well-defined porous materials.13−16 However, these methods always involve complicated steps, costly equipment, and toxic reagents, resulting in low reproducibility, high cost, and poor uniformity in porous features. In particular, when the building blocks of the hierarchical 3D structures are porous 1D nanostructures, the contact between electrolyte and electrocatalysts will be largely increased in electrochemical reactions. These features © XXXX American Chemical Society
Received: April 14, 2018
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DOI: 10.1021/acs.inorgchem.8b01020 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry performances could be modulated by changing the metallic elements’ states.22−24 For example, spinel copper cobaltite (CuCo2O4) as a low cost and nontoxic binary metal oxide exhibits high electrochemical activity and remarkable rate capability arising from the synergistic effects of Cu and Co ions.25 Various shaped CuCo2O4 such as nanowires, nanoparticles, and nanosheets have been obtained by corresponding synthetic methods.26−28 However, the low conductivity, activity, and stability restrict its electrochemical applications. Carbon quantum dots (CQDs) with fruitful functional surface groups such as hydroxyl and carboxyl could supply many favorable sites for the synthesis of electroactive catalysts.29−32 Furthermore, the negative charged surface of the CQDs offers an easy-make electrostatic interaction with positively charged transition metal cations, which has the capacity to drive and control over the crystal growth of shaped nanoparticles. Therefore, one-pot coupling CuCo2O4 and CQDs with welldefined micro-/nanostructures is expected to achieve efficient composite electrocatalysts with desirable performances because of their potential synergistic effects and good electronic conductivity. In this work, a CQDs-induced hydrothermal self-assembly strategy has been used to presynthesize CuCo-precursor with hierarchical structures. After being annealed in Ar, the CuCoprecursor was converted to hierarchical CuCo2O4@CQDs hollow microspheres. Benefiting from these structural characteristics, such as defects (CoII and the oxygen vacancy) enriched surface and good electronic conductivity, the asobtained CuCo2O4@CQDs electrode shows high overall electrochemical water splitting performance, such as good oxygen evolution reaction (OER) activity and hydrogen evolution reaction (HER) activity. In addition, an asymmetric supercapacitor (ASC) (CuCo2O4@CQDs//CNTs) displays an energy density of 45.9 Wh kg−1 at 763.4 W kg−1, as well as a long cycling lifespan.
Figure 1. (a) Schematic illustration of the synthesis of the hierarchical CuCo2O4@CQDs hollow microspheres, (b) XRD pattern of the CuCo2O4@CQDs, (c−e) TEM image and the HRTEM images of CuCo2O4@CQDs. The inset in (e) is the SAED pattern of the CuCo2O4@CQDs.
spheres (inset in the Figure 1e) shows they are polycrystalline and can be indexed to the cubic spinel CuCo2O4 and CQDs. In order to study the growth mechanism for the different morphologies, the effect of CQDs quantity on the 3D hierarchical CuCo2O4@CQDs was performed by field-emission scanning electronic microscopy (FE-SEM). As shown in Figure 2a, when CQDs are absent, the obtained CuCo2O4 displays irregular polyhedrons with a wide size distribution. When 5 and 10 mL of CQDs solution were added, irregular microflakes with different sizes and some echinus structures emerged (Figure S1c, d). Further increasing the amount of CQDs to 15 mL results in the formation of hierarchical echinus-like CuCo2O4@ CQDs with a diameter ∼2 μm, where numerous nanowires densely cover the surface of microspheres in an irradiative pattern (Figures 2b and S1e). The inset image in Figure 2b gives a broken sphere, which illustrates their interior hollow voids. SEM at high magnification (Figure 2c) reveals that these nanowires are porous and well-dispersed. The open space between neighboring porous nanowires with rough surfaces facilitates ion access to the electrode/electrolyte interface and promotes faradic reaction efficiency. In this case, a large number of electroactive sites for the redox reaction can be resulted. Nevertheless, excessive use of CQDs (30 mL) leads to the aggregation of hierarchical structure (Figure S 1f), implying that CQDs could regulate the morphology of the materials. The growth mechanism of the CuCo2O4@CQDs may be described as follows: The electrostatic interaction between Cu2+, Co2+ and negatively charged CQDs may drive the growth of the crystal faces and control over the shape of nanocrystals. Then, the induced crystal nuclei grows into low-dimensional nanoblocks, which then coalesces with adjacent ones and selfassembled into the 3D hollow structure because of the decrease of their surface energy. The detailed elemental composition of
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RESULTS AND DISCUSSION The preparation process of the hierarchical CuCo2O4@CQDs hollow microspheres is schematically shown in Figure 1a. The CuCo-precursor was first synthesized by a hydrothermal process of an aqueous mixture of Co2+, Cu2+, urea and CQDs (Figures 1a and S1a, eqs S1−S4). After thermal treatment in Ar at 400 °C, the porous product was obtained. Figure 1b displays the X-ray powder diffractometer (XRD) pattern of the CuCo2O4@CQDs. The peaks are consistent with the cubic spinel phase of CuCo2O4 (Powder Diffraction File No. 001−1155, Joint Committee on Powder Diffraction Standards, [1938]). There are no planes of graphite due to the low quantity and crystallinity of CQDs. Transmission electron microscopy (TEM) image (Figure 1c) reveals that these microspheres are composed of ultrafine porous nanowires, where a large void space could be clearly discerned. The high-resolution TEM (HRTEM) image clearly shows these nanowires are porous in nature, consisting of numerous interconnected nanoparticles (Figure 1d). A lattice fringe of 0.46 nm is consistent with (111) plane of the cubic CuCo2O4 (Figure 1e). The TEM image in Figure S1b shows the diameter of CQDs is 3 nm. The lattice spacing of 0.21 nm corresponds to carbon (Figures 1e and S1b), indicating that CQDs have been successfully incorporated into the CuCo2O4@CQDs composite. The selected area electronic diffraction (SAED) pattern of the hierarchical CuCo2O4@CQDs hollow microB
DOI: 10.1021/acs.inorgchem.8b01020 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 2. SEM images of (a) bare CuCo2O4 and (b, c) CuCo2O4@CQDs, (d−h) elemental mapping of Cu, Co, O, and C, (i) EDS results of the CuCo2O4@CQDs.
Figure 3. High-resolution XPS of the CuCo2O4 and CuCo2O4@CQDs: (a) Cu 2p, (b) Co 2p, (c) O 1s, and (d) C 1s for the CuCo2O4@CQDs.
spectra as shown in Figure 3b display two main peaks at 780 and 795 eV along with a spin-energy at around 15 eV, suggesting the coexistence of CoIII and CoII states.33 Accordingly, the CoII/CoIII surface ratio of CuCo2O4@CQDs is determined to be 1.16, which is higher than that of pristine CuCo2O4 (0.82). As shown in XPS spectra of O 1s (Figure 3c), two types of oxygen exist: O1 is attributed to oxygen bound to metals, while O2 is to a numerous defects.33−36 More importantly, the relative atomic ratio of O 2 /O 1 for CuCo2O4@CQDs is higher than that for the CuCo2O4, indicating that the created surface O-vacancies are balanced by the conversion of CoIII to CoII. The high-resolution C 1s spectrum (Figure 3d) shows a great number of functional groups were on the surface of the CQDs, which may bind to
the as-obtained sample was analyzed by EDS, clearly showing Co, Cu, O, and C are homogeneously dispersed in the hybirds (Figure 2d−h). The content of the CQDs is 8.9 wt % (Figure 2i). The CQDs content of the CuCo2O4@CQDs is further determined by thermal gravimetric analysis (TGA) (Figure S2). The mass loss below 150 °C is ascribed to the adsorbed water and gas, the major weight reduction of 10.3% between 400 and 600 °C is assigned to the combustion of carbon species, which is consistent with the EDS data. Figure 3a gives the Cu 2p X-ray photoelectron spectroscopy (XPS) spectra of the CuCo2O4 and CuCo2O4@CQDs. The two peaks at 934.1 and 953.8 eV are for the Cu 2p3/2 and Cu 2p1/2, respectively. Furthermore, the satellite peaks at 941.6 and 961.8 eV confirm the characteristic of Cu2+.32 The Co 2p C
DOI: 10.1021/acs.inorgchem.8b01020 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 4. (a) Polarization curves for the OER with CuCo2O4@CQDs, CuCo2O4 and CQDs catalysts at 10 mV s−1. (b) Tafel plots (Potential vs log (current density)) of CuCo2O4@CQDs, CuCo2O4 and CQDs catalysts. (c) CV curves of CuCo2O4@CQDs. (d) Current densities vs scan rates for CuCo2O4@CQDs and CuCo2O4 catalysts. (e) Nyquist plots of CuCo2O4@CQDs and CuCo2O4 catalysts. (f) Polarization curves of CuCo2O4@ CQDs before and after 1000 cycles at 10 mV·s−1.
The electrocatalytic OER and HER comparisons of the CuCo2O4@CQDs, CuCo2O4 and CQDs were comparatively investigated. The OER was measured by linear sweep voltammetry (LSV) measurments at 10 mV s−1 to minimize the capacitive current. The CQDs, CuCo2O4 and CuCo2O4@ CQDs electrocatalysts exhibit onset potentials of 1.79, 1.56, and 1.46 V vs RHE at 1 mA cm−2, respectively (Figure 4a). The lowest onset potential of CuCo2O4@CQDs indicates its best OER activity. In addition, the CuCo2O4@CQDs displays the overpotential of 290 mV at 10 mA cm−2, much lower than CuCo2O4 (420 mV) and CQDs (660 mV), respectively. To study the effect of CQDs content on the OER performance of the CuCo 2 O 4 @CQDs samples, LSV comparisons of CuCo 2 O 4 @CQDs with different CQDs loadings were performed at 10 mV s−1. It is shown that the introduction of CQDs could boost the electrochemical performance effectively. With the increase of CQDs solution to 10 mL, the CuCo2O4@ CQDs-10 shows the overpotential of 310 mV, which is much lower than CuCo2O4-5 (350 mV) at 10 mA cm−2. However, excessive use of CQDs, i.e., 30 mL used (Figure S5a), the overpotential of 380 mV is obtained due to serious aggregation of the product. As a result, a moderate ratio of CQDs to CuCo2O4 is important to realize the outstanding electrochemical performance. In addition, the Tafel slope was further derived to evaluate the predominant OER process. The Tafel curves were plotted using the equation η= a + b log(j), where a and b represent intercept and Tafel slope, respectively. The
CuCo2O4 through covalent bonding or hydrogen bonding. The specific surface area of the CuCo2O4@CQDs measured by Brunauer−Emmett−Teller (BET) is 84.6 m2 g−1, much larger than bare CuCo2O4 (20.1 m2 g−1) (Figure S3a). The BJH pore size distribution of the 3D porous CuCo2O4@CQDs hollow microspheres shows a pore size distribution centered at 27.3 nm (Figure S3b). It has been found that the OER activity of Co-based spinel oxides relies on the relative ratio of CoII/CoIII, where CoII is beneficial for the formation of CoOOH, which is responsible for the enhanced OER activity.34 In contrast, CoIII tends to improve the strength between the catalysts’ surface and hydroxide groups, which decreases the OER activity.35 As shown in Figure S4, both the CuCo2O4 and CuCo2O4@CQDs were subjected to cyclic voltammetry (CV) measurements. There are two pairs of redox peaks at around 1.1 and 1.4 V versus reversible hydrogen electrode (RHE), corresponding to Co2+/Co3+ and Co3+/Co4+ redox processes, respectively. It is obvious that the peaks at 1.1 V of CuCo2O4@CQDs is larger than pristine CuCo2O4, which indicates the formation of more CoOOH. Meanwhile, density-functional theory calculations indicate that the oxygen defects generate new defect states, which are in the band gap of CuCo2O4, and the electrons on the defect states can be easily excited, thereby facilitating the increased conductivity.36 Thus, highly comparable OER activity can be expected on the as-obtained CuCo2O4@CQDs. D
DOI: 10.1021/acs.inorgchem.8b01020 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 5. (a) Polarization curves for the HER with CuCo2O4@CQDs, CuCo2O4, and CQDs catalysts at 10 mV s−1. (b) Onset potentials at 1 mA cm−2 over CuCo2O4@CQDs, CuCo2O4, and CQDs catalysts. (c) Tafel plots of CuCo2O4@CQDs, CuCo2O4, and CQDs catalysts. (d) Polarization curves of CuCo2O4@CQDs before and after 1000 cycles at 10 mV·s−1.
Figure 6. (a) LSV curves of the CuCo2O4@CQDs in a three-electrode configuration at 10 mV s−1, showing overall electrolysis of water over CuCo2O4@CQDs. (b) LSV curve of the CuCo2O4@CQDs in a two-electrode configuration at 10 mV s−1. (c) Chronopotentiometric curve of the CuCo2O4@CQDs under 1.95 V vs RHE; the inset is the SEM image of the CuCo2O4@CQDs before and after cycling.
Tafel plots of the CuCo2O4, CQDs, and CuCo2O4@CQDs catalysts were obtained by fitting the polarization curves to the Tafel equation (Figure 4b). The Tafel slope over CuCo2O4@ CQDs catalyst is determined to be 64.0 mV dec−1, suggesting that the OER on CuCo2O4@CQDs electrode follows the Volmer−Heyrovsky mechanism. Noting that Tafel slop of CuCo2O4@CQDs is smaller than that of the CuCo2O4 (90.3 mV dec−1) and CQDs (108.1 mV dec−1), further indicating the more favorable OER reaction kinetics over CuCo2O4@CQDs. In addition, the value is also better than mostly reported Cobased oxide electrocatalysts.37−40
CV measurements were used to confirm double-layer capacitance (Cdl) to sustain their excellent electrochemical performance, which depends linearly on the electrochemically active surface area (ECSA). CV curves were performed without a redox process from 1.1 to 1.2 V (vs RHE) ranging from 40 to 200 mV s−1 (Figure 4c). According to the CV curves and the plots of the resultant current densities versus scan rates, the Cdl of CuCo2O4@CQDs is determined to be 5.86 mF cm−2, which is 2-fold that of CuCo2O4 (2.86 mF cm−2) (Figure 4d). Since Cdl is related to the surface area, the effective surface area of CuCo2O4@CQDs is also 2-fold that of CuCo2O4, demonstratE
DOI: 10.1021/acs.inorgchem.8b01020 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 7. Supercapacitors performance of the CuCo2O4 and CuCo2O4@CQDs electrode: (a) CV curves of the CQDs, CuCo2O4 and CuCo2O4@ CQDs electrodes at 5 mV s−1. (b) GCD curves of the CQDs, CuCo2O4 and CuCo2O4@CQDs electrodes at 1 A g−1. (c) Specific capacitances of the CuCo2O4 and CuCo2O4@CQDs electrodes calculated from different current densities. (d) Nyquist plots of the CuCo2O4 and CuCo2O4@CQDs electrode. (e) Schematic illustration of ion diffusion on the CuCo2O4@CQDs surfaces.
The HER activities of the as-prepared samples were evaluated in N2-saturated 1.0 M KOH electrolyte. The polarization curves show that the onset potential of CuCo2O4@CQDs (222 mV) is much less than that of the CuCo2O4 (439 mV), and CQDs (650 mV) (Figure 5a,b). The Tafel slope of CuCo2O4@CQDs is 65 mV·dec−1 (Figure 5c), smaller than those of the other two samples (83 mV·dec−1 for CuCo2O4 and 133 mV·dec−1 for CQDs, respectively), showing favorable reaction kinetics of CuCo2O4@CQDs electrode. The stability of CuCo2O4@CQDs electrode was evaluated by CV scans between −0.5 and 0.1 V vs RHE at 100 mV s−1. After 1000 cycles, the polarization curve overlays almost exactly with the initial one (Figure 5d). Therefore, the as-obtained CuCo2O4@CQDs can be used as excellent bifunctional electrocatalyst toward efficient OER and HER performance. For practical applications, we further examined its overall water splitting capacity. Figure 6a is the LSV curves of the HER and OER in 1 M KOH over CuCo2O4@CQDs electrode. The onset potential toward HER is −222 mV vs RHE and OER is +1460 mV vs RHE. Here, the electrolysis reaction toward water splitting starts at 1.73 V (Figure 6b). In addition, the CuCo2O4@CQDs electrode exhibits good stability upon long-term testing at 1.95 V vs RHE. As shown in Figure 6c, after reacting for 46 h, SEM (inset) indicates that the catalyst
ing the enhanced ECSA over 3D hollow microspheres. The large ECSA is beneficial for the water molecules adsorption, togther with luxuriant active sites for the process of electrochemical reactions. As shown in Figure 4e, the CuCo2O4@CQDs also reveals a lower series resistance than that of the CuCo2O4, implying good electron transfer ability. The improvement in charge transport may be mainly ascribed to the close electrical contact between the CQDs and CuCo2O4.41 The durability of CuCo2O4@CQDs was tested by performing the continuous scanning in the potential between 1.2 and 1.6 V vs RHE at 100 mV s−1. The polarization curve nearly keeps unchanged after 1000 cycles at 10 mV·s−1, demonstrating its good stability (Figure 4f). Therefore, the improved electrocatalytic performance and stability reveal that the CuCo2O4@CQDs can be used as an efficient OER catalyst. The electrocatalytic performance of CuCo2O4@CQDs is cocontributed by the surface area and oxygen defects. The LSV curves of Figure 4a were normalized by the BET surface area to eliminate the effect of the surface areas. Figure S6 shows the specific activity of CuCo2O4@CQDs is 0.33 mA cm−2BET at 1.6 V, which is 10-fold higher than that of CuCo2O4 (0.031 mA cm−2BET), implying the excellent catalytic performance of the CuCo2O4@CQDs is mainly affected by the surface oxygen vacancies. F
DOI: 10.1021/acs.inorgchem.8b01020 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 8. (a) CV curves and (b) GCD curves of the assembled CuCo2O4@CQDs//CNTs ASC, the inset is the zoom-in graph at high current densities. (c) Specific capacitance and the Coulombic efficiency of the CuCo2O4@CQDs//CNTs ASC. (d) Ragone plots of the CuCo2O4@ CQDs//CNTs ASC, the inset is an schematic cartoon for the as-fabricated ASC device. (e) Long-term cycle performance of the ASC device over 4000 cycles at 20 A g−1.
behavior arising from Faradaic reactions of the Co4+/Co3+ and Cu2+/Cu+.42 Figure 7c illustrates the specific capacitance values of the single CuCo2O4 and CuCo2O4@CQDs electrodes. It is obvious that the CuCo2O4@CQDs electrode shows higher specific capacitance at the same current density. The specific capacitance of the CuCo2O4@CQDs electrode is decreased from 1528.8 to 1252.7 F g−1 as the increase of the current densities from 1 to 20 A g−1, which is much better than that of pristine CuCo2O4 (from 877.2 to 398 F g−1). Electrochemical performances (area specific capacitance, F cm−2) of the electrodes based on CuCo2O4-based materials at various current densities (mA cm−2) have also been given in Table S1. Correspondingly, CuCo2O4@CQDs electrode shows a high capacitance retention of 81.9% even at 20 A g−1, which is also much better than CuCo2O4 with only 45.4% capacitance retained. The performances of CuCo2O4@CQDs with different loadings of CQDs were also performed by GCD at 1 A g−1 (Figure S5b). The specific capacitance values are determined to be 1255.4, 1414.0, and 1046.3 F g−1 over CQDs/CuCo2O4-5, CQDs/CuCo2O4-10, and CQDs/CuCo2O4-30, respectively. Figure 7d presents Nyquist plots of the CuCo2O4 and CuCo2O4@CQDs electrodes. It is shows that the CuCo2O4@ CQDs electrode has a lower resistance, which is consistent with its good electrochemical performance. The 3D hollow structures of the composites could supply rich active sites for redox reactions, and increase the specific capacitance. In addition, CQDs as well as oxygen vacancies can improve the conductivity of the CuCo2O4, which is good for the electrons migration, thus boosting the rate performance (Figure 7e).
still maintains original shape, further demonstrating its good stability. The reasons for boosting the electrocatalytic activities and durability of CuCo2O4 are as follows: (1) The 1D porous nanowires and assembled 3D hollow microspheres not only provide abundant surface areas, which are beneficial to the diffusion of active species and accelerate Faradaic reactions, but also supply short pathways to facilitate the infiltration of the electrolyte and increase the contact surface between the active species and electrolyte. (2) The CoII-rich surfaces and oxygen vacancies in the CuCo2O4@CQDs are beneficial for the formation of CoOOH, which is responsible for the enhanced Faradaic reaction and OER performance. In order to feature the multifunctional applications, the supercapacitive performance of the CuCo2O4@CQDs electrode was also evaluated with a three-electrode system. CV curves of the CQDs, CuCo2O4, and CuCo2O4@CQDs were performed at 5 mV s−1 (Figure 7a). The integrated area of the CuCo2O4@CQDs is larger than CQDs and pristine CuCo2O4, indicating the CuCo2O4@CQDs electrode has better performance. Figure 7b displays galvanostatic charge−discharge (GCD) of the CQDs, CuCo2O4, and CuCo2O4@CQDs electrodes at 1 A g−1. Both curves of the CuCo2O4 and CuCo2O4@CQDs electrodes are quasi-symmetrical, suggesting they possess a high Coulombic efficiency and an outstanding reversible redox process. The specific capacitance values are determined to be 84, 877.2, and 1528.8 F g−1 over CQDs, CuCo2O4, and CuCo2O4@CQDs at 1 A g−1, respectively. Typically, Figure S7a−d gives CV and GCD curves of CuCo2O4 and CuCo2O4@ CQDs. The shape of the plots indicates pseudocapacitive G
DOI: 10.1021/acs.inorgchem.8b01020 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry An ASC device with two electrodes was assembled using the CuCo2O4@CQDs and CNTs as the positive and negative electrode, respectively. As shown in Figure 8a, the ASC device possesses the pseudocapacitor-type and an electric double-layer capacitance. The specific capacitance values of the CuCo2O4@ CQDs//CNTs device reach 141.1, 128.5, 111.8, 86.3, 70.6, 62.7, and 54.9 F·g−1 at 1, 2, 3, 5, 10, 15, and 20 A·g−1, respectively (Figure 8b, c). Furthermore, the Coulombic efficiency values in a range of 1−20 A·g−1 are 85.2, 87.4, 94.9, 95.2, 95.9, 98.5, and 96.2%, respectively, demostrating the effective electron transport on the electrode/electrolyte surface. Figure 8d displays the energy density and power density are 45.9 Wh·kg−1 and 763.4 W·kg−1 at 1 A·g−1, respectively. Even at 20 A g−1, the energy density still remains 17.8 Wh kg−1 with a power density of 15257.1 W kg−1. Therefore, our device exhibits a high power density without significant loss in the energy density. This value is higher than those previously reported symmetric supercapacitors such as AC//AC (
