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Flower-Like Copper Cobaltite Nanosheets on Graphite Paper as HighPerformance Supercapacitor Electrodes and Enzymeless Glucose Sensors Shude Liu, K.S. Oscar Hui, and K.N. Hui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11001 • Publication Date (Web): 13 Jan 2016 Downloaded from http://pubs.acs.org on January 13, 2016

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ACS Applied Materials & Interfaces

Flower-Like Copper Cobaltite Nanosheets on Graphite Paper as HighPerformance Supercapacitor Electrodes and Enzymeless Glucose Sensors

Shude Liu,† K.S. Hui,*,‡ K.N. Hui,*,# †

Department of Materials Science and Engineering, Pusan National University, San

30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Republic of Korea. ‡

Department of Mechanical Convergence Engineering, Hanyang University, 17

Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea. #

Institute of Applied Physics and Materials Engineering, University of Macau,

Avenida da Universidade, Taipa, Macau, China.

*

Corresponding author:

E-mail: [email protected] (K.S. Hui) Tel: +82 2-2220-0441; Fax: +82 2-2220-2299 E-mail: [email protected] (K.N. Hui) Tel: +853 8822-4426; Fax: +853 8822-2426

1 ACS Paragon Plus Environment


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Abstract Flower-like copper cobaltite (CuCo2O4) nanosheets anchored on graphite paper have been synthesized using a facile hydrothermal method followed by a post-annealing treatment. Supercapacitor electrodes employing CuCo2O4 nanosheets exhibit an enhanced capacitance of 1131 F g−1 at a current density of 1 A g−1 compared with previously reported supercapacitor electrodes. The CuCo2O4 electrode delivers a specific capacitance of up to 409 F g−1 at a current density of as high as 50 A g−1, and a good long-term cycling stability, with 79.7% of its specific capacitance retained after 5000 cycles at 10 A g−1. Furthermore, the as-prepared CuCo2O4 nanosheets on graphite paper can be fabricated as electrodes and used as enzymeless glucose sensors, which exhibit good sensitivity (3.625 µA µM−1 cm−2) and an extraordinary linear response ranging up to 320 µM with a low detection limit (5 µM).

Keywords: supercapacitors; copper cobaltite; hydrothermal method; nanosheets; glucose sensor.


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Introduction

High-performance energy-storage and biosensor devices have attracted considerable attention because of the increasing demand for more efficient, lightweight, sustainable energy sources,1-6 and ultrahigh sensitivity clinical diagnostics.7 Supercapacitors, as promising energy-storage devices, have been of interest because of their long lifespan compared with secondary batteries and high capacitance and excellent reliability compared

with

conventional

dielectric

capacitors.8

The

performance

of

supercapacitors and glucose sensors largely depend on the composition and microstructure of the selected electrode materials. Generally, supercapacitors can be classified into two types according to their charge-storage mechanisms: electric double-layer capacitors (EDLCs) and pseudocapacitors.9 Unlike EDLCs, which store energy via the electrostatic storage of charge,10-11 pseudocapacitors exploit the reversible Faradaic reactions that occur at the electrode surface;12 as a result, pseudocapacitors offer a much higher specific capacitance. On the other hand, enzymeless glucose sensors are imperative to the diagnosis of Diabetes,13 as a global health care problem, owing to the advantageous of simplicity, high stability and reproducibility compared to enzymatic biosensors based on glucose oxidase enzyme that is highly affected by temperature, pH, and humidity.14 Thus far, Co and Cu based transition-metal

oxides,

carbon/Co3O4,17

3D

such

as

CoMoO4,15

Co3O4/Ni(OH)2,16

graphene/Co3O4,14 CuO/GO,18

and

activated

Cu@Cu2O,19

have

demonstrated as potential candidates for supercapacitors and glucose sensors because of their enhanced electrocatalyic behavior, superior electrochemical property, and good

chemical

stability.

Meanwhile,

the

stability,


electrochemical

and


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physicochemical properties of a Co3O4 electrode have been significantly improved by incorporating Cu2+ to Co2+ from the Co2+(Co2)3+O4 spinel structure.20-21

Among the transition-metal oxides available, spinel cobaltites (MCo2O4) possess higher electrical conductivity and higher electrochemical activity than monometallic oxides because of the transport of electrons between multiple transition-metal cations with a relatively low activation energy; this mechanism significantly enhances the storage performances of MCo2O4, particularly in terms of high-rate capability.22 To date, spinel CuCo2O4 electrodes with various nanostructures, such as nanowires,23 nanograsses,24 nanoparticles,20-21, 25-28 and nanocubes,29 have been prepared. Improved electrochemical

performance

has

been

demonstrated

in

Li-ion

batteries,

supercapacitors, catalyst, and O2 evaluation process with CuCo2O4 as electrodes. In a recent study by Liu et al., the morphology of a CuCo-carbonate hydroxide (CuxCo2xCH)

microsphere composed of well-defined porous nanoplates has been shown to

provide efficient transport kinetics of electrons and ions, and reduce the ions diffusion paths, resulting in high specific capacitance in supercapacitor applications.30 Despite breakthroughs, very little research has been done to investigate microflower-like CuCo2O4 nanosheets serving as electrodes for supercapacitors and enzymeless glucose sensor applications.

In particular, the electrochemical performance of supercapacitors and glucose sensors significantly depends on the nanostructures of the electrodes, in addition to the desirable composition of active materials. Ideal electrodes need to be constructed with a substantial amount of electroactive sites and short transport paths for both electrolyte ions and electrons in redox reactions. At present, active materials without


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binders or conductive substrates have exhibited great potential for decreasing the “dead surface” from the contact with the electrolyte to participate in the Faradaic reactions.31 Notably, graphite papers can be used as a scaffold to disperse nanosheetshaped metal oxide/hydroxide compounds and avoid self-aggregation.32 Furthermore, graphite papers provide extra active sites and simplify electrode processing with the advantages of low-cost, flexibility, high mechanical strength, and light weight.32

This paper reports flower-like CuCo2O4 nanosheets grown directly on graphite paper through a facile hydrothermal method followed by a simple thermal annealing treatment. The synthesized CuCo2O4 nanosheets deliver enhanced capacitance with good cycling stability when evaluated as an electrode material for supercapacitors. Notably, the supercapacitor electrode exhibits enhanced capacitance, desirable rate performance (1131 F g−1 at 1 A g−1 and 409 F g−1 at 50 A g−1), and long-term capacity retention (79.7% after 5000 cycles at 10 A g−1). In addition, the CuCo2O4 nanosheets electrode is tested as an enzymeless glucose sensor; it exhibits good electrocatalytic activity toward glucose oxidation with a high sensitivity of 3.625 µA µM−1 cm−2 and a low detection limit of 5 µM, outperforming other reported materials.

Experimental

Preparation of functionalized graphite paper All reagents were of analytical grade and used as received. The graphite paper was cleaned several times with acetone, ethanol, and deionized water, and dried at room temperature. Subsequently, the graphite paper was functionalized to increase the number of oxygen functional groups by an electrochemical treatment.32 A typical



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three-electrode configuration measurement was conducted, with graphite paper as the working electrode, Pt as the counter electrode, and a Hg/Hg2Cl2 saturated calomel electrode (SCE) as the reference electrode. An aqueous solution containing 1 M H2SO4 was used as the electrolyte at 2.2 V for 20 min. Subsequently, the samples were cleaned with deionized water several times and then dried at 120 °C in a vacuum oven.

Fabrication of flower-like CuCo2O4 nanosheets All chemicals were used as received. Typically, 1 mmol Co(NO3)2·6H2O and 2 mmol Cu(NO3)2·3H2O were dissolved in 40 mL of deionized water to form a homogeneous solution. Urea (14 mmol) and cetyltrimethyl ammonium bromide (CTAB) (1.2 g) were added to the homogeneous solution by magnetic stirring, and the reaction solution was transferred to a 50 mL Teflon-lined stainless steel autoclave. The cleaned graphite paper substrate was immersed into the reaction solution. Subsequently, the autoclave was heated to 120 °C for 24 h and then cooled naturally to room temperature. The graphite paper loaded with a precursor was washed several times with distilled water and ethanol and dried in an oven to yield the flower-like CuCo2O4 nanosheets on the graphite paper. The final product was obtained by annealing the precursor at 350 °C for 3 h in air. For comparison, Co3O4 nanoflakes grown on graphite paper were synthesized under the same conditions without the addition of Cu(NO3)2·3H2O. The mass loadings of the CuCo2O4 nanosheets and Co3O4 nanoflakes on the graphite paper were approximately 1.0 and 1.3 mg cm−2, respectively.

Characterization of materials


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The microstructure and morphology of the as-obtained samples were characterized by scanning electron microscopy (SEM, Hitachi, S−4800, 15 kV) equipped with energy dispersive X-ray spectroscopy (EDAX) and high-resolution transmission electron microscopy (HRTEM, JEOL 2100F). The crystallographic structure of the samples was tested by X-ray diffraction (XRD, D8-Discovery Brucker, 40 kV, 40 mA, Cu Kα, λ = 1.5406 Å), and the functional groups on the samples were examined by Fourier transform infrared absorption spectroscopy (FTIR, VERTEX70). X-ray photoelectron spectroscopy (XPS, VG Scientifics ESCALAB250) was performed to analyze the chemical bonding status of the spinel CuCo2O4 material. The XPS spectra of the CuCo2O4 powder were obtained after the sonication of the graphite paper and calibrated to the C 1s peak at 284.6 eV.

Electrochemical characterization The electrochemical performances of the samples were evaluated in a traditional three-electrode electrolytic cell by cyclic voltammetry (CV) at various scan rates ranging from 5 mV s−1 to 50 mV s−1 at a potential window of 0–0.8 V, and a galvanostatic charge−discharge test was performed in a 6.0 M KOH electrolyte between 0 and 0.6 V versus SCE at various current densities. The electrochemical impedance spectroscopy (EIS) measurement was conducted over a frequency range of 100 kHz–0.01 Hz at the open circuit potential. All electrochemical experiments were conducted using a three-electrode mode multichannel electrochemical workstation (IVIUMSTAT, Ivium Technologies). The specific capacitance (Cs) was calculated using Eq. (1).33

Cs =

I ×t ∆V × m 7

ACS Paragon Plus Environment

(1)


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where I (mA) is the constant discharge current, t (s) is the discharge time, ∆V (V) is the voltage drop upon discharging (excluding the IR drop), and m (mg) is the mass of the active materials. The electrochemical glucose sensing properties of the CuCo2O4 nanosheet electrodes were examined in a 0.1 M NaOH solution using the aforementioned three-electrode system. The current response was measured by constant-potential chronoamperometry at room temperature.

Results and discussions Fig. 1a schematically illustrates the fabrication process for flower-like CuCo2O4 nanosheets on graphite paper as supercapacitor electrode materials. First, the graphite paper is cleaned with acetone, ethanol, and deionized water to remove the possible metal impurities from the surface. An electrochemical treatment is used to generate oxygen functional groups on the graphite paper according to the previous work.32 Afterward, flower-like CuCo2O4 nanosheets are grown vertically on graphite paper using a simple hydrothermal and post-annealing process. The graphite paper is employed as a current collector because of its good electrical conductivity and lightweight.

The successful fabrication of CuCo2O4 nanosheets on the graphite paper is confirmed by powder XRD. As shown in Fig. 1b (red line), the XRD patterns of the bare Co3O4 nanoflakes in the 2θ range of 5°–80° could be indexed to the cubic phase Co3O4 (JCPDS Card No. 42-1467). The two peaks marked with “#” at approximately 26° and 55° are assigned to the graphite substrate. By contrast, the XRD patterns of the CuCo2O4 (black line) show well-fitting peaks (marked with “*”) in the 2θ range of 5°–80°, which are indexed to cubic spinel CuCo2O4 (JCPDS, Card No. 01-1155),34 in


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addition to the peaks from graphite, confirming the successful growth of CuCo2O4 on the graphite paper. The inset in Fig. 1b shows the crystal structure of spinel CuCo2O4, in which the cubic lattice parameter and space group (Fd3m) are evaluated by a leastsquares fitting of 2θ and (hkl). The green/blue/red small spheres represent the Cu atoms, Co atoms, and O atoms, respectively.

FTIR spectroscopy is used to examine the molecular structural changes and confirm the formation of CuCo2O4 nanosheets after annealing. Fig. 1c shows the FTIR spectra of the CuCo2O4 precursor and CuCo2O4 samples. The peaks at approximately 3410 and 3323 cm−1 correspond to the –OH stretching band, and the peak at 1633 cm−1 indicates the existence of the bending modes of absorbed water molecules.35 Furthermore, the peaks at approximately 1525, 1381, and 1051 cm−1 are assigned to νOCO2, νCO3, and νC=O, respectively.36 The spectra clearly show the characteristic in-plane and out-of-plane bending vibrations of CO32− at 876 and 750 cm−1. In addition, the broad peak at 3410 cm−1 and the weak peak at 1633 cm−1 are assigned to adsorbed water.37 Whereas, other absorption bands below 700 cm−1 are associated with metal–oxygen stretching and bending modes. Based on the above analysis, the precursor can be identified as a typical copper–cobalt hydroxide carbonate compound (both CO32– and OH– anions derived from urea acted as precipitants).36 After annealing treatment, two very strong peaks centered at 659 and 564 cm−1 are observed, confirming the presence of spinel CuCo2O4.38 As a comparison, two strong peaks of spinel Co3O4 are also observed at 657 and 551 cm−1, respectively, which are consistent with the aforementioned XRD patterns, further indicating a similar conversion process from a cobalt hydroxide carbonate compound to cobalt oxide.



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The elemental composition and oxidation state of CuCo2O4 nanosheets are characterized by XPS. The survey spectrum (Fig. 2a) indicates the presence of Cu, Co and O, as well as C. The Gaussian fitting results show that the Co 2p emission spectrum (Fig. 2b) presents two main peaks, Co 2p3/2 and Co 2p1/2 at 780.0 eV and 795.1 eV respectively, together with a spin-energy separation of around 15 eV, indicating the presence of the mixed Co2+ and Co3+.26, 39-40 The fitting peaks at the binding energies of around 779.6 eV and 794.8 eV are ascribed to Co2+, while the peaks at 782.4 eV and 796.1 eV represent the Co3+.39 The two shakeup satellite peaks (indicated by “Sat.”) located around 788.2 and 803.3 eV, revealing that cobalt has a spinel structure.39,

41

Cu 2p peaks are also fitted with two main peaks at binding

energies of 934.4 and 954.2 eV. In addition, two shakeup satellite peaks (indicated by “Sat.”) are also observed at binding energies of 941.7 and 962.1 eV (Fig. 2c), confirming the characteristic of Cu2+.39-40, 42 The high-resolution spectrum of the O 1s region (Fig. 2d) shows three oxygen contributions, which are marked with O1, O2, and O3. Notably, the O1 component at 529.7 eV is consistent with typical metal– oxygen bonds. The well-resolved O2 component at approximately 531.3 eV corresponds to a larger number of defect sites with low oxygen coordination. The O3 component at 532.6 eV is assigned to the multiplicity of physi-sorbed and chemisorbed water at or near the surface.30 These results clearly show that the chemical compositions of the prepared CuCo2O4 contain Co2+, Co3+, and Cu2+, indicating the composition is constituted by CuCo2O4, as further confirmed by EDX analysis (Figures S1 (d)).

Fig. 3 presents the morphology of the as-obtained spinel Co3O4 nanoflakes and CuCo2O4 nanosheets on the graphite paper. The existence of a graphite paper


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supporter is verified by Figure S1 (a,b). Figure S1 (c) shows the EDX data of the CuCo2O4 on graphite paper, confirming the existence of Cu, Co, O, and C elements in the prepared samples. Figure S1 (d) shows weight percentage of the elements corresponding to the EDX data. The atomic ratio of Co/Cu of the CuCo2O4 is approximately 2, confirming the formation of CuCo2O4. After hydrothermal growth and subsequent thermal annealing treatment, uniform Co3O4 hexagonal nanoflakes with high density grow aggregately on the graphite paper, forming a porous structure (Fig. 3a, b). The Co3O4 nanoflakes have a mean thickness of 200 nm and a length of up to approximately 1.5 µm; such thin features are beneficial to an efficient, reversible Faradic reaction and a short ionic diffusion path during the charge–discharge process.43 The field emission SEM (FESEM) images of CuCo2O4 (Fig. 3c, d) show that the structure is composed of well-defined nanosheet subunits, which form an open porous network. In addition, the CuCo2O4 nanosheets are aligned homogeneously and deposited uniformly without aggregation over the skeleton of the graphite paper, allowing for the rapid electrolyte transport and active-site accessibility for enhanced capacitance. Significantly, the thickness of the CuCo2O4 nanosheets is estimated to be only approximately 65 nm; thus, CuCo2O4 nanosheets are much thinner than Co3O4. The growth mechanism of CuCo2O4 nanosheets follows a preferred orientation growth process.44-45 In the hydrothermal process, the slow hydrolysis of urea results in the in situ release of OH− and CO32−, further initiating the precipitation of Cu2+ and Co2+ to form CuCo-based carbonate hydroxide species (Figure S2).30 The first step in the precursor generation is to form amorphous M salts, where M presents Cu2+ or Co2+, on the graphite paper. The nascent precursors attached to the graphite paper function as the nucleation sites. As the freshly formed nanonuclei are thermodynamically unstable due to the high surface energy, they tend



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to gather together to minimize the interfacial energy, resulting in the aggregation of supersaturated nuclei.44 As the reaction proceeds, the concentration of the reactants becomes lower, and new particles are continuously grown along the oriented direction on the initially formed precursor aggregations. As a result, small nanosheets form and further grow along the specific directions to interlace and overlap with each other into a network structure, constituting the flower-like morphology.45 Finally, the flowerlike CuCo-precursor species can be converted into CuCo2O4 structure through a simple oxidation reaction. The chemical reaction mechanism can be expressed in equations (2-4):46-47 2NH2(CO)NH2 + 5H2O → 4NH4+ + 2OH− + CO32 − + CO2

(2)

(Cu2+, Co2+) + 2OH¯ + CO32 − → (Cu,Co)2(CO3)(OH)2

(3)

Cu2(CO3)(OH)2 + 2Co2(CO3)(OH)2 + O2 → 2CuCo2O4 + 3CO2 + 3H2O

(4)

Owing to the growth mechanism of the present approach, the surfactant CTAB can selectively stabilize the exposed planes of precursor materials to improve their size uniformity through the electrostatic interactions between the ionic head groups of CTAB and the OH− groups of the precursor materials during hydrothermal reaction, as reported in a previous paper.48 Specifically, CTAB acts as a morphology director hampering aggregation and promoting the preferential formation of uniform hexagonal sheets or complex two-dimensional morphologies.48

TEM is further performed to investigate the structure of the CuCo2O4 nanosheets. The TEM image reveals that the CuCo2O4 nanosheets have a highly porous texture with an ultrathin feature, as shown in Fig. 4. The magnified images (Fig. 4a, b) clearly show white spots (4–10 nm), indicating that the pores are uniformly distributed throughout the surface of the nanosheets. The formation of the pores could be related to the


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release of gas or water molecules during the thermal decomposition of the CuCo2O4 precursor.23 Flower-like CuCo2O4 nanosheets exhibit transparent features, indicating their ultrathin nature, which is consistent with SEM observations. Ultrathin nanosheets with large electroactive surface areas and open-network features can ensure an efficient and rapid diffusion of electrolyte ions to the surface of the active material; as a result, more electroactive sites exist for Faradaic reactions, leading to high specific capacitance. The lattice fringes shown in Fig. 4b are indexed to the (220) and (400) crystal planes of the cubic CuCo2O4 phase, further confirming the formation of crystalline CuCo2O4 nanosheets. Fig. 4c shows some visible lattice fringes with an equal interplanar distance of 0.249 nm, corresponding to the (311) planes of cubic CuCo2O4. In addition, the corresponding selected-area electron diffraction (SAED) pattern in Fig. 4d indicates the polycrystalline structure of the nanosheets, and the diffraction rings can be readily indexed to the (200), (311), (400), (511), and (533) planes of the CuCo2O4 phase. These results are consistent with the XRD patterns. The energy-dispersive X-ray spectrometry (EDS) mapping of CuCo2O4 nanosheets (Fig. 4e) clearly shows that Cu, Co, and O are evenly distributed.

The typical CV curves of the Co3O4 nanoflake and CuCo2O4 nanosheet electrodes are obtained at scan rates of 5, 10, 20, 30, and 50 mV s−1, with potential windows ranging from 0 V to 0.8 V in a 6 M KOH solution, as shown in Fig. 5a–b. A typical redox couple is clearly observed in the Co3O4 electrode at the formal potentials of approximately 0.35 and 0.52 V, resulting mainly from the redox reactions related to the Co4+/Co3+/Co2+ redox couple with OH ̄ anions.34, 49 A slight shift in the peaks is observed with increasing scan rate. This shift corresponds to the increase in internal resistance to ion and electron transfer at higher scan rates, and such internal resistance



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increase causes a slight decrease in the electrochemical performance.50 In the CuCo2O4 nanosheet electrodes, typical redox couples are clearly observed at formal potentials of approximately 0.34 and 0.53 V; the expanded peaks are mainly attributed to the Cu2+/Cu+ and Co4+/Co3+/Co2+ reactions and facilitated by OH ̄ ions from the electrolyte.27,

34

An increase in current is observed with increasing scan rate. This

result further verifies the ability of nanosheets to conduct ions and electrons at higher rates. The shapes of these CV curves show no significant change as the scan rate is increased from 5 mV s−1 to 50 mV s−1, indicating the excellent reversibility of the Faradaic redox reaction. Figure S3 shows a comparison of the CV curves of the spinel Co3O4 nanoflakes, CuCo2O4 nanosheets, and bare graphite paper electrodes at a scan rate of 20 mV s−1. The graphite paper has a negligible contribution to the capacitance of the whole electrode (approximately 4.14% for the Co3O4 sample and 2.89% for the CuCo2O4 sample), revealing that the major capacitance contribution originates from the active materials. Remarkably, the CV integrated area of the CuCo2O4 electrode is much larger than that of the Co3O4 electrode, indicating a significant specific capacitance increase, which could be attributed to the advantageous morphology of nanosheets and the richer electrochemical properties provided by copper and cobalt ions.

The cathodic peak current densities of Co3O4 nanoflake electrodes (Fig. 5a) and CuCo2O4 nanosheet electrodes (Fig. 5b) are plotted as functions of the scan rates and the square roots of the scan rates, as respectively shown in Fig. 5c and 5d. For the Co3O4 nanoflake and CuCo2O4 nanosheet electrodes, the cathodic peak currents linearly increase with the square roots of the scan rates, / , confirming that the


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pseudocapacitive nature of both electrodes is limited by the diffusion of OH− to the active sites.33 According to Eq. (5),32 /

= 2.69 × 10 / ∗1/2

(5)

where ip is the peak current, n is the number of electrons transferred, A is the electrode area, Do is the diffusion coefficient, Co* is the reactant concentration, and ν is the scan rate. For comparison, the diffusion coefficients (DCo3O4 and DCuCo2O4) of the Co3O4 and CuCo2O4 composite electrodes are calculated using Eq. (6), with the assumption that both electrodes have the same n, A, and Co* values. The diffusion coefficient of the CuCo2O4 electrode (DCuCo2O4) is 2.62 times higher than that of the Co3O4 electrode, indicating the beneficial effect of the nanoflower-like CuCo2O4 nanosheets on performance. This conclusion is further confirmed by the subsequent galvanostatic charge–discharge tests, whose results are shown in Fig. 6. DCuCo2O4/DCo3O4 = [(ip/ν1/2)CuCo2O4/(ip/ν1/2)Co3O4]2 = (11.47/7.09)2 = 2.62

(6)

Fig. 6a presents the galvanostatic charge–discharge curves of the spinel Co3O4 nanoflake and CuCo2O4 nanosheet electrodes at a current density of 1 A g−1. The CuCo2O4 electrode delivers a higher capacitance than the Co3O4 electrode. Specifically, the charge–discharge measurements of the Co3O4 nanoflake and CuCo2O4 nanosheet electrodes are measured at different current densities of 1, 2, 3, 5, 10, 20, and 50 A g−1, as shown in Figure S4 and S5. The Co3O4 has a discharge plateau at approximately 0.36 V, whereas CuCo2O4 has a discharge plateau at approximately 0.35 V. These results are fairly consistent with the results observed in the CV curves. Fig. 6b shows the specific capacitances of the Co3O4 and CuCo2O4



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electrodes at different densities ranging from 1 A g−1 to 50 A g−1. The specific capacitances of the Co3O4 electrode are calculated to be 664, 623, 545, 478, 348, 177, and 102 F g−1 at current densities of 1, 2, 3, 5, 10, 20, and 50 A g−1, respectively. The capacitance loss at high current densities is attributed mainly to the ohmic drop and the sluggish kinetics of the redox couples in the electrode materials.34 Compared with the Co3O4 electrode, the CuCo2O4 electrode exhibits significantly enhanced specific capacitances of as high as 1131, 1055, 1012, 937, 802, 663, and 409 F g−1 at current densities of 1, 2, 3, 5, 10, 20, and 50 A g−1, respectively. Notably, the CuCo2O4 electrode exhibits higher electrochemical performance than the previously reported spinel cobaltites, such as CuCo2O4,23-24, 27, 34, 39 NiCo2O4,51-52 and CuCo2O4@MnO253 (Table S1). The proposed CuCo2O4 on graphite paper electrode plays an important role in achieving high specific capacitance. Specifically, (1) considerable spaces between the interconnected open-network nanosheet arrays shorten the diffusion distance from the external electrolyte to the interior surface and improve the utilization rate of electrode materials;50 (2) direct contact of CuCo2O4 nanosheets on the highly conductive graphite paper not only avoids the use of polymer binder/conductive additives, and reduces “dead volume” in the electrode substantially, but also favors the transfer of the electrons effectively;54 (3) the distinctive thin thickness of the nanosheet can ensure high-rate performance;55 (4) the large lateral size (~ 2 µm) can effective keep the structural integrity during charge–discharge process.55 Long-term cycling stability is an important criterion for practical supercapacitor applications. Fig. 6c presents the cycling performance of the spinel Co3O4 nanoflake and CuCo2O4 nanosheet electrodes. The Co3O4 electrode exhibits a specific capacitance of 293 A g−1 (∼68.0% capacitance retention) after 5000 cycles at a 16 ACS Paragon Plus Environment

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current density of 10 A g−1. The CuCo2O4 electrode exhibits an impressive higher capacitance of 639 F g−1 (∼79.7% capacitance retention) after 5000 cycles at a current density of 10 A g−1. As shown in Figure S6, the high specific capacitances of the CuCo2O4 electrode at various charge–discharge current densities ranging from 1 A g−1 to 50 A g−1 indicate the high-rate capability of the electrode. The CV curves (Figure S7) of the CuCo2O4 electrode before and after 5000 cycles at 20 mV s−1 reveal only a slight change in electrochemical performance, suggesting the long-term stability of the CuCo2O4 electrode. This performance might be due to the better mass transport of the electrolyte within the interconnected open-network structure of the electrode during the fast charge–discharge process, flexible buffering of the volume variation of the nanosheet structure caused by electrochemical reaction, and the robust structure of the interconnected nanosheets grown directly on the graphite paper substrate.

EIS is conducted to further evaluate the ion transport properties of the electrodes. Fig. 6d shows the impedance Nyquist plots of the samples. The top-right inset in Fig. 6d shows the fitted equivalent circuit for the measured impedance data. The highfrequency intercept of the semicircle on the real axis shows the series resistance (Rs), and the diameter of the semicircle corresponds to the charge-transfer resistance (Rct) of the Faradaic process. The bottom-right inset in Fig. 6d clearly shows that the CuCo2O4 electrode has a lower Rs (0.97 Ω) than the Co3O4 electrode (1.35 Ω). Moreover, the impedance spectrum of the CuCo2O4 electrode reveals a smaller semicircle, which indicates that the Rct of CuCo2O4 electrode (1.29 Ω) is lower than that of the Co3O4 electrode (3.62 Ω), suggesting that the CuCo2O4 electrode has a higher electrochemical activity. In the low-frequency region, the CuCo2O4 electrode shows a more ideal straight line, indicating a more efficient electrolyte and proton



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diffusion.33 More importantly, the slope of the linear part of the CuCo2O4 electrode shows no significant change after cycling, implying the good ionic diffusion and access to the electrolyte after cycling. These results may be due to the ultrathin feature of the nanosheets and the interconnected open-network CuCo2O4 architecture, which leads to enhanced ionic diffusion and charge transport. After 5000 cycles, the Rct of the CuCo2O4 electrode slightly increases to 4.34 Ω. The enhanced electrochemical properties of the CuCo2O4 electrode are attributed to the robustness of the interconnected open network, which prevents structural degradation and facilitates rapid ion and electron transfer within the electrode and at the electrode–electrolyte interface.

The SEM and TEM images of the sample after 5000 cycling measurements are taken to examine the structural stability of the CuCo2O4 electrode. Fig. 7a shows the morphology of the interconnected nanosheets. The results show that the open network is almost maintained, and no obvious agglomeration of CuCo2O4 nanosheets has occurred with the exception of the increased pore spacing between the adjacent nanosheets, indicating the mechanical robustness of the CuCo2O4 electrode and the efficient infiltration of the electrolyte in the electrode. The TEM image of the CuCo2O4 nanosheets after 5000 cycles shows that the nanosheets have maintained their structural integrity, and their single-crystalline structure is made up of numerous interconnected mesopores (Fig. 7b). The same crystal orientation and the crystalline structure of the nanosheets are still maintained after the cycling test (Fig. 7c, d), demonstrating the good structural stability of the CuCo2O4 nanosheets. This result supports the potential of CuCo2O4 nanosheets as electrodes for supercapacitors. The reversibility of the proposed CuCo2O4 nanosheets is further evaluated, and the


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evaluation results are shown in Figure S8. Only a slight degradation in the capacitance is observed based on the before and after results of the 5000-cycle test, suggesting that the proposed material is suitable for rapid charge–discharge applications.

CuCo2O4 nanosheet electrode for non-enzymatic glucose detection

Interestingly, aside from being high-performance supercapacitor electrode materials, flower-like CuCo2O4 nanosheets on graphite paper can be used to detect glucose for the potential diagnosis and management of diabetes. Figure S9 shows the CV curves of the bare graphite paper electrode and the CuCo2O4 electrode in a 0.1 M NaOH solution in the presence of 5 mM of glucose at a scan rate of 5 mV s−1. The bare graphite paper electrode shows no obvious electrocatalytic activity with the glucose solution. By contrast, the CuCo2O4 electrode demonstrates an evident electrocatalytic activity in response to glucose. Fig. 8a shows the CV curves of the CuCo2O4 nanosheets in 0.1 M NaOH with glucose concentrations varying from 0 mM to 3 mM at 20 mVs-1, broad catalytic current peaks with a peak potential of about 0.28/0.56 V (I/II) are observed in the CV curve without addition of glucose, corresponding to the cathodic/anodic peaks of Cu2+/Cu3+ redox couple.56-58 Another two pairs of redox peaks at 0.39/0.65 V (III/IV) and 0.58/0.72 V (V/VI) are observed, revealing the reversible transition between Co3O4 and CoOOH and transition between CoOOH and CoO2, respectively.14, 59-60 The reversible reaction mechanisms of Cu and Co species in the alkaline electrolyte can be expressed in Eq. (7-8).17, 57, 61 Introduction of glucose causes an obvious increase in the anodic peak currents (at about 0.65 and 0.72 V) in a concentration-dependent manner. This phenomenon can be ascribed to the glucose oxidation to gluconolactone, which is accompanied by the conversion of CoO2 to CoOOH and MOOH to M(OH)2 (M = Cu, Co). The possible electrooxidation



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mechanism of glucose can be expressed in Eq. (9-10).14,

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56, 61

Furthermore, with

increasing the glucose concentration, the oxidation current potential gives rise to a slight upshift to a positive direction, indicating the oxidized intermediates and the absorption of glucose on the active sites of the electrode.62 CuCo2O4 + OH− + H2O ↔ CuOOH + 2CoOOH + e−

(7)

CoOOH + OH− ↔ CoO2 + H2O + e−

(8)

2CoO2 + C6H12O6 (glucose) → 2CoOOH + C6H10O6 (gluconolactone) (9) 2MOOH + C6H12O6 → 2M(OH)2 + C6H10O6

(10)

The glucose-sensing performance of the CuCo2O4 nanosheet electrode is evaluated at an oxidation potential of 0.6 V by amperometric measurements in a homogeneously stirred NaOH solution (Fig. 8b). The oxidation peak currents sharply increase with the addition of glucose to the homogeneously stirred 0.1 M NaOH solution and reach 96% of the steady-state current within 6 s, implying the rapid response of the CuCo2O4 nanosheet electrode. Fig. 8c presents the dose response curve (amperometric current increase vs. glucose concentration). An extraordinary sensitivity of 3.625 µA µM−1 cm−2 is recorded in the linear response (correlation coefficient, R = 0.998) ranging up to 320 µM with a detection limit of ~5 µM. Furthermore, the performance of the asprepared CuCo2O4 nanosheet electrode is much higher than those previously reported for non-enzymatic sensors, such as NiO nanoparticles/GO-modified electrode (1.571 µA uM−1 cm−2),63 Ni(OH)2 nanoparticles/Ni foam-modified electrode (1950.3 µA mM−1 cm−2),64 CNT/MnO2 nanoflake electrode (3406.4 µA mM−1 cm−2),13 Cu2O nanosphere/RGO electrode (0.185 µA uM−1 cm−2),65 MnCo2O4 nanofiber electrode (679.5 µA mM−1 cm−2),61 and flower-like spherical and few dendrite-like Cu-Co/RGO electrode (1.921 µA µM−1 cm−2).66 These results confirm that the as-prepared


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CuCo2O4 nanosheet electrode is a promising functional material for the electroanalytical detection of glucose.

Non-enzymatic glucose detection aims to eliminate the electrochemical response generated by some easily oxidizable endogenous interfering compounds, such as ascorbic acid (AA), uric acid (KCl), and dopamine (DA), which normally co-exist with glucose in human blood. The amperometric response of the CuCo2O4 nanosheet electrode is evaluated to ensure the accurate detection of glucose instead of oxidizable endogenous interfering compounds. The amperometric response evaluation is conducted by sequentially adding 40 µM glucose, 1 mM AA, 1 mM DA, and 1 mM KCl. As shown in Fig. 8d, the electrode exhibits almost no current response for all three interferents compared with its response to 40 µM glucose. After the addition of the three interferents, 40 µM glucose is again added to the solution. The current response still reaches a similar value after the final addition of glucose, regardless of the previous addition of interferents. The exceptional performance results indicate the good anti-interference capability and high selectivity of CuCo2O4 nanosheet electrode.

Conclusions We present a rational design and fabrication of flower-like CuCo2O4 nanosheets grown on graphite paper via a facile hydrothermal method combined with a postannealing treatment. The as-fabricated CuCo2O4 for use as a supercapacitor electrode exhibits a high capacitance of 1131 F g−1 at a current density of 1 A g−1. The CuCo2O4 electrode exhibits high-rate performance. It achieves a specific capacitance of up to 409 F g−1 at a current density of 50 A g−1. It also demonstrates a good long-term cycling stability, with 79.7% of its specific capacitance retained after 5000 cycles.



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Furthermore, the CuCo2O4 nanosheet electrode exhibits remarkable non-enzymatic electrocatalytic activity toward glucose, including high sensitivity, good selectivity, and low detection limit.

Acknowledgements This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013R1A1A2007365) and the Start-up Research Grant (SRG2015-00057-FST) from Research & Development Office at University of Macau.

Supporting Information SEM and EDX analysis, XRD, CV, galvanostatic charge−discharge, specific capacitance, Coulombic efficiency results, and summary of spinel cobaltite electrodes for supercapacitors.


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Fig. 1

Fig. 1. (a) Schematic diagram of the fabrication process for CuCo2O4 nanosheets on graphite paper. (b) XRD patterns of Co3O4 nanoflakes (red line) and CuCo2O4 nanosheets (black line). The inset shows the crystal structure of the cubic spinel CuCo2O4. (c) FTIR spectra of the CuCo2O4 precursor, CuCo2O4 nanosheets, and Co3O4 nanoflakes.


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Fig. 2

Fig. 2. XPS spectra of CuCo2O4 nanosheets, (a) full spectrum, (b) Co 2p, (c) Cu 2p, and (d) O 1s.



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Fig. 3

Fig. 3. FESEM images of (a, b) Co3O4 nanoflakes and (c, d) CuCo2O4 nanosheets.


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Fig. 4

Fig. 4. Structure of CuCo2O4 nanosheets: (a) low-magnification TEM image; (b, c) HRTEM image; (d) SAED pattern; (e) EDS mapping of elements.



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Fig. 5

Fig. 5. CV curves of (a) Co3O4 nanoflake and (b) CuCo2O4 nanosheet electrodes, measured in a 6.0 M KOH at various scan rates (5−50 mV s−1). Plots of the cathodic peak current densities obtained from (a, b) versus (c) scan rates and (d) the square roots of scan rates.


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Fig. 6

Fig. 6. (a) Galvanostatic charge–discharge curves of Co3O4 nanoflake and CuCo2O4 nanosheet electrodes at a current density of 1 A g−1. (b) Specific capacitances of the two electrodes measured at various current densities ranging from 1 A g−1 to 50 A g−1. (c) Specific capacitances and capacitance retentions of the two electrodes. (d) EIS spectra of the two electrodes before and after cycle tests (top-right inset is the amplifier region; bottom-right inset is the fitted circuit of Nyquist plot).



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Fig. 7

Fig. 7. SEM and TEM images of CuCo2O4 nanosheet electrode after cycle test: (a) SEM image; (b) low-magnification TEM image; (c) HRTEM image; (d) SAED pattern of the CuCo2O4 nanosheets.


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Fig. 8

Fig. 8. (a) CV curves of the CuCo2O4 nanosheets in 0.1 M NaOH with glucose concentrations varying from 0 mM to 3 mM at 20 mV s−1. (b) Amperometric sensing of glucose by the successive addition of glucose to CuCo2O4 at 0.6 V in 0.1 M NaOH. (c) Average dose response curve (amperometric current response vs. glucose concentration) obtained with a linear fitting at lower concentration range and an exponential fitting at higher concentration range. (d) Interference test of CuCo2O4 with 40 µM glucose in the presence of 1 mM AA, 1 mM DA, and 1 mM KCl.



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