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Article Cite This: ACS Omega 2018, 3, 7204−7213
Two-Dimensional Double Hydroxide Nanoarchitecture with High Areal and Volumetric Capacitance Abhay D. Deshmukh,*,† Akanksha R. Urade,† Alisha P. Nanwani,† Kavita A. Deshmukh,§ Dilip R. Peshwe,§ Patchaiyappan Sivaraman,∥ Sanjay J. Dhoble,‡ and Bipin Kumar Gupta*,⊥
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†
Energy Materials and Devices Laboratory, Department of Physics and ‡Nanomaterials Research Laboratory, Department of Physics, RTM Nagpur University, Nagpur 440033, India § Department of MME, Visvesvaraya National Institute of Technology, Nagpur 440010, India ∥ Polymer Science and Technology Centre, Naval Materials Research Laboratory (DRDO), Ambernath (E) 421506, India ⊥ CSIR-National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India S Supporting Information *
ABSTRACT: The development of high volumetric or areal capacitance energy storage devices is critical for the future electronic devices. Hence, the hunting for next-generation electrode materials and their design is of current interest. The recent work in the twodimensional metal hydroxide nanomaterials demonstrates its ability as a promising candidate for supercapacitor due to its unique structure and additional redox sites. This study reports a design of freestanding high-mass-loaded copper-cobalt hydroxide interconnected nanosheets for high-volumetric/areal-performance electrode. The unique combination of hydroxide electrode with high mass loading (26 mg/cm2) exhibits high areal and volumetric capacitance of 20.86 F/cm2 (1032 F/cm3) at a current density of 10 mA/cm2. This attributes to the direct growth of hydroxides on porous foam and conductivity of copper, which benefits the electron transport. The asymmetric supercapacitor device exhibits a high energy density of 21.9 mWh/cm3, with superior capacitance retention of 96.55% over 3500 cycles.
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INTRODUCTION The proliferating demand for green and clean energy has excited tremendous research efforts in the design and fabrication of the energy storage devices, in particular, supercapacitor. Recent research has focused on improving the specific capacitance of supercapacitor, but a typical challenge for such a system is to increase the volumetric capacitance.1−3 The only way to increase the volumetric capacitance is higher mass loading with three-dimensional (3D) porous network. Nevertheless, the crucial disadvantage of higher mass loading per cubic centimeter is increasing the dead mass of an electrode.4 The highest volumetric capacitance achieved for hydrated ruthenium oxide5 is around 1000−1500 F/cm3, activated graphene6,7 can reach the capacitance of 200−350 F/cm3, activated carbon8,9 can reach 60−100 F/cm3, and Mxene10,11 can reach 1000 F/cm3 but only with low mass loading or in thin film. The two-dimensional solids are gaining tremendous attention, in particular, layered double hydroxide (LDH) with a general formula of [M12+−x M3+x(OH)2]12−22 (M2+and M3+, the bivalent and trivalent metal cations, respectively), due to the double benefit of their large active surface area23−25 and high density of redox active species. Metal hydroxides had been studied and proved to be a promising candidate for © 2018 American Chemical Society
supercapacitors exhibiting gravimetric capacitance exceeds that of reported metal oxide materials. However, volumetric capacitance is limited due to lower mass loading of active material. With increasing mass loading per centimeter cubic, metal oxide/hydroxides show decrease in cyclability as well as capacitance owing to lower surface active sites and etching of active material. To date, all oxide/hydroxides have been studied in thin film with lower mass loading.26−30 Layered double hydroxide (LDH) nanosheets as electrode for supercapacitor application have raised particular interest.30,31 Several LDHs have been studied, including Ni−Co LDHs, Ni−Mn LDHs, Ni−Fe LDHs, and Cu−Co LDHs.31−38 Although there are some publications on preparations39−43 and applications of Cu−Co LDHs, to our knowledge, the higher mass loading of Cu−Co LDHs for high areal/volumetric supercapacitor application has not been reported so far. The higher stability, good conductivity, excellent electrochemical activity, and multiple functionality over other metal hydroxides have let the Cu−Co LDHs to be an ideal electrode for supercapacitor application. The copper-cobalt double hydroxReceived: March 29, 2018 Accepted: June 18, 2018 Published: July 2, 2018 7204
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Figure 1. (a) Schematic representation of the synthesis procedure of L-CCH electrode showing different mass loadings. (b, c) Microscopic images of L-CCH electrode confirm the high mass loading with interconnected layered growth of double hydroxide. (d) Fourier transform infrared spectra of L-CCH electrode shed the light on types of bonding and surface groups. (e) X-ray diffraction spectra of L-CCH, which is a combination of copper and cobalt hydroxide results in double hydroxide electrode. (f) Energy-dispersive spectrum for L-CCH, which shows the pure formation of double hydroxide.
change in concentration of solute has been preferred over time to have higher density of L-CCH electrode instead of growth in the layered structure (Figure 1a). The growth mechanism and morphology of L-CCH network is well understood from the field emission scanning electron microscopy (FESEM) images that reveal the growth of 2D layered like interconnected structure (Figure 1b). The insightful observation indicates the dense network of L-CCH nanosheets that are approximately 100 nm in width and few microns in length (Figure 1c). This interconnected structure can play an important role in increasing the surface area and electrochemical sites for absorption. Further energy-dispersive spectroscopy (EDS) analysis was carried out, which confirms the presence of cobalt, copper, and oxygen, as shown in Figure 1f. The detailed spectroscopic analysis of L-CCH structure was explored by Fourier transform infrared spectroscopy (FTIR) (Figure 1d). The peaks at 3134 and 3438 cm−1 are assigned to O−H stretching vibration of water molecules in the interlayer and hydrogen-bonded hydroxyl group of copper-cobalt hydroxide. The other absorption bands observed below 700 cm−1 are associated with metal oxygen stretching and bending modes. The spectrum shows a distinct peak at 524 and 541 cm−1, indicating the absorptions associated with Cu−O stretching and Cu−OH bending vibrations, whereas that at
ides reported in this study were prepared by growing coppercobalt double hydroxide higher mass loading on the nickel foam with porous and layered nanostructures for supercapacitors. Here, we report a two-dimensional copper-cobalt double hydroxide with different mass loadings per centimeter square, demonstrating the improved electrochemical properties when used as electrode material in a supercapacitor. The unique combination of double hydroxide electrode with a high mass loading of 26 mg/cm2 exhibits a high volumetric capacitance of 1032 F/cm3 at a current density of 10 mA, which will open up new opportunities to develop a high volumetric supercapacitor and a new field in double hydroxide materials.
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RESULTS AND DISCUSSION A schematic representation of the deposition process of highdensity layered copper-cobalt hydroxide (L-CCH) architecture is shown in Figure 1. The electrodeposited L-CCH electrode results in the formation of a porous 3D-interconnected twodimensional layered framework with a large oxygen-containing functional group, which was beneficial for increasing the electrochemical sites within the network. The mass loading of the L-CCH can easily be tuned with a variation of concentration of solute in the electrodeposition process. The 7205
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Figure 2. (a) Cyclic voltammetry curve at 5 mV/s for different mass loadings. (b) Galvanostatic charge/discharge curves of different mass loadings at 20 mA/cm2 current density. (c) Cyclic voltammetry curves recorded at various scan rates for higher areal density electrode. (d) Galvanostatic charge/discharge curve of higher areal density electrode at different current densities. (e) Variation of areal and volumetric capacitance with current densities calculated from galvanostatic charge/discharge curve. (f) Cyclic stability performance for 2000 charge/discharge cycles; the inset figure shows last 10 charge/discharge cycles. (g) Ragone plot of the estimated volumetric energy density and volumetric power density at various charge/ discharge rates.
511 cm−1 corresponds to the Co−O stretching vibration.44 Figure 1e shows the X-ray diffraction (XRD) pattern of the LCCH electrode, which shows the 2θ values at 13°, 26°, 33°, and 36°, corresponding to (001), (002), (120), and (121) lattice plane, corresponding to copper hydroxide (JCPDS 420746), and the peaks observed at 43°, 51°, 59°, and 74° 2θ values, corresponding to (200), (102), (003), and (311) lattice plane, which are in good agreement with the XRD data of cobalt hydroxide of JCPDS 300443. The average grain size was estimated by using the Debye−Scherer formula: D = 0.9 λ/β cos(θ), where λ is 1.54A° for the wavelength of Cu Kα radiation, “β” is full width at half-maximum, and “θ” is the Bragg angle. The average grain size was found to be in the range of 100 nm, which is in agreement with the SEM results. Layered double hydroxides (LDHs) have been extensively studied as pseudocapacitive material and acknowledged as superior to metal oxides due to higher electrochemical activity. However, the performance of hydroxides/oxides completely depends on the electrode film thickness, which limits their use in requisite high volumetric or areal capacitance devices.
Moreover, thick-film electrode without insulating binder deteriorates the performance of metal oxide/hydroxide due to collapse of 3D architecture as well as limited ionic transport in the inner volume of the thicker electrode. To enhance the volumetric capacitance of L-CCH electrode, a higher mass loading of 26 mg/cm2 electrode was examined. In the initial experiments, we found that the volumetric capacitance of lower mass loading L-CCH1 is negligible compared to volumetric capacitance of high mass loading of L-CCH3. First, the electrochemical properties of different areal density L-CCH electrodes were tested in 2 M KOH electrolyte. The electrically conductive layer allows the thicker film with the high mass loading formation without any binder. The obtained cyclic voltammetry (CV) curves in 2 M KOH from 0.0 to 0.45 V potential versus Ag/AgCl are shown in Figure 2a. It is clearly seen from the typical CV curves of L-CCH electrode that the capacitance increases by a factor of 10 in the higher-arealdensity electrode. Galvanostatic charge/discharge measurement (Figure 2b) was carried out for the L-CCH1, L-CCH2, and L-CCH3 electrode at a current density of 20 mA/cm2, 7206
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Figure 3. (a) Plot of 1/Q* against V1/2 to find the total charge (Q*) stored by the electrode material. (b) Plot of Q* against V−1/2 to find the charge stored only on the outer surface of the electrode material (Q*outer). (c) Contributions of double-layer capacity and diffusion-controlled redox capacity to the total capacity of the electrode. (d) Plot of log I and log V to find the value of b (b = 0.53, 0.75, and 0.71 for L-CCH1, L-CCH2, and L-CCH3)
of L-CCH electrode. The cyclic stability of the electrode was examined for >2000 charge/discharge cycles at a very high current density of 60 mA/cm2 (Figure 2f). Usually, at higher current density, the electrode stability decreases rapidly. However, in this study, the capacity retention of 80% was found after 2000 cycles at 60 mA/cm2. To elucidate the charge/discharge kinetics and semirectangular behavior of L-CCH electrode for lower and higher mass loading, the method given by Trasatti45,46 et al. was employed. According to this method, the total charge storage of an electrode is a sum of contribution due to an inner surface, where faradic reaction occurs from the slow H+ ion donating species, and the outer surface, where redox reaction can occur from fast charge transfer. The capacitive charge (including both pseudocapacitance and electric double-layer capacitance) depends on the potential scan rate given by Trasatti in a way as follows
which shows the discharge time of 12.63, 32.72, and 192 s, respectively, suggesting the formation of more redox sites in higher mass loading, which is in agreement with CV results. Figure 2c shows the CVs of L-CCH3 electrode at a scan rate ranging from 5 to 100 mV/s showing the semirectangular shape retained even at a very high scan rate of 100 mV/s. The semirectangular shape may be explained by the existence of double-layer capacitance as well as pseudocapacitive behavior. The possible electrochemical faradic process of L-CCH electrodes in 2 M KOH as electrolyte is CoOOH + OH− ↔ CoO2 + H 2O + e−
(1)
CuOH + OH− ↔ Cu(OH)2 + e−
(2)
To study the areal and volumetric capacitance of the L-CCH3, electrode galvanostatic charge/discharge measurements were performed at 10, 20, 30, 40, 50, 60, 80, 102, 122, 204, and 306 mA/cm2 in 2 M KOH, shown in Figure 2d. A plot of current density verses areal/volumetric capacitance for the high mass loading (L-CCH3) electrode is shown in Figure 2e. The excellent volumetric capacitance of 1032 F/cm3 and areal capacitance of 20.86 F/cm2 were measured at a current density of 10 mA/cm2. The capacitance of L-CCH3 electrode was found to be 18.04 F/cm2, which is 18 times higher than that of L-CCH1 electrode (1.06 F/cm2) at the current density of 20 mA/cm2 and retains 50% of capacitance even at a high current density of 100 mA/cm2 (shown in Supporting Information Tables S1 and S2). This high capacitance value at a high scan rate is assumed due to the more porous nature of L-CCH3 electrode, which increases the rate of intercalation/deintercalation of an electrolyte ion in the L-CCH 2D-interconnected layered framework. In addition, it also reveals that the volumetric/areal capacitance is dependent on the mass loading
Q *(v) = Q *outer + const V −1/2
(3)
Q−1*(v) = Q−1*total + const V
(4)
1/2
where Q* is the total charge obtained by integrating the CV curve at different scan rates. The capacitive charge contribution and the total charge contribution can be calculated from the graphs, as shown in Figure 3a,b. In this process, the total charge storage can be calculated from the y-axis intercept of Q* verses V1/2 and the charge stored on the outer surface of the electrode can be calculated from the plot of Q* against V−1/2. Moreover, the difference between the total charge and the charge stored on the outer surface will give the value of the charge stored in the inner surface. As shown in Figure 3a, the value of the maximum total charge stored is found to be 8.3486 C/cm2, 7207
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Figure 4. (a) Nyquist plot (−Z″ vs Z′ plot) for the different areal density electrodes within the frequency range from 0.01 to 100 000 Hz. (b) Bode phase angle plot (phase angle against frequency for different areal density electrodes). (c) Plot of the real part of capacitance against frequency for different areal density electrodes. (d) Plot of the imaginary part of capacitance against frequency for different areal density electrodes. (e) Normalized reactive power |Q|/|S| and reactive power |P|/|S| vs frequency plots (f) Nyquist plot (−Z″ vs Z′ plot) for L-CCH3 before and after 2000 cycles; the inset shows the equivalent circuit used for fitting the data. (g) Bode phase angle plot before and after cycles for L-CCH3
which is equivalent to the maximum areal capacitance ∼20.86 F/cm2 for the potential window 0.45 V. Furthermore, the charge stored at the outer surface, (Figure 3b) was found to be 1.606 C/cm2 (∼3.56 F/cm2) and the charge stored in the inner surface (diffusion-controlled redox capacity) was found to be 6.7426 C/cm2 (∼14.98 F/cm2). From Figure 3c, it is clear that the diffusion-controlled redox capacity increases on increasing the mass loading. The L-CCH3 electrode exhibited 80.77% diffusion-controlled redox capacity, whereas L-CCH2 and L-CCH1 exhibited 45.94 and 43.66%, respectively. Increase in the total charge might be the reason for the corresponding higher areal and volumetric capacitance for LCCH3 electrode compared to that of L-CCH2 and L-CCH1. It is speculated that the surface Grotthuss mechanism18 is responsible for the diffusion of H+ ions to the inner region surface. The total electrochemical charge storage can be obtained by integrating the CV curve, and this charge originates from two contributions: the faradic contribution limited by ion diffusion together with fast charge-transfer process at the surface and the
non-faradic contribution from the fast electric double-layer effect. This effect can be analyzed by the CV curve at the different scan rates using the power law47−49 i = aVb, where “a” and “b” are adjustable parameters. b value can be determined by the slope of log i against log V. Ideally, if b value is ∼0.5, then the capacity response is diffusion controlled and it satisfies Cottrell’s equation9 i = V1/2 and b = 1 shows purely capacitive response (including both double-layer capacitance and diffusion-controlled redox capacitance). From Figure 3d, the b value of L-CCH3 is found to be 0.53, which demonstrates the dominancy of diffusion-controlled redox capacitance, which agrees with our results. For L-CCH1 and L-CCH2, b values were 0.75 and 0.71, respectively, which shows the capacitive response. This confirms the higher mass of our LCCH3 electrode increases the diffusion-controlled redox activity and is dominant in this study. The electrochemical impedance spectroscopy (EIS) is an important characterization tool to evaluate the resistive behavior of the electrode. Figure 4a shows the respective Nyquist plots for three different electrodes from f = 0.01 to 10 7208
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Figure 5. (a) Cyclic voltammogram of carbon cloth as a negative electrode and L-CCH3 as positive electrode at 5 mV/sec. (b) Cyclic voltammetry curves of an asymmetric L-CCH3//CC electrode at different scan rates. (c) Galvanostatic charge/discharge curves at different current densities from 3 to 10.5 mA/cm2. (d) Plot of areal and volumetric capacitance as a function of current densities. (e) Cyclic performance during 3500 charge/ discharge cycles. It is clearly seen from the figure that only 0.1% loss is observed during cyclic performance; inset figure shows the last 10 charge/ discharge cycles. (f) Nyquist plot (−Z″ vs Z′ plot) of L-CCH3//activated carbon cloth (ACC) electrode within the frequency range from 0.01 to 100 000 Hz. (g) Plot of real and imaginary capacitance against frequency. (h) Normalized power against frequency. (i) Ragone plot showing the relationship between the volumetric energy density and power density of a typical electrolytic capacitor, supercapacitor, Li thin-film batteries, and our prepared L-CCH3//ACC electrode.
000 Hz, where Z′ is the real part and Z″ is the imaginary part of the impedance, respectively. The Nyquist plots consist of three characteristic regions:50,51 (1) a low-frequency region that is represented by an inclined line along the imaginary axis which shows capacitive behavior also known as double-layer capacitive region; (2) a high-frequency region represented by a partial semicircle which shows the blocking behavior of the supercapacitor (in this region, the supercapacitor behaves as a pure resistor and the resistance values determines the chargetransfer and series resistance of the electrode); and (3) a middle-frequency range that shows the effect of electrode thickness and the porosity on the diffusion of ions from electrolyte to electrode. The inset of the Figure 4g shows the equivalent circuit [R(C{RQ})W] used for the fitting curve of L-CCH3. Rs is the equivalent series resistance of an electrolyte, Rct is the charge-transfer resistance during the faradic reaction, Cd is the electric double-layer resistance, Zw is the Warburg impedance that represents the impedance of the diffusion
controlling process in the electrolyte, and CPE is the constantphase element. Electrochemical impedance is defined as51,52 ZCPE = [Y0(jω)n]−1, where the unit of Y0 is 1 S/cm2 and is independent of frequency and n is the exponent whose values varies between −1 and +1; when n = −1, the CPE is pure inductor; when n = 0, the CPE behaves as a pure resistor; and when n = 1, CPE is pure capacitor. In our experiment, the CPE value for L-CCH3 is found to be n = 1.0, which shows that CPE behaves like a pure capacitor, which is in good agreement with our results. Nyquist plots clearly show that in the highfrequency region, L-CCH3 has a lower series resistance value (Rs = 0.59 Ω) than that of L-CCH1 and L-CCH2 (Supporting Information Figure S1), which implies that the resistance of an electrolyte solution is lesser in L-CCH3 electrode. Moreover, the impedance spectrum of L-CCH3 shows a smaller semicircle (Rct = 0.11 Ω) than that that of the L-CCH1 and L-CCH2, which suggests that the L-CCH3 has a good electrical conductivity between the prepared electrode and current 7209
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lower mass loading. The obtained relaxation times for L-CH1, L-CCH2, and L-CCH3 were 1, 2, and 12 s, respectively. To study the practical application of high mass loading LCCH3 architecture, an asymmetric supercapacitor is fabricated by using L-CCH3 as a positive electrode and the activated carbon cloth (ACC) as a negative electrode. Figure 5a shows the CV curves of carbon cloth within a 0.0 to 1.0 V and LCCH3 electrode within a 0.0 to 0.45 V voltage window. The prepared L-CCH3//ACC asymmetric device operates at a higher voltage window from 0.0 to 1.2 V at different scan rates ranging from 10 to 200 mV/s, exhibiting the combined behavior of carbon cloth and L-CCH electrode (Figure 5b). The CV curves show obvious reduction oxidation peak within a 0 to 1.2 V potential window. Interestingly, with the increasing scan rate, no major shifts are observed in the oxidation reduction peak, which shows the device ability to sustain a high charge/discharge rate and an excellent ability of ion diffusion into the electrode surface. Figure 5c shows the galvanostatic charge/discharge curve of L-CCH3//ACC at various current densities from 3 to 10.5 mA/cm2. The areal capacitance values calculated from charge-discharge (CD) curves are 2.43, 2.32, 2.24, 2.18, 2.14, 2.04, and 1.98 F/cm2 and 109.73, 104.38, 100.81, 98.14, 96.35, 92.11, and 89.22 F/ cm3 at the current densities of 3, 4.5, 6, 7.5, 9, 10.5, and 12 mA, respectively (shown in Supporting Information Table S4). The comparative study of different areal capacitance values and their energy densities is shown in Supporting Information Table S5, which shows the highest performance of L-CCH electrode in terms of area/volumetric capacitance as well as energy density. The electrochemical impedance spectroscopy of an asymmetric device is also studied to gain the understanding of electrochemical behavior. Figure 5d represents plot of areal and volumetric capacitance as a function of current densities and Figure 5e shows the cyclic performance during 3500 charge/discharge cycles. Figure 5f shows the Nyquist plot and the corresponding equivalent circuit. The asymmetric device shows a very low series resistance Rs = 0.9 Ω and a chargetransfer resistance of 2.11 Ω. The relatively low values of Rs and Rct represent the higher accessibility in the diffusion of ions in the electrolyte to the electrode material during charge/ discharge cycles, which is responsible for an excellent electrochemical performance of an asymmetric device. To study the stability of an asymmetric device, the cycle stability tests were carried out at 8.33 mA/cm2 current density, shown in Figure 5e. The asymmetric device exhibits an excellent cyclic stability with capacitive retention of 96.55% after 3500 cycles. Figure 5i shows the Ragone plot of several commercially available energy storage devices. It can be seen from the plot that the energy density of L-CCH3//ACC supercapacitor reached close to the energy density of Li thin-film batteries and power density approached close to the power density of the 25 mF supercapacitor. The imperative feature of this device is a very small characteristic relaxation time constant τ0 (the minimum time required to discharge all energy from the device), which is only 12 s. Thus, the areal/volumetric capacitance values described above are probably near to the maximum values possible for LDH materials in general. Thus, the fact is that high volumetric and areal capacitance of L-CCH material may also enable LDHs use in supercapacitors. Thus, this work opens up exciting possibilities of developing higher mass loading of LDH electrode supercapacitor devices using a large variety of combinations of metals and their chemistries.
collecting electrode, as well as lower charge-transfer resistance, which is attributed to the significant increase in the capacitance value of L-CCH3. Additionally, in the low-frequency region, the ideal straight line implies a higher rate of electrolyte diffusion and mass transfer. Figure 4c shows the good access to the electrolyte and ionic diffusion even after 2000 cycles attributed to the excellent electrical conductivity and very low internal resistance (Figure 4b). The Bode plots show phase angle values obtained for higher mass loading electrode is close to 74, which is almost near to that of the ideal capacitor, suggesting that the prepared material is suitable for the fabrication of low-leakage capacitor. Moreover, no significant change occurs in the phase angle after 2000 cycles. The EIS data strongly supports the excellent electrochemical activity and high rate capability. The relaxation time constant τ is a measure of how fast the device can discharge and obtained from the analysis of the complex plots. The complex capacitance is expressed as C(ω) = C′(ω) + jC″(ω), where C′(ω) is the real part of the capacitance that shows the amount of stored energy in farad and C″(ω) is the imaginary part of the capacitance that shows the energy dissipation as a function of the angular frequency and they are given by22 C′(ω) = −Z″(ω)/ω |Z(ω)|2
(5)
C″(ω) = Z′(ω)/ω |Z(ω)|2
(6)
where Z′ and Z″ are the real and imaginary part of the impedance, ω = 2πf, and Z(ω) = Z′(ω) + jZ″(ω), where ω is the angular frequency. The complex power is defined as S(ω) = P(ω) + jQ(ω), where P(ω) is the real part of complex power called active power and Q(ω) is the imaginary part of the complex power called reactive power, and they are given by P(ω) = ωC″(ω)|ΔVrms|2
(7)
Q (ω) = −ωC″(ω)|ΔVrms|2
(8)
where ΔVrms = ΔVmax/√2 and ΔVmax is the maximum value of the potential (here, 0.45 V). From Figure 4e, the relaxation time constant (τ = 1/f) can be calculated from either the frequency corresponding to the half of the maximum value of the real part of capacitance, that is, from C′(ω) against the frequency plot or peak frequency corresponding to the C″(ω) against frequency plots. As shown in Figure 4c, the real part of the capacitance decreases as the frequency increases, which is a characteristic of the electrode material and the electrode/ electrolyte interface. The more detailed characteristics of relaxation time constant can be obtained from the normalized power against frequency plot. Figure 4f shows the plot of the real part of power P/S and imaginary part Q/S against frequency plots. The power dissipated into the systems can be analyzed from the active power P/S. The impedance behavior of the supercapacitor varies from the ideal capacitive behavior at a low frequency where no power is dissipated to the pure resistive behavior at a high frequency where P = 100%. In fact, P/S and Q/S show the opposite trend with respect to the frequency. The crossing of the two plots P/S and Q/S appears at ϕ = 45° and will give the value of the frequency corresponding to the relaxation time. It defines the capacitive behavior at frequencies below 1/τ and resistive behavior at frequencies above 1/τ. From the crossing point of the two plots, the relaxation time has been calculated for higher and 7210
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Electrochemical Measurement. Electrochemical performance of L-CCH electrodes was studied by using a threeelectrode configuration and 2 M KOH as an electrolyte. Platinum foil and silver/silver chloride electrode are used as a counter and a reference electrode, respectively. Cyclic voltammetry was performed on a Metrohm Autolab-128N potentiostat. The electrodeposited L-CCH electrodes were used as a working electrode in a three-electrode system. CV measurement was carried out at different scan rates in the potential window 0 to 0.45 V. The charge/discharge study was performed at different current densities of 10, 20, 30, 40, 50, 60, 80, 102, 122, 142, 204, and 306 mA/cm2 in a potential window of 0 to 0.45 V.
CONCLUSIONS In summary, a simple and reproducible method has been adopted to demonstrate the electrochemical performance of copper-cobalt hydroxide with higher mass loading in aqueous electrolyte. Benefits of the double metal hydroxide and 2D structure contribute to the ultra-high volumetric/areal pseudocapacitive performance. The 2D hydroxide structure reveals an enhanced volumetric capacitance response of 1032 F/cm3. The enhanced capacitance was due to the high mass loading layered structure with porous structure leading to more electrochemical sites for redox response, as well as high ionic diffusion and better electron conductivity of copper in coppercobalt hydroxide electrode. The prepared asymmetric supercapacitor showed a high areal and volumetric capacitance of 2.43 F/cm2 (109 F/cm3) at 2.41 mA/cm2 and exhibited an excellent cyclic stability with 96.55% capacitive retention after 3500 cycles at the current density of 8.33 mA/cm2. Thus, this effective method of fabricating a freestanding metal hydroxide/ carbon asymmetric device paves the way for practical application of an energy storage device.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00596. Performance data for three independent CCH pseudocapacitive electrodes, areal and volumetric capacitance at different current densities from 10 to 122 mA/cm2, performance comparison of copper-cobalt hydroxide pseudocapacitor with oxides, areal and volumetric capacitance of an asymmetric device at different current densities, performance comparison of L-CCH//ACC two-electrode pseudocapacitor, equivalent circuit of LCCH-1 and L-CCH-2, CV curves at different cell voltages at a scan rate of 100 mV/s, CD curves at different cell voltages, plot of the bode modulus and phase angle against frequency for two-electrode, CV curve before and after 3500 cycles at a scan rate of 10 mV/s (PDF)
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METHODS Synthesis of L-CCH/Ni-Foam Electrode. All chemicals were of analytical grade and used without further purification. Before depositing copper-cobalt LDHs, nickel foam was washed with 0.1 M HCl solution for 3 min and cleaned with deionized water to remove NiO layer from its surface. The cleaned nickel foam was dried in a vacuum oven overnight at 60 °C and used as the working electrode (1.5 cm × 1 cm). Platinum and Ag/AgCl were used as the counter and reference electrode, respectively. The Cu−Co LDHs were electrochemically deposited by dissolving 4 mM Cu(NO3)2·3H2O and 8 mM of Co(NO3)2·6H2O in 30 mL of distilled water at a constant voltage of −1 V for 300 s. Further, the electrodeposited Cu−Co LHDs were cleaned with distilled water and dried in vacuum oven overnight at 80 °C. The mass loading of the electrode was measured before and after electrodeposition. The same procedure is repeated by taking different concentrations of copper nitrate trihydrate and cobalt nitrate hexahydrate to prepare a different mass loading electrode to compare the results. The different mass loading electrode of 4.69, 8.77, and 26.53 mg/cm2 are denoted as L-CCH1, LCCH2, and L-CCH3, respectively. Characterization. Powder X-ray diffraction (PXRD) patterns were recorded with a PAN-analytical diffractometer equipped with Cu Kα (λ = 1.5405 Å) X-ray source in geometry and 0.5 s dwelling time with a proportional detector. An antiscattering incident slit (2 mm) and nickel filter were used. The tube voltage and current were 40 kV and 40 mA, respectively. Scanning electron microscopy (SEM) was performed on a JEOL JSM 7610F FESEM equipped with energy-dispersive spectroscopy (EDS) Oxford instruments, X-Max with coating unit (make: JEOL, model: JEC-3000FC). The EDS mapping was obtained at low magnification at random points of the electrodeposited film. Fourier transform infrared spectra were obtained from the Perkin Elmer Spectrum one instrument. All electrochemical measurements were carried out using a Metrohm Autolab 128N potentiostat (Netherland) with 2 M KOH aqueous electrolyte in electrochemical cell. Ag/AgCl and platinum foil were used as pseudo reference electrode and counter electrode, respectively, whereas electrodeposited LCCH was the working electrode.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (A.D.D.). *E-mail:
[email protected] (B.K.G.). ORCID
Bipin Kumar Gupta: 0000-0002-0176-0007 Author Contributions
All authors contributed to the experimental design and data analyses. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.D.D. acknowledges the financial support from Naval Research Board (NRB), DRDO New Delhi (NRB Sanctioned no: DRDO/NRB/4003/PG/338). A.D.D. acknowledges ENVIRON Care Product for providing the activated carbon cloth (ACC) for this research work. A.D.D. also acknowledges Celgard, LLC, North Carolina for their support in making available the material for our research.
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