Pseudocapacitance of Mesoporous Spinel-Type MCo2O4 (M = Co, Zn, and Ni) Rods Fabricated by a Facile Solvothermal Route.

We present the structural properties and electrochemical capacitance of mesoporous MCo2O4 (M = Co, Zn, and Ni) rods synthesized by a facile solvothermal route without necessity to use templates. The Brunauer-Emmett-Teller specific surface areas of these mesoporous rods are found to be about 24, 54, and 62 m2 g-1 with major pore diameters of about 31, 15, and 9 nm for MCo2O4, M = Co, Zn, and Ni, respectively. X-ray photoelectron spectroscopy and X-ray diffraction studies reveal the phase purity of the samples with a predominant spinel-type crystal structure. The spinel crystal structure with lattice parameters of 8.118, 8.106, and 8.125 Å is obtained for MCo2O4, M = Co, Zn, and Ni, respectively. The transmission electron microscopy study reveals that the mesoporous rods are built by self-assembled aggregates of nanoparticles which are well-interconnected to form stable mesoporous rods. The electrochemical capacitor performance was investigated by means of cyclic voltammetry, galvanostatic charge/discharge cycling, and impedance spectroscopy in a three-electrode configuration. As a result, the spinel-type MCo2O4 rods exhibit high specific capacitances of 1846 F g-1 (CoCo2O4), 1983 F g-1 (ZnCo2O4), and 2118 F g-1 (NiCo2O4) at a scan rate of 2 mV/s. Furthermore, the mesoporous spinel-type metal oxides show desirable stability in alkaline electrolyte during long-term cycling with excellent cycling efficiency.

Introduction

Global demand for energy conversion and storage technology is anticipated to rise rapidly because of the depletion of fossil fuels. Therefore, researchers currently focus on developing electrochemical energy storage technologies for sustainable energies by simple, effective, and eco-friendly approaches.[1−5] Supercapacitors are a new category of energy storage devices. They have attracted much attention because of their fast charge/discharge rate, high power density, high reliability, and long life cycle.[6−9] RuO2 has been investigated extensively because of its high specific capacitance (700 F g–1), high reliability, and good reversibility.[10,11] However, the high cost and environmental harmfulness of RuO2 prevent its commercialization in supercapacitors. Hence, various spinel-type binary metal oxides A2+B23+O4 where A is a divalent cation (Zn, Ni, Mn, Cu, etc.) and B is a trivalent cation (Co, Fe, Mn, etc.) have been recently investigated as pseudocapacitors.[12−21] Among the spinel-type metal oxides, the cobalt-based metal oxides show better electrochemical properties.

Xiong et al. have investigated the pseudocapacitor performance of Co3O4 microspheres, and a specific capacitance (Cs) of 92 F g–1 at 5 mA cm–2 is reported.[15] Mousavi and co-workers have studied cauliflower-shaped nanostructured CuCo2O4 with the Cs of 338 and 88 F g–1 at current densities of 1 and 50 A g–1, respectively.[16] Krishnan et al. have studied the supercapacitance of nanostructured spinel-type MCo2O4 (M = Cu, Mn, and Mg) obtained by the molten salt method. The Cs values of MgCo2O4, MnCo2O4, and CuCo2O4 are 160, 288, and 152 F g–1, respectively, at a current density of 2 A g–1.[17] Zhou et al. have fabricated one-dimensional (1D) ZnCo2O4 porous nanotubes by electrospinning. They have reported the Cs of ZnCo2O4 to be 770 F g–1 at a current density of 10 A g–1 with excellent rate performance and good cycling stability.[18] Wu et al. have investigated binder-free ZnCo2O4 nanorods on a Ni fiber with a Cs of 10.9 F g–1 at 30 mV/s (scan rate).[19] Dong and co-workers have reported a selective preparation route for hierarchically mesoporous NiCo2O4 with a Cs of 1619.1 F g–1 at a current density of 2 A g–1.[20] Furthermore, the Cs of mesoporous NiCo2O4 was found to be 571.4 F g–1 at a high current density of 10 A g–1 with excellent stability. Moreover, the nanostructured spinel-type metal oxides are capable of delivering better electrochemical kinetics because of their larger surface area and shorter paths for charge carrier movement in comparison with their bulk counterparts. On the other hand, it is found that metal oxides with appropriate porous or hierarchical nanostructures can enhance the power density and also the cycling stability of supercapacitors.[20−23] Generally, mesoporous materials are fabricated by using soft templates (surfactants or long-chain polymers) and hard templates (mesoporous silica or carbon).[24−26] However, some complications occur for these methods because of the residual templates in the final samples and the use of multistep procedures.

In this work, we report the preparation of naturally self-assembled porous spinel-type MCo2O4 rods (M = Co, Ni, and Zn) by a facile solvothermal method. Structural properties of these samples were investigated by means of powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) with selected area electron diffraction (SAED), and Brunauer–Emmett–Teller (BET) specific surface area analysis. The pseudocapacitance of the mesoporous rods was comprehensively studied for all samples by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) cycling, and electrochemical impedance spectroscopy in a three-electrode configuration. KOH (6 M) was used as an electrolyte. The electrochemical capacitance Cs was 1846 F g–1 for CoCo2O4, 1983 F g–1 for ZnCo2O4, and 2118 F g–1 for NiCo2O4 at a scan rate of 2 mV/s, and an excellent cycling stability was observed.

Results and Discussion

Figure a shows the TGA–differential thermal analysis (DTA) curves for the MCo2O4 samples obtained at 80 °C. TG curves show two well-distinct steps of weight loss for all samples. The first weight loss occurs at temperatures up to 190 °C for CoCo2O4, 184 °C for ZnCo2O4, and 220 °C for NiCo2O4 which is due to the evaporation of H2O. The second weight loss occurring at temperatures up to 300, 405, and 405 °C for CoCo2O4, ZnCo2O4, and NiCo2O4, respectively, is associated with an exothermic peak in the DTA curve (the DTA curve of CoCo2O4 only shown for clarity) and is assigned to the decomposition of the anhydrous oxalate into spinel-type metal oxides. The evaporation of CO and CO2 by decomposing the oxalates leads to the formation of porous nanostructured materials. Furthermore, the distribution of pore size and microstructure of the samples are investigated by the Barrett–Joyner–Halenda (BJH) model and HR-TEM. The formation of mesoporous rods is schematically illustrated in Figure b. During the initial synthesis process, the divalent (M2+ = Co2+/Ni2+/Zn2+) cations and trivalent Co3+ cations are interlinked with ethylene glycol (EG) and H2C2O4 molecules to form the nanoclusters. Then, a large number of small crystals start to nucleate and form the MCo2(C2O4)3 oxalates which is confirmed by XRD, as shown in Figure c. EG acts as a cross-linking reagent to form the rod-shaped products.[30] Finally, the rods are heated in air to transform to the porous spinel-type metal oxide rods by the evaporation of CO and CO2.

Figure 1

(a) TGA/DTA curves of product precursors. (b) Schematic illustration of the formation of the mesoporous MCo2O4 rods where M = Co, Zn, and Ni. (c) XRD pattern of the mesoporous MCo2O4 rods.

The XRD patterns of the MCo2O4 (M = Co, Ni, and Zn) samples obtained by a facile solvothermal approach and a post-annealing approach at 450 °C for 2 h are shown in Figure c. All the peak positions at 2θ values of 31.16°, 36.86°, 38.64°, 44.80°, 55.60°, 59.30°, 65.24°, and 77.42° match well with the spinel-type structure (JCPDS #73-1701), and these peaks correspond to the following Miller indices (hkl): (220), (311), (222), (400), (422), (511), (440), and (533), respectively. These results reveal that the samples contain exclusively the spinel phase. The unit cell parameter a for all samples is determined using the formula a = d(h2 + k2 + l2)1/2. The lattice parameters of 8.118, 8.106, and 8.125 Å are obtained for the MCo2O4 rods where M = Co, Zn, and Ni, respectively. The (311) peak was used for the calculation of the mean crystallite sizes (D) of the samples by the Debye–Scherrer formula[31]D = kλ/β cos θ, where β is the full width at half-maximum of the (311) peak, θ is the Bragg angle, k is the structure factor, and λ = 0.1541 nm is the wavelength. The mean crystallite sizes of 30, 17, and 8 nm are obtained for MCo2O4, M = Co, Zn, and Ni, respectively, which are confirmed by TEM analysis.

The morphology of the samples was investigated by SEM. The SEM images of the MCo2O4 (M = Co, Zn, and Ni) samples obtained at 450 °C are shown in Figure a–c. Bundles of porous rods (log-like structure) are observed for CoCo2O4 and ZnCo2O4 (Figure a,b), whereas well-defined needlelike rods are observed for NiCo2O4 (Figure c). The length of these rods is found to be 5–10 μm for all samples. Figure a–f reveals the microstructural features of these samples investigated by TEM. The TEM images (Figure a–c) show the agglomerated rods (thickness from few nanometers to few microns) with a length of several micrometers. The SAED patterns (inset of Figure d–f) clearly reveal bright spots on the diffraction rings, indicating the nanostructure of the samples. The electron diffraction rings represent the (111), (220), (311), (400), (511), and (440) planes of the spinel structure which is in good agreement with the XRD results. Further, HR-TEM images (Figure d–f) also reveal that the rods are built by self-assembled nanoparticle aggregates which are well-interconnected to form stable mesoporous rods. The interplanar distances of MCo2O4 mesoporous rods (inset of Figure d–f) are found to be 0.245 nm for CoCo2O4, 0.244 nm for ZnCo2O4, and 0.245 nm for NiCo2O4 which matches well with the (311) plane of the spinel structure. The pore size distributions and specific surface area for the MCo2O4 rods were evaluated using the BET theory and the BJH model. The N2 adsorption/desorption isotherms and pore size distribution data for MCo2O4 (M = Zn and Ni) are shown in Figure . The BET data of CoCo2O4 are not included for the sake of clarity. The calculated BET specific surface areas of CoCo2O4, ZnCo2O4, and NiCo2O4 were about 24, 54, and 62 m2 g–1, respectively. The average pore sizes of CoCo2O4, ZnCo2O4, and NiCo2O4 are 31, 15, and 9 nm, respectively. It clearly reveals the mesoporous structure of the samples. The large surface area can effectively enhance the electrochemical reaction and allow better penetration of the electrolyte for the faradaic reaction.

Figure 2

SEM micrographs of the mesoporous MCo2O4 rods: (a) M = Co, (b) M = Zn, and (c) M = Ni.

Figure 3

TEM images of the mesoporous MCo2O4 rods: (a) M = Co, (b) M = Zn, and (c) M = Ni. High-resolution images of the MCo2O4 rods: (d) M = Co, (e) M = Zn, and (f) M = Ni. The inset shows corresponding SEAD pattern and high-resolution lattice image.

Figure 4

Nitrogen adsorption/desorption isotherm and the corresponding pore size distribution (inset) of the mesoporous MCo2O4 rods.

Raman spectroscopy is used to acquire details about the structural changes, lattice disorder, and compositional changes owing to the divalent cation replacement in the mesoporous cobaltite. Figure shows the Raman spectra of the mesoporous spinel-type rods. Raman active bands at 194, 482, 522, 618, and 690 cm–1 corresponding to F2g, Eg, 2F2g, and A1g modes are identified for single-crystal Co3O4 by Hadjiev et al.,[32] whereas Raman active vibration peaks at 186, 460, 505, and 659 cm–1 corresponding to F2g, Eg, F2g, and A1g modes, respectively, are reported for urchin-like structured NiCo2O4 by Umeshbabu et al.[33] The shifts in the Raman active mode of urchin-like structured NiCo2O4 in comparison to those of single-crystal Co3O4 were ascribed to Ni substitution for Co and the presence of the nanostructure. In this work, the vibrational peaks of mesoporous CoCo2O4 rods are observed at 478, 520, 617, and 688 cm–1 corresponding to Eg, F2g, F2g, and A1g modes, respectively, which are in agreement with the literature.[34] For NiCo2O4 rods, the vibration peaks are observed at 475, 516, 613, and 684 cm–1 corresponding again to Eg, F2g, F2g, and A1g modes, respectively. Similarly, the vibrational peaks of ZnCo2O4 rods are observed at 466, 510, 604, and 678 cm–1. The vibrational peaks are shifted toward lower wavenumbers with divalent cation substitution Co > Ni > Zn in the spinel structure. This result further supports the presence of the cubic spinel-type structure for the fabricated mesoporous rods.

Figure 5

Raman spectra of the mesoporous MCo2O4 rods where M = Co, Zn, and Ni.

The surface elemental composition and the component’s oxidation state of the different mesoporous rods were investigated by XPS, and their respective metal 2p spectra are shown in Figure . The corresponding main O 1s peaks at 530.3, 529.9, and 529.8 eV are attributed to the spinel lattice oxygen of CoCo2O4, ZnCo2O4, and NiCo2O4, respectively, and are shown in Figure S1 of the Supporting Information together with the C 1s spectra to prove the only weak topmost contamination.[35−37]

Figure 6

Co 2p, Ni 2p, Zn 2p, and Zn LMM spectra of CoCo2O4 (a), ZnCo2O4 (b–d), and NiCo2O4 (e,f) mesoporous rods. In particular, (a,c,e) Co3+ peaks and satellites: shaded and Co2+ peaks and satellites: gray; (b,d) Zn2+ tetrahedral coordination: gray and octahedral coordination: shaded; and (f) Ni peaks: shaded and loss features: gray.

For the transition-metal 2p multiplet fitting, we agreed to use only the 2p3/2 component of the spin–orbit doublet justified by the sufficient energy gap between both peaks. To address the complex multiplet structure of the third-row transition-metal photoelectron and loss feature multiplets, we follow the approaches of Biesinger and Grosvenor for the interpretation of the Co and Ni multiplets.[38,39] In particular, we used templates of pure Co2+ and Co3+ 2p multiplets to describe the CoCo2O4 compound in Figure a. Because of the strong overlap of the respective photoelectron peaks, the sole attempt to distinguish between the two binding states is to utilize the respective satellite features around 785.6 eV (Co2+) and 789.7 eV (Co3+) as guiding peaks for the respective multiplets. This is corroborated by the pure Co3+ multiplet of ZnCo2O4 in Figure c. The overall Co quantification for CoCo2O4 finally results in a Co3+ amount of 62 at % which is close to the expected content. The Co 2p spectrum of ZnCo2O4 in Figure c reveals, as already stated above, a pure Co3+ multiplet justified by the absence of the Co2+ satellite. By contrast, the corresponding Zn 2p3/2 multiplet in Figure d results in two Zn2+ components at 1021.4 and 1022.8 eV which indicate Zn2+ in tetrahedral and octahedral oxygen coordination, respectively.[40,41] This is supported by the fact that in nanostructured oxide spinels often an almost random arrangement of the different cations on the tetrahedral and octahedral cation sites is observed.[42,43] The chemical state of Zn2+ is additionally corroborated by the Zn LMM Auger peaks in Figure b.

The Co 2p spectrum of NiCo2O4 in Figure e was fitted according to the procedure described for the CoCo2O4 compound. In conclusion, we have to consider the presence of a certain amount of Co2+ because of the pronounced satellite feature at 785.4 eV. Finally, the Co3+ concentration amounts to 59% of the overall Co content. The corresponding Ni 2p spectrum in Figure f was fitted using an adapted template used in the study of Grosvenor et al. and checked by reference measurements using pure NiO (not shown);[27] as for Ni2+ in pure NiO, the ratio between the peaks at 855.9 and at 854.3 eV amounts to 1.5. In the case of the NiCo2O4 rods, this ratio is about 2.8; hence, in conclusion, a distinct amount of Ni3+ has to be considered because the main peak for Ni3+ is expected at around 856.0 eV. However, because of the lack of appropriate Ni3+ reference materials and, therefore, an unavailable template for the respective multiplet, this cannot be assigned unambiguously. Nevertheless, the assumption seems to be evidenced by the corresponding amount of Co2+, suggesting some Co/Ni exchange. From the results described above, the following conclusions can be drawn: (i) The Co3+ content of 62% found for CoCo2O4 is to some extent smaller than 2/3 and might hint at some oxygen deficiency. (ii) NiCo2O4 shows mixed valences for both the transition-metal elements which compensate each other. (iii) ZnCo2O4 shows site exchange for Zn and Co, that is, a partially inverse spinel. The Co2+/Co3+ and Ni2+/Ni3+ redox couples are the major active sites for the electrochemical reactions.

The CV of MCo2O4 (M = Co, Zn, and Ni) mesoporous rods with 6 M KOH as the electrolyte is performed to evaluate the pseudocapacitor performance. Figure a–c shows the CV curves for the MCo2O4 mesoporous rods in a potential window of 0.0–0.6 V versus Ag/AgCl at different scan rates. The well-defined oxidation and reduction peaks are observed in the cathodic and anodic scans in all CV curves, revealing that the capacitance of the mesoporous spinel-type metal oxide working electrodes is primarily based on the redox reaction.[16] All CV curves show an almost identical shape, and the redox current increases with the scan rate. The peaks in the CV are ascribed to the electrochemical redox reactions associated with M–O/M–O–OH (M = Co, Zn, and Ni).[44] The most probable electrochemical redox reactions can be described for the present mesoporous rods in the alkaline electrolyte solution as

Figure 7

CV of mesoporous rods as a function of scan rate for (a) CoCo2O4, (b) ZnCo2O4, and (c) NiCo2O4. (d) Anodic peak current density against the square root of the scan rate for mesoporous MCo2O4 rods.

With increasing scan rate, the anodic and cathodic peak position is shifted toward higher and lower potentials, respectively, which indicates the faradaic reaction.[12] The charge stored in the mesoporous electrode is given by the area enclosed by the CV curve. The Coulombic efficiency (η) is represented as the ratio between the area of oxidation and reduction peaks. The η of mesoporous MCo2O4 rods, where M = Co, Zn, and Ni, estimated from the CV curves at 5 mV/s (scan rate) is 95.23%. The η is enhanced up to 97.78% for a scan rate of 100 mV/s. The specific capacitance (Cs) of the mesoporous rods is determined from the CV data by using the following equation[9]where m is the mass of the active material, ν is the scan rate, (E2 – E1) is the potential window, and i(E) is the current at each potential. The Cs values of these mesoporous rods are found to be 1846 F g–1 for CoCo2O4, 1983 F g–1 for ZnCo2O4, and 2118 F g–1 for NiCo2O4 at a scan rate of 2 mV/s. The Cs values decrease to 1665 F g–1 for CoCo2O4, 1738 F g–1 for ZnCo2O4, and 1911 F g–1 for NiCo2O4 at a scan rate of 50 mV/s. It reveals that ion diffusion is restricted in the surface of the active materials at higher scan rate. Thus, the electric double layer capacitance dominates over the pseudocapacitance at higher scan rate. The faradaic reaction dominates at low scan rates (<10 mV/s) which leads to the enhancement of the specific capacitance because of the more effective usage of the active material in the working electrode. The diffusion of OH– ions is the rate-controlling process which is confirmed by observing a linear relation between the anodic peak current and the square root of the scan rate, as shown in Figure d. Among these mesoporous MCo2O4 rods (M = Co, Zn, and Ni), NiCo2O4 shows the highest specific capacitance because of larger surface area with smaller pore size.

To further investigate the specific capacitance of these samples, GCD experiments were performed. Figure a shows the GCD curves of the working electrodes at a given current density. The asymmetric shape of the GCD curves represents the faradaic nature of the materials. The CoCo2O4 electrode is quickly charged to 0.4 V at 3.57 A g–1 in comparison with the charging time of ZnCo2O4 (3.52 A g–1) and NiCo2O4 (3.30 A g–1) electrodes (51, 160, and 130 s for M = Co, Zn, and Ni, respectively). According to the BJH study, the pore size of the CoCo2O4 rods is higher than that of the other two samples and it facilitates ion movements into the electrode for the electrochemical reaction or screening of the external electric field. While comparing the discharge time of the electrodes (53, 71, and 94 s for M = Co, Zn, and Ni, respectively), the NiCo2O4 electrode shows the higher discharge capacity because of the larger surface area in comparison to the other two electrodes. This large surface area of the electrode allows to store more charge or to access more active sites for the redox reaction.

Figure 8

(a) GCD curves of working electrodes (mesoporous MCo2O4 rods; M = Co, Zn, and Ni) at a given current density. Discharge curves of mesoporous MCo2O4 rods (b) CoCo2O4, (c) ZnCo2O4, and (d) NiCo2O4 at different current densities, and the inset figure shows the specific capacitance vs current density obtained from the discharge curves.

To determine the practical specific capacitance of these electrodes, the charge/discharge measurements at different current densities were carried out, as shown in Figure b–d. All discharge curves exhibit three different regimes: (i) initially, a rapid potential drop occurs owing to internal resistance, (ii) a slow potential decay at intermediate times caused by the faradaic redox reaction, and (iii) finally, a fast potential decay owing to the electric double layer capacitance. Cs is estimated from the discharge curves by using the following expression[9]where I, Δt, m, and Δv are the applied current, discharge time, active mass, and potential difference, respectively. The Cs values of CoCo2O4, ZnCo2O4, and NiCo2O4 are 473 F g–1 at 3.57 A g–1, 590 F g–1 at 3.52 A g–1, and 788 F g–1 at 3.30 A g–1, respectively. The Cs of NiCo2O4 decreases with the current density, as shown in the inset of Figure d, and similar features are observed for the other samples too. The Cs of the NiCo2O4 electrode is superior in comparison with that of the other two electrodes. Considering the BET and HR-TEM results, the larger surface area and highly porous nature of NiCo2O4 are anticipated to result in the higher Cs. A good cycling stability is vital for real capacitor applications. Therefore, we have carried out continuous GCD measurement for about 2000 cycles, as shown in Figure . A few continuous GCD cycles for NiCo2O4 at a current density of 3.30 A g–1 is shown in the inset of Figure . After 2000 cycles, the MCo2O4 electrodes where M = Co, Zn, and Ni are maintained at 94.93, 95.94, and 97.11% of their initial Cs, respectively. It reveals that the mesoporous spinel-type metal oxide rods show the desirable stability in the 6 M KOH electrolyte during long-term cycling with excellent cycling efficiency. In Table , we summarized the Cs values of the samples from the present work in comparison with the Cs of 1D MCo2O4 where M = Co, Zn, and Ni obtained by the template route in the literature.[24,45−48] The Cs value of the present work is superior to some pioneering 1D MCo2O4 electrodes obtained by template routes. In particular, the porous nanorods are built by the aggregates of nanoparticles, and thus, the electrolyte can easily penetrate through the porous 1D structure which helps for efficient redox reactions during the faradaic charge storage mechanism. Hence, the majority of the electroactive sites of the 1D porous MCo2O4 electrodes are utilized efficiently, resulting in higher specific capacitance with better cycling efficiency.

Figure 9

Cycle performance of mesoporous MCo2O4 rods; M = Co, Zn, and Ni at a given density. The inset shows the continuous GCD cycle for NiCo2O4 at 3.30 A g–1.

Table 1

Specific Capacitance (Cs) of the Samples from the Present Work in Comparison with the Cs of 1D MCo2O4 Where M = Co, Zn, and Ni Obtained by the Template Route in the Literature

materials	synthesized by template route	Cs (F g–1)	current density/sweep rate	references
CoCo2O4 nanorods	CTAB as a template	456	1 A g–1	(45)
CoCo2O4 nanowires	SBA-15 as a template	373	3 mV/s	(46)
CoCo2O4 nanorods	H3BTC as a template	262	5 mV/s	(47)
CoCo2O4 nanorods	SBA-15 as a template	253	0.5 A g–1	(48)
CoCo2O4 nanorods	KIT-6 as a template	370	0.5 A g–1	(48)
CoCo2O4 nanofibers	P123 as a template	401	5 A g–1	(49)
CoCo2O4 mesoporous rods	template free	473	3.57 A g–1	present work
CoCo2O4 mesoporous rods	template free	1846	2 mV/s	present work
ZnCo2O4 rods	PVP as a template	382	3 A g–1	(24)
ZnCo2O4 nanoneedle	P123 as a template	367	2 mV/s	(50)
ZnCo2O4 porous rods	template-free route	590	3.52 A g–1	present work
ZnCo2O4 porous rods	template-free route	1983	2 mV/s	present work
NiCo2O4 nanofibers	cotton fibers as templates	981	1 A g–1	(51)
NiCo2O4 nanowires	P123 as a template	743	1 A g–1	(52)
NiCo2O4 nanowires	P123 as a template	722	1 A g–1	(53)
NiCo2O4  nanofibers	PVP as a template	987	1 A g−1	(54)
NiCo2O4 porous rods	template-free route	788	3.30 A g–1	present work
NiCo2O4 porous rods	template-free route	2118	2 mV/s	present work

The electrochemical impedance spectroscopic (EIS) measurements were carried out before and after GCD cycles to investigate the charge-transfer reaction mechanism of the electrodes. Figure a–c shows the complex impedance plots of working electrodes before and after 2000 cycles at a given current density. The following features are observed in the complex impedance spectra. An offset in the real part of impedance (Rs) at high frequencies represents a sum of intrinsic resistance of the active material, contact resistance, and electrolyte resistance. A small semicircle is observed at high to intermediate frequencies. It represents the faradaic charge-transfer resistance (Rct) of the electrochemical reactions between the electrode and the electrolyte. A straight line is observed in the low-frequency region which originates from the diffusive resistance of anions (OH–) from the liquid electrolyte to the solid oxide electrode surface. To quantify the bulk resistance and faradaic charge-transfer resistance of the electrodes before and after the GCD cycles, the EIS data were analyzed with electrical equivalent circuits (ECs) using ZView (version 2.70) software. A representative EC is shown in the inset of Figure d. In the EC, Rs, Rct, W, CPEdl, and CPEps refer to the bulk resistance, faradaic charge-transfer resistance, Warburg impedance, constant phase element of double layer capacitance, and pseudocapacitance, respectively. All electrodes show almost the same bulk resistance before and after the GCD cycles, as shown in Figure d. However, the bulk resistance of the NiCo2O4 electrode is slightly higher than that of the other two electrodes (Figure d). It might be caused by a larger grain boundary resistance between grains as expected because of the smaller grain sizes. The charge-transfer resistance of the CoCo2O4 and ZnCo2O4 electrodes increased after the GCD cycling, indicating that the fading of the faradaic electrochemical reaction may result from the formation of inactive sites or a loss of adhesion of some active material to the current collector. The resistance Rct of the NiCo2O4 electrode increased slightly after cycling. This could be the major benefit of this electrode which possesses a highly porous structure that leads to fast ion/electron transfers at the electrode/electrolyte interface.

Figure 10

Complex impedance spectra of mesoporous MCo2O4 rods in 6 M KOH solution before and after 2000 cycles test for (a) CoCo2O4, (b) ZnCo2O4, and (c) NiCo2O4. (d) Resistance Rs and Rct of mesoporous MCo2O4 rods before and after 2000 cycles test. The inset of the figure shows a representative EC.

Conclusions

The spinel-type MCo2O4 (M = Co, Zn, and Ni) mesoporous rods were successfully synthesized by a simple solvothermal route without surfactants and a post-annealing approach. The length of these mesoporous rods is found to be of several micrometers. The XRD measurements reveal a phase-pure spinel-type crystal structure with lattice parameters of a = 8.118, 8.106, and 8.125 Å for MCo2O4, M = Co, Zn, and Ni, respectively. The HR-TEM study reveals that the mesoporous rods are built by self-assembled nanoparticle aggregates which are well-interconnected to form stable mesoporous rods. The spinel-type MCo2O4 rods exhibit high specific capacitances of 1846 F g–1 (CoCo2O4), 1983 F g–1 (ZnCo2O4), and 2118 F g–1 (NiCo2O4) at a scan rate of 2 mV/s. The mesoporous MCo2O4, M = Co, Zn, and Ni, rods show an excellent cycling stability with 94.93% at 3.57 A g–1, 95.94% at 3.52 A g–1, and 97.11% at 3.30 A g–1, respectively, of their initial Cs even after 2000 cycles. The mesoporous spinel-type metal oxides reveal the desirable stability in the alkaline electrolyte during long-term cycling with excellent cycling efficiency.

Materials and Methods

The spinel-type MCo2O4 (M = Co, Zn, and Ni) mesoporous rods were fabricated by a simple solvothermal approach without the assistance of surfactants. The NiCo2O4 mesoporous rods were prepared by mixing 1 mmol Ni(SO4)2·7H2O (Loba Chemie) and 2 mmol CoSO4·7H2O (Loba Chemie) and dissolving this mixture in EG and deionized water (3:1 v/v ratio) under magnetic stirring. H2C2O4 (3 mmol, Loba Chemie) was added to this solution slowly under continuous stirring for about 1 h at room temperature (RT). The final mixture was poured into an autoclave (Teflon-lined stainless steel) with a volume of 100 mL and hydrothermally treated at 140 °C for 24 h in an oven. The products were collected by centrifugation, washed with deionized water several times, and dried in air at 80 °C for 10 h. The ZnCo2O4 rods were prepared by mixing 2 mmol CoSO4·7H2O and 1 mmol Zn(SO4)2·7H2O (Loba Chemie), and the Co3O4 mesoporous rods were synthesized by taking 3 mmol CoSO4·7H2O, while the other steps were identical to those of the NiCo2O4 rods. Finally, the product precursors were heated at 450 °C for 2 h with a heating rate of 5 °C min–1 and cooled down slowly to RT.

The powder XRD data of the samples were acquired with a MiniFlex-II (Rigaku) diffractometer with λCu = 1.541 Å operating at a voltage of 40 kV and a current of 30 mA. The TGA–DTA were performed using a simultaneous thermal analyzer (Hitachi, STA7000 series) at temperatures from 30 to 1000 °C at a heating rate of 5 °C min–1 in air. The microstructural properties were analyzed by SEM (JEOL JSM-6390LV) and TEM (Tecnai G220 S-Twin and Jeol JEM 2100) with SAED. The specific surface area and pore size distributions were evaluated according to the BET theory and BJH model, respectively (Quantachrome, QUADRASORB SI). The Raman spectra were measured with a Renishaw laser Raman spectrometer.

The XPS measurements were performed using a K-Alpha XPS+ instrument (Thermo Fisher Scientific, East Grinstead, UK). Data acquisition and processing using the Thermo Avantage software is described elsewhere.[27] The mesoporous rods were analyzed using a microfocused, monochromated Al Kα X-ray source with 30–400 μm spot size. The K-Alpha charge compensation system was employed during the analysis, using electrons of 8 eV energy and low-energy argon ions to prevent any localized charge build-up. The spectra were fitted with one or more Voigt profiles. All spectra were referenced to the C 1s peak of hydrocarbon at a 285.0 eV binding energy controlled by means of the well-known photoelectron peaks of metallic Cu, Ag, and Au. For intense peaks and/or peaks clearly evidenced by the peak shape, the binding energy uncertainty was around ±0.1 eV. In the case of weak peaks and no direct justification by the peak shape, the uncertainty was set to ±0.2 eV. The analyzer transmission function, Scofield’s sensitivity factors,[28] and effective attenuation lengths (EALs) for photoelectrons were applied for quantification. The EALs were calculated using the standard TPP-2M formalism.[29]

The electrode slurry was prepared by mixing the MCo2O4 (M = Co, Zn, and Ni) mesoporous rods (80 wt %), activated charcoal carbon black (SD FINE-CHEM, 10 wt %), polyvinylidene fluoride (Alfa Aesar, 10 wt %) with N-methyl pyrrolidinone (Loba Chemie) as a solvent. Initially, the Ni mesh (Alfa Aesar) substrate was cleaned by ultrasonication. Then, the slurry of the electrode materials was pasted on the cleaned Ni mesh substrate as a working electrode, and it was dried at 80 °C for 24 h. Finally, the working electrode was compacted with a hydraulic press (load of 5 t). The working electrode area was 1 cm2 (geometrical area) with an active electrode mass of ∼1 mg. The electrochemical performance was investigated by CV, GCD cycling, and impedance spectroscopy in a three-electrode configuration. KOH (6 M) was used as an electrolyte. A potentiostat/galvanostat (Biologic, SP-240) was used for the electrochemical measurements with EC lab software. Ag/AgCl was used as a reference electrode and a platinum rod was used as a counter electrode.