Synthesis of Ultra long MnO2@CeO2 Core-Shell Hetero Structure Nanowires for Super Capacitor Application
Corresponding author: Lei Wang and Shengliang Zhong, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, 330022, P.R. China; Tel.: +86 791 88120386; fax: +86 791 88120386; E-mail: firstname.lastname@example.org (L Wang); email@example.com (SL Zhong).
With more and more global ecological and environmental attention, the ever-accelerating energy shortage and environmental deterioration have aroused enthusiasm for sustainable and economically affordable energy storage and release devices . Super capacitors are one of most promising electrochemical energy storage devices [2-3]. Compared to traditional capacitors, super capacitors have drawn particular attentions because of their advantageous properties like high rate capability and quick charging and discharging processes. Nevertheless, extant super capacitors have unsatisfactory specific capacitances, conductivities, and energy densities . The reaction of an electrical double layer capacitor (EDLC) is charge separation at the electrode/electrolyte interface, as for pseudo capacitors, the reactions are faradaic reactions . Metal oxides are one of most studied pseudo capacitance materials because they often show swift, invertible redox reactions at the interface of electrode-electrolyte, and they show better specific capacitance than EDLC. For a super capacitor, it is most important to obtain favourable electrode materials in order to realize higher energy density and cycling rates. The optimized electrode materials should have both high capacitance and high operation voltage, because energy density is the product of capacitance and voltage squared . To enhance the specific capacitance and cycle life of electrodes, different sorts of active materials have been tested during the research and development (R&D) of super capacitors. Transition metal oxides, having multiple valence states, are good for redox reactions, which is propitious for working as a pseudo capacitive electrode material . RuO2 is a superlative electrode material for pseudo capacitor devices in the reports that already exist, yet it has been replaced by other materials owing to its high cost and toxicity . Therefore, researchers have turned to other species of metal oxides like CuO , Co3O4, MnO2, MoO3, NiO , and ZnO, etc.
Among these metal oxides, MnO2 has captured much attention due to its lower price, abundance and lower toxicity . Owing to their conspicuous electronic properties compared to bulk materials, significant efforts are being made to synthesize one-dimensional (1D) nanostructured MnO2 materials . Prepared macro porous α-MnO2 nanowire electrodes and reported a maximal capacitance of 280 F g-1. Wei et al.  prepared β-MnO2 nanowires with (NH4)3PO4 and Mn(NO3)2, incubated at 220 °C for 6 h in a hydrothermal environment, and reported the Csp of 453 F g-1 at 0.5 A g-1. Nevertheless, the applications of MnO2 are confined because of its deficient cycling stability, which can result in its crystal expanding and contracting during iterative cycling processes [14-15]. To address the issue, there are efforts aimed at developing a convenient method to prepare ultra-long one-dimension MnO2 nanostructures that would provide more active sites due to their large surface area . Li et al.  synthesized ultra-long MnO2 nanowires by a hydrothermal route and nanowires showed an excellent specific capacitance of 345 F g-1. What’s more, some composite materials consisting of MnO2 together with accessional metal oxides/hydroxides have been studied. For example, combining MnO2 with Co3O4 , NiO/Ni(OH)2, TiO2, and CuO as well as utilizing the synergistic effect with other metal oxides/hydroxides is a high priority for amplifying the electrochemical character. Unlike other traditional transition metal oxide electrode materials, CeO2 has 4f valence electrons (the electronic configuration of Ce3+ is [Xe]4f1), therefore CeO2 exhibits exceptional electrochemical performance. Otherwise, because of the reversible reaction of Ce3+ and Ce4+, CeO2 materials may be applied in electrochemical energy storage [19-20]. CeO2 has negligible diffusion and its cycle capacitance is just what MnO2 needs. Cerium mixed oxides such as CeO2-TiO2  and CeO2–SiO2  were studied. Reports about MnO2/CeO2 are still rare. Zhang et al.  synthesized hierarchical porous MnO2/CeO2 by a hydrothermal method with a Csp up to 274.3 F g-1. However, there is no report on ultra-long MnO2@CeO2 core-shell hetero structured nanowires.
In this work, considering of synergies between MnO2 and CeO2, we report, for the first time, an easy approach to fabricate ultra-long MnO2@CeO2 core shell nanowires and a hydrothermal method at 120 ℃ was selected in order to prepare the precursor. The nanowire structure and sufficient electronic conductivity of MnO2@CeO2 core shell nanowires can facilitate the diffusion of reactants and electrons, thus making the MnO2@CeO2 composites electrochemical performance preferable. Furthermore, the MnO2@CeO2 core shell nanowires exhibit good electrochemical performance, indicating its potential to act as super capacitor electrode material.
Potassium permanganate (KMnO4, 99.5%) and hex methylene tetra amine (HMT) (C6H12N4, 99%) were obtained from Aladdin Industrial Inc. (Shanghai, P. R. China) and used in the synthesis of MnO2 nanowires. Cerium(III) nitrate hex hydrate (Ce(NO3)3·6H2O, 98%) was obtained from Sino pharm Chemical Reagent Co., Ltd. (Shanghai, P. R. China) and used as the cerium source in the hydrothermal process to fabricate MnO2/CeO2 nanowires.
2.2. Synthesis of MnO2 Nanowires and CeO2 nanoparticles
The experimental steps are as follows: 0.1 mM KMnO4 and 3 mM HMT were dispersed into 15 mL deionized (DI) water. The mixture was stirred vigorously for 10 min, moved into a Teflon lined reactor, and held at 120 oC for 6 hours. Just after that, the mixture was allowed to cool to ambient temperature and washed thrice alternately with deionized water and absolute ethyl alcohol, then dried overnight in a vacuum drying oven at 60 °C. The as-synthesized precursor was calcined at 250 °C for 4 h to obtain MnO2 nanowires. CeO2 nanoparticles were obtained by the same procedures.
2.3. Synthesis of MnO2@CeO2 Nanowires
0.015 g MnO2 synthesized in the previous step was dissolved in 15 mL deionized water, sonicated for 50 min to obtain a homogeneous dispersion. With vigorous magnetic stirring, 0.03 g Ce(NO3)3∙6H2O was then added to the above solution at ambient temperature. After sufficient mixing, the mixture obtained was placed in a water bath and then retained there at 90 oC for 1 h. After reaction, the precipitates were gathered and washed thrice alternately with deionized water and absolute ethyl alcohol, respectively. Then, the obtained product was kept dry all night long in a drying oven at 60 oC and stored under these conditions for future use. After calcination for 4 h at 250 oC in air, the as-synthesized product was obtained.
The following instruments were used for characterization: X-ray diffraction (XRD) (D/MAX-2004), scanning electron microscopy (SEM) (LEO SUPRA 55 microscope (ZEISS, Germany)), energy-dispersive X-ray spectroscopy (EDX) (D/MAX-2004), transmission electron microscopy (TEM) (JEOL Ltd., Tokyo, Japan), N2 adsorption-desorption isotherms and pore size distribution (Quantach-rome Instruments, Boynton Beach, FL, USA), X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific Inc., Waltham, MA, USA).
2.5. Electrochemical measurements
The working electrodes were prepared by mixing the active material of CeO2, MnO2 and MnO2@CeO2 nanowires with acetylene black and polyvinylidene fluoride (PVDF) binder at a mass ratio of 8:1:1, then dispersing in ethanol sonicating for 20 min to obtain a slurry, respectively. Thereafter, the slurry was dried at 60 oC for 12 h and then manually rolled into a sheet to form the working electrodes. The mass of per electrode was 1~2 mg. In this study, the electrochemical properties of MnO2@CeO2 were evaluated using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements in a 2 M KOH electrolyte solution in a three electrode setup (with Ni mesh as the counter electrode, Ag/AgCl as the reference electrode, and MnO2@CeO2 as the working electrode) on an electrochemical workstation (CompactStat.h IVIUM Technolo-gies). CV measurements were performed in the potential window of 0.25 V to 0.45 V using scan rates ranging from 1 to 20 mV/s. GCD curves were recorded at different current densities within the potential ranging from 0.25 V to 0.45 V. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range from 0.1 Hz to 100 kHz. In this work, the mass of CeO2, MnO2 and MnO2@CeO2 working electrodes is 1.3 mg, 1.2 mg and 1.2 mg, respectively.
The capacitance of the electrode active material was calculated as follows:
3. Results and discussion
3.1. Structural and morphological studies
Figure1: SEM and TEM images of CeO2 (a-c), MnO2 (d-f) and MnO2@CeO2 (g-i).
Fig. 1 presents SEM and TEM images of CeO2, MnO2 and MnO2@CeO2. CeO2 consists of relatively regular hexagonal Nano sheets with a smooth surface. The SEM image 1(a) and TEM images 1(b) and 1(c) show pure CeO2 Nano sheets with a diameter of around 20 nm. TEM images of the ultra-long MnO2 nanowires are exhibited in Fig. 1(e) and (f). The nanowires are around 10-30 nm in diameter and 20-50 micro meters in length. Compared to ultra-long MnO2 nanowires (~40 nm in length, ~15 nm in diameter) synthesized by Li et al. , these ultra-long nanowires provide more active sites because of their bigger length and diameter (~several micro meters in length, 10-30 nm in diameter) in this work. TEM images of the MnO2@CeO2 are presented in Fig. 1(h) and (i); the MnO2@CeO2 products have a nanowire nanostructure and the surfaces are coarse. The diameter is around 30~80 nm, which is much wider than that of the MnO2 nanowires, indicating CeO2 growth on the surface of MnO2 nanowires forming core-shell structure (Fig. 1(i)). Moreover, the SEM images (Fig. 1(g-h)) reveal that the length to diameter ratio of the nanowires is extremely high. Their lengths are dozens of times more than their diameters, and the diameters of the nanowires are around ~50 nm. From Fig. 1(h) and (i), the CeO2 granules evenly gather around the MnO2 nanowires exterior. Dissimilar to its unaccompanied constitution, the molecular dimension of CeO2 in these complexes is remarkably low, generally it is less than 5 nm.
Figure 2: EDX images of CeO2 (a), MnO2 (b) and MnO2@CeO2 (c).
The elemental ingredients of CeO2 NSs, MnO2 NWs and CeO2@MnO2 are displayed in Fig. 2(a). The elements of the CeO2 NSs are 68.39 wt% of Ce, 23.98 wt% of O and 7.63 wt% of C (carbon). Fig. 2(b) shows the elements of the MnO2 NWs are 49.18 wt% of Mn, 46.34 wt% of O and 4.48 wt% of C (carbon). The Fig. 2(c) shows that the elements of the CeO2/MnO2 CNWs are 21.37 wt% of Mn, 31.35 wt% of Ce, 30.83 wt% of O and 16.45 wt% of C (carbon). The results show that CeO2@MnO2 composite materials have been prepared.
Figure 3: HRTEM pictures and XRD spectra of (a), (d) CeO2 NPs; (b), (e) MnO2 NWs; (c), (f) CeO2@MnO2.
The HRTEM pictures and XRD patterns of CeO2 NSs, MnO2 NWs as well as CeO2@MnO2 are emerged in Fig. 3. Diffraction signals along (111), (200), (220), (311), (400), (331), (420), and (422) planes confirm the cubic fluorite structure of CeO2 which matches well with JCPDS PDF Card no. 34-0394 . Signals are intensive and palpable, showing the great crystallinity of CeO2. This indicates that the CeO2 has a face-centred cubic structure and can be assigned to Fm3m point group . Fig. 3(d) shows a HRTEM image of CeO2 Nano sheets with an interplanar spacing distance of 0.312 nm mapping to the (111) plane of cubic CeO2 (JCPDS 34-0394). MnO2 signal peaks at 2θ = 28.9o, 37.6o, 42.0o and 56.4o are acute and strong, indicating that the MnO2 is α-MnO2 which has the fine crystallographic texture found on standard JCPDS card No. 44-0141 . Besides, Fig. 3(e) illustrates the HRTEM image of MnO2 nanowires with a lattice pitch of 0.272 nm based on (222) lattice spacing of MnO2. At the same time, the XRD pattern of MnO2@CeO2 is showing that both MnO2 and CeO2 are maintaining their initial crystal structure. Fig. 3(f) proves the HRTEM picture of MnO2/CeO2 CNWs; it is observed that the MnO2@CeO2 consists of CeO2 with an interplanar spacing distance of 0.312 and 0.191 nm belongs to (111) and (220) planes of cubic CeO2, and MnO2 NWs has lattice spacing’s of 0.270, 0.170 and 0.161 nm which corresponded to the (104), (116) and (018) planes of MnO2 (JCPDS no. 33-0664) . Thus, the XRD and TEM spectra of pure CeO2, MnO2 and CeO2@MnO2 are consistent with EDAX results.
Figure 4: Adsorption-desorption isotherms and pore size distributions of (a) and (d): CeO2 NSs, (b) and (e): MnO2 NWs and (c) and (f): CeO2@MnO2.
The nitrogen adsorption/desorption isotherm plots as well as the derived pore size distribution of CeO2 NSs, MnO2 NWs and CeO2@MnO2 are shown in Fig. 4. The CeO2 NSs belong to a type-Ш isotherm. The almost perpendicular line at heavy pressure (~1.0 P/P0) proves the existence of macro porosity, which is probably attributed to the congestion of the Nano sheets . Obviously, the holes of CeO2 NSs are 0~100 nm. The pore size of MnO2 could be the micro and mini-sizes that belong to type I and IV (judging from intervenient isotherm) . As shown in Fig. 4(e), the pore size distribution of MnO2 is less than 5 nm, which illustrates that it is dominated by micro and mesopores, and an unconspicous hysteresis loop illustrates a small amount of mesopore. As shown in Fig. 4(a), the quantity of N2 absorbed grew noticeably when the relative pressure (P/P0) came to more than 0.8, which certified that the macropores are geared to CeO2 and MnO2  and the bore diameters of individual MnO2 particles are more than 15 nm. In line with IUPAC assortment, these N2 adsorption-desorption isotherms (MnO2 and MnO2@CeO2) are attributed to an H3 type hysteresis loop and can belong to a type IV isotherm, proving the mesopores features . In fig. 4(f), the pore-size distribution curves of samples are calculated with the desorption branch of the N2 isotherms. The pore size distribution curves of materials contain mesopores (2~50 nm), and the primary mesopores are mainly slit-like pores and seats at 48.48 nm. In addition, the average pore diameter of MnO2@CeO2 is ~3.7 nm, shortening the electronic transmission path is propitious to electrical conductivity and super capacitor performance . The BET surface area of MnO2@CeO2 is ca. 38 m2 g-1. Results are much in accordance with the illumination of SEM and TEM.
3.2. XPS analysis
Figure 5: XPS spectra of MnO2@CeO2 nanowires. (a) Survey scan. (b) Mn 2p state. (c) Ce 3d state. (d) O 1s state.
Fig. 5(a) is a total scanning map of MnO2@CeO2 manifesting the existence of Ce, Mn, and O. The carbon signal might be derived from the external environment , which would enhance electric conductivity as well as stabilization by a carbon skeleton . The oxidation state of cerium was acquired from Fig. 5(c). The six signals, u1, u2, u3, v1, v2 and v3, are found in Fig. 5(c) (Ce 3d XPS picture). Among them, u1, u2 and u3 signals correspond to Ce4+ 3d3/2 as well as v1, v2 and v3 signals refer to Ce4+ 3d5/2 that are in response to three pairs of spin-orbit double peaks . As known from the literature , missing oxygen elements of CeO2 exterior (forming oxygen holes) can give rise to Ce3+. Accordingly, in a way, the appearance of oxygen holes affects its interplanar crystal spacing, lattice constant, and crystalline network. For Mn 2p in Fig. 5(b), two main peaks of 2p3/2 and 2p1/2 can be resolved into two signals. One signal at 654.08 eV is attributed to the binding energy of Mn3+, while the other at 642.23 eV is indexed to the existence of Mn4+; these are consistent with MnO2 in the XPS data bank (653.9±0.2 and 641.9±0.2) . Furthermore, the O1s peaks in Fig. 5(c) were measured to illustrate the valence of the oxygen elements of MnO2@CeO2. The O 1s signal in Fig. 5(d) displays the signal at 529.8 eV, which can be attributed to the MnO2 . The XPS results are in common with the previous XRD pattern. Considering the above analysis, we have triumphantly prepared the ultimate MnO2/CeO2. Moreover, the reaction process and reaction mechanism are listed in following formula:
C6H12N4 + 6H2O → 6HCHO + 4NH3
NH3 + H2O → NH4+ + OH–
KMnO4 + OH– →MnO2 + 2H2O
2MnO2 +2NH4+ → 2MnO(OH)+2NH3
Ce3+ + 3OH– → Ce(OH)3
Mn(OH)4 → MnO2 + 2H2O
4Ce(OH)3 + O2 → 4CeO2 + 6H2O
3.3. Electrochemical studies
Figure 6: Cyclic voltammetry curves of (a) CeO2 NSs, (b) MnO2 NWs, and (c) CeO2@MnO2 with different scan rates; (d) typical CV curves of CeO2 NSs, MnO2 NWs and CeO2@MnO2 electrodes at a scan rate of 20 mV s-1.
Figure 7: Specific capacitances of CeO2 NSs, MnO2 NWs and CeO2@MnO2 at different scan rates (5 mv/s, 10 mv/s, 20 mv/s, 50 mv/s and 100 mv/s).
To demonstrate the possible application of the MnO2@CeO2 sample as a super-capacitor electrode material, the charge-discharge principle of CeO2 NSs, MnO2 NWs and CeO2@MnO2 in 2 M KOH were delved into with an electrochemical workstation. Fig. 6(a-c) exhibits the CV curves of CeO2 NSs, MnO2 NWs and CeO2@MnO2 at diverse scanning rates of 5~100 mV s-1 with 0.18 to 0.45 V. All in all, along with increasing the scanning rate (5 to 100 mV s-1), the redox current signal swells and the redox peak signals come to a minimum and maximum, because of the multiplication of hydroxyl ions . Nevertheless, because of the stabilization of CeO2 NSs, MnO2 NWs and CeO2@MnO2, their CV curves have almost no change.
Distinguished from traditional double layer capacitors, CeO2 NSs, MnO2 NWs and CeO2@MnO2 monopolize palpable reversible redox peaks, which proves the three kinds of samples have emblematical faradic pseudo-capacitance features, because of the oxidation-reduction reaction between Ce3+ and Ce4+ associated with OH anions . As for CeO2 NSs electrode. The following is the reaction process:
CeO2 + e– + H2O → Ce3+OOH + OH–
Ce3+OOH – e– + OH– → CeO2 + H2O
Similar to CeO2 NSs, the cycle CV curves of MnO2 also presents obvious redox peaks. The following is the reaction process:
MnO2 + e– + H2O →Mn3+OOH + OH–
Mn3+OOH – e– + OH– →MnO2 + H2O
Fig. 6(c) shows the redox peaks gradually become broader. This is attributed to the dedication of MnO2 together with the assistance of CeO2. The equation for the possible reversible procedure is expounded as follows:
CeO2 + OH– → 4CeOOH + e–
MnO2 + OH– → 4MnOOH + e–
The capacitance of the electrode actually is decided by the area of the CV curve that explains the actual active surface area of electrode materials. Fig. 6(d) shows the CV curve of CeO2 NSs, MnO2 NWs and CeO2@MnO2 at a scanning velocity of 20 mV s-1. The CeO2@MnO2 electrode has a greater area of the CV curve and higher current signal than that of the CeO2 NSs and MnO2 NWs electrodes, ascribing to the coordination between CeO2 and MnO2: (1) the contact between effective areas of the composite electrode materials and electrolyte ions is enlarged; (2) CeO2 NSs has accumulated around MnO2 surface, which could quicken the Faradaic reaction between CeO2 and MnO2. With the CV curves, the formula for specific capacitance (Cs) is as follows:
For the aforesaid expression, Vi and Vf are the initial and final voltages, i(V)dV is the charge during the anodic and cathodic scan, m is the mass of the active material, υ is the scan rate and ΔV is a potential window. The calculated specific capacitance values are 221, 145 and 265 F g-1 at a scanning rate of 5 mV s-1 for CeO2 NSs, MnO2 NWs and CeO2@MnO2, respectively.
Figure 8: (a-c): GCD curves at diverse ampere densities (0.9, 1, 1.5, 2 and 2.5 A/g); (d): GCD curves at the ampere density of 0.9 A g-1 and (e): specific capacitances at different ampere densities (0.9, 1, 1.5, 2 and 2.5 A/g) of CeO2 NSs, MnO2 NWs and CeO2@MnO2.
As is well known, the specific capacitances can be described by the discharge curves as follows:
In the formula: I is the discharge current density (A), Δt is the discharge time (s), m is the mass of active materials (g), and ΔU is the voltage window (V). In Fig. 6(e), the calculated specific capacitances are 68, 54 and 72 F g-1 at the ampere densities of 0.9 A g-1. Obviously, the specific capacitance of CeO2@MnO2 core shell nanowires was the highest among the three electrode materials, primarily because the CeO2@MnO2 had broader specific surface area that provides more active sites, which would be in favour of the transport of e-electrolyte ions.
Figure 9: EIS study and analogue circuit of CeO2 NSs, MnO2 NWs and CeO2@MnO2.
Figure 10: Cycling stability of CeO2, MnO2 and CeO2@MnO2 at 1 A/g for 5000 cycles.
Fig. 9 reveals the EIS of CeO2 NSs, MnO2 NWs and CeO2@MnO2 electrodes. In papers, the depressed arc at high frequency is called the charge transfer resistance (RCT) which is on the electrode/electrolyte interfaces . The resistances of CeO2 NSs, MnO2 NWs and CeO2@MnO2 were 0.5 Ω, 4 Ω and 0.3 Ω, which reveal that CeO2@MnO2 had a faster charge transfer rate. The better electrochemical performance of CeO2@MnO2 can be credited to the greater possibility of charge contact and less diffusive impedance. The comparison further suggests that CeO2@MnO2 exhibits improved super capacity characteristics.
The cycling performances of the CeO2, MnO2 and CeO2@MnO2 electrodes were evaluated by the continuous charging-discharging test at a current density of 1 A g-1 (Fig. 10). Impressively, the CeO2@MnO2 manifest exceptional cycling stability and deliver 87.02% of its initial capacitance even after 5000 cycles. The cycling performance of CeO2 and MnO2 remained 72% and 72.94%, respectively. The cycling performances of CeO2@MnO2 is superior to CeO2 and MnO2, which must be related to advantageous structural features of the CeO2@MnO2 , to be discussed above.
Ultra long MnO2@CeO2 were successfully prepared by a facile approach. The MnO2@CeO2 nanowires were chosen as super capacitor electrode material. Thanks to a synergistic reaction, this is a super capacitor electrode material which can embody material oxidation and reduction qualities. Super capacitor performance of the prepared MnO2@CeO2 electrode material was estimated by CV. The Cs of MnO2@CeO2 was 265 F/g at a scanning rate of 5 mV/s in 2 M KOH. The results suggested that this core-shell hetero structure morphology can enhance the charge transfer rate of MnO2 and CeO2. In addition, its ultra-long nanowire morphology exposes more active sites for oxidation-reduction reactions.
We acknowledge the funding support from the National Natural Science Foundation of China (Nos. 21301078 and 91622105), Jiangxi Provincial Education Department (GJJ13215). Postdoctoral Scientific Research Foundation of Jiangxi Province, the Sponsored Program for Cultivating Youths of Outstanding Ability in Jiangxi Normal University, the Initial Fund for Doctors from Jiangxi Normal University, Youth Foundation of Jiangxi Normal University, and Postdoctoral Scientific Research Foundation of Jiangxi Normal University.
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