Elsevier

Chemical Engineering Journal

Volume 361, 1 April 2019, Pages 1030-1042
Chemical Engineering Journal

Cobalt-doped zinc manganese oxide porous nanocubes with controlled morphology as positive electrode for hybrid supercapacitors

https://doi.org/10.1016/j.cej.2018.12.152Get rights and content

Highlights

  • ZnMn2O4 (ZMO) or Co-doped ZMO porous nanocubes (PNCs) were prepared by a solvothermal method.

  • NH4F played a vital role in the growth process of the nanocubes.

  • Co-doped ZMO PNCs greatly enhanced the electrochemical performance.

  • The Cs value of the ZMO:5Co electrode is 4 times higher compared to the pristine ZMO.

  • The ZMO:5Co-based HSC delivered higher energy and power densities.

Abstract

Rationally designed porous structured electrode materials have attracted significant potential interest in hybrid supercapacitors owing to their more predominate surface area and endow the superior energy storage capability. The synthesis strategy of these multifunctional porous structures is more desirable and still inferior. Herein, we synthesized novel ZnMn2O4 (ZMO) or cobalt (Co)-doped ZMO porous nanocubes (PNCs) by a facile solvothermal method, followed by calcination in air. The NH4F played an important role as a template in the formation of nanocubes morphology and detailed growth process was investigated. Impressively, with the incorporation of different molar concentrations of Co ions into pristine ZMO, the electrochemical performance was enhanced and the capacitance values were significantly increased due to the porosity and multi-metal ions synergistic effect. The pristine ZMO and the optimized Co-doped ZMO PNCs exhibited maximum specific capacitance values of ∼267 and ∼1196 F g−1, respectively, at 1 A g−1 of current density. The optimized ZMO:5Co PNCs electrode exhibited more than 4 times in its specific capacitance value with respect to the pristine ZMO PNCs at a constant current density (1 A g−1), and it also showed excellent cycling performance (∼85.5%) at higher current density (7 A g−1). Furthermore, a hybrid supercapacitor (HSC) device was made by utilizing ZMO:5Co PNCs (positive) and activated carbon (negative) electrodes, exhibiting a maximum specific capacitance of ∼68 F g−1 at 1 A g−1 of current density and a high energy density of 27.38 W h kg−1 at a high power density of 1059 W kg−1 within a potential window of 1.45 V. The HSC device also showed excellent cycling stability with ∼80.5% of capacitance retention after performing the 4000 cycles.

Introduction

Nowadays, renewable energy sources have been great and increasing concerns for personalized energy technologies owing to the limited sources from fossil fuels in environments [1], [2], [3], [4], [5], [6], [7]. The portable and various electronic devices such as smartphones, flashlights, laptops, cameras, memory backup-power systems, etc. are expected to be a clean and new class of devices for renewable energy sources [4], [8], [9], [10], [11], [12], [13], [14], [15]. High power density and as well as high energy density plays a key role in the rapid growth process of portable electronic devices for the conventional energy technology [4], [16], [17]. Usually, supercapacitors and batteries are being implemented to increase both the energy and power densities. Supercapacitors possess several benefits such as quick charge/discharge times, easy operation mode, and enhancement of cyclic life-time. However, they still exhibit poor energy density and severely limit their potential for widespread practical applications [18], [19], [20]. On the other hand, batteries also produce chemical energy storage owing to their self-discharge and good cyclic performance, but a mismatch problem arises in their excellent performance due to the low specific capacity of anode materials [21], [22]. In this regard, development of the electrochemical materials with such high energy and high power densities is an essential and challenging study. In general, hybrid supercapacitors (HSCs) which are a combination of battery-type/pseudocapacitive Faradaic electrode and non-Faradic electric double layered capacitive electrode could achieve the higher energy and power densities due to their hybridization, and further they can enhance the operating voltage to achieve the high-performance hybrid supercapacitors [23], [24]. In HSCs, a battery-type Faradaic electrode (positive electrode) has been fabricated from lithium intercalated or transition metal (TM) related compounds. On the other side, the capacitive electrode (negative electrode) materials are based on graphene or activated carbons (ACs) [23], [25]. Thus, HSCs are actively pursuing the desirable energy and power densities with assured dual characteristics of the batteries and supercapacitors for practical applications.

Generally, TM oxides are the common battery-type or pseudocapacitive materials and have been extensively applied in their respective energy storage fields due to their low cost and multiple oxidation states [26], [27], [28], [29], [30]. Recently, mixed or bimetallic TM oxide composites have received a significant attention to show better capacitance performance as compared with the monometallic composites in aqueous electrolyte [21], [27]. More importantly, these mixed or bimetallic TM oxide composites serve as the best electrode materials for high power applications. Rational engineering of metal cations doping into the TM ions as an electrode material is very important to improve the electrochemical performance as an anode material in batteries and next-generation supercapacitors [31], [32], [33], [34]. Up to date, there have been many research reports on metal ions incorporated into various TM oxide host lattices (Cu-doped Mn2O3, [35] Mn-doped V2O5, [23] cobalt (Co)-doped ZnSnO4 [27] and Mn-doped Zn2GeO4 [22]) with improved electrochemical performance and double or triple times enhanced specific capacities for battery applications. In the above battery studies, the incorporation of metal cations not only enhanced the specific capacities of the TM oxide electrode materials, but also increased the cycle performance and maintained the good rate capability due to the multi-metal synergistic effect. Thus, it is believed that the diverse metal cations incorporated into desirable TM oxide composites offer a new pathway to design HSC electrode materials with improved energy density for energy storage device applications.

The spinal-like structure of bimetallic TM oxides having the general composite formula of AX2O4 (A, X = Mn, Cu, Co, Fe, Ni, Zn, etc.) has attracted much attention as anode materials for high-performance lithium ion batteries (LIBs) [36], [37], [38]. Among the AX2O4 materials, particularly, the Mn-based ZnMn2O4 composite is eco-friendly, non-toxic, low-cost, and naturally abundant compared with other battery-type materials like NiCo2O4. Thus, the advantages of the spinal ZnMn2O4 composite are being a good novel anode electrode material in LIBs [39], [40], [41], [42] and it is also a positive electrode material in supercapacitors to store the energy [43], [44], [45]. Furthermore, limited electrochemical reports were found in the field of supercapacitors for the excellent featured ZnMn2O4 composite as an electrode material. Accordingly, we considered the ZnMn2O4 composite as a pristine material to study the electrochemical properties for energy storage applications. Typically, the electrochemical performance of electrode material depends on the physical properties such as surface area and mechanical stability. Furthermore, the performance was improved by tailoring the nano/microstructure morphology and its porosity, and this has been proven by several research studies [46], [47], [48], [49]. Especially, the porous nanostructured morphologies often increase the electrochemical activity of the metal oxide electrode materials, resulting in enhanced electrochemical properties with good specific capacitance and better cycling life-time [48]. At higher current densities, these porous nanostructured materials possess volume expansion with good void space, which helps the diffusion of the electrolyte ions easily at the electrode/electrolyte interface in the continuous cycling process, enabling the good structure and cycling stability. In this work, we rationally designed an easy and cost-effective way for preparing the porous ZnMn2O4 or Co ions doped ZnMn2O4 nanocubes by a facile solvothermal method. The synthesized ZnMn2O4 or Co ions doped ZnMn2O4 porous nanocubes were further used as an electrode material for HSCs. Furthermore, to evaluate the practical functionality, a HSC device was made by assembling the optimized Co-doped ZnMn2O4 electrode materials as a cathode and carbon electrode as an anode material.

Section snippets

Materials

In a typical synthesis process, zinc acetate dihydrate (Zn(CH3COO)2·2H2O, ≥99.9%), manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O, ≥99.9%), cobalt acetate tetrahydrate ((CH3COO)2Co·4H2O, ≥99.9%), ammonium fluoride (NH4F, ≥99.9%), isopropyl alcohol (IPA, (CH3)2CHOH, ≥99.7%) and hexamethylenetetramine (HMTA, C6H12N4, ≥99.9%) were purchased from Sigma Aldrich Co., South Korea. Nickel (Ni) foam was obtained from MTI Corporation, South Korea. Potassium hydroxide (KOH, ≥85%), N-methyl-2-pyrrolidone

Results and discussion

To derive the growth process of the ZMO PNCs, the NH4F concentration-dependent experiments were carried out and the obtained FE-SEM images are depicted in Fig. S1. The FE-SEM images clearly show a closer inspection of the morphological properties of the prepared hydroxyl form of ZnMn2(OH)6 powder samples at the fixed amount of HMTA and varied molar concentrations of NH4F, i.e., 0, 5, 10 and 15 mmol. The reaming all the solvothermal conditions were maintained constant. Fig. S1(a) and (b) shows

Conclusions

In summary, the spinal-like structured ZMO and Co ions doped ZMO PNCs were successfully prepared by a facile solvothermal route followed by calcination at 450 °C for 3 h in air. The growth of the hydroxyl nanocubes was investigated by tuning the amount of NH4F. It was found that with the incorporation of Co into ZMO PNCs, the porosity of the materials was increased and the electrochemical properties were greatly enhanced. The optimized ZMO:5Co PNCs electrode showed good reversible Faradaic

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B4011998 and No. 2018R1A6A1A03025708).

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