Synthesis of CuO@CoNi LDH on Cu foam for high-performance supercapacitors
Graphical abstract
Introduction
Supercapacitors, as a kind of energy storage device with fast charging-discharging capability, high power density, long cycling life, and eco-friendliness, play an important role in the new energy field nowadays [1], [2]. To a large extent, the electrochemical performances of supercapacitors are determined by electrode materials [3], which can be classified into electric double layer capacitive (EDLC), pseudocapacitive (PC), and battery-like electrode materials in the light of different energy storage mechanisms [4], [5]. Among them, battery-like electrode materials have recently become a research hotspot due to their high theoretical specific capacity originating from redox reactions on the electrode/electrolyte interface [6]. In particular, transition metal hydroxides with abundant reserves and low cost, such as Co(OH)2 [7] and Ni(OH)2 [8] are typical battery-like materials [7], yet demonstrating much lower specific capacities than their theoretical values and unsatisfactory rate capability owing to limited ion-diffusion pathways and poor intrinsic electronic conductivity (≈ 10−5 to 10−9 S·cm−1) [9], [10], [11]. Consequently, it’s still desirable to further boost the electrochemical performances of battery-like materials by tailoring their nanostructures with large specific surface area, highly exposed active sites, and rapid charge carrier transfer [5], [12], [13]. One strategy to solve this problem is to construct nanoscale 3D core-shell architectures that make use of highly conductive cores to support electroactive shells such as Ni(OH)2 and render large accessible surface [14], thus showing superb electrochemical properties by accelerating the electron transport and electrolyte permeation. For instance, Chen et al. prepared the core-shell Ni3S2@Co(OH)2 nanowires with a charge transfer resistance of 0.79 Ω, as well as a high specific capacitance of 2139.4F·g−1 and good cycling stability [7]. Li et al. reported the P-Ni(OH)2@MnO2 hybrid with hierarchical core-shell structures that exhibited improved electrochemical performances [15].
Copper oxide has been considered as an ideal core material because of its low cost, substantial reserve, and desirable electrical conductivity. In recent years, various nanostructured CuO including nanowires [16], [17], [18], nanorods [19], [20], nanonetworks, and nanoflowers [12], [21] have been reported as core materials. Besides, fabricating binder-free core-shell electrodes via growing CuO on porous current collectors like Cu foam (CF) has been an effective way to provide more active sites and channels, thus improving the overall electrochemical performances of the electrodes. For example, Ruan et al. reported the synthesis of Ni(OH)2/Cu2O/CuO nano-cluster on Ni foam, which exhibited a high capacitance of 1474F·g−1 at 15 mA·cm−2, but only poor cyclic stability of 82% capacitance retention over 1500 cycles [22]. Zhang and co-workers prepared core-shell structured NiMn-LDH@CuO on Cu foam which displayed a capacitance of 2430.8 F·g−1 at 0.8 A·g−1, and the assembled ASC device showed a low energy density of 10.8 Wh·kg−1 at a power density of 100 W·kg−1 [23]. Despite these efforts, most reported CuO-based binder-free core-shell heterostructures are still suffering from inferior cyclic stability, low energy density, and low charge-discharge rate. As for shell materials, Ni(OH)2 is a widely studied potential candidate. Furthermore, researchers have found that the hybrid composed of two or more transition metal hydroxides can exhibit better electrochemical performances than individual transition metal hydroxide as a consequence of the synergistic effect between multiple components. For example, Chen et al. took a hydrothermal method to synthesize CoMn LDH nanowires on Ni foam which demonstrated a high specific capacitance (1409 F·g−1) and good cycling stability (93.2% after 3000 cycles at 1 A·g−1) [1]. Especially, a series of CoxNi1-x(OH)2 hybrid was integrated onto CuO nanowires, and the results showed that electrochemical performances of the optimized hybrid were strikingly better than individual Co or Ni hydroxide onto CuO nanowires [24]. However, those previously reported LDHs still showed unsatisfactory electrochemical performances, such as low specific capacitance, poor rate capability, and short cycle life owing to the irrational structure of CuO, weak combination, and shedding of active materials. Although CuO@LDH-based core-shell materials on Cu foams have been reported in previous works using a two-step method, the formation mechanism and the effect of experimental conditions on the morphology and composition of products have not yet been systematically studied, which hinders the design of novel CuO@LDH-based core-shell materials with excellent electrochemical performances. Besides, no previous works have taken the rational design of interfaces and the adhesion of CuO on substrates into consideration, which is important for enhancing the stability and electrochemical performances of core-shell nanomaterials. Poor adhesion of the electrode materials on substrates would lead to the rapid decay of electrochemical performances such as reversibility, rate performance, and cycle life. Especially, the shedding of CuO can highly affect the followed coating step of shell materials, which thus largely reduces the loading mass of active materials. Therefore, it is critical to design a rational 3D structured CuO core on CF with robust adhesion to solve this issue and enhance the electrochemical performances of the electrode, and investigate the relationship between experimental conditions and the morphology of products to provide guidance for designing novel core-shell nanostructures.
At this point, we prepared a series of CuO/Cu(OH)2-based materials with strong adhesion on CF as the core via alkali-assisted wet chemical oxidation of CF, which was based on an orthogonal experimental method in this paper [13], [18], [25]. We discussed the formation mechanism of the products and how the experimental conditions influenced the morphology and composition of the products. Several kinds of substrates were selected as the potential core materials by considering the anti-shedding and electrochemical properties of the 18 orthogonal experimental products [26]. After investigating their nanostructures and electrochemical performances, the final candidate was orientated towards the CF@CuO substrate with the morphology of cross-linked nanosheet aggregates, whose well-distributed 3D architecture of CuO nanosheets could provide a more accessible area and robust adhesion for the growth of CoNi LDHs. Afterward, we constructed the core-shell CF@CuO@CoNi LDH composites by electrodepositing ultra-thin CoNi LDH sheets onto CuO cross-linked nanosheet aggregates and optimized the molar ratio of Co2+ and Ni2+. As a result, this well-designed core-shell structure is particularly conducive to the better exposure of active sites of electrode materials and ions transfer in the electrolyte, thus acquiring excellent electrochemical properties as expected, such as high specific capacity (319.4 mAh·g−1 at 1 A·g−1). The as-fabricated ASC delivered an ultrahigh energy density (92.5 Wh·kg−1 at the power density of 400 W·kg−1), and super-long cycle life.
Section snippets
Materials and chemical
All chemicals were of analytic grade and used without further purification. Ammonium persulfate ((NH4)2S2O8), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), sodium hydroxide (NaOH), potassium hydroxide (KOH), hydrochloric acid and polyvinyl alcohol (PVA-1775) were all purchased from Kelong Chemical Technology Co. Ltd. (Chengdu, China). Cu foams with a thickness of 1 mm were offered by Jiaojiao Chamber Commerce limited (Chongqing, China).
Synthesis of CF@CuO substrates
The CF@CuO
Results and discussion
A binder-free CuO@CoNi LDH core-shell heterostructures with robust adhesion on Cu foams were successfully fabricated by a two-step process. As shown in Fig. 1a, the reaction for the synthesis of CuO involves the oxidation of Cu foams with APS and NaOH, and the obtained substrate was designated as CF@CuO. During the oxidation process of CF in an aqueous system containing NaOH and APS, the following chemical reactions were involved.
Conclusions
In summary, we have prepared a novel CuO@CoNi LDH cross-linked core-shell nano-architecture assembled by ultrathin nanosheets grown on Cu foam via a wet-chemical oxidation process followed by fast electrodeposition. An orthogonal design was initially used to choose the optimum CF@CuO core with the highest specific capacity and most robust adhesion between Cu foam and CuO. Moreover, the electrochemical performances of the CoNi LDH shell were further optimized via tunning the molar ratio of Co2+
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Fundamental Research Funds for the Central Universities (No. 106112016CDJXZ228803), Chongqing Innovation Fund for Graduate Students (No. CYB18042), Opening Project of State Key Laboratory of Advanced Chemical Power Sources, Doctoral Program of Guizhou Education University (No. 2019BS022) and Hundred Talents Program of Guizhou Province (No. QKHPTRC [2016] 5675). The authors would like to thank Dr. Bin Zhang, Dr. Xiao Zhang and Dr. Yang Zhou at Analytical and
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