Zn-Ni-Co trimetallic carbonate hydroxide nanothorns branched on Cu(OH)2 nanorods array based on Cu foam for high-performance asymmetric supercapacitors
Graphical abstract
Introduction
Among all kinds of energy storage devices, supercapacitors (SCs) have received significant attention owning to their fast charge and discharge rate, long lifespan, high power density, and high reliability [[1], [2], [3], [4]]. However, it is difficult to attain desired energy density for conventional capacitors, which are unsuitable for the green and sustainable development of energy storage devices [5]. It has been manifested that the structure and electrochemical properties of the electrode materials were very crucial for supercapacitors to achieve an excellent electrochemical performance [6,7]. Therefore, designing and synthesizing electrode materials with desirable structures and outstanding electrochemical performances have been consistently pursued as an important task for the development of high performance supercapacitors [8,9].
Pseudocapacitive materials store energy by interfacial reversible faradaic reactions instead of charge adsorptions compared with electric double-layer capacitive (EDLC) materials like pure carbon-based materials, which can afford enhanced specific capacitance and high energy density [10,11]. Moreover, pseudocapacitive materials can also combine with EDLC materials to construct asymmetric supercapacitors to further enhance the energy and power densities [[12], [13], [14]]. Traditional pseudocapacitive materials mainly include typical transitional metal oxides/hydroxides, such as RuO2, MnO2, Co3O4, Fe3O4, Ni(OH)2, NiO, Co(OH)2 and their binary systems, which have been explored widely [[15], [16], [17], [18], [19], [20], [21]]. The metal carbonate hydroxides (CH), possessing rich redox reaction sites, have been widely utilized to synthesize nanostructured transition metal oxides with supercapacitive behaviors, for example, Co3O4 can be prepared by annealing the Co2(CO3)(OH)2 (CoCH) [22]. Actually, some metal carbonate hydroxides themselves can easily form hierarchical nanostructures with large active surface areas, which could be directly used as electrodes to develop high-performance supercapacitors [23]. However, these applications are rarely reported. In addition, incorporating different metal cations into a host to achieve multi-transition-metal composites is an effective way to enhance the electrochemical performance of supercapacitors [1,[24], [25], [26], [27]]. In this way, the compositional complexity is increased which could offer more redox active sites and higher conductivity than the corresponding mono-metal or binary-metal composites, which may enhance electrochemical performance in the end owning to the potential synergetic effects [28,29]. Hence, the polymetallic carbonate hydroxides could be directly utilized as potential electrode materials for supercapacitor. For active electrode materials, the existence of inactive components (current collector, conductive additive, adhesives and so on) makes the efficient use of active sites especially difficult [20,30]. Thus, some porous materials such as copper foam were usually selected as substrates avoid self-aggregation, which helps to create extra active sites and improve the electrochemical performances [1].
In this work, a hierarchical Cu(OH)2@ZnNi-CoCH core-shell nanorods array directly grown on three-dimensional (3D) conductive copper foam was synthesized by a simple in situ oxidation reaction combined with hydrothermal strategy. Specifically, as we have previous reported [31], a facile and cost-effective in situ oxidation reaction process was employed to fabricate the Cu(OH)2 nanorods array on three-dimensional conductive Cu foam substrate, which can be directly utilized as electrode in the absence of any conductive additives and binders. This design can effectively reduce the “dead surface” in traditional slurry-derived electrodes, facilitate more efficient interfacial contact between the Cu(OH)2 nanorod backbones and Cu substrate and thus provide prominent conductivity to the electrode [9,[32], [33], [34]]. Then, the Zn and Ni co-substituted Co carbonate hydroxide (ZnNi-CoCH) nanothorns were grown uniformly on the surface of Cu(OH)2 nanorods by a simple hydrothermal strategy to form a 3D hierarchical core-shell structure (Cu(OH)2@ZnNi-CoCH). Such a hierarchical architecture offers some remarkable advantages including large surface area, short diffusion distance, one-dimensional electron transport pathway, and component synergistic effect, all of which are beneficial to the improvement of electrochemical properties of electrodes [34].
As expected, the as-obtained Cu(OH)2@ZnNi-CoCH core-shell nanorod array electrode presents a brilliant capacitive performance. Specifically, the Cu(OH)2@ZnNi-CoCH electrode exhibits significantly enhanced electrochemical performance with a high specific capacitance (2746 F g−1 at 1 A g−1), outstanding rate capability (76.91% from 1 A g−1 to 20 A g−1), and extraordinary cycling stability (maintained 95.61% of the initial capacitance after 6000 cycles at 20 A g−1), which are much superior to those obtained from the electrodes fabricated with Cu(OH)2, Cu(OH)2@CoCH, Cu(OH)2@Ni-CoCH, or bare ZnNi-CoCH nanospheres. Meanwhile, an asymmetric supercapacitor (ASC) was assembled using Cu(OH)2@ZnNi-CoCH nanorods array as the positive electrode and active carbon (AC) as the negative one. The as-fabricated ASC exhibited a fascinating electrochemical performance with the maximum specific capacitance of 111.2 F g−1, a high energy density of 34.75 Wh kg−1 at a high power density of 375 W kg−1, and good cycling stability (86.54% capacitance retention rate after 6000 cycles). Furthermore, the practical applicability of the ASC device was also demonstrated by lighting up a red LED indicator (about 2.5 V) and a heart-shaped combination of ten LED indicators connected in parallel.
Section snippets
Materials
The copper foam and nickel foam were purchased from Kunshan Guangxinyuan New Material Co., Ltd. Ethanol (AR) and hydrochloric acid (HCl) were supplied by Tianjin Fuyu Fine Chemical Co., Ltd. Sodium hydroxide (NaOH, AR), ammonium persulfate ((NH4)2S2O8, AR), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, AR), nickel nitrate hexahydrate (Ni(NO3)2·6H2O, AR), zinc nitrate hexahydrate (Zn(NO3)2·6H2O, AR), urea (CH4N2O, AR), potassium hydroxide (KOH, AR), acetone, acetylene black, polyvinylidene fluoride
Results and discussion
The formation procedure of the Cu(OH)2@ZnNi-CoCH core-shell nanorods array on Cu foam is schematically illustrated in Fig. 1. First, large-scale high-density Cu(OH)2 NRs array grown vertically on Cu foam was obtained by a simple in situ oxidation reaction as reported in our previous work [31]. Subsequently, the facile and high-efficiency hydrothermal synthesis process induced the formation of ZnNi-CoCH nanothorns on the surface of Cu(OH)2 NRs backbone, forming a core-shell heterostructured
Conclusion
In conclusion, the 3D hierarchical core-shell Cu(OH)2 nanorods array branched with ZnNi-CoCH nanothorns has been successfully designed and synthesized via an in situ oxidation reaction with the combination of hydrothermal strategies. The well-aligned Cu(OH)2 nanorods array grown directly on Cu foam could not only serve as a high-surface-area scaffold for anchoring the ZnNi-CoCH nanothorns but also allow fast electron transfer and ion diffusion, thus accelerating the rate of faradaic redox
Acknowledgments
The authors are thankful to funds from the Qingdao Innovation Leading Talent Program, National Natural Science Foundation of China (21805124), and Natural Science Foundation of Shandong Province (ZR2018BEM020).
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Ying Liu and Xueying Cao contributed equally to this work.