A novel hierarchical core-shell structure of NiCo₂O₄@NiCo-LDH nanoarrays for higher-performance flexible all-solid-state supercapacitor electrode materials

Highlights

• The unique NiCo₂O₄@NiCo-LDH cover-shell heterostructure greatly improve the electrochemical performance.

• The NiCo₂O₄@NiCo-LDH//ACC electrode shows the high area capacitance of 6092 mF/cm².

• The NiCo₂O₄@NiCo-LDH//AC ASC device has good flexibility, high energy density and power density.

Abstract

In the present work a 3D hierarchical NiCo₂O₄@NiCo-LDH core-shell structure has been fabricated on activated carbon cloth through a facile hydrothermal strategy and an electrodeposition step. The core of the NiCo₂O₄ nanowires forms the nanoarray backbone on carbon cloth while the shell of NiCo-LDH nanosheets extends the electrochemical active sites and generates mesoporous hierarchical channels to facilitate electron and ion transport. Therefore, with the optimized mass ratio of 1.9 between NiCo-LDH and NiCo₂O₄, such a core-shell structure enhanced the areal capacitance of the as-prepared NiCo₂O₄@NiCo-LDH electrode up to 6092 mF/cm² (or areal capacity of 0.846 mAh/cm² at 2 mA/cm² for the battery type pseudocapacitor) and obtain high specific capacitance of 1882 F/g under 1 A/cm². The capacity retention rate can maintain 83.9 % after 5000 cycles at 30 mA/cm². Furthermore, an all-solid-state asymmetric supercapacitor (ASC) was assembled using NiCo₂O₄@NiCo-LDH//activated carbon. Such an ASC exhibits a very high energy density of 49 Wh/kg at the power density of 750 W/kg and retains 30 Wh/kg at 7500 W/kg. The ASC shows its excellent cycling stability by keeping 83.3 % of initial capacity retention rate after 2000 cycles under 5 A/g. The assembled flexible all-solid-state supercapacitor with NiCo₂O₄@NiCo-LDH//activated carbon exhibits good mechanical stability and high specific capacity of 157 F/g under 1 A/g. The significantly improved electrochemical properties demonstrates the practical applications of the core-shell structure of NiCo₂O₄@NiCo-LDH on carbon cloth for novel ASC configuration design and development of next energy storage devices.

Introduction

Accompanying the depletion of fossil fuel energy sources, renewable and practical green energy systems have been encouraged to undergo continual improvement. Energy storage is important for energy system to extend applications of renewable energy and a better economical society [1]. Among various energy storage types, supercapacitors (SC) or called electrochemical capacitor (EC) are in the electrochemical category based on an interfacial pseudocapacitive reaction to store electrical energy [2], [3]. SCs have been developed to deliver high power density, high capacity, fast charge-discharge capability and long cycling life for almost unlimited times [4]. Based on these advantages, SCs have been widely used in portable electronic devices, electric vehicles and other energy conversion storage devices where a large of energy needs to be stored/released quickly [5]. However, SCs are susceptible to self-discharge and have 1–2 orders of magnitude less energy density storage than the batteries [3], [6]. There are great interests in the development of the ion and electron transport kinetics in a SC electrode and the rate of charge transfer at the interfaces between the electrode and the electrolyte in order to increase the storage capacity and power density of SCs [4]. A facile and rational design of electrode nanostructure can greatly increase the kinetics of the electrodes and thereafter significantly increase the energy density and reduce the cost of supercapacitors [7], [8], [9]. In this way, the working voltage and the capacitance capacity of the SCs can be simultaneously increased.

SCs have two categories: conventional electrical double-layer capacitors (EDLCs) and battery-type Faradaic capacitors (also called pseudocapacitors) [10]. EDLCs typically have low energy density. The battery-type electrode materials often comprise metal oxides and/or hydroxides with capability of fast reversible and multiple ion-participating faradic reactions, therefore achieving high specific capacity and energy density [10]. It is well known that the metal, hydroxide, and alloy compounds have some disadvantages, such as their low resistance to the aggressive influence of environmental factors including temperature, oxygen and electromagnetic radiation. Oxide compounds are much more stable when used up 1000 ºC. Materials such as the complex iron oxides, called ferrites, contain excellent electronic properties that are also promising for practical applications [11], [12]. Furthermore, metal compounds and alloys produce high active losses due to eddy currents. Hexaferrites can avoid such active losses due to high microwave properties and high stability under extreme conditions [13], [14]. Many strategies have been developed to obtain metallic oxides/ferrites [15], [16]. Some other surface/interface engineering strategies are also employed to enhance charge storage capability of transition metal oxides, such as Mn₃O₄, NiO and Cu₂O [17], [18]. One effective route to enhance the energy density of SCs is the construction of asymmetric supercapacitors (ASCs) by the combination of a battery-type capacitor and a EDLC [3]. In such a way, the working potential, and the capacitance storage ability of SCs are improved tremendously to obtain high energy and power densities.

Many reports indicate the novel design of heteroarchitectures of transition metal oxides (TMO)/hydroxides is an effective route to develop electrode materials, which can significantly facilitate the synergistic effect arising from a hierarchical structure and complex of different components [9], [19], [20]. The combination of different compounds with excellent electronic properties can lead to new composite materials, which have earned great technological interests in recent years. The addition of a second phase can significantly optimize the structure and improve the electronic properties of the resulting composite material [21], [22]. Different types of polymers also can be combined with oxides and carbon-based materials to become the new composites with increased and attractive electronic properties could be fabricated [23], [24]. Specifically, a unique core-shell structure has an effective impact to enhance the electrochemical capacitance and energy density of supercapacitors [25], [26], [27]. Li et al. reported a hierarchical core-shell structure of NiGa₂O₄@MnO₂, which showed perfect electrochemical performance when anchored on carbon cloth [28]. Sheng et al. also showed a core-shell structure of dendritic Co₃O₄@Co₂(CO₃)(OH)₂ on carbon cloth, which has high area-specific capacitance, excellent rate capacitance and electrochemical cycle stability [29]. Chen et al. successfully constructed NiCo₂O₄@Ppy core-shell structure with good electrochemical performance. Nevertheless, they found the polymerization in preparation of such a structure was often out of control and required an addition of dopants [30]. Therefore, it has great significance to develop an effective core-shell structure by a highly efficient strategy in a low cost and environmentally friendly way for assembling ASCs.

As a result of their abundant resources in nature and many advantages, various TMOs have been widely used as pseudocapacitors. Among the reported cobaltites, spinel cobaltites (MCo₂O₄ where M = Ni, Zn, Fe, Cu, etc.) show ultrahigh specific capacitance and rate capabilities by taking advantages of synergistic effects of two types of metal ions [31]. NiCo₂O₄ possesses much better conductivity and higher electrochemical activities than conventional transition metal oxides by at least 2 orders of magnitude. Furthermore, low-cost, abundant resources and environmentally friendly nature of NiCo₂O₄ distinguish it as a significant pseudocapacitive electrode material. Many methods are reported for electrode preparation, including hydrothermal [32], biomass extraction [33], solid phase reaction [34], sol-gel technology [35], microwave-assisted [36], [37] and pulse electrodeposition [38], etc. Among the reported techniques, the constant piezoelectric deposition method has been widely used in the rapid preparation of active materials because of its advantages in safety, environmental protection, simple preparation process, controllable synthetic rate and short period [22].

Recently, layered double hydroxides (LDHs) attract extended interests due to their advantages, including low-cost, simple and environmentally friendly fabrication route, large surface area, tunable host metal ions and exchangeable interlayer anions, abundant active sites for high electrochemical activities and excellent redox activities, theoretical specific capacity with intercalation phenomena etc. [16] However, LDHs have low electrical conductivity and are apt to aggregation. Many works have been conducted to construct the NiCo₂O₄@LDH core/shell nanostructure in order to solve the above problems. The construction of core/shell through TMO-LDHs nanostructure provides unique properties for electrochemical supercapacitor devices [27]. Specifically, Acharya et al. fabricated leaf-like hierarchical NiCo₂O₄ nanorods@Ni-Co-LDH nanosheets core/shell nanostructures on Ni foam by a two-step oxalic acid-assisted and metal organic framework (MOF)-derived method [31]. The as-prepared hybrid core/shell NCO/NiCo-LDH electrodes have the high capacitance of 2370 F/g under 1 A/g, rate capability (78.2 % at 30 A/g) and long lifespan of 86.4 % after 5000 charge/discharge cycles. They demonstrate the synergistic effects of NCO nanorods and NiCo LDH nanosheets in a single core/shell structure framework on Ni foam can provide abundant accessible electroactive sites for rapid redox reactions with superior stability for long cycles. Although growing NiCo₂O₄@LDH directly on Ni foam have many advantages, such as high-capacitance and excellent cycling stability, there are still some drawbacks in the practical applications. For example, it is difficult to study the energy density of such supercapacitors in a three-electrode system. The oxalic acid assistant and MOF synthesis/etching techniques to grow LDH nanosheets are still not easy to control in present time. The oxidation of Ni foam affects the total capacitance of the core/shell composite. Furthermore, the high cost of Ni foam hinders its application. The high weight of Ni metal increases the mass loading of active material and affects the flexibility of the all-solid-state supercapacitor electrodes. The employment of carbon cloth as flexible electrode is more meaningful for two electrode electrochemical evaluation [39], [40], [41].

In the present work, dendritic NiCo₂O₄@NiCo-LDH nanoarrays were grown on flexible carbon cloth through a facile hydrothermal process with a following electrodeposition step, one step synthesis was involved in the whole experiment process. The 1D NiCo₂O₄ nanowires anchored on the ACC film function as the backbone to grow a shell of NiCo-LDH nanosheets to form a unique core-shell heterostructures with excellent flexibility. The novelty and improvements of the present technique are its easy operation, the well-controlled core/shell nanostructure, low cost and environmentally friendly manner. No polymer binder, surfactant or additive/acid are used in this technique, which avoid 20 % loss of the battery’s energy by the electrical contact resistance. The chosen carbon cloth substrate also can provide high mechanical properties for flexible all-solid-state supercapacitor electrodes with low cost. Specifically, the 2D NiCo-LDH nanosheets have high specific surface area, which can provide abundant electrochemical sites while the core of NiCo₂O₄ backbone has good electrical conductivity [41]. The NiCo₂O₄@NiCo-LDH electrode provides a superior areal capacitance of 6092 mF/cm² (or areal capacity of 0.846 mAh/cm² at 2 mA/cm² for the battery type pseudocapacitor) and better cycling characteristics of 83.9 % capacity retention after 5000 cycles to the NiCo₂O₄ or NiCo-LDH electrode. One flexible all-solid ASC using NiCo₂O₄@NiCo-LDH nanoarrays on carbon cloth shows high energy density of 49 Wh/kg at 750 W/kg power density and excellent cycling stability.