Hierarchical Ni_0.54Co_0.46O₂ nanowire and nanosheet arrays grown on carbon fiber cloth for high-performance supercapacitors

Highlights

• Ni_0.54Co_0.46O₂ grown on carbon fibers as binder-free electrode for supercapacitor directly.

• The coexistence of Ni²⁺ and Ni³⁺ in Ni_0.54Co_0.46O₂ could boost the performance.

• The products exhibit outstanding specific capacity and rate performance.

• Symmetrical device delivers excellent energy density and power density.

Abstract

Hierarchical Ni_0.54Co_0.46O₂ architectures composed by nanowires or nanosheets were successfully grown on bio-mass carbon fiber cloth (CFC) by hydrothermal method. The morphology of Ni_0.54Co_0.46O₂ can be effectively controlled by using different precipitators. The structural effects of the two kinds of morphologies were researched. the results suggest that the Ni_0.54Co_0.46O₂ nanosheet arrays grown on CFC (NCO-NSs/CFC) shows a higher Faradaic areal capacity of 438 μAh cm⁻² (238.1 mAh g⁻¹) at a current density of 1 mA cm⁻² and still about 90.3% initial capacity retention even at the high current density of 50 mA cm⁻². Moreover, an all-solid-state flexible symmetric supercapacitor device has been successfully assembled. The optimized device delivers superior electrochemical performance with an outstanding energy density of 92.4 Wh kg⁻¹ at a power density of 207.2 W kg⁻¹. Such hierarchical nanostructure composed by well-aligned uniform Ni_0.54Co_0.46O₂ nanosheet arrays grown on bio-mass carbon fiber cloth might hold great promise as battery-type electrode material for high-performance supercapacitor.

Introduction

Energy is one of the most important topics that attracting more and more attention of the whole world in the 21st century. To meet the urgent requirements of renewable and sustainable energy sources, there is a growing demand for developing cleaner and more efficient energy storage systems such as supercapacitors and lithium-ion batteries [1]. Supercapacitor is regarded as one of the most promising avenue to the future for its relatively higher power density, faster charge/discharge capability and longer cycling life [1], [2], [3].

However, the realization of that strategic conception depends on many aspects particularly a breakthrough in obtaining higher energy meanwhile keeping power delivery and cycling stability [4]. One promising approach is to introduce a battery-type faradaic electrode into hybrid electrochemical capacitors [2]. Transition metal oxides as battery-type materials used in capacitors explored relatively higher performance than EDLCs due to their rich redox reactions on the surfaces as well as in the bulk of the electrode [5], [6], [7], [8], [9], [10]. At present, a series of transition metal oxides electrode materials, such as MnO₂ [11], [12], NiO [13], [14], [15] and Co₃O₄ [16], have been researched. However, the applications of these materials are mainly restricted by various aspects as low energy capacity and poor cycle life [17]. NiCo₂O₄ with spinel structure sparked worldwide interest as an attractive battery-type faradaic electrode for advanced capacitor devices for the presence of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ redox couples in the spinel structure [18], [19], [20]. What is remarkable is that the addition of Co can largely reduce the specific capacity though effectively improve the cycling life and the conductivity of the NiCo oxides [21]. Significantly, according to some researches, the specific capacity is found to be relied on the molar ratio of Co/Ni, getting highest at Co/Ni molar ratio of close to 1 when forming the NaCl-type NiCoO₂ [22], [23]. NiCoO₂ has also been used in many other fields such as water oxidation [24] as well as Li-ion batteries [25] and both of them exhibit excellent performance confirming NiCoO₂ has enormous potential in the application of electrochemical reactions.

In recent researches, there were two main nanostructures have raised a widely attention called nanorods/nanowires and nanosheets synthesized by hydro-/solvo-thermal method [26], [27]. There were some reports demonstrated that the electrochemical performances such as ion transfer rates and electrode/electrolyte interface properties are strongly influenced by the morphologies and microstructures of electrode materials [28]. In addition, it is well known that defects in materials can improve the performance of supercapacitors, because crystalline imperfection can provide additionally electrochemical active sites. Thus, it is very significate and prospective to study the difference between the different shapes of NiCoO₂ with abundant lattice imperfection and its electrochemical properties, which were few researches on before.

Traditionally, when discrete particles are used as electrode materials for supercapacitors, electrodes have to be fabricated with a lot of insulating polymer binder and conductive agents, which can largely affect the capacity performance, portability, price and practicability of supercapacitors. Carbon fiber supported electroactive nanocomposites seems to be a promising plan, with the unprecedented superiority of excellent flexibility, lightweight as well as corrosion resistance. However, most of the as-prepared carbon fibers used for electrode materials are carbon nanotubes, graphene and commercial carbon fiber [29], [30], [31]. Given their complex preparation, cost and abundance, the bio-mass carbon derived from bio-base resources such as hemp [8], flax [11] and cotton [32] has attracted intense interest lately and is widely used in SCs. As far as we know, the previous study of NiCoO₂ used as electrodes for supercapacitors had to be blended with binders and affect the performance seriously due to the lower conductivity than NiCo₂O₄, so it is urgent need to develop an effective binder-free method to achieve the best performance of NiCoO₂ [32].

In this work, we successfully synthesized controllable morphology of hybrid nanostructures composed of Ni_0.54Co_0.46O₂ nanowire and nanosheet arrays grown on bio-mass carbon fiber cloth (CFC) through a facile hydrothermal method. A large number of lattice defects and crystalline imperfection were obtained in both morphologies which could boost the chemical performance of supercapacitors effectively and thus form non-stoichiometry Ni_0.54Co_0.46O₂, denoted as NCO-NWs/CFC and NCO-NSs/CFC, respectively. Urchin-like and flower-like Ni_0.54Co_0.46O₂ were prepared by the same method without supporting on the CFC, denoted as NCO-NWs and NCO-NSs separately. As expected, two different nanostructures loaded on CFC directly as binder-free for SCs exhibit high Faradaic capacity. In comparison, NCO-NSs/CFC as a hybrid battery-type electrode exhibit a higher capacity (438.6 μAh cm⁻² at a current density of 1 mA cm⁻²), better rate capability (387.0 μAh cm⁻² at 50 mA cm⁻²) and capacity retention (∼70% after 10000 cycles at 10 mA cm⁻²) than NCO-NWs/CFC. Moreover, a symmetric supercapacitor device composed of as-prepared NCO-NSs/CFC electrodes has been successfully assembled. The excellent device delivers an outstanding energy density of 92.4 Wh kg⁻¹ at a power density of 207.2 W kg⁻¹.