Hierarchical mesoporous NiO nanoarrays with ultrahigh capacitance for aqueous hybrid supercapacitor

Highlights

• Hierarchical mesoporous NiO nanoarrays show an ultrahigh specific capacitance beyond its theoretical faradaic capacitance.

• A self-generated sacrificial template approach was developed to prepare the 3D hierarchical and mesoporous array structure.

• The ultrahigh capacitance is ascribed to the combination of faradaic and electrical double-layer capacitance.

• The hybrid supercapacitor made of NiO-HMNAs and MGMs showed a high energy density and outstanding cycleability.

Abstract

Hybrid supercapacitors (HSCs), which usually involve faradaic or pseudocapacitive positive materials and electric double-layer capacitive negative materials, have demonstrated great potentials with enhanced energy density outdistancing traditional electrical double-layer capacitors. To endow materials with higher energy density and power density, the rational design and synthesis of electrodes with hierarchical and mesoporous structure are highly desired. In this work, we report the fabrication of hierarchical mesoporous NiO nanoarrays (NiO-HMNAs) as a battery-type electrode for hybrid supercapacitor with an ultrahigh specific capacitance (3114 F g⁻¹ at the current density of 5 mA cm⁻²), which is beyond the theoretical faradaic capacitance value of NiO. NiO-HMNAs were prepared by a self-generated sacrificial template approach, which involves the preparation of hierarchical ZnO/NiO composites by co-deposition of Zn²⁺ and Ni²⁺ and the removal of ZnO by an alkali etching process to construct mesoporous structure. The ultrahigh capacitance of NiO-HMNAs is ascribed to the nearly full redox reaction of NiO in the unique hierarchical mesoporous architecture, and the raised electrical double-layer capacitance at the enlarged surface of nanoarrays. Moreover, the optimized HSC fabricated by using NiO-HMNAs as the positive electrode and macroporous graphene monoliths (MGMs) as the negative electrode has demonstrated a high energy density of 67.0 W h kg⁻¹ at a power density of 320 W kg⁻¹ with a maximum voltage of 1.6 V and outstanding cycleability (capacitance retention of 89.6% after 6000 cycles).

Introduction

Supercapacitors have attracted great interest as one of the most promising electrical energy storage devices to alleviate the energy and environmental challenges because of their desirable properties such as fast energy delivery, high power density, and long life cycling behavior [1], [2], [3], [4]. Nevertheless, their further applications are restricted by the limited energy density. Developing high-performance supercapacitors with higher energy density without sacrificing the power delivery and cycling stability is highly demanded but still facing a major challenge [5], [6], [7]. Recently, hybrid supercapacitors (HSCs), which involve pseudocapacitive or faradaic battery-type materials as the positive electrode and porous carbon as the negative electrode have demonstrated great potentials to deliver enhanced energy density because of the improved specific capacitance from the pseudocapacitive or faradaic battery-type electrode and extended operating voltage from carbon electrode [8], [9]. With the purpose of enhancing electrochemical performance of HSCs, considerable efforts have been devoted to promote the performance of the positive electrode materials [1], [10], [11], [12]. NiO is a promising battery-type material due to its high theoretical specific capacitance (2935 F g⁻¹ at voltage window of 0.44 V), well-defined redox behavior and low cost. However, despite the fact that various NiO nanostructures, including nanoflakes [13], nanobelts [14], nanocolumns [15] and hierarchical porous [16], [17] structures had been employed in supercapacitors, most of them showed capacitances less than 1500 F g⁻¹ and usually accompanied with poor rate capability and stability. Theoretically, these issues can be addressed by constructing a structure with following features: 1. Large surface area, which increases the exposing active sites and thus enlarges the utilization of active material. 2. Large porosity, which facilitates penetration of the electrolyte into the whole electrode matrix, and thus promotes the electrochemical reaction efficiency. 3. High electronic transport efficiency between the metal oxides and current collector, enabling a fast electrochemical energy conversion process. 4. Structural stability, which is necessary to prevent agglomeration or collapse of electrode materials.

In recent years, three dimensional (3D) hierarchical architectures, such as nano- or microarrays on conductive substrate, have shown potentials related to above features [18], [19], [20], [21]. For example, Lou and his coworkers reported hierarchical NiMoO₄ nanosheets and nanorods arrays grown on conductive substrates for hybrid supercapacitors with high capacitance and stability [22]. We prepared hierarchical core-shell Co₃O₄@NiO nanowire@rod arrays, which exhibited an ultrahigh areal capacitance (39.6 F cm⁻² at the current density of 5 mA cm⁻²) and remarkable rate capability [23]. However, because the sizes of the rod or sheet of the arrays are commonly hundreds of nanometers or even bigger, it is hard to achieve a full use of the active materials due to the penetration depth limitation of electrolyte (approximately 20 nm [24]). Increasing porosity of the active materials is an effective way to address this issue. The mesoporous structure provided abundant active sites accessible to charge storage, thus leading to excellent capacitive properties [25], [26]. For example, mesoporous cobalt carbonate hydroxide nanosheet arrays derived from CoAl-LDH were reported to exhibit enhanced specific capacitance (1075 F g⁻¹ at 5 mA cm⁻²), high rate capability and cycling stability (92% maintained after 2000 cycles) [27].

As we know, the capacitance of transitional metal oxides mainly comes from the multi-electron transfer faradaic processes. Nevertheless, the capacitance from electrochemical double layer (EDLCs) will not be neglected when the specific area of a pseudocapacitive or battery-type electrode increases greatly. Theoretically, when integrating both the faradaic capacitance from redox and the EDLCs on large surface area, an ideal structure of metal oxide electrode can provide reversible capacitance even higher than its theoretical faradaic capacitance value. However, such an expectation was rarely demonstrated [28], [29]. Herein, hierarchical mesoporous NiO nanoarrays (NiO-HMNAs) with ultrahigh capacitance of 3114 F g⁻¹ were developed for an aqueous HSC. NiO-HMNAs were prepared on conductive foam by a self-generated sacrificial template method. Hierarchical ZnO/NiO nanoarrays (ZnO/NiO-HNAs), where both the targeted mesoporous NiO material and the ZnO templates belong to the composites, were firstly prepared by hydrothermal co-deposition and calcination treatment. Mesoporous structures were obtained after removing ZnO matrix by alkali etching. Such hierarchical and mesoporous structure empowered the NiO-HMNAs electrode with excellent energy storage performance. An ultrahigh specific capacitance of 3114 F g⁻¹, higher than NiO's theoretical faradaic capacitance (2935 F g⁻¹ at voltage window of 0.44 V) was obtained at the current density of 5 mA cm⁻², which is ascribed to the combination of high faradaic and electrical double-layer capacitances. A HSC with the NiO-HMNAs as the positive electrode and macroporous graphene monoliths (MGMs) as the negative electrode (NiO-HMNAs//MGMs) was fabricated, which achieved a high energy density of 67.0 Wh kg⁻¹ at a high power density of 320 W kg⁻¹, and an excellent electrochemical stability with specific capacitance retention of 89.6% after 6000 cycles.