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Three-Dimensional Hierarchical NixCo1-xO/NiyCo2-yP@C Hybrids on Nickel Foam for Excellent Supercapacitors Yubo Shao, Yong-Qing Zhao, Hua Li, and Cai-Ling Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12881 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 4, 2016
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ACS Applied Materials & Interfaces
Three-Dimensional Hierarchical NixCo1-xO/NiyCo2-yP@C Hybrids on Nickel Foam for Excellent Supercapacitors Yubo Shao, Yongqing Zhao, Hua Li, Cailing Xu* State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Laboratory of Special Function Materials and Structure Design of the Ministry of Education, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China * Corresponding author:
C. L. Xu: Tel: +86-931-891-2589, FAX: +86-931-891-2582, Email:
[email protected];
[email protected] KEYWORDS: Carbon coverage, Excellent performance, Nickel cobalt phosphide, Nickel cobalt oxide, Supercapacitor ABSTRACT Active materials and special structures of the electrode have decisive influence on the electrochemical properties of supercapacitors. Herein, three-dimensional (3D) hierarchical NixCo1-xO/NiyCo2-yP@C (denoted as NiCoOP@C) hybrids have been successfully prepared by a phosphorization treatment of hierarchical NixCo1-xO@C grown on nickel foam. The resulting NiCoOP@C hybrids exhibit an outstanding specific capacitance and cycle performance because they couple the merits of the superior cycling stability of NixCo1-xO, the high specific capacitance of NiyCo2-yP, the mechanical stability of carbon layer and the 3D hierarchical structure. The specific capacitance of 2638 F g-1 can be obtained at the current density of 1 A g-1 and even at 1
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the current density of 20 A g-1, the NiCoOP@C electrode still possesses a specific capacitance of 1144 F g-1. After 3000 cycles at 10 A g-1, 84% of the initial specific capacitance is still remained. In addition, an asymmetric ultracapacitor (ASC) is assembled through using NiCoOP@C hybrids as anode and activated carbon as cathode, respectively. The as-prepared ASC obtains a maximum energy density of 39.4 Wh kg-1 at a power density of 394 W kg-1 and still holds 21 Wh kg-1 at 7500 W kg-1. 1. INTRODUCTION Supercapacitors, which can supply superior energy density compared to dielectric capacitors and higher power density than batteries, have attracted considerable research attention as a hopeful energy conversion and storage device for different applications such as man-carried electronic equipment, electric automobile and backup energy systems.1-3 Electrode materials, as is known to all, play the decisive role for the performance of supercapacitors.4, 5 To date, various electrode materials such as carbon-based materials,6, 7 conducting polymers,8 and transition metal oxides/hydroxieds9,
10
have been extensively studied. However,
despite great progress, the inherent flaws of these electrode materials such as the low energy density for carbonaceous materials, the poor cycle stability for conducting polymers, and low electric conductivity for transition oxides/hydroxides have seriously hindered their practical applications. Consequently, there is a strong desire to develop novel electrode materials for improving the performance of supercapacitors. 2
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ACS Applied Materials & Interfaces
In recent years, transition metal phosphides (e.g., Ni2P,11 CoP,12 MoP,13 FeP14), as an important class of functional materials, have been extensively studied in lithium-ion batteries,15 photocatalytic degradation,16 hydrodesulfurization (HDS)17 and electrocatalytic water splitting18 due to their excellent catalytic performance and superior electric conductivity. Especially, transition metal phosphides are recently investigated as an attractive new class of electrode materials for supercapacitors. Most noteworthy is metal-rich phosphides, which may show the better capacitive performance because of the metalloid characteristics or superconductivity induced by strong electron delocalization always occurring in the metal sublattice of phosphides.19, 20 For instance, Wang et al. reported the Ni2P@rGO composites with a specific capacitance of 2266 F g-1 at 5 mA cm-2.21 Kong and co-workers successfully synthesized Ni2P and Ni5P4 nanoparticles by the ball milling process and annealing method, which exhibited a specific capacitance of 843 F g-1 and 801.5 F g-1, respectively.22 More recently, Chen et al. reported the Ni2P nanosheets supported on Ni foam demonstrated a specific capacity of 2141 F g-1 at a scan rate of 50 mV s-1 and still remained a specific capacitance of 1109 F g-1 even at 83.3 A g-1.23 Unfortunately, two obvious deficiencies of poor cycle stability and unsatisfactory specific capacitance still generally exist in the current phosphide electrodes. Furthermore, an optimized integration of phosphide materials and other electroactive materials is considered as an efficient method to improve the cycling stability and specific capacitance of supercapacitor. For example, Liu and co-workers have prepared Ni2P/Co3V2O8 nanocomposites by a facile chemical 3
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precipitation technique, which showed the negligible capacity attenuation (
