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Multishelled Nickel-Cobalt Oxide Hollow Microspheres with Optimized Compositions and Shell Porosity for High-performance Pseudocapacitors Xiangcun Li, Le Wang, Jianhang Shi, Naixu Du, and Gaohong He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04654 • Publication Date (Web): 21 Jun 2016 Downloaded from http://pubs.acs.org on June 24, 2016
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ACS Applied Materials & Interfaces
Multishelled
Nickel-Cobalt
Oxide
Hollow
Microspheres
with
Optimized
Compositions and Shell Porosity for High-performance Pseudocapacitors
Xiangcun Li,* Le Wang, Jianhang Shi, Naixu Du, Gaohong He* State Key Laboratory of Fine Chemicals, Dalian University of Technology Linggong Road 2#, Dalian 116024, China E-mail:
[email protected],
[email protected] 1
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Abstract Nickel-cobalt oxides/hydroxides have been considered as promising electrode materials for high-performance supercapacitor. However, their energy density and cycle stability are still very poor at high current density. Moreover, there are few reports on the fabrication of mixed transition metal oxides with multishelled hollow structures. Here, we demonstrate a new and flexible strategy for the preparation of hollow Ni-Co-O microspheres with optimized Ni/Co ratios, controlled shell porosity, shell numbers and shell thickness. Owing to its high effective electrode area and electron transfer number ( n3 / 2 A ), mesoporous shells and fast electron/ion transfer, the triple-shelled Ni-Co1.5-O electrode exhibits an ultrahigh capacitance (1884 F/g at 3A/g) and rate capability (77.7%, 3-30A/g). Moreover, the assembled sandwiched Ni-Co1.5-O//RGO@Fe3O4 asymmetric supercapacitor (ACS) remains 79.4% of its initial capacitance after 10,000 cycles and shows a high energy density of 41.5 W h kg-1 at 505 W kg-1. Importantly, the ACS device delivers a high energy density of 22.8 W h kg-1 even at 7600 W kg-1, which is superior to most of the reported asymmetric capacitors. This study has provided a facile and general approach to fabricate Ni/Co mixed transition metal oxides for energy storage. Keywords: Ni-Co Oxides, Multishell, Hollow Microspheres, Shell Porosity, Pesudocapacitor
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ACS Applied Materials & Interfaces
1. Introduction Nickel- and cobalt-based oxides/hydroxides and their mixed forms have been widely used as ideal electrode materials because of their high availability and excellent pseudocapacitive properties.1-6 Particularly, Ni-Co mixed oxides such as NiCo2O4 have been attracting tremendous interest in electrochemical capacitors due to the tendency to form high electroactive surface area structures with the presence of various valence state of nickel and cobalt (Ni2+/Ni3+ and Co2+/Co3+).7,8 Moreover, the Ni-Co mixed oxides possess much higher electrical conductivity of 10-1-10 S cm-1 than the monometallic NiO and Co3O4 (10-3-10-2 S cm-1).9 However, the molar ratio of Co/Ni in the NiCo2O4 crystalline is fixed at 2:1. Accordingly, synthesis of Ni-Co mixed oxides with variable compositions and unique structures and their influences on the supercapacitor properties is still a challenge work. To date, different Ni-Co oxides/hydroxides such as NiCo2O4 spheres/nanosheets,4,10
[email protected](OH)2 rods,11 NixCo3-xO4 nanowires,9 C/CoNi3O4 nanoarrays,12 Ni-Co-Al-LDH layered hydroxide,8 Co0.45Ni0.55O/RGO composites,13 etc., have been developed for capacitors. However, previous attempts to fabricate Ni, Co-based oxide supercapacitor electrode materials encountered unsatisfactory performances such as low capacitance (
