Template-free synthesis of hierarchical porous Co 3 O 4 microspheres and their application for electrochemical energy storage G.X. Pan a, *, X.H. Xia b , F. Cao a , J. Chen a, c , Y.J. Zhang a a Department of Materials Chemistry, Huzhou University, Huzhou, 313000, China b Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore c College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 313000, China A R T I C L E I N F O Article history: Received 30 March 2015 Received in revised form 12 May 2015 Accepted 13 May 2015 Available online 19 May 2015 Keywords: Hybrid batteries Cobalt oxide Nanoflake microspheres Porous materials Electrochemical energy storage A B S T R A C T Tailored high-activity cathode materials are of great importance for construction of high-performance capacitors and hybrid batteries. In this work, we report a facile template-free chemical bath deposition method to fabricate porous Co 3 O 4 nanoflake microspheres consisting of self-assembled nanoflakes with thicknesses of 10 nm. The as-prepared Co 3 O 4 microspheres have average diameters of 2 mm and the secondary Co 3 O 4 nanoflakes are interconnected with each other forming a highly open net-structure with pore diameters of 20-200 nm. When evaluated as cathode materials for pseudocapacitive hybrid batteries, the Co 3 O 4 nanoflake microspheres deliver a specific capacity of 83 mAh g 1 at 1 A g 1 after 10,000 cycles, higher than the Co 3 O 4 nanowire microspheres counterpart (70 mAh g 1 ). In addition, the electrode exhibits excellent long-term cycling stability with 94.5% capacity retention after 10,000 cycles at 1 A g 1 . The enhancement of high-rate electrochemical performances is due to the unique nanoflake microspheres architecture with large surface area and open porous structure. ã2015 Elsevier Ltd. All rights reserved. 1. Introduction Energy is the cornerstone on which people rely for existence. In the past decades, intensive research has been dedicated to developing advanced electrochemical energy storage devices with high energy/power density to respond to the increasing energy demand [1–3]. In particular, pseudocapacitive batteries (also called as high-rate hybrid batteries), usually consist of metal oxide/ hydroxide cathodes and carbon anodes, are regarded as one of the most important power sources due to their interesting character- istics in terms of fast recharge capability, high power density and long cycling life. It should be mentioned that, previously, the term “pseudocapacitive hybrid batteries” were, sometimes, wrongly called as “pseudo-capacitors” in the reference [4]. This conceptual confusion causes some mistakes and misleads the readers. More recently, Brousse and co-workers elaborated the difference between “pseudocapacitive hybrid batteries” and “pseudo-capaci- tors” in detail, and gave the right definitions of “pseudo-capacitors” and “pseudocapacitive hybrid batteries” [4]. It is accepted that the performance of pseudocapacitive hybrid batteries is mainly determined by the cathode materials, which are the core part and the key to high performance. In recent years, the research on high-capacity cathode of alkaline pseudocapacitive hybrid batter- ies focuses on metal oxides (NiO [5,6], and Co 3 O 4 [7–9], etc.), binary metal oxides (NiCo 2 O 4 [10], and ZnCo 2 O 4 [11]), hydroxides (e.g., Co (OH) 2 [12,13], Ni(OH) 2 [14]) and metal sulfides (NiS and CoS [15,16]), which have significantly high-rate capacities and energy densities arising from faradic redox reactions and pairing with porous carbon anode, which stores energy using fast ion adsorption. These cathode materials generally possess high theoretical values (200–300 mAh g 1 ), but their practical utiliza- tion is still low because these active materials, albeit their other advantages, have drawbacks of poor rate capability and reversibil- ity since they are kinetically unfavorable to support fast electron/ ion transport required by high power density. One of solutions is to achieve rational-designed nanostructures of the cathode materials [17,18]. The basic rule for designing new high-performance electrode materials of alkaline pseudocapacitive hybrid batteries is to boost the energy density as high as possible without sacrificing high power density. The energy density is controlled by the intrinsic electrochemical activity and surface area of the electrode material, for both nanostructured metal oxides/hydroxides are favorable. The power density is controlled by kinetics of charges and ions [19,20]. Therefore, it is crucial to enhance the kinetics of ion and electron transport in electrodes and at the electrode/electrolyte * Corresponding author. Tel.: + 86 572 232 1166. E-mail address: hipgxzjut@hotmail.com (G.X. Pan). http://dx.doi.org/10.1016/j.electacta.2015.05.078 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved. Electrochimica Acta 173 (2015) 385–392 Contents lists available at ScienceDirect Electrochimica Acta journa l home page : www.e lsevier.com/loca te/ele cta cta