Journal of Power Sources 195 (2010) 6977–6981 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Short Communication Capacitance decay of nanoporous nickel hydroxide Guangxia Hu a , Chunxiang Li b , Hao Gong a,∗ a Department of Materials Science & Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore b Advanced Materials Technology Centre, Singapore Polytechnic, 500 Dover Road, Singapore 139651, Singapore article info Article history: Received 17 February 2010 Received in revised form 16 March 2010 Accepted 22 March 2010 Available online 3 April 2010 Keywords: Nanoporous nickel hydroxide Capacitance decay High current density Supercapacitor Phase change abstract Nanoporous nickel hydroxide Ni(OH) 2 coated on nickel foam by using a chemical bath deposition method shows a high specific capacitance of 2200 F g -1 at a discharging current density of 1 Ag -1 . After 500 charge–discharge cycles, the specific capacitance is stabilized at 1470 Fg -1 , and there is only a 5% fall in specific capacitance during the following 1500 cycles. The relationship between the capacitance decay and changes in the microstructure and morphology of nanoporous Ni(OH) 2 is investigated. The results show that phase transformation and the growth of particle/crystal size, rather than the formerly proposed flaking off of Ni(OH) 2 , are the major factors contributing to the capacitance decay. © 2010 Published by Elsevier B.V. 1. Introduction Nickel hydroxide (Ni(OH) 2 ) has been used for many decades as an active material for the positive electrode of batteries. These nickel-based batteries can perform well at relatively low discharge rates (0.1–1 C). At very high discharge rates (>10 C), however, only a few percent of the storage capacity can be used [1]. At high dis- charge rates/current densities, the best energy-storage device is the supercapacitor. Well-developed carbon-based supercapacitors (electric double-layer capacitors) have very high discharge current densities, but they suffer from a low energy density or a low specific capacitance of about 200 F g -1 [2]. In recent years, nanosized Ni(OH) 2 , which can function well at high discharge current densities, has been identified as a very promising material for supercapacitors [3–10]. Two types of energy-storage mechanism play a role in a supercapacitor, namely, non-faradic charging as in electric double-layer capacitors and Faradaic charging similar to the processes in batteries. Studies [11,12] show that high current density discharge performance is improved greatly by using nanostructured Ni(OH) 2 because of the high specific surface area, fast redox reaction and shortened diffu- sion path in the solid phase. Cheng et al. [8] reported a specific capacitance of ∼696 F g -1 for sol–gel-derived Ni(OH) 2 xerogels, i.e., a performance that is significantly higher than that of well- developed carbon-based materials. Yuan et al. [13] reported a specific capacitance of ∼710 F g -1 for spherical superstructured ∗ Corresponding author. Tel.: +65 65164632; fax: +65 68742081. E-mail address: msegongh@nus.edu.sg (H. Gong). Ni(OH) 2 . Lang et al. [5] and Yang et al. [14] further reported a higher specific capacitance (>2000 F g -1 ) for loosely packed, nanoflake Ni(OH) 2 structures. Although a high specific capacitance is achieved, nanostructured Ni(OH) 2 suffers from significant capac- itance decay during charge–discharge cycles and thereby prevents it from industry applications such as electrical vehicles and hybrid electrical vehicles. Some researchers propose that the capacitance decay may be due to the flaking off of Ni(OH) 2 [14]. There is no strong evidence to support this view, however, and no other possible reasons have been advanced, according to the best of our knowledge. A systematic investigation is needed to alleviate the ambiguities for a better understanding of this important phe- nomenon and thus pave the way to industrial applications of high capacitance Ni(OH) 2 supercapacitors. In this work, a Ni(OH) 2 film with interconnected nanoflakes is deposited directly on a nickel foam substrate, and a high specific capacitance of up to 2200 F g -1 is achieved. The decay in capaci- tance during charge–discharge cycles is studied systematically by means of multi-characterization techniques including XRD, SEM, TEM, and electrochemical methods. Capacitance decay behaviour under different current regimes is also examined. 2. Experimental details The solution for chemical bath deposition (CBD) was prepared by mixing 40 mL of 1 M nickel sulfate, 30 mL of 0.25 M potassium persulfate, 10 mL of aqueous ammonia (22–24% NH 3 ), and 20 mL of deionized water in a 250 mL Pyrex beaker at room temperature. The Ni foam substrate was chemically cleaned with acetone, methanol, 0378-7753/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.jpowsour.2010.03.093