Cite this: DOI: 10.1039/c2ra22743f Received 8th August 2012, Accepted 9th November 2012 Nano a-NiMoO 4 as a new electrode for electrochemical supercapacitors3 DOI: 10.1039/c2ra22743f www.rsc.org/advances Baskar Senthilkumar, ab Kalimuthu Vijaya Sankar, a Ramakrishnan Kalai Selvan,* a Meyrick Danielle b and Minakshi Manickam b Nickel molybdate (a-NiMoO 4 ) nanoparticles were prepared by a solution combustion synthesis (SCS) technique and, for the first time, were studied as a potential electrode material for super- capacitors. High specific capacitance (1517 F g 21 ) and energy density (52.7 W h Kg 21 ) were delivered by nano-a-NiMoO 4 at a current density of 1.2 A g 21 , due to the pseudocapacitive nature of the material. Electrochemical supercapacitors are electrical energy storage devices that are used in power applications such as uninterrupted power supplies, pacemakers, electric vehicles, airbags, forklifts and load cranes. They can store electrical energy based on either the formation of an electrical double layer (EDL) or fast surface redox reactions (pseudocapacitive). 1–3 Pseudocapactive-type elec- trode materials, such as transition metal oxides and conducting polymers, can deliver high energy density relative to that delivered by electrical double layer capacitors (EDLCs) using carbon-based active materials. 1–3 In the last few decades, research has focused on pseudocapacitive metal oxides and hydroxides, including RuO 2 , 4 IrO 2 , 5 MnO 2 , 6 Fe 2 O 3 , 7 V 2 O 5 , 8 MoO 3 , 9 NiO, 10 Co 3 O 4 , 11 Ni(OH) 2 , 12 Co(OH) 2 , 13 MnMoO 4 14 and CoMoO 4 . 15 Among the metal oxides, RuO 2 is a promising electrode material that can supply a high energy density, 16 high specific capacitance (SC) (up to 1300 F g 21 ) 17 and good reversibility. 16,17 However, the high cost and toxicity of RuO 2 limits its commercialization. In recent years, research has been particularly centred on Ni(OH) 2 , Co(OH) 2 , MnMoO 4 , and CoMoO 4 due to their superior electrochemical properties, environmentally benign nature and, in the case of nickel and manganese, their low cost. 12–15 Recently, Yang et al. reported a very high SC of 3152 F g 21 for electrodeposited Ni(OH) 2 in a KOH electrolyte, 12 the highest SC value reported for any material thus far. Similarly, Ni(OH) 2 prepared by a hydrothermal method exhibits a high capacitance of 2675 F g 21 . 18 The high SC exhibited by these Ni(OH) 2 materials is due to the diffusion-controlled reversible redox reaction: Ni( II) « Ni(III)+e 2 . 18–22 While Ni(OH) 2 exhibits a high SC, and there are several reports of its suitability as a supercapacitor electrode, 19–22 it is disadvantaged by poor cycling stability. More recently, MnMoO 4 , CoMoO 4 , MnMoO 4 /CoMoO 4 nano- composites and heterostructured MnMoO 4 /CoMoO 4 nanowires have been synthesised by a micro-emulsion method. Heterostructured MnMoO 4 /CoMoO 4 nanowires show a SC of 187.1 F g 21 and excellent cycling stability. 10 Likewise, a CoMoO 4 / MWCNT composite material prepared by Xu et al. has demon- strated an improved capacitance relative to many other cobalt materials of 170 F g 21 and good cycling stability. 11 Metal molybdates have also been studied as electrode materials for Li- ion batteries 23–25 and NiMoO 4 and Ni 0.75 Co 0.25 MoO 4 nanowires synthesised by a hydrothermal method exhibit a good reversible capacity of 520 mA h g 21 after 20 cycles. 23 The studies based on Ni(OH) 2 , MnMoO 4 , CoMoO 4 and NiMoO 4 have motivated us to investigate NiMoO 4 as an electrode material for supercapactiors. NiMoO 4 has many advantages, such as chemical stability, low cost and enhanced electrochemical performance, 23–25 but it has not yet been studied in a super- capacitor context. In the present work, we have attained a high SC and a superior energy density of 1517 F g 21 and 52.7 W h Kg 21 , respectively, at a power density of 300 WKg 21 for nano- a-NiMoO 4 , due to the Faradic reversible reaction of Ni(II) « Ni(III)+e 2 . The electrochemical characteristics of the NiMoO 4 obtained in this study have been compared with those of MnMoO 4 and CoMoO 4 ?xH 2 O synthesised by the same technique. These studies reveal the potential application of this cost effective, environmen- tally-friendly material as an electrode material for supercapacitors. Nanosized a-NiMoO 4 was synthesised by the solution combus- tion synthesis (SCS) technique using Ni(NO 3 ) 2 ?6H 2 O (2.66 g), (NH 4 ) 6 Mo 7 O 24 ?4H 2 O (1.615 g) as precursors and urea, CO(NH 2 ) 2 (0.68 g), as a fuel. An oxidant-to-fuel ratio of 1 : 1 was used. A detailed experimental procedure of a-MnMoO 4 is given else- where. 26 The phase formed was identified by powder X-ray diffractometry 5635 (Siemens, D500 advance) with Cu–Ka radia- tion. Infrared absorption spectra were obtained in the wave a Solid State Ionics & Energy Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore 641 046, India. E-mail: Tel: +91-422-2428446; Tel: +91-422-2428446 b School of Chemical and Mathematical Sciences, Murdoch University, Murdoch, WA 6150, Australia 3 Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra22743f RSC Advances COMMUNICATION This journal is ß The Royal Society of Chemistry 2012 RSC Adv. Downloaded on 24 November 2012 Published on 23 November 2012 on http://pubs.rsc.org | doi:10.1039/C2RA22743F View Article Online View Journal