In-situ hydrothermal synthesis of MnO 2 /NiO@Ni hetero structure electrode for hydrogen evolution reaction and high energy asymmetric supercapacitor applications Sanjit Saha a,b , Suman Chhetri a,b , Partha Khanra c , Pranab Samanta a,b , Hyeyoung Koo d , Naresh Chandra Murmu a,b , Tapas Kuila a,b, * a Surface Engineering and Tribology Division, CSIR-Central Mechanical Engineering Research Institute, Durgapur 713209, India b Academy of Scientific and Innovative Research (AcSIR), CSIR-CMERI Campus, Durgapur 713209, India c Soft Innovative Materials Research Centre, Korea Institute of Science and Technology (KIST), Jeonbuk 565905, South Korea d Soft Innovative Materials Research Centre, Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Jeonbuk 565905, South Korea A R T I C L E I N F O Article history: Received 18 November 2015 Received in revised form 17 February 2016 Accepted 17 February 2016 Available online xxx Keywords: Multi-metal oxide Binder free electrode Hydrogen evalution reaction Thermaly reduced graphene oxide Asymmetric device Energy density A B S T R A C T In-situ deposition of MnO 2 /NiO hetero structure on Ni-foam has been carried out through simple one- step hydrothermal reaction. The in-situ deposited multi-metal oxide shows extraordinary electro- catalytic activity in hydrogen evolution reaction with a small Tafel slope of 38 mV per decade and very low onset potential of 0.17 V. An electrolyser has been fabricated with MnO 2 /NiO deposited Ni-foam which effectively achieves a current density of 24 mA cm 2 at an applied voltage of 1.57 V. Furthermore, the metal oxide deposited on Ni-foam is directly used as the supercapacitor electrode which shows good rate capability as positive electrode materials. An asymmetric capacitor (ASC) has been assembled by using the MnO 2 /NiO deposited Ni-foam as positive electrode and thermally reduced graphene oxide as negative electrode. The assembled ASC has a large specific capacitance of 218 F g 1 at a current density of 3 A g 1 and can deliver high energy and power density of 59.5 Wh kg 1 and 25,350 W kg 1 , respectively. The ASC shows very good electrochemical stability throughout 10,000 charge–discharge cycles along with the capability to work in the high frequency range. ã 2016 Elsevier Ltd. All rights reserved. 1. Introduction The ever rising demand of energy accompanied with global warming and environmental deterioration has obliged us to develop an alternative source of clean energy fuel carrier [1–5]. Low pollutant emission and high efficiency make hydrogen as a future candidate for the replacement of fossil fuels [3,4]. The electrochemical splitting of water into hydrogen and oxygen is an effective technique of producing high-purity hydrogen [5]. The hydrogen evolution reaction (HER) and oxygen evolution reaction are the two half reactions of water splitting and plays crucial role for the overall efficiency [2]. Commercial electrolyzers suffer from large operating cell voltage which in turn increases the overall energy consumption resulting poor electrical efficiency [4,5]. The execution of efficient electrocatalysts can decrease the over potentials ensuing the whole process less energy-intensive [2]. On the other hand, the depletion of fossil fuels and environ- mental change has obliged us to develop an alternative energy storage device which can provide high energy density correspond- ing to a large power density [6–8]. Supercapacitors provide higher energy density as compared to the conventional dielectric capacitors and large power density than that of the batteries [9,10]. In addition, fast charge–discharge (CD) rate and long cyclic stability of supercapacitor suggests its convenient service in electrical vehicles, flexible and portable electronics [10–12]. Current research approaches of supercapacitor emphasis on the enhancement of specific capacitance of the electrode materials as well as extending the potential window due to its quadratic relationship with the energy density [13,14]. Development of different positive and negative electrode materials along with proper charge or mass balancing can provides wide potential window as well as high specific capacitance [15,16]. Therefore, two different electroactive materials can be assembled to fabricate an asymmetric device to generate both pseudocapacitance and pure * Corresponding author at: Surface Engineering and Tribology Division, CSIR- Central Mechanical Engineering Research Institute, Durgapur 713209, India. E-mail addresses: tkuila@gmail.com, kuila@cmeri.res.in (T. Kuila). http://dx.doi.org/10.1016/j.est.2016.02.007 2352-152X/ ã 2016 Elsevier Ltd. All rights reserved. Journal of Energy Storage 6 (2016) 22–31 Contents lists available at ScienceDirect Journal of Energy Storage journal homepa ge: www.elsev ier.com/locate/est