Electrochemical investigation of copper/nickel oxide composites for supercapacitor applications
Introduction
Nowadays, the imminent shortage of fossil fuels and growing environmental concerns are pushing scientists and engineers to exploit sustainable, clean, and highly efficient technologies to supply and store energy, which are ultimately related to our human health, daily lives, environment and global economy. Among various energy supply systems, fuel cells have experienced remarkable rapid growth because these sources are pollute-free and provide with abundant power and energy performance [1]. However, a single fuel cell system is not always sufficient to satisfy the load demands of a vehicle, and the response of fuel cells during transient and instantaneous peak power demands is relatively poor. To overcome the problem, the fuel cell systems should be hybridized with energy storage systems (ESS) with both high energy and power performance to meet the total power demand of a hybrid electric vehicle (HEV). Supercapacitors, also known as electrochemical capacitors or ultra-capacitors, are considered as promising candidates to improve the performance of the fuel cell systems [2]. The fuel cell systems hybridized with supercapacitors could be charged or discharged at a high current, beyond the level at which the battery can function. Its fuel economy is also higher, and it performs more efficiently than the fuel cell/battery. Researchers have proved that supercapcitors are able to provide higher power density and longer cycle life than batteries and higher energy density than traditional electric capacitors [3], [4], [5], [6]. Supercapacitors can be classified into electrical double layer capacitors (EDLCs) and pseudocapacitors based on the charge storage mechanisms. EDLC is operated by storing electrostatic charges at the interface of the electrode and electrolyte, which is a non-Faradaic process. Usually, carbon based materials such as active carbon [7], [8], [9], carbon nanotubes (CNTs) [10], [11], [12], graphene based materials [13], [14] and carbon aerogel [15], [16] are developed for EDLCs due to their ultra high specific surface area and excellent electrochemical stability in electrolytes. Pseudocapacitors store the energy through the redox reaction of the electrode material with the electrolyte. The accumulation of the electrons at the electrode is a Faradaic process involving the passage of the electrons produced by the redox reaction across the electrolyte–electrode interface. The energy storage mechanism is similar with that of the rechargeable batteries, but the special relation between the extent of charge acceptance (Δq) and the charge of the potential (ΔV), the derivative (d(Δq)/d(ΔV)), is equivalent to a capacitance and can be experimentally measured [3]. Usually, transition metal oxides [17], [18], [19] and conductive polymers [20], [21], [22] are widely used as electrode materials for pseudocapacitors. Nowadays, transition metal oxides are wildly researched because of their high theoretical specific capacitance, environmental friendly nature and low cost. However, the poor electric conductivity and densely packed structure limit their power properties for wider applications. Meanwhile, contact resistance across the carbon-binder and carbon–metal interfaces could be another issue since it dissipates power and decreases energy density. As we know, the maximal power density is inversely proportional to the equivalent series resistance, or ESR. Thus, decreasing inner resistance becomes important for developing supercapacitors with higher performances.
Recently, researchers have demonstrated that metal electrodes have several attractive advantages as supercapacitor electrodes, such as relatively high conductivity, electrochemical stability and high power density [23], [24], [25], [26], [27]. For instance, Hideyuki Nakanishi and Bartosz Grzybowski [23] developed a porous Au electrode by etching the silver from the Au–Ag alloys and achieved high power density for supercapacitor application. However, the high cost of gold does not make it suitable for large-scale applications. Nickel could achieve high electrochemical performance due to the conversion from Ni to Ni (II), which possesses multiple oxidation states and enables rich redox reactions for pseudocapacitance generation [28]. Lately, nickel based composites with various micro/nano structures were developed as supercapacitor electrode materials. For instant, Lengyuan Niu et al. developed Ni@C core–shell composite by using hydrothermal synthesis and annealing processes [24]. The composite exhibits a high specific capacitance of 530 F/g. Qi Lu and his partners fabricated monolithic Ni–NiO nano composite using a modified polyol process and thermal annealing process [26]. And Jae-Hun Kim et al. have synthesized 3D ordered Ni core–NiO shell inverse opal using the polystyrene bead template-assisted electrochemical approach [27].
Based on above considerations, the goal of this work is to develop novel composite electrodes with high surface area by using simple and scalable fabrication processes and optimize the electrode performance by using a well-defined electrode network with decreased inner resistance. In detail, three-dimensional copper core–nickel shell composite electrode is deposited on the Ti/Cu coated substrate by using two simple and continuous electroplating processes. The micro-porous and branched structure can provide ultra-high surface area for the location of electro-active nickel materials. Then nickel oxide formed on the nickel surface in a chemical oxidation process using hydrogen peroxide solution as oxidant. This prepared micro branched Cu/NiO composite material possesses increased specific surface area and is expected to result in more facile electrical conductivity compared with that of conventional transition metal oxides and carbon based composite materials.
Section snippets
Material preparation
As described in Fig. 1, the micro branched Cu/NiO composite electrode was prepared by five steps. Firstly, Thin Ti/Cu seed layers with thickness of 300 Å/1500 Å were sputtered on the silicon oxide coated wafer, then the as-prepared wafer was cleaned and rinsed in the acetone, methanol and DI water for 10 min respectively, serving as the substrate. Then copper was electroplated from a mixture solution consisting of 0.4 M CuSO4 and 1.5 M H2SO4, where 0.1 M acetic acid was added to stabilize
Results and discussion
The morphology of the as-prepared materials has been investigated by the scanning electron microscopy (SEM) technique. Fig. 2 (a, b, c) shows the SEM images of the electroplated copper material at different magnifications. As clearly shown in these figures, the porous Cu was formed with micro porous and branch-like structure as desired. This novel structure resulted from the fast and vigorous evolution of hydrogen bubbles during the copper electroplating process under the applied high current
Conclusion
In summary, copper/nickel oxide composites with porous and branch-like microstructure have been synthesized on the Ti/Cu seed layer by two continuous electroplating processes, chemical oxidation and annealing process. The prepared composite material exhibited the maximum specific capacitance of 296.2 F/g at the scan rates of 10 mV/s. The enhancement of the electrochemical performance (9.65 Wh/kg at 1.25 kW/kg) was obtained due to its high surface area, novel structured morphology, and low inner
Acknowledgments
This research was partially supported by the research grant of Kwangwoon University in 2013, the Pioneer Research Center Program (2010-0019313), and the Basic Science Research Program (2010-0024618) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. The authors are grateful to MiNDaP (Micro/Nano devices and Packaging Lab.) group members of Kwangwoon University.
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