Electrochemical investigation of copper/nickel oxide composites for supercapacitor applications

Highlights

• Novel micro branched copper/nickel oxide composite with high surface area has been developed for supercapacitor application.

• Simple and continuous electroplating processes are adopted to fabricate the composite material.

• The measured maximum specific capacitance was 296.2 F/g at the scan rate of 10 mV/s.

• The fabricated metal-based electrode has good conductivity and capacitive properties with low internal resistance (2.05 Ω).

Abstract

Micro-branched copper/nickel oxide composites were synthesized in this article by using two simple and continuous electroplating processes, chemical oxidation, and annealing process. Morphology and electrochemical properties of the as prepared sample were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), and electrochemical measurements. The SEM images indicated that the as prepared micro branched copper/nickel oxide composites had porous and branched microstructure, which derived from the rapid generation and dispersion of the hydrogen bubbles while electroplating copper under the high current density and the well-formed nickel layer on the copper surface. The measured maximum specific capacitance was 296.2 F/g at the scan rate of 10 mV/s. In addition, cyclic voltammetric curves indicated good ions accessibility and charge storage ability of the prepared composite material, which was mainly due to its high surface area, novel branch-like microstructure and low inner resistance (2.05 Ω). Moreover, the developed composite material exhibited long cycle life along with 97% specific capacitance retained after 500 cycles. The enhancement of the electrochemical performance also benefited from good adhesion and low contact resistance between the copper and nickel layers.

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.