Surface Roughening Process of Nickel Electrodes for Application of Supercapacitors

Abstract

Increase in energy demand and depletion of natural resources would be the major causes for inducing development of Energy Storage Systems. Supercapacitor has been known one of the most important electrochemical energy storing devices which can have better power density and excellent stability than batteries in some sense and higher energy density than conventional capacitors. To improve the performance of supercapacitors, many of research works have been focused on improving surface area, diffusion rate, electrical conductivity of materials and long cyclic stability. Various metal oxides including NiO, CoOx, MnO2, RuO2have been used for promising electrode materials for supercapcitors. Among these materials, Nickel has drawn significant attention as an efficient electrode material. Because Nickel has distinct features to be a main electrode material for supercapacitors such as light weight, low cost, chemical stability, good corrosion-resistance, fine electrical and thermal conductivity and excellent electrochemical redox performance. We used Ni foil as a basic electrode material and tried to enhance the surface area of the Ni foil using electrochemical deposition of NiZn fine structures on Ni foil. Also we observed the effects of temperature and current density on morphology of the product. As a result, inhomogeneous NiZn structures were obtained on Ni foil. The difference of morphology is mainly due to the position on a Ni foil electrode. So, the inhomogeneous structures can be divided into three categories such as edge, interval and center. The edge morphology was in shape of keyhole, interval showed cylinder structures and center gave sphere and hemisphere structures at a macro level. However, at a micro level, rough and porous surface was found in all three parts. We concluded that the reason for the inhomogeneous structures were due to mass transport effect. The homogeneity depends on the temperature and current density. Consequently, NiZn/Ni structure with rough, porous and large surface area was fabricated and showed enhanced areal capacitance about 13 times than Ni foil itself. To get much higher areal capacitance, we tried to make one more step after fabrication of the NiZn/Ni structure. That is electrochemical deposition of Nickel on NiZn/Ni structure to make Ni/NiZn/Ni structure. As a result, roughened surface was produced and Ni/NiZn/Ni structure showed much better areal capacitance than Ni foil itself and NiZn/Ni structure. The reason for the increased areal capacitance is based on roughened surface of the electrode which enabled to enhance the active material-electrolyte interfacial area. Consequently Ni/NiZn/Ni structure has more suitable morphology than Ni foil for application of supercapacitors. Materials were characterized by Scanning electron microscope (SEM), X-ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Energy dispersive X-ray spectroscopy (EDS). Electrochemical properties were measured by cyclic voltammetry (CV), galvanostatic charge/discharge measurement using electrochemical workstation (VersaSTAT3, Princeton Applied Research).