Fabrication and Characterization of Thin-Film Nickel Hydroxide Electrodes for Micro-Power Applications

Abstract

INTRODUCTION Micro power sources are very attractive for nano- and micro-scale devices, such as Point-of-Care medical diagnostics and Micro Electro Mechanical Systems, where only low power densities and capacities are required. In the present work, we report on the micro-fabrication of patterned α-Ni(OH)2 films which are suitable for rechargeable nickel metal hydride (Ni-MH) and nickel-zinc (Ni-Zn) micro-batteries as well as for micro-capacitors. The main objective of the current research is to investigate the influence of three-dimensional (3D) patterned structures on the mechanical stability as well as on the electrochemical performance of the electrode. ELECTRODE FABRICATION The patterned electrode is fabricated through multi-layered films which are deposited by employing e-beam evaporation, UV photolithography (using a negative KMPR photoresist), and electro-deposition techniques. In detail, thin films of chromium and nickel with a thickness of 40nm are deposited on glass and used as adhesion (seed) and current collector layers, respectively. NiOx/Ni(OH)2 is electro-deposited from NiCl2 solution followed by electro-precipitation of Ni(OH)2 films from Ni(NO3)2.6H2O aqueous solution to form a grid-like patterned layer. Figure 1 schematically depicts the fabrication process. MATERIALS & ELECTROCHEMICAL CHARACTERIZATION The chemical composition of the micro fabricated electrodes is characterized by employing X-Ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS). The feature size of the grid-like pattern is determined to be roughly 20 microns in height by using a mechanical profilometry technique. Figure 2 illustrates the XPS spectrum of the Ni(OH)2 electrode material based on the Ni2p3/2 spectrum. The electrochemical characteristics are studied using cyclic voltammetry (CV) in a 1M KOH electrolyte. Measurements are performed for unpatterned as well as patterned electrodes to investigate the influence of the 3D structure as shown in Figure 3. CONCLUSIONS We observe that electrode patterning considerably improves the adhesion of the α-Ni(OH)2 film to the nickel current collector layer. It is also found that the micro fabricated α-Ni(OH)2 electrode is reversible and diffusion limited according to the Randles-Sevcik linearity. Furthermore, the electrolyte concentration dependency is studied via CV which shows minimum concentration of around 0.2M required for good electrode REDOX reactions.