Abstract
This study is about the preparation of (Ni-Co)(OH)2 by cathodic deposition method and its applications on supercapacitors. The experiments were divided into three parts. The first part was focused on the design of experiment, which was used to study the factors that influence electrochemical performance of the (Ni-Co)(OH)2 and find an optimal condition for this synthesis. The second part was focused on synthesizing a sandwich-like (Ni-Co)(OH)2/carbon nanotubes (CNT) composite in order to increase the areal capacitance, then assembled as asymmetric supercapacitor to enlarge energy density. The goal is to enhance its potential for practical applicatipons. In the third part, we used chemical vapor deposition (CVD) to grow graphene on different substrates which acted as the current collectors for cathodic deposition of (Ni-Co)(OH)2. Then, we discussed the reasons that affected the electrochemiacal performance of (Ni-Co)(OH)2/graphene composite. The followings are the contents of each part. In the first part, we deposited the (Ni-Co)(OH)2 by cathodic deposition and made a 24 full factorial design of experiment (DOE). The four factors chosen were as below, Ni2+/(Ni2++Co2+), pH value of the precursor solution, deposition temperature, and deposition current. The goal is to increase the specific capacitance and improve the rate capability of the (Ni-Co)(OH)2. Finally, we did a steepest ascent experiment according to the regression model obtained from DOE, and the optimal conditions were obtained as below, 0.35 of Ni2+/(Ni2++Co2+), 3.60 of pH value, 70℃ of deposition temperature, and 5 mA/cm2 of current density. The highest specific capacitance was 1218.5 F g-1 (at 5 mV s-1) and rate retention (100 mV s-1) was 71.8%. Finally, using the optimal (Ni-Co)(OH)2 as the cathode combined with activated carbon (AC), the assembling asymmetric supercarpacitor can charge from 0.166 V to 1.6 V, exhibiting a energy density of 16.7 W h kg-1 and a power density of 7.2 kW kg-1 at 10 A g-1. In the second part, the sandwich-like were (Ni-Co)(OH)2/CNT exhibited a improved areal capacitance of 1.55 F cm-2. Seen from the SEM images, compared to the galvanostatic method, the pulse-rest method facilitated the (Ni-Co)(OH)2 depositing into the pores of CNT, which further improved the utilization of active materials and increase the specific capacitance and redox reversibility of the composite (Ni-Co)(OH)2/CNT. The composite (Ni-Co)(OH)2/CNT containing four layers of hydroxides exhibited a areal capacitance of 3.68 F cm-2 at 2 A g-1. Besides, combined with AC, the assembling asymmetric supercapacitor can charge from 0.166 V to 1.6 V, exhibiting a energy density of 17.1 W h kg-1 and a power density of 7.2 kW kg-1 at 10 A g-1. In the third part, comparing the composites (Ni-Co)(OH)2/graphene grown on different substrates including Ni foam, stainless steel mesh, and Ti foil, we found that (Ni-Co)(OH)2/graphene on Ni foam was uniformly deposited and highly porous, exhibiting the highest specific capacitance of 1593 F g-1 and areal capacitance of 9.9 F cm-2 . Besides, combined with AC, the assembling asymmetric supercapacitor can charge from 0.2 V to 1.6 V, exhibiting a energy density of 17.1 W h kg-1 and a power density of 7.2 kW kg-1 at 10 A g-1.