Nickel-based Nanomaterials for Electrochemical Supercapacitors

Abstract

The demand for energy storage technologies is rapidly increasing in portable electronics, transportation, and renewable energy systems. Thus, the objective of this research is to develop and enhance the performance of Ni-based electrochemical supercapacitors by optimizing synthesis conditions and design of the electrode materials. Conventional and on-chip supercapacitors were developed with notable performance enhancement.

For conventional supercapacitors, a uniform and conformal coating process was developed to deposit Ni(OH)2 nanoflakes on carbon microfibers in-situ by a simple chemical bath deposition at room temperature. The microfibers conformally-coated with Ni(OH)2 make direct physical contacts with essentially every single nanoflakes, leading to more efficient electron transport. Using this strategy, we have achieved devices that exhibit five times higher specific capacitance compared to planar (non-conformal) Ni(OH)2 nanoflakes electrodes prepared by drop casting of Ni(OH)2 on the carbon microfibers (1416 F/g vs. 275 F/g).

For on-chip storage applications, microfabricated supercapacitors were developed using a combination of top-down photolithography and bottom-up CBD. The resulting Ni(OH)2 micro-supercapacitors show high-rate redox activity up to 500 V/s and an areal cell capacitance of 16 mF/cm2 corresponding to a volumetric stack capacitance of 325 F/cm3. This volumetric capacitance is 2-fold higher than carbon and metal oxide based micro-supercapacitors. Furthermore, these micro-supercapacitors show a maximum energy density of 21 mWh/cm3, which is superior to the Li-based thin film batteries.

To enhance cycling stability, Ni-Cu-OH and Ni-Co-OH ternary electrodes have been prepared with different Ni:Cu and Ni:Co ratios by CBD at room temperature on carbon microfibers. It is observed that the electrodes with Ni:Cu and Ni:Co composition ratio of 100:10 results in an optimum capacitance and cycling stability. For the optimum composition, Ni-Co-OH with graphene and carbon nanofibers electrode was tested, with resultant improvement in electrode potential window, equivalent series resistance, and cyclic stability.

To further increase energy density, Ni(OH)2//Graphene asymmetric supercapacitor were fabricated with areal capacitance of 253 mF/cm2 at 5 mA/cm2 which is higher than NiO//rGO prepared by hydrothermal method. Ni-Co-OH/G-CNF//Graphene asymmetric supercapacitor results in a maximum power of 23 mW within an operating voltage of 2.2 V which are higher than of Ni(OH)2//Graphene (15.94 mW within 1.8 V). Our asymmetric supercapacitors have flexible-electrodes, low-cost fabrication process and environmentally friendly materials.