Nano-Oxide Enhancement of Biomass-Derived Activated Carbons for Supercapacitor Applications
Authors: Yumak Tugrul , Yakaboylu Gunes Alp , Bragg Dustin , Oginni Oluwatosin , Singh Kaushlendra , Sabolsky Edward M. 
Published: February 2020

Activated carbons (AC) are most widely used electrodes materials due to their large surface area, low cost and ease of processing, but they can suffer from poor energy storage capacity and inferior rate capability. A capacitance higher than EDLC can be achieved through enhancing the ACs with a pseudocapactive mechanism, pseudocapacitive electrodes often suffer from poor electrical conductivity, and sometimes the electrodes demonstrate low cycling stability. Besides the type of electrode material, the microstructure and surface chemistry also affect the electrochemical performance. This study further studied the effect of modifying the surface chemistry and microstructure of ACs to activate a pseudocapacitive mechanism. Both commercial ACs and ACs derived from lignocellulosic biomass precursors were incorporated into the supercapacitors and tested.
Lignocellulosic biomass can be utilized by cost-effective and environmental-friendly techniques to produce carbonaceous materials with unique porosity and surface chemistry. The aim of this study was to compare the biomass-derived and modified activated carbon with commercial activated carbon in supercapacitor applications. For this purpose, different activated carbon samples were synthesized from naturally soft and hard biomasses, such as grass and woody type. Physical/chemical activation methods were applied in order to improve the porous structure and surface chemistry. In addition, the activated carbons were combined with different metal oxides by different techniques in order to investigate the effect of synthesis route on the surface chemistry, microstructure and electrochemical performance of metal-oxide/activated carbon composite electrodes. XPS, SEM, TGA and N2 physisorption techniques were used to determine the effect of experimental parameters on the surface chemistry, morphology, thermal stability (the content of metal oxide), and surface area and pore characteristics.
Electrode inks were prepared by mixing the synthesized material (modified activated carbon or metal oxide/activated carbon), carbon black (CB) and polyvinylidene fluoride (PVDF) at a weight ratio of 85:5:10. The electrodes were manufactured by a casting technique. Stainless steel as a current collector, KOH as an electrolyte, Nafion as a separator and stainless steel CR-2032 parts used to assemble the supercapacitors. The electrochemical performance evaluation was performed with self-discharge behavior for 50 cycles and constant current charge/discharge test for 5000 cycles in the voltage range of 0.1 V - 1 V with a constant current of 0.1 A/g. Due to the N2 physisorption results, chemical/physical activation process resulted in an increase in specific surface area for the activated carbons. The metal oxide loading led to a slight decrease in surface area but no significant effect on the porous structure of composite materials. The linear and symmetrical shape of the constant current charge-discharge curves proves that MnO2/AC and NiO/AC based electrodes possess the required electrochemical reversibility and charge/ discharge capabilities. A significant decrease in the ESR drop was obtained by metal oxide loading. The ESR drop for commercial AC, MnO2/AC (hydrothermal method) and NiO/AC (precipitation method) is 0.35 V, 0.077 V and 0.066 V, respectively. The calculated specific capacitance increased 160% and 56% for NiO/AC (precipitation method) and MnO2/AC (hydrothermal method), respectively, when compared to pristine commercial activated carbon based electrodes.

Acknowledgements:

The WVU Energy Institute through the O’Brien Energy Research Fund supported this work. The authors would also like to acknowledge West Virginia University Shared Research Facilities for support through materials characterization. Dr. Yumak acknowledges financial support from the Scientific and Technological Research Council of Turkey (TUBITAK) BIDEB-2219 Postdoctoral Research Program.