Browsing by Author "Enock, Talam"

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Biogas slurry derived mesoporous carbon/metal oxides composites for supercapacitors application
(NM-AIST, 2017-12) Enock, Talam
With the increasing utilities of intermittent renewable energies, hybrid electric vehicles and portable electronic devices, supercapacitors have been identified as one of the solution for energy storage since they can deliver high power. The main drawback of the supercapacitors is low energy densities which can be increased either by synthesizing nanostructured porous materials or improving the voltage window of the electrolyte. In this study the mesoporous carbons were synthesized from biogas slurry by varying the carbonization temperature (450, 550, and 650 ºC); activation temperature (600, 700, and 800 ºC); activation time from 30 to 120 min; and KOH activation agent to carbon mass ratio (1:1, 2:1, and 3:1). The mesoporous carbon samples synthesized were characterized by XRD, SEM, EDX, TEM, XPS, nitrogen sorption at 77 K, and the relevant electrochemical performances were investigated using cyclic voltammetry and electrochemical impedance spectroscopy. All samples exhibited type IV isotherms demonstrating the existence of mesopores. The specific surface area increased from 148 for unactivated carbon to 514 m2 g‒1 for samples activated at 700 ºC with the mass ratio of 3:1. The total volume increased from 0.12 to 0.38 cm3 g‒1 while the Smicro/ Smeso decreased with increasing activation time and KOH/carbon mass ratio. The carbon materials activated at 700 ºC, 3:1 KOH to carbon mass ratio for 120 min exhibited high specific capacitance of 289 F g–1 at a scan rate of 5 mV s‒1. When mesoporous carbon /MnO2 composites were synthesized through co-precipitation route; the BET surface area decreased from 514 to 110 m2 g‒1 while total pore volume decreased from 0.52 to 0.17 cm3 g‒1 for samples loaded with 40 mL of 5×10‒4 and 5×10‒2 M KMnO4, respectively and 0.2 g of mesoporous carbon. The electrode fabricated from the MnO2/ mesoporous carbon composites exhibited high specific capacitance of 709 F g‒1 at scan rate of 5 mV s‒1 in three electrodes cell system. XRD studies of the NiCo2O4/ mesoporous carbon composites revealed that the spinel structure of the NiCo2O4 was maintained in the composites. The nitrogen uptake increased with increasing annealing temperature to 300 ºC then decreased at 400 ºC. The NiCo2O4/ mesoporous carbon composites exhibited high specific capacitance of 835 F g‒1 at scan rate of 5 mV s‒1 for sample annealed at 350 ºC. The cyclic stabilities of the electrodes were above 90% after 50000 cycles. Electrochemical impedance spectroscopy studies demonstrate that synthesized materials have high conductivities. This study shows that high performance electrodes can be designed from biogas slurry derived porous carbon and its MnO2 and NiCo2O4 composites.
Biogas-slurry derived mesoporous carbon for supercapacitor applications
(Elsevier Ltd., 2017-09) Enock, Talam; King’ondu, Cecil; Pogrebnoi, Alexander; Jande, Yusufu
This study reports on the transformation of biogas slurry into mesoporous carbon for supercapacitor electrodes. Pore structures have been modified by altering activation time, temperature and KOH/carbon mass ratio. The mesoporous carbons are successively developed as evidenced by type IV isotherms obtained in nitrogen sorption studies. BET, micropore and mesopore surface area of 515, 350, and 165 m2 g−1, respectively as well as a narrow pore width distribution of 3–4.5 nm are obtained. X-ray photoelectron results have confirmed the presence of functional groups of oxygen and nitrogen in the samples which facilitates the pseudocapacitance. The electrochemical measurements in 6 M KOH using a three electrode cell with Ag/AgCl as reference electrode and platinum as counter electrode has been performed. The materials activated at 700 °C, 3:1 KOH to carbon mass ratio, and for 120 min exhibit high specific capacitance of 289 F g−1 at a scan rate of 5 mV s−1. Shortening activation time to 30 and 60 min reduces specific capacitance to 163 and 182 F g−1, in that order. Additionally, at 3:1 KOH to carbon mass ratio and 60 min activation time, specific capacitances of 170 and 210 F g−1 at 600 and 800 °C, respectively are obtained. Moreover, specific capacitance increases with increasing the KOH to carbon mass ratio from 148 F g−1 for 1:1–163 F g−1 for 3:1 at 700 °C. Electrochemical impedance spectroscopy studies demonstrate that material has high conductivity. In addition; capacity retention of 96% after 20,000 cycles is shown at scan rate of 30 mV s−1. The study shows that high performance electrodes can be designed from biogas slurry derived porous carbon.
Status of Biomass Derived Carbon Materials for Supercapacitor Application
(Hindawi, 2017-01-31) Enock, Talam; King’ondu, Cecil; Pogrebnoi, Alexander; Jande, Yusufu
Environmental concerns and energy security uncertainties associated with fossil fuels have driven the world to shift to renewable energy sources. However, most renewable energy sources with exception of hydropower are intermittent in nature and thus need storage systems. Amongst various storage systems, supercapacitors are the promising candidates for energy storage not only in renewable energies but also in hybrid vehicles and portable devices due to their high power density. Supercapacitor electrodes are almost invariably made of carbon derived from biomass. Several reviews had been focused on general carbon materials for supercapacitor electrode. This review is focused on understanding the extent to which different types of biomasses have been used as porous carbon materials for supercapacitor electrodes. It also details hydrothermal microwave assisted, ionothermal, and molten salts carbonization as techniques of synthesizing activated carbon from biomasses as well as their characteristics and their impacts on electrochemical performance.