Biogas slurry derived mesoporous carbon/metal oxides composites for supercapacitors application

Abstract

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.