Physicochemical and electrochemical properties of pristine and modified ordered mesoporous carbon synthesized using SBA-15 as template

Abstract

Ordered mesoporous carbon based materials (OMC) are suitable for many applications because of their unique physicochemical properties. In this research, OMC materials with large surface areas up to 1226 m2 g‒1 were synthesized using the nanocasting method with SBA-15 silica as the hard template and sucrose as the carbon precursor. The mesoporous silica SBA-15 template was prepared by hydrothermal synthesis using tetraethyl orthosilicate (TEOS) and Pluronic P123 copolymer under acidic conditions which was later used to synthesize OMC. Based on small angle X-ray scattering (SAXS) data analysis, the (1 0 0) peak position of the OMC was exactly the same as for the template SBA-15 (1.0° 2), which is an indication of long-range structural ordering, implying a complete replication of the carbon from the SBA-15 silica template. Additionally, both the transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) images of SBA-15 and OMC were similar, showing well-ordered hexagonal pores and uniform pores sizes of less than 5 nm in diameter with rod-like particle morphology. Oxygen and nitrogen containing functional groups, respectively, were incorporated into the OMC surface in order to improve the electrochemical performance and hydrophilic properties. The oxygen functionalized OMC (OMC-O) was formed through oxidative treatment with 2M HNO3 solution. X-ray photoelectron spectroscopy (XPS) measurements showed an increase in the amount surface oxygen (13.7%) in the form of carbonyl, carboxyl and quinone groups which were supported by the Fourier transform infrared (FT-IR) spectra. Modification of OMC with nitrogen containing functional group was performed by noncovalent functionalization method via adsorption of Basic Red 2 (BR2) dye, C20H19ClN4, from aqueous solution to afford the nitrogen functionalized OMC (OMC-N). The highest adsorption capacity q of BR2 was obtained with solution concentration of 1000 mg L‒1 at 60 °C, pH of 10, OMC loading of 0.2 g L‒1 and contact time of 180 min as determined by ultraviolet-visible (UV-Vis) spectroscopy. The experimental data showed excellent fit with Langmuir isotherm model (R2 = 0.989) giving a maximum capacity qmax of 1000 mg g‒1. BR2 adsorption onto the OMC obeyed pseudo-second order kinetic model and the process is thermodynamically favourable, spontaneous, physical (ΔHads = 37.9 kJ mol‒1) and endothermic. Electrochemical behaviours of OMC, OMC-O and OMC-N were evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge (GCD) using a threeelectrode assembly in 1M KOH aqueous electrolyte. Both OMC-O and OMC-N demonstrated higher specific capacitance than OMC and good capacitance retention (> 50%) at the optimum scan rate (10 mV s‒1) and current density (0.5 A g‒1). However, BR2 functionalized OMC-O (OMC-ON) containing oxygen and nitrogen functionalities exhibited the highest capacitance (356.4 F g‒1) among the samples, but has the lowest capacitance retention (45%) upon increasing scan rate. The enhancement in specific capacitance of modified OMC was correlated to the pseudo-capacitance induced by redox reactions of oxygen and nitrogen functionalities in OMC prepared by oxidative modification and noncovalent functionalization method. The synergistic effects of the oxygen and nitrogen increase the hydrophilicity, while the presence of mesopores also promoted the formation of electrical double-layer, and consequently increased the specific capacitance of OMC in aqueous medium.