Graphene composited activated carbon nanofibers for carbon dioxide adsorption

Abstract

Activated carbon nanofibers (ACNFs) is a newly modified structure of carbon-based adsorbent that could adsorb carbon dioxide (CO2) due to its high specific surface area (SSA), wide distribution of pores as well as high volume of active sites on its fibrous structure. Meanwhile, graphene is a single layer of pure carbon atoms known for its great properties such as high SSA, high thermal and chemical stability, and high electrical and thermal conductivities. It is hypothesized that the incorporation of graphene as nanofiller in the polyacrylonitrile (PAN)-based ACNFs may improve the overall properties of the ACNFs. Nevertheless, pure graphene has been found to be very expensive and this factor hindered its utilization in wide range of applications. Due to that, rice husk which is known as abundantly available agricultural waste was introduced in this study to obtain cost-effective graphene-based materials. Herein, the main highlight of this current study is to fabricate PAN-based graphene composited activated carbon nanofibers (gACNFs) with enhanced physicochemical properties and to evaluate its adsorption performance behaviours towards CO2, especially in flue gas. The study was performed by varying several experimental and adsorption parameters including the PAN to graphene ratio (0, 1, 5, 10% of graphene-derived rice husk char (GRHC) relative to PAN weight), types of graphene-based materials (GRHC and reduced graphene oxide (rGO)), polyethyleneimine (PEI)-impregnated and non-impregnated gACNFs, as well as variation of pressure (5, 10, and 15 bar) and temperature of adsorption (0, 25, 50 °C). The resultant gACNFs with 1 wt.% of GRHC displayed the greatest improvement in their porous structure including largest SSA up to 597 m2/g and highest micropore volume (0.2606 cm3/g) which was twice the values of pristine ACNFs (202 m2/g and 0.0976 cm3/g). These tailorable surface properties are superior factors for effective CO2 adsorption. Additionally, gACNFs with diameter ranging between 250-350 nm was obtained, which was smaller than the pristine ACNFs. This was due to electrical conductivity contributed by the GRHC that enhanced the solution conductivity during electrospinning, resulting in fibers with smaller diameter. Moreover, under the activation temperature of 700 °C, the yield of gACNFs obtained (44.5%), was almost double the value of pristine ACNFs (25.1%) due to the thermal stability properties of GRHC. The resultant GRHC/ACNF0.01 with the best porous structures and physicochemical properties exhibited the highest volume of CO2 uptakes among other samples up to 3.1 mmol/g at atmospheric pressure and 25 °C. Meanwhile, the PEI-gACNFs have shown increment in CO2 uptake from 3.1 to 4.8 mmol/g under the same conditions. Notably, the adsorption performance of CO2 was directly proportional with the pressure increment, however it was inversely proportional with the increased temperature. Interestingly, both gACNFs and PEI-gACNFs fitted the pseudo-first order kinetic model (physisorption) at 1 bar, however, best fitted the pseudo-second order kinetic model (chemisorption) at 15 bar. Both gACNFs samples obeyed the Langmuir adsorption isotherm model. The stability performance of both gACNFs was reduced up to 23% after 5 complete cycles at 50 °C and atmospheric pressure.