Valorization of oil palm by-product materials as a new sustainable carbon source for carbon-based nanomaterial synthesis and energy storage application

Energy and environmental issues are the two major problems that our world is facing today. These, combined with developing consumer demands have stirred researchers’ interest in inexpensive, environmentally friendly functional materials. With Malaysia as an indicator, and based on a projected annual production of palm oil in Malaysia of over 15.4 million metric tons by 2020, it is estimated that about 46.6 tons of lignocellulosic wastes will be generated. Transforming these wastes into wealth could be integrated into a global paradigm shift towards sustainable development. Thus, in this research, a new approach was proposed to produce reduced graphene oxide (rGO) from graphene oxide (GO), and activated carbons (AC) using various oil palm by-product materials, namely oil palm leaves (OPL), palm kernel shells (PKS) and empty fruit bunches (EFB). The effect of heating temperature on the formation of graphitic carbon and the yield was examined prior to the GO and rGO synthesis. Carbonization of the starting materials was conducted in a furnace under nitrogen gas for 3 h at temperatures ranging from 400 to 900 °C and a constant heating rate of 10 °C/min. The GO was further synthesized from the as-carbonized materials using the ‘improved synthesis of graphene oxide’ method. Subsequently, the GO was reduced by low-temperature annealing reduction at 300 °C in a furnace under nitrogen gas for 1 h to produce rGO. It was found that the IG/ID ratio calculated from the Raman spectral analysis increases with the increasing of the degree of the graphitization in the order of rGO from oil palm leaves (rGOOPL) < rGO palm kernel shells (rGOPKS) < rGO commercial graphite (rGOCG) < rGO empty fruit bunches (rGOEFB) with the IG/ID values of 1.06, 1.14, 1.16 and 1.20, respectively. The surface area and pore volume analyses of the as-prepared materials were performed using the Brunauer Emmett Teller (BET) nitrogen adsorption-desorption method. The lower BET surface area of 8 and 15 m²g−¹ observed for rGOCG and rGOOPL, respectively could be due to partial restacking of GO layers and locally-blocked pores. Relatively, this lower BET surface area is inconsequential when compared to rGOPKS and rGOEFB, with a surface area of 114 and 117 m² g−¹, respectively. Furthermore, electrochemical energy storage performances of the rGOs, and also the as-prepared activated carbons were also all carried out using cyclic voltametry (CV) method, and were found to be good electrode materials for supercapacitors applications. One of the OPL-based AC electrodes was found to have very high capacitance values of 434 F g−¹ at 5 mVs−¹, which is much higher than the specific capacitance value (343 F g−¹) of the only oil palm leaf-derived porous carbon nanoparticles ever reported in the literature. On the other hand, a novel phase change material (PCM) made of n-nonadecane infused by capillary forces in a compressed reduced graphene oxide-activated carbon composite matrix was also investigated. Reduced graphene oxide (rGOEFB) - activated carbon (AC) composite was successfully prepared and exhibited an improved thermal conduction property, and was used as a matrix or framework for the fabrication of shape-stabilized composite phase change material (SCPCM). During this SCPCM set-up, which was achieved by simple impregnation method, the pores of the rGOEFB-AC composite serves as the matrix/framework while n-nonadecane as the central envelope. The molten n-nonadecane was successfully stabilized by this porous carbon matrix via the capillary force and surface tension forces which mitigated the seepage of the molten n-nonadecane throughout the phase change cycle. The phase change temperatures and latent heats of composite SCPCM-5 were 37.25 and 25.58 ᵒC and 82.72 and -62.22 J/g, respectively. On the whole, the novel carbon- based nanomaterials produced in this project demonstrate excellent features that enabled them to be used as both thermal and electrochemical energy storage materials.

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