Hybrid Nanocomposites of rice husk derived graphene-like material incorporated zeolitic imidazolate frameworks-8 for hydrogen storage

Abstract

Global warming is happening and human activities are the major causes of this issue. For instance, the burning of fossil fuels will release large amounts of carbon dioxide (CO2) into the air and finally trap heat in our atmosphere, causing global warming. Therefore, a clean fuel such as hydrogen (H2) is crucial for the environment. However, effective H2 storage remains a challenge since it is usually stored at -196 °C. Currently, H2 can be stored via adsorption in carbon-based material that has high surface area, is light-weight and chemically stable. Graphene is one of the common materials used to store H2 but pure carbon-based material is not practical for energy storage as it has low H2 storage capacity at ambient temperature. H2 storage in graphene can be further enhanced by some modifications. Addition of graphene into metal organic frameworks has been a promising approach to improve H2 storage capabilities as this material has excellent gas storage capacity at ambient temperature. In this study, rice husk is used as a biomass precursor to prepare rice husk derived graphene (GRHC) which was then added into zeolitic imidazolate framework-8 (ZIF- 8) to form a hybrid nanocomposite. Herein, the main objective of this study was to synthesize hybrid nanocomposites of ZIF-8/GRHC with an enhanced physicochemical property for a better H2 storage capacity at ambient temperature. The study was performed by varying several experimental and adsorption parameters including the type of activating agent to produce GRHC (potassium hydroxide, KOH and phosphoric acid, H3PO4), rice husk char (RHC) to activating agent ratio (1:1, 1:2, 1:3, 1:4 and 1:5), loading of GRHC in the hybrid nanocomposites (0.04, 0.08, 0.12, 0.16 and 0.20 g) and variation of H2 pressure (3, 6, 9 and 12 bar). The resultant hybrid nanocomposites with 0.04 g (ZGK 0.04) of GRHC activated with KOH (GRHC-KOH) displayed the greatest improvement in their porous structure including largest specific surface area of up to 1065.51 m2/g and highest micropore volume (0.4784 m3/g) which was higher than the value of pristine ZIF-8 (687.32 m2/g and 0.0419 m3/g). Additionally, the ZGK 0.04 with pore diameter of 0.81 nm was obtained which was smaller than pure ZIF-8, 1.98 nm. This was due to the addition of GRHC-KOH which was able to shrink the pore diameter of ZGK 0.04. The introduction of GRHC-KOH also enhanced the accessibility of hydrogen molecules to the open metal sites in the main structure of ZIF-8. These tailorable surface properties are superior factors for effective H2 adsorption at ambient condition. ZGK 0.04 with the best porous structures and physicochemical properties illustrated the highest volume of H2 adsorbed at ambient temperature and 12 bar (1.82 wt. %) as compared to pristine ZIF-8 and GRHC-KOH which were around 0.41 wt. % and 0.74 wt. % respectively. Notably, the adsorption performance of H2 was directly proportional with the pressure increment. ZIF-8 obeyed Langmuir adsorption isotherm model while ZGK 0.04 and GRHC-KOH obeying Freundlich adsorption isotherm model. At 3 bars, all the samples showed that pseudo-first order kinetic model (physisorption) was the fitted model but as the pressure increased, pseudo-second order kinetic model (chemisorption) was found to be the best fitted model. ZGK 0.04 exhibited the highest stability where the H2 adsorption only dropped around 6.51 % after 5 complete cycles at -196 °C and atmospheric pressure. The optimization of H2 storage depicts that 0.5 g of ZGK 0.04 at 15 bars of H2 pressure and 60 mins of reaction time was the best condition to achieve the highest adsorption at room temperature, 1.95 ± 2.50 wt. %.