(663c) Renewable Bio-Graphene Synthesis and Its Application As an Anti-Corrosive Coating

Graphene, the thinnest 2D material ever known have been widely used in many applications including sensors, displays, solar, panels, etc. State-of the-art for synthesis of graphene is chemical vapor deposition (CVD) which uses high vacuum and hydrogen for removing the oxide layers and etching the surface for graphene formation. High vacuum makes the process expensive and hydrogen use require careful handling due to its flammability. In this study, graphene is grown over metals at atmospheric pressure in a novel method where hydrogen and vacuum are not required. Unlike CVD which uses a gaseous carbon source, a solid renewable carbon source (biochar) is the graphene precursor. Biochar is prepared from biomass model compound (cellulose) by hydrothermal carbonization (HTC) at 300⁰ C and 8.5 MPa in subcritical water. The solid carbon source is then placed on the metal (Fe, Co) foils and heated to 1000⁰ C using a quartz tube furnace. On cooling, it was observed that graphene can form on both sides of the metal foil through the carbon dissolution and precipitation mechanism. Usually, at the high temperatures, the carbon atoms from biochar dissolve into the metals and upon cooling, carbon precipitates to form graphene. From the cobalt-carbon phase diagram, cobalt forms a cobalt carbide which is unstable at room temperature and dissociates into graphene. Due to the absence of carbides over the surface, graphene coated cobalt can theoretically provide a better corrosion resistance. XRD analysis showed a sharp peak corresponding to graphene. Interestingly, Raman analysis revealed that there were no defects on the graphene coated cobalt as no significant D band was observed. Based on the 2D/G ratio, both single and multilayer graphene were found on Co. On the other hand, from the phase diagram of iron-carbon, the solubility of carbon in iron is 1.4wt% at 1000⁰ C where a mixture of γ-Fe, Fe₃C and graphitic carbon are present. On cooling to the eutectic temperature (727⁰ C), the carbon atoms either precipitate to form graphene or form a stable iron carbide. From the XRD analysis, both graphene and iron carbide are detected over the iron foil. From Raman analysis, the 2D/G ratio was less than 1, corresponding to multilayer graphene. The anti- corrosive property of graphene was then tested in 3.5 wt% NaCl. Open circuit potential (OCP), Electrochemical Impedance Spectroscopy (EIS) and Linear Polarizing Resistance (LPR) technique were employed to analyze the corrosion behavior of metals and graphene coating in the presence of an electrolyte. Linear Polarizing Resistance (LPR) technique was used to generate Tafel plots by scanning the measured OCP from -300mV to +300mV vs OCP. The auto fit method was then applied to extrapolate the Tafel slopes to determine the corrosion current from which the corrosion rates of metals with and without graphene can be calculated. Initial corrosion measurements show that iron and cobalt metal without graphene showed a corrosion rate of 0.195 and 0.06 mmpy (millimeter per year). Graphene coated iron and cobalt showed a corrosion resistance of 0.20 and 0.61 mmpy. In the case of iron, non-homogenous coverage of graphene and the presence of the carbide phase gave rise to micro-galvanic corrosion, hence showing no improvement in corrosion resistance. Usually, multilayer graphene provides more corrosion resistance than monolayer graphene. Since both monolayer and multilayer graphene were present on Co, the monolayer did not give provide resistance to the electrolyte and hence the corrosion rate was similar to that of pure Co. Operational parameters will now be fine-tuned so that better multilayer coverage can be obtained on the iron foils for better corrosion resistance.