Jianbang Ge

Capture and electrochemical conversion of CO2 to ultrathin graphite sheets in CaCl2-based melts

Molten CaCl2 is reported to be a potential dopant for reactivation of CaO and enhancement of the cyclic capture ability of CaO. The present work showed that O2− in molten CaCl2–CaO has a strong affinity for CO2 at 850 °C, with resulting formation of carbonates. Using a RuO2·TiO2 inert anode, the formed carbonates were successfully electrochemically split into value-added ultrathin graphite sheets, which look like a kind of graphene, accompanied by evolution of carbon monoxide at the cathode and environmentally friendly by-product oxygen at the anode. The reduction mechanism of CO32− was investigated by cyclic voltammetry and square wave voltammetry. Results demonstrated that there are two steps in electrochemical reduction of CO32−, and the transferred electron numbers calculated for each step are 1.76 and 1.99, respectively. The kind of graphene generated at the cathode may have applications in fields such as energy storage and electronic devices. The molten CaCl2–CaO has potential applications and prospects in large-scale capture of CO2, and electrochemical conversion of CO2 into high value added carbon material such as ultrathin graphite sheets with renewable energy sources.

Direct Conversion of Greenhouse Gas CO2 into Graphene via Molten Salts Electrolysis

Producing graphene through the electrochemical reduction of CO2 remains a great challenge, which requires precise control of the reaction kinetics, such as diffusivities of multiple ions, solubility of various gases, and the nucleation/growth of carbon on a surface. Here, graphene was successfully created from the greenhouse gas CO2 using molten salts. The results showed that CO2 could be effectively fixed by oxygen ions in CaCl2-NaCl-CaO melts to form carbonate ions, and subsequently electrochemically split into graphene on a stainless steel cathode; O2 gas was produced at the RuO2-TiO2 inert anode. The formation of graphene in this manner can be ascribed to the catalysis of active Fe, Ni, and Cu atoms at the surface of the cathode and the microexplosion effect through evolution of CO in between graphite layers. This finding may lead to a new generation of proceedures for the synthesis of high value-added products from CO2, which may also contribute to the establishment of a low-carbon and sustainable world.

Electrochemical deposition of carbon nanotubes from CO2 in CaCl2–NaCl-based melts

As part of the efforts to address global climate change, the identification of methods for the capture of carbon dioxide and its selective electrochemical conversion into value-added carbonaceous materials in molten salt electrolytes is a research topic of scientific and technological significance. In most cases, metal electrodes such as nickel and stainless steel are used as the cathode to investigate the nucleation and growth of a variety of carbon nanostructures. In this study, the electrochemical deposition of carbon nanotubes (CNTs) and carbon microstructures was performed in molten CaCl2–NaCl–CaO using glassy carbon and graphite rod as the cathode and RuO2–TiO2 as the anode. The capture formula was established and the capture coefficient was defined and calculated to be 1.8 s−1. Cyclic voltammetry, constant voltage electrolysis, as well as on-line outlet gas analysis were conducted to investigate the electrode reactions, and the results indicated that the captured CO2 can be electrochemically converted to carbon and environmentally-friendly oxygen as the only by-product. SEM and TEM images showed that quasi-spherical and nanotubular carbon were deposited on the graphite and glassy carbon cathodes at 750 °C. However, by regulating the temperature, quasi-spheres and nanosheets were observed at the glassy carbon cathode.

Advancement in knowledge of phenomena and processes: general discussion

Hongmin Zhu replied: We want to analyze the effect of F ion on the equilibrium of the titanium ions and metallic titanium. By adding F ion into a chloride molten salt, you can tune the coordination situation of anions around the cation. In molten salts each cation is always coordinated by some anions. When F ions are introduced into chloride salts, the F ions will be preferentially coordinated to the higher valance cations (in this case, Ti and Ti); this will drive the equilibrium of the disproportionation reaction to the higher valance cation side. Therefore, by analyzing carefully the effect of the F ion addition to the equilibrium, you will be able to know the relative stability (chemical potential) of the cations. This is also linked with the electrode reaction steps.