Carbon from carbon dioxide via molten carbonate electrolysis: fundamental investigations

Conversion of carbon dioxide into useful products has become highly desirable in recent years in order to both mitigate carbon dioxide emissions to the atmosphere and develop non-fossil energy sources. A variety of methods exist for the electro-reduction of carbon dioxide in solution to useful products, such as carbon monoxide and hydrocarbons. However, none of these processes are able to directly convert carbon dioxide to carbon. In this thesis, the conversion of carbon dioxide into solid carbon via molten carbonate electrolysis has been investigated. Both the past literature and the present work have shown that carbon nanopowder can be produced via this process, so it is highly likely that the electro-deposited carbon obtained is a valuable product. Although this process has been known since the 1960s, there are still many areas where our knowledge of the process is lacking. Hence, this thesis is focussed primarily on the reactions occurring in the molten carbonate electrolyte, the properties of the electro-deposited carbon and the re-oxidation of the electro-deposited carbon. Using cyclic voltammetry carried out at platinum working electrodes, it was found that carbon was electro-deposited at the cathodic limit in the Li2CO3-Na2CO3 and Li2CO3-K2CO3 electrolytes at temperatures of ca. 600 °C and ca. 700 °C, probably by the following reaction: CO32- + 4e- → C + 3O2- One novel finding of this research is that carbon electro-deposition competed with other cathodic reactions at the cathodic limit, which included alkali metal formation, carbon monoxide formation and alkali metal carbide formation. However, the carbon electro-deposition reaction dominated over the other cathodic reactions once the metal working electrode surface had become covered with a layer of electro-deposited carbon. This was probably because a lower overpotential is required to deposit carbon onto carbon, as opposed to carbon onto metal. Moreover, the other cathodic reactions may have been catalysed by the bare metal working electrode surface before it became covered with carbon. Electrochemical re-oxidation of electro-deposited carbon was found to occur via a process consisting of at least two stages, which was deduced using cyclic voltammetry in conjunction with the re-oxidation of electro-deposited carbon via galvanostatic chronopotentiometry. These stages may have corresponded to the oxidation of portions of the carbon with different morphologies. Carbon was electro-deposited onto mild steel working electrodes via chronoamperometry in the Li2CO3-Na2CO3, Li2CO3-K2CO3 and Li2CO3-Na2CO3-K2CO3 electrolytes. The highest apparent electro-deposition rate obtained was 0.183 g/cm2.h at an applied potential of -2.98 V vs. Ag/AgCl, using the Li2CO3-K2CO3 electrolyte at 708 °C. The average current efficiencies obtained for carbon electro-deposition were: 74.4 % for Li2CO3-Na2CO3, 79.0 % for Li2CO3-K2CO3 and 51.2 % for Li2CO3-Na2CO3-K2CO3. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) spectroscopy revealed that the washed carbon deposits mostly consisted of fine quasi-spherical carbon particles, some as small as 60 nm in diameter. All of the electro-deposited carbon appeared to be amorphous.