Study of the chemistry of the carbon fuel cell electrolyte at near critical conditions

Abstract

The research in this thesis was performed at the Hawaii Natural Energy Institute based at the University of Hawaii at Manoa; two principal foci were addressed: (1) to design and investigate the performance of an aqueous-alkaline biocarbon fuel cell that generates power at temperatures ~500 K, (2) to determine the electrolyte chemistry at conditions similar to those of the fuel cell. Our Lab has been working in the first focus, the design of an aqueous-alkaline biocarbon fuel cell, since 2000. In 2007 T. Nunoura et al. [12] published the first paper in this topic. They studied the thermodynamics of the anode and cathode reactions and showed the experimental results of a firstgeneration, aqueous-alkaline biocarbon fuel cell built in the Lab. They showed that an aqueous-alkaline fuel cell operating at 518 K and 35.8 bar was able to realize an open-circuit voltage of 0.57 V, a short circuit current density of 43.6 mA/cm2 and a maximum power of 19 mW, using a 6M KOH/ 1 M LiOH mixed electrolyte with a catalytic silver screen/platinum foil cathode and an anode composed of 0.5 g of compacted corncob charcoal previously carbonized at 950°C. A second paper published by M. Antal and G. Nihous [13] proved that the reactions of a moderate temperature aqueous-alkaline biocarbon fuel cell may be favored at temperatures as high as 300 °C and the carbonate electrolyte may be as effective as the hydroxide electrolyte. Based on these promising findings, the work in the biocarbon fuel cell continued. Different fuel cells were designed and built to overcome the problems that arose. The fuel cell working with high concentrations of potassium carbonate solution as electrolyte showed the best performance. However, unexpected crystals determined by TG-MS as potassium bicarbonate appeared in this fuel cell that caused our research focus to take another direction: The study of the electrolyte. We decided to focus on the carbonate/bicarbonate chemistry and the understanding of the formation of these potassium bicarbonate crystals. To study the formation of the crystals, a "tubing bomb" that can stand pressures as high as 2000 psi and can be quickly heated in a sand bath was built. Previous literature [14] indicates the decomposition of dry bicarbonate into carbonate and CO2. This research focuses on the thermodynamics of this decomposition reaction in solution at conditions close to the fuel cell conditions. By determining the equilibrium constant, the thermodynamic properties (enthalpy, entropy and Gibbs free energy) of the bicarbonate decomposition reaction and the temperature at which the potassium bicarbonate in solution will completely decompose, we can determine the allowable operation temperature for the fuel cell.