265459 Determination of a Biocarbon Fuel Cell Electrolyte Chemistry
The electrochemical combustion of carbon converts carbon particles directly into electricity with a theoretical efficiency of 100%. As a result of this surprising aspect, different attempts have been undertaken to develop a reliable carbon fuel cell for more than a century. In our laboratory, an aqueous-alkaline carbon fuel cell that works at moderate temperatures (over 200 °C) and high pressures (≈48 bar) has been under development since 2000. Our findings indicated that under these conditions, biocarbon can indeed undergo vigorous oxidation and thereby generate power in a fuel cell at efficiencies exceeding 80%. However, serious engineering challenges confront the development of a reliable carbon fuel cell. Due to the precipitation of bicarbonate crystals that were found when disassembling the fuel cell, we became concerned about the electrolyte instability that results from the reaction of CO2 with carbonate ions to produce bicarbonate. Therefore, we decided to study the bicarbonate/carbonate chemistry at the fuel cell conditions. To understand the KHCO3/K2CO3 equilibria at the fuel cell conditions, we built a “tubing bomb” reactor that was suitable for use at pressures of 2000 psi and temperatures of 300 °C. Moreover, it could be quickly heated and cooled in a fluidized sand bath. The liquid phase chemistry was studied by exposing solutions of potassium bicarbonate in the pressure vessel to temperatures near 300 °C at their saturation pressure, different time frames and different cooling treatments. Three different analyses of the final solutions were utilized: (1) pH measurement, (2) TGMS of the dry crystals in the Hungarian Academy of Science and (3) Titration.
The significant pH increase of the final solutions suggested the potassium bicarbonate decomposed via the reaction 2HCO3-↔ CO32-+ CO2+ H2O. We learned that the TG-MS results were not reliable because of the high sensitivity of the carbonate/bicarbonate solutions to the presence of CO2. The drying of the sample under atmospheric conditions shifts the above reaction to the left due to the atmospheric CO2 and acidifies the sample; on the other hand, we also observed that the drying by exposing the solution to vacuum shifts the reaction to the right as the CO2 escapes to the gas phase and the sample becomes more alkaline. Therefore, the bicarbonate/carbonate solutions drying process modifies the composition of the sample. To preserve the original solution, the bicarbonate/carbonate solutions storage should be done by capping them in bottles with little dead air space to minimize its exposure to air. The solution can be determined reliably by titration. This titration analysis confirmed the bicarbonate decomposition into carbonate and CO2 and gave more accurate values of the extents of reaction.
Based on the titration analysis of the final solutions, we determined the dependence of the equilibrium constant with the temperature in this range and extracted the thermodynamic properties - Gibbs free energy, enthalpy and entropy- from a Van’t Hoff plot. We found that bicarbonate ions are not stable at fuel cell conditions. Our findings indicate that at these conditions, the bicarbonate decomposition reaction into CO2 and carbonate is spontaneous and endothermic, with a positive change in entropy. The troublesome crystals observed in the fuel cell disassembly were formed while the vessel was cooled under pressure and precipitated thereafter. Our carbon fuel cell should experience no problems with the formation of KHCO3, providing the cell is quickly cooled and pressure is quickly released. These findings have been a cause of optimism for the carbonate fuel cell. In this presentation, we describe the results of both the electrolyte study and the ongoing carbon fuel cell study.