The Sodium-O2/Co2 Electrochemical Cell
Metal-air batteries, especially lithium and sodium air systems, have attracted significant research effort in the past decade. The high theoretical specific energy (3500Wh/kg for Li-O2 and 1600Wh/kg for Na-O2) and moderate equilibrium potential (2.96V for Li-O2 and 2.3V for Na-O2) make these chemistries attractive energy storage platforms for transportation, autonomous aircraft, and emergent robotics technologies. The term metal-air battery, however, is often used to describe the system which is actually a "metal-O2" battery, for in most studies O2 is used in place of air as the active material in the battery cathode. This change is employed to eliminate formation of electrochemically stable metal hydroxide and metal carbonate discharge products when CO2 and moisture present in ambient air react with metal ions in the cathode. Therefore, when it comes to practical design and operation of metal-air battery, significant new complications that largely defeat the competitive advantages of this storage technology will be introduced into the metal-air battery system due to the impurities in the ambient air. Recent work has shown that when a mixture of O2 and CO2 is used as the active material in the cathode, it is possible to recharge a metalO2/CO2 cell provided steps are taken to prevent electrolyte decomposition during the recharge. In the current work, we studied electrochemical processes in model sodiumO2/CO2 (Na-O2/CO2) cells. We find that provided that with the formation of the electrochemically stable electrode/electrolyte interfaces, such cells are able to deliver both exceptional energy storage capacity and stable long-term charge-discharge cycling behaviors at room temperature. NaHCO3 is shown to be the principal discharge product through in- and ex-situ chemical analysis of the cathode. The modification on the electrolyte to make it stable under high voltage is crucial for the rechargeability of the system. By means of differential electrochemical mass spectrometry (DEMS), we show that addition of as little as 10% of electrolyte additive extends the high-voltage stability of the electrolyte by at least 1 V, allowing recharge of the Na-CO2/O2 cells. We also report a novel primary Li-CO2 battery that consumes pure CO2 gas in its cathode. The battery exhibits a high discharge capacity of around 2500 mAh/g at moderate temperatures. The metal-O2/CO2 platform provides a novel approach for simultaneous capturing of CO2 emissions and producing electrical energy.
Joo,Yong L.; Suntivich,Jin
Ph.D. of Chemical Engineering
Doctor of Philosophy
dissertation or thesis