Electrochemical carbon dioxide concentration

Abstract

"The average man requires some 1.84 lbs of oxygen per day and generates approximately 2.2 lbs CO2 per day. In manned spacecraft missions, the expense of maintaining separate oxygen supply and carbon dioxide removal systems direct current attention to a dual performance system, one which not only removes carbon dioxide but generates oxygen simultaneously. An early system which performed this task was the aqueous alkaline carbon dioxide concentrator. The aqueous carbon dioxide concentrator using cesium carbonate as the electrolyte performed successfully with high relative humidities ([greater than] 60 percent RH). Below 60 percent relative humidity, drying of the electrolyte induces CsHCO3 precipitation at the anode which in turn allows hydrogen crossover to the cathode to take place. Although control of humidity is possible, it is not desirable since such control not only increases capital and operating cost but also increases total system equivalent weight. Thus, it is desirable to use an electrolyte which can function at a broad range of relative humidity. One such electrolyte is tetramethyl amonium carbonate. Its solution, having a lower vapor pressure than cesium carbonate, permits cell operation at relative humidities as low as 35 percent. The success of the aqueous carbon dioxide concentrator suggests using a similar device utilizing a molten salt as the electrolyte. Operation of a concentrator using a molten salt electrolyte offers the advantage of being insensitive to relative humidity. The molten salt electrolyte also possesses a more highly concentrated ionic field, yielding high thermal and electrical conductivities for the electrolyte. High conductance is a desirable property, requiring less area of contact. Use of noble metal electrodes in the aqueous cell permits high power and current density and long cell duration. In a high temperature cell the need for noble metal electrodes is reduced since non-noble metals are available, which, at cell operating temperatures, are catalytically active enough for both cathodic and anodic reactions. An electrochemical cell utilizing a molten carbonate electrolyte is an obvious extension to higher temperature from the aqueous concentrator. While these devices have been used for fuel cells, they have not been tested as CO2 concentrator."--Introduction.