Design and development of a water vapor electrolysis unit. NASA CR-607.

A water vapor electrolysis cell utilizing sulfuric acid as the electrolyte has been shown to be capable of operating efficiently on the typical humidity of cabin air. Concentrated sulfuric acid can absorb enough water from a 50% relative humidity air stream at 75°F to permit an electrolysis cell to c&rate at a current density as high as 150 mA/cm2. . Sulfuric acid was examined for physical properties applicable to its use in the watervapor electrolysis cell. An evaluation of the factors affecting the water absorption properties indicated that the vapor pressure difference between the inlet air stream and the acid coupled with the air flow-rate were controlling factors for water absorption. Experimental tests were conducted to prove the theoretical equations; the results agreed closely with the equation. Electrode performance in bulk sulfuric acid was evaluated; it was found that the anode overvoltage is the largest contributor to the cell overvoltage. This is especially true in the acid that is sufficiently concentrated to absorb enough water from the air stream. An electrode made from tantalum screen with black platinum held in place with a fluorocarbon resin was shown to reduce both anodic and cathodic overvoltage compared to a black-platinized platinum electrode. Secondary cell reactions, principally ozone, were studied and the ozone threshold anodic overvoltage was found to be approximately 2.07 V. Measurements in test cells .verified the result. Immobilizing gel matrix materials were studied including a synthetic zeolite, silicic acid, and a finely divided silica. These materials were characterized. Test cells were built and operated with these materials as the immobilizing matrix. The zeolite slowly reacted with the concentrated sulfuric acid resulting in poorer cell performance.