Historically, despite its compelling cost and performance advantages, the use of molten hydroxide electrolytes has been ignored by DCFC researches, primarily due to the potential for formation of carbonate salt in the cell. This paper describes the electrochemistry of a patented medium-temperature DCFC based on molten hydroxide electrolyte, which overcomes the historical carbonate formation. An important technique discovered for significantly reducing carbonate formation is to ensure high water content of the electrolyte. Water helps hydrolysis of the carbonates and reduces formation of peroxide and superoxide ions that may react with carbon dioxide producing carbonate ions. High water content can be achieved by maintaining a humid atmosphere above the melt. To date, four successive generations of medium temperature DCFC prototypes have been built and tested at SARA Inc. to demonstrate the technology, all using graphite rods as their fuel source. The cells all used a simple design in which the cell containers served as the air cathodes and successfully demonstrated delivering more than 40 A at 0.3 V with the current density exceeding 200 mA/cm2. The basic feature of this simple cell design is that the cathode is not traditional gas fed electrode type. It is a non-porous electrode structure made of an inexpensive Fe-Ti alloy and gaseous oxygen is introduced into the cell by bubbling humid air through the electrolyte. Results obtained indicated that the cell operation was under a mixed activation-Ohmic-mass transfer control. The activation control is mainly due to slow anode oxidation of carbon, the Ohmic control is mainly due to a large electrode spacing whereas the mass transfer control is most likely because of slow diffusion of oxygen species (O2, O22−, O2, and H2O) to the cathode surface. Cell performances are improved in the new generation cell design, which has been recently built, and which enables faster mass transfer of the reaction species and a lower voltage drop across the electrolyte. In the new design, the cathode is a separate perforated component of the cell that allows the use of a larger surface area electrode and for the electrode spacing to be varied.

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