Molten open circuit voltage

Typical dimensions for the /5-alumina electrolyte tube are 380 mm long, with an outer diameter of 28 mm, and a wall thickness of 1.5 mm. A typical battery for automotive power might contain 980 of such cells (20 modules each of 49 cells) and have an open-circuit voltage of lOOV. Capacity exceeds. 50 kWh. The cells operate at an optimum temperature of 300-350°C (to ensure that the sodium polysulfides remain molten and that the /5-alumina solid electrolyte has an adequate Na" " ion conductivity). This means that the cells must be thermally insulated to reduce wasteful loss of heat atjd to maintain the electrodes molten even when not in operation. Such a system is about one-fifth of the weight of an equivalent lead-acid traction battery and has a similar life ( 1000 cycles). 

Replacement of the sulfur melt by a solution of sulfur and/or its compounds offers some advantages in that the operating temperature can be lowered, the faradaic utilization of the reactant increased (i.e., the redox reactions of sulfur may involve reduction to lower polysulfide, solid at 350°C, as well as oxidation to positive valences of sulfur), the open-circuit voltage made higher, and the two-phase operation avoided. In exploratory systems which follow this route, use is made of sulfur dissolved in molten chloroalumi-... 

Owing to the high temperature, H2 oxidation is largely insensitive to the chosen anode materials. The exchange current densities of Co [13], Cu [15], Pt [16], Ir [16], Au [16], and Ag [16] were about 45, 69, 85, 27, 26, and 19 mA cm-, respectively at 923 K in Li-K carbonate melts. Considering the possible experimental error, these values indicate that the materials have a similar H2 oxidation rate at the anode. In addition, the oxidation potential of Ni in molten carbonate is about —0.802 V under a 1 atm O2 condition at 925 K [2]. Thus, Ni is metallic at open circuit voltage, and is not oxidized up to a certain potential. 

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