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tance of 40 pF is to be measured to better than 50%and io = 250 mA/cm2, then the slope of the potential-time trace has to be measured at times shorter than 2 psec. Experimental In view of the facts mentioned above, it was necessary to use pulses with a very short rise time and high current density. It was also necessary to be able to record small potential changes with time, which required a low noise level in the experimental setup and the compensation of the ohmic potential drop in the solution. Fig. 6 shows a schematic of the measuring circuit. For the constant current pulses, the output of a Wavetek function generator was used directly. This instrument was gated by a Tyco-built pulse generator to allow multiple pulse application. By the use of two diodes, either cathodic or anodic current pulses could be selected. A dual trace oscilloscope (Tektronix 556with two type W plug-in units) was used to record current and potential as a function of time. Current was measured by the potential drop across a known resistance. The i-R compensated potential-time traces were obtained by feeding the potential of the two-electrode cell and the output of the i-R compensator into the inputs of the differential amplifier. The measurements were conductedatanA1diskelectrode(0.385cm2)e A15-cm2sheetofpureA1(Alpha Chemical Co.) served a s the counterelectrode. The electrolyte was AlCL -KC1-NaCl ( 57.5-12.5-30.0 mol %) . Since the freshly prepared melt showed some gassing, the temperature was raised and kept at 160 " C for about 2 hr. At that time, all gassing had stopped and the temperature of the melt was lowered to 130 "C and kept constant at this value by a temperature controlled silicone oil bath, as described earlier. Again, a slightly positive Ar pressure was kept in the cell at all times. Results Figs. 7 and 8show two typicaloscilloscopic potential-time traces at anodic and cathodic current pulses. From the initial slope of the galvanostatic voltage-time curves, we determined an average value for the double layer capacitance of 40*10 pF/cmZ. Considering the surface roughness, one would expect to find a double layer capacitance of approximately this size in the absence of specific adsorption effects. Fig. 9 is an example of overvoltage versus square root of time plots at vari- ous current densities, according to Eq. (5)e At times larger than 10 psec, the po- tential varies linearly with t1'2. As expected, the slope of these lines increasesPDF Image | NASA ALUMINUM CHLORINE BATTERY
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