Influence of Carbon Structure and Physical Properties on The Corrosion Behavior in Carbon Based Air Electrodes for Zinc
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(1)
Zn + 40H- - Zn(OH) 42 + 2 e-
(2)
Zn(OH) 4 2
->
ZnO + 2 OH" + H 20
(3)
half reaction (equations 2 and 3). The need for hydroxide is demonstrated in equation 2. There is literature evidence [1,2] that the CO 2 was the result of corrosion of the carbon matrix of the air electrode. We report here the results of our investigation of this corrosion reaction and what properties of the carbon contribute to the corrosion reaction rate. Experimental All bifunctional air electrodes were made according to the standard AER Energy formulation [3], using different carbons in the active layer. The carbons tested were Shawinigan Acetylene Black (AB50; Chevron Chemical), Vulcan XC-72 (Cabot), Ketjen EC-600J (KB, Akzo-Nobel). and two proprietary carbon black samples, labeled Carbon A and Carbon B. One of the proprietary (Carbon B) samples was graphitized by the manufacturer at 2700 'C. Air electrodes made with AB50 are also referred to as A100 electrodes. Samples of Vulcan and KB were graphitized at 2700 'C (Fiber Materials, Biddeford, ME). Carbons treated with Co tetra(4-methoxyphenyl)porphyrin (CoTMPP; Aldrich) were prepared by mixing the appropriate carbon and CoTMPP in a V-blender, followed by heat treatment at 400 'C for 1 hour under N 2. All corrosion experiments were carried out using the AER Energy 13220 prismatic cell [5], without an anode. 45% KOH was used as the cell electrolyte. The two cathodes were electronically isolated from each other and one was discharged against the other at 1 A for 200 hours. The electrolyte was then analyzed, using standard acid/base titration techniques, for KOH and K 2C0 3 . The corrosion rate was calculated as a corrosion current based on the amount of OH consumed and only oxidation of carbon to CO 2 (i.e. 4 e- oxidation) is considered in the rate calculation. Relative corrosion rates are the corrosion current 43 Mat. Res. Soc. Symp. Proc. Vol. 496 01998 Materials Research Society
normalized to the rate for A 100 electrodes. Table I shows the data obtained from this test procedure. Both corrosion cell cycling and standard cell cycling studies were conducted using a MACCOR automated cell cycling system. Carbon
Icorr ýtA cm-2
Rel I,,,
AB50 61.6 1.0 AB50/2% CoTMPP 226.1 3.7 AB50/3% CoTMPP 178 2.9 Graphitized Vulcan (GV) 38.4 0.6 GV/2% CoTMPP 104.9 1.3 Carbon A 57.7 0.9 Carbon A/2% CoTMPP 119.8 1.9 70% AB50/30% KB 198.4 3.2 Carbon B 41.4 0.7 Carbon B/2% CoTMPP 69.8 1.1 70/30 AB50/GK 61.6 1.0 AB50 Acetone washed air dry 175.6 2.1 AB50 Acetone washed oven dry 143.5 1.7 Table I: Corrosion data for air electrodes with active layers produced with different carbons. Current density is based on geometric area. Double layer capacitance measurements were made by dc measurements at constant current of 0.2 A during the discharge of the 13220 cell, measuring the slope of the voltage vs. time curve between 1.55 V and 1.28 V and using equation 4,
I
= Cd,
dV
dt
(4)
where I is the current in A and the term dV/dt is the slope of the discharge profile in V s-1 , and Cdl is the double layer cap
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