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Abstract

Approaches were investigated for extending the operating life of Tantalum Hybrid faradaic/dielectric capacitors by reducing electrochemical changes that limit life. This work concentrates on electrolytic additives designed to eliminate or greatly suppress hydrogen formation that normally results from potentiostatic leakage current. Sample capacitors with electrolytes containing hydrogen limiting reagents were subjected to accelerated aging conditions. The results are compared to similar tests on standard capacitors with no electrolyte additives.

Background

The Tantalum Hybrid capacitor is a series combination of a dielectric oxide film capacitance, Ta2O5 and a high faradaic capacitance, a film of the conductive metal oxide, RuO2. The result is a polar capacitor with the Ta2O5 being the positive electrode (anode) and the RuO2 being the negative electrode (cathode). A high potential can be maintained across the thin electrochemically formed Ta2O5 film, while the RuO2 coating remains at low potential. This allows high cell voltage without fear of reaching the electrolyte breakdown potential.

The advantages of the Hybrid capacitor can be better grasped with a basic understanding of common electrolytic capacitors. These devices employ thin oxide films on both electrodes, but they are usually asymmetric, using a material of higher surface area for the negative electrode. The film on the positive electrode is thicker than the negative electrode film, and sets the working voltage of the capacitor. The negative electrode has a higher capacitance, but the two electrodes often have similar physical sizes. The overall capacitance, C, can be determined by analysis of the equivalent series circuit for an electrolytic capacitor. For Hybrid capacitors, 1/C = 1/Ca + 1/Cc, where Ca and Cc respectively are the positive and negative electrode capacitances. Since Cc>>Ca, the overall capacitance is equivalent to Ca. Because the RuO2 negative electrode requires little volume, available space can be used to enlarge the positive electrode. The result is a capacitor with much higher energy density.

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