Electrochemistry of ferrocene derivatives relevant to their use in measuring state of charge of lead acid batteries

Touma B. Issa

The electrochemistry of some ferrocene compounds coated on gold electrodes in concentrated sulphuric acid solutions in the concentration range 1–5 M has been investigated. The objective of this work was to investigate whether such compounds could be used to determine, potentiometrically, the state of charge of lead-acid batteries. The investigated ferrocenes included, simple ferrocene, poly(vinylferrocene), bridged ferrocenes in which the two cyclopentadienyl groups are linked by –CH2– chains of vary lengths and alkanethiol substituted ferrocenes. Each of the investigated ferrocenes were found to undergo one electron reversible oxidation/reduction at the surface of the electrode. Their anodic and cathodic peak potentials were independent of pH in the range 2–10 in aqueous solutions of K2SO4, KCl and NaClO4. However, in concentrated acid solutions (H2SO4, HCl and HClO4) the anodic and cathodic peak potentials shifted, almost linearly, to less positive potentials with increasing acid concentration. The attachment of alkanethiol group to the ferrocene molecules improved the retainability at the gold electrode surface which is possibly linked to strong chemisorption of the thiol group on the gold surface. The retainability and chemical stability of the ferrocene compounds at the gold electrode surface were found to be higher in ClO4– than in Cl– and SO42– media which is postulated to be related to the salt formation between the anion and the ferricenium ion generated on oxidation. The bridged ferrocenes were found to be chemically more stable than simple ferrocene. The presence of a carbonyl group in the alkanethiol chain generally decreased the chemical stability of the ferrocene compounds in aqueous acidic media but more so in SO42– than in ClO4– media. Of all the investigated ferrocenes, the potentiometric response of the 1,1`-Bis(11-mercaptoundecyl)ferrocene redox couple was found to be the most stable and reproducible in concentrated sulphuric acid. Hence this couple has the potential of measuring the change in the acid concentration corresponding to the state of charge of lead-acid battery. Long term stability of the material in concentrated sulphuric acid, however, is not good. The material survived for only 15 days. Further work on identifying more stable ferrocene couples is needed.

Electrochemistry of ferrocene derivatives relevant to their use in measuring state of charge of lead acid batteries

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