In Situ Determination of Gravimetric and Volumetric Charge Densities of Battery Electrodes: Polyaniline in Aqueous and Nonaqueous Electrolytes

Abstract

Polyaniline (PAM) electrodes with redox capacities >1 C/cm² were investigated by in situ electrogravimetry in aqueous electrolytes of pH values varying from <0 to 3 and in nonaqueous, propylene carbonate-based media. Film mass, volume, and bulk density were determined in situ as a function of electrode potential. For aqueous electrolytes, the gravimetric analysis supports a charge–transfer mechanism which depends on pH and involves the exchange of protons, electrolyte anions, and H₂O. Protonation equilibria of reduced PANI were monitored gravimetrically in aqueous HCl, H₂SO₄, and HClO₄. In nonaqueous electrolytes, however, the main charge-compensating process appears to be reversible insertion of anions. For LiClO₄/propylene carbonate solutions, it was shown that up to 0.9 electrons can be transferred per aniline unit, resulting in experimental charge densities of 270 Ah/kg, based on dried reduced PANI. Upon immersion and cycling in nonaqueous electrolytes, the practically usable charge density was lowered to 160 Ah/kg due to sorption of solvent and incomplete oxidation. Based on in situ gravimetry, a detailed analysis of mass and volume requirements for electroactive material and electrolyte in rechargeable PANI batteries (e.g. Li/PANI) was derived.