Anodic oxidation of p-phenylenediamines in battery grade electrolytes

The use of anion-inserting organic electrode materials represents an interesting opportunity for developing ‘metal-free’ rechargeable batteries. Recently, crystallized conjugated diamines have emerged as new host materials able to accommodate anions upon oxidation at potentials higher than 3 V vs. Li+/Li0 in carbonate-based battery electrolytes. To further investigate the electrochemical behavior of such promising systems, comparison with electroanalytical data of soluble forms of conjugated diamines measured in battery grade electrolytes appeared quite useful. However, the literature on the topic is generally poor since such electrolyte media are not common in molecular electrochemistry. This contribution aims at providing relevant data on the characterization by cyclic voltammetry of unsubstituted, diphenyl-substituted and tetramethyl-substituted p-phenylenediamines. Basically, these three molecules revealed two reversible one-electron reaction upon oxidation corresponding to the electrogenerated radical cation and dication, respectively, combined with the association of electrolyte anions (i.e., PF6−, ClO4− and TfO−). The nature of the counter-anion did not show much influence on the electrochemical activity, which remained governed by the solvation process in high-polarity solvents (i.e., PC- or EC-based battery electrolytes). However, in presence of PF6−, the emergence of a pre-peak prior to the second oxidation step was observed when labile protons exist in the radical cation state. This contribution is attributed to a deprotonation reaction of the radical cation induced by the catalytic decomposition of PF6− in presence of H+, which supports well the few other experimental data reported on the decomposition issues of LiPF6. In addition, substitution of the amine redox centers with appropriate functional groups (that increase the molecule π-delocalization) allowed to reach higher formal potentials without impacting the bi-electronic behavior and the reversibility of the processes. [Display omitted]