Electrochemically Driven Ion Insertion Processes across Liquid|Liquid Boundaries: Neutral versus Ionic Redox Liquids

Abstract

The oxidation and rereduction of the redox liquids para-N,N,N‘,N‘-tetrahexylphenylenediamine (THPD) and para-N,N,N‘-trihexylphenylenediamine (TriHPD) associated with anion and proton insertion and expulsion are studied as a function of the proton concentration in aqueous NaClO4 electrolyte. Voltammetric, in situ UV/vis-spectroelectrochemical, and quartz crystal microbalance techniques are employed. The biphasic acid−base equilibria of the redox liquids involving protonation and simultaneous anion transfer from the aqueous phase are shown to exhibit only small deviation from ideal behavior and well-defined biphasic dissociation constants, pKA,biphasic have been determined. However, the protonation of the bulk redox liquids is shown to be dominated by intermolecular rather than intramolecular factors. In particular, the ability of THPD to undergo bulk protonation by HClO4 is higher (pKA2,biphasic = 5.1) compared to that of TriHPD (pKA2,biphasic = 3.9); this is opposite to the behavior predicted on the basis of the estimated values for the aqueous protonation equilibrium constants, pKA2 = 7.5 ± 0.5 and pKA2 = 8.8 ± 0.5 for THPD and TriHPD, respectively. Further, the electrochemically driven deprotonation occurs irrespective of protonation constants at essentially the same potential for both materials. The three-phase junction electrode|redox liquid|aqueous electrolyte for the initiation of the anion and proton insertion−electrochemical reactions is shown to be the key to processes observed for neutral redox liquids, whereas ionic redox liquids show reactivity independent of the three-phase junction due to sufficient ionic bulk conductivity.