Abstract
In this study, kinetic and thermodynamic aspects associated with the selective uptake/release of divalent cations from/into an equimolar (0.1 M) aqueous electrolyte mixture of Co2+ and Ni2+ cations at TCNQ((s)) vertical bar electrode((s)) vertical bar electrolyte((aq)) triple phase boundary junction are probed. Results derived from cyclic voltammetry demonstrate that competitive insertion of either Co-(aq)(2+) or Ni-(aq)(2+) or both cations can be kinetically controlled and that the thermodynamic properties can be described in terms of midpoint potentials (E-m). The effect of voltammetric scan rate, electrolysis time, electrolyte concentration, temperature, and method of electrode modification on the preferential selection of Co-(aq)(2+) or Ni-(aq)(2+) cations have been explored. Importantly, the large separation in peak potential (Delta E-p observed for the redox-based TCNQ/[M(TCNQ)(2)(H2O)(2)] solid-solid transformation; Delta E-p=250 mV for Co2+ vs 140 mV for Ni2+) under conditions of cyclic voltammetry are consistent with an electrochemically irreversible, but chemically reversible interconversion for both systems. The kinetic and/or thermodynamic implications of the Delta E-p values are discussed in terms of nucleation-growth and miscibility gap theories. From a thermodynamic perspective, the similar to 55 mV difference in Em for the two systems suggests that TCNQ(center dot-) prefers to accommodate Co-(aq)(2+) cations so that the reaction [Ni(TCNQ)(2)(H2O)(2)]((s))+Co-(aq)(2+) reversible arrow [Co(TCNQ)(2)(H2O)(2)]((s))+Ni-(aq)(2+) is thermodynamically favored and the estimated equilibrium constant (K-eq=50) attests that the reaction lies in favor of the Co-TCNQ system. Atomic force microscopy (AFM) monitoring of the changes that accompany the TCNQ/[M(TCNQ)(2)(H2O)(2)] transformations reveals that the morphology and crystal size of electrochemically generated Co- and Ni-TCNQ systems are substantially different from each other and from the parent TCNQ crystals with the kinetically favored [Ni(TCNQ)(2)(H2O)(2)] needles being much shorter than the thermodynamically favored [Co(TCNQ)(2)(H2O)(2)] analogue, thereby enabling their facile identification in AFM images.