Kinetic and thermodynamic interplay of cation ingress and egress at a TCNQ-modified electrode in contact with aqueous electrolyte mixtures containing Co(II) and Ni(II) cations

摘要

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 Co~(2+) and Ni~(2+) cations at TCNQ_(s) electrode_(s) electrolyte(aq) triple phase boundary junction are probed. Results derived from cyclic voltammetry demonstrate that competitive insertion of either Co~(2+) (aq) or Ni~(2+) (aq) 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~(2+) _((aq)) or Ni~(2+) _((aq)) cations have been explored. Importantly, the large separation in peak potential (ΔE _p observed for the redox-based TCNQ/[M(TCNQ)_2(H_2O) _2] solid-solid transformation; ΔE _p = 250 mV for Co~(2+) vs 140 mV for Ni~(2+)) 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 ΔE _p values are discussed in terms of nucleation-growth and miscibility gap theories. From a thermodynamic perspective, the ~55 mV difference in E _m for the two systems suggests that TCNQ?- prefers to accommodate Co~(2+) (aq) cations so that the reaction [Ni(TCNQ)_2(H 2O)2]_(s) + Co~(2+) (aq) [Co(TCNQ)_2(H_2O)2]_(s) + Ni ~(2+) (aq) 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(H_2O) _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(H_2O)2] needles being much shorter than the thermodynamically favored [Co(TCNQ)_2(H_2O) _2] analogue, thereby enabling their facile identification in AFM images.