Electrochemical Mechanism of Ferrocene-Based Redox Molecules in Thin Film Membrane Electrodes

Abstract

2 GRAPHICAL ABSTRACTCartoon illustrating NaCl permeation through water nanodroplets and Cl -exchange at the membrane/sample interface, both of which create an environment to enable the formation of FeCln 3-n complexes, leading to a non-reversibility in the redox chemistry of the membrane. RESEARCH HIGHLIGHTS electrochemistry of ferrocene derivatives in polymeric membranes  role of chloride on the irreversible oxidation of ferrocene in thin film membranes  synchrotron radiation study of chloride on the ferrocene reactivity in membranes  elimination of chloride induced irreversibility of the ferrocene reaction chemistry ABSTRACTCyclic voltammetry (CV) in chloride-based aqueous electrolytes of ferrocene molecule doped thin membranes (~200 nm in thickness) on glassy carbon (GC) substrate electrodes, both plasticized poly(vinyl chloride) (PVC) and unplasticized poly(methyl methacrylate)/poly(decyl methacrylate) (PMMA-PDMA) membranes, has shown that the electrochemical oxidation behavior is irreversible due most likely to 3 degradation of ferrocene at the buried interface (GC|membrane). Furthermore, CV of the ferrocene molecules at GC electrodes in organic solvents employing chloride-based and chloride-free organic electrolytes has demonstrated that the chloride anion is inextricably linked to this irreversible ferrocene oxidation electrochemistry. Accordingly, we have explored the electrochemical oxidation mechanism of ferrocenebased redox molecules in thin film plasticized and unplasticized polymeric membrane electrodes by coupling synchrotron radiation-X-ray photoelectron spectroscopy (SR-XPS) and near edge X-ray absorption fine structure (NEXAFS) with argon ion sputtering to depth profile the electrochemically oxidized thin membrane systems. With the PVC depth profiling studies, it was not possible to precisely study the influence of chloride on the ferrocene reactivity due to the high atomic ratio of chloride in the PVC membrane; however, the depth profiling results obtained with a chlorine-free polymer (PMMA-PDMA) provided irrefutable evidence for the formation of a chloride-based iron product at the GC|PMMA-PDMA interface. Finally, we have identified conditions that prevent the irreversible conversion of ferrocene by utilizing a high loading of redox active reagent and/or an ionic liquid (IL) membrane plasticizer with high ionicity that suppresses the mass transfer of chloride.

Electron Hopping between Fe 3 d States in Ethynylferrocene‐doped Poly(Methyl Methacrylate)‐poly(Decyl Methacrylate) Copolymer Membranes

“…Figure a presents the observed SR‐VBS data. Importantly, these results are consistent with the notion that, at the buried interface, the membrane film comprises near to pure oxidation products of EtFc + Cl − and/or (EtFc + ) n‐3 FeCl n 3−n ion association complexes arising from the nucleophilic attack of Cl − as illustrated in a previous work , noting that previous Fe 2p NEXAFS spectroscopic evidence , not the present SR‐VBS data, provided evidence for the formation of Fe 3+ associated with oxidation of EtFc to EtFc + . Here, the SR‐VBS spectra at 13 to 20 min of Ar sputtering or depths of 156 to 240 nm (buried interface) correspond to the presence of strong levels of Fc and the gradient from the membrane surface to the buried interface reveals Fe 3d states at 2–3 eV (although Fe 2+ and Fe 3+ have 3d 6 and 3d 5 electron configurations respectively, such subtle changes in the Fe 3d DOS were not discernable from the convolution of PMMA‐PDMA SR‐VBS spectral features) together with C 2p π bonding states in Fc aromatic moieties at approximately 6–10 eV and C 2 s from EtFc at 12–13 eV .…”

“…Very recently, it has been shown that the Cl − ‐degraded reaction product may irreversibly block the available Fc in the membrane to be oxidized, resulting in loss of the electrochemical signal . While this effect is drastic with chloride ions, where there is a gradual conversion of EtFc to EtFc + to irreversible FeCl n 3−n complexes after continuous CV cycling, it is gradual or imperceptible with larger anions (i. e. ClO 4 − , PF 6 − or organic anions).…”

“…While this effect is drastic with chloride ions, where there is a gradual conversion of EtFc to EtFc + to irreversible FeCl n 3−n complexes after continuous CV cycling, it is gradual or imperceptible with larger anions (i. e. ClO 4 − , PF 6 − or organic anions). Note that copolymer membranes of PMMA‐PDMA were used in the previous study to eliminate any spectroscopic detection of chloride from the PVC.…”

“…Having closely inspected these spectra, we evaluated the highly doped (11.1 wt.% of Fe) PMMA‐PDMA membrane after polarization (with repetitive scans by CV to electrochemically accumulate Fc + at the buried electrode/membrane interface) . Figure a presents the observed SR‐VBS data.…”

“…Reagent grade ethyl acetate and 1,4‐dioxane (Chem. Supply), along with MMA (99.5 %) and DMA (99 %) sourced from Polysciences, Inc. (Gymea, New South Wales, Australia) and polymerization initiator 2,2’‐azobis(isobutyronitrile) (AIBN sourced from Sigma Aldrich) were used for the synthesis of the PMMA‐PDMA copolymer as previously described .…”