Mixed ion transfer in redox processes of poly(3,4-ethylenedioxythiophene)

Mass Transfer from Ion-Sensing Component-Loaded Nanoemulsions into Ion-Selective Membranes: An Electrochemical Quartz Crystal Microbalance and Thin-Film Coulometry Study

Recent work has shown that ion-selective components may be transferred from nanoemulsions (NEs) to endow polymeric membranes with ion-selective sensing properties. This approach has also been used for nanopipette electrodes to achieve single-entity electrochemistry, thereby sensing the ion-selective response of single adhered nanospheres. To this date, however, the mechanism and rate of component transfer remain unclear. We study here the transfer of lipophilic ionic compounds from nanoemulsions into thin plasticized poly(vinyl chloride) (PVC-DOS) films by chronoamperometry and quartz crystal microbalance. Thin-film cyclic coulovoltammetry measurements serve to quantify the uptake of lipophilic species into blank PVC-DOS membranes. Electrochemical quartz crystal microbalance data indicate that the transfer of the emulsion components is insignificant, ruling out simple coalescence with the membrane film. Ionophores and ion-exchangers are shown to transfer into the membrane at rates that correlate with their lipophilicity if mass transport is not rate-limiting, which is the case with more lipophilic compounds (calcium and sodium ionophores). On the other hand, with less lipophilic compounds (valinomycin and cation-exchanger salts), transfer rates are limited by mass transport. This is confirmed with rotating disk electrode experiments in which a linear relationship between the diffusion layer thickness and current is observed. The data suggests that once the nanoemulsion container approaches the membrane surface, transfer of components occur by a three-phase partition mechanism where the aqueous phase serves as a kinetic barrier. The results help better understand and quantify the interaction between nanoemulsions and ion-selective membranes and predict membrane doping rates for a range of components.

High performance printed organic electrochromic devices based on an optimized UV curable solid-state electrolyte

Manufacturing cost is a major concern for electrochromic device (ECD) applications in smart windows for energy saving and low-carbon economy. Fully printing instead of a vacuum-based chemical vapor deposition (CVD) process is favored for large-scale fabrication of ECDs. To adapt to the screen printing process, a UV curable solid-state electrolyte based on lithium bis(trifluoromethane-sulfonyl) imide (LiTFSI) was specially formulated. It contains poly(ethylene glycol) diacrylate (PEG-DA), LiTFSI, water, and ethyl acetate. The optimized ECDs have achieved a 0.6 s bleaching time at 0.6 V and a 1.4 s coloring time at -0.5 V. The ECDs also exhibited excellent stability, which could endure 100 000 cycles of color switching while still maintaining 35% of transmittance change at a 550 nm wavelength. A demo ECD has been fabricated with a screen printed electrolyte, exhibiting stable switching between the clear state and patterned color state.

Ion Mobility in Thick and Thin Poly-3,4 Ethylenedioxythiophene Films-From EQCM to Actuation

Conductive polymer actuators and sensors rely on controlled ion transport coupled to a potential/charge change. In order to understand and control such devices, it is of paramount importance to understand the factors that determine ion flux at various conditions, including the synthesis potential. In this work, the ion transport in thinner poly-3,4-ethylenedioxythiophene (PEDOT) films during charge/discharge driven by cyclic voltammetry is studied by consideration of the electrochemical quartz crystal microbalance (EQCM) and the results are compared to the actuation responses of thicker films that have been synthesized with the same conditions in the bending and linear expansion modes. The effects of polymerization potentials of 1.0 V, 1.2 V, and 1.5 V are studied to elucidate how polymerization potential contributes to actuation, as well the involvement of the EQCM. In this work, it is revealed that there is a shift from anion-dominated to mixed to cation-dominated activity with increased synthesis potential. Scanning electron microscopy shows a decrease in porosity for the PEDOT structure with increasing synthesis potential. EQCM analysis of processes taking place at various potentials allows the determination of appropriate potential windows for increased control over devices.

Cation exchange behavior during the redox switching of poly (3,4-ethylenedioxythiophene) films

Poly (3,4-ethylenedioxythiophene), PEDOT, films were synthesized at room temperature by potentiodynamic and potentiostatic step deposition in aqueous solutions containing EDOT monomer and LiClO4. In some solutions, the effect of small amounts of sodium dodecylsulfate, SDS, on the polymerization rate of EDOT and on the stiffness of the obtained PEDOT film was studied. The obtained PEDOT films were transferred in aqueous solutions containing cations with different molar mass, such as H+, Li+, Na+, K+, and Cs+. The apparent molar masses of the exchanged species during potentiodynamic experiments were determined by in situ microgravimetry. These measurements underlined the importance of the electrolyte chosen for electropolymerization process. It is known that SDS anions can be trapped inside the polymer layer during electropolymerization, providing them with a cation exchange behavior. However, even if the PEDOT films were deposited from an electrolyte without SDS, they still acted as cation exchangers.

Charge Storage Properties of Nanostructured Poly (3,4-ethylenedioxythiophene) Electrodes Revealed by Advanced Electrogravimetry

PEDOT nanowires (NWs) directly grown on the conducting electrode of quartz resonators enable an advanced electrogravimetric analysis of their charge storage behavior. Electrochemical quartz crystal microbalance (EQCM) and its coupling with electrochemical impedance spectroscopy (ac-electrogravimetry or AC-EG) were used complementarily and reveal that TBA⁺, BF₄^- and ACN participate in the charge compensation process with different kinetics and quantity. BF₄^- anions were dominant in terms of concentration over TBA⁺ cations and the anion transfer results in the exclusion of the solvent molecules. TBA⁺ concentration variation in the electrode was small compared to that of the BF₄^- counterpart. However, M_w of TBA⁺ is much higher than BF₄^- (242.3 vs. 86.6 g·mol^-1). Thus, TBA⁺ cations' gravimetric contribution to the EQCM response was more significant than that of BF₄^-. Additional contribution of ACN with an opposite flux direction compared with BF₄^-, led to a net mass gain/lost during a negative/positive potential scan, masking partially the anion response. Such subtleties of the interfacial ion transfer processes were disentangled due to the complementarity of the EQCM and AC-EG methodologies, which were applied here for the characterization of electrochemical processes at the PEDOT NW electrode/organic electrolyte interface.

Effects of different electrolytes and film thicknesses on structural and thermoelectric properties of electropolymerized poly(3,4-ethylenedioxythiophene) films

The effects of the type of electrolyte and film thickness on the structural and thermoelectric properties of poly(3,4-ethylenedioxythiophene) (PEDOT) thin films on indium-tin-oxide (ITO) substrates prepared using electropolymerization were investigated. Two electrolytes were prepared using two different solvents: a water/methanol solvent (protic solvent) and acetonitrile (aprotic solvent) with 3,4-ethylenedioxythiophene (EDOT) and LiCF₃SO₃, typically included in electrolytes as dopants. The electrochemical properties of the two electrolytes were analyzed; it was found that the polymerization process for EDOT on an ITO substrate varied based on the electrolyte used. When the electropolymerization time was increased, the surface morphology of the PEDOT films prepared using the water/methanol solvent appeared to contain grains approximately 100 nm in size whereas the PEDOT films prepared using acetonitrile appeared to contain aggregated grains connected by polymeric networks. Even though there were differences in the surface morphology and chemical bonds determined using Fourier-transform infrared spectroscopy/attenuated total reflectance analysis, the thermoelectric properties were strongly dependent on the film thickness and were only weakly dependent on the type of electrolyte used. The highest power factor was 41.3 μW (m⁻¹ K⁻²) for a PEDOT film with a thickness of 0.5 μm prepared using the water/methanol solvent electrolyte.

The effects of electrolyte type and film thickness on the structural and thermoelectric properties of poly(3,4-ethylenedioxythiophene) (PEDOT) thin films on indium-tin-oxide (ITO) substrates prepared using electropolymerization were investigated.

The Influence of Molecular Dipole Moment on the Redox-Induced Reorganization of α-Helical Peptide Self-Assembled Monolayers: An Electrochemical SPR Investigation

Self-assembled monolayers (SAMs) of ferrocene-labeled α-helical peptides were prepared on gold surfaces and studied using electrochemical surface plasmon resonance (EC-SPR). The leucine-rich peptides were synthesized with a cysteine sulfhydryl group either at the C- or N-terminus, enabling their immobilization onto gold surfaces with control of the direction of the molecular dipole moment. Two electroactive SAMs were studied, one in which all of the peptide dipole moments are oriented in the same direction (SAM1), and the other in which the peptide dipole moment of one peptide is aligned in the opposite direction to that of its surrounding peptide molecules (SAM2). Cyclic voltammetry combined with SPR measurements revealed that SAM reorientations concomitant with the oxidation of the ferrocene label were more significant in SAM2 than in SAM1. The substantially greater change in the peptide film thickness in the case of SAM2 is attributed to the electrostatic repulsion between the electrogenerated ferrocinium moiety and the positively charged gold surface. The greater permeability of SAM1 to electrolyte anions, on the other hand, appears to effectively neutralize this electrostatic repulsion. The film thickness change in SAM2 was estimated to be 0.25 ± 0.05 nm using numerical simulation. The timescale of the redox-induced SPR changes was established by chronoamperometry and time-resolved SPR measurements, followed by fitting of the SPR response to a stretched exponential function. The time constants measured for the anodic process were 16 and 6 ms for SAM1 and SAM2 respectively, indicating that the SAM thickness changes are notably fast.

Solvent effects on the electrochemical p-doping of PEDOT

EQCM experiments were carried out on PEDOT films exposed to TEABF(4)/CH(3)CN and TEABF(4)/CH(2)Cl(2) under permselective conditions and subjected to cyclic voltammetry in the potential range corresponding to p-doping. Current and frequency responses were used to obtain time-resolved ion and solvent flux data as functions of potential. Normalization of these fluxes with potential scan rate distinguishes thermodynamically (ir)reversible elementary steps in the overall redox process. The specific mechanisms are different in the two solvents, although both show mechanistic switches at partial redox conversion during both p-doping and undoping. These different mechanistic signatures are characterized according to the solvent identity, by different patterns of deviation from scan rate normalization for the experimentally measured ion and solvent fluxes. Comparison of these ion and solvent fluxes demonstrates that the rates of solvent expulsion (during doping) and entry (during undoping) are key determinants of mechanism. In both switching directions there are changes between kinetically limiting and rapid solvent transfer that depend upon solvent identity, i.e. the mechanism depends substantially upon charge state, switching direction and solvent. These mechanistic pathways and shifts can be visualized by a scheme-of-cubes representation.

Polymer films on electrodes: investigation of ion transport at poly(3,4-ethylenedioxythiophene) films by scanning electrochemical microscopy

Electropolymerization, morphology characterization, and ion transport of poly(3,4-ethylenedioxythiophene) (PEDOT) films doped with different counterions (chloride, ferrocyanide (FCN), and poly(p-styrenesulfonate) (PSS-)) on a platinum electrode were investigated using scanning electrochemical microscopy (SECM) during both potential step (chronocoulometric) and cyclic voltammetric scans. An ultramicroelectrode (UME) tip was positioned close to the surface of a PEDOT-modified substrate electrode, and the responses of both electrodes to a substrate potential step or linear sweep were monitored simultaneously. Chloride or ferrocyanide (FCN) ejection during PEDOT reduction was shown to be a function of the reduction potential. The nature of the cation in the bulk solution was not found to be important in the kinetics of ion transport in PEDOT+/FCN- films. Direct evidence for the incorporation of cations of Ru(NH3)6(3+/2+) in a PEDOT film during its reduction was also obtained by SECM measurements. The adsorption of Ru(NH3)6(3+) in fully oxidized PEDOT+/PSS- films was observed and attributed to ion exchange between the Na+ co-ion of PSS- and Ru(NH3)6(3+) in the bulk solution.