Electrochemical impedance spectroscopy of thin films with two mobile charge carriers: effects of the interfacial charging

Phosphates Transfer in Pristine and Modified CJMA-2 Membrane during Electrodialysis Processing of NaxH(3-x)PO4 Solutions with pH from 4.5 to 9.9

Phosphate recovery from different second streams using electrodialysis (ED) is a promising step to a nutrients circular economy. However, the relatively low ED performance hinders the widespread adoption of this environmentally sound method. The formation of "bonded species" between phosphates and the weakly basic fixed groups (primary and secondary amines) of the anion exchange membrane can be the cause of decrease in current efficiency and increase in energy consumption. ED processing of Na_xH_(3-x)PO₄ alkaline solutions and the use of intense current modes promote the formation of a bipolar junction from negatively charged bound species and positively charged fixed groups. This phenomenon causes a change in the shape of current-voltage curves, increase in resistance, and an enhancement in proton generation during long-term operation of anion-exchange membrane with weakly basic fixed groups. Shielding of primary and secondary amines with a modifier containing quaternary ammonium bases significantly improves ED performance in the recovery of phosphates from Na_xH_(3-x)PO₄ solution with pH 4.5. Indeed, in the limiting and underlimiting current modes, 40% of phosphates are recovered 1.3 times faster, and energy consumption is reduced by 1.9 times in the case of the modified membrane compared to the pristine one. Studies were performed using a new commercial anion exchange membrane CJMA-2.

Catalytic oxidation of ibuprofen over bulk heterojunction photocatalysts based on conjugated donor-acceptor configured benzoselenadiazole molecule

The use of photocatalysts for acquiring direct photon energy from sunlight is a promising way to clean the environment, particularly the remediation of contaminants from water. In this work, firstly π-conjugated organic semiconductor configuring benzoselenadiazole, 4-(3,5-bis(trifluoromethyl) phenyl)-7-(5'-hexyl-[2,2'-bithiophen]-5-yl)-benzo [c] (Kümmerer, 2009; Chen et al., 2018; Randeep et al., 201) selenadiazole, abbreviated as (RTh-Se-F), was synthesized. The designed RTh-Se-F with an extended π-conjugation showed good optical properties in the visible region and estimated a low optical band gap of ∼2.02 eV . The molecular orbitals i.e. HOMO (-5.33 eV) and LUMO (-3.31 eV) for RTh-Se-F organic semiconductor were suitably aligned to energy levels of (Madhavan et al., 2010Madhavan et al., 2010)-Phenyl-C₇₁-butyric acid methyl esters (PC₇₁BM) which resulted in the broadening of absorption and covering of entire visible region. RTh-Se-F was integrated with varied weight percentages (wt %) of PC₇₁BM to obtain bulk heterojunction (BHJ) and applied as efficient visible light driven BHJ photocatalyst for an effective oxidation of ibuprofen. RTh-Se-F@PC₇₁BM (1:2, wt %) BHJ photocatalyst showed the superior ibuprofen degradation of ∼93% within 90 min under visible light illumination. The maximum degradation rate by BHJ photocatalyst might be accredited to the broadening of absorption capacity and improved lifetime of photogenerated electron-hole pairs which might be resulted from high absorption properties of RTh-Se-F organic semiconductor.

Measuring the Pores’ Structure in P3HT Organic Polymeric Semiconductor Films Using Interface Electrolyte/Organic Semiconductor Redox Injection Reactions and Bulk Space-Charge

The article is another in a series of follow-up articles on the new spectroscopic method Energy Resolved–Electrochemical Impedance Spectroscopy (ER-EIS) and presents a continuation of the effort to explain the method for electronic structure elucidation and its possibilities in the study of organic polymeric semiconductors. In addition to the detailed information on the electronic structure of the investigated organic semiconductor, the paper deals with three of the hitherto not solved aspects of the method, (1) the pores structure, which has been embedded in the evaluation framework of the ER-EIS method and shown, how the basic quantities of the pores structure, the volume density of the pores’ density coefficient β = (0.038 ± 0.002) nm⁻¹ and the Brunauer-Emmet-Teller surface areas SABET SA == 34.5 m²g⁻¹ may be found by the method, here for the archetypal poly(3-hexylthiophene-2,5-diyl) (P3HT) films. It is next shown, why the pore’s existence needs not to endanger the spectroscopic results of the ER-EIS method, and a proper way of the ER-EIS data evaluation is presented to avoid it. It is highlighted (2), how may the measurements of the pore structure contribute to the determination of the, for the method ER-EIS important, real rate constant of the overall Marcus’ D-A charge-transfer process for the poreless material and found its value k_ctD-A = (2.2 ± 0.6) × 10⁻²⁵ cm⁴ s⁻¹ for P3HT films examined. It is also independently attempted (3) to evaluate the range of k_ctD-A, based on the knowledge of the individual reaction rates in a chain of reactions, forming the whole D-A process, where the slowest one (organic semiconductor hopping transport) determines the tentative total result k_ctD-A ≅ 10⁻²⁵ cm⁴ s⁻¹. The effect of injection of high current densities by redox interface reactions in the bulk of OS with built-in pores structure may be very interesting for the design of new devices of organic electronics.

Impedance Analysis of Electrochemical Systems

Interpretation of impedance spectroscopy data requires both a description of the chemistry and physics that govern the system and an assessment of the error structure of the measurement. The approach presented here includes use of graphical methods to guide model development, use of a measurement model analysis to assess the presence of stochastic and bias errors, and a systematic development of interpretation models in terms of the proposed reaction mechanism and physical description. Application to corrosion, batteries, and biological systems is discussed, and emerging trends in interpretation and implementation of impedance spectroscopy are presented.

Porous Semiconducting Polymers Enable High-Performance Electrochemical Transistors

Organic polymer electrochemical transistors (OECTs) are of great interest for flexible electronics and bioelectronics applications owing to their high transconductance and low operating voltage. However, efficient OECT operation must delicately balance the seemingly incompatible materials optimizations of redox chemistry, active layer electronic transport, and ion penetration/transport. The latter characteristics are particularly challenging since most high-mobility semiconducting polymers are hydrophobic, which hinders efficient ion penetration, hence limiting OECT performance. Here, the properties and OECT response of a series of dense and porous semiconducting polymer films are compared, the latter fabricated via a facile breath figure approach. This methodology enables fast ion doping, high transconductance (up to 364 S cm^-1 ), and a low subthreshold swing for the hydrophobic polymers DPPDTT and P3HT, rivalling or exceeding the metrics of the relatively hydrophilic polymer, Pg2T-T. Furthermore, the porous morphology also enhances the transconductance of hydrophilic polymers, offering a general strategy for fabricating high-performance electrochemical transistors.

Synthesis of Bi2S3 thin films based on pulse-plating bismuth nanocrystallines and its photoelectrochemical properties

The solubility of Bi3+ in aqueous solution is an important factor that limits the fabrication of high-quality Bi2S3 thin films. In order to find a low-cost method to manufacture high-quality Bi2S3 thin films, we are reporting the preparation of the Bi2S3 thin films based on pulse-plating method in this paper for the first time. The nano-bismuth particles were obtained by electroplating on fluorine-doped SnO2 (FTO)-coated conducting glass substrates with saturated bismuth potassium citrate solution as the electroplating bath, and then it was put into a muffle furnace to oxidize. Finally, the thin films depositing on FTO glass substrates were put into the thioacetamide solution for vulcanization. In the end, the Bi2S3 thin films were successfully prepared on FTO glass substrates. Different characterization techniques were used to characterize the structure, morphology and photoelectrochemical properties of the prepared thin films. The test results revealed that we used this method to synthesize the high-quality Bi2S3 thin films, thus the Bi2S3 materials synthesized through this method are promising candidates in photoelectrochemical application.

An Insight into Ionic Conductivity of Polyaniline Thin Films

The work addresses an issue of the conductivity phenomenon in conductive polymer thin films. Polyaniline was chosen as a broadly used and thoroughly investigated conductive polymer in order to test and show capabilities of the developed original approach based on impedance spectra analysis. A number of films of different thickness were deposited onto a Pt electrode surface and consequently investigated in aqueous solution containing perchloric acid as an electrolyte. The processes that occur in polyaniline film were studied by cyclic voltammetry, electrochemical quartz crystal microgravimetry (EQCM) and electrochemical impedance spectroscopy (EIS). The role of incorporated ions as charge carriers was investigated with respect to the control of the conductivity properties of the film. Along with detailed polyaniline behavior study, the work makes up a fundamental scientific impact on theoretical electrochemistry and electroanalytical techniques.

The Development of Electroconvection at the Surface of a Heterogeneous Cation-Exchange Membrane Modified with Perfluorosulfonic Acid Polymer Film Containing Titanium Oxide

One way to enhance mass transfer and reduce fouling in wastewater electrodialysis is stimulation of electroconvective mixing of the solution adjoining membranes by modifying their surfaces. Several samples were prepared by casting the perfluorosulfonic acid (PFSA) polymer film doped with TiO₂ nanoparticles onto the surface of the heterogeneous cation-exchange membrane MK-40. It is found that changes in surface characteristics conditioned by such modification lead to an increase in the limiting current density due to the stimulation of electroconvection, which develops according to the mechanism of electroosmosis of the first kind. The greatest increase in the current compared to the pristine membrane can be obtained by modification with the film being 20 μm thick and containing 3 wt% of TiO₂. The sample containing 6 wt% of TiO₂ provides higher mass transfer in overlimiting current modes due to the development of nonequilibrium electroconvection. A 1.5-fold increase in the thickness of the modifying film reduces the positive effect of introducing TiO₂ nanoparticles due to (1) partial shielding of the nanoparticles on the surface of the modified membrane; (2) a decrease in the tangential component of the electric force, which affects the development of electroconvection.

Normal and inverted regimes of charge transfer controlled by density of states at polymer electrodes

Conductive polymer electrodes have exceptional promise for next-generation bioelectronics and energy conversion devices due to inherent mechanical flexibility, printability, biocompatibility, and low cost. Conductive polymers uniquely exhibit hybrid electronic-ionic transport properties that enable novel electrochemical device architectures, an advantage over inorganic counterparts. Yet critical structure-property relationships to control the potential-dependent rates of charge transfer at polymer/electrolyte interfaces remain poorly understood. Herein, we evaluate the kinetics of charge transfer between electrodeposited poly-(3-hexylthiophene) films and a model redox-active molecule, ferrocenedimethanol. We show that the kinetics directly follow the potential-dependent occupancy of electronic states in the polymer. The rate increases then decreases with potential (both normal and inverted kinetic regimes), a phenomenon distinct from inorganic semiconductors. This insight can be invoked to design polymer electrodes with kinetic selectivity toward redox active species and help guide synthetic approaches for the design of alternative device architectures and approaches.

An insight into the hybrid dye-sensitized solar cell from polyaniline-CdS nanotubes through impedance spectroscopy

Composites of polyaniline (PANI) and cadmium sulfide nanoparticles (CdS NPs) are in situ synthesized by polymerizing aniline in acetic acid medium for different concentrations of CdS NPs. The composites, characterized by scanning and transmission electron microscopy, exhibit nanotubular morphology of PANI decorated with nanospheres of CdS NPs at their outer surface. The FTIR and UV-vis spectra indicate strong interaction between PANI and the CdS NPs suggesting a good conjugate for photovoltaic applications. The dc conductivities of the composite increase with an increase of the CdS concentration showing a maximum of 1.17 × 10^-2 S cm^-1 for the C200 sample (obtained by polymerizing 182 μL aniline with 200 mg CdS). The current-voltage plots indicate that the photocurrent is higher from the dark current and the separation between the dark current and photocurrent is maximum in C200. The effectiveness of these composites in DSSCs is studied under illumination of 100 mW cm^-2 and the C200 device exhibits a maximum open circuit voltage of 0.73 V, a short circuit current of 8.22 mA cm^-2, and a power conversion efficiency of 3.96%. The impedance study of the cells indicates that the lifetime of the photo-injected electrons is highest (1.34 ms) for the C200 sample, explaining the maximum PCE among the composites.

In situ Electrochemical Impedance Spectroscopy of Electrostatically Driven Selective Gold Nanoparticle Adsorption on Block Copolymer Lamellae

Electrostatic attraction between charged nanoparticles and oppositely charged nanopatterned polymeric films enables tailored structuring of functional nanoscopic surfaces. The bottom-up fabrication of organic/inorganic composites for example bears promising potential toward cheap fabrication of catalysts, optical sensors, and the manufacture of miniaturized electric circuitry. However, only little is known about the time-dependent adsorption behavior and the electronic or ionic charge transfer in the film bulk and at interfaces during nanoparticle assembly via electrostatic interactions. In situ electrochemical impedance spectroscopy (EIS) in combination with a microfluidic system for fast and reproducible liquid delivery was thus applied to monitor the selective deposition of negatively charged gold nanoparticles on top of positively charged poly(2-vinylpyridinium) (qP2VP) domains of phase separated lamellar poly(styrene)-block-poly(2-vinylpyridinium) (PS-b-qP2VP) diblock copolymer thin films. The acquired impedance data delivered information with respect to interfacial charge alteration, ionic diffusion, and the charge dependent nanoparticle adsorption kinetics, considering this yet unexplored system. We demonstrate that the selective adsorption of negatively charged gold nanoparticles (AuNPs) on positively charged qP2VP domains of lamellar PS-b-qP2VP thin films can indeed be tracked by EIS. Moreover, we show that the nanoparticle adsorption kinetics and the nanoparticle packing density are functions of the charge density in the qP2VP domains.

Dual-transmission line modeling of electrochemical processes in polyaniline-cellulose ester composite porous membranes

The charge transport processes in polyaniline (PANI) composite porous membranes have been elaborated in this study using dual-transmission line impedance model conventionally used for macroscopically homogeneous (nanoporous) membranes. Mixed cellulose ester (ME)-PANI porous membranes were prepared using various in situ chemical polymerization techniques including solution- and vapor-phase polymerizations, and two-compartment cell diaphragmatic polymerization. Each technique yielded different PANI deposition site and content in the membranes. As a result, the modeling of electrochemical impedance spectroscopy (EIS) data yielded different model parameters that have been correlated with the PANI content and deposition site (i.e., surface layering versus in-bulk deposition) in the membranes. The modeling results showed that PANI deposition enhanced charge transport by shifting the interfacial transfer mechanism at pore walls from simple double layer charging to the charge transfer involving oxidation of PANI molecular chains deposited at the pore walls of the composite membranes. In addition, in-bulk PANI deposition in the membranes by means of two-compartment cell polymerization showed several orders of magnitude faster charge transport as compared to the membranes where PANI deposited only at the surface. This study shows that pore-controlled diffusion in PANI composite porous membranes can be satisfactorily modeled using dual-transmission line model and correlated with PANI deposition site in the membranes.

Nanochannel-based electrochemical sensor for the detection of pharmaceutical contaminants in water

Effective real-time monitoring is the key to understanding and tackling the issue of pharmaceutical contamination of water. This research demonstrates the utility of an alumina nanochannel-based electrochemical sensor platform for the detection of ibuprofen in water derived from various sources. Our results indicate that the sensor is highly sensitive with a limit of detection at 0.25 pg mL(-1). The novel sensor described here has potential for application as a simple, rapid, inexpensive and highly reliable method for real-time environmental water quality assessment.

Hydrogenation-induced surface polarity recognition and proton memory behavior at protic-ionic-liquid/oxide electric-double-layer interfaces

The electric-double-layer (EDL) formed at liquid/solid interfaces provides a broad and interdisciplinary attraction in terms of electrochemistry, photochemistry, catalysts, energy storage, and electronics. Especially in recent years, much effort has been devoted to the fundamental understanding and practical applications of transistor configurations with EDLs because of their ability for high-density charge accumulation. However, to exploit additional new functionalities of such an emerging interface is not only of great importance but also a huge challenge. Here, we demonstrate that, by introducing protic ionic liquid (PIL) as the gate dielectric for ZnO EDL transistors (EDLTs), small and chemically active ions, such as protons and hydroxyls, can serve as an adsorption medium to extend the interfacial functionalities of EDLTs. By selectively driving the H(+) or OH(-) groups onto ZnO channel surfaces with an electric field, the charged adsorbates interact with surface atoms in different adsorption mechanisms, showing remarkable variations in electron transport and providing a possibility for the recognition of surface polarity. Most significantly, the large hysteresis in the transfer characteristics of PIL-EDLTs makes the device available and promising for nonvolatile proton memory devices via surface hydrogenation and dehydrogenation processes. Such a finding provides us with new opportunities to understand liquid/solid heterogeneous interface phenomena and to extend the practical functions of EDLs through controllable interfacial interaction.