The paper is concerned with the small signal ac impedance of porous film electrodes in contact with solution. An overview is presented of the standard transmission line model with two transport channels and a crosswise element. The simplest configurations are discussed: a single resistance in one of the channels, and either an interfacial capacitor or a RC transfer circuit at the pore's wall. The resulting relaxation functions are classified in terms of two characteristic frequencies: one for coupled transport and interfacial polarization and another one for the interfacial reaction. Subsequently, these models are extended in order to describe porous electrodes where the interfacial polarization displays complex properties, i.e., frequency dispersion. The capacitive element is described by a constant-phase element (CPE), and it is shown that the fractionary exponent provides an additional and measurable degree of freedom in the parameter space of the relaxation function, whose determination can be exploited as a supplementary tool for analysis. The analysis of impedance measurements of TiO 2 nanoporous photoelectrodes in negative bias voltage shows that the suggested approach is capable of identifying two characteristic relaxation frequencies in a frequency-resolved measurement on this system.

Layered molybdenum disulfide (MoS2) is deposited by microwave heating on a reduced graphene oxide (RGO). Three concentrations of MoS2 are loaded on RGO, and the structure and morphology are characterized. The first layers of MoS2 are detected as being directly bonded with the oxygen of the RGO by covalent chemical bonds (Mo‐O‐C). Electrochemical characterizations indicate that this electroactive material can be cycled reversibly between 0.25 and 0.8 V in 1 m HClO4 solution for hybrids with low concentrations of MoS2 layers (LCMoS2/RGO) and between 0.25 and 0.65 V for medium (MCMoS2/RGO) and high concentrations (HCMoS2/RGO) of MoS2 layers on graphene. The specific capacitance measured values at 10 mV s−1 are 128, 265, and 148 Fg−1 for the MoS2/RGO with low, medium, and high concentrations of MoS2, respectively, and the calculated energy density is 63 W h kg−1 for the LCMoS2/RGO hybrid. This supercapacitor electrode also exhibits superior cyclic stability with 92% of the specific capacitance retained after 1000 cycles.

We merged the microwave synthesis approach with an extension of the nonhydrolytic sol-gel method to induce highly selective crystallization of MoS(2) layers over graphene sheets. This hybrid material showed superior electrocatalytic activity in hydrogen evolution reactions.

The electrical properties of tin oxide varistors doped with CoO, Nb 2 O 5 , and Cr 2 O 3 , were investigated using the impedance spectroscopy technique with the temperature ranging from 25 to 400°C. The impedance data, represented by means of Nyquist diagrams, show two time constants with different activation energies, one at low frequencies and the other at high frequencies. These activation energies were associated with the adsorption and reaction of O 2 species at the grain boundary interface. The Arrhenius plots show two slopes with a turnover at 200°C for both the higher and lower frequency time constants. This behavior can be related with the decrease of minor charge carrier density. The barrier formation mechanism was associated with the presence of Cr Sn -at the surface, which promotes the adsorption of the OЈ and OЉ species which are in turn proposed as being responsible for the barrier formation.

The aim of this study is to immobilize an enzyme, namely, organophosphorus hydrolase (OPH), and to detect the presence of paraoxon, which is an organophosphorus compound, using the layer-by-layer (LbL) deposition technique. To lift the OPH from the solid substrate, a pair of polyelectrolytes (positively charged chitosan (CS) and negatively charged poly(thiophene-3-acetic acid) (PTAA)) were combined. These species were made charged by altering the pH of the solutions. LbL involved alternate adsorption of the oppositely charged polyions from dilute aqueous solutions onto a hydrophilic quartz slide. This polyion cushion was held together by the electrostatic attraction between CS and PTAA. The growing process was monitored by fluorescence spectroscopy. OPH was then adsorbed onto the five-bilayer CS/PTAA system. This five-bilayer macromolecular structure compared to the solid substrate rendered stability to the enzyme by giving functional integrity in addition to the ability to react with paraoxon solutions. The ultimate goal is to use such a system to detect the presence of organophosphorus compounds with speed and sensitivity using the absorption and fluorescence detection methodologies.

No abstract

In this paper, the electrochemical degradation of polypyrrole film was studied by means of overpotential application. The overpotential was 0.58 V versus SCE, and after every 5 min of application of 0.58 V, a cyclic voltammogram was recorded in the range of -0.7 to 0.5 V as well as an electrochemical impedance spectroscopy and electrochemical quartz crystal microbalance (EIS and EQCM). The main characteristic is the huge increase in the charge transfer resistance (r(ct)), which indicates that the insertion process of ions in the polymer matrix is hindered by the electrochemical degradation. Once the process of insertion is damaged, the number of intercalated ions in the matrix should decrease, which is expressed by the low-frequency capacitance, which is proportional to the number of intercalated ions in the polymeric matrix. The decrease of intercalated ions has an influence in the mass variation of the polymer film, which is confirmed by EQCM measurements.

This work investigates the loss in performance induced by molecular oxygen in bulk heterojunction solar cells. We observe that upon exposure to molecular oxygen both formation of P3HT