Hans-Jürgen Grande

A new model for electrochemical oxidation of polypyrrole under conformational relaxation control

Abstract The electrochemical behaviour of conducting polymers was modelled using a conformational relaxation treatment, defined as a rearrangement of the chain conformations along the polarization time. After relaxation other electrochemical processes are controlled and completed under diffusion control. The relaxation process is quantified by a relaxation time, which depends on the anodic overpotential (oxidation level) and the cathodic overpotential (closure and compaction of the structure related to the neutral state). Under these conditions and assuming nucleation-like kinetics for the advance of the oxidation front, anodic chronoamperograms for polypyrrole films in 0.1 M LiClO 4 + propylene carbonate solutions obtained by applying potential steps from different positive potentials to several negative values were simulated and fitted to experimental results.

Organo-metal halide perovskite-based solar cells with CuSCN as the inorganic hole selective contact

CuSCN is proposed as a cost-competitive hole selective contact for the emerging organo-metal halide perovskite-based solar cells. The CuSCN films have been deposited by a solution casting technique, which has proven to be compatible with the perovskite films, obtaining planar-like heterojunction-based glass/FTO/TiO2/CH3NH3PbI3−xClx/CuSCN/Au solar cells with a power conversion efficiency of 6.4%. Among the photovoltaic parameters, the fill factor (i.e. 62%) suggests good carrier selectivity and, therefore, efficient functionality of the TiO2 and CuSCN charge carrier selective contacts. However, the open-circuit voltage (Voc), which remains low in comparison with the state of the art perovskite-based solar cells, appears to be the main limiting parameter. This is attributed to the short diffusion length as determined by impedance spectroscopy. However, the recombination losses are not only affected by the CuSCN, but also by the perovskite film. Indeed, variations of 20 °C in the thermal annealing of the perovskite films result in changes larger than 200 mV in the Voc. Furthermore, a detailed analysis of the quantum efficiency spectra contributes significant insights into the influence of the selective contacts on the photocurrent of the planar heterojunction perovskite solar cells.

Conformational movements explain logarithmic relaxation in conducting polymers

Logarithmic relaxation associated with the electrochemical oxidation of conducting polymer films after polarization to high cathodic potentials for long periods of time can be explained by means of the electrochemically stimulated conformational relaxation (ESCR) model. The key idea in the ESCR treatment is that conducting films attain a compacted and closed structure as they are submitted to cathodic polarization. Moreover, longer reduction times promote a higher compactness of the polymeric structure, making difficult the exchange of counterions between the polymer and the solution during further oxidation, which therefore must be preceded by the relaxation of the structure. A logarithmic dependence between the coefficient of cathodic polarization (zc) and the wait time was obtained from chronoamperometric analysis. The ESCR model makes use of this result to explain the retard observed in both anodic chronoamperograms and voltammograms as the cathodic polarization time is shifted to higher values.

CONFORMATIONAL RELAXATION DURING POLYPYRROLE OXIDATION: FROM EXPERIMENT TO THEORY

Abstract The closure of the polymeric entanglement is proposed to occur during the expulsion of small anions from an oxidized polypyrrole film during reduction. The compactness of the polymeric structure rises by polarization at more cathodic potentials. So increasing anodic overpotentials are required to promote conformational relaxation, allowing the penetration of counterions into the polymer matrix, as was observed experimentally from thick (15 μm) polypyrrole films in 0.1 M LiClO 4 /acetonitrile solutions. The relaxation controlled oxidation is not uniform: it is initiated on different points on the polymer-electrolyte interface, as was confirmed by chronoamperometry. The developed electrochemical relaxation model, including this like-nucleation behaviour of the relaxation process, is able to simulate experimental chronoamperograms. A good agreement between theoretical and experimental results was obtained.

Solvent effects on the charge storage ability in polypyrrole

Abstract Solvent effect on the charge storage ability of the polypyrrole have been studied in two different ways: analyzing both the ion–solvent and polymer–solvent interactions and by a multiple regression procedure. The first way was not sufficient to explain all the results obtained. By the multiple regression, influence of the four different variables of the solvents simultaneously has been obtained. Solvents having high dipole moments and low polarizability and having a high capacity to donate electrons are the best solvents among those investigated in this paper to obtain high charge storage abilities.

Electrodeposition of antimony selenide thin films and application in semiconductor sensitized solar cells.

Sb2Se3 thin films are proposed as an alternative light harvester for semiconductor sensitized solar cells. An innovative electrodeposition route, based on aqueous alkaline electrolytes, is presented to obtain amorphous Sb2Se3. The amorphous to crystalline phase transition takes place during a soft thermal annealing in Ar atmosphere. The potential of the Sb2Se3 electrodeposited thin films in semiconductor sensitized solar cells is evaluated by preparing TiO2/Sb2Se3/CuSCN planar heterojunction solar cells. The resulting devices generate electricity from the visible and NIR photons, exhibiting the external quantum efficiency onset close to 1050 nm. Although planar architecture is not optimized in terms of charge carrier collection, photocurrent as high as 18 mA/cm(2), under simulated (AM1.5G) solar light, is achieved. Furthermore, the effect of the Sb2Se3 thickness and microstructural properties on the photocurrent is analyzed, suggesting the hole transport is the main limiting mechanism. The present findings provide significant insights to design efficient semiconductor sensitized solar cells based on advanced architectures (e.g., nanostructured and tandem), opening wide possibilities for progresses in this emerging photovoltaics technology.

Electrochemical oxidation of polypyrrole under conformational relaxation control. Electrochemical relaxation model

Abstract A simple model of polymeric relaxation, associated with the electrochemical switching of polypyrrole films between their reduced (insulating) and oxidized (electronically conducting) states, offers a reasonably precise description of the form of chronoamperograms obtained after previous submission of the polymer film to cathodic potentials (which control the compactness of the neutral polymer) for long periods of time. Opening of the structure driven by anodic potential is not uniform: nucleation of conducting zones inside the neutral polymer and their overlap at long times of anodic polarization are taken into account in the model. Diffusion of counterions from the solution across the oxidized zones are also included. The definition of a relaxation time (depending on both cathodic and anodic overpotentials), and the inclusion of nucleation and diffusion processes, allows a theoretical modelling of chronoamperograms in good agreement with experimental data. This electrochemical relaxation theory is an open model including both electrochemical and polymeric magnitudes.

Reversible electrochemical reactions in conducting polymers: A molecular approach to artificial muscles

Electrical currents trigger oxidation or reduction reactions in conducting polymers. Changes in volume associated with these redox processes can be transformed into macroscopic movements of more than 180° by the construction of a bilayer: polypyrrole-flexible and inactive polymer (artificial muscle). The effects of the applied potential, the nature of the solvent and the electrolyte concentration on the angular movement of the free end of the bilayer were analysed. The movement accelerates with increasing anodic (or cathodic, when the movement is reversed) overpotentials, with increasing electrolyte concentration or by using more polar solvents, leading to the conclusion that the movement is linked to electrochemically driven exchange of hydrated counterions between the solution and the conducting polymer. Geometrical considerations give a simple equation for both the microscopic and macroscopic changes of volume associated to the penetration of counterions during oxidation, which is able to explain the experimental behaviour.

Injectable and Self-Healing Dynamic Hydrogels Based on Metal(I)-Thiolate/Disulfide Exchange as Biomaterials with Tunable Mechanical Properties

Despite numerous strategies involving dynamic covalent bonds to produce self-healing hydrogels with similar frequency-dependent stiffness to native tissues, it remains challenging to use biologically relevant thiol/disulfide exchange to confer such properties to polymeric networks. Herein, we report a new method based on Metal(I) [Au(I) or Ag(I)] capping to protect thiolates from aerial oxidation without preventing thiolate/disulfide exchange. Dynamic hydrogels were readily prepared by injecting simultaneously aqueous solutions of commercially available HAuCl4 and 4-arm thiol-terminated polyethylene glycol [(PEGSH)4], resulting in a network containing a mixture of Au(I)-thiolate (Au-S) and disulfide bonds (SS). While the dynamic properties of the hydrogel were closely dependent on the pH, the mechanical properties could be easily tuned by adjusting (PEGSH)4 concentration and amount of Au-S, as judged by dynamic rheology studies. Permanent Au-S/SS exchange at physiological pH conferred self-healing behavior and frequency-dependent stiffness to the hydrogel. In addition, in vitro studies confirmed that Au-based dynamic material was not cytotoxic to human dermal fibroblasts, demonstrating its potential use as a medical device. Dynamic hydrogels obtained using Ag(I) ions demonstrated that the exchange reaction was not affected by the nature of the Metal(I) capping. Finally, this efficient thiolate capping strategy offers a simple way to produce injectable and self-healing dynamic hydrogels from virtually any thiol-containing polymers.

Influence of the counterion size on the rate of electrochemical relaxation in polypyrrole

Abstract Oxidation chronoamperograms of highly compacted polypyrrole films were performed in propylene carbonate solutions containing different electrolytes (LiF, LiCl, LiBr and LiI), allowing the study of the effect of the counterion size on the rate of electrochemical switching controlled by conformational relaxation processes.