Two-Dimensional CVD-Graphene/Polyaniline Supercapacitors: Synthesis Strategy and Electrochemical Operation.

Nanocomposites of graphene materials and conducting polymers have been extensively studied as promising materials for electrodes of supercapacitors. Here, we present a graphene/polyaniline heterostructure consisting of a CVD-graphene and polyaniline monolayer and its electrochemical operation in a supercapacitor. The synthesis employs functionalization of graphene by p-phenylene sulfonic groups and oxidative polymerization of anilinium by ammonium persulfate under reaction conditions, providing no bulk polyaniline. Scanning electron microscopy, atomic force microscopy, and Raman spectroscopy showed the selective formation of polyaniline on the graphene. In situ Raman spectroelectrochemistry and cyclic voltammetry (both in a microdroplet setup) confirm the reversibility of polyaniline redox transitions and graphene electrochemical doping. After an increase within the initial 200 cycles due to the formation of benzoquinone-hydroquinone defects in polyaniline, the specific areal capacitance remained for 2400 cycles with ±1% retention at 21.2 µF cm-2, one order of magnitude higher than the capacitance of pristine graphene.

Host-Guest Interactions in Metal-Organic Frameworks Doped with Acceptor Molecules as Revealed by Resonance Raman Spectroscopy.

Metal-organic frameworks (MOFs) represent a class of porous materials whose properties can be altered by doping with redox-active molecules. Despite advanced properties such as enhanced electrical conduction that doped MOFs exhibit, understanding physical mechanisms remains challenging because of their heterogeneous nature hindering experimental observations of host-guest interactions. Here, we show a study of charge transfer between Mn-MOF-74 and electron acceptors, 7,7,8,8-tetracyanoquinodimethane (TCNQ) and XeF2, employing selective enhancement of Raman scattering of different moieties under various optical-resonance conditions. We identify Raman modes of molecular components and elucidate that TCNQ gets oxidized into dicyano-p-toluoyl cyanide (DCTC-) while XeF2 fluorinates the MOF upon infiltration. The framework's linker in both cases acts as an electron donor as deduced from blue shifts of the C-O stretching mode accompanied by the emergence of a quinone-like mode. This work demonstrates a generally applicable methodology for investigating charge transfer in various donor-acceptor systems by means of resonance Raman spectroscopy.

Conducting polyaniline prepared in the solutions of formic acid: Does functionalization with carboxyl groups occur?

Polyaniline is a conducting polymer with an application potential in the field of biomedical engineering. By employing FTIR spectroscopy and conductivity measurements, it has been shown that the oxidation at stoichiometric peroxydisulfate-to-aniline mole ratio 1.25 in the solutions of formic acid in the range 0-10 M provides samples of a moderate conductivity of the orders 0.01-0.1 S cm-1. They consist of polyaniline and aniline oligomers as typical of the aniline oxidation in weak acids. The detailed investigation of the infrared spectra indicates a partial ring-carboxylation of polyaniline at high acid concentrations. The extent of structural defects is higher for a series prepared at over-stoichiometric peroxydisulfate-to-aniline mole ratio 2.5, which provided only non-conducting samples. The reference sample series represented by poly(aniline-co-o-aminobenzoic acid) was also prepared and is used in the discussion of the infrared spectra.

Role of p-Benzoquinone in the Synthesis of a Conducting Polymer, Polyaniline.

Polyaniline (PANI) and 2,5-dianilino-p-benzoquinone both are formed by oxidation of aniline in an acidic aqueous environment. The aim of this study is to understand the impact of addition of p-benzoquinone on the structure of PANI prepared by the oxidation of aniline hydrochloride with ammonium peroxydisulfate and to elucidate the formation of low-molecular-weight byproducts. An increasing yield and size-exclusion chromatography, Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy, and nuclear magnetic resonance analyses of the products show that p-benzoquinone does not act as a terminating agent in the synthesis of PANI and the content of 2,5-dianilino-p-benzoquinone increases with the increasing molar concentration of p-benzoquinone in the reaction mixture, [BzQ]. Regarding the structure of PANI, Raman and UV-visible spectra show that the doping level and the charge delocalization both decrease with the increase of [BzQ], and the FTIR spectra of the PANI bases indicate an increased concentration of benzenoid units at higher [BzQ]. We explain these observations by an increasing concentration of structural defects in PANI chains and propose a 2,5-dianilino-p-benzoquinone-like structure of these defects present as pendant groups. The bands typical of 2,5-dianilino-p-benzoquinone-like moiety are observed even in the vibrational spectra of the sample prepared without addition of p-benzoquinone. This confirms in situ oxidation of aniline to p-benzoquinone within the course of the oxidation of aniline hydrochloride to PANI.

Thermally Induced Protonation of Conducting Polyaniline Film by Dibutyl Phosphite Conversion to Phosphate.

The blue thin polyaniline base film changes its color to green after immersion of the film into dibutyl phosphonate. The green color of the film converts to a greenish-blue after heating to 200 °C in air, which is characteristic for the protonated conducting form of polyaniline. This is in contrast to the "standard" polyaniline hydrochloride, which is transformed into a cross-linked polyaniline base under such conditions. To explain this unexpected observation, the interaction of polyaniline base with dibutyl phosphonate at ambient conditions and after heating to 200 °C was studied using UV-visible, FTIR and Raman spectroscopies. On the basis of these studies, we propose that the dibutyl phosphite tautomeric form of dibutyl phosphonate, which interacts with polyaniline base at 20 °C, converts to the oxidized form, dibutyl phosphate, at 200 °C and subsequently protonates the film. Quantum-chemical modeling of the interaction of polyaniline base with dibutyl phosphite and dibutyl phosphate supports this explanation.