Probing Oxygen-to-Hydrogen Peroxide Electro-Conversion at Electrocatalysts Derived from Polyaniline.

Hydrogen peroxide (H2O2) is a key chemical for many industrial applications, yet it is primarily produced by the energy-intensive anthraquinone process. As part of the Power-to-X scenario of electrosynthesis, the controlled oxygen reduction reaction (ORR) can enable the decentralized and renewable production of H2O2. We have previously demonstrated that self-supported electrocatalytic materials derived from polyaniline by chemical oxidative polymerization have shown promising activity for the reduction of H2O to H2 in alkaline media. Herein, we interrogate whether such materials could also catalyze the electro-conversion of O2-to-H2O2 in an alkaline medium by means of a selective two-electron pathway of ORR. To probe such a hypothesis, nine sets of polyaniline-based materials were synthesized by controlling the polymerization of aniline in the presence or not of nickel (+II) and cobalt (+II), which was followed by thermal treatment under air and inert gas. The selectivity and faradaic efficiency were evaluated by complementary electroanalytical methods of rotating ring-disk electrode (RRDE) and electrolysis combined with spectrophotometry. It was found that the presence of cobalt species inhibits the performance. The selectivity towards H2O2 was 65-80% for polyaniline and nickel-modified polyaniline. The production rate was 974 ± 83, 1057 ± 64 and 1042 ± 74 µmolH2O2 h-1 for calcined polyaniline, calcined nickel-modified polyaniline and Vulcan XC 72R (state-of-the-art electrocatalyst), respectively, which corresponds to 487 ± 42, 529 ± 32 and 521 ± 37 mol kg-1cat h-1 (122 ± 10, 132 ± 8 and 130 ± 9 mol kg-1cat cm-2) for faradaic efficiencies of 58-78%.

Insights on the Electrocatalytic Seawater Splitting at Heterogeneous Nickel-Cobalt Based Electrocatalysts Engineered from Oxidative Aniline Polymerization and Calcination.

Given the limited access to freshwater compared to seawater, a growing interest surrounds the direct seawater electrolysis to produce hydrogen. However, we currently lack efficient electrocatalysts to selectively perform the oxygen evolution reaction (OER) over the oxidation of the chloride ions that are the main components of seawater. In this contribution, we report an engineering strategy to synthesize heterogeneous electrocatalysts by the simultaneous formation of separate chalcogenides of nickel (NiSx, x = 0, 2/3, 8/9, and 4/3) and cobalt (CoSx, x = 0 and 8/9) onto a carbon-nitrogen-sulfur nanostructured network. Specifically, the oxidative aniline polymerization in the presence of metallic cations was combined with the calcination to regulate the separate formation of various self-supported phases in order to target the multifunctional applicability as both hydrogen evolution reaction (HER) and OER in a simulated alkaline seawater. The OER's metric current densities of 10 and 100 mA cm-2 were achieved at the bimetallic for only 1.60 and 1.63 VRHE, respectively. This high-performance was maintained in the electrolysis with a starting voltage of 1.6 V and satisfactory stability at 100 mA over 17 h. Our findings validate a high selectivity for OER of ~100%, which outperforms the previously reported data of 87-95%.

Iridium and Ruthenium Modified Polyaniline Polymer Leads to Nanostructured Electrocatalysts with High Performance Regarding Water Splitting.

The breakthrough in water electrolysis technology for the sustainable production of H2, considered as a future fuel, is currently hampered by the development of tough electrocatalytic materials. We report a new strategy of fabricating conducting polymer-derived nanostructured materials to accelerate the electrocatalytic hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and water splitting. Extended physical (XRD, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX)) and electrochemical (cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS)) methods were merged to precisely characterize the as-synthesized iridium and ruthenium modified polyaniline (PANI) materials and interrogate their efficiency. The presence of Ir(+III) cations during polymerization leads to the formation of Ir metal nanoparticles, while Ru(+III) induces the formation of RuO2 oxide nanoparticles by thermal treatment; they are therefore methods for the on-demand production of oxide or metal nanostructured electrocatalysts. The findings from using 0.5 M H2SO4 highlight an ultrafast electrochemical kinetic of the material PANI-Ir for HER (36 - 0 = 36 mV overpotential to reach 10 mA cm-2 at 21 mV dec-1), and of PANI-Ru for OER (1.47 - 1.23 = 240 mV overpotential to reach 10 mA cm-2 at 47 mV dec-1), resulting in an efficient water splitting exactly at its thermoneutral cell voltage of 1.45 V, and satisfactory durability (96 h).

Nanostructured Carbon-Nitrogen-Sulfur-Nickel Networks Derived From Polyaniline as Bifunctional Catalysts for Water Splitting.

The development of reliable production routes for sustainable hydrogen (H2), which is an essential feedstock for industrial processes and energy carrier for fuel cells, is needed. It appears to be an unavoidable alternative to significantly reduce the dependence on conventional energy sources based on fossil fuels without increasing the atmospheric CO2 levels. Among the different power-to-X scenarios to access high purity H2, the electrochemical approach based on electrolysis looks to be a promising sustainable solution at both the small and large industrial scales. However, the practical realization of this important opportunity faces several challenges, including the efficient design of cost-effective catalytic materials to be used as a cathode with improved intrinsic and durable activity. In this contribution, we report the design and development of efficient nanostructured catalysts for the electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in aqueous media, whereby noble metal-free elements are embedded in a matrix of a conducting polymer, polyaniline (PANI). To increase the electrical conductivity and further the electrocatalytic ability toward HER of the chemically polymerized PANI in the presence of nickel (II) salt (nitrate), the PANI-based materials have first been stabilized at a mild temperature of 250-350°C in air and then carbonized at 800-1,000°C under nitrogen gas to convert the chemical species into nitrogen, sulfur, nickel, and carbon nanostructured networks (CNNs). Different physicochemical (TGA-DSC, Raman spectroscopy, XRD, SEM, EDX, ICP, CHNS, BET, and XPS) and electrochemical (voltammetry and electrochemical impedance spectrometry) methods have been integrated to characterize the as-synthesized CNNs materials and interrogate the relationship of material-to-performance. It has been found that those synthesis conditions allow for the substantial increase of the electrocatalytic performance toward HER and OER in alkaline media in terms of the onset potential and charge transfer resistance and overpotential at the specific activity of 10 milliamps per square centimeter, thus ranking the present materials among the most efficient noble metal-free catalysts and making them possible candidates for integration in practical low-energy consumption alkaline electrolyzers.

One- and two-photon absorption and emission properties of an oligo(phenylenethienylene)s series.

The photophysical and nonlinear absorption properties of an oligo(phenylenethienylene)s series (nTBT) are investigated in this article. The length of the chromophore is gradually increased from one to four phenylenethienylene repeating units in order to evaluate the effects of the electronic delocalization on the two-photon absorption cross sections (δ). According to the excitation anisotropy measurements and quantum chemical calculations, two electronic transitions with distinctive symmetries, 1Ag → 1Bu and 1Ag → 2Ag, are present in the low energy region of the linear absorption spectrum. The lowest-energy transition 1Ag → 1Bu is one-photon allowed but two-photon forbidden and implies an electronic charge delocalization all along the oligomer segment whereas the weakly-allowed 1Ag → 2Ag transition exhibits a transition moment perpendicular to the average plane of the chromophore. The latter transition mainly contributes to the two-photon absorption ability of the oligomers. All derivatives are poorly solvatochromic and the breakdown of the mirror symmetry rule observed between absorption and fluorescence spectra at room temperature has been attributed to a photoinduced geometrical relaxation leading to a very efficient planarization process of the oligomer irrespective of its size. Increasing the oligomer length results in a slight shift of the two-photon absorption band (∼1300 cm(-1)) and in a drastic increase of δ from 2 ± 1 GM up to 802 ± 160 GM for 1TBT and 4TBT respectively. Based on a three-level model, it was found that main contributions to the strong increase of δ stem from the transition moments Mge and Mee' which are multiplied by a factor of 2.8 and 5 when going from 1TBT to 4TBT.

Comprehensive analysis of fragment orbital interactions to build highly π-conjugated thienylene-substituted phenylene oligomers.

π-Conjugated thienylene-phenylene oligomers with fluorinated and dialkoxylated phenylene fragments have been designed and prepared to understand the interactions in fragment orbitals, the influence of the substituents (F, OMe) on the HOMO-LUMO gap, and the role of intramolecular non-covalent cumulative interactions in the construction of π-conjugated nanostructures. Their strong conjugation was also evidenced in the gas phase by UV photoelectron spectroscopy and theoretical calculations. These results can be explained by the crucial role of the relative energetic positions of the π orbitals of the dimethoxyphenylene, which was used to model the dialkoxyphenylene entity, in determining the π/π(*) orbital levels of the fluorinated phenylene entity. Dialkoxyphenylenes raise the HOMO orbitals, whereas fluorinated phenylenes lower the LUMO orbitals in the oligomers. In addition, the presence of S⋅⋅⋅F and H⋅⋅⋅F interactions in the fluorinated phenylene-thienylene compounds add to the S⋅⋅⋅O interactions in the mixed targets and contribute to the full conjugation in the oligomer, inducing weak inter-ring angles between the involved aromatic cycles. These results, which showed extended conjugation of the π system, were corroborated by a narrow HOMO-LUMO gap (according to DFT calculations) and by a relatively strong maximum wavelength (as obtained by TD-DFT calculations and experimental UV/Vis measurements). The crystallographic data of two mixed thienylene-(fluorinated and dialkoxylated phenylene) five-ring oligomers agree with the above results and show the formation of quasi-planar conformations with non-covalent S⋅⋅⋅O, H⋅⋅⋅F, and S⋅⋅⋅F interactions. These studies in the solid and gas phases show the relevance of associating dialkoxyphenylene and fluorinated phenylene fragments with thiophene to lead to oligomers with improved electronic delocalization for electronic or optoelectronic devices.

Generating long supramolecular pathways with a continuous density of states by physically linking conjugated molecules via their end groups.

Self-assembly of conjugated 2,5-dialkoxy-phenylene-thienylene-based oligomers on epitaxial monolayer graphene was studied in ultrahigh vacuum by low-temperature scanning tunneling microscopy (STM). The formation of long one-dimensional (1D) supramolecular chain-like structures has been observed, associated to a physical linking of their ends which involved the rotation of the end thiophene rings in order to allow π-π stacking of these end-groups. dI/dV maps taken at an energy corresponding to the excited states showed a continuous electronic density of states, which tentatively suggests that within such molecular chains conjugation of electrons is preserved even across physically linked molecules. Thus, in a self-organization process conjugation may be extended by appropriately adapting conformations of neighboring molecules. Our STM results on such self-organized end-linked molecules potentially represent a direct visualization of J-aggregates.

Further characterization of a putative serine protease contributing to the γ-secretase cleavage of ß-amyloid precursor protein.

The 3-alkoxy-7-amino-4-chloro-isocoumarins JLK-6 and JLK-2 have been shown to markedly reduce the production of Amyloid ß-peptide (Aß) by Amyloid-ß Precursor Protein (APP) expressing HEK293 cells by affecting the γ-secretase cleavage of APP, with no effect on the cleavage of the Notch receptor. This suggested that these compounds do not directly inhibit the presenilin-dependent γ-secretase complex but more likely interfere with an upstream target involved in γ-secretase-associated pathway. The mechanism of action of these compounds is unknown and there are high fundamental and therapeutical interests to unravel their target. Isocoumarin compounds were previously shown to behave as potent mechanism-based irreversible inhibitors of serine proteases, suggesting that the JLK-directed target could belong to such enzyme family. To get further insight into structure-activity relationships and to develop more potent isocoumarin derivatives, we have synthesized and evaluated a series of isocoumarin analogues with modifications at positions 3, 4 and 7. In particular, the 7-amino group was substituted with either acyl, urethane, alkyl or aryl groups, which could represent additional interaction sites. Altogether, the results highlighted the essential integrity of the 3-alkoxy-7-amino-4-chloro-isocoumarin scaffold for Aß-lowering activity and supported the involvement of a serine protease, or may be more generally, a serine hydrolase. The newly reported 7-N-alkyl series produced the most active compounds with an IC(50) between 10 and 30µM. Finally, we also explored peptide boronates, a series of reversible serine protease inhibitors, previously shown to also lower cellular Aß production. The presented data suggested they could act on the same target or interfere with the same pathway as isocoumarins derivatives.