Elucidation of the mechanism of redox grafting of diazotated anthraquinone

Electroreduction of Viologen Phenyl Diazonium Salts as a Strategy To Control Viologen Coverage on Electrodes

A majority of the reported electrografting of aryldiazonium salts result in the formation of covalently attached films with a limited surface coverage of below 5 nmol·cm^-2. Herein, we report the preparation of higher-thickness redox-active viologen-grafted electrodes from the electroreduction of viologen phenyl diazonium salts, by either cyclic voltammetric (CV) sweeps or electrolysis using a fixed potential. Both of the methodologies were successfully applied for various conductive surfaces, including glassy carbon (GC), gold disc, indium tin oxide glass, mesoporous TiO₂ electrodes, and 3D compacted carbon fibers. A robust maximal viologen coverage, Γ_viologen = 9.5 nmol·cm^-2, was achieved on a GC electrode by CV electroreduction. Electroreduction held at a fixed potential at E_appl. = -0.3 V can fabricate viologen-grafted electrodes with Γ_viologen in the range of 0-37 nmol·cm^-2 in a controllable way, by simply adjusting the electrodeposition time t_appl.. Time-dependent Γ_viologen were found to be 10 nmol·cm^-2@2 min, 20 nmol·cm^-2@4.2 min, and 30 nmol·cm^-2@7 min. Furthermore, a TiO₂ electrode coupled with Γ_viologen of 140 nmol·cm^-2 exhibited electrochromic performance, with the color changing from pale yellow to blue and red brown.

Organic Redox Species in Aqueous Flow Batteries: Redox Potentials, Chemical Stability and Solubility

Organic molecules are currently investigated as redox species for aqueous low-cost redox flow batteries (RFBs). The envisioned features of using organic redox species are low cost and increased flexibility with respect to tailoring redox potential and solubility from molecular engineering of side groups on the organic redox-active species. In this paper 33, mainly quinone-based, compounds are studied experimentially in terms of pH dependent redox potential, solubility and stability, combined with single cell battery RFB tests on selected redox pairs. Data shows that both the solubility and redox potential are determined by the position of the side groups and only to a small extent by the number of side groups. Additionally, the chemical stability and possible degradation mechanisms leading to capacity loss over time are discussed. The main challenge for the development of all-organic RFBs is to identify a redox pair for the positive side with sufficiently high stability and redox potential that enables battery cell potentials above 1 V.

Selective Molecularly Mediated Pseudocapacitive Separation of Ionic Species in Solution

We report the development of a dual-electrode pseudocapacitive separation technology (PSST) to capture quantitatively, remotely, and in a reversible manner value-added carboxylate salts of environmental and industrial significance. The nanostructured pseudocapacitive cell exhibits elegant molecular selectivity toward ionic species: upon electrochemical oxidation, a poly(vinylferrocene) (PVF)-based anodic electrode shows high selectivity toward carboxylates based on their basicity and hydrophobicity. Simultaneously, on the other side of the electrochemical cell, a poly(anthraquinone) (PAQ)-based cathodic electrode undergoes electrochemical reduction and captures the counterions of these carboxylates. The separation and regeneration capability of the electrochemical cell was evaluated through the variations in concentration of the carboxylates in polar organic solvents (often used in electrocatalytic processes) upon electrochemical charging and neutralization of the polymeric cargo of the electrodes, respectively. The strong separation efficiency of the system was indicated by its ability to capture an individual carboxylate (acetate, formate, or benzoate) selectively over other competing ions present in solution in significant excess, with an electrosorption capacity in the range of 122-157 mg anions/g_cell (polymer and CNT components on the anodic and cathodic side of the cell). The ion sorption capacity of the cell was high even after five adsorption/desorption cycles (18 000 s of continuous operation). In addition, the cell exhibited molecular selectivity even between two carboxylates (e.g., between benzoate and acetate or formate) which differ only in terms of basicity and hydrophobicity. We anticipate that this strategy can be employed as a versatile platform for selective ion separations. In particular, the functionalization of electrochemical cells with the proper polymers would enable the remote and economically viable electro-mediated separation of the desired ionic species in a quantitative and reversible manner.

Robust All-Carbon Molecular Junctions on Flexible or Semi-Transparent Substrates Using "Process-Friendly" Fabrication

Large area molecular junctions were fabricated on electron-beam deposited carbon (eC) surfaces with molecular layers in the range of 2-5.5 nm between conducting, amorphous carbon contacts. Incorporating eC as an interconnect between Au and the molecular layer improves substrate roughness, prevents electromigration and uses well-known electrochemistry to form a covalent C-C bond to the molecular layer. Au/eC/anthraquinone/eC/Au junctions were fabricated on Si/SiOx with high yield and reproducibility and were unchanged by 10(7) current-voltage cycles and temperatures between 80 and 450 K. Au/eC/AQ/eC/Au devices fabricated on plastic films were unchanged by 10(7) current density vs bias voltage (J-V) cycles and repeated bending of the entire assembled junction. The low sheet resistance of Au/eC substrates permitted junctions with sufficiently transparent electrodes to conduct Raman or UV-vis absorption spectroscopy in either reflection or transmission geometries. Lithographic patterning of Au/eC substrates permitted wafer-scale integration yielding 500 devices on 20 chips on a 100 mm diameter wafer. Collectively, eC on Au provides a platform for fabrication and operation of chemically stable, optically and electrically functional molecules on rigid or flexible materials. The relative ease of processing and the robustness of molecular junctions incorporating eC layers should help address the challenge of economic fabrication of practical, flexible molecular junctions for a potentially wide range of applications.

Electrochemical properties of gold and glassy carbon electrodes electrografted with an anthraquinone diazonium compound using the rotating disc electrode method

The RDE method was combined with the electrografting procedure to prepare thick AQ films on Au and glassy carbon electrodes.

Electron transport through a diazonium-based initiator layer to covalently attached polymer brushes of ferrocenylmethyl methacrylate

A versatile method based on electrografting of aryldiazonium salts was used to introduce covalently attached initiators for atom transfer radical polymerization (ATRP) on glassy carbon surfaces. Polymer brushes of ferrocenylmethyl methacrylate were prepared from the surface-attached initiators, and these films were thoroughly analyzed using various techniques, including X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRRAS), ellipsometry, and electrochemistry. Of particular interest was the electrochemical characterization of the electron transfer through the diazonium-based initiator layer to the redox centers in the polymer brush films. It was found that the apparent rate constant of electron transfer decreases exponentially with the dry-state thickness of this layer. To investigate the electron transfer in the brushes themselves, scanning electrochemical microscopy (SECM) was applied, thereby allowing the effect from the initiator layer to be excluded. The unusual transition feature of the approach curves recorded suggests that an initial fast charge transfer to the outermost-situated ferrocenyl groups is followed by a slower electron transport involving the neighboring redox units.

Electrochemical modification of gold electrodes with azobenzene derivatives by diazonium reduction

An electrochemical study of Au electrodes electrografted with azobenzene (AB), Fast Garnet GBC (GBC) and Fast Black K (FBK) diazonium compounds is presented. Electrochemical quartz crystal microbalance, ellipsometry and atomic force microscopy investigations reveal the formation of multilayer films. The elemental composition of the aryl layers is examined by X-ray photoelectron spectroscopy. The electrochemical measurements reveal a quasi-reversible voltammogram of the Fe(CN)6 (3-/4-) redox couple on bare Au and a sigmoidal shape for the GBC- and FBK-modified Au electrodes, thus demonstrating that electron transfer is blocked due to the surface modification. The electrografted AB layer results in strongest inhibition of the Fe(CN)6 (3-/4-) response compared with other aryl layers. The same tendencies are observed for oxygen reduction; however, the blocking effect is not as strong as in the Fe(CN)6 (3-/4-) redox system. The electrochemical impedance spectroscopy measurements allowed the calculation of low charge-transfer rates to the Fe(CN)6 (3-) probe for the GBC- and FBK-modified Au electrodes in relation to bare Au. From these measurements it can be concluded that the FBK film is less compact or presents more pinholes than the electrografted GBC layer.