Two-dimensional redox-active covalent organic frameworks (COFs) are ideal materials for energy storage applications due to their high surface area, extended π conjugated structure, tunable pore size and adjustable functionalities.[1-3] Herein, we report the synthesis and supercapacitor application of two redox active COFs [TpPa-(OH)2 and TpBD-(OH)2] along with the role of their redox active functional groups for the enrichment of specific capacitance.3 Of these COFs, TpPa-(OH)2 exhibited the highest specific capacitance of 416 F g−1 at 0.5 A g−1 current density in three electrode configuration while the highest specific capacitance was 214 F g−1 at 0.2 A g−1 current density in two electrode configuration. Superior specific capacitance was due to emergence of excellent pseudocapacitance by virtue of precise molecular level control over redox functionalities present in the COF backbone. This COF also demonstrated 66% capacitance retention after 10000 cycles along with 43% accessibility of the redox-active hydroquinone (H2Q) moieties in three electrode configuration while the capacitance retention was 88% after 10000 cycles in two electrode configuration. Exceptionally high specific capacitance of TpPa-(OH)2 was due to the reversible proton-coupled electron transfer (2H+/2e−) of hydroquinone/benzoquinone (H2Q/Q) moieties wherein H2Q and Q had comparable chemical stabilities during redox cycling that originated from H-bonding, which was supported by calculated structures.
Iron oxyhydroxide thin films electrochemically deposited from a non-aqueous medium using metal inorganic complexes as a metal ion precursor have been demonstrated as an efficient electrochemical water oxidation catalyst under near neutral as well as alkaline pH conditions.
A detailed electrochemical investigation of a series of iron complexes (biuret-modified tetraamido iron macrocycles Fe -bTAML), including the first electrochemical generation of Fe (O), and demonstration of their efficacy as homogeneous catalysts for electrochemical water oxidation (WO) in aqueous medium are reported. Spectroelectrochemical and mass spectral studies indicated Fe (O) as the active oxidant, formed due to two redox transitions, which were assigned as Fe (O)/Fe (OH ) and Fe (O)/Fe (O). The spectral properties of both of these high-valent iron oxo species perfectly match those of their chemically synthesised versions, which were thoroughly characterised by several spectroscopic techniques. The O-O bond-formation step occurs by nucleophilic attack of H O on Fe (O). A kinetic isotope effect of 3.2 indicates an atom-proton transfer (APT) mechanism. The reaction of chemically synthesised Fe (O) in CH CN and water was directly probed by electrochemistry and was found to be first-order in water. The pK value of the buffer base plays a critical role in the rate-determining step by increasing the reaction rate several-fold. The electronic effect on redox potential, WO rates, and onset overpotential was studied by employing a series of iron complexes. The catalytic activity was enhanced by the presence of electron-withdrawing groups on the bTAML framework. Changing the substituents from OMe to NO resulted in an eightfold increase in reaction rate, while the overpotential increased threefold.
A novel, easy, quick, and inexpensive integrated electrochemical methodology composed of cyclic voltammetry and amperometry has been developed for the determination of the kinetic stability of higher oxidation states for inorganic complexes. In this study, ferrocene and its derivatives have been used as model systems and the corresponding ferrocenium cations were generated in situ during the electrochemical experiments to determine their kinetic stabilities. The study found that the ferrocenium cations decompose following the first-order kinetics at 27 ± 3 °C in the presence of ambient oxygen and water. The half-lives of the ferrocenium, carboxylate ferrocenium, and decamethyl ferrocenium cations were found to be 1.27 × 10(3), 1.52 × 10(3), and ≫11.0 × 10(3) s, respectively, in acetonitrile solvent having a 0.5 M tetrabutylammonium hexafluorophosphate electrolyte. These results are in agreement with the previous reports, i.e. the ferrocenium cation is unstable whereas the decamethyl ferrocenium cation has superior stability. The new methodology has been established by performing various experiments using different concentrations of ferrocene, variable scan rates in cyclic voltammetry, different time periods for amperometry, and in situ spectroelectrochemical experiments.
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