An
electrochemical analysis strategy based on the Marcus–Hush
approximation is presented to analyze the kinetic component of organic
redox flow battery (RFB) electrolytes. The procedure was applied to
aqueous solutions of methyl viologen (MV) and 2,2′-bipyridyl
(diquat, DQ) derivatives as model redox-active electrolytes;
although these systems are promising negolyte candidates in organic
RFBs, their electrode kinetics continues to be unclear. For compound MV, the voltammetric analysis revealed an adsorption process
of electrogenerated species to the glassy carbon electrode surface,
so its electron transfer rate constant k
s should not be estimated by applying outer sphere electron transfer
formulations. For the remaining compounds studied, experimental k
s values were obtained and they range from 0.22
to 0.62 cm s–1. Quantum chemical modeling was applied
not only to decipher properties of the adsorption process of the MV structure but also to rationalize the kinetic differences
in compounds studied through their total and inner reorganization
energies. This experimental and theoretical approach allowed elucidation
of the kinetic component of compounds studied, revealing that k
s values for MV and DQ compound derivatives should not exhibit the reported differences
of at least one order of magnitude. Finally, the experimental k
s value (0.62 cm s–1) obtained
for compound 5,5′-DMDQ is the largest value reported
to date in the literature of aqueous organic RFBs, which makes it
a strong anolyte candidate.
Evaluation of the substituent effect in reaction series is an issue of interest, as it is fundamental for controlling chemical reactivity in molecules. Within the framework of density functional theory, employment of the chemical potential, μ, and the chemical hardness, η, leads to the calculation of properties of common use, such as the electrodonating (ω(-)) and electroaccepting (ω(+)) powers, in many chemical systems. In order to examine the predictive character of the substituent effect by these indexes, a comparison between these and experimental binding constants (Kb) for binding of a series of radical anions from para- and ortho-substituted nitrobenzenes with 1,3-diethylurea in acetonitrile was performed, and fair correlations were obtained; furthermore, this strategy was suitable for all of the studied compounds, even those for which empirical approximations, such as Hammett's model, are not valid. Visual representations of substituent effects are presented by considering the local electrodonating power ω(-)(r).
Analyte concentration effects on the first reduction process of methyl viologens and diquat redox flow battery electrolytes were examined by cyclic voltammetry in aqueous media. A simple one-electron transfer mechanism to form radical cations was detected for diquat, 4,4′-dimethyl diquat, and bis(3trimethylammonio)-propyl viologen compounds. The radical cations attach to the electrode surface when the source of their electrogeneration is methyl viologen molecules bearing PF 6 − ions as a counterpart. However, this inner sphere reduction mechanism was not observed in methyl viologen having an I − counterion. For the latter compound, as well as for 5,5′-dimethyl diquat and 1,1′bis(3-sulfonatopropyl)-4,4′-bipyridinium, a piece of experimental evidence for unexpected, fast, and reversible dimerization interactions between their electrogenerated radical cations is presented. To get information on these bimolecular interactions, a screening methodology (using different levels of theory) was employed in finding suitable dimeric structures and their related interaction energies. By using diquat as a reference system, a relationship between calculated interaction energies and the corresponding experimental dimerization constants was obtained. The examination of redox-active molecules using this experimental and theoretical approach will allow a better selection of redox flow battery electrolytes.
Electron transfer controlled hydrogen bonding was studied for a series of nitrobenzene derivative radical anions, working as large guest anions, and substituted ureas, including dihomooxacalix[4]arene bidentate urea derivatives, in order to estimate binding constants (Kb) for the hydrogen-bonding process. Results showed enhanced Kb values for the interaction with phenyl-substituted bidentate urea, which is significantly larger than for the remaining compounds, e.g., in the case of 4-methoxynitrobenzene a 28-fold larger Kb value was obtained for the urea bearing a phenyl (Kb ∼ 6888) vs tert-butyl (Kb ∼ 247) moieties. The respective nucleophilic and electrophilic characters of the participant anion radical and urea hosts were parametrized with global and local electrodonating (ω(-)) and electroaccepting (ω(+)) powers, derived from DFT calculations. ω(-) data were useful for describing trends in structure–activity relationships when comparing nitrobenzene radical anions. However, ω(+) for the host urea structures lead to unreliable explanations of the experimental data. For the latter case, local descriptors ωk(+)(r) were estimated for the atoms within the urea region in the hosts [∑kωk(+)(r)]. By compiling all the theoretical and experimental data, a Kb-predictive contour plot was built considering ω(-) for the studied anion radicals and ∑kωk(+)(r) which affords good estimations.
In this work, experimental evidence of the influence of the electron transfer kinetics during electron transfer controlled hydrogen bonding between anion radicals of metronidazole and ornidazole, derivatives of 5-nitro-imidazole, and 1,3-diethylurea as the hydrogen bond donor, is presented. Analysis of the variations of voltammetric EpIcvs. log KB[DH], where KB is the binding constant, allowed us to determine the values of the binding constant and also the electron transfer rate k, confirmed by experiments obtained at different scan rates. Electronic structure calculations at the BHandHLYP/6-311++G(2d,2p) level for metronidazole, including the solvent effect by the Cramer/Truhlar model, suggested that the minimum energy conformer is stabilized by intramolecular hydrogen bonding. In this structure, the inner reorganization energy, λi,j, contributes significantly (0.5 eV) to the total reorganization energy of electron transfer, thus leading to a diminishment of the experimental k.
In recent years interestint he development of protocols that facilitate the oxidativea ddition of gold to access mild cross-coupling processes mediated by this metal hasincreased. In this context, we report herein that ascorbic acid, an atural and readily accessible antioxidant, can be used to accelerate the oxidative addition of aryldiazonium chlorides onto Au I .T he aryl-Au III speciesg enerated in this way,h as been used to prepare 3-arylindoles in ao ne-pot protocol startingf rom anilines and para-, meta-, and ortho-s ubstituted aryldiazonium chlorides. The mechanism underlying the oxidative addition has been examined in detail based on EPR analyses, cyclic voltammetry,a nd DFT calculations. Interestingly, we have found that in this protocol, the chloride atom induces the Au II /Au III oxidation step.
Finding two-electron storage aqueous organic negolytes is crucial for increasing the charge storage capacity
in next-generation
flow batteries. Inspired by the ability of gallocyanine compound (GAL) to act as an efficient two-electron transfer mediator
in biology, we investigated its reduction process by cyclic voltammetry
at alkaline conditions, detecting ion pairing (with Na+, K+, or Li+ ions) and H+ transfer
pathways in electrogenerated species. The voltammetric responses obtained
were dependent on the concentrations of analyte and supporting electrolyte
and exhibited values of peak-to-peak potential distance (ΔE
p) close to the limiting value established in
flow battery protocols for screening irreversible systems (>200/n mV). The results presented in this work can not be explained
by irreversible mechanisms. This atypical behavior was related to
the evolution of a mixed and reversible process of adsorption and
diffusion pathways, where GAL species near the electrode
first attach to the electroactive surface without passivating it,
so at high concentrations of GAL compound, the reactions
can take place by a mainly diffusive process (at the modified electrode).
The reversible redox chemistry in compound studied was corroborated
by testing a flow cell of 0.83 V composed of a GAL-KOH
system and the posolyte ferrocyanide-KOH solution.
The cell exhibited a Coulombic efficiency close to 100% and retained
87% of its capacity after 200 charge–discharge cell cycles.
This is the first report concerning the use of a phenoxazine-based
derivative for storing two electrons per molecule in an aqueous flow
cell.
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