Electrochemical analysis

In this dissertation the synthesis, characterization, and binding properties of
carbohydrate receptors 34-38 was described. Macrocyclic receptor 34 and macrobicyclic
receptor 35 bind monosaccharides in aqueous media through combination of
hydrophobic, electrostatic and hydrogen bonding interactions. The dissociation constants
(Kd) for the complexes between 1 ,8-naphthyridine receptors 34, and 35 with a variety of
neutral and negatively charged monosaccharides in aqueous media were determined by
fluorimetric and UV /vis titration. The observed values are in the range from ~0.3 to >10
mM, within the Kd range reported for lectin/monosaccharide complexes. However,
among monosaccharide substrates tested receptor 34 showed the strongest affinity for
sialic acid (Kd = ~0 . 3 mM), a monosaccharide that plays many important roles in a wide
variety of physiological and pathological processes. Macrocyclic receptor 34 recognizes not only sialic acid in solution, but also binds selectively in vitro to human
cancer cell surface carbohydrate antigens containing terminal sialic acid moieties. In
addition, besides their binding selectivity, receptors 34 and 35 display also the ability to
discriminate between closely related monosaccharide substrates by opposite variation of
the fluorescence emission intensity. Structure-binding relationship study of receptor 34
revealed that H-bonding donor/acceptor pattern and presence of positive charge on
receptor's side arms are crucial for selective monosaccharide binding in aqueous media.

The intercalation of anions into carbon fiber from organic electrolytes containing lithium salts was studied. The reversible intercalation of anions into carbon could lead to the possible substitution of conventional metal oxide cathode materials in lithium-ion cells. EWC300 was selected as the most suitable carbon fiber material based on data from preliminary tests. Experiments were performed with LiClO4 in EC/DMC and LiPF6 in EC/DMC electrolytes. Slow scan cyclic voltammetry (0.1 mV/s) and galvanostatic charge/discharge experiments at various C rates were used. Intercalation of PF6- occurred by staging and was highly dependent on the current density. High current density (20 mA/g) was necessary to reach potentials over 5 V vs Li to achieve intercalation capacities over 80 mAh/g. Powder x-ray diffraction revealed that carbon fibers became less crystalline after anions were intercalated into their structure. Scanning electron microscopy showed longitudinal cracking on the carbon fibers after 120 cycles indicating dimensional instability.

Experiments were conducted to investigate the effectiveness of ion vapor deposited (IVD) aluminum as a base coat for polyurethane topcoat systems in marine environments. Three test environments were used including marine atmospheric atmosphere, quiescent filtered seawater open to laboratory air and aerated seawater. Visual observation and electrochemical impedance spectroscopy techniques were primary testing methods; other characterizations included adhesion tape test, coating thickness measurements, holiday detection, optical microscopy, open circuit potential measurements and potentiodynamic polarization. It was found that IVD aluminum was a good candidate for replacement of environmentally sensitive coatings on AA2219-T87 and AISI 4340 steel substrates, respectively. The IVD aluminum coating provided good sacrificial protection and served as a good base for polyurethane topcoat systems. Electrochemical impedance spectroscopy was a suitable method to predict coating performance in the early stages of exposure.

Full nickel-hydrogen (Ni-H2) boilerplate batteries were cycled and impedance measurements were made at different states-of-charge (SOC), electrolyte concentrations and charge/discharge rates. Experiments were conducted on cells containing new and cycled (11,000 cycles) electrodes. Additionally, an EIS study of Ni-H2 flightweight IPV satellite cells was performed. A number of experiments were conducted on silver oxide-metal hydride batteries. The interest was focused on both negative and positive electrodes and upon the system itself. This work was preliminary and aided in describing the general performance of the battery. For analysis, the data was fitted to an equivalent electrical circuit using the Nonlinear Least Squares Method (NLSM). The correlation between theoretical and empirical data was sufficiently good.

An automated procedure for integrated cycling and electrochemical impedance spectroscopy (EIS) testing of nickel sinter individualized pressure vessel electrodes for secondary nickel/hydrogen batteries was developed. Nickel electrodes from three major U.S. manufacturers were cycled under various conditions. The condition of the electrodes was monitored using both EIS and traditional electrochemical methods. In order to establish relationships between the status of the electrodes and the acquired impedance spectra, various cycling and electrode parameters were analyzed and compared with the EIS data. Nonlinear least squares (NLS) regression was used for analysis of the impedance data. An equivalent circuit was developed which produced good correlation with the impedance data at all states-of-charge and discharge rates. Problems with the experimental procedure which limit the validity of EIS testing were discussed.

Binuclear oxo- and sulfide-bridged Mo(V)-cysteine and Mo(V)-EDTA
complexes are chosen as model compounds for nitrogenase enzyme.
Both groups of complexes undergo electrochemical reduction in a single
four- electron step to Mo(III) dimers. The reduced binuclear Mo (III)
units dissociate into catalytically active species capable of reducing
the nitrogenase substrate, acetylene. Ease of dissociation increases
as oxygen is replaced by sulfur in the bridging unit. Electrocatalytic
reduction products of acetylene consist primarily of C2H4 and C2H6 at
low partial pressures, but C4H6, C4H8, and higher hydrocarbons predominate
at higher pressures. The reduction of substrate is believed
to occur by a Mo(III) catalyst adsorbed on the electrode surface. A
mechanism is proposed for the e lectrocatalysis, and its relationship
to the enzymic catalysis is discussed.

The elect;rochemical reduction of Fe2(salen)2O at a platinum electrode
in dimethylsulfoxide is investigated using the techniques of cyclic voltammetry,
controlled potential coulometry and chronoamperometry.
The effects of concentration and addition of a proton source on the
reduction mechanisrn are investigated. The reduction proceeds by two
related mechanisms. At short-times, Fe2(salen)2O is reduced by two
sequential one-electron steps producing an Fe(III)Fe(II) dimer and an
unstable Fe(II) dimer. At long-times the mixed-valence dimer reacts
leading to transfer of 1.5 electrons per Fe2 unit. Mechanisms consistent
with the experimental data are proposed involving the formation of
a tetrameric structure.