Studies of the intrinsic electrochemical, structural, and electronic propertiesof microparticles of energy storage materials can provide much needed insight into the factors that control various aspects of the performance of technical electrodes for battery applications. This Account summarizes efforts made in our laboratories toward the development and implementation of methods for the in situ electrical, optical, and spectroscopic characterization of microparticles of a variety of such materials, including Ni hydroxide, Zn, carbon, and lithiated Mn and Co oxides. In the case of Ni hydroxide, the much darker appearance of NiOOH compared to the virtually translucent character of virgin Ni(OH)2 allowed for the spatial and temporal evolution of charge flow within spherical microparticles of Ni(OH)2 to be monitored in real time during the first scan toward positive potentials using computer-controlled video imaging. In situ Raman scattering measurements involving single microparticles of Zn harvested from a commercial Zn|MnO2 battery revealed that passive films formed in strongly alkaline solutions by stepping the potential from 1.55 V to either 0.7 or 0.8 V vs SCE displayed a largely enhanced feature at ca. 565 cm(-1) ascribed to a longitudinal optical phonon mode of ZnO, an effect associated with the presence of interstitial Zn and oxygen deficiencies in the lattice. In addition, significant amounts of crystalline ZnO could be detected only for passive films formed at the same two potentials after the electrodes had been roughened by a single passivation-reduction step. Quantitative correlations were found in the case of LiMn2O4 and KS-44 graphite between the Raman spectral properties and the state of charge. In the case of KS-44, a chemometrics analysis of the spectroscopic data in the potential region in which the transition between dilute phase 1 and phase 4 of lithiated graphite is known to occur made it possible to determine independently the fraction of each of the two phases present as a function of potential without relying on the coulometric information. Also featured in this Account are methods we developed for the assembly and electrochemical characterization of Zn|MnO2 and nickel|metal-hydride Ni|MH alkaline batteries incorporating single microparticles of the active materials. As evidenced from the data collected, the voltage-time profiles for constant current operation for both types of devices were found to be similar to those of commercially available batteries involving the same chemistries. The ability to monitor the state of charge of individual particles based strictly on spectroscopic data is expected to open exciting new prospects for visualizing the flow of charge within electrodes in Li-ion batteries, an area that is being vigorously pursued in our laboratories.
The oxidation of hydroxylamine on Au electrodes in aqueous phosphate buffer solutions (pH 7) was examined using electrochemical and in situ infrared reflection absorption spectroscopy techniques. Polarization curves recorded with a rotating Au disk electrode showed that the onset of NH2OH oxidation occurs at ca. 0.0 V vs SCE, reaching a well-defined peak at a disk potential, E disk peak, ca. 0.2 V vs SCE. Plots of the disk current, i disk, at E disk peak vs ω1/2 were linear with a close to zero intercept. Measurements in which E disk of the rotating ring-disk electrode was scanned, while E ring was fixed at a value negative enough for solution phase NO to undergo reduction, yielded plots of i ring vs E disk, which mirrored the peak found for i disk and thus was consistent with NO being one of the predominant products of NH2OH oxidation. In situ infrared measurements provided evidence for N2O being produced at the same onset potential of NH2OH oxidation. The disk polarization curves could be reproduced by theoretical simulations involving an EEECE mechanism in which nitrite, one of the products of NH2OH oxidation, reacts with NH2OH yielding an electrochemically inert species. In accordance with theory, plots of i disk at E disk peak as a function of [NH2OH] bent downward as [NH2OH] increased.
Changes in the electrocatalytic activity of Pt for the oxygen and hydrogen peroxide reduction reactions (ORR and HPRR, respectively) in an aqueous acidic electrolyte induced by the adsorption of bromide, as a model impurity, have been investigated using chronoamperometric techniques under forced convection. Experiments were carried out using a polycrystalline Pt|Pt rotating ring-disk electrode in O 2 -saturated 0.1 M HClO 4 containing 10 μM KBr (800 ppb). Potential steps were applied to the Pt disk from E o , at which Br − is fully desorbed, to more positive values, E step , at which Br − undergoes adsorption. During this period, the currents both at the disk, i disk , and at the ring, i ring , were monitored, with the ring polarized at a potential at which H 2 O 2 (aq) oxidation proceeds under diffusion-limited conditions. The results obtained, assuming the ORR does not interfere with Br − adsorption, made it possible to correlate i disk and i ring with the coverage of adsorbed Br − , θ Br − , and E step . As evidenced from the data collected, significant drops in the diffusion-limited currents for O 2 reduction induced by the presence of 10 μM KBr could only be observed for θ Br − > 0.25 regardless of E step . Similar measurements involving the same Pt(poly) rotating disk electrode performed in 1 mM H 2 O 2 in deaerated 0.1 M HClO 4 devoid of O 2 displayed a similar trend. This behavior was found to be consistent with a simple blockage of surface-active sites by adsorbed bromide, as predicted by the attenuation model proposed by Levart. shielded i ring using the method reported originally by Markovic et al. 1,7,16 This information made it possible to correlate the disk current, i disk and i ring , associated, respectively, with the ORR and the oxidation of H 2 O 2 (aq) under diffusion-limited conditions, with θ Br − in O 2 -saturated solutions. Also to be
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