The influence of the precursors, namely potassium ferrocyanide and potassium ferricyanide on the particles sizes of Prussian Blue (PB) and Prussian Green (PG), under identical reaction conditions have been investigated. It was found that the particle sizes influence the gravimetric capacity utilization of these materials as cathodes for aqueous potassium (K +) ion batteries. The PG particle sizes were on the order of 50-75 nm, whereas PB particles size were on the order of 2-10 microns. The PG cathodes demonstrated a reversible capacity of 121.4 mAhr/g, with a coulombic efficiency of 98.7% compared to PB cathodes which demonstrated 53.8 mAhr/g, with a coulombic efficiency of 100%. We interpret the increased capacity of PG batteries relative to PB batteries as being a result of the smaller particle size of PG, which results in greater accessibility of the cathode to K+ ions.
In this paper the general question of laser-induced photochemistry on metal surfaces is addressed. Specifically, we have studied resonant photodecomposition of a variety of aromatic molecules on roughened silver surfaces in ultrahigh vacuum. A continuous ion laser source at a number of different wavelengths in the region 350-410 nm was used to produce graphitic carbon on the surface which was monitored by Raman spectroscopy at the 1580-cm"1 band of surface carbon. Laser power-dependence studies of fragmentation rate for several molecules at 406.7 nm indicate that the initial absorption step is a two-photon process. Energetic considerations imply that photochemistry for other molecules studied is also due to multiphoton absorption, except for benzaldehyde fragmentation with 350.7-nm excitation where the photodecomposition rate is linear in intensity. In this case photochemistry apparently occurs directly from the first excited singlet state following one-photon absorption. Distance-dependence studies of photofragmentation rates by use of an inert spacer layer to separate the molecule undergoing photochemistry from the surface indicate that energy transfer to the metal surface is important in determining the reaction rate. A maximum decomposition rate is observed for pyridine 15-20 Á from the surface, resulting from the different distance dependences of enhanced molecular absorption at rough silver surfaces and damping by energy transfer to the surfaces. These competing processes produce a maximum photochemical rate where surface-enhanced absorption is still present but the energy damping rate has decreased substantially. Intermediate species in the decomposition of pyridine and pyrazine to carbon have been observed as CN peaks in the surface Raman spectrum during photolysis of these molecules. The CN stretch frequency of these fragments is shifted from that observed for cyanide ions adsorbed on silver to a frequency typical of a nitrile such as C3H3N. The fragmentation mechanism for these and other surfaces adsorbed molecules studied remains to be established. Evidence indicates that radical formation is a likely pathway for some molecules (e.g., benzaldehyde) while other molecules may decompose through surface ionic species.
Photofragmentation has been observed for molecules near a roughened Ag( 110) surface using low intensity UV excitation at·. 363.8 nm.The effect shows a nonlinear dependence on illumination intensity at intensities less than 100 W/cm2, and is dependent on molecule-surface separation. intensities a saturation of rate is seen.· The ionization of the molecules studied would be expected to be a three photon.process at the wavelength used (see table I).The dependence of reaction rate on distance from the si~v~t$urface was -3-also investigated by simple coverage experiments and by using a spacer layer of NH3 molecules. A monotonic increase in reaction rate with coverage was observed for pyridine between S and SOL (-1 and 10 monolayers). Careful measurements of carbon peak _ rate of change for a monolayer of pyridine on a 10L NH3 spacer layer showed an initial rate of change half that observed for a monolayer of pyridine adjacent to the surface.. '!he coverage dependence of surface enhanced Raman scattering on a similar surface with visible excitation (S14.S run) shows that th~ Raman enhancement for the layer of molecules closest to the surface is at least lOX that of subsequent layers. 12 Thus the distance dependence of photodecomposition -is clearly much longer ranged than that of surface Raman enhancementoThe identity of some of the molecule fragments observed-may be ascertained from Raman vibrational frequencies. Shown in fig. 1 is the spectrum of SL pyridine with 363.8 run excitation. Although surface enhancement of Raman -signal is not nearly as large with tN as with visible excitation (residual molecular pyridine peaks other than the 990 cm-1 band which are visible with S14.S nm excitation are not visible in this spectrum), several peaks in addition to the carbon peaks at 1300 cm-1 and 1600 cm-1 are observed. CH stretching modes are seen at 31S0and -28S0 cm-1 , and a peak at 219S cm-1 has been assigned to a CN vibration. CN is a likely initial product in the fragmentation of CSHSN+ .13In conclusion, photodecomposition has been observed for several molecules near. rough silver surfaces. The dependence on excitation intensity is nonlinear at low intensities «100 mW), and the effect is dependent on molecule-surf ace separation. Further experiments are underway to determine the exact nature of the excitation and fragmentation process.During the course of this work it was learried that multiphoton -4-• ir..f photofragmentation has also been observed for high intensity excitation of pyridine on po1ycrysta11ine silver surfaces at 532 nm. 14 Acknowledgements: 1biswork was supported by the D_irector, Office of Energy
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