Impedance parameters and the state-of-charge. III. Zinc-manganese dioxide and magnesium-manganese dioxide dry batteries

The problem of non-destructive determination of the state-of-charge of zinc- and magnesium-manganese dioxide dry batteries is examined experimentally from the viewpoint of internal impedance and open-circuit voltage at equilibrium. It is shown that the impedance is mainly charge-transfer controlled at relatively high states-of-charge and progressively changes over to diffusion control as the state-of-charge decreases in the case of zinc-manganese dioxide dry batteries. On the other hand, the impedance is mainly diffusion controlled for undischarged batteries but becomes charge-transfer controlled as soon as there is some discharge in the case of magnesium-manganese dioxide batteries. It is concluded that the determination of state-of-charge is not possible for both types of batteries by the measurement of impedance parameters due to film-induced fluctuations of these parameters. The measurement of open-circuit voltage at equilibrium can be used as a state-of-charge indicator for Zn-MnO2 batteries but not for Mg-MnO2 batteries.

Impedance parameters and the state-of-charge. II. Lead-acid battery

The determination of the state-of-charge of the lead-acid battery has been examined from the viewpoint of internal impedance. It is shown that the impedance is controlled by charge transfer and to a smaller extent by diffusion processes in the frequency range 15–100 Hz. The equivalent series/parallel capacitance as well as the a.c. phase-shift show a parabolic dependence upon the state-of-charge, with a maximum or minimum at 50% charge. These results are explained on the basis of a uniform transmission-line analog equivalent circuit for the battery electrodes.

Impedance parameters and the state-of charge. I. Nickel-cadmium battery

The problem of nondestructive determination of the state-of-charge of nickel-cadmium batteries has been examined experimentally as well as theoretically from the viewpoint of internal impedance. It is shown that the modulus of the impedance is mainly controlled by diffusion at all states of charge. Even so, a prediction of the state of charge is possible if the equivalent series/parallel capacitance or the alternating current phase shift is measured at a sufficiently low a.c. test frequency (5–30 Hz) which also avoids inductive effects. These results are explained on the basis of a uniform transmission-line analog equivalent circuit for the battery electrodes.

Methanol oxidation on carbon-supported Pt---Sn electrodes in silicotungstic acid

Electro-oxidation of methanol was studied on carbon-supported Pt---Sn/C electrodes in silcotungstic acid (SiWA) at various concentrations. The porous-carbon electrodes employing Pt---Sn/C catalyst have been characterized using chemical analyses, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) in conjunction with electrochemistry. The presence of Pt---Sn and Pt3Sn alloys along with Pt and SnO2 phases in the catalyst were identified by XRD. XPS analysis showed a lower amount of PtO species in the Pt---Sn/C catalyst with respect to the corresponding Pt/C sample. From the steady-state galvanostatic polarization data on Pt---Sn/C electrodes in SiWA, it is inferred that a one-electron process is the rate determining step. The performance of the electrodes in 0.084 M SiWA was better than in 2.5 M H2SO4 under similar conditions up to load currents of about 100 mA cm−2 indicating the promoting behaviour of the electrolyte. At currents larger than 100 mA cm−2, the performance of the electrodes in 0.084 SiWA was poorer than that in 2.5 M H2SO4 mainly due to the dominance of mass polarization in the former owing to the large size of keggin units associated with the structure of SiWA. This aspect was supported by cyclic voltammetry and ac impedance studies on Pt---Sn/C electrodes. Simulation of the electrochemical impedance response for the oxidation of methanol in SiWA was carried out using the equivalent electrical circuit model.

Methanol oxidation on carbon-supported Pt---Sn electrodes in silicotungstic acid

Electro-oxidation of methanol was studied on carbon-supported Pt-Sn/C electrodes in silcotungstic acid (SiWA) at various concentrations. The porous-carbon electrodes employing Pt-Sn/C catalyst have been characterized using chemical analyses, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) in conjunction with electrochemistry. The presence of Pt-Sn and Pt3Sn alloys along with Pt and SnO2 phases in the catalyst were identified by XRD. XPS analysis showed a lower amount of PtO species in the Pt-Sn/C catalyst with respect to the corresponding Pt/C sample. From the steady-state galvanostatic polarization data on Pt-Sn/C electrodes in SiWA, it is inferred that a one-electron process is the rate determining step. The performance of the electrodes in 0.084 M SiWA was better than in 2.5 M H2SO4 under similar conditions up to load currents of about 100 mA cm-2 indicating the promoting behaviour of the electrolyte. At currents larger than 100 mA cm-2, the performance of the electrodes in 0.084 SiWA was poorer than that in 2.5M H2SO4 mainly due to the dominance of mass polarization in the former owing to the large size of Keggin units associated with the structure of SiWA. This aspect was supported by cyclic voltammetry and ac impedance studies on Pt-Sn/C electrodes. Simulation of the electrochemical impedance response for the oxidation of methanol in SiWA was carried out using the equivalent electrical circuit model.

Kinetics of hydrogen evolution on submicron size Co, Ni, Pd and CoNi alloy powder electrodes by d.c. polarization and a.c. impedance studies

Submicron size Co, Ni and Co-Ni alloy powders have been synthesized by the polyol method using the corresponding metal malonates and Pd powder by reduction of PdOx in methanol. The kinetics of the hydrogen evolution reaction ( HER) in 6 M KOH electrolyte have been studied on electrodes made from the pressed powders. The d.c. polarization measurements have resulted in a value close to 120 mV decade(-1) for the Tafel slope, suggesting that the HER follows the Volmer-Heyrovsky mechanism. The values of exchange current density (i(o)) are in the range 1-10 mA cm(-2) for electrodes fabricated in the study. The a.c. impedance spectra measured at several potentials in the HER region showed a single semicircle in the Nyquist plots. Exchange current density (i(o)) and energy transfer coefficient (alpha) have been calculated by employing a nonlinear least square-fitting program.

Temperature and potential dependence electrochemical impedance studies of LiMn2O4

Detailed analysis of alternating current impedance data of LiMn2O4 electrodes measured at several temperatures and potentials was carried out. The Nyquist plots generally consisted of semicircles corresponding to two time constants. However, at low temperatures (-10 to 10 A degrees C) and potential region between 3.90 and 4.20 V, three time constants were present. The third semicircle present at the middle to high frequency range was attributed to electronic resistance of LiMn2O4. Impedance parameters were evaluated using appropriate electrical equivalent circuits. From the temperature dependence of resistive parameters, activation energy values for the corresponding processes were calculated.

Pristine and graphitized-MWCNTs as durable cathode-catalyst supports for PEFCs

Long-term deterioration in the performance of PEFCs is attributed largely to reduction in active area of the platinum catalyst at cathode, usually caused by carbon-support corrosion. Multi-walled carbon-nanotubes (MWCNTs) as cathode-catalyst support are found to enhance long-term stability of platinum catalyst (Pt) in relation to non-graphitic carbon. In addition, highly graphitic MWCNTs (G-MWCNTs) are found to be electrochemically more stable than pristine MWCNTs. This is because graphitic-carbon-supported-Pt (Pt/MWCNTs) cathodes exhibit higher resistance to carbon corrosion in-relation to non-graphitic-carbon-supported-Pt (Pt/C) cathodes in PEFCs during accelerated stress-test (AST) as evidenced by chronoamperometry and carbon dioxide studies. The corresponding change in electrochemical surface area (ESA), cell performance, and charge-transfer resistance are monitored through cyclic voltammetry, cell polarization, and impedance measurements, respectively. The extent of crystallinity, namely amorphous or graphitic nature of the three supports, is examined by Raman spectroscopy. X-ray diffraction and transmission electron microscopy studies both prior and after AST suggest lesser deformation in catalyst layer and catalyst particles for Pt/G-MWCNTs and Pt/MWCNTs cathodes in relation to Pt/C cathodes, reflecting that graphitic carbon-support resists carbon corrosion and helps mitigating aggregation of Pt particles. It is also found that with increasing degree of graphitization, the electrochemical stability for MWCNTs increases due to the lesser surface defects.

Oxygen-participated electrochemistry of new lithium-rich layered oxides Li3MRuO5 M = Mn, Fe)

We describe the synthesis, crystal structure and lithium deinsertion-insertion electrochemistry of two new lithium-rich layered oxides, Li3MRuO5 (M = Mn, Fe), related to rock salt based Li2MnO3 and LiCoO2. The Li3MnRuO5 oxide adopts a structure related to Li2MnO3 (C2/m) where Li and (Li0.2Mn0.4Ru0.4) layers alternate along the c-axis, while the Li3FeRuO5 oxide adopts a near-perfect LiCoO2 (R (3) over barm) structure where Li and (Li0.2Fe0.4Ru0.4) layers are stacked alternately. Magnetic measurements indicate for Li3MnRuO5 the presence of Mn3+ and low spin configuration for Ru4+ where the itinerant electrons occupy a pi*-band. The onset of a net maximum in the chi vs. T plot at 9.5 K and the negative value of the Weiss constant (theta) of -31.4 K indicate the presence of antiferromagnetic superexchange interactions according to different pathways. Lithium electrochemistry shows a similar behaviour for both oxides and related to the typical behaviour of Li-rich layered oxides where participation of oxide ions in the electrochemical processes is usually found. A long first charge process with capacities of 240 mA h g(-1) (2.3 Li per f.u.) and 144 mA h g(-1) (1.38 Li per f.u.) is observed for Li3MnRuO5 and Li3FeRuO5, respectively. An initial sloping region (OCV to ca. 4.1 V) is followed by a long plateau (ca. 4.3 V). Further discharge-charge cycling points to partial reversibility (ca. 160 mA h g(-1) and 45 mA h g(-1) for Mn and Fe, respectively). Nevertheless, just after a few cycles, cell failure is observed. X-ray photoelectron spectroscopy (XPS) characterisation of both pristine and electrochemically oxidized Li3MRuO5 reveals that in the Li3MnRuO5 oxide, Mn3+ and Ru4+ are partially oxidized to Mn4+ and Ru5+ in the sloping region at low voltage, while in the long plateau, O2- is also oxidized. Oxygen release likely occurs which may be the cause for failure of cells upon cycling. Interestingly, some other Li-rich layered oxides have been reported to cycle acceptably even with the participation of the O2- ligand in the reversible redox processes. In the Li3FeRuO5 oxide, the oxidation process appears to affect only Ru (4+ to 5+ in the sloping region) and O2- (plateau) while Fe seems to retain its 3+ state.