Voltage excursion

Stabilization of Pt Catalysts toward High-Voltage Excursions... 

The use of graphitized carbons can impart significant stability to high-voltage excursions and appears suitable for automotive fuel cell use. However, a number of workers have reported fhaf the performance of Pt on these supports is poorer than that found for conventional Pt catalysts. For example. 

Cou p de fouet — (crack of the whip, whiplash) A sudden and dramatic voltage excursion during the early stages of charge or discharge of a battery (esp. the lead-acid battery) [i]. The effect can be understood as due to a -> nu-cleation overpotential [ii]. 

Ball, S.C. et ah. The effect of dynamic and steady state voltage excursions on the stability of carbon supported Pt and PtCo catalysts, ECS Trans., 3, 595, 2006. 

Li-Ion batteries require electronic controls to prevent cell voltage from exceeding predetermined high- and low-voltage limits. Voltage excursions outside these limits can damage the cell and may create safety issues in the cell. The electronic controls also determine the state-of-charge and stop-cell operation. 

The ability of the material to withstand transient voltage excursions outside the reversible window is also a function of time and, mainly, electrolytic solution composition. In the literature on the subject there is no common opinion concerning the region of potentials where poly pyrrole is chemically stable. Christensen and Hamnet [116], for example, have found that overoxidation of polypyrrole in aqueous 1 M NaC104 starts with potentials higher than 700 mV versus SCE. Nov ... 

An adequate safety factor is necessary for peak and average power ratings. Most transmission Hnes are tested at two or more times their rated peak power before shipment to the customer. This safety factor is intended as a provision for transmitter transients, Hghtning-induced effects, and high-voltage excursions resulting from unforeseen operating conditions. 

There are also purely electrochemical methods that can be used to produce a bared surface. In one technique, the entire exposed surface is activated by a large positive voltage excursion, which is followed by a voltage step back to (or towards) E< . In this way, any potential pitting sites are initiated, and the test measures the ability of the material to resist propagation and to repassivate. One version of this test (Fig. 9) involves a step to +2 V for 3 s followed by a step back to E ,fr> during which time the current is monitored. The potential is held there for 5 min before it is again... 

Under current flow, the kinetics of the electrode reactions comes into effect. The Butler-Volmer equation describes the dependency of the current passing through the electrode interface (current density per unit geometric area) on a small voltage excursion (called overvoltage q) from the respective equilibrium potential E . ... 

In certain cases, a diode is placed in parallel with the cell to limit the voltage excursion in reversal. 

A neural stimulation system typically uses constant current to deliver charges required to excite cells or neurons however, this often causes voltage excursion to exceed 1 V on the stimulation electrode. In comparison, impedance spectroscopy traditionally operates in the 5-50 mV range, typically 10 mV, without dc bias for simplicity in data interpretation [34, 109]. However, the small potential without dc bias does not accurately reflect the performance at high potentials that are involved in oxidation or reduction reactions for conducting polymers. In fact, electrode... 

Fig. 16 Current and voltage waveforms of a PEDOT-coated Pt electrode under pulse stimulation (a). The electrode was stimulated in PBS under a charge-balanced, cathodic-first, biphasic pulse current at 1 mC/cm at 50 Hz (b). Voltage excursion was measured under pulse stimulation currents to evaluate the PEDOT electrode charge injection capacity...

Similar to metal electrode materials, conducting polymers stimulation requires reversible reactions driven under stimulation currents to ensure low voltage excursion and reduced production of by-products. Reports suggest that conducting polymers, especially PPy, are susceptible to irreversible oxidation [120]. Overoxidation of conducting polymers was reported to cause permselectivity changes, loss of conductivity, and de-doping [58,121-122]. 

Fig. 6.9 Scanning electron microscope photograph of an AIROF electrode showing delamination due to cathodic voltage excursions below —0.6 V against Ag AgCl. This damage is irreversible and deteriorates the charge injection capacity of the electrode considerably. From [28], with permission 2004 IEEE...

Ball, S., Hudson, S., Theobald, B., and Thompsett, D. (2006) The effect of dynanric and steady state voltage excursions on the stabihty of carbon supported Pt and PtCo catalyst, ECS Trans., 3(1), 595. Carter, R.N., Brady, B.K., Subramanian, K., Tighe, T, and Gasteiger, H.A. (2007) Spatially resolved electrode diagnostic technique for fuel cell applications, ECS Trans., 11(1), 423. 

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