Time constant, electrochemical

Filters have a time constant r = R x C which increases the damping of the measuring instrument. The time constant depends on the required attenuation and the interfering frequency, but not on the internal resistance of the measuring instrument. The time constants of the shielding filter are in the same range as those of the electrochemical polarization, so that errors in the off potential are increased. Since the time constants of attenuation filters connected in tandem are added, but the attenuation factors are multiplied, it is better to have several small filters connected in series rather than one large filter. 

Then let us examine the rate relaxation time constant x, defined as the time required for the rate increase Ar to reach 63% of its steady state value. It is comparable, and this is a general observation, with the parameter 2FNq/I, (Fig. 4.13). This is the time required to form a monolayer of oxygen on a surface with Nq sites when oxygen is supplied in the form of 02 This observation provided the first evidence that NEMCA is due to an electrochemically controlled migration of ionic species from the solid electrolyte onto the catalyst surface,1,4,49 as proven in detail in Chapter 5 (section 5.2), where the same transient is viewed through the use of surface sensitive techniques. 

There is an additional important observation to be made in Fig. 9.25 regarding the magnitude of the relaxation time constant, x, upon current imposition Electrochemical promotion studies involving both solid electrolytes and aqueous alkaline solutions have shown that x (defined as the time required for the catalytic rate increase to reach 63% of its final steady-state value upon current application) can be estimated from ... 

These considerations demonstrate that an electrochemical experiment may be lacking in reliability if the time constant of the cell is not sufficiently low compared to the time scale of the experiment. 

As has been shown in Eigure 68, since the time constants for these two electrochemical components, Rsei and Ra, are comparable at anode/electrolyte and cathode/electrolyte interfaces, respectively, the impedance spectra of a full lithium ion could have similar features in which the higher frequency semicircle corresponds to the surface films on both the anode and the cathode, and the other at lower frequency corresponds to the charge-transfer processes occurring at both the anode and the cathode. ... 

Dynamic characteristics of a fuel cell engine are of paramount importance for automotive application. Three primary processes govern the time response of a PEFC. They are (1) electrochemical double-layer discharging, (2) gas transport through channel and GDL, and (3) membrane hydration or dehydration (i.e., between a dry and a hydrated state). The time constant of double-layer discharging is between micro- and milliseconds, sufficiently short to be safely ignored for automotive fuel cells. The time constant for a reactant gas to transport through GDL can be estimated simply by its diffusion time, i.e.,... 

Specifically, Figure 16 shows that the current density in a cell with dry cathode gas feed drops nearly instantaneously once the cell voltage is relaxed from 0.6 to 0.7 V due to the fact that the electrochemical double-layer effect has a negligibly small time constant. Further, there exists undershoot in the current density as the oxygen concentration inside the cathode catalyst layer still remains low due to the larger consumption rate under 0.6 V. As the... 

Fig. 8.5. In electrochemical reactions involving one or more adsorbed reaction intermediates (sometimes involved in the rate-determining step), the steady-state concentration of the intermediate changes with the potential. However, each intermediate has a time constant to reach the surface coverage corresponding to a given overpotential. The downside of too low a pulse time, or too fast a sweep rate, is that the intermediate concentration does not relax to its appropriate concentration in time. The Tafel slope (sometimes a significant mechanism indicator) may then differ from that calculated for the assumed path and rate-determining step.

Consideration of the equivalent circuit diagram of an electrochemical cell, such as that given in Figure 5.1, reveals the major limitation on the rate at which the potential of an electrode can be varied, namely, the time constant of the electrochemical cell, RuCd. When a potential sweep is applied across the cell, the nonfaradaic charging current that flows is described by [24]... 

The measurement system sensitivity threshold does not exceed 1 ppb. The time constant is determined by the electrochemical cell inertia and ranges 30 s, which corresponds to a 4-5-km and less than 300-m spatial resolution over the horizontal and vertical, respectively, for measurements during the ascent and descent of the aircraft. Atmospheric ozone concentration can be measured in-situ within the range of 0 - 4E+12 cm 3, with an accuracy not worse than 5 %. 

Wang15 investigated heat and mass transport and electrochemical kinetics in the cathode catalyst layer during cold start, and identified the key parameters characterizing cold-start performance. He found that the spatial variation of temperature was small under low current density cold start, and thereby developed the lumped thermal model. A dimensionless parameter, defined as the ratio of the time constant of cell warm-up to that of ice... 

When the electrochemical properties of some materials are analyzed, the timescale of the phenomena involved requires the use of ultrafast voltammetry. Microelectrodes play an essential role for recording voltammograms at scan rates of megavolts-per-seconds, reaching nanoseconds timescales for which the perturbation is short enough, so it propagates only over a very small zone close to the electrode and the diffusion field can be considered almost planar. In these conditions, the current and the interfacial capacitance are proportional to the electrode area, whereas the ohmic drop and the cell time constant decrease linearly with the electrode characteristic dimension. For Cyclic Voltammetry, these can be written in terms of the dimensionless parameters yu and 6 given by... 

Figure 12. Plots of the reduced current against the applied potential theoretically calculated based upon the TLM as functions of o of PLD at a scan rate of 20 mV s 1. Solid bold line denotes the ideal doublelayer capacitor where the time constant is zero. Reprinted with permission from G. -J. Lee, S. -I. Pyun, and C. -H. Kim,./. Solid State Electrochem., 8 (2004) 110. Copyright 2003, with kind permission of Springer Science and Business Media.

Figure 17. Plots of the reduced current density against the applied potential obtained from the as-activated carbon (dotted line) and as-reactivated carbon (dashed line) electrode specimens. Solid line represents the ideal double layer capacitor where the time constant is zero. Reprinted from C.-H. Kim, S.-I. Pyun, and H.-C. Shin, J. Electrochem. Soc. 149 (2002) A93. Copyright 2001, with permission from The Electrochemical Society.

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