Limiting Currents change with time

Figure 1.12. Current interruption method showing the voltage change with time [3], (From Larminie J, Dicks A. Fuel cell systems explained. 2003 John Wiley Sons Limited. Reproduced with permission of the publisher and the authors.)...

In order to get the current—potential relationship on the RDE, particularly the expression of limiting current density as the function of the electrode rotating rate and the reactant concentration, Pick s second law has to be used to give the equations of reactant concentration change with time at the steady-state situation of diffusion—convection. When the surface concentration of oxidant reaches zero during the reaction at the steady-state situation, the concentration distribution within the diffusion—convection layer is not changing with time anymore, meaning that the diffusion rate is... 

Among the special-purpose cells are those vessels used for continuous analysis. In this type of measurement the limiting current is measured for control or automation purposes over a long period of time. Thus both the reference electrode and the indicator electrode properties must not change with time. This... 

Several current-voltage curves were recorded and if changes with time was observed, the limiting current was measured at the various times and extrapolated to t = 0. All measured currents were corrected for changes in the viscosity of the solution by multiplying the measured current by (v/Vo) /. ... 

Calibration instability in the detector is typically caused by dark current changes over time and temperature as well as cutoff wavelength shifts with temperature which can cause small changes in apparent responsivity. Instability in the ROIC is primarily limited to offset changes in MOSFETs over time, temperature, and environmental exposure. The extent to which these impact the calibration frequency of a system is often determined through test and validation of the ROIC at the sensor-chip level, and can take minutes to days of data collection depending on the needs of the application. 

In general it will be necessary to measure via impedance measurements using a four electrode cell. A schematic diagram of the cell which would be used for such measurements is shown in Fig. 10.15. The expected behaviour will be as described in Eqn (10.3) except that Warburg impedances can arise from either or both phases. An example of an impedance spectrum of the H2O/PVC interface is shown in Fig. 10.16. The application of a constant overpotential will, in general, lead to a slowly decaying current with time due to the concentration changes which occur in both phases, so that steady state current potential measurements will be of limited use. 

However, when one gets down to detailed quantitative equations to represent real, actual reactions with several steps in consecutive sequence, the mathematics become very complex. Thus, the change in the limiting current with time introduces complications that one tries to avoid in other transient methods by working at low times (constant current or constant potential approaches) or at times sufficiently high that the current becomes entirely diffusion controlled. However, taking into account the... 

If a constant current i equal to 60% of the limiting current found for the most concentrated solution of precursor (i.e., i = 0.6 i,) is applied to the cell, then the ratio i/i, is 0.6 and is represented by line A of Figure 25.1. The potential of the electrode at the time the constant current is applied would be the potential at the intersection of line A with the curve. Now, consider a second precursor solution which is only 90% as concentrated as the first one. The new limiting current, i[, would be nine-tenths of the original limiting current, or ij = 0.9i,. However, in a constant-current experiment, the applied current would not change, so the new ratio, i/ij, is larger than the former ratio and can be calculated as... 

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