Cell potential changes during operation

Ceil potential (Eceii) changes during operation of the cell. The Nemst equation shows that Eceii depends on Ecew and a term for the potential at nonstandard-state concentrations. During the operation of a typical voltaic cell, reactant concentration starts out higher than product concentration, gradually becomes equal to it, and then less than it, until Q = K and the cell can do no more work. 

Standard Cell Potential and K Effect of Concentration on E ei Changes in Eoen During Cell Operation Concentration Cells... 

The potential of the zinc-copper cell changes as concentrations change during cell operation. The only concentrations that change are [reactant] = [Cu ] and [product] = [Zn ] ... 

As the cell operates, [Zn" ] increases (as the Zn electrode deteriorates) and [Cu ] decreases (as Cu plates out on the Cu electrode). Although the changes during this process occur smoothly, we can identify four general stages of operation. Figure 21.11A shows the first three. The main point to note is as the cell operates, its potential decreases ... 

As a result of in the changes in temperature and moisture, the membrane, GDL and bipolar plates will all experience expansion and contraction. Because of the different thermal expansion and swelling coefficients between these materials, hygrothermal stresses are expected to be introduced into the unit cell during operation. In addition, the non-uniform current and reactant flow distributions in the cell result in non-uniform temperature and moisture content of the cell which could in turn, potentially causing localized increases in the stress magnitudes. 

As with any voltaic cell, the potential of the zinc-copper cell changes during cell operation as the concentrations of the components change. With two of the four components solids, the only variables are [Cu +] and [Zn +l ... 

It is largely observed that the nano/microstractural properties of the Pt or Pt-alloy-based CL and the PEM evolve during PEMFC operation (both hydrogen-fed and DMFCs), even at normal (i.e. not below zero) operation temperatures. These spatio-temporal nano/microstmctural changes translate into irreversible longterm cell potential degradation (Fig. 11.1). 

The cell potential for a concentration cell is typically small and decreases continually during the operation of the cell as the concentrations in the two compartments approach each other. When the concentrations of the ions in the two compartments are equal, E becomes zero and no further change occurs. 

The other effect considered in this section deals with transients in a single fuel cell. The transient models examine step changes in potential and related phenomena (e.g., gas flow rates, water production, and current density). Hence, they are aimed at examining how a fuel-cell system handles different load requirements, which may occur during automotive operation or start up and shut down. They are not trying to model slow degradation processes that lead to failure or the transients associated with impedance experiments (i.e., potential or current oscillations). These types of models are discussed in section 7. 

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