Potential During Cell Operation

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 ]  

The positive °eii value (1.10 V) means that this reaction proceeds spontaneously from the standard-state conditions, at which [Zn ] = [Cu ] = 1 M (2 = 1), to some point at which [Zn ] [Cu ] (Q 1). If we start the cell when 

Stage ]. EceW ceii when Q 1 When the cell begins operation, [Cu ] [Zn- ], so Eceii ii- 

As cell operation continues, [Zn ] increases and [Cm ] decreases thus, Q becomes larger, and fceii decreases. 

Stage 3. fceii Elew when Q 1 As [Cu ] falls below [Zn ], ceii eii-Stage 4. ceii = 0 when Q = K Eventually, E w is zero. This occurs when the system reaches equilibrium no more free energy is released, so the cell can do no more work. At this point, we say that a battery is dead.  

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Changes in Potential During Cell Operation 709 Concentration Cells 710... 

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

Some FF+ will be converted back to Ft at lower potentials during the operation of a fuel cell, but it may not deposit to the original particles. Some FF+ may migrate into the FEM, where it is reduced by H2 diffusing into the FEM from the anode to form Ft according to Reaction 1.21. With time a band of Ft may appear in the FEM when observed under SEM. The location of the Ft band is determined by where Ft and H2 meet in the FEM according to their transport rates. 

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 ... 

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. 

In [53], oscillatory wave patterns observed during electrochemical dissolution of a nickel wire in acidic media was reported. It was shown that space-averaged potential or current oscillations are associated with the creation of an inhomogeneous current distribution, and that the selection of a specific spatial current pattern depends on the current control mode of the electrochemical cell. In the almost potentiostatic (fixed potential) mode of operation, a train of traveling pulses prevails, whereas antiphase oscillations occur in the galvanostatic (constant average current) mode. 

Fig. 4.6.1. Schematic diagram showing the direction of the emf, electrochemical potential gradient, electrostatic potential gradient, and electric field during spontaneous operation of a cell operating in conformity with Conventions 1 and 2, described below.

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