Electrolytes convection

Usually, this phenomenon limits the lifetime of a battery because the storage capacity falls below a reasonable lower limit. One reason for this zinc migration was identified by McBreen [35] an inhomogeneous current distribution makes the zinc move away from high current density areas. Another mechanism seems to be active as well an electrolyte convection induced by electro-osmosis through the separator [36],... 

Both share more or less the same merits but also the same disadvantages. The beneficial properties are high OCV (2.12 and 1.85 V respectively) flexibility in design (because the active chemicals are mainly stored in tanks outside the (usually bipolar) cell stack) no problems with zinc deposition in the charging cycle because it works under nearly ideal conditions (perfect mass transport by electrolyte convection, carbon substrates [52]) self-discharge by chemical attack of the acid on the deposited zinc may be ignored because the stack runs dry in the standby mode and use of relatively cheap construction materials (polymers) and reactants. 

A roughness on the millimeter scale is observed for micro, meso and macro PS if the HF concentration or temperature varies over the interface. HF concentration variations may be caused by inhomogeneous electrolyte convection at the electrolyte-electrode interface, another common cause are bubbles that stick to the electrode surface. Lateral temperature variations may be a problem during illumination-assisted PS formation. 

The third form of mass transport is convection driven by pressure. When forced circulation exists in electrolyte, convection may be the dominant form of mass transport. Thus, in general, a flux Jj (mol/s cm) of species j may occur due to the above three types of mass transport mechanisms. The flux can be described by the Nernst-Planck equation [5]... 

In the present treatment, both charging and discharging modes are considered in addition to open circuit operation however, only diffusive trauisport is permitted in the electrolyte—convection is not. Consequently, the results are most directly applicable to solid electrolyte media however, liquid media (acqueous, molten salt, etc.) in a previous matrix should also conform to such assumptions. 

Current flow in electrolytes results from the movement of ions. There are three mechanisms of ion transport in electrolytes convection, diflusion, and migration. 

L The speed of deposition is usually much lower than 75pmh (although forced electrolyte convection may improve this). 

Figure 1-12. Rotating-disk electrode with electrolyte convection in front.

A very frequently used technique for the study of electrode reactions is measuring the impedance of an electrode at variable frequency. This technique can be applied to electrodes at equihbrium where the external ac current causes concentration changes of both components of the redox reaction in opposite directions. The ac current can also be superimposed upon a constant current, provided a steady state can be reached for this dc current. This requires the presence of convection in the transport process. Since in solid electrolytes convection is impossible, such cases will not be discussed here. 

The current density is directly related to the ionic fiux, Nj, in the electrochemical cell. The fiux is typically described in terms of three major components diffusion of ions across a concentration gradient, migration of charged ions down the electric field, and transport of ions due to bulk electrolyte convection. Consequently, the flux of an ionic species j is given by... 

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