Concentration polarization electrolytic cell

In addition to this, one distinguishes that there are two other sources of voltage difference. One has its origin within the electrolyte of the cell, and the other is referred to rate processes taking place at the electrodes. The former is called concentration polarization, while the latter is called overvoltage. 

The influence of porosity on the electrochemical activity has not been studied much for electrolyte-supported cells because anode pastes for electrolyte-supported cells are made for screen printing, and thus contain significant amounts of organics, which almost guarantees sufficient porosity. In addition, since the anode thickness for electrolyte-supported cells is only on the order of 50 pm, the concentration polarization itself becomes much less of an issue. In fact, Jiang et al. [44] showed that anode overpotential for cermet anodes prepared with extra graphite pore formers... 

To reduce and overcome this concentration polarization, several techniques are available. For reactants soluble in the electrolyte, high bulk concentrations are used and also the electrolyte is circulated by pumping which uses a fraction of the electricity produced by the fuel cell and hence reduces the available power. For gaseous reactants, porous gas electrodes are used to achieve larger contact of the three phases, namely the gas, the liquid and the solid phases. There are different types of such electrode and two examples are shown in Figures 3a et 3b. 

Electrode polarization of the anode is less severe than that for the cathode and is mainly concentration polarization resulting from accumulation of zinc chloride near the electrode surface. During rest periods after discharge, diffusion of ZnCl2 into the bulk of the electrolyte reduces the anode polarization and this also contributes to the recovery of the cell voltage. Polarization behaviour of the cathode and anode of a D-size Leclanche cell is shown in Fig. 3.7. 

Under working conditions, with a current density j, the cell voltage E(J) decreases greatly as the result of three limiting factors the overvoltages r a and r c at both electrodes due to a rather low reaction rate of the electrochemical processes (activation polarization), the ohmic drop RJ in the electrolyte and interface resistance Re, and mass transfer limitations for reactants and products (concentration polarization). 

Chutaux cell — This was a chromic acid (carbon electrode)-zinc - battery constructed in such a way that the electrolyte solutions could flow along the electrodes to reduce concentration polarization. 

Higgins cell — This was a chromic acid (carbon electrode) - zinc - battery constructed in such a way that the chromic acid electrolyte was stirred to reduce concentration polarization. See also -> chromic acid battery, -> Daniell cell, -> zinc, - Zn2+/Zn electrodes, -r Zn2+/Zn(Hg) electrodes, -> zinc-air batteries (cell), and - Leclanche cell. 

The Importance of Concentration Polarization As noted earlier, concentration polarization occurs when the effects of diffusion, migration, and convection are insufficient to transport a reactant to or from an electrode surface at a rate that produces a current of the magnitude given by Equation 22-2. Concentration polarization requires applied potentials that are larger than calculated from Equation 22-2 to maintain a given current in an electrolytic cell (see Figure 22-2). Similarly, the phenomenon causes a galvanic cell potential to be smaller than the value predicted on the basis of the theoretical potential and the IR drop. 

In the electrolytic cell of Fig. 6.1.1 the cupric ions and sulfate ions both contribute to the conduction mechanisms, but only the cupric ions enter into the electrode reaction and pass through the electrode-solution interface. The electrode therefore acts like a semipermeable membrane which is permeable to the Cu " ions but impermeable to the ions. Anions accumulate near the anode and become depleted near the cathode, resulting in concentration gradients in the solution near the electrodes of both ions. This is termed concentration polarization, in accord with the meaning of the phrase when applied to neutral species. 

By way of example, let us consider the simple electrolytic cell of Fig. 6.1.1, containing a motionless dilute binary metal electrolyte. We wish to determine the current-voltage characteristic of the cell, that is, the concentration polarization. To do this, we must calculate the flux of metal ions (cations) arriving at the cathode and depositing on it. As noted above, we assume that the overall rate of... 

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