Resistance and overpotential

For the developments of solid-state electrochemical devices and optimization of their operation regimes, it is necessary to accurately evaluate the overall performance of individual electrodes, reflected primarily by the area-specific polarization resistance and overpotentials, and to collect reliable information on the electrochemical reaction mechanisms. In the three-electrode arrangement, the working electrode is polarized with respect to the reference electrode (RE), i.e., a voltage is applied between the WE and counter electrode (CE) and electrical current... 

Obviously the anode material should not only be an adequate electronic conductor, but also electrocatalytically active such that a rapid charge exchange can be established. Resistive and overpotential losses are thereby minimised. However, the catalytic behaviour of anode materials should not extend to the promotion of unwanted side reactions, hydrocarbon pyrolysis followed by deposition of carbon being an example. The electrochemical reaction takes place in the region (Figure 6.1) where oxygen ions available from the electrolyte can... 

If step (a), the transition of charged components, is a limiting step, an inhomogeneous electric field is set up at the surface of the deposit (D) to guide the charged components into the active positions, and a linear relationship should, therefore, exist between the process rate and overpotential. This overpotential, which is due to distortion of the electric field, is usually small and is part of the resistance overpotential. 

In Section 11.1 we viewed the overpotential as the stimulus (or perturbation), which causes the reaction to proceed at a certain rate. We could equally well have inverted the roles and looked at the current density as the stimulus and the resulting overpotential as the response. In this case the value of the overpotential is an indication of the sluggishness of the reaction. To be more precise, the ratio between the response T] and the perturbation i is a measure of the effective resistance of the system to proceed at the desired rale. In the example given earlier, applicable to small perturbations, the ratio (ri/j) is constant, but in general it is a function of potential, as implied by Fig. lA. It will be noted that r /i has the dimensions of an electrical resistance, and indeed it represents the total reaction resistance R. For the simple case discussed here we can write... 

The relationship between the solution resistance and the shortest relaxation time of the reaction that can be studied can perhaps be clarified by the following numerical example. Consider a small elec-trode of 0.05 cm, for which C =1.0 pF, and assume that the charge injected is 0.01 pC/cm yielding a value of T = 10 mV. employing high quality instrumentation one can measure the decay of overpotential with, sufficient accuracy if iR is n be expressed by the inequality... 

Under tlie circumstances, it is reasonable to say that the reaction kinetics on the surface of carbon electrode is facilitated and hence the activation overpotential is not observed. However, the system resistance such as electrode, electrolyte, polymer electrolyte membrane, and carbon block could be dominant, influencing cell overpotential over all current ranges. Therefore, it is necessary to reduce the system resistance by selecting the cell constituents with low resistance and optimising the cell assembly. 

From the experimental results, we concluded that electrical energy demand AG for electrodialysis of Hix solution decreases with increasing temperature. Moreover, the electrodialysis cell overpotential is primarily influenced by the system resistance and hence the thermal efficiency of the cell could be improved by reducing the constituent resistance and by optimizing the cell assembly. 

The chosen electrode material should be conductive and inert within the potential range of the cell. Materials composed of allotropes of carbon and, to a lesser extent, gold are most commonly used. The cell should be designed to minimize overpotentials due to kinetics, ohmic resistance, and mass transfer of fuel in order to maximize cell voltage (A ) and current (i) generation. In addition, all cell components should be mechanically stable within their operating environment... 

O2 (Air) fuel cell is ca. 0.5-0.6 V [28, 29]. This deviation is primarily caused by the large overpotential associated with the reduction of O, but the decrease also can be attributed to other resistances in the fuel cell itself, including solution resistance and electrical contact resistance. 

To minimize the cell voltage in an electrochemical reactor, the anode and cathode electrodes are placed as close as possible to minimize the resistance (IR) overpotential. Such voltage minimization is achieved in zero-gap and capillary cells [31, 32], by placing the electrodes directly adjacent to the membrane/diaphragm that separates the anode and cathode compartments. Figure... 

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