Cell voltage overpotentials

Voltage distribution/V estimated reversible cell voltage overpotentials electrolyte iR membrane iR total... 

Seconday Current Distribution. When activation overvoltage alone is superimposed on the primary current distribution, the effect of secondary current distribution occurs. High overpotentials would be required for the primary current distribution to be achieved at the edge of the electrode. Because the electrode is essentially unipotential, this requires a redistribution of electrolyte potential. This, ia turn, redistributes the current. Therefore, the result of the influence of the activation overvoltage is that the primary current distribution tends to be evened out. The activation overpotential is exponential with current density. Thus the overall cell voltages are not ohmic, especially at low currents. 

Corresponding to the charge in the potential of single electrodes which is related to their different overpotentials, a shift in the overall cell voltage is observed. Moreover, an increasing cell temperature can be noticed. Besides Joule-effect heat losses Wj, caused by voltage drops due to the internal resistance Rt (electrolyte, contact to the electrodes, etc.) of the cell, thermal losses WK (related to overpotentials) are the reason for this phenomenon. 

However, under working conditions, with a current density j, the cell voltage E(j) decreases greatly as the result of three limiting factors the charge transfer overpotentials r]a,act and Pc,act at the two electrodes due to slow kinetics of the electrochemical processes (p, is defined as the difference between the working electrode potential ( j), and the equilibrium potential eq,i). the ohmic drop Rf. j, with the ohmic resistance of the electrolyte and interface, and the mass transfer limitations for reactants and products. The cell voltage can thus be expressed as... 

The third limitation is concerned with the numerous contributions to the cell voltage Vceii, which, along with the difference in the electrode reversible potentials AEeq, comprises overpotentials at the cathode, tjc, and the anode, as well as the ohmic drop A ohmic ... 

The cell voltage was acceptable at the higher temperature of 820 °C about 600 mV at 30 mA/cm2 (Fig. 26). At the lower temperature, 710 °C, the higher overpotential probably indicates partial freezing of the electrolyte. 

Useful work (electrical energy) is obtained from a fuel cell only when a reasonable current is drawn, but the actual cell potential is decreased from its equilibrium potential because of irreversible losses as shown in Figure 2-2". Several sources contribute to irreversible losses in a practical fuel cell. The losses, which are often called polarization, overpotential, or overvoltage (ri), originate primarily from three sources (1) activation polarization (r act), (2) ohmic polarization (rjohm), and (3) concentration polarization (ricoiic)- These losses result in a cell voltage (V) for a fuel cell that is less than its ideal potential, E (V = E - Losses). 

For hydrogen production from water, pure water (pH=7.0) is seldom used as an electrolyte. Water is a poor ionic conductor and hence it presents a high Ohmic overpotential. For the water splitting reaction to proceed at a realistically acceptable cell voltage the conductivity of the water is necessarily increased by the addition of acids or alkalis. Aqueous acidic and alkaline media offer high ionic (hydrogen and hydroxyl) concentrations and mobilities and therefore possess low electrical resistance. Basic electrolytes are generally preferred since corrosion problems are severe with acidic electrolytes. Based on the type of electrolytes used electrolyzers are... 

Figure 6.14. Cell Voltage vs. Cell Current profile of a hydrogen - oxygen fuel cell under idealized (dotted-dashed curve) and real conditions. Under real conditions the cell voltage suffers from a severe potential loss (overpotential) mainly due to the activation overpotential associated with the electroreduction process of molecular oxygen at the cathode of the fuel cell. Smaller contributions to the total overpotential losses (resistance loss and mass transport) are indicated.

Figure 25 shows the evolution of cell voltage with time of Raney-nickel anodes that are deliberately operated at too high current densities so that the effectively applied overpotential was above the threshhold for nickel oxidation, which amounts to +80 mV vs the reversible hydrogen electrode. Evidently at a current density of 400 mA/cm2 and at 80°C the oxidation of Raney nickel proceeds within hours and at 300 mA/cm2 still within a week. 

Overpotential, ohmic potential, and concentration polarization make electrolysis more difficult. They drive the cell voltage more negative, requiring more voltage from the power supply in Figure 17-1 to drive the reaction forward. 

AE is the thermodynamic cell voltage depending on the nature of the electrode reactions, rj is the total overpotential and represents the surplus of electrical energy required to drive the process at a practical rate and to overcome mass transfer resistances. AFn = IR is the ohmic drop in the interelectrode gap, the electrode... 

The solution of Equation (2.59) at a constant Nemst voltage shows a linear dependency of the cell voltage from the overpotential. The Ohmic law is similar to the term, where the voltage drop is proportional to the current density represented by the quotient of the utilised part of the maximum current and the cell area. 

The boundary conditions for the electrical model are the overpotentials on the cathodic side and on the anodic side. In both cases these conditions are set at the interconnections. The first overpotential is set to zero, while the latter is calculated as the difference between the open reversible voltage and the cell voltage, namely... 

Figure 3.3.7 Theoretical (dashed dotted) and real (solid) cell voltage (V) - current density (I) performance characteristics of a fuel cell. Overpotentials are responsible for the difference between theoretical and real performance and cause efficiency losses. They split into (i) activation polarization overpotentials at anode and cathode due to slow chemical kinetics, (ii) ohmic polarization overpotential due to ohmic voltage losses along the circuit, and (iii) concentration polarization overpotentials due to mass-transport limitations. The activation overpotentials of the cathode are typically the largest contribution to the total overvoltage.

Finite electrolyte conductivities and ionic current flow lead to ohmic voltage components in electrochemical cells. It is constructive at this point to review the effects of ohmic voltage contributions to driven and driving cells in the case of uniform current distributions. It will be shown that for each type of cell, the ohmic resistance lowers the true overpotential at the electrode interface for a fixed cell voltage even in the case of a uniform current distribution at all points on the electrode. 

To develop any electrochemical process, a voltage should be applied between anodes and cathodes of the cell. This voltage is the addition of several contributions, such as the reversible cell voltage, the overvoltages, and the ohmic drops, that are related to the current in different ways. One of these contributions, the overvoltage, controls the rate of the transfer of electrons to the electrochemically active species through the electrode-electrolyte interface when there is no limitation in the availability of these active species on the interface (no mass-transfer control and no control by a preceding reaction). In this case, the relationship between the current that flows between the anodes and the cathodes of a cell and the overpotential is... 

As the cathodic reaction is the reverse of the anodic one, the theoretical thermodynamic cell potential is 0 V. In actual practice, the cell voltage required to drive the process accounts for the voltage drops in the electrolyte, anode and cathode electrical connections, electrical circuit losses, and overpotentials for both electrode reactions when occurring at a reasonable rate. 

FIGURE 12.2 Schematic view of various overpotential losses ideal and apparent fuel cell voltage-current characteristics. 

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