Activation polarization losses

The activation polarization loss is dominant at low current density. At this point, electronic barriers have to be overcome prior to current and ion flow. Activation losses show some increase as current increases. Ohmic polarization (loss) varies directly with current, increasing over the whole range of current because cell resistance remains essentially constant. Gas transport losses occur over the entire range of current density, but these losses become prominent at high limiting currents where it becomes difficult to provide enough reactant flow to the cell reaction sites. 

On closing the circuit and allowing a small current to ran through the system, the activation overpotentials are the greatest source of voltage loss. Activation polarization losses can be determined by the Tafel equation [20] when concentration polarization is not taken into account (Figure 6.2, section A, and Figure 6.3) [2, 36, 65]. 

The Ni/YSZ anode has been widely used and studied for H2 SOFCs [53]. The Ni/YSZ cermet provides a satisfactory performance at 800-1000 °C, but suffers from low tolerance to the sulfur compounds and a substantial activation polarization loss at low temperature (<600 °C). The H2 SOFCs require the use of a ZnO-based sorbent to remove sulfur compoimds in the H2 feed stream. An alternative to the Ni anode is the perovskites. Perovskites such as Sro.sLao.4Ti04 have exhibited sulfur resistance, but low performance [18,54]. 

Activation Polarization Activation polarization losses are highly nonlinear with current and manifest as a sharp initial drop in cell voltage from open-circuit conditions followed by diminishing additional losses as the current is increased through ohmic and concentration polarization dominated regions, as shown in region 1 of Figure 4.1. 

In this section, we derive a general expression to describe activation polarization losses at a given electrode, known as the Butler-Volmer (BV) kinetic model. The BV model is not the only (or necessarily the most appropriate) model to describe a particular electrochemical reaction process. Nevertheless, it is a classical treatment of electrode kinetics that is widely applied to study and model a majority of the electrode kinetics of fuel cells. The BV model describes an electrochemical process limited by the charge transfer of electrons, which is appropriate for the ORR, and in most cases the HOR with pure hydrogen. The fundamental assumption of the BV kinetic model is that the reaction is rate hmited by a single electron transfer step, which may not actually be true. Some reactions may have two or more intermediate charge transfer reactions that compete in parallel or another intermediate step such as reactant adsorption (Tafel reaction from Chapter 2) may limit the overall reaction rate. Nevertheless, the BV model of an electrochemical reaction is standard fare for a student of electrochemistry and can be used to reasonably fit most fuel cell reaction behavior. 

COMMENTS Notice the strong effect of the exchange current density near the open circuit. There are still activation polarization losses accumulating throughout the entire polarization curve, but the effect is most dramatic at low current density. 

Example 4.5 Activation Polarization Loss Calculation Given the table below, solve for the following ... 

Figure 3-8 shows how the cell polarization curve is formed, by subtracting the activation polarization losses, ohmic losses, and concentration polarization losses from the equilibrium potential. Anode and cathode activation losses are lumped together, but, as mentioned before, a majority of the losses occiu on the cathode because of sluggishness of the oxygen reduction reaction. 

Fig. 6. Discharge behavior of a battery where is the open circuit voltage (a) current—potential or power curve showing M activation, ohmic, and M concentration polarization regions where the double headed arrow represents polarization loss and (b) voltage—time profile.

Figure 3.5 [36], For the 02 reduction reaction on freshly prepared LSM electrodes, the initial polarization losses are very high and decrease significantly with the cathodic polarization/current passage (see Figure 3.5b). Consistent with the polarization potential, the impedance responses at open circuit decrease rapidly with the application of the cathodic current passage. For example, the initial electrode polarization resistance, RE, is 6.2 Qcm2 and after cathodic current treatment for 15 min RK is reduced to 0.7 Qcm2 see Figure 3.5 (a). The reduction in the electrode polarization resistance is substantial. The analysis of the impedance responses as a function of the cathodic current passage indicates that the effect of the cathodic polarization is primarily on the reduction in the low-frequency impedance [10]. Such activation effect of cathodic polarization/current on the electrochemical activity of the cathodes was also reported on LSM/YSZ composite electrodes [56-58], Nevertheless, the magnitude of the activation effect on the composite electrodes is relatively small.

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). 

To determine actual cell performance, three losses must be deducted from the Nernst potential activation polarization, ohmic polarization, and concentration polarization. Definition of the ohmic polarization is simply the product of cell current and cell resistance. Both activation polarization and concentration polarization required additional description for basic understanding. 

Figure 2-1 shows that the reversible cell potential for a fuel cell consuming H2 and O2 decreases by 0.27 mV/°C under standard conditions where the reaction product is water vapor. However, as is the case in PAFC s, an increase in temperature improves cell performance because activation polarization, mass transfer polarization, and ohmic losses are reduced. 

The improvement in cell performance at higher pressure and high current density can be attributed to a lower diffusion polarization at the cathode and an increase in the reversible cell potential. In addition, pressurization decreases activation polarization at the cathode because of the increased oxygen and water partial pressures. If the partial pressure of water is allowed to increase, a lower acid concentration will result. This will increase ionic conductivity and bring about a higher exchange current density. The net outcome is a reduction in ohmic losses. It was reported (33) that an increase in cell pressure (100% H3PO4, 169°C (336°F)) from 1 to 4.4 atm (14.7 to 64.7 psia) produces a reduction in acid concentration to 97%, and a decrease of about 0.001 ohm in the resistance of a small six cell stack (350 cm electrode area). 

Ihe actual from the thermodynamic electrode potential. The polarization is Ihe result of the irreversibility of the electrode process, that is, the activation polarization and the voltage loss, which develops from concentration gradients nf the reactants. This leads to the current-voltage characteristics as shown in Fig. 2,... 

Such a comparative study has been made by Byakov and his collaborators.29 255 They have shown that in the case of water the main contribution to the loss rate given by formula (6.3) comes from excitation of intramolecular vibrations rather than from dipole relaxation. This is all the more so in nonpolar media where the main channel of continuous losses is not the relaxation of constant dipole moments (which are zero) but the polarization losses due to the electron-inducing dipole moments in molecules. The possible exceptions are the media consisting of molecules with a high degree of symmetry, such as methane and neopentane, which have no active vibrations in the IR region. 

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.

The main drawbacks of the Ni-YSZ anode are intolerance to the sulfur compounds as low as 2 ppm in H2 steam and a substantial polarization loss at low temperatures (< 600 °C). The majority of the H2 SOFCs require the use of a ZnO-based sorbent to remove H2S in the H2 feed stream. The alternative to Ni anode is the perovskite which possess both electric conductivity and oxidation activity at high temperature. Perovskite such as Sro.5Lao.4Ti04 has shown excellent resistance to sulfur poisoning, however, its overall performance is a factor of 5 less than that of the Ni-cermet. ... 

In other words, in the kinetic regime with a predominant simple Tafel dependence a change in I by a factor of k shifts polarization losses by — Mn/c. i is the most important parameter here, and a larger active surface due to increased thickness improves the performance. 

The experimental plots of iR-free voltage vs. current density obtained for O2 or air and hydrogen as a fuel have been used for the estimation of the factors, which determine the cell polarization losses, namely activation potential, Tafel slope, and mass transport limitations. 

---