349 cell-potential loss

Figure 3.26. Modelled main cell-potential loss terms. Ohmic bulk losses in electrolyte Act. an activation losses at negative electrode Act. cat activation losses at positive electrode. (Reprinted from S. Campanari and P. lora (2004). Definition and sensitivity analysis of a finite volume SOFC model for a tubular cell geometry. /. Power Sources 132,113-126. Used by permission from Elsevier.)...

To minimize the cell potential losses due to the rate of proton transport and reactant gas permeation in the depth of the electro catalyst layer, this layer should be made reasonably thin. At the same time, the metal active surface area should be maximized, for which the Pt particles should be as small as possible [1]. 

Kima et al. recently proposed a model allowing the calcrflation of the steady-state cell potential loss as a function of an interfacial delamination between the CL and the GDL, but without a link with materials degradation phenomena. Rong et al. 215 presented a discretized model to describe debonding and delamination... 

By the half-cell potentials, we conclude the Zn-Zn+2 half-reaction has the greater tendency to release electrons. It will tend to transfer an electron to silver ion, forcing (54) in the reverse direction. Hence we obtain the net reaction by subtracting (54) from (52). But remember that this subtraction must be in the proportion that causes no net gain or loss of electrons. If two electrons are lost per atom of zinc oxidized in (52), then we must double half-reaction (54) so that two electrons will be consumed. 

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. 

Fig. 1 Plots for a typical PEM fuel cell showing (a) potential loss contributions, (b) corresponding characteristic I-V curves, and (c) resulting power density curves showing efficiency at maximum power, where 8 is the efficiency of the conversion of chemical energy into electrical energy is Ceii/ theoreticai.

This means that the Ni electrode is the anode and must be involved in oxidation, so its reduction half-reaction must be reversed, changing the sign of the standard half-cell potential, and added to the silver half-reaction. Note that the silver half-reaction must be multiplied by two to equalize electron loss and gain, but the half-cell potential remains the same ... 

When using a multiplier to equalize electron loss and gain in reduction half-cell potentials, do not use the multiplier on the voltage of the half-cell. 

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). 

The performance of fuel cells is affected by operating variables (e.g., temperature, pressure, gas composition, reactant utilizations, current density) and other factors (impurities, cell life) that influence the ideal cell potential and the magnitude of the voltage losses described above. Any number of operating points can be selected for application of a fuel cell in a practical system, as illustrated by Figure 2-4. 

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). 

Electrolyte Structure Ohmic losses contribute about 65 mV loss at the beginning of life and may increase to as much as 145 mV by 40,000 hours (15). The majority of the voltage loss is in the electrolyte and the cathode components. The electrolyte offers the highest potential for reduction because 70% of the total cell ohmic loss occurs there. FCE investigated increasing the porosity of the electrolyte 5% to reduce the matrix resistance by 15%, and change the melt to Li/Na from Li/K to reduce the matrix resistivity by 40%. Work is continuing on the interaction of the electrolyte with the cathode components. At the present time, an electrolyte loss of 25% of the initial inventory can be projected with a low surface area cathode current collector and with the proper selection of material. 

The cell potential is simply the work that can be accomplished by the electrons produced in the SOFC, and this potential decreases from the equilibrium value due to losses in the electrodes and the electrolyte. For YSZ electrolytes, the losses are purely ohmic and are equal to the product of the current and the electrolyte resistance. Within the electrodes, the losses are more complex. While there can be an ohmic component, most of the losses are associated with diffusion (both of gas-phase molecules to the TPB and of ions within the electrode) and slow surface kinetics. For example, concentration gradients for either O2 (in the cathode) or H2 (in the anode) can change the concentrations at the electrolyte interface,which in turn establish the cell potential. Similarly, slow surface kinetics could result in the surface at the electrolyte interface not being in equilibrium with the gas phase. 

The two main techniques for measuring electrode losses are current interrupt and impedance spectroscopy. When applied between cathode and anode, these techniques allow one to separate the electrode losses from the electrolyte losses due to the fact that most of the electrode losses are time dependent, while the electrolyte loss is purely ohmic. The instantaneous change in cell potential when the load is removed, measured using current interrupt, can therefore be associated with the electrolyte. Alternatively, the electrolyte resistance is essentially equal to the impedance at high frequency, measured in impedance spectroscopy. Because current-interrupt is simply the pulse analogue to impedance spectroscopy, the two techniques, in theory, provide exactly the same information. However, because it is difficult to make a perfect step change in the load, we have found impedance spectroscopy much easier to use and interpret. 

The decay of a radical-anion can be followed directly by generating the intermediate within the cavity of an esr spectrometer through application of a controlled potential pulse to the cathode of a thin electrochemical cell [46]. Loss of the radical-anion is then followed by decay of the esr signal. Decay is second order in radical-ion concentration for dimethyl fumarate (k = 160 M s ) and for cin-namonitrile (k = 2.1 x 10 M s ) in dimethylformamide with tetrabutylammonium counter ion. Similar values for these rate constants have been obtained using purely electrochemical techniques [47]. 

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.

Voltammetry experiments are not often performed in flow cells for analytical purposes. One reason for this is the special problem of ohmic potential losses (iR drops) at an electrode in a confined stream. Another reason is the problem of precisely pumping solution at a carefully controlled velocity. In general, rotating electrodes are more easily controlled and do not involve serious plumbing problems. On the other hand, flow cells operated at a fixed potential (i.e., at one point along the steady-state voltammetric curve) are eminently useful for electrosynthesis, chromatographic detection, and automated analysis systems. These features will be described in later chapters. 

This effect apparently could be ascribed to the occurrence of the so-called ohmic potential drop (IR-drop)71 with concentrations higher than ca. lSxlO molT1. This ohmic potential drop is the product of the electrical resistance of the cell solution between working electrode and reference electrode and the electrical current, resulting in a high value of the last-mentioned parameter in a potential loss that cannot be neglected. Consequently, the effective potential of the working electrode compared... 

The corrected cell potential, Ecot, is obtained by subtracting the ohmic (jjohm), concentration (jjcone), and activation (//act) losses (i.e. overpotentials) from the ideal Nemst potential, E ... 

From the basic principles we can make preliminary design estimates. Inefficiencies in a system arise because of voltage losses and because all of the current does not enter into the desired reactions. The minimum potential required to perform an electrolytic reaction is given by the reversible cell potential, a thermodynamic quantity. Additional voltage that must be applied at the electrodes represents a loss that is manifested in a higher energy requirement. The main causes of voltage loss are ohmic drops and overpotentials. The applied potential is equal to the sum of the losses plus the thermodynamic requirement ... 

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