Half-cell polarization voltage

Figure 23.S The time dependence of the half-cell polarization voltage rjo for the indicated dimensionless current densities Jo-...

However, oxygen and methanol fluxes in the DMFC are affected by crossover. Thus, to construct a full ID model of this cell we should write the balance of methanol and oxygen fluxes taking crossover into account. From this balance we will deduce the methanol (cf) and oxygen (cj) concentrations in the respective catalyst layer. Making a substitution similar to (3.4), we will obtain the half-cell polarization voltages and then construct the polarization curve of the whole cell. 

Suppose that both sides of the DMFC operate in a low-current regime. The half-cell polarization voltages are then determined by Eq. (2.29), where on the anode side ct = c is given by (3.16) and on the cathode side we should set Ct = Ct (Eq. (3.25)). 

Here AS is the entropy change in the half-cell reaction, rj is the half-cell polarization voltage, j is the mean current density in the cell, I is the thickness of the respective catalyst layer, at is the proton conductivity of the catalyst layer, am is the proton conductivity of the bulk membrane, and A and Am are the thermal conductivities of the catalyst layers and membrane, respectively. Note that the thermal conductivities of the ACL and CCL are assumed to be the same. 

Equations (3.58) and (3.59) provide a means of measuring the thermal conductivities of the catalyst layers and membrane in a working fuel cell environment. The temperature difference on both sides of the thick and thin membranes and simultaneously the half-cell polarization voltage rjc have to be measured as a function of cell current. In the region of currents where rjc is approximately constant, the slope of the straight line AT versus j would give A and A in experiments with thick and thin membranes, respectively. If the variation of rjc with j is not small, it can be taken into account this would only slightly complicate the procedure. 

The growth of current ahead of the wave is accompanied by the rise of the half-cell polarization voltage rj. Evidently, with the growth of current the transport losses dominate and to rationalize the dependence r] i) we have to consider the transport term in Eq. (4.34) ... 

The half-cell polarization voltages (4.215) and (4.216) depend on the local current density at the inlet rather than on the total current in the system. Thus, the formation of the jumper induces finite rf and rf in the cell. Furthermore, since rf and rf are constant along z, the cell potential immediately feels the jumper. 

Here AS is the entropy change in the overall reaction, j is the local current density in the cell, and r] is the sum of half-cell polarization voltages r) = q° +... 

Here AS is the entropy change in the half-cell electrochemical reaction, r] is the half-cell polarization voltage, the subscripts a, c and e indicate the anode, the cathode and the electrolyte, respectively, and the superscript n enumerates the MEAs (Figure 5.20). Parameters Xa, Xe, and ka are... 

The net result of current flow in a fuel cell is to increase the anode potential and to decrease the cathode potential, thereby reducing the cell voltage. Figure 2-3 illustrates the contribution to polarization of the two half cells for a PAFC. The reference point (zero polarization) is hydrogen. These shapes of the polarization curves are typical of other types of fuel cells. 

Electrochemical cells employed to carry out voltammet-ric or amperometric measurements can involve either a two or three electrode configuration. In the two electrode mode, the external voltage is applied between the working and a reference electrode, and the current monitored. Since the current must also pass through the reference electrode, such current flow can potentially alter the surface concentration of electroactive species that poises the actual half-cell potential of the reference electrode, changing its value by a concentration polarization process. For example, if an Ag/AgCl reference electrode were used in a cell in which a reduction reaction for the analyte occurs at the working electrode, then an oxidation reaction would take place at the surface of the reference electrode ... 

To begin our discussion, it is useful to consider current-voltage curves for an ideal polarized and an ideal nonpolarized elearode. Polarization at a single electrode can be studied by coupling it with an electrode that is not easily polarized. Such electrodes have large surface areas and have half-cell reactions that are rapid and reversible. Design details of nonpolarized electrodes are described in subsequent chapters. 

A DC potential may develop at the electrode metal/solution interphase. The absolute potential of this interphase (half-cell electrode potential) cannot be measured—it must be considered unknown. However, the potential difference between two electrodes can be measured with an ordinary voltmeter connected to the two metal wires from the electrodes. If file metals were different, then they could generate a potential difference of 1 V or more. However, here we presume that the same electrode material is used and that the measured potential difference is small. We will discuss the case for three different electrode materials important in biological work platinum, silver coated with silver chloride (AgCl), and carbon. To the extent that both electrodes are equal, we have a symmetrical (bipolar) system, and the voltage—current dependence should not be dependent on polarity. 

With an external DC power supply connected to the electrolytic cell, the applied voltage that gives no DC current flow in the external circuit corresponds to the equilibrium potential of the half-cell (or actually the cell). It is the same voltage as read by a voltmeter with very high input resistance and virtually no current flow (pH meter). In electrochemistry, potentiometry is to measure the potential of an electrode at zero current flow, which is when the cell is not externally polarized. To understand the equilibrium potential with zero external current, we must introduce the concept of electrode reaction... 

The linear polarization technique requires us to polarize the steel whth an electric current and monitor its effect on the half cell potential. It is carried out with a sophisticated development of the half cell incorporating an auxiliary electrode and a variable low voltage DC power supply. The half cell potential is measured and then a small current is passed from the auxiliary electrode to the reinforcement. The change in the half cell potential is simply related to the corrosion current by the equation ... 

Figure 1.3 Schematic for the calculation of voltage loss in a fuel cell (for discussion see text). ACL and CCL are the abbreviations for the anode and cathode catalyst layers, respectively. Yellow shaded areas indicate the local polarization voltage r]. For simplicity, the proton conductivity of catalyst layers is taken to be equal to the proton conductivity of the bulk membrane (otherwise the curve loses smoothness at the membrane interfaces). Note that the half-cell voltage loss is given by the value of the overpotential at the catalyst layer/membrane interface.

When a conductive particle is exposed to an electric field, it causes the particle to polarize. As a consequence an overpotential r varying according to a cosine law is induced at the surface of the particle (Eq. 1, Fig. l) Thus, a maximum potential difference will occur at opposite poles of the particle. In order to carry out electrochemistry at the surface of the particle a critical voltage difference corresponding to the sum of two half-cell reactions must be reached. Thus, for a given particle of radius r and an applied electric field E there will exist two polar regions defined by a critical angle 0 within which electrochemistry will occur. (Eq. 2, Fig. 2). This forms the theoretical basis of toposelective electrodeposition. 

This mixed potential is explained in Fig. 5 through an Evans diagram. In an operating fuel cell, along with this polarization close to open circuit voltage (OCV), there are losses due to hydrogen permeation into cathode electrode from anode chambers in PEMFC and methanol crossover in direct methanol fuel cell (DMFC). In a half-cell system, the crossover losses do not exist, but the polarization due to the carbon oxidation or any other contaminant participating in a side-reaction depresses the OCV. 

Oxidation half-reaction (occurs at the anode) 2C1 ( ) -> Chy -i- 2e Because of the external voltage of the electrolytic cell, the electrodes do not have the same polarities in electrolytic and galvanic cells. In a galvanic cell, the cathode is positive and the anode is negative. In an electrolytic cell, the anode is positive and the cathode is negative. 

The polymer discotic material (PDM) developed by Fuji Photo Film has a hybrid alignment, which mimics half of the bend alignment structure of the OCB cell. In contrast to the discotic film developed for TN LCDs, the azimuthal alignment direction of the PDM layer is oriented at 45° to the transmission axis of polarizer, and the in-plane retardation of the PDM layer compensates for the in-plane retardation of the on-state OCB cell. The total in-plane retardation of the PDM layer should be the same as that of the on-state OCB cell so that the voltage-on state becomes black at a voltage lower than 5 Vj s. 

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