Open circuit cells

Figure 14.10 gives a schematic presentation of the current-voltage characteristics of hydrogen and oxygen on platinum. The theoretical open circuit cell voltage at 25 °C and standard conditions is 1.229 V, for both the fuel cell and electrolyzer. 

Figure 10-30(a) applies to an open circuit cell in the dark Fig. 10-30(b) applies to a short circuit cell in the dark. After the cell circuit is closed in the dark, the Fermi level is equilibrated between the two electrodes thereby forming a space charge layer both in the n-tyi>e anode and in the p-type cathode. The overall potential, AE, generated in the two space charge layers nearly equals the difference of the flat band potential between the n-type anode and the p-type cathode as eiqpressed in Eqn. 10-57 ... 

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

Figure 2 gives an overview on the definition of and relation between quantities used in surface science and in electrochemistry such as work function or surface potential. In the following we distinguish between (i) cells to which we apply a current (I >0, the current direction being opposite to the short-circuit current direction, U = E+ LaIRa > E), named polarization cells (cells under load), (ii) cells from which we extract current (7<0, in short-circuit direction, U < E), named current-generating cells, and finally (iii) open-circuit cells (I-0, U-E). In all cases we... 

Open-circuit cell Measurement of thermodynamic formation data Potentiometric composition sensors... 

Assuming that oxidation drives the phosphorylation process, thenA) < 0 andX2 > 0, and J U2 is the conventional P/O ratio, while X IX2 is the ratio of phosphate potential to the applied redox potential. The following relations are from Stucki (1980). At static head (sh), analogous to an open circuited cell, the net rate of ATP vanishes, and the rate of oxygen consumption and the force (the phosphate potential) are expressed in terms of a as follows... 

The other was an open circuit cell from the reference electrode to the working electrode ... 

The energy efficiency of the AFC itself is similar to or slightly better than that of other low-temperature cells, lying in the range of 45-60%, with an open circuit cell voltage around 0.9 V and a current density of 0.2-1.0 A cm highest for space cells (Jo and Yi, 1999 Spakovsky and Olsommer, 2002 McLean et al, 2002). 

Figure 18-4 Change in cell potential after passage of current until equilibrium is reached. In (a), the high-resistance voltmeter prevents any significant electron flow, and the full open circuit cell potential is measured. For the concentrations shown this is + 0.412 V. In (b), the voltmeter is replaced with a low-resistance current meter, and the cell discharges with time until eventually equilibrium is reached. In (c), after equilibrium is reached, the cell potential is again measured with a voltmeter and is found to be 0.000 V. The concentrations in the cell are now those at equilibrium as shown.

As far as electrochemical cells relevant for applications or electrochemical measurements are concerned, we must distinguish between polarization cells, galvanic cells and open-circuit cells, depending on whether an outer current flows and, if so, in which direction this occurs. Table 1.1 provides examples of the purposes for which such cells may be used. In terms of application, we can distinguish between electrochemical sensors, electrochemical actors and galvanic elements such as batteries and fuel cells. These applications offer a major driving force for dealing with solid-state electrochemistry. 

Open-circuit cells (chemical sensors) (for further details, see Chapter 13). 

Open circuit cell voltage Two voltage plateaus observed... 

With the activity expressed as p =p/po, (partial pressure/standard pressure) the pressure dependence of the open circuit cell voltage can be written as ... 

The mixed conducting medium considered here is generalized in the sense that the number of mobile ionic species is left open but the treatment is specialized in that steady state conditons are presumed. A more rigorous treatment of a still more generailized medium has been given by Wagner (33), but it seems to be restricted to open circuit cells only. 

A//q2-= 0, Aju = -FE, E = open circuit cell voltage). If the on one side is known (which is achieved by a mixture of oxygen with an inert gas or a reactive buffer mixture such as H2/H2O, CO/CO2), this so-called /I-cell can measure the oxygen partial pressure on the other side (see Figure 4). The signal can also be used to control Pq directly. 

Changes in Temperature, H2SO4 Concentration and Open Circuit Cell Voltage During Active Mass Formation... 

Figure 9.24 Power plot of a photovoltaic cell, f/pi, i the open-circuit cell voltage, the cell voltage at maximum power, ip, the short circuit photocurrent, ip j the photocurrent at maximum power, and the power maximum.

Open-circuit cell voltage (V) Energy density (Wh kg-i Power density (W kg-1) Cycles to deep discharge Energy efficiency on cycle (%)... 

A Nernst equation may be written for every galvanic cell at equilibrium. Therefore the equation is the basis of all thermodynamic applications of potentiometry (i.e. measurement of open-circuit cell potentials by means of a potentiometer or other zero-current device). Cells may be constructed and appropriate Nernst equations written to find, for example, the dissociation constant of water, and many electrolyte activity coefficients and stability and solubilit,v constants. Potentiometric titration curves are also interpieted by means of the appropriate Nernst equation. 

For the reasons discussed below, in a system with only a few molecules shown in Figure 1.1 the OCV is very small. If the number of hydrogen and oxygen molecules is large enough, this voltage is about 1.2 V. The open-circuit cell potential can be calculated from thermodynamic relations, as discussed in the following sections. 

The open-circuit cell voltage of this system is reported to be 3.0 V, energy density 303 Whkg and cathode energy density 0.1 mA cm. However, the life cycle has not been reported. 

E is also called theoretical open circuit cell voltage, OCV, in dependence of temperature and reactant, respectively product concentrations. Practical OCVs may observed to be lower, due to losses (mixed potentials due to side reactions or gas crossover, see practical OCV of H2/O2 fuel cell) already occurring at zero current flow. 

The Nemst potential, E, gives the ideal open circuit cell potential. This potential sets the upper limit or maximum performance achievable by a fuel cell. 

The theoretical OCV has the same value as the reversible eell potential. However, even when no current is drawn from a fuel cell, there is irreversible voltage loss, which means that the actual values of the OCV are always lower than the theoretically expected values. To date, a quantitative explanation for such OCV behavior has not been clear in the literature. One explanation attributes this behavior to H2 crossover and/or internal current, as described in the fuel cell book written by Larminie and Dicks [26]. A mixed potential [121-124] has also been widely used to interpret the lower OCV. The combined effects of fuel crossover, internal short, and parasitic oxidation reactions occurring at the cathode are the source of the difference between the measured open circuit cell voltage and the theoretical cell potential. Therefore, the actual OCV is expressed as... 

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