System open circuit

Figure 2.40 - Particular working points (U, I) of an electrochemical system open circuit ( ) short circuit (M)...

Battery System Open-Circuit Voltage (V) Operating Voltage (V) Capacity (mAh) Voltage versus Time Response... 

System Open-circuit Discharge energy Wh/kg power W/kg ... 

Baaery system Open circuit voltage per cell (V) On-load voltage per-cell (V) Practical energy density Operating temperature CO... 

Integrated microelectromechanical systems Open circuit current (solar cell)... 

Fig. 1. The energy cycle of a thermal electric generating station having two alternative cooling systems (—) the open-circuit or once-through system and (-------------------------------------) a representative closed-cycle, cooling-tower system. Reproduced by permission (3).

Open circuit voltage is the voltage across the terminals of a cell or battery when no external current flows. It is usually close to the thermodynamic voltage for the system. 

Valve or non-linear resistor In this version, a nonlinear SiC resistance is provided across the gap and the whole system works like a preset valve for the follow current. The resistance has an extremely low value on surge voltages and a very high one during normal operations to cause a near-open circuit. It is now easier to interrupt the follow currents. 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

Typical dimensions for the /5-alumina electrolyte tube are 380 mm long, with an outer diameter of 28 mm, and a wall thickness of 1.5 mm. A typical battery for automotive power might contain 980 of such cells (20 modules each of 49 cells) and have an open-circuit voltage of lOOV. Capacity exceeds. 50 kWh. The cells operate at an optimum temperature of 300-350°C (to ensure that the sodium polysulfides remain molten and that the /5-alumina solid electrolyte has an adequate Na" " ion conductivity). This means that the cells must be thermally insulated to reduce wasteful loss of heat atjd to maintain the electrodes molten even when not in operation. Such a system is about one-fifth of the weight of an equivalent lead-acid traction battery and has a similar life ( 1000 cycles). 

The zinc electrode is probably the most widely used metallic negative. The material is relatively cheap, has a good electrochemical equivalent (820 Ah/kg), and shows high open-circuit voltages (OCVs) in most systems (Table 1). 

Table 1. Open-circuit voltages of battery systems with zinc anodes...

PCT diagrams of AB2 (electrode alloys) /H systems reflect multiphase or nonideal behaviour [54], This is illustrated in Fig. 19, in which both the equilibrium pressure and the open—circuit equilibrium voltage, Er are plotted for Zr()5Ti05 Vo.sNij Fe02Mn02. 

Table 4.2 lists the same catalytic systems but now grouped in terms of different reaction types (oxidations, hydrogenations, reductions and others). In this table and in subsequent chapters the subscript D denotes and electron donor reactant while the subscript A denotes an electron acceptor reactant. The table also lists the temperature and gas composition range of each investigation in terms of the parameter Pa/Pd which as subsequently shown plays an important role on the observed r vs O global behaviour. Table 4.3 is the same as Table 4.2 but also provides additional information regarding the open-circuit catalytic kinetics, whenever available. Table 4.3 is useful for extracting the promotional rules discussed Chapter 6. 

Figure 5,14. Dependence of Ow-Or on catalyst potential eUWR for the systems (a) Pt(W)-Au(R) and (b) Pt(W)-Ag(R) at T=400°C. Open symbols Open-circuit operation in 02-He mixtures. Filled symbols Closed circuit operation at po2=12 kPa.32 Reprinted by permission of The Electrochemical Society.

F/gwre 5 JO, (a) Complex impedance spectra (Nyquist plots) of the CH4,02) Pd YSZ system at different Pd catalyst potentials. Open circuit potential U R =-0.13 V. Dependence on catalyst potential of the individual capacitances, C4i (b) and of the corresponding frequencies, fmii, at maximum absolute negative part of impedance (c).54 Reprinted with permission from Elsevier Science. 

Figure 5.44. In situ SERS spectra69 of oxygen adsorbed on Ag/YSZ at 300°C under (a) open circuit conditions and with die cell operating in the potentiostatic mode with (b) UWR = -2 V and (c) Uwr = +2 V. Spectra (b) and (c) were obtained after the system had reached steady state, w = 200 mW, photon counter time constant, x = 2 s, ssw = 2 cm l. Reprinted with permission from WILEY-VCH.

Figure 7.8. (a) Dependence of (Ptr RfAg) on potential UWR for the system Pt(W)-Ag(R) exposed to H2-He mixtures (open-symbols, Ph2 varying between 0.53 and 0.024 kPa) and H2-02 mixtures (filled symbols, p02=12 kPa, Ph2 varying between 0.28 and 7.8 Pa) open-circuit operation, T=673 K, Au counter electrode, (b) Work function of working (W) and reference (R) electrode as a function of open-circuit potential UWR.21 Symbols and conditions as in figure 7.8a. Diamonds show the literature41 values of 0,w(pt) and o,R(Ag)- Reprinted with permission from The Electrochemical Society. 

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