Nernst voltage

Fig. 2.8 Local resolution of the Nernst voltage and the molar hydrogen fraction.

In Case B, the hydrogen flow rate is chosen with 2.72-10-6 mobs to approximate a linear change in the molar hydrogen fraction from 0.7 at the entry to 0.299 at the outlet of the anode along the cell area. In this range of the molar fraction, the Nernst voltage changes approximately inversely proportional with the fuel utilisation. 

Even if the Nernst voltage, the overpotential, the current density and the power density change inversely proportional with the fuel utilisation, the local distribution... 

Reversible Nernst voltage at unit activity of reactants and products, V Thermoneutral voltage V... 

The effect of an ambiguous pumping current characteristic at 7<1 can be avoided by exposing the anode to a reference atmosphere, favorably to ambient air (Fig. 17.15.9) like a potentiometric cell generating a superimposed Nernst voltage,... 

VN, which is compared with an applied bias voltage of typically VB=0.5 V. The pumping voltage, VP, is determined by the difference between the bias and the Nernst voltage... 

Because the maximum Nernst voltage varies in the range 0-1 V and is 0.5 V at 7=1, we can distinguish three cases... 

The Nernst voltage, V, is, in SSE, a theoretical quantity, having the dimension of voltage, that reflects a chemical potential difference. For a given chemical component, X, with mobile ions of charge Zion. Vth is defined as. 

Relations Between Open-Circuit Voltage and the Nernst Voltage... 

The argumentation in the SSE approach would be as follows There is a selective membrane for Cl ions, that is, an electrolyte, made of two parts connected in series AgCl and the LE. The Nernst voltage is determined by AfiiCli) according to Eq. (9),... 

Vth equals Vqc of Eq. (22) since the electronic component in the combined electrolyte AgCl/LE vanishes (see Eq. (16a)). This demonstrates that the two definitions of the Nernst voltage are indeed equal when the membrane is a pure ionic conductor. This would not be so if electronic conduction would be present (as can be the case in SSE), as then V(h 7 l oc (Eq. 16a). Different values for the Nernst voltage defined as Voc rather than V(h would be obtained also when the LE contains variable charge-mobile ions, for example, Cu" " and Cu++, as then Voc as can be... 

The difference in the Nernst voltages calculated by using the partial pressures of the reacting gas species at the bulk and the TPB gives the concentration overvoltage. The reversible Nernst voltage of the cell is represented in terms of the species partial pressures and temperature as... 

Figure 10.7 shows the reversible cell voltage as a function of the FU with the system pressure as the parameter. From these curves, it is evident that the reversible cell voltage decreases with the increase in the FU and also that increasing system pressure results in increased Nernst voltage. However, this increase is quite small. 

The codes, based on the integration of the electrode, cell, and stack models, allow the calculation of the maps of the current density, Nernst voltage, polarizations, internal resistance, gas and solid temperatures, pressure drops, and the compositions and flow rates of the gaseous streams. 

The cell voltage Tceii is obtained by calculating the Nernst voltage liNemst and subtracting the losses caused by ohmic resistance R, activation overpotential tjact. and mass transport limitations t trans- The nomenclature follows that used in Section 29.2 (see Table 29.2). The following assumptions were made in order to discuss a very simple model ... 

Fig. 2.4 The calculated Nernst voltage Eff as a function of the fuel utilisation Uf.

In this equation, the Nernst voltage loss term represents the voltage loss owing to the variation in reactant and product gas concentrations in the supply gas stream. If we just have only single reactant specie for one electrode side, the Nernst voltage is given as... 

Concentration overpotential or loss is estimated as the change in Nernst voltage loss owing to the variation of reactant gas concentration from the bulk gas flow stream to the gas concentration at the electrode reaction surface as follows ... 

Maximum Expected Open-Circuit Voltage Nernst Voltage 107... 

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