Pressure equilibrium cell voltage

By integration the dependence of the equilibrium cell voltage on the partial pressure of the dissolved gas (with the integration constant K equivalent to A%, [10]) is obtained. 

Eeq is the equilibrium cell voltage under the temperature and pressure working conditions... 

Electrochemical cells may consist of two electrodes of the same type, but with different concentrations of the electroactive species in the electrolyte. Such cells are known as concentration cells. For example, two platinum electrodes operate in two H+/H2 solutions of different activity, separated by a membrane. The equilibrium cell voltage is defined by Equation (21a). As the standard potential is the same for both electrode reactions, the measurable cell voltage will depend only on the activity ratios, Equation (21b). If in this system both electrolytes were in equilibrium with the same EE pressure, the measured E would respond linearly to the pH difference between the two electrolytes, Equation (21c) (i.e. a pH electrode). 

The gases are discharged to their respective storage reservoirs at pressure P. When P = 1 atm and T=25 °C, the required ( standard ) equilibrium cell voltage is E° = —1.229 V. Estimate the equilibrium cell voltage if feed and products are all at 100 atm. Assume that O2 and H2 are ideal gases and that the specific volume of water is negligible. 

In Example 13.12 we computed the effect of pressure on the equilibrium cell voltage for Reaction 13.BL. Now we wish to conduct the same operation under conditions where the water will enter the cell at 1 atm and the hydrogen will leave the cell at 1 atm but the oxygen will leave the cell at 100 atm. Everything else is exactly the same as in Example 13.12. What is the equilibrium cell voltage for this situation ... 

Chapters 7 to 9 apply the thermodynamic relationships to mixtures, to phase equilibria, and to chemical equilibrium. In Chapter 7, both nonelectrolyte and electrolyte solutions are described, including the properties of ideal mixtures. The Debye-Hiickel theory is developed and applied to the electrolyte solutions. Thermal properties and osmotic pressure are also described. In Chapter 8, the principles of phase equilibria of pure substances and of mixtures are presented. The phase rule, Clapeyron equation, and phase diagrams are used extensively in the description of representative systems. Chapter 9 uses thermodynamics to describe chemical equilibrium. The equilibrium constant and its relationship to pressure, temperature, and activity is developed, as are the basic equations that apply to electrochemical cells. Examples are given that demonstrate the use of thermodynamics in predicting equilibrium conditions and cell voltages. 

Figure 6.17 Composition of Ti02 reduced by Coulombic titration to TiOv plotted versus the corresponding equilibrium partial pressure of oxygen, determined by cell voltage, for a sample at 900°C. [Data taken from D-K. Lee, J-I. Jeon, W. Choi, and H-I. Yoo, J. Solid State Chem., 178, 185-193 (2005).]...

Therefore such sensors are called Nernstian sensors. As a reference air with defined humidity is used. In reducing gases that are in chemical equilibrium (e.g., H2, H2Oj CO, C02 water gas) the oxygen partial pressure is determined by the mass law constant Kv and this in turn depends on the temperature. In the case of H2,H20-mixtures the cell voltage is obtained by insertion of a temperature function of log Kp into the Nernst equation... 

Under theoretical cell voltage conditions, for both half-cell reactions (HOR and ORR) there is no net reaction. In other words, both half-electrochemical reactions are in equilibrium, and no net current passes through the external circuit. The cell voltage can be considered the OCV. At 25 °C, if the pressures of both H2 and 02 are 1 atm, the OCV should be 1.23 V. However, in reality the OCV is normally lower and an OCV of 1.23 V is never observed. This is due to the mixed potential at the cathode side, and hydrogen crossover from the anode side to the cathode side [22, 23], At 1.23 V, Pt is not stable so oxidation of Pt occurs ... 

Introducing Nemst s law for the equilibrium case, the simation when no current (and hence power) is delivered by the cell, the equilibrium ceU voltage under nonstandard conditions for a H2/O2 ceU in dependence of the respective reactant/ product concentration (partial pressures) can be expressed as ... 

In case of thermodynamic equilibrium of k phases of a system of k components, the cell voltage is related to the Gibbs energies of formation of the k phases, independent of their relative amounts at constant temperature and total pressure. The determinant formula turned out to be very convenient in this regard to calculate the emf E, especially in the case of multicomponent systems and complicated stoichiometric ratios ... 

In Example 13.12 we computed the effect of pressure on the reversible equilibrium steady-flow cell voltage for the Reaction 13.BL. We assumed that the specific volume of water was negligible. This is not rigorously true. Calculate the numerical value of the error caused by this assumption. Assume that Vwater = 1.00 L/kg, independent of pressure. Present your answer as... 

To predict the local polarisation in a full-scale cell or stack at any point, its dependence on composition, pressure, and temperature of the gas flowing in the gas channel contacting the electrode must be known. In a large cell, these bulk gas properties vary from one point to the next. Electrode polarisation or overpotential - the difference between the local potential of the electrode under load and the potential at open circuit (equilibrium potential) - is also a local quantity because it depends not only on the bulk gas composition but also on the current density. In a large cell the current is usually distributed nonuniformly, as discussed in Sections 11.2-11.5. Similar to Eq. 7, one can express the local cell voltage under load, i.e., when current is passed, as the thermodynamic cell potential minus three loss terms the ohmic loss, the cathode polarisation, and the anode polarisation ... 

Conversely, at maximum O content (M20i+j ) there is phase equilibrium with the external partial pressure of oxygen. Further oxydation would correspond to the evolution of oxygen. The result is that the cell voltage falls to zero if no hydrostatic pressure differences occur and P02 is also 1 bar on the other side ... 

The maximum electrical energy available from a fuel cell is determined by the Gibbs free energy difference across the electrolyte membrane, AG. This determines the equilibrium voltage of the cell, E, through the Nemst equation, which is nothing more than a restatement in electrical units of how AG (= 2FE) changes with pressure. 

The SOFC consists of cathode, electrolyte and anode collectively referred to as the PEN - positive electrode, electrolyte, negative electrode. A single cell operated with hydrogen and oxygen provides at equilibrium a theoretical reversible (Nernst) or open circuit voltage (OCV) of 1.229 V at standard conditions (STP, T = 273.15 K. i> = 1 atm). With the standard electrode potential E°, universal gas constant R. temperature T. Faraday s constant F, molar concentration x and pressure p, the OCV is given by... 

No bridge is again needed since the SE serves as the separating membrane. (For this cell to work properly and Pq should be kept below the equilibrium pressure of Zn/ZnCl2, otherwise the chloride ZnCl2 would be formed at the electrodes, and both Pcij-s would obtain the (same) equilibrium value and the voltage would vanish). In SSE the metal electrodes in concentration cells are, in many cases, inert. A typical example is the cell 02(Pi ),Pt SE(02-) Pt,02(Pi ). 

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