Cell voltage thermal cells

Corresponding to the charge in the potential of single electrodes which is related to their different overpotentials, a shift in the overall cell voltage is observed. Moreover, an increasing cell temperature can be noticed. Besides Joule-effect heat losses Wj, caused by voltage drops due to the internal resistance Rt (electrolyte, contact to the electrodes, etc.) of the cell, thermal losses WK (related to overpotentials) are the reason for this phenomenon. 

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. 

A single cell produces a voltage of about 1 V. To obtain higher voltages the cells are connected in series to form a stack. Furthermore, heat rejected in the process can be used for different on-site thermal consumption, which makes the... 

In many cases, it will be impossible to prevent unwanted reactions at the counter electrode. Then a separation of the anolyte and catholyte is needed. An optimal compromise has to be found for the separator between separation effectiveness and ion conductivity, that is, minimized electrical resistance and low energy consumption. Moreover, chemical, thermal, and mechanical stability and price of the separator have to be considered. Naturally, a complete separation is impossible, because a slight diffusion rate is inevitable. In laboratory scale experiments, probably a high cell voltage is acceptable in order to realize a maximal separation. 

The efficiency of an actual fuel cell can be expressed in terms of the ratio of the operating cell voltage to the ideal cell voltage. The actual cell voltage is less than the ideal cell voltage because of the losses associated with cell polarization and the iR loss, as discussed in Section 2.1.2. The thermal efficiency of the fuel cell can then be written in terms of the actual cell voltage. 

It has been observed that solid oxide fuel cell voltage losses are dominated by ohmic polarization and that the most significant contribution to the ohmic polarization is the interfacial resistance between the anode and the electrolyte (23). This interfacial resistance is dependent on nickel distribution in the anode. A process has been developed, PMSS (pyrolysis of metallic soap slurry), where NiO particles are surrounded by thin films or fine precipitates of yttria stabilized zirconia (YSZ) to improve nickel dispersion to strengthen adhesion of the anode to the YSZ electrolyte. This may help relieve the mismatch in thermal expansion between the anode and the electrolyte. 

IFC has been marketing the PC25, a 200 kW atmospheric PAFC unit, since 1992. Details of this commercial cycle are proprietary and not available for publication. In order to discuss an example PAFC cycle, a pressurized (8 atm) 12 MW system will be presented (50). This cycle is very similar to the 11 MW IFC PAFC cycle that went into operation in 1991 in the Tokyo Electric Power Company system at the Goi Thermal Station, except that two performance enhancements have been incorporated. Limited data are available regarding the Goi power plant. However, it is understood that the average cell voltage is 750 mV and the fuel utilization is 80% (51). The enhanced 12 MW cycle presented here utilizes values of 760 mV and 86%. This enhanced cycle (Figure 9-8) is discussed below with selected gas compositions presented in Table 9-6. 

The deviation of the flux from a purely radial configuration can lead to several consequences on the operational conditions of the fuel cell, thus affecting the resultant performance. First of all, if the gas is not well distributed within the cell surface, the reaction rate varies from area to area. This implies the existence of preferred zones for the electrochemical reaction, and, consequently, of local high current density, thus reducing the overall cell voltage. Secondly, different reaction rates throughout the cell causes temperature gradients and, consequently, thermal stresses, which can cause mechanical failure of the cell [2-4], Finally, the existence of (some) preferred zones for the electrochemical reaction implies that part of... 

It is important to improve the endurance and the reliability of the electrolyte plate for the commercialization of MCFC. The electrolyte loss from the matrix plate increases the cell resistance and deteriorates the cell voltage. The formation of cracks in the electrolyte plate causes a gas cross-leakage between the fuel gas and the oxidizer gas. The pore structure of the matrix plate must be stable and fine to support liquid electrolyte under MCFC operation. It is necessary to prevent the formation of cracks in electrolyte plates during thermal cycling. Because of the immediate and large... 

The theoretical efficiency in such systems is close to unity, but in practice, the practical efficiency is significantly lower. This is attributable to the different mechanical and thermal losses in the system but also to the direct chemical reactions between the reactants and secondary electrochemical reactions in the cell. The voltage efficiency of the cell, q, is defined as the quotient between the cell voltage, V. at a given cell current, I, and the cell voltage at open circuit, (the maximum value of the cell voltage, equivalent, if the cell is in equilibrium, to the reversible cell potential, Ej) ... 

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