Theoretical fuel cell potential

Because AG and F are known, the theoretical fuel cell potential of hydrogen/oxygen can also be calculated ... 

In Table 15 are given some theoretical energy capacities [335] calculated by means of Eq. (129) and the data from Table 14. These data are based on the hypotheses that the fuel cell potential is the reversible potential, the Faradaic efficiency is 100% and the weight taken into the calculation is that of the re-... 

Table 1 Thermodynamics and theoretically reversible cell potential for fuel cell reactions... 

At 25 °C, the theoretical hydrogen/oxygen fuel cell potential is 1.23 Volts. This equation gives the reversible open circuit voltage of the hydrogen fuel cell. 

The theoretical hydrogen/oxygen fuel cell potential is 1.23 Volts. As temperature rises from room temperature to that of an operating fuel cell (80 °C), there is a small cell voltage decrease from 1.23 V at 25 °C to 1.18 V at 80 °C. Fuel cell voltage is in general the summation of the thermodynamic potential ENemst/ the activation overpotential ijact (from both anode and cathode overpotentials, i.e., i]act(cathode)- act(anode))/ and the ohmic overpotential ijohmic/ which can be expressed as... 

At 25°C, the theoretical hydrogen/oxygen fuel cell potential is 1.23 Volts. 

A fascinating point, especially to physical chemists, is the potential theoretical efficiency of fuel cells. Conventional combustion machines principally transfer energy from hot parts to cold parts of the machine and, thus, convert some of the energy to mechanical work. The theoretical efficiency is given by the so-called Carnot cycle and depends strongly on the temperature difference, see Fig. 13.3. In fuel cells, the maximum efficiency is given by the relation of the useable free reaction enthalpy G to the enthalpy H (AG = AH - T AS). For hydrogen-fuelled cells the reaction takes place as shown in Eq. (13.1a). With A//R = 241.8 kJ/mol and AGr = 228.5 under standard conditions (0 °C andp = 100 kPa) there is a theoretical efficiency of 95%. If the reaction results in condensed H20, the thermodynamic values are A//R = 285.8 kJ/ mol and AGR = 237.1 and the efficiency can then be calculated as 83%. 

The portion AQ = AH - AG = TAS of AH is transformed into heat. Ideal theoretical efficiencies % determined by the types and amounts of reactants and by the operating temperature. Fuel cells have an efficiency advantage over combustion engines because the latter are subdued to the Carnot limitation. High thermodynamic efficiencies are possible for typical fuel cell reactions (e.g., e,h = 0.83 (at 25°C) for H2 + I/2O2 -> H20(i)). The electrical potential difference between anode and cathode, = -AG/W(f, which is also called the electromotive force or open-circuit voltage, drives electrons through the external... 

As shown in Figure 18, the potential is almost proportional to the logarithm of H2 concentration diluted in air. When H2 is diluted in N2, the observed potential corresponds to the electromotive force of a H2-02 fuel cell, and in fact the EMF was as large as about 1.0 V with a theoretical slope of 30 mV/decade, as shown in the same figure. It has been shown that in the case of H2 diluted in air, the following electrode reaction, i.e., electrochemical oxidation of hydrogen (2) and electrochemical reduction of oxygen (3), are important. 

Celik, I., Pakalapati, S. and Salazar-Villalpando, M. (2005) Theoretical calculation ofthe electrical potential at the electrode/electiolyte interfaces of solid oxide fuel cells, Journal of Fuel Cell Science and Technology 2(4), 238-245. 

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