Reversible fuel cell potential

Effect of Operation Conditions on Reversible Fuel Cell Potential... 

The first and the second law of thermodynamics allow the description of a reversible fuel cell, whereas in particular the second law of thermodynamics governs the reversibility of the transport processes. The fuel and the air are separated within the fuel cell as non-mixed gases consisting of the different components. The assumption of a reversible operating fuel cell presupposes that the chemical potentials of the fluids at the anode and the cathode are converted into electrical potentials at each specific gas composition. This implies that no diffusion occurs in the gaseous phases. The reactants deliver the total enthalpy J2 ni Hi to the fuel cell and the total enthalpy J2 ni Hj leaves the cell (Figure 2.1). 

Reversible fuel cells are also the focus of much research. A fuel cell can be run backward to take an external source of electricity and turn it into hydrogen, which can then be stored and later used by the fuel cell to generate electricity. If it turns out to be more cost-effective than other means of storing electricity, such as batteries, this strategy could have a variety of potential applications in military and space missions. Reversible fuel cells could also help address one of the barriers to more widespread use of renewable electricity—its intermittency. For instance, excess electricity generated by windmills during windy times could be stored and used during less windy times. 

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-... 

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. 

A fuel cell uses the reverse process. Hydrogen along with oxygen from the air are applied to the cell. The hydrogen splits to release its electrons to the external circuit and provide power to the load. The protons move across the membrane, attracted by the oxygen potential, and combine with the oxygen to form water at the opposite electrode surface. 

Hydrogen is increasingly recognized as a potential fuel for industry and transport. It can be produced by electrolysis or photolysis of water, and its oxidation produces no greenhouse gases. Moreover, it is the best fuel for fuel cells, which generate electricity directly by the reversal of electrolysis. Fuel cells have been known for almost two centuries they are a type of battery in which electricity is produced by the redox reaction between H2 and O2 in solution. They offer high thermodynamic efficiencies... 

Because the entropy change for the H2/O2 reaction is negative, the reversible potential of the H2/O2 fuel cell decreases with an increase in temperature by 0.84 mV/°C (assuming reaction product is liquid water). For the same reaction, the volume change is negative therefore, the reversible potential increases with an increase in pressure. 

An analysis by Cairns and Liebhafsky (3) for a H2/air fuel cell shows that a change in the gas composition that produces a 60 mV change in the reversible cell potential near room temperature corresponds to a 300 mV change at 1200°C (2192°F). Thus, gas composition changes are more significant in high temperature fuel cells. 

The reversible potential of a fuel cell at temperature T is calculated from AG for the cell reaction at that temperature. This potential can be computed from the heat capacities (Cp) of the species involved as a function of T and from values of both AS° and AH° at one particular temperature, usually 298K. Empirically, the heat capacity of a species, as a function of T, can be expressed as... 

Figure 2-1 shows that the reversible cell potential for a fuel cell consuming H2 and O2 decreases by 0.27 mV/°C under standard conditions where the reaction product is water vapor. However, as is the case in PAFC s, an increase in temperature improves cell performance because activation polarization, mass transfer polarization, and ohmic losses are reduced. 

Table 6-4 Equilibrium Composition of Fuel Gas and Reversible Cell Potential as a...

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