Fuel cell irreversible heat loss

A cell that is operated infinitesimally close to electrochemical equilibrium (or open circuit conditions) will not produce any useful power output. To produce a significant power output, sufficient to propel a vehicle, for instance, the cell must be operated at a current density on the order of 1 A cm . Under load, the value of the current density jo of fuel cell operation determines the power output. The current density is directly related to reaction rates at catalyst layers, as well as flows of electrons, protons, reactants, and product species in the cell components. Each of these processes contributes to irreversible heat losses in the cell. These losses diminish the amount of electrical work that the cell could perform. 

FIGURE 1.4 Illustration of basic fuel ceU processes and their relation to the thermodynamic properties of a cell. The electrical work performed by the cell, corresponds to the reaction enthalpy, — A//, minus the reversible heat due to entropy production, —TAS, and minus the sum of irreversible heat losses at finite load, Qi. These losses are caused by kinetic processes at electrochemical interfaces as well as by transport processes in diffusion and conduction media. 

As discussed above, the electrochemical oxidation of a fuel can theoretically be accomplished at very high efficiencies (e.g. 96% for gas-phase product water or 83% for liquid product water for the H2/O2 reaction at 25 °C, see Fig. 8.3) as compared to heat engines utilizing the combustion of a fuel. However, in practice, fuel cells experience irreversible losses due to resistive and reaction kinetic losses (see Fig. 8.4), and efficiencies of fuel cell stacks rarely exceed 60% at rated load. The irreversible losses appear as heat and, for example, a 1 kW fuel cell operating... 

Fig. 1 Thennodynamic potentials and definitions of a hydrogen fuel cell, where the heat lost (Q) is composed of reversible and irreversible losses and the cell is operating at a potential V, the efficiency is also shown...

In fine, this additional electrical power will be transformed into heat which will be released into the enviromnent (the component itself to begin with). This time, the losses will be irreversible. In other words, even if we were to reverse the function and have the component operate in fuel-cell mode, it is not possible to recover these losses from the enviromnent. 

The heat liberated by these irreversible losses can be put to good use, to feed the component itself and satisfy all or part of the entropic heat requirement TASrev of the electrochemical reaction. It is for this reason that the electrical efficiency of an electrolyzer (a concept which we shall discnss later on) could be greater than that of a fuel cell, and even surpass 100% under certain considerations. Indeed, in a fuel cell, the irreversible losses, which are identical to those of an electrolyzer in terms of the nature of the phenomena involved, hamper electricity production. 

Steam cycle. This would be much simpler than the MCFC/GT-ST cycle and would also allow the fuel cell to operate at atmospheric pressure. This cycle, again, would likely be more expensive than the MCFC/GT cycle and be less efficient because of the irreversible losses in heat exchange. In the case of directly creating steam from the fuel cell exhaust, the fuel cell would replace the combustor. 

Irreversible losses cause a difference in the efficiency of reversible and real processes. These losses can be described and quantified by their irreversible entropy production. The consideration of the ohmic losses shows that the irreversible entropy production in a SOFC is smaller than in another low-temperature fuel cell. This is caused by the lower irreversible entropy production of the heat dissipated at a higher temperature. The effects of the irreversible mixing of reactants and products lead to an irreversible entropy production as well that reduce the cell voltage. The changes in the Nernst voltage can be understood by the analysis of the fuel utilisation. 

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