Open circuit voltage mixed potential effect

The theoretical cell voltage of the reaction is 1.214 V according to the free Gibbs energy of the reaction of ArG = —702.5 kJ/mol. In reality various effects lead to a reduction of the achievable open circuit voltage. The formation of CO-species as intermediate at the anode [12, 13] and the methanol permeation through the membrane are well known causes for reduced power density and the formation of mixed potentials. Special anode catalysts for lower sensitivity towards... 

Unexpectedly, however, Voc depends on stoichiometry with the growth of A, Voc increases (Figure 4.26(b)). At A = oo, the cell open-circuit voltage reaches the value where E < 100 mV is the voltage loss under open-circuit conditions (see below). The effect of V(,c lowering due to methanol crossover (mixed potential) is well known in DMFC studies. The model above allows us to explain the dependence of OCV on feed molecule stoichiometry A. 

Polarization curves of protCHi exchange membrane fuel cells deviate from the simulated curve in Fig. 3a. Most significant is the low open-circuit potential of 0.9-1.0 V, as opposed to the reversible potential of approximately 1.2 V. The low open-circuit potential has a number of contributing factors, which include multiple reactions that set up a mixed potential, crossover of H2 or O2 through the membrane, and finite resistance effects of voltage measurement devices. These effects cannot be modeled with the overpotential method discussed here. Nonetheless, it is interesting to see how well the overpotential model can approximate a real curve. 

The theoretical OCV has the same value as the reversible eell potential. However, even when no current is drawn from a fuel cell, there is irreversible voltage loss, which means that the actual values of the OCV are always lower than the theoretically expected values. To date, a quantitative explanation for such OCV behavior has not been clear in the literature. One explanation attributes this behavior to H2 crossover and/or internal current, as described in the fuel cell book written by Larminie and Dicks [26]. A mixed potential [121-124] has also been widely used to interpret the lower OCV. The combined effects of fuel crossover, internal short, and parasitic oxidation reactions occurring at the cathode are the source of the difference between the measured open circuit cell voltage and the theoretical cell potential. Therefore, the actual OCV is expressed as... 

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