Open circuit voltage exchange current density

Owing to the high temperature, H2 oxidation is largely insensitive to the chosen anode materials. The exchange current densities of Co [13], Cu [15], Pt [16], Ir [16], Au [16], and Ag [16] were about 45, 69, 85, 27, 26, and 19 mA cm-, respectively at 923 K in Li-K carbonate melts. Considering the possible experimental error, these values indicate that the materials have a similar H2 oxidation rate at the anode. In addition, the oxidation potential of Ni in molten carbonate is about —0.802 V under a 1 atm O2 condition at 925 K [2]. Thus, Ni is metallic at open circuit voltage, and is not oxidized up to a certain potential. 

Electron transfer according to Eq. (18) occurs also at an open circuit when no current flow is observed through the electrode. The electrode then automatically attains a potential that is characterized by equal rates of the reaction in both directions as a dynamic equilibrium, and this equilibrium voltage (U°) is determined by the point at which the forward and reverse reaction rates are equal. Then the current flow in both directions is balanced which means i+(0) = — i (0) = io. This balancing current is called exchange current density (necessarily it is related to the surface area, therefore it is a current density given, for example, in units of mA/ cm ). 

The coefficient on the right-hand side of the equation is 61 mV. Now, assuming no external current is drawn, = 0, and a cathode exchange current density of 10 A/cm, the overpotential caused by current losses versus the internal current density is shown in Figure 5.26. For an internal current density of 1 mA/cm, the cell overpotential is 0.28 V. From the figure, we see a steep increase in overpotential at small internal current density. Thus, even if the cell current density to the load is zero, the open-circuit voltage of the cell is 0.92 V for an internal current density of 1 mA/cm. ... 

H2/O2 fuel cell performance was measured using PVA/PDDA membrane-based MEA (1 0.5 by mass). The AFC exhibits an open circuit voltage (OCV) of 0.81 V and a peak power density of 32.7 mW cm" at a current density of 72.9 mA cm" (Figure 10.11). Although it is much lower than some alkaline exchange membranes, TMA functionalized (LDPE-co-VBC) [130] and poly(ETFE-g-VBC) [131] membrane by introducing nonfluorinated or partially fluorinated groups such excellent... 

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