Tafel slope operating conditions

This is an area of much practical importance for research and development in electrolyzer technology more work is currently required for elucidation of the behavior of high-area porous and composite electrode materials with regard especially to the values of Tafel slope and conditions under which low b values can be achieved for H2, O2, and CI2 evolution reactions, thus minimizing activation overpotential energy losses in high current-density operations. 

There are very few studies of the ORR under hot and dry fuel cell operating conditions. Recently, methods have been devised to separate the mass transport effects from the kinetic effects [74, 75], but none of these have been applied to hot and dry fuel cell operation. These studies showed that, under fully humidified conditions up to 70°C, oxygen reduction had a tenfold higher specific performance for platinum black at 0.90 V compared to Pt on carbon as has been previously reported in the literature [76]. However, this significant benefit of platinum black is shown to rapidly decrease when the potential is shifted to lower, more fuel cell relevant potentials. This is manifested in the Tafel slope, which decreased from 360 to 47 mV/decade in the region where the overpotential was <0.35 V. The effect of hot and dry conditions has been studied in a 5 cm MEA where mass transport and kinetics are difficult to separate [73]. At 120°C, the Tafel slope is found to increase inversely with RH. It is speculated that this is due to the decrease in ionic conductivity in the electrode. RH can also influence water oxidation to form Pt-OH and Pt-O and thereby change the surface condition of the platinum crystals. 

A polarization curve, i.e., an 1-V curve of a fuel cell stack under certain operating conditions, is frequently used to describe fuel cell performance and determine performance decay with time. Polarization curve analysis can provide the most important kinetic parameters, such as the Tafel slope, exchange current density, cell resistance, limiting current density, and specific activity of Pt. A single fuel cell potential can be described by ... 

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