Current densities nanostructured catalysts

In this chapter, the authors review some developments in the application of Pd-based nanostructures for the electrocatalytic oxidation of alcohols in alkaline media. Special focus is given to the role of such nanostructures (mono-, hi-, or ternary metallic catalysts) and/or their supporting platforms in lowering the anodic overpotential (onset potential), enhancing the catal3dic current density, and improving the stability or lifetime of the catalysts. 

Figure 19.8 illustrates the effect of hold time at each current density point in MEA tests using different nanostructured thin film (NSTF) MEAs, showing increased MEA mass activity at 900 mV using 5 s holds (matching RDE values more closely) compared with lower MEA mass activity using 1,050 s holds at each point in the polarization curve [13] similar variations in activity between the MEA and RDE techniques may also be expected for core-shell catalyst materials. 

In order to be able to properly examine the inherent activity of minute amounts of OER catalysts, one needs a substrate with minimal interference, extremely slow OER kinetics of its own and extraordinary stability at high positive electrode potentials. The unique featiues of 3M s Pt-NSTF (nanostructured thin film) catalyst [12] such as superior durability, electrochemical inertness at high potentials, and the absence of corrosion interference due to exposed carbrui, made it a logical choice as a support [13, 14]. It is well known that pure platinum has a high overpotential for OER. For instance, at a current density of 1 mA/cm, the OER on platinum proceeds at a potential that is 0.47 V higher than oti single crystal ruthenium oxide [15]. Thus, the OER partial current density oti the Pt-NSTF substrate wiU be orders of magnitudes lower than on ruthenium, iridium, and other similar OER-active materials. 

So, under such a circumstance, only when the combined thickness of the GDM and the CL is around 0.11 pm will the diffusion of O2 be enough to support a current density of 1.5 A cm Therefore, under a completely flood condition, an actual fuel cell with a combined thickness of the GDM and the CL of around 300 pm cannot generate a current density of 1.5 A cm" at all. Even for a nanostructured thin film electrode structure developed by 3M with a catalyst layer thickness as thin as 0.5 pm and in the absence of any diffusion medium, it is not possible to provide such a current density if the electrode is completely flooded. 

Figure 2.18. Tafel plots for ORR in 0.5 M H2SO4 on different catalysts bjcdi Tafel slope at low current density bhcd Tafel slope at high current density [47]. (With kind permission from Springer Science+Business Media Journal of Applied Electrochemistry, Electrocatalytic hehaviour for oxygen reduction reaction of small nanostructured erystaUme himetaUic Pt-M supported catalysts, 36, 2006, 1143-1149, A. Stassi, Figure 9, Springer.)...

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