NSTF electrodes

Fig. 22.18 HAADF-STEM image with EDS Pt and Ir maps of NSTF electrode subjected to cell reversal conditions. EDS mapping showed that significant amounts of Ir precipitated within the MPL. Agglomerates of Ir/Pt particles were found in the membrane, while pure Ir particles were found in the MPL [47]...

From a fundamental point of view, a more interesting event occurs around 1.75 V. It appears that this is the voltage limit of the Ir-Ru catalysts, since beyond this voltage a rapid increase in ceU voltage takes place. STEM studies were performed on the end-of-Ufe MEAs for each loading and indicated that little to no Ru or Ir remains on the whisker surfaces once the critical 2 V limit was reached. Thus, dissolution is the key deactivation mechanism. Some Ir was found with Pt in the membrane near the NSTF electrodes, while most was present within the MPL as small, pure metallic nanoparticles (Fig. 22.18). 

Superior durability aspects of the NSTF electrode in comparison to carbon-support-dispersed catalysts are related to (i) its non-electron-conducting support that eliminates carbon corrosion that can cause significant increases in gas transport resistance and (ii) its bulk-like-Pt surface that is more resistant to Pt dissolution [67, 75, 76]. However, NSTF faces higher sensitivity to contaminants due to its lower Pt surface area [77] and other operational challenges related to its unique structure [78-81], notably its electrode thickness which is l/30th of conventional electrode. Figure 13.8... 

NSTF electrodes also may offer some benefits from a manufacturing standpoint thanks to their fabrication method which can be solely via dry roll-to-roll processes... 

This kind of local-current-density-dependent falloff is not observed on fresh NSTF electrodes, despite their lower Pt surface areas than those of dispersed-catalyst electrodes (about a factor of 6 lower at 0.15mgp/cm ) [114,115]. Debe ascribed this difference to a larger number of collisions per unit time (reaction attempts) of a given... 

Another interesting implication from the above result is that if the space between all the colunms is filled with ionomer completely to form a non-porous CL (NPCL) as shown in Figure 2.14, even though the oxygen permeability in Nafion is about 4 times as low as in water, oxygen should also have no problem to diffuse through a few nm thick ionomer film (Table 2.14). In such an ionomer-filled NPCL water and protons will transport through the ionomer film easily to achieve 100% catalyst utilization. Actually, the NSTF electrode made by 3 M (Debe 2006) is similar to such a CL, but it is about 250 nm thick. 

In general, thin catalyst layers will have the lower proton resistance. However, they may also fill up more easily with water as is, for example, the case with the very thin NSTF electrodes by 3M, which only seem to function well at non-saturated conditions. Also for start-up under freezing conditions, a thicker electrode, or at least a higher pore volume, seems to be an advantage as complete filling with ice is even more detrimental. 

Non-nano-sized Pt is not immune for oxide formation or dissolution, but the equilibrium potential of oxide formation is higher compared to nano sized particles. Pt black and NSTF electrodes have either much larger Pt particles or even a continuous phase Pt. This results in lower specific surface area but more stable activity as was convincingly demonstrated by the life-time studies carried out on NSTF electrodes by 3M [49]. 

Nanostructured Thin Film (NSTF) Electrode. Debe et al. [59, 60] employed sputter technology and deposited catalyst on a nanostructured thin film (NSTF). This NSTF is an oriented crystalline organic whisker. Perylene red (PR) is a highly useful organic material for growing the NSTF. To form an electrode, the... 

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