Degradation Mechanisms of Platinum Nanoparticle Catalysts in Proton Exchange Membrane Fuel Cells: The Role of Particle Size

Abstract

Five membrane-electrode assemblies (MEAs) with different average sizes of platinum (Pt) nanoparticles (2.2, 3.5, 5.0, 6.7, and 11.3 nm) in the cathode were analyzed before and after potential cycling (0.6 to 1.0 V, 50 mV/s) by transmission electron microscopy. Cathodes loaded with 2.2 and 3.5 nm catalyst nanoparticles exhibit the following changes during electrochemical cycling: (i) substantial broadening of the size distribution relative to the initial size distribution, (ii) presence of coalesced particles within the electrode, and (iii) precipitation of submicron-sized particles with complex shapes within the membrane. In contrast, cathodes loaded with 5.0, 6.7, and 11.3 nm size catalyst nanoparticles are significantly less prone to the aforementioned effects. As a result, the electrochemically active surface area (ECA) of MEA cathodes loaded with 2.2 and 3.5 nm nanoparticle catalysts degrades dramatically within 1000 cycles of operation, while the electrochemically active surface area of MEA cathodes loaded with 5.0, 6.7, and 11.3 nm nanoparticle catalysts appears to be stable even after 10 000 cycles. The loss in MEA performance for cathodes loaded with 2.2 and 3.5 nm nanoparticle catalysts appears to be due to the loss in electrochemically active surface area concomitant with the observed morphological changes in these nanoparticle catalysts.

In-Operando Anomalous Small-Angle X-Ray Scattering Investigation of Pt3Co Catalyst Degradation in Aqueous and Fuel Cell Environments

“…20 The similar growth for nanoparticles of significantly different size is not consistent with what would be expected if all changes were due to the effect of particle size (Gibbs-Thomson) on Pt dissolution rates. [14][15][16] In order to understand this discrepancy another experiment was performed on a Pt catalyst with an initial mean diameter similar to that of the Pt 3 Co catalyst. The results of the Pt PSD evolution and the resulting GSA loss and growth in mean diameter are shown in Figure 6 for up to 2000 triangle potential cycles in an aqueous cell.…”

“…This minimal amount of degradation (less than 0.2 nm growth in the mean diameter, as was seen for a Pt catalyst with an initial mean diameter of ∼3 nm) 20 was expected for the Pt catalyst with an initial mean diameter of ∼5 nm under the cycling conditions and over the number of cycles due to the particle size effect yielding a smaller Pt dissolution rate as the PSD increases. [14][15][16] Through in-situ SAXS in an aqueous environment, Yu et al showed limited growth in the mean diameter of a 30 wt% Pt/C with an initial mean diameter of ∼6 nm. 53 They reported a mean diameter increase of ∼0.06 nm after 1800 triangle cycles between 0.56 V and 1.16 V at 50 mV/s; a cycling regime more degrading than the one used here due to the higher upper potential limit (1.16 vs. 1.0 V).…”

“…5,6,[11][12][13] Some studies suggest that alloying has a positive effect while others show no significant impact on stability. It is known that relative stability is dependent on particle size, [14][15][16] the larger the initial particle size the more stable, and the majority of Pt alloy synthesis techniques are based on high temperature annealing processes which result in larger initial particle sizes. Therefore any comparisons between larger annealed Pt-alloys and typical commercial Pt catalysts with smaller mean particle sizes are not straightforward.…”

“…14,[18][19][20][21][22] A comprehensive discussion of the ASAXS method and its use in characterizing carbon-supported metal catalysts can be found in several publications and textbooks. [23][24][25][26][27][28][29] One advantage this technique has is its ability to obtain element specific PSDs.…”

Long-Term Stability of Nanostructured Thin Film Electrodes at Operating Potentials

“…7 It is well known that Pt/C catalysts can also be stabilized by growing them to diameters above ∼5 nm, but at the expense of decreased mass activity. 8 Compared to nano-Pt/C catalysts, NSTF ternary catalysts have also exhibited superior stability and 7x longer lifetime in accelerated load cycles that consisted of current density varying between 0.02 A/cm 2 , as at near open circuit voltage, and 1 A/cm 2 in H 2 -air at 80…”