Electrocatalyst decay protocols were used to accelerate cathode performance loss for Pt catalysts. Four electrodes with average platinum particle sizes of 1.9, 3.2, 7.1 and 12.7 nm were evaluated to elucidate the impact of particle size on initial performance and subsequent decay, when subjected to identical potential cycles. The decay rates of Pt electrochemical surface area (ECA) and mass activity (i m ) were significantly greater for 1.9 and 3.2 nm Pt-on-carbon catalysts (Pt-C) compared to 7.1 nm Pt-C, which was stable for 10,000 potential cycles. As expected, the performance decay rate of the electrodes with the smallest Pt particle size was the highest and that of the largest Pt particle size was lowest. However, the initial performance of the largest Pt particle size electrode was significantly lower. Thus, a Pt particle size was identified that balanced performance and durability. The relative impact of operational conditions, such as relative humidity, cell temperature and upper potential limit on 3.2 nm Pt electrodes was also evaluated. Highest decay rates were found when the cathode was subjected to a higher upper potential limit. The decay was attributed to a combination of Pt dissolution, particle growth and carbon support corrosion.

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.

The increase in oxygen binding energy was previously proposed to account for the lower oxygen reduction activity of a Pt monolayer supported on Au(111) single crystal than that on Pd(111) and pure Pt(111) surfaces. This single-crystal based understanding, however, cannot explain the new finding of a 1.6-fold increase of oxygen reduction activity on Pt monolayer-modified 3-nm Au nanoparticles (Pt/Au/C) in comparison with that on Pt/Pd/C with a similar particle size. The Pt/Au/C catalyst also has an activity higher than that of a state-of-the-art 2.8-nm Pt/C catalyst. Our new experimental results and density functional theory calculations demonstrate that a significant compressive strain in the surface of the core nanoparticles plays a role in the observed activity enhancement.

A carbon corrosion model is developed based on the formation of surface oxides on carbon and platinum of the polymer electrolyte membrane fuel cell electrode. The model predicts the rate of carbon corrosion under potential hold and potential cycling conditions. The model includes the interaction of carbon surface oxides with transient species like OH radicals to explain observed carbon corrosion trends under normal PEM fuel cell operating conditions. The model prediction agrees qualitatively with the experimental data supporting the hypothesis that the interplay of surface oxide formation on carbon and platinum is the primary driver of carbon corrosion.

Abstract:The initial performance and decay trends of polymer electrolyte membrane fuel cells (PEMFC) cathodes with Pt3Co catalysts of three mean particle sizes (4.9 nm, 8.1 nm, and 14.8 nm) with identical Pt loadings are compared. Even though the cathode based on 4.9 nm catalyst exhibited the highest initial electrochemical surface area (ECA) and mass activity, the cathode based on 8.1 nm catalyst showed better initial performance at high currents. Owing to the low mass activity of the large particles, the initial performance of the 14.8 nm Pt3Co-based electrode was the lowest. The performance decay rate of the electrodes with the smallest Pt3Co particle size was the highest and that of the largest Pt3Co particle size was lowest. Interestingly, with increasing number of decay cycles (0.6 to 1.0 V, 50 mV/s), the relative improvement in performance of the cathode based on 8.1 nm Pt3Co over the 4.9 nm Pt3Co increased, owing to better stability of the 8.1 nm catalyst. The electron OPEN ACCESSCatalysts 2015, 5 927 microprobe analysis (EMPA) of the decayed membrane-electrode assembly (MEA) showed that the amount of Co in the membrane was lower for the larger particles, and the platinum loss into the membrane also decreased with increasing particle size. This suggests that the higher initial performance at high currents with 8.1 nm Pt3Co could be due to lower contamination of the ionomer in the electrode. Furthermore, lower loss of Co from the catalyst with increased particle size could be one of the factors contributing to the stability of ECA and mass activity of electrodes with larger cathode catalyst particles. To delineate the impact of particle size and alloy effects, these results are compared with prior work from our research group on size effects of pure platinum catalysts. The impact of PEMFC operating conditions, including upper potential, relative humidity, and temperature on the alloy catalyst decay trends, along with the EMPA analysis of the decayed MEAs, are reported.