Durability of cathode catalyst components of PEM fuel cells

Proton-exchange membrane fuel cells (PEMFC) are electrochemical devices that convert a fuel with the aid of oxygen directly into electrical energy with high efficiency without being limited by the Carnot cycle. With hydrogen as the preferred fuel, which can in principle be produced from renewable feedstocks, fuel cells may become important devices for electricity generation for stationary, mobile and portable applications. Commercial implementation of PEMFCs for mobile applications requires bringing down the current high costs of this technology. A major contributor is the catalyst cost and especially the ORR (oxygen reduction reaction) electrocatalysts because of high Pt loadings. Besides the rather slow rate of oxygen reduction, Pt catalysts also suffer from limited stability under PEMFC operating conditions. Deactivation of the electrocatalyst is primarily influenced by the loss in electrochemical surface area for Pt catalysts. Pt dissolution at high potential followed by particle sintering due to Oswald ripening, coalescence and particle migration characterize the surface area and mass activity loss. For alloys, the dissolution of the non-noble metal also contributes to deactivation of the catalysts. Additionally, carbon support corrosion also plays a role. This thesis addressed the issue of durability of carbon-supported Pt-based ORR catalysts. Specifically, the potential benefit of non-noble metal alloying (Co, Ni, Cu) on the ORR activity and stability of Pt catalysts is investigated. To learn about the intrinsic properties of such alloys the work involved studies of electrodeposited PtM layers followed by studies of carbon corrosion and the activity and stability of carbonsupported alloys. The main electrochemical technique was cyclic voltammetry at room temperature and 80 °C. The ORR activity and durability of unsupported Pt and PtM alloys with respect to non-noble mental dissolution and Pt surface area (ECSA) loss was discussed in Chapters 2 – 6. Unsupported Pt and PtM alloys were prepared through electrodeposition because of the ease of preparation of alloys with a wide compositional variety. In general, an enhancement in the ORR activity was achieved for all the alloys when compared to Pt after 15 CV scans. The ECSA loss was found to be more substantial in these first scans for the non-noble metal-rich alloys. Further potential cycling led to similar losses in the ECSA for Pt and the alloys. Regarding non-noble metal dissolution, Co and Ni were found to be more resistant towards dissolution than Cu during the initial stages of potential cycling. However, at the end of 1000 CV scans, the amount of non-noble metal in the catalyst layer was around 15 atom% irrespective of the alloying element and the initial Pt:M ratio. The CV and XPS studies pointed to the formation of a Pt-enriched catalyst surface with the non-noble metals being in subsurface layers. In spite of having a similar catalyst surface, non-noble metal-rich alloys were found to be more stable towards potential cycling. In other words, the durability of the alloys at room temperature depends on the initial Pt:M ratio. Structural and elemental studies on the near-surface regions are necessary to understand these differences in more detail. The durability of the alloys studied at elevated temperature (Chapter 6) revealed that the PtM alloys maintained their enhanced ORR activity even after 1000 potential cycles. However, no difference in the ORR between the Pt-rich and non-noble metal rich alloys was found. Nevertheless, PtNi was found to be the most durable among the alloys followed by PtCo and PtCu. On the issue of non-noble metal dissolution, the alloys still retained about 15-20 atom% non-noble metals, even after extensive potential cycling. The investigation of the influence of chloride ions on the ORR activity and durability of Pt and PtNi alloys described in Chapter 3 shows that a chloride ion concentration as low as 5 ppm is sufficient to poison the catalyst and reduce the ORR activity by several orders of magnitude. However, among the catalysts studied in chloridecontaining electrolyte, Pt10Ni90 was found to be the most active one. Chloride ions, even in minute quantity, were found to accelerate Ni dissolution. To examine whether the enhanced durability of the unsupported alloys can in principle be useful for the development of actual fuel cell catalysts, Pt and PtM alloy nanoparticles were prepared on a carbon support and annealed at different temperatures (Chapter 7). The effect of particle size and the alloying element is discussed in this chapter. Non-noble metal rich alloys exhibited the highest activity at room temperature after initial dealloying. The electrocatalytic activities of the fresh alloys were found to be dependent on the particle size, alloying element and nonnoble metal concentration. Nonetheless, after 1000 potential cycles at 80 °C with almost complete dissolution of non-noble metal, the activities of the alloys were quite similar to that of Pt. Besides, the aged catalysts showed only a modest dependence on the particle size. Comparing electrodeposited and carbon supported alloys, it is noted that in both cases room temperature Cu dissolution is rapid as compared to Co and Ni. Also, an enhancement in the ORR activity was achieved for the alloys. However, unlike supported alloys, the electrodeposited layers were able to retain their enhanced activity after the durability tests. This could be related to the amount of non-noble metal retained: electrodeposited alloys retained about twice as much of the non-noble metal than the supported ones. To conclude with, the significance of PtM alloys as an alternative to Pt/C relies on how to retain a considerable amount of non-noble metal in the catalyst. The last part (Chapter 8) of this thesis deals with the electrochemical corrosion behavior of various commercial carbon supports at elevated potential (1.2 V) and temperature under potentiostatic conditions by employing on-line electrochemical mass spectrometer (OLEMS). The corrosion rate of the carbons decreased with time. The CVs revealed that the onset potential of carbon oxidation and CO2 evolution shifted towards higher values after the potential hold experiments again confirming the resistance of carbon towards corrosion. The carbon weight loss was found to be depending on their BET surface area. The BET-surface area normalized weigh loss is similar for all the carbons, which indicate that the corrosion behavior of these carbon supports is quite similar.