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

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

Determination of the potentiostatic stability of PEMFC electro catalysts at elevated temperatures

The electrochemical stability of platinum on carbon catalyst (Hispec TM 4000, Johnson Matthey) has been investigated predominantly at constant potentials ranging from 0.95 to 1.25 V at elevated temperatures. By combining a quartz crystal microbalance (QCM) with electrochemical techniques, dynamic insight is obtained on the oxidation and corrosion of both platinum and carbon during potentiostatic hold. From the cyclic voltammetry (CV) data, it can be concluded that at all conditions, the platinum surface area decreases when Pt on carbon catalysts are exposed to a constant potential of 1.05 to 1.25 V. Under the applied conditions, this loss of surface area is primarily caused by the dissolution of platinum. Both the QCM as well as on-line electrochemical mass spectrometry (OLEMS) experiments show that the corrosion of carbon is catalysed by the presence of platinum at 80 °C, as long as the platinum surface is not passivated by an oxide layer. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Influence of chloride ions on the stability of PtNi alloys for PEMFC cathode

The dependence of the rate of Ni dissolution from PtNi alloys on the chloride concentration was studied electrochemically in 0.5 M HClO4 at room temperature. Electrodeposited PtNi catalysts were subjected to extensive potential cycling between 20 mV and 1.3 V at various Cl- concentrations and the cyclic voltammograms (CV) response and the oxygen reduction reaction (ORR) activity of the catalysts were determined at different intervals. Energy dispersive X-ray spectroscopy (EDS) and inductively coupled plasma spectroscopy (ICP) analyses were carried out to determine the elemental composition of the alloys and the amount of dissolved Ni at different stages of the potential cycling. It was found that the presence of Cl- increases the rate of Ni dissolution and by this accelerates the dealloying process relative to potential cycling in chlorine-free solutions. Dealloying is most pronounced during the initial stages of potential cycling. Already a small amount of Cl- is sufficient to dissolve the majority of the non-noble metal from the alloys. Even then, under oxygen reduction conditions, the blockage of Pt surface by Cl- is less pronounced for the alloys than for pure Pt catalysts, leading to marginally improved ORR activity for the PtNi alloys at low Cl- concentrations. From a practical point of view, the effect of chloride ion leakage from a commercially available saturated KCl reference electrode on the electrocatalytic activity was also investigated

Oxygen reduction kinetics on electrodeposited PtCo as a model catalyst for proton exchange membrane fuel cell cathodes: Stability as a function of PtCo composition

PtCo catalysts with composition varying between Pt80Co20 and Pt10Co90 were prepared by electrochemical underpotential codeposition. The bimetallic catalysts were subjected to 1000 electrochemical cycles in 0.5 M HClO4 at room temperature. The activity and stability of these electrodes for oxygen reduction was determined, in conjunction with the characterization of these catalysts with energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. Although Pt-rich electrodes had better activity in the initial stages of potential cycling, higher Co atomic ratios led to higher stability and higher oxygen reduction reaction (ORR) activity after electrochemical cycling. Pt10Co90 turned out to be the best electrode among the alloys considered, in terms of ORR activity and stability, which is linked to a higher concentration of Co on the surface