Durability of PEM Fuel Cells

Durability is a fundamental and necessary feature for PEM fuel cells to see a wide diffusion as a practical power source in any application field. The requirements for fuel cell lifetime vary with the specific application, in particular a duration of at least 5000 h is mandatory for use on cars, while even longer periods are needed for bus and stationary employments (for most applications an acceptable degradation rate is considered to be comprised in the range 2-10 pV/h [51]). However, the wide variability of operative conditions usually encountered in automotive applications, such as dynamic driving cycles, startup/shutdown phases, and freeze/thaw, makes also the target for car very difficult to be met with the current technologies. 

As discussed before the amount of platinum catalyst used in electrocatalysts of a PEM fuel cell is supported on carbon in the form of nanoparticles, in order to 

Also corrosion problems of the carbon support have been considered as a cause of electrocatalyst durabihty loss [32], in particular carbon oxidation can occur through electrochemical oxidation at the cathode, with formation of CO2 (C -I- 2H2O = CO2 -I- 4H -F 4e ), or through water gas shift reaction, with the production of CO (C H2O = CO H2). Both these routes are catalyzed by Pt [56, 57] and subtract caibon useful for platinum loading, with consequent metal sintering and decrease of the electrochemical surface area [58]. 

Since Pt dissolution is favored by high electrode potential, relative humidity, and temperature, the possibility to limit the risk of electrocatalyst aging is based on the use of Pt-alloy catalyst instead of pure platinum, at least for the cathode, which is characterized by higher potential with respect to anode, and by adoption of operative conditions not too severe in terms of humidity and temperature. While this last point requires interventions on the membrane structure, the study of catalyst materials has evidenced that a minor tendency to sintering can be obtained by the addition of non-noble metals, such as Ni, Cr, or Co, to the Pt cathode catalyst [59, 60], suggesting a possible pathway for future work. On the other hand also the potential application of non-platinum catalysts is under study, in particular transition metal complexes with structures based on porphyrines and related derivatives have been proposed to substitute noble metals [61], but their activity performance is still far from those of Pt-based catalysts. 

The carbon corrosion issues are faced by the study of different carbonaceous supports, such as carbon black or carbon nanostructures. Recent results evidence the superiority of graphitized carbon with respect to amorphous carbon black in terms of corrosion resistance, and the promising characteristics of carbon nanocages when Pt sintering effects are considered [62]. 

---

Another critical issue regarding durability of PEM fuel cells is the reliability of the electrolytic membrane, which can undergo mechanical, thermal, and chemical/ electrochemical degradations [63]. The mechanical degradation consists in different types of failure (cracks, pinholes, perforations) which are favored by... 

A review of the main parameters influencing long-term performance and durability of PEM fuel cells. J. Power Sources, 180, 1-14. 

Hinds G. Performance and durability of PEM fuel cells a review. Teddington, UK National Physical Laboratory 2004. NPL Report No. DEPC-MPE 002. 

In order to establish a fundamental understanding of contamination mechanisms, develop effective contamination control strategies, and then improve the reliability/durability of PEM fuel cells, it is important to recognize the sources and their possible effects on fuel cell operation. In this chapter, we review the origins of contaminants and their chemistries, with some description of their effects on PEM fuel cell operation. 

Many impurities other than those discussed here may also affect both the anode and the cathode reactions in fuel cells. Gaseous impurities are known to be the most serious factors (see also chapters Air Impurities and Performance and Durability of PEM Fuel Cells Operating with Reformate ) at the fuel cell anode and the cathode. In this chapter, those impurities were excluded from discussion, and only the impacts of cationic and organic substances (that may occur as water-soluble species) on the ORR were considered, but the results indicated that these contaminants were equally serious problems for fuel cell degradation. 

---