Cathodic durability

To improve the PEFC cathode durability, both materials and system approaches have been proposed in the last years. From the system point of view, improvements in cathode durability can be achieved by minimizing the residence time of the hydrogen-air front in the anode compartment. On the materials side, Pt alloys have shown higher durability compared relative to Pt-based electrodes [34]. However, since transition metal reduction potential is below that of hydrogen. 

Figure 6 shows examples of potential cycling test protocols that are used to simplify this test of carbon cathode durability when it is conducted on MEAs. These test protocols do not require the air supply system to the anode or the voltage-hmiting circuit shown in Fig. 3. As one example of their application. Fig. Vshows the results that were obtained when a potential-cycling test was conducted under the conditions in Fig. 6a. An analysis of the results indicates that CO was generated by potential cycling and that it was accompanied by a dechne in cell performance. This suggests that the test protocol is one effective method of evaluating start-stop degradation.

Diaphrag m Cell Technology. Diaphragm cells feature a porous diaphragm that separates anode and cathode compartments of the cell. Diaphragms should provide resistance to Hquid flow, requite minimum space between anode and cathode, produce minimum electrical resistance, and be durable. At the anode, which is generally a DSA, chloride ions are oxidized to chlorine (see eq. 1) and at the cathode, which is usually a woven steel wine mesh, water is reduced to hydrogen. 

Cathodic protection by means of impressed current is very adaptable and economic because of the long durability of anodes and the large number of anode materials and shapes. Some examples are described here. Internal cathodic protection of fuel oil tanks has already been dealt with in Section 11.7. The internal protection of water tanks is described in detail in Chapter 20. 

Numerous materials fall into the category of electronic conductors and hence may be utilised as impressed-current anode material. That only a small number of these materials have a practical application is a function of their cost per unit of energy emitted and their electrochemical inertness and mechanical durability. These major factors are interrelated and —as with any held of practical engineering—the choice of a particular material can only be related to total cost. Within this cost must be considered the initial cost of the cathodic protection system and maintenance, operation and refurbishment costs during the required life of both the structure to be protected and the cathodic protection system. 

Recent experience has confirmed that, by adopting the recommendations of the British Standards Institution or similar codes of practice operating in other countries, the likelihood of corrosion damage to buried structures adjacent to cathodically protected installations is negligible. This is because recently installed cathodically protected structures are usually coated with eflicient and durable insulating coverings such as epoxy resins and the protective current applied is consequently small. In many cases the small protective currents that can be applied by means of galvanic anodes is adequate. 

Liu, X., Chen, J., Liu, G., Zhang, L., Zhang, H., and Yi, B. (2010) Enhanced long-term durability of proton exchange membrane fuel cell cathode by employing Pt/Ti02/C catalysts. Journal of Power Sources, 195 (13), 4098-4103. 

Newly developed alloys have improved properties in many aspects over traditional compositions for interconnect applications. The remaining issues that were discussed in the previous sections, however, require further materials modification and optimization for satisfactory durability and lifetime performance. One approach that has proven to be effective is surface modification of metallic interconnects by application of a protection layer to improve surface and electrical stability, to modify compatibility with adjacent components, and also to mitigate or prevent Cr volatility. It is particularly important on the cathode side due to the oxidizing environment and the susceptibility of SOFC cathodes to chromium poisoning. 

C.H. Lee, Enhanced efficiency and durability of organic electroluminescent devices by inserting a thin insulating layer at the Alq3/cathode interface, Synth. Met., 91 125-127 (1997). 

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

The stability of electrocatalysts for PEMFCs is increasingly a key topic as commercial applications become nearer. The DoE has set challenging near-term durability targets for fuel cell technology (automotive 5,000 h by 2010 stationary 40,000 h by 2011) and has detailed the contribution of the (cathode) catalyst to these. In particular, for automotive systems as well as steady-state stability, activity after simulated drive cycles and start-stop transients has been considered. In practice, both these treatments have been found to lead to severe degradation of the standard state-of-the-art Pt/C catalyst, as detailed next. 

Polyphosphates also act as corrosion inhibitors. They are cathodic polarizers and form a durable corrosion-inhibiting film that includes adsorbed calcium. 

On the cathode side, on the other hand, the primary purpose of gas purge is to remove water from the cathode compartment, particularly in preparation for cold start from subzero temperatures. As gas purge defines the initial condition of water distribution in a cell, it is a crucial step in PEFC cold start. Recent experimental studies have amply shown that not only performance but also material durability of PEFC hinges strongly upon the gas purge process prior to cool down and cold start. This is because an effective gas purge can remove water from the catalyst layer and membrane, thereby creating space for water produced in cold start to be stored. 

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