Fuel cell catalysts, drawbacks

A number of fuel cell catalysts have been synthesized in this maimer, such as Pt colloids, Pt/Sn colloids, and Pt/Ru colloids of different Pt to Ru ratios. A drawback of the Boennemann synthesis method is that oxidative removal of the stabilizer molecule requires temperatures higher than 300 °C [69]. High-temperature treatment of Pt-based catalysts should be avoided as changes in the structure, namely preferential surface segregation of Pt or RUO2, typically take place (this is further discussed in Section 9.4.1). To the best of our knowledge the Boennemann method has not been used to make catalysts of the same composition but different sizes. 

Although ORR catalysts for DMFCs are mostly identical to those for the PEM fuel cell, one additional and serious drawback in the DMFC case is the methanol crossover from the anode to the cathode compartment of the membrane electrode assembly, giving rise to simultaneous methanol oxidation at the cathode. The... 

Platinum is generally acknowledged as the most effective catalyst for the electroreduction of oxygen in a wide range of conditions (e.g. fuel cells). In the instance of aqueous HC1 electrolysis, the basic drawback is corrosion or deactivation of the catalyst during cell shutdown, owing to chemical attack from HC1 and chlorine that diffuse across the membrane. 

Saha et al. [109] have proposed an improved ion deposition methodology based on a dual ion-beam assisted deposition (dual IBAD) method. Dual IBAD combines physical vapor deposition (PVD) with ion-beam bombardment. The unique feature of dual IBAD is that the ion bombardment can impart substantial energy to the coating and coating/substrate interface, which could be employed to control film properties such as uniformity, density, and morphology. Using the dual lABD method, an ultralow, pure Ft-based catalyst layer (0.04-0.12 mg Ft/cm ) can be prepared on the surface of a GDL substrate, with film thicknesses in the range of 250-750 A. The main drawback is that the fuel cell performance of such a CL is much lower than that of conventional ink-based catalyst layers. Further improvement... 

The individual components used in an AFC are not necessarily expensive compared to those of other fuel cell t3q>es under development. Use of Ft catalysts can be avoided, while the bipolar plates collecting the electron flows typically have to be made of fairly expensive black carbon to avoid corrosion. The peripherals needed for water management and electrolyte draining add to the cost, but do not necessarily lead to drawbacks such as long start-up... 

The application to fuel cells was reopened by Ballard stacks using a new Dow membrane that is characterized by short side chains. The extremely high power density of the polymer electrolyte fuel cell (PEFC) stacks was actiieved not only by the higher proton conductance of the membrane, but also by the usage of PFSA polymer dispersed solution, serpentine flow separators, the structure of the thin catalyst layer, and the gas diffusion layer. Although PFSA membranes remain the most commonly employed electrolyte up to now, their drawbacks, such as decrease in mechanical strength at elevated temperature and necessity for humidification to keep the proton conductance, caused the development of various types of new electrolytes and technologies [7], as shown in Fig. 2. 

At the current technology stage, Pt-based electrocatalysts are the most practical materials in terms of both activity and stability, although their performance is still insufficient and needs further improvement. The major drawback of these Pt-based catalysts is the limited availability and high cost, contributing to the excessive production costs of fuel cell systems. 

However, DMFCs do suffer some drawbacks such as lower electrical efficiency and higher catalyst loadings as compared to H2 fuel cells. Efforts should be continued on developing anode catalysts with improved methanol oxidation kinetics, cathode catalysts with a high tolerance to methanol, membranes with lower methanol permeation rates, and strategies to reduce the methanol crossover rate. Attention should also be given to other direct-feed fuel cells using other liquid fuels (such as formic acid). 

The present chapter summarized the fundamental aspects and recent advances in electrocatalysts for the oxidation reactions of H2/CO, methanol, and ethanol occurring at fuel cell anodes emphases were placed on the state-of-the-art Pt-Ru- and Pt-Sn-based catalytic systems. Pt-based catalysts are still considered to be the most viable for the anodic reactions in acidic media. The major drawback however, is the price and limited reserves of Pt. To lower the Pt loading, the core-shell strucmre comprising Pt shells is more beneficial than the alloy structure, since all the Pt atoms on the nanoparticle surfaces can participate in the reactions (and those in cores do not) particularly, the Pt submonolayer/monolayer approach would be an ultimate measure to minimize the Pt content [30-35]. The architectures in nanoscale also have a significant effect on the reactivity and durability [54, 94] and thus should be explored continuously in the future. As for the ethanol oxidation, Rh addition is shown to enhance the selectivity towards C-C bond splitting [70, 71] however, Rh is even more expensive than Pt, and thus less expensive constituents replacing Rh are necessary to be found. 

Considerable ongoing research aims to increase the electro-catalytic activity of platinum for the ORR. A particularly difficult problem to resolve is the large loss in potential (0.3. 4 V) mostly at the cathode, which is the source of major decline in the fuel cell s efficiency. Another drawback of existing electro-catalyst technology is the high Pt loading in cathode electro-catalysts, t)T)ically in the range of 0.1-0.5 mg cm. ... 

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