Enhanced Activity Cathode Catalysts

As discussed previously, the DoE has set a target of catalyst activity of four times fhaf of pure Pt/carbon. This can be expressed as activity per mass of Ft or acfivify per cost equivalent. Three general approaches have been investigated to achieve this target Pt alloys, Pt core-shells, and non-Pt catalysts 

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For example, the small scale of the device was intended as a demonstration of architecture suitable for implanted applications. Mano et al. demonstrated a miniature fuel cell with bilirubin oxidase at the cathode catalyst that is more active at pH 7 and tolerates higher halide concentrations than does laccase. Additionally, the long-side-chain poly-(vinylpyridine)—Os(dialkyl-bis-imidazole)3 redox polymer discussed above was employed to both lower the anode potential and, via the long side chains, enhance electron transport from the biocatalyst. The cell achieved a current density of 830 at 0.52 V... 

Further enhancement of the specific activity of PEFC air cathode catalysts has been achieved by moving on from carbon-supported Pt to carbon-supported Pt alloy catalysts [41, 78-81]. The gain in activity per unit mass of Pt in moving over from Pt to PtCo alloy cathode catalysts is demonstrated in Fig. 43 (explanation for this enhancement in activity has been given above). With the rise by factor... 

Zeis R, Mathur A, Fritz G, Lee J, Erlebacher J (2007) Platinum-plated nanoporous gold an efficient, low Pt loading electrocatalyst for PEM fuel cells. J Power Sources 165 65-72 Wu H, Wexler D, Wang G (2009) PtjNi alloy nanoparticles as cathode catalyst for PEM fuel cells with enhanced catalytic activity. J Alloy Compd 488 195-198... 

Pt monolayer catalysts show a promising pathway toward solving one of the major problems facing PEM fuel cells by enhancing the Pt-specific activity and the utilization of Pt atoms and therefore reducing the cost of the cathode catalyst, although more fuel cell tests of durability are needed before the monolayer catalysts can be put in fuel cell vehicles. There is still a need for a reduction in the total noble metals in these catalysts. 

The heat of adsorption of the intermediate remains the most straightforward parameter on which development of new cathode catalysts can be based. It has been suggested that a combination of two metals from the two branches of the volcano curve could result in enhanced activity [20], which implies that a direct correlation exists between composition... 

Metal deposition on the electrocatalysts active sites can take place as well. Although elements such as chromium are used to enhance the activity of cathode catalysts, it must be in the alloyed form for such enhancement, while in the case of corrosion of metal plates, it will form an adlayer on the catalytic surface, blocking platinum atoms becoming inactive for oxygen or hydrogen dissociation. 

High electrochemical activity and contamination tolerance. An eleetroeatalyst should be of high intrinsic activity, to diminish electrochemical reaction polarization and enhance energy conversion effieiency. More specifieally, the anode catalyst should have the ability to resist detrimental by-products such as CO and other intermediates that poison the active reaction surface the cathode catalyst should have the ability to restrain crossover methanol oxidation when liquid methanol is seleeted as the fuel. 

The Li/SOCl2 cell consists of a lithium anode, a porous carbon cathode, and a nonaqueous SOCl2 LLAlCl4 electrolyte. Other electrolyte salts, such as LiGaCl4 have been employed for specialized applications. Thionyl chloride is both the electrolyte solvent and the active cathode material. There are considerable differences in electrolyte formulations and electrode characteristics. The proportions of anode, cathode, and thionyl chloride will vary depending on the manufacturer and the desired performance characteristics. Significant controversy exists as to the relative safety of anode-limited vs. cathode-limited designs. Some cells have one or more electrolyte additives. Catalysts, metallic powders, or other substances have been used in the carbon cathode or in the electrolyte to enhance performance. 

HCMSC-supported Pt cathode catalyst was observed for other conditions. As well, Fig. 9.18 shows the highest initial and final cnrrent of the HCMSC-supported Pt cathode catalyst among all the carbon-snpported cathode catalysts. The enhancement in the electrocatalytic activity is attribnted to the nnique stractural properties of the HCMSC. The electrochemical active layer was also calculated to be 106, 43 and 55 m /g for the HCMSC-supported Pt cathode catalyst, for the IWC cathode catalyst and the PtA C (E-TEC) cathode catalyst, respectively. 

The current density of a single-wall carbon nanotube sheet electrode, with infused platinum nanoparticles as the cathode in a microbial fuel cell, was approximately an order of magnitude higher than that with an e-beam-evapo-rated platinum cathode. The enhancement of catalytic activity can be associated with the increase of the catalyst surface area in the active cathode layer [61]. In another study, MFCs with carbon nanotube mat cathodes produced a maximum power density of 329 mW m , more than twice of that obtained with carbon cloth cathodes (151 mW m ) [62]. A similar twofold improvement was obtained by electrochemically depositing Pt nanoparticles on a CNT textile cathode for aqueous cathode MFCs, with only 19.3% Pt loading of a commercial Pt-coated carbon cloth cathode [63]. 

Activity loss can also be accessed directly using potentiodynamic cycling. In this case electroactive species (reaetants) for specific electrocatalytic reaction under consideration should be present in the solution, so the decay of Faradaic current can be measured directly. These types of measurements are erucial for solving the problems related to catalyst stability, and this is a field of active research, with different solutions offered. In the case of Pt supported cathode catalysts for PEMFCs enhanced stability can be achieved in different ways. For example, this can be done by suppressing dissolution processes through alteration of surface electronic structure by the designing well defined monolayer catalysts [41] or by stabilization of the surface by gold clusters [42]. In addition, suitably chosen support can also increase catalyst stability, for example different carbon nanoarchitectures with or without heteroatom [43]. 

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