Pd-based electrocatalysts

In the case of ethanol, Pd-based electrocatalysts seem to be slightly superior to Pt-based catalysts for electro-oxidation in alkaline medium [87], whereas methanol oxidation is less activated. Shen and Xu studied the activity of Pd/C promoted with nanocrystalline oxide electrocatalysts (Ce02, C03O4, Mn304 and nickel oxides) in the electro-oxidation of methanol, ethanol, glycerol and EG in alkaline media [88]. They found that such electrocatalysts were superior to Pt-based electrocatalysts in terms of activity and poison tolerance, particularly a Pd-NiO/C electrocatalyst, which led to a negative shift of the onset potential ofthe oxidation of ethanol by ca 300 mV compared... 

Pt and Pd-Based Electrocatalysts for Ethanol and Ethylene Glycol Fuel Cells... 

The real attraction for Pd-based electrocatalysts is originated by the fact that, unlike Pt-based electrocatalysts, they can be highly active for the oxidation of a large variety of substrates in alkaline environmerrt where also non-noble metals are sirffrciently stable for electrocherrrical apphcatiorts. 

The progress achieved over the last five years in the design of Pd-based electrocatalysts for the oxidation of alcohols in alkaline media is reviewed in this chapter, which is particularly concerned with renewable substrates such as ethanol and glycerol. 

Current synthetic methods for the preparation of Pd-based electrocatalysts for anodes of DAFCs are manifold, including reduction of high-valent metal compounds with chemical agents, colloidal metal deposition, electrodeposition and transmetalation... 

Some applications of the spontaneous deposition technique to the preparation of Pd-based electrocatalysts for fuel cells have been recently reported,among these there are examples of active anode catalysts in DAFCs. " ° Their preparation commonly proceeds by simply stirring a water solution of either a Pd or Pd" salt in the presence of an appropriate electropositive metal phase. For example, stirring a water solution of K2PdCl4 in the presence of Vulcan-supported Ni-Zn and Ni-Zn-P alloys gives the highly efficient electrocatalysts Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C. °... 

In the following Sections the most relevant studies of alcohol oxidation in both half cells and single monoplanar fuel cells using electrodes containing Pd-based electrocatalysts are reviewed. 

It is worth noticing that, irrespective of the metal oxide, all Pd-based catalysts promote the selective conversion of ethanol to acetate ion. " No analytical data for the oxidation products of methanol, ethylene glycol and glycerol have been provided by K. P. Shen, yet one can surmise that the latter two substrates are mainly oxidized to carboxylic acid as found for other Pd-based electrocatalysts vide... 

Scheme 5. Selective conversion of aldehydes into carboxylates on Pd-based electrocatalysts in alkaline media.

If DAFCs fuelled with ethanol and higher alcohols have a commercial future, this seems to be indissolubly linked to Pd-based electrocatalysts and anion-exchange membranes. As shown in this Chapter, the known catalytic architectures for alcohol oxidation are extremely valid, yet they suffer the scarce ability to cleave C-C bond in a selective way as well as poisoning by COads. Therefore increasing research efforts are required to design new catalysts with better performance and higher electrochemical stability. 

The Chapter by C. Bianchini provides a comprehensive and authoritative overview of recent advances on the use of Pd-based electrocatalysts in direct alcohol fuel cells. The Chapter shows lucidly how the dilution of Pd with non-noble metals in a smart catalytic architecture can lead to inexpensive and highly active anode electrodes which can knock down the main barriers for the commercialization of direct alcohol/fuel cells (DAFC). 

To improve the electrocatalytic activity of platinum and palladium, the ethanol oxidation on different metal adatom-modified, alloyed, and oxide-promoted Pt- and Pd-based electrocatalysts has been investigated in alkaline media. Firstly, El-Shafei et al. [76] studied the electrocatalytic effect of some metal adatoms (Pb, Tl, Cd) on ethanol oxidation at a Pt electrode in alkaline medium. All three metal adatoms, particularly Pb and Tl, improved the EOR activity of ft. More recently, Pt-Ni nanoparticles, deposited on carbon nanofiber (CNE) network by an electrochemical deposition method at various cycle numbers such as 40, 60, and 80, have been tested as catalysts for ethanol oxidadmi in alkaline medium [77]. The Pt-Ni alloying nature and Ni to ft atomic ratio increased with increasing of cycle number. The performance of PtNi80/CNF for the ethanol electrooxidation was better than that of the pure Pt40/CNF, PtNi40/CNF, and PtNi60/CNF. 

Palladium is more abundant in nature and sells at half the current market price of platinum. Unlike Pt, the Pd-based electrocatalysts are more active towards the oxidation of a plethora of substrates in alkaline media. The high activity of Pd in alkaline media is advantageous considering that non-noble metals are sufficiently stable in alkaline for electrochemical applications. Importantly, it is believed that the integration of Pd with non-noble metals (as bimetallic or ternary catalysts) can remarkably reduce the cost of the membrane electrode assemblies (MEAs) and boost the widespread application or commercialization of DAFCs [1]. Palladium has proved to be a better catalyst for alcohol electrooxidation in alkaline electrolytes than Pt [2]. Palladium activity towards the electrooxidation of low-molecular weight alcohols can be enhanced by the presence of a second or third metal, either alloyed or in the oxide form [3]. 

Pd-based electrocatalysts have shown comparable ORR activity to state-of-the-art Pt/C. It may play a role to replace or partially replace costly Pt catalysts in the cathode of low-temperature fuel cells. 

As expected, the oxygen reduction rate strongly depends on the orientations of the Pd surface with Pd(lOO) having a much higher ORR activity than any other facets in acid. This trend has been confirmed both in the studies of bulk single crystals and nanocrystals of Pd. Synthesis and characterization of shape-controUed Pd and Pd alloy nanomaterials are of importance to design next generatirai Pd-based electrocatalysts. 

The long-term stability of Pd-based electrocatalysts is one of the unavoidable issues for PEM fuel cell applications. Pd-Pt-based ORR catalysts are more stable than Pd-transition metal alloys under harsh fuel cell conditions, but may still not meet the long-term fuel cell operation requirement due to the Pd leaching out. Future research may focus on improving the durability of Pd-based catalysts by surface modification and composition optimization. Core-shell type of catalyst with Pd-based materials as the core and Pt as the shell may be one of the most promising candidates to be used in the automotive fuel cell due to its low Pt content and high activity and stability. 

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