Carbon-Supported Palladium-Cobalt-Noble Metal (Au, Ag, Pt) Nanocatalysts as Methanol Tolerant Oxygen-Reduction Cathode Materials in DMFCs

Abstract

The carbon-supported nanoparticles of Pd-Co-M ͑M = Pt, Au, Ag͒ catalysts for direct methanol fuel cells ͑DMFCs͒ in a ratio of ͑70:20:10͒ were prepared through reverse microemulsion method. The X-ray diffraction ͑XRD͒ analysis showed well-defined reflections corresponding to a face centered cubic phase of palladium. From transmission electron microscopy analysis, the particle size after heat-treatment at 500°C was found to be approximately 20 nm, which was also confirmed by XRD analysis. Polarization data indicated Pd-Co-Pt to have better oxygen reduction reaction ͑ORR͒ activity than the other combinations with Ag and Au, in terms of shift in onset potential to a positive value of more than 100 mV and increased reduction current. The ORR kinetics on Pd-Co-Pt was analyzed by using rotating disk electrode to follow a 4 electron pathway, the order of the reaction being unity. The peroxide formation estimated from the rotating ring disk electrode measurements was found to be a negligibly small amount of 1.1%. An additional advantage observed with Pd-Co-Pt was its high methanol tolerance and ORR activity nearly equal to Pt. The major problem which concerns the developments in the direct methanol fuel cells ͑DMFCs͒ is the crossover of methanol through the polymer electrolyte membrane to the cathode compartment.1 This results in establishment of mixed potentials and depolarization of cathode potential, reduction of cell power and conversion losses due to fuel loss. [2][3][4][5][6][7][8][9] The simultaneous reduction of oxygen and suppression of fuel oxidation at the cathode requires electrode materials selective to oxygen reduction reaction ͑ORR͒ and further the catalyst should be tolerant to methanol.Therefore efforts are under way in developing new generation electrocatalysts to address these problems, by examining the requirements for selective ORR and improvements in the electrocatalytic characteristics. Platinum is the best known cathode catalyst, which has not been replaced by other materials so far. But the problem with the Pt catalyst is the high affinity towards adsorption of methanol on its surface. This leads to the catastrophic loss in the performance of DMFCs. Hence, Pt catalyst materials are developed by suitably alloying with secondary elements and studies have shown that platinum-based binary alloyed electrocatalysts such as PtFe, PtCo, PtNi, PtBi, and PtCr exhibit a higher catalytic activity for ORR in acid electrolytes than pure platinum. [10][11][12][13][14][15][16][17][18][19][20][21][22] These catalysts have already shown nearly the same activity for the ORR in the absence as well as in the presence of methanol.Recent research has shown the practicality of using nonplatinum metal alloys for ORR with resistance to methanol oxidation. Several nonplatinum electrocatalysts are reported for their adequate oxygen reduction activity to be considered as potential catalysts in commercial fuel cell applications. Nonprecious catalysts such as metal phthalocyanines, 23,24 transition metalloporphyrins, 25,26 tra...

Methanol tolerant oxygen-reduction activity of carbon supported platinum–bismuth bimetallic nanoparticles

“…It is now appropriate to bring in the cases of Pt bimetallics where this type of nonideal methanol-tolerant behaviour was observed. It appears that a ''trade off'' is common with all the known so called methanol tolerant combinations of Pt [18,30,50]. The voltammograms for ring and disk in a RRDE setup at different rotation rates for the Pt-Bi/C modified GC electrode (both as-prepared and heat treated) in oxygensaturated acid solutions at 25°C were performed.…”

Temperature dependence of morphology and oxygen reduction reaction activity for carbon-supported Pd–Co electrocatalysts

“…From Fig. 1a, it was observed that all of the XRD patterns exhibit five main characteristics peaks of the fcc crystalline Pd (JCPDS Card 00-005-0681) [15], namely the planes (111), (200), (220), (311), and (222) demonstrating that all the alloy catalysts mainly resemble the single-phase disordered structure (solid solution). The five diffraction peaks in the Pd-Co (32:68 atom%) alloy catalysts are shifted to higher 2h values compared to those of Pd-Co upon heattreatment suggesting incorporation of Co into the Pd lattice.…”

“…Pd-transitional metal alloys exhibited a high catalytic activity and a good selectivity for the ORR, and it might be attributed to an electronic stabilization of the added alloy element [11,12]. In addition, Pd-based electrocatalysts with suitable metal combinations and optimum compositions exhibited high catalytic activity for the ORR; the ORR activity of the Pd-Co/C catalysts with a Co content of 10-30% was found to be close to that of Pt/C [13][14][15]. Also, three kinds of carbon-supported Pd-based alloy electrocatalysts, Pd-CoAu, Pd-Co-Ti, and Pd-Co-Mo, have been prepared, and their electrocatalytic activity was found to be comparable with Pt for the ORR in a PEMFC [16].…”

“…However, the particle size of Pd-based catalysts as reported in the literature [13][14][15][16] was relatively large, and thus there is a significant space to improve ORR mass activity. Challenges to be met for the preparation of improved Pd alloy catalysts include the need for synthesis procedures resulting in catalysts with desirable alloy composition, small particle size, and a narrow size distribution [24], especially for alloy electrocatalysts.…”

Pt–Au/C cathode with enhanced oxygen-reduction activity in PEFCs

“…* Author for correspondence (psridhar55@gmail.com) Accordingly, efforts are being made to find a long-lasting and active catalyst for PEFCs. In this context, introduction of second noble metal, such as palladium, gold, to Pt has received considerable attention in recent years due to their attractive stability and activity towards ORR in fuel cells (Li et al 2004;Shao et al 2006a, b;Hernandez-Fernandez et al 2007;Mathiyarasu and Phani 2007). Among the various noble metals, gold is of particular interest in view of its inertness in the bulk state but high catalytic activity at nano-scales.…”