Carbon-Supported Pt-Ni Alloys Prepared by the Borohydride Method as Electrocatalysts for DMFCs

Abstract

Carbon-supported Pt-Ni electrocatalysts in the Pt:Ni atomic ratio 90:10 and 70:30 were prepared by reduction at room temperature of Pt and Ni salts with sodium borohydride and tested for the first time in direct methanol fuel cells ͑DMFCs͒ both as anode and as cathode materials. The performance of Pt-Ni as cathode in DMFC was higher than that of Pt-Ni as anode. Pt 90 Ni 10 as cathode showed better performance than Pt both in terms of mass activity and specific activity. When used as an anode material, the cell performance increased with increasing nickel content in the material. The enhanced performance of Pt-Ni as a cathode is ascribed both to a better activity for the oxygen reduction ͑electronic effect͒ and a better methanol-tolerance ͑ensemble effect͒ than Pt, related mainly to the alloyed nickel, while the improvement of Pt-Ni as anode for methanol oxidation is attributed to the presence of nonalloyed NiO species.Direct methanol fuel cells ͑DMFCs͒ have been receiving increasing attention because of the advantages in transportation and storage of the fuel, the high-energy efficiency, simplicity, and convenience. However, DMFCs also present serious concerns, such as the slow kinetics of the anodic and cathodic reactions and methanol crossover. 1,2 The anode electrocatalyst that can be used in DMFCs with proton exchange membranes ͑PEMs͒ as electrolyte has to be stable in acid media, should oxidize methanol at low overpotentials, and be stable over time. Methanol oxidation is a slow reaction that requires multiple active sites for the adsorption of methanol and sites that can donate OH species for the oxidative desorption of the adsorbed methanol residues. One of the best known and currently the most efficient anode electrocatalyst for DMFCs is Pt-Ru. 3-5 But even using Pt-Ru in the anode the power density of a DMFC is about an order of magnitude lower than that of a polymer electrolyte membrane fuel cell operated on hydrogen if the same Pt loading is used. Therefore, a number of alternative elements to Ru, as W, Os, Co, Sn, and Mo, showing a cocatalytic activity for the anodic oxidation of methanol, if used either as platinum alloys or as adsorbate layers on platinum, have been investigated. 6-11 Two models have been proposed to explain the effect of the second metal M: the first is the bifunctional mechanism, 12,13 according to which Pt sites are adsorption and dehydrogenation centers producing carbonaceous species, while on M sites the OH species are adsorbed at a less positive potential than on pure Pt; the second is the ligand model, 12,14 which proposes the modification of the Pt electronic structure by the presence of M rendering Pt atoms more susceptible for OH adsorption and for the dissociative adsorption of methanol.On the other hand, to overcome the problem of the methanol crossover, attempts are directed to the development of new cathode electrocatalysts with higher methanol tolerance than Pt. The current direction is to test the activity of some alloys of the first-row transition metals, which pres...

Structure of PtnNi Nanoparticles Electrocatalysts Investigated by X-ray Absorption Spectroscopy

“…14 The Pt 3 Ni NPs specifically showed better activity enhancement, and similar phenomenon was observed in other Pt-M (M = Fe, Co, etc.) bimetallic NPs systems.…”

Fuel Cells

“…Only recent studies on PtCo/C catalysts by Salgado et al [195] and on PtNi/C catalysts by Antolini et al [194] seem to mark a breakthrough in achieving both high oxygen reduction activity and high methanol tolerance, similarly as is claimed for Pt deposited on carbon-supported Fe-porphyrins [225].…”

“…Voltage efficiency and fuel utilization drawbacks have thus Thomas et al [190], by permission from Elsevier.) stimulated the continued search for both highly active methanol-tolerant cathode catalysts [191][192][193][194][195] and also membranes with reduced methanol permeability [13]. (iii) Additional mass transport losses occur on the anode when low methanol feed concentrations are used in order to reduce methanol cross-over [196].…”

Low-Pt Content Carbon-Supported Pt–Ni–TiO2 Nanotube Electrocatalyst for Direct Methanol Fuel Cells

“…Hence, it is essential to find less expensive alternatives. Recently, Ni was added as alloy metal to the Pt-based catalysts, and the resulting catalysts were found to exhibit better catalytic stability and activity for methanol electrooxidation than commercial Pt catalysts [6][7][8][9][10]. In this paper, we report an improvement of these Pt-Ni alloy catalysts by introduction of a new oxide component.…”