Electrochemical performance of dispersed Pt-M (M =V, Cr and Co) nanoparticles for the oxygen reduction electrocatalysis

Abstract

A cinética da reação de redução de oxigênio (RRO) foi estudada em eletrólito de KOH em eletrodos de camada ultra fina, formados por eletrocatalisadores de Pt-V, Pt-Cr e Pt-Co dispersos em pó de carbono (Pt-M/C). As características eletrônicas e estruturais dos eletrocatalisadores foram estudadas por XANES (X-ray Absorption Near Edge Structure) e EXAFS (Extended X-ray Absorption Fine Structure) in situ. Voltametria cíclica e curvas de polarização do estado estacionário para a RRO foram obtidas usando a técnica de disco/anel rotatório. Os resultados de XANES para as ligas no estado oxidado mostraram um aumento na ocupação da banda 5d da Platina, enquanto que os resultados de EXAFS mostraram uma pequena redução na distância interatômica Pt-Pt em relação à Pt/C. As medidas eletroquímicas indicaram que o mecanismo via 4 elétrons é principalmente seguido para a RRO nas ligas de platina. A maior atividade eletrocatalítica foi atingida pela liga Pt-V/ C, depois de tratamento térmico a 850 o C em atmosfera de hidrogênio.The kinetics of the oxygen reduction reaction (ORR) was studied in KOH electrolyte on ultra thin layer electrodes formed by Pt-V, Pt-Cr and Pt-Co electrocatalysts dispersed on a carbon powder (Pt-M/C). The electronic and structural features of the materials were studied by in situ XANES (Xray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure). Cyclic voltammograms and steady state polarization curves for the ORR were obtained using the rotating ring/disk electrode technique. The XANES results for the composites in the oxidized state have shown an increase of the occupancy of the Pt 5d band while the EXAFS analyses showed some reduction of the Pt-Pt interatomic distance in the different metal alloys compared to Pt/C. The electrochemical measurements indicated that the 4-electrons mechanism is mainly followed for the ORR in these platinum alloys. The highest electrocatalytic activity was shown by the Pt-V/C alloy, after treated at 850 o C under hydrogen atmosphere.Keywords: oxygen reduction, platinum alloys, X-ray absorption IntroductionSeveral studies have been carried out in the last years aiming to increase the platinum electrocatalytic performance for the oxygen reduction reaction (ORR). 1-3Some authors have reported that on Pt alloys, Pt-V, P-Co, Pt-Cr, Pt-Fe, etc, there is an increase of the kinetics of the ORR electrocatalysis compared to pure Pt, and this fact has been attributed to changes in the Pt-Pt bond distance, number of Pt nearest neighbors, electronic density of states of the Pt 5d band, and nature and coverage of surface oxide layers.Toda et al. 4 explained the improvement of catalytic activity based on the increases by the Pt 5d band vacancy, leading to a stronger Pt-O 2 -interaction. This causes weakening and lengthening of the O-O bond and its easier scission, resulting in an increase of the reaction rate. Arico et al. 5 and Shukla et al. 6 have attributed the increase of the catalytic activity to a decrease of the coverage of surface oxides and an enri...

Degradation mechanism study of perfluorinated proton exchange membrane under fuel cell operating conditions

“…On comparing the Pt/C catalysts with Pt-Co/C alloy catalysts, it is seen that the two Pt-Co/C catalysts exhibit an anodic shift of about 30 mV in the mixed kinetic-diffusion potential region and hence 30 mV lower over potential for ORR. This lower overpotential of the alloy catalysts towards ORR is due to the fact that the presence of alloying element decreases the desorption free energy ( G des ) of Pt-OH, Pt-O or Pt-O 2 such that the adsorption of oxygen containing intermediate species on Pt surface sites is inhibited in the supported alloy catalysts compared to Pt/C [47]. This discussion based on the ORR profile for each catalyst involves interference from diffusion limited current densities i lim , and hence Tafel plots are extracted using the following Eq.…”

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“…Pt-Cr alloys have much promise for improved oxygen reduction in fuel cells [73][74][75] and there have been reports on ternary Pt-Co-Cr compositions [63,76,77]. Our tests of the Pt-Co-Cr system for activity in the oxygen reduction reaction were promising.…”

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