Strategies for probing Pt-based electrocatalyst and ionically conductive membrane materials: voltammetry of Pt, PtRu and PtV nanoparticles and infrared studies of membrane hydration
Basnayake, Rukma Sanjeewanie Tissera
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Materials aspects of low temperature (< 100 °C) fuel cell systems were investigated. The first studies presented examine properties of nanometer scale catalyst materials toward the electrochemical oxidation of carbon monoxide (CO) and methanol (CH3OH) in acid electrolyte solutions. Catalyst samples studied include carbon supported Pt (C/Pt), Pt black, PtRu and PtV. Special focus was on bimetallic materials prepared using sonochemistry. The second area discussed explores hydration in Nafion thin film and freestanding membrane materials. Effects of hydration on Nafion backbone and side-chain groups and interactions of solvent with the polymer were investigated with the use of transmission infrared spectroscopy. For the C/Pt containing 40 wt. % Pt catalyst, the average yields of CO2 from electrochemical oxidation of 0.3 M CH3OH in 0.1 M acid electrolyte solutions determined following 180 s electrolysis periods exceeded 50 % for potentials ¡Ý 0.6 VRHE. Experiments on PtRu nanoparticles prepared through a sonochemical method show the PtRu materials have properties of uniform bulk alloys. In studies of CH3OH and CO oxidation, sonochemically prepared (SC) PtRu samples containing 10 at. % Ru (XRu = 0.1) and 50 at. % Ru (XRu = 0.5) displayed activities relative to the catalyst active surface area consistent with responses for bulk PtRu having similar composition. Studies of SC PtV showed the material has high resistance toward poisoning by adsorbed CO. However, PtV was not as active as Pt black toward CH3OH oxidation. For the membrane material Nafion, vibrational modes of polymer bands and water inside membrane pores and channels were identified by infrared spectroscopy and observed to be sensitive to film hydration. In the H+ exchanged form, -SO3- groups were shifted to -SO3H and water was easily removed upon exposure to a few torr of vacuum at 95 °C. In contrast, residual water was retained by membrane exchanged with Na+ after exposure to these conditions for up to 72 hours. The permeation of CH3OH and acetone ((CH3)2CO) into Na+ exchanged freestanding (Nafion 112) membrane was also examined. The C-H and O-H stretching modes of CH3OH were perturbed in a manner that suggests the polymer disrupts hydrogen bonding interactions within the solvent, similar to the effect it exerts on pure water. For acetone, the C-H stretching modes were not shifted appreciably compared to those of the bulk liquid. However, the carbonyl band was affected, indicating the likely importance of dipolar interactions between solvent molecules and polar groups on the polymer. Control experiments performed with poly(hexafluoropropylene-co-tetrafluoroethylene) (FEP) membrane did not show evidence for water or CH3OH permeation, which demonstrates the critical role played by the ion filled channels and pores in facilitating solvent transport within Nafion membrane.