Deposition of Ru-Ni-S Nanoparticles on Carbon by Spray-Pyrolysis: Effects of Solvent and other Processing Parameters

Abstract

Nanoparticles of Ru-Ni-S were synthesized in a single-step spray-pyrolysis process as potential catalysts for fuel cells and other applications. The liquid precursors containing ruthenium, nickel, and sulfur were nebulized by an ultra-sonic atomizer to generate aerosol droplets, which were subsequently decomposed to form uniformly distributed nanoparticles for deposition on a carbon thin film. It was observed that the application of methanol as solvent has a strong effect on the particle morphology, size, and composition. The morphology of the Ru-Ni-S nanoparticles changed from spherical with water as solvent, to dendrites upon increase in the methanol con- centration in the precursor solution. It was also found that the pyrolysis temperature strongly affected the particle morphology when methanol was used as solvent. High temperatures promote dendrite formation. When a water/methanol mixture was used as solvent, crys- talline ternary nanoparticles of Ru-Ni-S on a carbon layer were formed at lower temperatures. A very interesting and unique structure of spherical clusters of crystalline particles attached by a chain of crystalline nanoparticles was synthesized. Elemental analysis obtained with EDS attached to the SEM used for particle characterization has confirmed the existence of all elements of interest, and X-ray map- ping showed all elements were distributed uniformly in the nanoparticles. Spray-pyrolysis processing is a versatile technique for produc- tion of inorganic materials of a wide range of composition, size, and morphology. It typically consists of several steps that may include: precursor preparation, precursor atomization, droplet evaporation, droplet precipitation, droplet drying, droplet coagulation, thermoly- sis, and sintering. An review on spray-pyrolysis processing by Messing et al. (1) has discussed the fundamental process parame- ters enabling the formation of particles with controlled morphology and composition. Among the important process parameters in spray-pyrolysis processing, the effects of precursor properties on particle size, composition, and morphology are probably the least understood. Synthesis of electrocatalysts in a spray-pyrolysis process for different types of fuel cells is a relatively new application of the spray-pyrolysis technique. Tolerance to small amounts of carbon monoxide and sulfur is important for proton exchange membrane fuel cells operating on hydrogen obtained by reforming carbon- based fuels. Conventional nanoparticles (2-5 nm) of platinum-based metal alloys are used as both anode and cathode catalysts for proton exchange membrane fuel cells due to the high activity for both hy- drogen oxidation and oxygen reduction (2). However the platinum- based catalysts used today suffer high polarization losses, reducing performance and fuel efficiency due to particle agglomeration, car- bon monoxide and sulfur poisoning (2, 3). The search for carbon monoxide and/or sulfur-tolerant non-platinum electrocatalysts for fuel cell applications have been a very active research endeavor (4- 10). Only a few researchers have studied the platinum- and ruthe- nium-based nanoparticles for methanol oxidation in fuel cell appli- cations. The binary and ternary crystalline nanoparticles were sup- ported on carbon nanofilm as fuel cell catalysts. Waszczuk et al. (11) studied the methanol adsorption on platinum-ruthenium sur- faces and observed that the decomposition of methanol is quite different at ultra-high vacuum. It was observed that the behavior of simple molecules would be different under ultra-high vacuum. Con- trol of size, morphology, and composition of nanoparticles is im- portant in the synthesis processing of ceramic powders. It was