Exploitable results
New PEMFC anodes have been developed, using an unknown, non-conventional novel synthesis of electrocatalysts technique, which is the combustion method. New Pt-based alloys, Pt-Ru-M (M = Ni, Co, Fe) has been prepared easily and rapidly, as well as characterized. The so prepared materials were found to be single phased and to consist in soft agglomerates, easily to disgregate. The electrocatalytical behaviour of the materials prepared showed a good performance in the i-V tests.
Wet chemical methods predominantly rely on so called bottom-up syntheses of metal nanoparticles. The advantages are that one can control the sizes of the generated nanoparticles and also their monodispersity by the reaction conditions. The two general methods to prevent particle agglomeration are 1. electrostatic and 2. steric stabilizationL
Electrostatic stabilization is brought about by the Coulombic repulsion between particles, caused by the electrical double layer formed by ions adsorbed at the particle surface (e.g. sodium citrate) and the corresponding counter ions on a metal particle surface. Steric stabilization, in contrast is achieved via the co-ordination of sterically demanding organic molecules at the metal surface that act as protective shields. Consequently, the nanometallic cores are separated from each other thus agglomeration is prevented.
The distinct advantage of having these colloidal nanometal particles is that, they can be readily dispersible in various solvents enabling them to be laid on any given support. While preparation of colloidal catalyst precursors of controlled particle size, shape and structure can be achieved at the synthetic step, the colloidal catalyst preparation, in addition, offers ways to fine tune the electrocatalyst properties during deposition of nanoparticles on supports.
The so-called "precursor-concept", developed in our laboratories, facilitates tailoring of precursors with controlled structures as well as control of the interface between metals. Homogeneous alloys, segregated alloys, layered bimetallics, and "decorated" particles are all readily accessible using this step of catalyst synthesis. The primary incentive for using this technique is that it is possible to pre-prepare and thoroughly characterize the active components of electrocatalysts using modern analytical techniques.
Several mono, bi- and trimetallic nanoparticulate colloids were prepared (particle sizes between 1-4nm) by co-reduction and surface doping to generate particles having bulk alloy, core-shell or even gradient morphology, while applying alkalitriethylhydroborate or tetraoctylammoniumtriethylhydroborate trialkylaluminum as reducing agents.
The ceria-based transition metal (oxide) catalysts are known for their good redox characteristics. Depending on the reduction/oxidation state (level) they behave either as hydrogen storage (intermetallics) or as oxygen storage (mixed oxides) capacitors [Wrobel, G., Lamonier, C., Bennani, A. D'Huysser, A., Aboukaïs, A. J. Chem. Soc. Faraday Trans. 92 (1996) 2001]. The reduced transition metal (Cu) catalyst highly dispersed on CeO2 can undergo reversible oxidation at very low oxygen concentration in the reductive atmosphere already at room temperature. These findings have led us to the conclusion that it is worth to try this catalyst also in the electrochemical reactions, especially as the CO-resistant electro catalyst for hydrogen oxidation reaction (HOR) and as the electro catalyst for oxygen reduction reaction (ORR) in the low temperature PEM fuel cells.
The sol precursor of the nano-structured Cu/CeO2 electro catalyst was first obtained and mixed with Vulcan active carbon in order to prepare conductive substrate. After the appropriate activation procedure of this substrate, the Pt or Pt-Ru precursors were deposited on it in order to obtain a shell of noble metal over the core of Cu/CeO2. Thus , one of the preparation procedures for bi- and tri-metallic combinations supported on oxides and Vulcan have been used. The catalysts were subject to the detailed structural characterization and electrochemical tests. Based on tests with co-reduced and »onion«-type Pt-Cu catalysts and the characteristics of Cu/CeO2 support, we have obtained a new, CO-resistant electrocatalyst for low temperature PEMFC and DMFC.
The new Cu/CeO2 catalyst exhibits substantial electro catalytic activity in both, hydrogen oxidation (HOR) and oxygen reduction (ORR) reactions. The gas diffusion electrode prepared with Cu/CeO2 catalyst behaves as the reversible electrode for hydrogen reactions (HOR and HER) with relatively high exchange current density. No copper dissolution is observed at anodic potentials that indicate important stabilisation of metallic copper when dispersed on CeO2 substrate. It shows no decay during potential sweep cycling in oxygen.
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