Taina Rauhala Fuel Cell Catalysts Based on Metal Nanoparticles

Contents Introduction to fuel cells Why use nanoparticle catalysts? Preparation of supported NP catalysts Size effect The effect of different crystallographic planes on the catalytic activity Alloying Conclusions References

Low temperature fuel cells Low temperature fuel cells °C Polymer electrolyte fuel cells Phosphoric acid fuel cells Alkaline fuel cells Generally the catalysts are made of platinum or platinum alloys Expensive Limited availability

Hydrogen Fuel Cell Reactions Anode Cathode Overall reaction

The benefits of using nanoparticles The reduction of the amount of catalyst needed The surface area increases when the particle size decreases The lowering of overpotential losses Better utilization of the catalyst makes the catalyst surface more accessible to mass transport of reactants Changes in the electronic properties of the catalyst

Nanoparticle FC catalyst carriers Catalyst particles are supported on carbon carriers Carbon black Nanotubes Nanofibres Carriers increase the dispersion of catalyst and decrease the agglomeration Conductive Reduction of ohmic potential losses

Preparation of supported NPs 1. Activation Electrochemically Strong oxidizing acids Ozonolysis 2. Addition of metal complex 3. Reduction of metal complexes to form NPs

Size effect When the size of the particle is reduced, the relative amount of surface atoms in edge and corner positions increases Edge and corner atoms are electrocatalytically more active than other surface atoms Change in the catalytic activity occurs Applicable to <10 nm NPs

Effect of crystallographic planes Pt has a face centered cubic crystal structure 3 basal planes: (111), (100), (110) Catalytic activity for oxygen reduction reaction Pt(110) > Pt(100) > Pt(111)

Effect of crystallographic planes High-index planes High density of step and edge atoms Electrochemically more active than basal planes More stable than basal planes Pt(211) is the most active for ORR

Preparation of shape-controlled NPs Liquid phase colloidal process Organic capping agents Block some of the active sites and, thus, inhibit the growth in some directions and enhance the growth in others E.g. polyvinylpyrrolidone, polyacrylate Electrochemical shape control High-index planes Applying a square-wave potential destroys the low- index planes

Alloying Alloying with another metal might change Pt´s electronic structure The mechanisms are not known for most metals Alloy catalysts are used at the anode Added metals increase the catalysts CO-tolerance Hydrogen gas feed usually contains small amounts of CO which adsorbs on the surface and poisons the catalyst

Example: PtRu anode catalysts Ruthenium increases the affinity for adsorption of oxygen containing species in less positive potentials The rate determining step in CO oxidation is the reaction between OH and CO species –> The amount of poisoned surface sites decreases

Conclusions Nanoparticle catalysts show different electronic properties than bulk catalysts Size effect Crystallographic planes Catalysts based on NPs are crucial in the commercialization of fuel cells Cost reduction

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