Fuel Cell Catalysts Based on Metal Nanoparticles

1. Fuel Cell Catalysts Based on Metal Nanoparticles 4.5.2011 Taina Rauhala

2. 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

3. Low temperature fuel cells • Low temperature fuel cells 60-200 °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

4. Hydrogen Fuel Cell • Reactions • Anode • Cathode • Overall reaction

5. 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

6. 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

7. Preparation of supported NPs • Activation • Electrochemically • Strong oxidizing acids • Ozonolysis • Addition of metal complex • Reduction of metal complexes to form NPs

8. 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

9. 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)

10. 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

11. 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

12. 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

13. 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

14. 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

15. References • Tian, N., Zhou, Z.-Y. and Sun, S.-G., Platinum Metal Catalysts of High-Index Surfaces: From Single-Crystal Planes to Electrochemically Shape-Controlled Nanoparticles, J. Phys. Chem. C, 112 (2008) 19801-19817. • Sánchez-Sánchez, C.M., Solla-Gullón, J., Vidal-Inglesias, F.J., Aldaz, A., Montiel, V. and Herrero, E., Imaging Structure Sensitive Catalysis on Different Shape-Controlled Platinum Nanoparticles, J. Am. Chem. Soc., 132 (2010) 5622-5624. • Wildgoose, G.G., Banks, C.E. and Compton, R.G., Metal Nanoparticles and Related Materials Supported on Carbon Nanotubes: Methods and Applications, Small, 2 (2006) 182-193. • Markovic, N.M., Radmilovic, V. And Ross, P.N., Jr., Physical and Electrochemical Characterization of Bimetallic Nanoparticle Electrocatalysts, in Catalysis and Electrocatalysis at Nanoparticle Surfaces, Edited by Wieckowski, A., Savinova, E.R. and Vayenas, C.G., Marcel Dekker, New York 2003, pp.311-342. • Mukerjee, S., In-Situ X-Ray Absorption Spectroscopy of Carbon-Supported Pt and Pt-Alloy Electrocatalysts: Correlation of Electrocatalytic Activity with Particle Size and Alloying, in Catalysis and Electrocatalysis at Nanoparticle Surfaces, Edited by Wieckowski, A., Savinova, E.R. and Vayenas, C.G., Marcel Dekker, New York 2003, pp.501-530.