Abstract
Abstract
Pt-skeleton alloys show great potential as Proton Exchange Membrane Fuel Cell (PEMFC)catalysts. This PhD thesis studies different synthesis methods for nanoparticle fuel cell catalysts ina controlled way and the manner in which properties like nanoparticle size and composition can beeasily tuned. Finally, we investigate how these properties affect Oxygen Reduction Reaction(ORR) activity and stability of the prepared catalysts. In order to understand how the structures ofthe prepared nanoalloys affect their catalytic activity, different synthesis routes were used. Amodified impregnation/precipitation technique, which is commonly used in industrial synthesis offuel cell catalysts, will serve as reference. In this particular method, due to the slow rate of themetal precursor reduction and conversion to the metallic state and the even slower diffusion of themetal species on the surface of the carbon support, a reducing agent was used. It was observedthat, the rate of the reducing agent addition clearly affects the structural and the catalyticproperties of the nanoparticles (NPs), by changing the lattice constant of the nanoalloys, inducinga strain effect. Furthermore, upon heat treatment further enhancement of the activity is observeddue to the introduction of an electronic or ligand effect.
The above method is however difficult to control and its reproducibility is poor. For thatreason, a colloidal method is used as a toolbox to conduct systematic studies on PtxCo1-xnanoalloys. This method provides improved reproducibility and control of the particle size andcomposition of the NPs synthesized. As a result, a more thorough investigation of de-alloying andstrain effect on ORR performance was conducted, by preparing Pt-skeleton PtxCo1-x nanoalloys,i.e. by alloying Pt with different amount of cobalt and then dealloying them. Even without postheat treatment, the observed catalytic activity enhancement was significant after acid treatment,indicating that dealloying plays a crucial catalytic role, without having a detrimental effect on thestructure of the NPs. Apart from alloying though; changing the interparticle distance of Pt-basednanocatalysts is another proposed method to enhance significantly ORR performance. Therefore,by supporting both pure Pt and PtCo nanoalloys on HSA carbon with different Pt loadings, wewere able to investigate the effect of interparticle distance, the so-called proximity effect.
Still though, a clear understanding of the actual mechanisms that lead to activity enhancementof Pt-alloys is not yet achieved. Many studies have been conducted on Pt alloys; however, themajority treat the observed activity enhancement because of either a geometric/structural or anelectronic effect. However, here raises the question, if such small NPs have a defined crystalstructure capable to justify such an effect on the catalytic activity of nanoalloys. X-rayPhotoelectron Diffraction (XPD) spectroscopy measurements connect the thermal and staticdisplacement of Pt atoms within the crystal lattice of PtxCo1-x nanoalloys, the so-called particledisorder, with the lattice strain. Alloying Pt with increasing amount of Co increases strain andparticle disorder and subsequently both ORR performance and NPs stability. However, excessivealloying and de-alloying, as in the case of PtCo6, leads to particle collapse and significant losses inactivity and stability.
Pt-skeleton alloys show great potential as Proton Exchange Membrane Fuel Cell (PEMFC)catalysts. This PhD thesis studies different synthesis methods for nanoparticle fuel cell catalysts ina controlled way and the manner in which properties like nanoparticle size and composition can beeasily tuned. Finally, we investigate how these properties affect Oxygen Reduction Reaction(ORR) activity and stability of the prepared catalysts. In order to understand how the structures ofthe prepared nanoalloys affect their catalytic activity, different synthesis routes were used. Amodified impregnation/precipitation technique, which is commonly used in industrial synthesis offuel cell catalysts, will serve as reference. In this particular method, due to the slow rate of themetal precursor reduction and conversion to the metallic state and the even slower diffusion of themetal species on the surface of the carbon support, a reducing agent was used. It was observedthat, the rate of the reducing agent addition clearly affects the structural and the catalyticproperties of the nanoparticles (NPs), by changing the lattice constant of the nanoalloys, inducinga strain effect. Furthermore, upon heat treatment further enhancement of the activity is observeddue to the introduction of an electronic or ligand effect.
The above method is however difficult to control and its reproducibility is poor. For thatreason, a colloidal method is used as a toolbox to conduct systematic studies on PtxCo1-xnanoalloys. This method provides improved reproducibility and control of the particle size andcomposition of the NPs synthesized. As a result, a more thorough investigation of de-alloying andstrain effect on ORR performance was conducted, by preparing Pt-skeleton PtxCo1-x nanoalloys,i.e. by alloying Pt with different amount of cobalt and then dealloying them. Even without postheat treatment, the observed catalytic activity enhancement was significant after acid treatment,indicating that dealloying plays a crucial catalytic role, without having a detrimental effect on thestructure of the NPs. Apart from alloying though; changing the interparticle distance of Pt-basednanocatalysts is another proposed method to enhance significantly ORR performance. Therefore,by supporting both pure Pt and PtCo nanoalloys on HSA carbon with different Pt loadings, wewere able to investigate the effect of interparticle distance, the so-called proximity effect.
Still though, a clear understanding of the actual mechanisms that lead to activity enhancementof Pt-alloys is not yet achieved. Many studies have been conducted on Pt alloys; however, themajority treat the observed activity enhancement because of either a geometric/structural or anelectronic effect. However, here raises the question, if such small NPs have a defined crystalstructure capable to justify such an effect on the catalytic activity of nanoalloys. X-rayPhotoelectron Diffraction (XPD) spectroscopy measurements connect the thermal and staticdisplacement of Pt atoms within the crystal lattice of PtxCo1-x nanoalloys, the so-called particledisorder, with the lattice strain. Alloying Pt with increasing amount of Co increases strain andparticle disorder and subsequently both ORR performance and NPs stability. However, excessivealloying and de-alloying, as in the case of PtCo6, leads to particle collapse and significant losses inactivity and stability.
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2014 |