Development of nanostructured materials for PEM fuel cells application

Mit Flüssigbrennstoffen betriebene Brennstoffzellen (direct liquid fuel cells, DLFC) haben als aussichtsreiche Energiequelle für elektronische Kleingeräte und tragbare Anwendungen viel Aufmerksamkeit erhalten. Diese Arbeit zielt auf die Entwicklung von hoch aktiven und beständigen Elektrokatalysatoren für die Anwendung in DLFCs. Es wurden nanostrukturierte Katalysatorsysteme geträgert auf mehrwandigen Kohlenstoffnanoröhren (MWCNTs), als Katalysatoren für die Methanol- sowie für die Ameisensäure-Oxidation dargestellt. Diese wurden mit strukturaufklärenden und elektrochemischen Methoden charakterisiert, um ihre strukturellen Eigenschaften zu untersuchen und ihre elektrokatalytische Aktivität gegenüber ausgewählten Elektrooxidationsreaktionen zu bewerten. Die elektrochemische Charakterisierung verdeutlichte, dass die Katalysatoren höher Aktivität für die Methanol und Ameisensäure-Elektroxidationund höhere Stabilität als industrielle Katalysatoren zeigte. In Versuchen mit der Direkt-Methanol-Brennstoffzelle (DMFC) zeigten die aus hergestellten und temperaturbehandelten PtRu-Katalysatoren gefertigten Membran-Elektroden-Einheiten (MEA) höhere Leistung und Stabilität als die aus dem industriellen PtRu/C-Katalysator.Direct liquid fuel cells (DLFCs) have gained much attention as promising power sources for small electronic devices and portable applications. This thesis focuses on the development of highly active and durable electrocatalysts for DLFCs application. Nanostructured catalyst supported on multiwalled carbon nanotubes (MWCNTs) were prepared to be used as catalysts for methanol oxidation and formic acid oxidation. The prepared catalysts were characterized with structural and electrochemical techniques to study their structural properties and to evaluate their electrocatalytic activity toward the respective fuel electrooxidation. The electrochemical characterization revealed that the catalysts exhibited higher catalytic activity for methanol and formic acid electrooxidation and higher stability than the commercial catalysts. In a direct methanol fuel cell (DMFC), the membrane electrode assemblies (MEAs) fabricated from the as-prepared and the heat treated PtRu catalysts exhibited high DMFC performance and they are more durable than the MEAs fabricated from the commercial PtRu/C catalyst when subjected to an accelerated durability test (ADT).von Abu Bakr Ahmed Amine Nass

CuPt/CNT catalysts for oxygen reduction prepared by a facile two-step synthesis : chemical vs. electrochemical leaching

Cu@Pt/CNT electrocatalysts were prepared by a facile two-step impregnation process. Pt nanoparticles supported on CNTs were prepared by microwave-assisted polyol reduction followed by impregnation with a copper precursor. The final catalysts were obtained by thermal alloying at different temperatures; namely 600, 800, 1000 °C. The alloying extent between Pt and Cu was found to increase significantly with the alloying temperature as revealed from XRD analysis, as does the particle size obtained from TEM. Electrochemical dealloying was performed with the three different Cu@Pt/CNT catalysts, aiming to form a core shell structure. The formation of a Pt enriched surface upon dealloying was confirmed electrochemically by cyclic voltammtery and CO stripping. The electrocatalytic activity for oxygen reduction of the electrochemically dealloyed catalysts depends on the alloying temperature where the catalyst alloyed at 800 °C showed the highest surface specific activity, which is explained in view of the high Cu content in the alloy core compared to the sample alloyed at 600 °C. Alloying at 1000 °C and chemical/electrochemical leaching leads to porous structures with high activity, albeit lower than that of the 800 °C sample. Notably these porous particles provide high activity also with a low copper content in the core. In general, the dealloyed Cu@Pt/CNT catalysts showed higher catalytic activity both in terms of surface specific activity (SSA) and mass specific activity (MSA) than their Pt/CNT counterpart

Contrasting the EXAFS obtained under air and H₂environments to reveal details of the surface structure of Pt-Sn nanoparticles

Understanding the surface structure of bimetallic nanoparticles is crucial for heterogeneous catalysis. Although surface contraction has been established in monometallic systems, less is known for bimetallic systems, especially of nanoparticles. In this work, the bond length contraction on the surface of bimetallic nanoparticles is revealed by XAS in H2at room temperature on dealloyed Pt-Sn nanoparticles, where most Sn atoms were oxidized and segregated to the surface when measured in air. The average Sn-Pt bond length is found to be ∼0.09 Å shorter than observed in the bulk. To ascertain the effect of the Sn location on the decrease of the average bond length, Pt-Sn samples with lower surface-to-bulk Sn ratios than the dealloyed Pt-Sn were studied. The structural information specifically from the surface was extracted from the averaged XAS results using an improved fitting model combining the data measured in H2and in air. Two samples prepared so as to ensure the absence of Sn in the bulk were also studied in the same fashion. The bond length of surface Sn-Pt and the corresponding coordination number obtained in this study show a nearly linear correlation, the origin of which is discussed and attributed to the poor overlap between the Sn 5p orbitals and the available orbitals of the Pt surface atoms.</p

Rapid Microwave-Assisted Polyol Reduction for the Preparation of Highly Active PtNi/CNT Electrocatalysts for Methanol Oxidation

PtNi nanoparticle catalysts supported on oxygen functionalized carbon nanotubes were prepared by microwave-assisted polyol reduction using two different modes of irradiation, namely, continuous or pulsed irradiation. The influence of irradiation time or pulse number on catalyst structure and activity in methanol electrooxidation has been studied. Characterization was done with ICP-OES, XRD, TEM, XPS, and XAS to determine composition, morphology, crystal structural and chemical state. The electrocatalytic activity has been evaluated by cyclic voltammetry (CV) and chronoamperometry (CA). PtNi nanoparticles are present in alloy form and are well dispersed on the carbon nanotubes. Pt is in its metallic state, whereas Ni is present in metallic and oxidized form depending on the preparation conditions. The electrocatalytic activity both in terms of surface and mass specific activity is higher than that of the state-of-the-art-catalyst Pt/C (E-TEK). The enhancement of the electrocatalytic activity is discussed with respect to PtNi alloy formation and the resulting modification of the electronic properties of Pt by Ni in the alloy structure. The microwave assisted polyol method with continuous irradiation is more effective in the preparation of PtNi electrocatalysts both in terms of reaction time and activity than the pulsed microwave method

Contrasting the EXAFS obtained under air and H2 environments to reveal details of the surface structure of Pt-Sn nanoparticles

Understanding the surface structure of bimetallic nanoparticles is crucial for heterogeneous catalysis. Although surface contraction has been established in monometallic systems, less is known for bimetallic systems, especially of nanoparticles. In this work, the bond length contraction on the surface of bimetallic nanoparticles is revealed by XAS in H2 at room temperature on dealloyed Pt-Sn nanoparticles, where most Sn atoms were oxidized and segregated to the surface when measured in air. The average Sn-Pt bond length is found to be ~0.09 Å shorter than observed in the bulk. To ascertain the effect of the Sn location on the decrease of the average bond length, Pt-Sn samples with lower surface-to-bulk Sn ratios than the dealloyed Pt-Sn were studied. The structural information specifically from the surface was extracted from the averaged XAS results using an improved fitting model combining the data measured in H2 and in air. Two samples prepared so as to ensure the absence of Sn in the bulk were also studied in the same fashion. The bond length of surface Sn-Pt and the corresponding coordination number obtained in this study show a nearly linear correlation, the origin of which is discussed and attributed to the poor overlap between the Sn 5p orbitals and the available orbitals of the Pt surface atoms.<br/

Effects of heat treatment atmosphere on the structure and activity of Pt3Sn nanoparticle electrocatalysts: a characterisation case study

Comprehensive identification of the phases and atomic configurations of bimetallic nanoparticle catalysts are critical in understanding structure–property relationships in catalysis. However, control of the structure, whilst retaining the same composition, is challenging. Here, the same carbon supported Pt3Sn catalyst is annealed under air, Ar and H2 resulting in variation of the extent of alloying of the two components. The atmosphere-induced extent of alloying is characterised using a variety of methods including TEM, XRD, XPS, XANES and EXAFS and is defined as the fraction of Sn present as Sn0 (XPS and XANES) or the ratio of the calculated composition of the bimetallic particle to the nominal composition according to the stoichiometric ratio of the preparation (TEM, XRD and EXAFS). The values obtained depend on the structural method used, but the trend air &lt; Ar &lt; H2 annealed samples is consistent. These results are then used to provide insights regarding the electrocatalytic activity of Pt3Sn catalysts for CO, methanol, ethanol and 1-butanol oxidation and the roles of alloyed Sn and SnO2