Sean J. Ashton

The Particle Size Effect on the Oxygen Reduction Reaction Activity of Pt Catalysts: Influence of Electrolyte and Relation to Single Crystal Models

The influence of particle size on the oxygen reduction reaction (ORR) activity of Pt was examined in three different electrolytes: two acidic solutions, with varying anionic adsorption strength (HClO(4) < H(2)SO(4)); and an alkaline solution (KOH). The experiments show that the absolute ORR rate is dependent on the supporting electrolyte; however, the relationship between activity and particle size is rather independent of the supporting electrolyte. The specific activity (SA) toward the ORR rapidly decreases in the order of polycrystalline Pt > unsupported Pt black particles (~30 nm) > high surface area (HSA) carbon supported Pt nanoparticle catalysts (of various size between 1 and 5 nm). In contrast to previous work, it is highlighted that the difference in SA between the individual HSA carbon supported catalysts (1 to 5 nm) is rather trivial and that the main challenge is to understand the significant differences in SA between the polycrystalline Pt, unsupported Pt particles, and HSA carbon supported Pt catalysts. Finally, a comparison between measured and modeled activities (based on the distribution of surface planes and their SAs) for different particle sizes indicates that such simple models do not capture all aspects of the behavior of HSA carbon supported catalysts.

The effect of particle proximity on the oxygen reduction rate of size-selected platinum clusters

The diminished surface-area-normalized catalytic activity of highly dispersed Pt nanoparticles compared with bulk Pt is particularly intricate, and not yet understood. Here we report on the oxygen reduction reaction (ORR) activity of well-defined, size-selected Pt nanoclusters; a unique approach that allows precise control of both the cluster size and coverage, independently. Our investigations reveal that size-selected Pt nanoclusters can reach extraordinarily high ORR activities, especially in terms of mass-normalized activity, if deposited at high coverage on a glassy carbon substrate. It is observed that the Pt cluster coverage, and hence the interparticle distance, decisively influence the observed catalytic activity and that closely packed assemblies of Pt clusters approach the surface activity of bulk Pt. Our results open up new strategies for the design of catalyst materials that circumvent the detrimental dispersion effect, and may eventually allow the full electrocatalytic potential of Pt nanoclusters to be realized.

Design, development, and demonstration of a fully LabVIEW controlled in situ electrochemical Fourier transform infrared setup combined with a wall-jet electrode to investigate the electrochemical interface of nanoparticulate electrocatalysts under reaction conditions

We present a detailed description of the construction of an in situ electrochemical ATR-FTIR setup combined with a wall-jet electrode to investigate the electrocatalytic properties of nanoparticulate catalysts in situ under controlled mass transport conditions. The presented setup allows the electrochemical interface to be probed in combination with the simultaneous determination of reaction rates. At the same time, the high level of automation allows it to be used as a standard tool in electrocatalysis research. The performance of the setup was demonstrated by probing the oxygen reduction reaction on a platinum black catalyst in sulfuric electrolyte.

Differential Electrochemical Mass Spectrometry

The intention of this chapter is to provide an overview of the DEMS technique, the instrument designs and example research applications. This involves the presentation and discussion of previous design solutions, which are separated into three parts: the electrochemical cell, membrane interface and the vacuum system of the mass spectrometer.

Practical Aspects of the DEMS Instrument

This chapter concerns the more practical aspects of the DEMS instrument, covering the instrument characterisation, influence of experimental variables and measurement parameters, calibration procedures and finally, error and maintenance considerations. The characterisation and evaluation of the DEMS instrument is crucial to finding optimum experimental parameters, discovering possible areas of improvement during the development process, and to understanding the capabilities and constraints of the instrument before any research is conducted. This process is separated into the evaluation of the three principle DEMS components, to re-iterate the electrochemical cell, the membrane interface, and the vacuum system including QMS whose capabilities combine towards the overall performance of the instrument. Following this, the methodologies and setups used to optimise and calibrate the QMS and the DEMS instrument as a whole are presented. The contents of this chapter should serve to provide a practical understanding of the DEMS instrumental variables and document calibration procedures so that the instrument can be used in confidence by future users.

Methanol Oxidation on HSAC Supported Pt and PtRu Catalysts

In this first demonstration of this DEMS instrument research application, the study of the methanol oxidation reaction (MOR) on HSAC supported -Pt and -PtRu catalysts is revisited. The objective of this study was to not only demonstrate the research capabilities of the DEMS instrument constructed in this thesis but to also examine the electroanalytical techniques that are commonly employed in order to study and assess the MOR reaction and electrocatalyst activities using RDE. The results presented here, however, do not simply repeat previous observations but elucidate the contrasting potential dependent conversion of the MOR to CO2 on Pt and PtRu catalysts. Three-dimensional voltammetry is furthermore applied for the first time combined with DEMS and utilised to describe the MOR, encapsulating the potential, current and time relationship of the reaction system within a single contour plot.

Design and Construction of the DEMS Instrument

In this chapter, details of the DEMS instrument design and construction created as part of this thesis are presented. The instrument employs a dual thin-layer electrochemical flow cell and microporous PTFE membrane interfaced to a high vacuum system containing a QMS. The vacuum system design possesses a tubular aperture to control and direct the flux of gas through the cross-beam ion source, whilst a 3-stage differentially pumped vacuum construction provided optimum operating pressures of the QMS in order to maximise instrument sensitivity.

The Electrochemical Oxidation of HSAC Catalyst Supports

A fundamental understanding of the electrochemical oxidation behaviour of carbon blacks used to support finely dispersed Pt particles is crucial to the design of mitigating strategies that prevent the deterioration of PEMFC performance, particularly in automotive applications. In this study, DEMS is utilised to investigate and compare the electrochemical oxidation tendencies of a pristine EC300 carbon black and five graphitised carbon blacks (heat-treated between 2,100 and 3,200 °C). The study is then briefly extended to EC300 HSAC supported Pt nanoparticle catalysts. By monitoring the CO2 and O2 produced in situ, it is possible to elucidate the contributions of the different electrochemical reactions to the overall faradaic electrode current observed during high potential excursions >1.35 VRHE. The initial partial and complete electrochemical oxidation of EC300 was found to be considerably greater than the graphitised samples. However, during repeated potential excursions the electrochemical oxidation tendencies were observed to be dynamic, exhibiting three trends that are not only influenced by the amount of electrochemical oxidation but also the lower electrode potential limit. Finally, the behaviour of the EC300 HSAC supported Pt catalysts was found to be largely the same as the pure EC300 carbon black, however, an additional peak is observed after electrochemical oxidation attributed to the oxidation of CO species on Pt.