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Visualising Electrocatalysis at the Nanoscale

Final Report Summary - VISELCAT (Visualising Electrocatalysis at the Nanoscale)

Electrochemical processes are ubiquitous, spanning a large array of research fields and applications, including alternative energy systems (fuel cells, solar cells and batteries), development of electroanalytical sensors, waste-water purification and electro-organic synthesis. In most of these fields there is a demand to develop more efficient, durable and selective technologies, in which the concept and understanding of electrocatalysis is of pivotal importance. Of particular interest is the development and optimization of electrocatalysts for low temperature fuel cells, which are vital to the development of future technology to fulfill projected energy demands.

Many electrocatalysts utilise metallic nanoparticles (NPs) immobilised on a conductive (carbon) support, in order to maximise catalyst dispersion and enhance the electrocatalytically active surface area of the catalyst so as to minimise the amount of metal required to achieve a given activity. Typically, conventional studies focus on ensembles of large amounts of NPs, wherein the electrocatalytic properties of an individual NP is almost certainly obscured. For example, the properties of one NP might be completely different from another one in the same ensemble due to differences in size, shape and local interactions with the support material. Therefore, t0 understand the intrinsic reactivity of NPs, it is imperative to study them at the single-NP level.

In the EU-funded project VISELCAT at the University of Warwick (UK), we have set out to study electrocatalytic processes at the level of an individual NP. This is challenging and represents the frontier of this important area. To this end, we have developed scanning electrochemical cell microscopy (SECCM), a novel electrochemical imaging technique to probe electrochemical processes directly with high spatial and temporal resolution. In particular, the spatial resolution of SECCM is sufficient to probe individual types of sites on electrode surfaces.

Using SECCM, we have been able to (re-)assess the electron transfer properties of many electrode materials, including various types of carbon materials, often employed as supports for electrocatalytic NPs and catalytic metal electrodes with different morphologies. We have addressed and provided definitive answers to outstanding issues regarding the active sites at which electron transfer takes place on highly oriented pyrolytic graphite (HOPG) and polycrystalline boron-doped diamond. In particular, our findings on HOPG are of key importance to the understanding of electron transfer at many carbon materials, and demonstrating the need for a revision of current textbook models.

Furthermore, we have employed SECCM to study electronucleation and growth of metal NPs. Our findings suggest that the interaction between nanoparticles and the substrate material play a much larger role than currently understood, and we are currently expanding on this research line to gain a thorough understanding of electrodeposition processes.

Finally, we have studied electrocatalytic materials with SECCM. First, we have studied the relationship between the local surface structure and electrocatalytic activity on polycrystalline platinum for a number of reactions, including reactions relevant to alternative energy systems, and found that the grain structure and surface orientation of individual grains are important descriptors for localised reactivity. Second, we have studied electrocatalytic processes at single, individual nanoparticles, through two different approaches. In the first approach, we employed SECCM to locate and map the reactivity of individual NPs within an electrocatalytic ensemble, consisting of platinum NPs supported on a single carbon nanotube. Significantly, our studies show that subtle variations in the morphology of NPs lead to dramatic changes in (potential-dependant) reactivity, which has important implications for the design and assessment of NP catalysts. By combining the electrochemical properties of single NPs with the physical properties obtained through high-resolution electron microscopy, it was found that a subtle difference in morphology can lead to significant changes in reactivity. Furthermore, this instrumental approach is general and can be adapted to for research in functional imaging, nanoscale electron transfer and catalysis. In the second approach, SECCM was employed to study the landing of individual nanoparticles NPs on various electrode materials, without any need for encapsulation or fabrication of complex substrate electrode structures. The pipette-based methodology of SECCM provided great flexibility with respect to electrode materials. Due to the small electrode areas defined by the pipet dimensions, the background current is low, allowing for the detection of minute current signals with good time resolution. This approach was used to characterize the potential-dependent activity of Au NPs and to measure the catalytic activity of a single NP on a TEM grid, combining electrochemical and physical characterization at the single NP level for the first time. Such measurements open up the possibility of studying the relation between size and activity of catalyst particles unambiguously. In both approaches.

The findings of this project have led eight peer-reviewed papers in leading journals, with several more in preparation. Moreover, there results have been presented at various national and international symposia.