Combining UHV-STM and electrochemistry for surface studies of model catalysts

Abstract

Electrocatalysis is expected to play a crucial role in global society moving to an energy infrastructure largely based on renewable energy such as solar and wind power. Due to the intermittency of energy sources such as these we need an efficient way of averaging out the energy production in order for us to have access energy from renewable sources when the wind is not blowing and the sun is not shining. Electrocatalysis allows us to take the excess electricity from sources such as wind and solar power and convert it into (i) useful industrial chemicals thereby reducing the need to produce them by other means and (ii) energy rich fuels thereby lowering the demand for fossil fuels. However, if this is to become a reality we need to design catalysts that can make an impact on the large energy scales needed in a world of over 7 billion people. That means designing catalysts of active, stable and abundant materials. Here we present an experimental method for investigating model electrocatalysts on the atomic scale, which has been developed throughout this PhD project. The method aims to further our understanding of e.g. corrosion processes and electrocatalytically active sites. This is done through being able to prepare well-defined model systems under the controlled conditions of UHV. The samples prepared in this way are subsequently transferred under vacuum to the electrochemical cell meaning that contaminants from ambient conditions are avoided. The electrochemical measurement itself is then performed in an inert atmosphere before transferring the sample back to the UHV chamber again. The UHV chamber is equipped with an STM making it possible to investigate the sample before and after the electrochemical measurements thereby facilitating the correlation of the surface sites with the electrochemical response. Furthermore, the chamber is equipped with various equipment for forming interesting surface geometries. We present data from three different metal surfaces: Pt(111), Cu(111) and Cu(100). The Pt(111) surface’s electrochemistry is well-established and thus serves as a good test of whether our experimental setup works or not. Thus we will showcase the setup’s capabilities by (i) investigating clean Pt(111) and (ii) investigate the corrosion process of Pt(111) in 0.1 M HClO4 and compare the results to the literature in the field. The Cu single crystal facets are not nearly as well understood as the Pt(111) facet but let us test our setup with a different metal and different electrolyte, namely KOH. Finally, we try to replicate the Cu CVs on Cu single crystals prepared under ambient conditions through electropolishing. The obtained Pt(111) results are in great agreement with the literature, both in terms of the shape of the CV and the observed corrosion phenomena. Through designing samples with different surface geometries we correlate an electrochemical feature at 0.12 V vs. RHE with the presence of many (111) steps on the Pt(111) surface and find an inversely proportional relationship between upper potential limit and the size of the observed adislands on the surface when corroding the surface. For Cu(111) we find a CV between −0.2 and 0.45 V vs. RHE with just one sharply peaked redox feature due to OH adsorption and desorption. This peak corresponds roughly to an OH coverage of 0.28 ML. The results is reproduced under laboratory conditions by electropolishing a sample for 10 s at 3 V in 66 % H3PO4. We also show that this CV is very dependent on the lower potential limit. The CV measured on Cu(100) contains an OH feature at −0.15 V vs. RHE corresponding to an OH coverage 0.25 ML. This CV was also dependent on the exact potential limits. In both the case of Cu(111) and Cu(100) we speculate that the OH peak’s dependence on the lower potential limit is due to a restructuring of the surface. In situ methods will have to be used to confirm this. The Cu(100) CV has so far not been reproduced under laboratory conditions, in fact it mostly looks like a polycrystalline one. We conclude that the setup works as intended and could be very useful for understanding corrosion processes and active sites through the engineering of different surface structures using either the UHV equipment or techniques such as Pb UPD in the electrochemical cell.