Igor Nahálka
Many industrial processes are based on electrochemical reactions using engineered electrocatalysts. However, the current lack of theoretical knowledge obscures efforts to maximize catalysts' efficiency, and the optimization process is mostly empirical. To fully understand the structure/chemical function relationship on complex surfaces, new experimental tools are needed that can evaluate electrochemical processes on nm to tens of ÎŒm length scales. These tools have to be surface-specific and ideally label-free in order not to influence the reactions. In this thesis, we apply high-throughput wide-field second harmonic (SH) microscopy together with in-situ electrochemical techniques to i) study the extent of surface heterogeneity, ii) surface changes during electrochemical cycling, and iii) to identify and characterize surface active sites.
We first investigated surface heterogeneity at the glass/electrolyte interface with the novel SH microscope developed in our laboratory, as this interface has been widely discussed in the literature with inconsistent results. The spatial resolution of our SH microscope, ~400 nm, together with the temporal resolution of 250 ms, allowed us to spatially resolve the surface pKa,s for the silica deprotonation reaction with values ranging from 2.3 to 10.7. The average value, 6.7, coincides well with the reported values from experimental studies lacking spatial resolution. Furthermore, we demonstrated the ability of SH microscopy to image the orientation of bulk water molecules under an externally-applied electric field in a confined 1 ÎŒm-sized glass pore.
We further applied SH imaging coupled in-situ with cyclic voltammetry to characterize the surface of gold electrodes during electrochemical cycling. Analyzing the voltage dependence of the SH signal and utilizing a novel correlation coefficient procedure for wide-field SH imaging, we have identified two types of surface areas on the polycrystalline gold. The first type remains stable during electrochemical cycling, while the second type undergoes surface reconstruction. We assign the second type of gold surface areas to domains of higher roughness, where anion adsorption occurs at lower potentials than expected.
Lastly, we focused on studying the oxygen evolution reaction (OER) on polycrystalline gold working electrodes by coupling SH imaging with electrochemical methods. We observe a large spatial heterogeneity of the OER, and we quantify that only < 1 % of the electrode surface is responsible for oxygen production. We further identify two types of active sites. The first type is observed at potentials > 2 V vs. the reverse hydrogen electrode (RHE), which is the onset of the OER. This type is stable under potential cycling, presumably extending multiple layers deep into the gold electrode. The second, anomalous type, is observed at potentials < 2 V vs. RHE. This type is removed by potential cycling, suggesting it involves a structural motif only 1-2 gold layers deep.
The findings in this thesis highlight the importance of surface heterogeneity on a sub-ÎŒm level, showing that the surface structure influences the chemical properties and reactivity. This heterogeneity needs to be considered and utilized in the industrial context to establish more efficient electrochemical processes with lower energy requirements.
EPFL2020
