Mapping Electrochemical Heterogeneity at Gold Surfaces: A Second Harmonic Imaging Study

Designing efficient catalysts requires correlating surface structure and local chemical composition with reactivity on length scales from nanometers to tens of microns. While much work has been done on this structure/function correlation on single crystals, comparatively little has been done for catalysts of relevance in applications. Such materials are typically highly heterogeneous and thus require methods that allow mapping of the structure/function relationship during electrochemical conversion. Here, we use optical second harmonic imaging combined with cyclic voltammetry to map the surface of gold nanocrystalline and polycrystalline electrodes during electrooxidation and to quantify the spatial extent of surface reconstruction during potential cycling. The wide-field configuration of our microscope allows for real-time imaging of an area similar to 100 mu m in diameter with submicron resolution. By analyzing the voltage dependence of each pixel, we uncover the heterogeneity of the second harmonic signal and quantify the fraction of domains where it follows a positive quadratic dependence with increasing bias. There, the second harmonic intensity is mainly ascribed to electronic polarization contributions at the metal/electrolyte interface. Additionally, we locate areas where the second harmonic signal follows a negative quadratic dependence with increasing bias, which also show the largest changes during successive cyclic voltammetry sweeps as determined by an additional correlation coefficient analysis. We assign these areas to domains of higher roughness that are prone to potential-induced surface restructuring and where anion adsorption occurs at lower potentials than expected based on the cyclic voltammetry.

Mapping of Chemical Heterogeneity at Surfaces with High-Throughput Wide-Field Second Harmonic Microscopy

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

Simultaneous determination of chemical diffusion and surface exchange coefficients of oxygen by the potential step technique

Alexandre Closset, Stefan Diethelm, Jan Van Herle

Oxygen diffusion is treated in a dense electronically conducting cobaltate pellet blocked ionically on one surface, electronically on the other, and sealed on its cylindrical periphery. A procedure is developed for extracting the chemical diffusion and surface exchange coefficients for oxygen by use of the asymptotic equations derived for the current response to a potential step at short and long times. It is shown that, while the formation of interfacial phases by reaction between the sample and the electrolyte may affect the surface exchange coefficient, the chemical diffusion coefficient data determined by the present approach are independent of such interfacial phenomena. The consistency of data obtained from several specimens with varying thickness and manner of interfacing with the electrolyte validates the diffusion model and the method used for data analysis. An oxygen permeation cell is also developed in this work as a modification of the diffusion cell. The new cell allows monitoring of the permeation rate by electrochemical means. The steady-state permeation data obtained by the permeation cell are consistent with the chemical-diffusion and surface- exchange coefficients measured by the blocked diffusion cell as long as the assumptions of the related theoretical models are satisfied. This is a further validation of the diffusion model and the related methodology developed here for obtaining the necessary data for characterizing oxygen exchange and transport in such materials.

1999

Electrochemical Promotion and Metal-Support Interactions

Christos Comninellis

Ethylene oxidn. on IrO2, which is a metal-type conducting metal oxide, and on Pt and Rh was used as a model reaction to compare the rate enhancement and kinetic modification induced by (i) electrochem. promotion (non-Faradaic electrochem. modification of catalytic activity effect) via electrochem. O2- supply from YSZ (Y2O3-stabilized ZrO2) and TiO2 and by (ii) metal-support interactions obtained by interfacing IrO2 with TiO2 (with and without electrochem. promotion) or by depositing dispersed Rh on TiO2 and YSZ porous supports. It was found that the addn. of TiO2, which is catalytically inactive, to IrO2 submicrometer particles enhances the activity of IrO2 for C2H4 oxidn. by a factor of 12 and that the same max. rate enhancement is obtained via electrochem. promotion of the IrO2 catalyst (i.e., via electrochem. O2- supply to the IrO2 catalyst from a YSZ solid electrolyte). Furthermore it was found that the IrO2-TiO2 catalyst mixts. can only be marginally promoted electrochem. These observations show conclusively that the mechanism of metal (IrO2)-support (TiO2) interaction in the system IrO2-TiO2 is identical to that of the electrochem. promoted IrO2-YSZ system (i.e., continuous O2- supply to the IrO2 catalyst surface). This conclusion is also corroborated by independent kinetic and XPS studies of electrochem. promotion of Pt utilizing TiO2 (instead of YSZ) as the O2- donor. The kinetics of C2H4 oxidn. were investigated on Rh films interfaced with YSZ at various imposed potentials and thus imposed work function values, F (i.e., under conditions of electrochem. promotion and also on finely dispersed Rh catalysts deposited on various doped and undoped TiO2 and YSZ porous supports of measured work function F). Again it was found that the reaction kinetics are affected in the same way upon varying the work function of the Rh film via elec. polarization of the Rh/YSZ interface or upon varying the work function of the support of the dispersed Rh catalysts. This observation confirms the equivalence of the promoting mechanism of metal-support interactions and electrochem. promotion (i.e., O2- migration onto the catalyst surface). The results show conclusively that electrochem. promotion is an elec. controlled metal-support interaction and that at least certain types of metal-support interactions are induced by reverse spillover of oxygen anions from the carrier onto the surface of the metal crystallites. (c) 2001 Academic Press. [on SciFinder (R)]