Imaging the Heterogeneity of the Oxygen Evolution Reaction on Gold Electrodes Operando: Activity is Highly Local

Understanding the mechanism of the oxygen evolution reaction (OER), the oxidative half of electrolytic water splitting, has proven challenging. Perhaps the largest hurdle has been gaining experimental insight into the active site of the electrocatalyst used to facilitate this chemistry. Decades of study have clarified that a range of transition-metal oxides have particularly high catalytic activity for the OER. Unfortunately, for virtually all of these materials, metal oxidation and the OER occur at similar potentials. As a result, catalyst surface topography and electronic structure are expected to continuously evolve under reactive conditions. Gaining experimental insight into the OER mechanism on such materials thus requires a tool that allows spatially resolved characterization of the OER activity. In this study, we overcome this formidable experimental challenge using second harmonic microscopy and electrochemical methods to characterize the spatial heterogeneity of OER activity on polycrystalline Au working electrodes. At moderately anodic potentials, we find that the OER activity of the electrode is dominated by

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

Promotion électrochimique des catalyseurs à base de rhodium et d'iridium

Justyna Eaves

In this thesis, the electrochemical promotion of lr and Rh based catalysts was studied for the complete combustion of ethylene and propylene. The study was performed by using two types of electrocatalytical cells: a classical, pellet type cell and a tubular cell. This work has two major goals. The first one is the optimization of the employed electrocatalytical systems from electrochemical point of view. The second one concerns the study of the mechanism of the electrochemical promotion. The major part of this work concerns the electrochemical characterization of two cell types: a pellet type cell and a tubular cell. On the basis of the obtained results the modélisation of the potential and current distribution was performed, providing key parameters for the development of more efficient cell configurations. On one hand, these configurations ensure an uniform current distribution on the working electrode and enable correct catalyst potential measurement. On the other hand, it minimizes the current bypass in the bipolar tubular cell. The aim of the investigation in a two-compartment reactor was to evaluate and to compare the Au and Ir electrodes used in this work. Under 400°C, gold was found to be much worse material than lrÛ2 both as reference and counter electrode. Its potential was affected by oxygen and propylene adsorption and no equilibrium of electrochemical reaction was obtained at such low temperature. On the base of this knowledge, several materials better adapted to electrochemical promotion conditions have been proposed. Using the two-compartment reactor potential measurements under steady state conditions allowed to verify the mechanism of the propylene combustion on Rh and that of ethylene combustion on lr based catalysts proposed in the literature. Ameliorations of the existent kinetic models were proposed. The kinetic measurements were then analyzed with the additional help of ex situ XPS analysis. This study allowed to link the oxidation state of the catalyst surface with the extent of electrochemical promotion of the catalysts. It also allowed the ex situ observation of the appearance of new oxygen (oxide) species on the gas exposed surface of Ir catalyst. This new type of oxygen was different from the oxygen of Ir02 oxide, and it can represent the promoting species, according to backspiïïover theory. The same oxygen species was observed on well oxidized Ir02 catalyst surface, even before the polarization of this catalyst. Its presence was associated to the catalyst-support effect. It explains the high activity of Ir02 catalyst by comparison with the catalytic activity of lr metal. XPS analysis of Rh based catalysts indicated the possibly formation of an inferior Rh oxide (close to RhO) due to polarization. This may be a possible explanation of the observed permanent electrochemical promotion. Voltammetric charge measurements in very narrow potential window were performed while changing the catalyst thickness. The charge was demonstrated to be proportional to the catalyst thickness, suggesting the presence of the charged species on the gas exposed catalyst surface. The analogy between this situation and that of the emerged electrodes in liquid state electrochemistry was emphasized. The voltammograms obtained in this study were interpreted in the quite new, original way, which allowed to calculate the capacity of the different catalyst interfaces, the gas/catalyst interface, and the triple phase boundary, in good agreement with reference values from impedance spectroscopy measurements. Finally, the reaction rate transients during galvanostatic polarization of lr based catalyst have been moralized. The model fits well the experimental results.

EPFL2004

Electrochemical promotion of Pt catalysts for gas phase reactions

Arnaud Jaccoud

The subject of this work is the study of the electrochemical promotion of catalysis (EPOC), also called non-Faradaic electrochemical modification of catalytic activity (NEMCA). In this phenomenon, application of small currents or potentials on catalysts in contact with solid electrolytes leads to very pronounced strongly non-Faradaic and reversible changes in catalytic activity and selectivity. This work concerns the system composed of platinum film electrodes, deposited on the YSZ electrolyte (ZrO2 doped with 8% Y2O3), in contact with an oxygen containing atmosphere (the O2(g),Pt/YSZ system). The aim of this work was to focus on two points in the phenomenon of EPOC: 1) The nature of the species present on the surface of the catalyst and their impact on the catalytic activity. 2) The phenomenon of permanent promotion of catalysis (P-EPOC), for which the catalytic rate enhancement is not totally reversible, leading to a long lasting activated state. The Pt electrodes were prepared via three different methods (sputtering, thermal decomposition and screen-printing) and, as expected, the resulting microstructure, observed by scanning electron microscopy (SEM), was found to depend greatly on the preparation technique. The electrochemical response of each electrode was characterized by the technique of cyclic voltammetry. Due to the occurrence of different site-related electrochemical reactions, the voltammogram shape was highly influenced by the microstructure of the Pt/YSZ contact. Based on the preliminary studies, the cermet electrode was chosen for additional electrochemical experiments, using chronoamperometry, chronopotentiometry, cyclic voltammetry, steady state polarization and impedance spectroscopy. The results of all these techniques suggest that under anodic polarization, two parallel reactions take place. The first reaction is the oxygen evolution, giving rise to a steady state current under a constant applied potential. The other reaction is the oxygen storage (oxygen injection) in the form of Pt-O species. This second process gives rise to a time dependant current under a constant applied potential, and is therefore responsible for the pseudocapacitive behavior of the electrode. Moreover, linear sweep voltammetry measurements indicated that, by application of an anodic potential, at least three types of Pt-O species were stored, following distinct kinetics. Based on the amount of stored Pt-O and on the corresponding storage kinetics, they were attributed to three different locations on the electrode: 1) at the Pt/YSZ interface, 2) diffusing from the tpb toward the Pt/gas interface, 3) diffusing from the Pt/YSZ interface toward the bulk of the platinum electrode. It has been shown that the Pt film exhibited various activity states toward the catalytic oxidation of ethylene, depending on the oxygen species present at its surface. As evidenced by XPS measurements and in agreement with the thermodynamic data for the given system, the inactive (deactivated) state has been attributed to the formation of surface platinum oxide at T > 550°C. Decreasing the reaction temperature to 525°C caused the slow spontaneous reactivation of the catalyst and several days were necessary to reach the active state, where most of the catalyst was present in its metallic state. The catalyst activity recovery could be accelerated by the application of anodic potential pulses. In agreement with the mechanism of EPOC, this was explained by the oxide destabilizing effect of the anodically produced Oδ- species. Anodic polarization also caused reversible electrochemical promotion of catalysis at unusual high temperature (600°C). In addition, very intriguing catalytic relaxation transients were observed after prolonged anodic polarization times. Indeed, it was found that enhanced catalytic activity could be maintained for up to 10 hours after current interruption. Combination of these latter results, the related literature, the state of the art model of EPOC, and the results obtained by electrochemical techniques led to the proposition of an original model based on the processes of Pt-O storage/release at various locations of the O2(g),Pt/YSZ system. According to this model, anodic polarization produces Oδ- back-spillover species, promoting the catalytic activity, in agreement with the mechanism of EPOC. In parallel, hidden oxygen species are stored at the Pt/YSZ interface and diffuse toward the Pt phase. When the polarization is switched off, these hidden oxygen species diffuse to the gas-exposed surface and cause non-Faradaic promotion, as back-spillover Oδ- do. The large amount of stored charge and its slow diffusion controlled emergence causes the rate enhancement to last for hours.

EPFL2007