Surface Chemistry of Perovskite-Type Electrodes During High Temperature CO2 Electrolysis Investigated by Operando Photoelectron Spectroscopy

Any substantial move of energy sources from fossil fuels to renewable resources requires large scale storage of excess energy, for example, via power to fuel processes. In this respect electrochemical reduction of CO2 may become very important, since it offers a method of sustainable CO production, which is a crucial prerequisite for synthesis of sustainable fuels. Carbon dioxide reduction in solid oxide electrolysis cells (SOECs) is particularly promising owing to the high operating temperature, which leads to both improved thermodynamics and fast kinetics. Additionally, compared to purely chemical CO formation on oxide catalysts, SOECs have the outstanding advantage that the catalytically active oxygen vacancies are continuously formed at the counter electrode, and move to the working electrode where they reactivate the oxide surface without the need of a preceding chemical (e.g., by H2) or thermal reduction step. In the present work, the surface chemistry of (La,Sr)FeO3−δ and (La,Sr)CrO3−δ based perovskite-type electrodes was studied during electrochemical CO2 reduction by means of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at SOEC operating temperatures. These measurements revealed the formation of a carbonate intermediate, which develops on the oxide surface only upon cathodic polarization (i.e., under sufficiently reducing conditions). The amount of this adsorbate increases with increasing oxygen vacancy concentration of the electrode material, thus suggesting vacant oxygen lattice sites as the predominant adsorption sites for carbon dioxide. The correlation of carbonate coverage and cathodic polarization indicates that an electron transfer is required to form the carbonate and thus to activate CO2 on the oxide surface. The results also suggest that acceptor doped oxides with high electron concentration and high oxygen vacancy concentration may be particularly suited for CO2 reduction. In contrast to water splitting, the CO2 electrolysis reaction was not significantly affected by metallic particles, which were exsolved from the perovskite electrodes upon cathodic polarization. Carbon formation on the electrode surface was only observed under very strong cathodic conditions, and the carbon could be easily removed by retracting the applied voltage without damaging the electrode, which is particularly promising from an application point of view.

Impedance spectra were measured by means of a Novocontrol Alpha-A High Performance Frequency Analyzer equipped with a POT/GAL 30V 2A interface. e typically recorded frequency range was 5 mHz -1 MHz but was adjusted if necessary; the AC voltage was 5 mV root mean square. Impedance spectra with and without an additional DC polarization obtained on La0.6Sr0.4FeO3-δ (LSF) and La0.8Sr0.2Cr0.9Ni0.1O3-δ (LSCrNi8291) working electrodes are shown in Figures S1 and S2, respectively. All spectra consist of a high frequency intercept on the real axis -cf. Figures S1b and S2b -and an electrode feature in the medium to low frequency range. e high frequency intercept contains contributions of the ohmic resistance of ion transport in the YSZ electrolyte as well as contact and wire resistances. Since at lower temperature the electrolyte resistance is by far dominant and the relationship of ionic conductivity in YSZ and temperature is well known, it was used for temperature calibration of the employed pyrometer at 400 and 600 °C. [1][2] Without an applied DC bias the electrode features of both materials appear rather huge without reaching the real axis (see Figures S1a and S2a). Upon applying about -1 V cathodic bias, however, the electrode feature is signi cantly smaller -see Figures S1b and S2b. is behavior can be understood by considering the reaction at the working electrode: CO2 + 2 e -⇄ CO + O 2-. Since without polarization virtually only CO2 is present in the atmosphere the exchange rate of this reaction is extremely low; in this respect please compare also the very shallow slope at 0 V of the I-V-curve in Figure 2a. us the electrode resistance measured by impedancewhich is inversely proportional to this exchange rate -is huge. DC bias speeds up the electrode reaction, the slope of the I-V-curve becomes larger, and the electrode's impedance feature is thus smaller.

CURRENT-VOLTAGE CHARACTERISTICS
Current voltage curves on LSCrNi7291 and LSF were also measured at lower temperatures of 600 °C and 400 °C, respectively. e resulting Tafel diagrams are shown in Figure S3. As for the I-V-curves measured at higher temperatures the data were ed to Equ. 6b -the resulting t parameter I0 and α are summarized in Table S1. Table S1: Resulting t parameters of Tafel Fits in Figure S3 using Equ. 6b.

SCANNING ELECTRON MICROSCOPY (SEM)
SEM images were recorded on a FEI Quanta 200 FEG. Due to preparation reasons each sample contains parts of the perovskite thin lm, which are electrically not connected to the working electrode. us it was possible to investigate parts of the same sample with and without cathodic polarization but with the same thermal prehistory in 0.25 mbar CO2 atmosphere. e SEM results for LSF and LSCrNi7291 are shown in Figure S4. On both samples cathodic polarization obviously caused evolution of small particles, which together with spectroscopic results can be regarded as exsolved metallic particles. S-4

INTERPRETATION OF XPS-DATA
Identi cation of gas phase peaks was supported by suppressing them by applying a 90 V DC bias to the electron collecting nozzle of the spectrometer. is leads to a very strong broadening of the gas phase peaks, causing them virtually vanishing in the background. A comparison of C 1s spectra measured with and without the bias applied to the nozzle is shown in Figure S5. Analysis of XPS spectra was done using the so ware CasaXPS. For ing of O 1s spectra a Shirley-type background was used, whereas for C 1s spectra a combination of Shirley-type and linear background was employed. For ing of background corrected peaks of O 1s spectra a combination of a Gaussian and Lorentzian line shape was used. In case of C 1s spectra a combination of a Gaussian and Lorentzian line shape was used for gas phase and carbonate peak, while the graphite peak was ed with an asymmetric line shape according to literature 3 Binding energies obtained by the above mentioned t routine for di erent XPS peaks are plo ed as a Figure S5: Comparison of C 1s spectra with and without 90 V DC bias at spectrometer nozzle.
function of the applied electrochemical polarization in Figure S6. ese plots support the conclusion drawn from Figure 7a in the main manuscript that the carbonate is located on the oxide surface and is neither adsorbed on exsolved metal particles nor a kind of gas molecule. S-5