Formation and Detection of High-Pressure Oxygen in Closed Pores of La_0.6Sr_0.4CoO_3−δ Solid Oxide Electrolysis Anodes

Abstract

The chemical capacitance of La 0.6 Sr 0.4 CoO 3−δ (LSC) thin film microelectrodes with different microstructures was investigated upon varying anodic DC voltages. Dense and porous electrodes (open porosity) were prepared by using different parameters during pulsed laser deposition (PLD). Furthermore, electrodes with closed porosity were fabricated by depositing a dense capping layer on a porous film. Electrochemical impedance spectroscopy (EIS) was performed in synthetic air at 460 and 608 °C with anodic DC voltages up to 440 mV. Chemical capacitance values of the electrodes were derived from the obtained spectra. While the chemical capacitance of dense and porous electrodes decreased as expected with increasing anodic overpotential, electrodes with closed pores exhibited very unusual peaks with extremely high values of >8000 F/cm 3 at overpotentials of >100 mV. We demonstrate that this huge capacitance increase agrees very well with calculated chemical capacitances deduced from a real gas equation. Hence, we conclude that the formation of highly pressurized oxygen (up to gas pressures of ∼10 4 bar) in closed pores is responsible for this strong capacitive effect at anodic overpotentials. Such measurements can thus detect and quantify the buildup of high internal gas pressures in closed pores at the anode side of solid oxide electrolysis cells.

Closed-Pore Formation in Oxygen Electrodes for Solid Oxide Electrolysis Cells Investigated by Impedance Spectroscopy

“…Under anodic polarization, high-pressure oxygen then forms in these closed pores, leading to the observed chemical capacitance peak. Remarkably, these closed pores seem to withstand pressures in the range of 10 4 bar (calculated via the Soave−Redlich−Kwong real-gas equation, as shown in a previous study 29 ) because consecutive measurements of the chemical capacitance on a porous electrode yielded almost identical curves (Figure 7).…”

“…Therefore, we suggest that they all can be traced back to the same underlying mechanism. As shown in a former study 29 and briefly discussed above, porous/ capped electrodes exhibit a capacitance peak already in the pristine state due to high pressure oxygen gas formation in closed pores. Closed porosity seems to be the key requirement for the occurrence of the capacitance peak because this is the distinctive property of pristine porous/capped electrodes.…”

“…Further details regarding the ICP-MS measurements were reported in a previous paper. 29 2.5. In Situ NAP-XPS.…”

“…For the purpose of validation, we analyzed the surface compositions of pristine and annealed LSC thin films of different sample types via ICP-MS measurements using an approach already applied in previous studies. 29,30,33,59 Furthermore, by employing this method, we can get information about the morphology of the studied LSC films, which is another important aspect of our mechanistic model.…”

“…XRD diffractograms of pristine poly/dense, pristine porous/capped, pristine, and annealed porous films measured in the grazing-incidence geometry (a) as well as of epi/dense and cracked/dense (“frozen” at 195 mV and 600 °C and without polarization history at 600 °C) films measured in the Bragg–Brentano geometry (b).…”