Determination of Au and Pt Current Collector Activity to Avoid Interference in the Screening of Metal Oxide Catalyst Activity in SOECs

Abstract

Since solid oxides electrolysis cells (SOECs) represent a promising technology for efficient CO2 conversion to useful chemical products, there is a demand for durable, highly active, and inexpensive electrode materials for use in these systems. Many metal oxide catalysts suitable for CO2 reduction have been studied, including La0.3Me0.7Fe0.7Cr0.3O3-δ (Me = Ca, Sr; LMFCr), developed by our research group. These catalysts have comparable activity and are much more stable than conventional metal-ionic conductor cermets, known for coke formation and sulfur poisoning. However, while observed catalyst stability can be easily compared for different materials, their activity is harder to differentiate. This is because the overwhelming majority of materials tested are porous in nature, thus making their true active area unknown. To avoid this issue, we are working on determining LMFCr activity and the CO2 reduction mechanism using thin, smooth catalyst layers, obtained by pulsed laser deposition on a YSZ-(001) electrolyte, then using noble metals as the current collector.Preliminary studies of current collector-LMFCr thin film-CO2/CO combinations suggest that the current collector/LMFCr interfaces may have an electrochemical activity that would contribute measurably to that of the LMFCr-gas dual phase boundary interface. Assuming that the metal/LMFCr interface activity would be similar to that at the metal/ionic conductor triple phase boundary, the goal of the present work is to determine the catalytic activity of both Pt and Au, which are the most common current collector materials used in fuel environments, towards the CO2 reduction reaction, with the activity of these metal/YSZ interfaces toward oxygen reduction used as a reference.Two different full cell configurations were used in this work. In one direction, porous Au or Pt layers were produced by metal paste deposition onto both sides of the YSZ electrolyte and then sintered at 600°C. A polymeric YSZ precursor was then infiltrated into the porous metal backbone and sintered at 750°C to form the YSZ phase. To ensure that the counter was as non-polarizable as possible, Pt in air was chosen due to its high activity in oxygen-containing environments, while also depositing it over 5X the area and ensuring 2X the thickness of the catalyst layer at the working electrode. In the porous electrode configuration, the working electrode was exposed to the testing gas, while the counter electrode was exposed in air to achieve as high an activity as possible. In the second configuration, 0.3 mm thick Pt or Au wires were attached to a 250 µm thick YSZ electrolyte to form a point-contact working electrode. A porous LSM-YSZ composite with a geometric surface area of 0.5 cm2 was painted on the opposite side of the electrolyte to serve as the counter electrode, exposing it to the same gas as the working electrode. Electrochemical performance testing for both cell types was conducted in air and CO2 environments at 750°C with a flow rate of 50 SCCM. Morphological and compositional investigation as well as microelectrode triple phase length determination was done using SEM and EDS elemental mapping.Analysis of the EIS and CV data for the porous electrodes showed that the Pt/YSZ and Au/YSZ electrodes are both active towards the oxygen and CO2 reduction reactions. Au is less so, showing up to 40 times lower activity towards oxygen reduction and 3 times lower activity than for CO2 reduction. This would imply that Au is the preferred current collector material for future electrochemical evaluation of metal oxide catalysts. However, SEM studies indicated that Pt and Au cells had different metal particle sizes, so the triple phase boundary length may be not comparable. In addition, the double layer capacitance, often used to estimate the triple phase boundary length, could not be used here due to the high surface inhomogeneity and distribution of time constants across these catalyst layers.To eliminate these issues, microelectrode studies are currently being conducted. Preliminary results show that Au/YSZ is at least 8 times less active in air and 60 times less active in the CO2 environment than Pt/YSZ, with the activity of both of these electrodes controlled by precisely controlled triple phase boundary length determination via SEM. The cyclic voltammetry data also indicate that CO2 reduction does not occur at Au/YSZ through the full potential range (0-1.6 V vs. the counter electrode), which also implies a very low catalytic activity of Au/YSZ. Further studies, including comparison with the porous composite electrode data, are underway. In addition, determination of the reaction steps is being carried out from EIS data equivalent circuit fitting as well as verification through the use of distribution relaxation times technique.