Tuning Surface Acidity of Mixed Conducting Electrodes: Recovery of Si‐Induced Degradation of Oxygen Exchange Rate and Area Specific Resistance

Abstract

metal oxide-based gas sensors, for which device performance relies on rapid oxygen exchange reactions at the gas-solid interfaces. [1][2][3][4][5][6][7] In SOFCs, the oxygen reduction reaction (ORR) occurs at the cathode and the corresponding polarization resistance depends on the adsorption/dissociation of oxygen species on the surface, followed by charge transfer between the surface and the adsorbed species, and finally the incorporation of charged oxygen ionic species into the electrode lattice. Although many studies related to improving the initial performance of mixed conducting metal oxides have been pursued to date, particularly focused on tuning the oxygen exchange coefficient (k chem ), the performance of such devices often degrades rapidly due to the inevitable poisoning by extrinsic sources over time during device operation. [8][9][10][11] Sources of Si-impurities are not only widely present in material processing and testing (e.g., the use of quartz reactors, furnace refractories, glass or glass-ceramic sealants, and thermal insulation materials), but also during device operation (e.g., volatile organic silicon compounds (VOSCs) and gas impurities). [11][12][13][14][15][16][17][18][19][20][21][22] Si-contamination is known to cause significant performance degradation of many electrochemical devices. [8] For example, Perz et al. showed that silica reacts with the La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) surface, a mixed ionic and electronic conducting (MIEC) cathode commonly applied in SOFCs, forming a continuous La-Sr-silicate layer. [22] This impurity layer limits adsorption/dissociation of oxygen molecules on the LSCF surface, increasing the area-specific resistance (ASR) by a factor 6 after long-term operation (1340 h) at 700 °C. Zhao et al. reported the formation of a blocking siliceous layer on dense thin films of Pr 0.1 Ce 0.9 O 2-δ (PCO), originating from quartz tubes or furnace insulation utilized during measurement, leading to the degradation of the oxygen surface exchange coefficient (k chem ) by a factor 40. [19] Hertz et al. demonstrated the ability to reduce the ASR associated with thin film interdigitated Pt electrodes on a yttria-stabilized zirconia (YSZ) electrolyte by 1000-fold following HF etching of surface silica species, thereby allowing oxygen gas to readily access the triple phase reaction sites. [15] To date, most previous studies point to silica forming a continuous glassy blocking layer as the source of the degradation.Metal oxides are an important class of functional materials, and for many applications, ranging from solid oxide fuel/electrolysis cells, oxygen permeation membranes, and oxygen storage materials to gas sensors (semiconducting and electrolytic) and catalysts, the interaction between the surface and oxygen in the gas phase is central. Ubiquitous Si-impurities are known to impede this interaction, commonly attributed to the formation of glassy blocking layers on the surface. Here, the surface oxygen exchange coefficient (k chem ) is examined for Pr 0.1 Ce 0.9 O 2...

Surface electron modulation of metal oxide‐based electrochemical devices by surface additives—linking sensors and fuel cells

“…Through a systematic study of different additives, it was found that oxides described by Smith as basic, for example, lithia or calcia, resulted in an increased response rate, while those considered acidic, for example, silica or alumina, suppressed the response speed 18 . In line with the findings of Nicollet et al., the relaxation kinetics of PCO onto which Si‐species had been added to the surface responded considerably slower to stepwise oxygen changes than the pristine material, Figure 4B (blue) 3,18,27,43,44 . On the other hand, the basic additive lithia, resulted in an increased response rate, see Figure 4B (yellow).…”

“…Additionally, the basic additive can react with the Si-species, forming a silicate (shown in green in Figure 5D) which is expected to exhibit intermediate levels of acidity. 44 As expected, significant degradation was also found following infiltration with acidic Cr-additives. Here, also subsequent recovery was achieved by follow-up infiltration with basic additive (Lior Ca-species).…”

“…18 In line with the findings of Nicollet et al, the relaxation kinetics of PCO onto which Si-species had been added to the surface responded considerably slower to stepwise oxygen changes than the pristine material, Figure 4B (blue). 3,18,27,43,44 On the other hand, the basic additive lithia, resulted in an increased response rate, see Figure 4B (yellow). The addition of these two surface additives, resulted in a variation of k chem (derived from an analysis of relaxation profiles) by 6 orders of magnitude.…”

“…3,27,50 Inspired by previous results on the effect of single additives on performance, we tested if the ORR activity at the PCO/gas interface following degradation with Si-species could be regenerated by subsequent infiltration with basic Li-additives. 18,44,48 Indeed, infiltration with 0.1 at % Li.-additive significantly reduced the polarization resistance of the cell, and after 0.2 at% infiltration, the cell performance was nearly completely regenerated, Figure 5C. Taken in sum, based on these results it is likely that the significant degradation of the surface activity already at low Si-levels is due to the depletion of surface electrons induced by the acidic additives that impedes the charge transfer step in the oxygen exchange reaction.…”

“…Modified from Seo et al. with permission from Wiley‐VCH GmbH 44 . (B) Nyquist plots of the pristine cell, after infiltration with acidic Si‐ additive, followed by lower and higher level of Li‐additive infiltration.…”