Silicon Contamination of the Praseodymium Doped Ceria Oxygen Surface Exchange Coefficient

Abstract

Chemical oxygen surface exchange coefficients are used to quantify and rank the performance of oxygen exchange catalysts used in solid oxide fuel cell, solid oxide electrolysis cell, oxygen separation membrane, catalytic converter, and other oxygen-exchange-enabled devices. Unfortunately, during the manufacture and operation of these devices it is easy to introduce siliceous contaminants that degrade their oxygen exchange performance. Surprisingly, despite the well-known negative impact siliceous impurities have on the oxygen transport properties of ceria-based materials, quantitative measurements of how siliceous impurities impact the chemical oxygen surface exchange coefficient of praseodymium doped ceria are largely absent from the literature. In this work, the crystal structure, film thickness, surface composition, bulk composition, and chemical oxygen exchange coefficient of (100)-oriented PCO thin films with, and without, surface silica contaminants were evaluated via X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), Secondary Ion Mass Spectroscopy (SIMS), and Curvature Relaxation (κR) experiments, respectively. The results show that siliceous surface phases only a few nanometers thick are capable of (1) reducing the PCO chemical oxygen surface exchange coefficient by approximately 3 orders of magnitude and (2) approximately doubling the activation energy for oxygen exchange.

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

“…[ 12,54–56 ] Recently, Ma et al found that even if the siliceous surface phase is only a few nanometers thick, the chemical oxygen surface exchange coefficient of PCO decreases by ≈3 orders of magnitude and the activation energy is doubled. [ 57 ] In the context of this work, it is interesting to relate our findings for the selected levels of silica infiltration with what might be typical in applications. Horita et al., for example, examined the effects of impurities on the performance degradation of flatten tubular SOFC cells.…”

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

“…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.…”

“…For example, Si-species are recognized as an ubiquitous poison in metal oxide processing steps, for example, originating from furnace refractories and sealants/greases. 3,27 The widespread use of volatile organic silicon compounds (VSCs) in consumer products, for example, in electronics, furniture, healthcare, pharmaceuticals, cosmetics, and cookware, is already known to degrade SMOX-based sensors 8,22,27 ; and therefore there is a significant push in the industry to develop stable sensor options. 3,8 Si-species are known to significantly degrade SOFC cathode materials as well.…”

“…This process was thermally activated and had an activation energy comparable to that previously attained using PCO dense thin films, indicating that this process represents the ORR (see Figure 5C). 27,48,49 The significant degradation already at low concentrations, far below what would be needed for full surface coverage, indicates that an effect beyond the commonly accepted mechanism of physical blocking by Si-species must additionally play a role. 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.…”

“…27,48,49 The significant degradation already at low concentrations, far below what would be needed for full surface coverage, indicates that an effect beyond the commonly accepted mechanism of physical blocking by Si-species must additionally play a role. 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.…”