Stuart B. Adler

Electrode Kinetics of Porous Mixed‐Conducting Oxygen Electrodes

In this paper we use continuum modeling to analyze the mechanism of the oxygen reduction reaction at a porous mixed-conducting oxygen electrode. We show that for La 0.6 (Ca, Sr) 0.4 Fe 0.8 Co 0.2 O 3-δ at 700 C, solid-state oxygen diffusion and O 2 surface exchange dominate the electrochemical behavior, producing effective chemical resistances and capacitances. This behavior can be explained both qualitatively and quantitatively in terms of the known bulk and surface properties of the materials. This mechanism appears to be generally valid for mixed conductors with high rates of internal mass transfer, but breaks down for mixed conductors that have poor ionic transport. Our analysis also suggests that, for the best electrode materials, extension of the reaction zone beyond the three-phase boundary is limited to a few micrometers. We also show that gas phase diffusion resistance can contribute significantly to cell impedance at P o2 ≤ 0.1 atm

Reference Electrode Placement in Thin Solid Electrolytes

To separate anode and cathode contributions to the impedance of a thin solid electrolyte cell, many workers place a reference electrode on the surface of the inactive electrolyte, coplanar with the working electrode. This practice is evaluated theoretically using finite-element calculations of the real and imaginary potential distributions in the electrolyte under the conditions of ac impedance. The results of this analysis show that minor errors in the alignment of the anode and cathode can create significant errors in the measured half-cell overpotential. These errors involve not only quantitative scaling factors, but also cross-contamination of anode and cathode frequency response in the measured half-cell impedances. Even if electrodes are perfectly aligned, differences between the anode and cathode kinetics and/or frequency response may cause inherent distortion of the impedance, including frequency dispersion and inductive artifacts. We evaluate two approaches (pellets and microelectrode arrays) that can he used to avoid ambiguities.

Limitations of charge-transfer models for mixed-conducting oxygen electrodes

Abstract A framework is presented for defining charge-transfer and non-charge-transfer processes in solid state electrochemical systems. We examine why charge-transfer models have difficulty modeling non-charge-transfer effects, and walk through several examples including the ALS model for oxygen reduction on a porous mixed-conducting oxygen electrode. These examples illustrate that electrode ‘overpotential’ is often better described in terms of macroscopic thermodynamic gradients of chemical species. In the case of a porous mixed conducting oxygen electrode, oxygen reduction is limited by chemical reaction and diffusion, and may occur up to 20 microns from the electrochemical (charge-transfer) interface.

Reference electrode placement and seals in electrochemical oxygen generators

We report measurements and numerical calculations of the potential distribution within a thin solid electrolyte near active (current-bearing) electrodes. These studies demonstrate two principles: (1) In a flat-plate geometry, the electrolyte is approximately equipotential beyond a distance of about three electrolyte thicknesses from the edge of the active electrodes. (2) If one of the active electrodes on one side of the electrolyte extends beyond the other, it strongly biases the potential of the electrolyte far from the active region. We show that these effects make it challenging to measure electrode overpotential accurately on thin cells. However, we also show that these effects can be useful for protecting glass-ceramic seals in an oxygen generator stack against electrochemical degradation/delamination.

An electrical conductivity relaxation study of La0.6Sr0.4Fe0.8Co0.2O3−δ

Abstract Electrical conductivity relaxation (ECR) experiments were carried out on La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3− δ (LSFCO) in the temperature range of 840 to 910 °C and in the oxygen partial pressure range of 0.01 to 0.05 atm. The effects of annealing at 900 °C on the oxygen transport properties were investigated for individual samples. The values of D O 2− =D O / Γ O and k ex = k chem / Γ O agree well with the result from isotope-exchange depth profile (IEDP) experiments.

Anomalous Interface and Surface Strontium Segregation in (La1–ySry)2CoO4±δ/La1–xSrxCoO3−δ Heterostructured Thin Films

Heterostructured oxides have shown unusual electrochemical properties including enhanced catalytic activity, ion transport, and stability. In particular, it has been shown recently that the activity of oxygen electrocatalysis on the Ruddlesden-Popper/perovskite (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ heterostructure is remarkably enhanced relative to the Ruddlesden-Popper and perovskite constituents. Here we report the first atomic-scale structure and composition of (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ grown on SrTiO3. We observe anomalous strontium segregation from the perovskite to the interface and the Ruddlesden-Popper phase using direct X-ray methods as well as with ab initio calculations. Such Sr segregation occurred during the film growth, and no significant changes were found upon subsequent annealing in O2. Our findings provide insights into the design of highly active catalysts for oxygen electrocatalysis.

Imaging space charge regions in Sm-doped ceria using electrochemical strain microscopy

Nanocrystalline ceria exhibits a total conductivity several orders of magnitude higher than microcrystalline ceria in air at high temperature. The most widely accepted theory for this enhancement (based on fitting of conductivity data to various transport and kinetic models) is that relatively immobile positively charged defects and/or impurities accumulate at the grain boundary core, leading to a counterbalancing increase in the number of mobile electrons (small polarons) within a diffuse space charge region adjacent to each grain boundary. In an effort to validate this model, we have applied electrochemical strain microscopy to image the location and relative population of mobile electrons near grain boundaries in polycrystalline Sm-doped ceria in air at 20–200 °C. Our results show the first direct (spatially resolved) evidence that such a diffuse space charge region does exist in ceria, and is localized to both grain boundaries and the gas-exposed surface.

Microelectrode Array for Isolation of Electrode Polarization on Planar Solid Electrolytes

Planar cells incorporating a microelectrode as the working electrode were prepared using materials and techniques commonly employed in fabrication of planar solid oxide fuel cells. Initial results of ac and dc polarization measurements suggest that these cells potentially offer excellent isolation of working electrode frequency response (impedance), and quantification of steady-state current-overpotential relationships. The success of the technique relies heavily on how precisely the geometry of the microelectrode pattern is defined and characterized. Some of the challenges in implementing this technique are discussed.

Scanning thermo-ionic microscopy for probing local electrochemistry at the nanoscale

Conventional electrochemical characterization techniques based on voltage and current measurements only probe faradaic and capacitive rates in aggregate. In this work we develop a scanning thermo-ionic microscopy (STIM) to probe local electrochemistry at the nanoscale, based on imaging of Vegard strain induced by thermal oscillation. It is demonstrated from both theoretical analysis and experimental validation that the second harmonic response of thermally induced cantilever vibration, associated with thermal expansion, is present in all solids, whereas the fourth harmonic response, caused by local transport of mobile species, is only present in ionic materials. The origin of STIM response is further confirmed by its reduced amplitude with respect to increased contact force, due to the coupling of stress to concentration of ionic species and/or electronic defects. The technique has been applied to probe Sm-doped Ceria and LiFePO4, both of which exhibit higher concentrations of mobile species near grain bo...

Influence of Electrolyte Surface Planarization on the Performance of the Porous SOFC Cathodes

The electrochemical characteristics of porous Pt and La 0 . 8 Sr 0 . 2 CoO 3 - δ (LSC) electrodes has been found to depend strongly on polishing of the underlying Ce 0 . 8 Sm 0 . 2 O 2 - δ (SDC) electrolyte using SiC and/or diamond prior to cell fabrication. The effect of surface treatment on the microstructure and composition of SDC was investigated using scanning electron microscopy and electron spectroscopy for chemical analysis. After high-temperature sintering, the surface of SDC is Sm-enriched and has granular morphology; grinding or polishing removes this grained layer and leads to a decrease in Sm concentration. Refiring of the polished electrolyte at 1450-1650°C results in restoration of the grain structure and Sm composition on the surface. It was found using electrochemical impedance spectroscopy that the surface treatment most strongly influences the high-frequency component of the impedance, suggesting the changes are associated primarily with an interfacial resistance.