The oxygen reduction reaction in solid oxide fuel cells: from kinetic parameters measurements to electrode design

Abstract

The research and development of new Solid Oxide Fuel Cell cathode materials is an area of intense activity. The kinetic coefficients describing the O2-reduction mechanism are the O-ion diffusion ( D chem ) and the O-surface exchange coefficients ( k chem ). These parameters are strongly dependent on the nature of the material, both on its bulk and surface atomic and electronic structures. This review discusses the method for obtaining the kinetic coefficients through the combination of electrochemical impedance spectroscopy with focused ion-beam 3D tomography measurements on porous electrodes (3DT-EIS). The data, together with oxygen non-stoichiometry thermodynamic data, is analysed using the Adler-Lane-Steele model for macro-homogeneous porous electrodes. The results for different families of oxides are compared: single- and double-layered perovskites with O-vacancies defects, based on La-Sr cobalt ferrites (La0.6Sr0.4Co1-xFexO3-δ , x = 0.2 and 0.8) and La/Pr-Ba cobaltites (La0.5-xPrxBa0.5CoO3-δ , x = 0.0, 0.2 and 0.5), as well as Ruddlesden-Popper nickelates (Nd2NiO4 +δ ) with O-interstitial defects. The analysis of the evolution of molar surface exchange rates with oxygen partial pressure provides information about the mechanisms limiting the O2-surface reaction, which generally is dissociative adsorption or dissociation-limited. At 700 °C in air, the La-Ba cobaltite structures, La0.5-xPrxBa0.5CoO3-δ , feature the most active surfaces ( k chem ≃0.5–1 10−2 cm.s−1), followed by the nickelate Nd2NiO4 +δ and the La-Sr cobalt ferrites, with k chem ≃1–5 10−5 cm.s−1. The diffusion coefficients D chem are higher for cubic perovskites than for the layered ones. For La0.6Sr0.4Co0.8Fe0.2O3-δ and La0.6Sr0.4Co0.2Fe0.8O3-δ , D chem is 2.6 10−6 cm2.s−1 and 5.4 10−7 cm2.s−1, respectively. These values are comparable to D chem = 1.2 10−6 cm2.s−1, observed for La0.5Ba0.5CoO3-δ . The layered structure drastically reduces the O-ion bulk diffusion, e.g. D chem = 1.3 10−8 cm2.s−1 for the Pr0.5Ba0.5CoO3-δ double perovskite and D chem ≃2 10−7cm2.s−1 for Nd2NiO4 +δ . Finally, the analysis of the time evolution of the electrodes shows that the surface cation segregation affects both the O-ion bulk diffusion and the surface exchange rates.

A Highly Efficient Bifunctional Electrode Fashioned with In Situ Exsolved NiFe Alloys for Reversible Solid Oxide Cells

“…For humidified syngas, the R p of NiFe-R.P.PSFNNb symmetric cell is 0.112, 0.216, and 0.278 Ω·cm 2 at temperatures of 850, 800, and 750 °C, respectively, which are substantially smaller than the NiFe-R.P.PSFNNb fuel electrode cell that shows 0.55, 0.699, and 1.039 Ω·cm 2 at temperatures of 850, 800, and 750 °C, respectively. In SOFC operation, the cathodic polarization resistance seems to possess substantial portion of total polarization because of the sluggish kinetics of the oxygen reduction reaction. , Also, largely produced heterointerface between alloy and oxide by in situ exsolution process is expected to reduce the anodic polarization resistance of the hydrogen oxidation reaction. On the other hand, relatively large polarization resistance compared with the fuel electrode under a 30% CO/CO 2 atmosphere (0.471, 0.952, and 1.99 Ω·cm 2 at temperatures of 850, 800, and 750 °C) could indicate slower kinetics of CO 2 electroreduction rather than the oxygen evolution reaction in SOEC operation.…”

Correlation between Structural and Transport Properties of Ca-Doped La Nickelates and Their Electrochemical Performance

“…The Gerischer resistance makes a larger contribution to the polarization resistance; therefore, it can be assumed that the behavior of the electrodes is due to the diffusion impedance. The Gerischer impedance [62,63] or in other words, the chemical impedance [64,65], is often used for fitting the spectra of the electrodes based on mixed ionic and electronic conductors (MIEC):…”

“…According to the Adler-Lane-Steele model, the polarization resistance of the electrodes is influenced by many factors such as the electrode microstructure and transport properties of the electrode material [63][64][65][66][67][68][69][70]. According to mathematical transformations presented in [63,66], the equation for the Gerischer impedance can be written in the following form:…”

“…Thus, for other conditions of the electrode preparation being equal, considering the electrodes' morphological characteristics as being similar across the entire series and assuming that the other parameters (A 0 , c 0 , x O ν ) do not contribute significantly to the dependence qualitative behavior, a qualitative correlation of the polarization resistance (R η ) and kinetic parameters (k * , D eff * ) can be estimated as an inverse proportion of k * D * e f f . A similar approach was successfully applied in [21,41,63,69] to establish a correlation between the transport properties of Pr and Nd nickelates and the electrochemical characteristics of the related electrodes.…”

Sr Substituted La2−xSrxNi0.8Co0.2O4+δ (0 ≤ x ≤ 0.8): Impact on Oxygen Stoichiometry and Electrochemical Properties

“…However, one of the major challenges is to mitigate the degradation rate and increase the widespread use of solid oxide cells, especially at high current densities during longterm operation [6,7]. In addition, the oxygen electrodes contribute more to the polarization resistance of single cells owing to the slow kinetics of the oxygen reduction reaction (ORR) [8,9]. Therefore, further modifications in the existing materials, as well as the search for new materials, are important in order to improve the oxygen transport properties, electrochemical activity, and durability of the cells.…”