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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.

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Oxygen reduction kinetic parameters -oxygen ion diffusion D δ , molar surface exchange rate O and surface exchange coefficient k -were determined for porous Nd 2 NiO 4+δ solid oxide fuel cell cathodes as a function of temperature and oxygen partial pressure by analyzing electrochemical impedance spectroscopy data using the Adler-Lane-Steele model. Electrode microstructural data used in the model calculations were obtained by three-dimensional focused ion beam-scanning electron microscope tomography. Cathodes were fabricated using Nd 2 NiO 4+δ powder derived from a sol-gel method and were tested as symmetrical cells with LSGM electrolytes. The oxygen surface exchange rate exhibited a power-law dependency with oxygen partial pressure, whereas the oxygen diffusivity values obtained varied only slightly. The present analysis suggests that the O-interstitial diffusion has a bulk transport path, whereas the surface exchange process involves dissociative adsorption on surface sites followed by O-incorporation. For Nd 2 NiO 4+δ at 700 • C and 0.2 atm oxygen pressure, D δ = 5.6 · 10 −8 cm 2 s −1 , O = 2.5 · 10 −8 mol · cm −2 s −1 . The present D δ and O values and their activation energies are slightly different to those previously reported for Nd 2 NiO 4+δ using other measurement methodologies, and lower than typical state-of-the-art Co-rich perovskites. However, the average k δ = 1.0 10 −5 cm · s −1 at 700 • C is comparable to those of fast oxygen exchange rate perovskites.

Electrochemical response was measured as a function of oxygen pressure pO2 up to 10 bar for four different mixed-conducting oxygen electrode materials, the oxygen-vacancy-conducting perovskites (Sm0.5Sr0.5)CoO3 (SSC) and (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF), and the interstitial-oxygen-conducting nickelates Pr2NiO4 (PNO) and Nd2NiO4 (NNO). The impedance spectroscopy (IS) measurements were done on symmetrical cells with either single-phase or two-phase infiltrated electrode structures. The polarization resistance decreased with increasing pressure in all cases, but the nickelates decreased more rapidly than the perovskites. It is proposed that this difference is a direct result of the different pO2 dependences of the defect concentrations – the oxygen vacancy concentration decreases with increasing pO2, whereas interstitial concentrations increase. In order to test this hypothesis, point defect concentrations were calculated for LSCF and NNO single-phase electrodes using the Adler-Lane-Steele model from electrochemical data and electrode microstructural parameters obtained by three-dimensional tomography. The results verified that the observed changes with increasing pO2 can be explained by reasonable decreases in LSCF vacancy concentration and increases in NNO interstitial concentration. These results suggest that nickelate electrodes can be advantageous for pressurized devices.

This work presents the study of the O 2 -Reduction Reaction (ORR) by electrochemical impedance spectroscopy of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ cathodes as a function of temperature and pO 2 . The combination of the impedance data, modeled with a Transmission Line Model, with the microstructural data obtained by FIB-SEM tomography, allowed to obtain and compare the chemical diffusion coefficients (D chem ), O 2 equilibrium molar exchange rates ( 0 ) and the oxygen surface exchange rates (k chem ) for both compounds. The obtained values were, at 700°C in air, D chem = 5.4.10 −7 cm 2 .s −1 and k chem = 1.4.10 −6 cm.s −1 for La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ , while D chem = 2.6.10 −6 cm 2 .s −1 and k chem = 3.1.10 −6 cm.s −1 were obtained for La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ . The detailed analysis of these parameters as a function of pO 2 (10 −4 < pO 2 ≤ 1) and temperature (500 • C ≤ T ≤ 700 • C) by means of the Adler-Lane-Steele model, adapted to a finite length porous electrode, allowed identifying the O-ion diffusion and surface exchange as processes co-limiting the ORR. From this analysis, a predominantly surface limited ORR was found for La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ , changing to a more bulk limited ORR for La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ , which has higher oxygen-vacancy concentration.

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