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.

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.

Glyphosate [N-phosphono-methylglycine (PMG)] is the most used herbicide worldwide, particularly since the development of transgenic glyphosate-resistant (GR) crops. Aminomethylphosphonic acid (AMPA) is the main glyphosate metabolite, and it may be responsible for GR crop damage upon PMG application. PMG degradation into AMPA has hitherto been reckoned mainly as a biological process, produced by soil microorganisms (bacteria and fungi) and plants. In this work, we use density functional calculations to identify the vibrational bands of PMG and AMPA in surface-enhanced Raman spectroscopy (SERS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra experiments. SERS shows the presence of AMPA after glyphosate is deposited from aqueous solution on different metallic surfaces. AMPA is also detected in ATR-FTIR experiments when PMG interacts with metallic ions in aqueous solution. These results reveal an abiotic degradation process of glyphosate into AMPA, where metals play a crucial role.

This work presents a comparative study of the diffusion (Dchem) and surface exchange coefficients (kchem) of porous La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and Co3O4 nanoparticles decorated LSCF electrodes. The study was carried out using the 3DT-EIS method, which combines Electrochemical Impedance Spectroscopy experiments with FIB-SEM tomography data through an adapted Transmission Line - Adler Lane Steele electrochemical model. A reduction of the polarization resistance of about 60% was measured for the Co3O4 decorated LSCF respect to the reference LSCF cathode, in air at 700 °C. The Co3O4 decoration was found to modify the ORR surface reaction limiting mechanism from O2 dissociation to O-ion incorporation, whereas the diffusion coefficient was not modified by the decoration, which represents a surface diffusion process for both electrodes. After the EIS measurements, the Co3O4 particles were almost no longer visible by Field-Emission SEM on the surface of the decorated sample, but signs that these particles play an active role in Sr Segregation were observed by STEM-EDS, in particular by concentrating the segregated SrO in the surroundings of the decorated particles.

In this work, a Solid Oxide Fuel Cell (SOFC) cathode surface modification by nanoparticle impregnation was carried out. La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ (LSCF) ceramic cathodes impregnated with Ce 0.8 Gd 0.2 O 3-δ (GDC) nanoparticles were characterized with scanning (SEM) and transmission (TEM) electron microscopy. The electrochemical response as a function of temperature, oxygen partial pressure (pO 2 ) (10 -4 < pO 2 < 1 atm) and time was evaluated by electrochemical impedance spectroscopy (EIS). We obtained reproducible and homogeneous impregnations, which reduce the total cathode´s polarization resistance more than 20% in the temperature range between 400ºC and 800ºC and during 500 h at 700ºC. The impregnated cathode was found to be electrochemically limited by the same processes as the nonimpregnated cathode, namely gas diffusion, ion bulk diffusion and dissociative adsorption. The impregnation produced a reduction of more than 20% of the oxygen surface resistance; such difference remained stable through 500h of continuous operation at 700ºC.