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.
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