Display omitted
•Significant dependence of surface pretreatments on the TCP coating homogeneity.•Lower TCP coverage on pure Al and AA2024 alloy after pickling step pretreatment.•Strong Cu enrichment ...on AA2024 alloy not detrimental for TCP quality.•Thicker TCP coating and/or presence of Al oxy-fluorides leading to coating failure.
The effects of surface pretreatments (degreasing and pickling) on the characteristics of the Trivalent Chromium Process (TCP) coating on pure aluminum and on AA2024-T351 aluminum alloy were investigated here by means of surface sensitive techniques: X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The XPS and ToF-SIMS results evidence that the TCP coating homogeneity is strongly dependent on the pretreatment process used. The TCP coverage factor, calculated from XPS results, is significantly lower, on both pure aluminum and AA2024-T351 alloy surface, when a pickling step is applied. One of the main effects of pickling pretreatment is strong metallic copper enrichment at the surface of the 2024 alloy, associated with chemical dissolution of Al-Cu intermetallic particles. However, it is evidenced here, that the copper enrichment is not detrimental for the quality of the TCP coating. The coating failure, observed when the pickling step is applied, can be assigned to a faster kinetics of the coating growth leading to formation of thicker conversion coating more susceptible to cracking or to the localized presence of aluminum fluoride species leading to the appearance of coating defects or detachment.
•Different coating thickness over the Mg alloy matrix and particles was revealed.•Cr(OH)3 and ZrO2 were main components evidenced by XPS with Cr/Zr at.% ratio of ∼ 0.5.•Coated AZ91D alloy revealed ...5.5x increased corrosion resistance.•Reduced corrosion current density by a factor of 2 on coated alloy.
Trivalent chromium conversion (TCC) coating formation on AZ91D magnesium alloy has been investigated using scanning electron microscopy, energy X-ray dispersive spectroscopy (EDS), X-ray photoelectron spectroscoy (XPS). The coating was formed in a commercial SurTec 650 bath. XPS showed that coatings contained mainly Cr, Zr, O, S and F species, which were assigned to hydrated Cr(III) (Cr(OH)3/CrOOH), ZrO2, Cr2(SO4)3, and CrF3, and lower amounts of Mg2+ and Al3+ species assigned to oxide, hydroxide and fluoride. The Cr/Zr atomic ratio was around 0.5. The coating was formed above both α matrix and β (Mg17Al12) grain boundary regions, and it was significantly thicker above the cathodic β phase. Raman spectroscopy evidenced the presence of Cr(VI) species, formed due to oxidation of Cr(III) by H2O2, particularly in the coating above the cathodic sites. Electrochemical impedance spectroscopy revealed that corrosion protection was provided mainly by the barrier layer at the base of the coating, which resulted in a reduction by about a factor of 2 in the anodic and cathodic current density in potentiodynamic polarization curves.
A combination of electrochemical measurements, H2 volume measurements and surface analysis using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS) ...was used to investigate the role of the metallic Fe impurities (60 ppm) in high purity (99.9 wt%) magnesium and the effect of sulfur on the hydrogen evolution. The Mg corrosion was studied in NaCl solutions containing dissolved hydrogen sulfide. At OCP and under anodic polarization the hydrogen evolution reaction and the growth of dark corroded areas were significantly inhibited in the presence of H2Saq. ToF-SIMS and XPS analyses showed the presence of sulfur adsorbed on Mg and on Fe segregated at grain boundaries. The observed inhibition of hydrogen evolution at OCP and above is assigned to a poisoning effect by adsorbed S of the H adsorption sites on segregated iron which, in the absence of sulfur, provides catalytic sites for excess HER. The equilibrium Pourbaix diagram for the Mg-Fe-S system including the adsorbed species predicts that S is adsorbed on Mg and Fe in the E-pH conditions of this work, which confirms the role assigned to adsorbed sulfur in the inhibition of excess hydrogen evolution at OCP and under anodic polarization in the presence of H2Saq.
A composite consisting of cobalt and graphitic porous carbon (Co@GC-PC) is synthesized from bimetallic metal–organic frameworks and employed as the sulfur host for high-performance Li–S batteries. ...Because of the presence of a large surface area (724 m2 g–1) and an abundance of macro-/mesopores, the Co@GC-PC electrode is able to alleviate the debilitating effect originating from the volume expansion/contraction of sulfur species during the cycling process. Our in situ UV/vis analysis indicates that the existence of Co@GC-PC promotes the adsorption of polysulfides during the discharge process. Density functional theory calculations show a strong interaction between Co and Li2S and a low decomposition barrier of Li2S on Co(111), which is beneficial to the following Li2S oxidation in the charge process. As a result, at 0.2C, the discharge capacity of the S/Co@GC-PC cathode is stabilized at 790 mAh g–1 after 220 cycles, much higher than that of a carbon-based cathode, which delivers a discharge capacity of 188 mAh g–1.
Fe-based air alkaline batteries are promising candidates for large scale energy storage due to their low cost, eco-friendliness and high energy density. In this work, pure iron was studied as ...negative electrode of Fe-air batteries with and without Na2S added to the KOH alkaline electrolyte in order to bring new insight on the mechanisms of inhibition of the hydrogen evolution reaction (HER) and improvement of the electrochemical performances of the Fe electrode. Electrochemical (CV, galvanostatic discharge), spectroscopic (XPS, ToF-SIMS) and microscopic (SEM) analysis were applied to characterize the effect of the sulfide additive (Na2S) on the electrochemical behavior of Fe electrodes and their chemical and morphological modifications. Surface analysis performed by XPS and ToF-SIMS revealed the formation of a markedly thicker oxide/hydroxide layer. The mechanism of formation of this thick oxide/hydroxide layer is initiated by the adsorption of sulfur (Sads) at the oxide/metal interface that inhibits the HER during the first stages of electrochemical process and promotes subsequent anodic oxidation. With the Na2S electrolyte additive, Fe(0)/Fe(II) conversion/deconversion is suppressed and charge/discharge proceeds mostly by Fe(II)/Fe(III) conversion/deconversion with improved capacity owing to the porous morphology of the oxide/hydroxide layer.
The solvent 3-methoxytetrahydrothiophene 1, 1-dioxide (MESL) was synthesized and its cycling performances of grapshite and LiNi1/3Mn1/3Co1/3O2 (NMC) electrodes were investigated in view of the high ...anodic stability of MESL in the presence of lithium bis-trifluoromethanesulfonimidate (LiTFSI). Galvanostatic charge-discharge of graphite electrode was possible in MESL + LiTFSI (1 M) only in the presence of 5% (vol.) fluoroethylene carbonate (FEC) at 40 °C. On the other hand, a good cycling ability of NMC electrode with this electrolyte was observed at room temperature even at a cut-off voltage of 4.5 V vs Li+/Li. However, it was necessary to reduce the cut-off voltage from 4.5 V to 4.3 V vs Li+/Li in order to keep a good cycling ability <<< when the temperature was increased from 25 °C to 40 °C due to an exaltation of oxidation reactions onto the cathode surface. X-Ray Photoelectron Spectroscopy (XPS) analyses were performed to investigate the electrode/electrolyte interphase and formation of passive layer on the electrode surfaces in order to explain the differences of cycling ability of NMC and graphite electrodes at room temperature and 40 °C.
In situ electrochemical (by electrochemical impedance spectroscopy, EIS) and ex situ surface (by time-of-flight secondary ions mass spectrometry, ToF-SIMS) analysis were applied to investigate ...solid-state diffusion coefficient (D Li) into conversion-type α-Fe2O3 negative electrode for Li-ion batteries. D Li values obtained from EIS were in the range of 10–16 to 10–15 cm2 s–1 for electrodes partially and fully lithiated, respectively, showing that pulverization of the converted material promotes Li-ion migration. ToF-SIMS ion depth profiling performed after partial lithiation enabled discriminating the surface solid electrolyte interphase (SEI) region, a converted electrode region (Li2O/Fe0 matrix) of slow diffusion (D Li = 6 × 10–16 cm2 s–1) and an unconverted region (intercalated Fe2O3 matrix) of faster diffusion (D Li = 2 × 10–13 cm2 s–1) ahead of the conversion front. Comparison of the ex situ and in situ results indicates that the electrode conversion kinetics is limited by Li-ion diffusion in the converted matrix and suggests a hindering effect of the passivating SEI layer. ToF-SIMS depth profile analysis appears as a most appropriate and direct methodology to measure Li-ion diffusion solely in electrode materials, excluding SEI layer effects.
Iron oxide (mostly α-Fe2O3) model thin-film electrodes were prepared by thermal oxidation of pure metal iron substrates at 300 ± 5 °C in air and used for comprehensive investigation of the ...lithiation/delithiation mechanisms of anode material undergoing an electrochemical conversion reaction with lithium ions. Surface (X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS)) and electrochemical (cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)) analytical techniques were combined. The results show that intercalation of Li in the Fe2O3 matrix and solid electrolyte interphase (SEI) layer formation both precede conversion to metallic iron and Li2O upon lithiation. Depth profile analysis evidences stratification of the converted thin-film electrode into fully and partially lithiated outer and inner parts, respectively, due to mass transport limitation. The SEI layer has a stable composition (Li2CO3 with minor ROCO2Li) but dynamically increases/decreases in thickness upon lithiation/delithiation. Conversion, proceeding mostly in the outer part of the electrode, causes material swelling accompanied by SEI layer thickening. Upon delithiation, lithium is trapped in the deconverted electrode subjected to shrinking, and the SEI layer mostly decomposes and reduces in thickness after deconversion. The nonreversibility of both conversion and surface passivation mechanisms is demonstrated.
PDF
