AC and DC studies of the pitting corrosion of Al in perchlorate solutions

The pitting corrosion behaviour of Al in aerated neutral sodium perchlorate solutions was investigated by potentiodynamic, cyclic voltammetry, galvanostatic, potentiostatic and electrochemical impedance spectroscopy (EIS) techniques, complemented by ex situ scanning electron microscopy (SEM) examinations of the electrode surface. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al₂O₃ on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film by ClO₄⁻ ions. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allow the pitting potential (E_pit) and the repassivation potential (E_rp) to be determined. E_pit decreases with increase in ClO₄⁻ concentration, but increases with increase in potential scan rate. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involve pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation (t_i⁻¹) increases with increase in ClO₄⁻ concentration and applied step anodic potential (E_s,a). EIS measurements showed that at E_s,a < E_pit, a charge-transfer semicircle is obtained. This semicircle is followed by a Warburg diffusion tail at E_s,a > E_pit. An attempt is made to compare the values of E_pit and E_rp obtained through different methods and to determine the factors influencing these values in each particular method.     

Localized corrosion of Al90Fe5Gd5 and Al87Ni8.7Y4.3 alloys in the amorphous, nanocrystalline and crystalline states: resistance to micrometer-scale pit formation

The role of isothermal aging on the localized corrosion behavior of Al₉₀Fe₅Gd₅ and Al₈₇Ni_8.7Y_4.3 alloys was characterized in 0.6 M NaCl solution. The pitting (E_pit) and repassivation (E_rp) potentials were both increased ∼400 mV by the presence of transition and rare earth metal additions in supersaturated solid solution and amorphous structure. A statistical distribution in E_pit observed on small electrodes was due, in part, to the sensitivity of this critical potential to the presence of a population of critical surface flaws that serve as pit initiation sites. Mechanistic insight on the spacing of critical flaws was enabled by varying the tested electrode surface area. E_rp was not dependent on electrode surface area due to the similarity of pit depths in all electrode sizes. The critical potentials were also characterized after heat treating the amorphous ribbons isothermally at 150 °C for 25 h and 550 °C for 1 h. The former produced Al-rich nanocrystals embedded in the remaining amorphous matrix while the latter produced a fully crystalline condition containing intermetallic phases. Notably, the improved resistance to the formation of micrometer-scale pits was not lost compared with the fully amorphous condition when small Al-rich nanocrystals were present in an amorphous matrix. However, improvement in pitting corrosion resistance was completely lost in the fully crystallized condition as indicated by values for E_pit and E_rp that were similar to those of high purity, polycrystalline aluminum.     

Electrochemical performances of Al-based composites as anode materials for Li-ion batteries

Al-C, Al-Fe and Al-Fe-C composite materials have been prepared by high-energy ball milling technique. The electrochemical measurements demonstrated that the Al-Fe-C composites have greatly improved electrochemical performances in comparison with Al, Al-C and Al-Fe anode. For example, Al₇₁Fe₉C₂₀ can deliver the reversible capacity of 436 mAh g⁻¹ at first cycle and 255 mAh g⁻¹ at 15th cycle. This improved electrochemical performance could be attributed to the alloying formation of Al with Fe and the buffering effect by the graphite matrix. This suggests that the Al-Fe-C composite has a potential possibility to be developed as an anode material for lithium-ion batteries.     

The influence of temperature on the anodic oxidation/dissolution of uranium dioxide

The anodic dissolution of U^IVO₂ has been studied at 60 °C in 0.1 mol dm⁻³ KCl using a range of electrochemical methods and X-ray photoelectron spectroscopy (XPS). The results were compared to previous results obtained at 22 °C. This comparison shows that the threshold for the onset of anodic dissolution (−400 mV versus SCE) is not noticeably changed by this increase in temperature. However, both the oxidation of the surface (to U^IV/VO_2+x) and the rate of anodic dissolution (as U^VIO₂²⁺) leading to the formation of a U^VIO₃·yH₂O deposit are accelerated at the higher temperature. The XPS analysis shows that the conversion of U^V-U^VI occurs at lower potentials at 60 °C. Consequently, once the surface becomes blocked by the presence of a U^VIO₃·yH₂O deposit, rapid dissolution coupled to uranyl ion hydrolysis causes the development of locally acidified sites within the fuel surface at lower potentials at the higher temperature.     

Electrochemical characterization of chemical species formed during the electrochemical treatment of chalcopyrite in sulfuric acid

The dissolution mechanism of chalcopyrite, and the potential range in which its passivation phenomenon takes place, were studied on carbon paste electrodes with chalcopyrite (99.46% purity, +300 mesh, 53 μm size) (CPE-CP) in 1.7 mol/dm³ H₂SO₄. A sequence of anodic potential pulses was applied to the CPE-CP to characterize its electrochemical behavior. Copper ions, dissolved by the potential pulses, were determined using a mercury film electrode (MFE) and the anodic stripping voltammetry (ASV) on a vitreous carbon disk. In addition, the modified surface of CPE-CP was characterized, before and after the potential pulses, by cyclic voltammetry (CV). The characterization of the final surface state of each electrochemically modified CPE-CP and the amount of dissolved copper showed five potential regions where the chalcopyrite dissolution mechanism changed. The initial dissolution occurs at 0.615 V ≤ E_anod < 1.015 V versus SHE forming a non-stoichiometric polysulfide (Cu_1−rFe_1−sS_2−t). The absence of copper ions in the solution indicates a passive sulfide. However, at 1.015 V ≤ E_anod < 1.085 V versus SHE, the passive product decomposes forming porous layers of non-stoichiometric polysulfide (Cu_1−xFe_1−yS_2−z) that allow the diffusional transport of charged species and the dissolution of the mineral. In the region of 1.085 V ≤ E_anod < 1.165 V versus SHE, formation covellite (CuS) was identified. At E > 1.165 V versus SHE, CuS is unstable and gives rise to complete dissolution of the chalcopyrite. Due to the experimental conditions, the mineral dissolution is inhibited by possible jarosite precipitation.     

Co-deposition of Al-Cr-Ni alloys using constant potential and potential pulse techniques in AlCl3-NaCl-KCl molten salt

To improve the oxidation resistance of TiAl intermetallic compound under high temperature condition, cathodic co-deposition of Al-Cr and Al-Ni alloy was carried out by constant potential control or potential pulse control in AlCl₃-NaCl-KCl molten salt containing CrCl₂ and/or NiCl₂ at 423 K. Cathodic reduction of Ni and Cr starts at potential of 0.8 and 0.15 V versus Al/Al³⁺ in the molten salt, respectively. The co-deposition of Al, Cr, and Ni occurred at potentials more negative than −0.1 V to form a mixture of intermetallic compounds of Cr₂Al, Ni₃Al, and Al₃Ni. Concentration of Cr in the deposit was enhanced to 43 at% at −0.1 V; however, concentration of Ni in the deposit was 6 at% at the same potential. The concentration of Ni further decreased with more negative potential to 1 at% at −0.4 V. The potential pulse technique enhanced the Ni concentration in the deposit to about 30 at%, due to anodic dissolution of Al content from the deposit at the higher side of potential on the potential pulse electrolysis.     

Inhibitory effect of tungstate, molybdate and nitrite ions on the carbon steel pitting corrosion in alkaline formation water containing Cl ion

The pitting corrosion of carbon steel in carbonate-formation water solution in the presence of chloride ions and the effect of addition WO₄²⁻, MoO₄²⁻ and NO₂⁻ anions on the pitting corrosion were studied using cyclic voltammetry and potentiostatic current-time measurements and complemented by scan electron microscope (SEM), energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) investigations. Cyclic voltammograms of carbon steel in the presence of chloride ions in carbonate-formation water solution show one anodic peak, corresponding to the formation green rust carbonate and the two cathodic peaks. As the addition of Cl⁻ ions concentration increases, the anodic peak current density increases and pitting potential E_pit shifts to more negative potential. It is shown that the rate of pit initiation () decreases and the pitting potential E_pit moves to more positive direction upon the addition of inorganic anions. It was found that pitting inhibition of carbon steel increases in the sequence: (WO₄)²⁻ > (MoO₄)²⁻ > (NO₂)⁻.     

Preparation and characterization of Six-Co0.6B0.6Al0.2/modified graphite sphere composites as anodes for lithium-ion batteries

In order to improve the cyclic property of Si as anode for lithium-ion battery, Si_x-Co_0.6B_0.6Al_0.2/modified graphite sphere composites (labeled as Si_x-Co_0.6B_0.6Al_0.2/MGS, x = 0.72, 1.12, 1.68) were synthesized using high energy ball milling (HEBM) technique. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to characterize the compositions and morphology of composite materials. The electrochemical behaviors of Si_x-Co_0.6B_0.6Al_0.2/MGS were investigated by galvanostatic charge-discharge technique and electrochemical impedance spectroscopy (EIS). The results indicate that Co, B, Al and MGS have a good synergistic effect in improving the electrochemical performance of Si powder-based electrode. Compared to Si/MGS and Si_1.12-Co_0.6B_0.6Al_0.2, the cyclic performance and coulombic efficiency of Si_1.12-Co_0.6 B_0.6Al_0.2/MGS are significantly enhanced from the 2nd cycle. The first reversible capacity of Si_1.12-Co_0.6B_0.6Al_0.2/MGS is 533 mAh g⁻¹ and 90% of the capacity may be obtained after 80 cycles. EIS and SEM measurements indicate that the microstructural stability of the composite during cycling appears to be the main reason contributing to the good cycleability.     

The effect of Ga addition on the sinterability and microwave dielectric properties of RE3Al5O12 (Tb,Y, Er and Yb) garnet ceramics

The RE₃Al₅O₁₂ (RE=Tb, Y, Er, Yb) ceramics have been prepared by the mixed oxide route and the influence of Ga³⁺ doping on their properties is investigated. The intrinsic Y₃Al₅O₁₂ (YAG) ceramic sintered at 1650 °C for 4 h showed good dielectric properties; (ε_r=10.1, Q_u×f=65,000 GHz, τ_f=−45 ppm/°C). Addition of Ga₂O₃ was found to be beneficial in improving the densification of Tb₃Al₅O₁₂, Er₃Al₅O₁₂ and Yb₃Al₅O₁₂ except Y₃Al₅O₁₂ where Nb₂O₅ is the better choice. Among Ga³⁺ added samples, the composition Yb₃Al₅O₁₂+1 wt% Ga₂O₃ showed good microwave dielectric properties: ε_r=10.3, Q_u×f=50,000 GHz, τ_f=−58 ppm/°C. The Y₃Al₅O₁₂ doped with 1 wt% Nb₂O₅ has ε_r=10.7, Q_u×f=120,000 GHz and τ_f=−45 ppm/°C. The ceramics have good thermal properties (CTE=2–3 ppm/°C, λ=2–12 W/m K).     

Comparative study of Li[Co1−zAlz]O2 prepared by solid-state and co-precipitation methods

Li[Co_1−zAl_z]O₂ (0 ≤ z ≤ 0.5) samples were prepared by co-precipitation and solid-state methods. The lattice constants varied smoothly with z for the co-precipitated samples but deviated for the solid-state samples above z = 0.2. The solid-state method may not produce materials with a uniform cation distribution when the aluminum content is large or when the duration of heating is too brief. Non-stoichiometric Li_x[Co_0.9Al_0.1]O₂ samples were synthesized by the co-precipitation method at various nominal compositions x = Li/(Co + Al) = 0.95, 1.0, 1.1, 1.2, 1.3. XRD patterns of the Li_x[Co_0.9Al_0.1]O₂ samples suggest the solid solution limit is between Li/(Co + Al) = 1.1 and 1.2. Electrochemical studies of the Li[Co_1−zAl_z]O₂ samples were used to measure the rate of capacity reduction with Al content, found to be about −250 ± 30 (mAh/g)/(z = 1). Literature work on Li[Ni_1/3Mn_1/3Co_1/3−zAl_z]O₂, Li[Ni_1−zAl_z]O₂ and Li[Mn_2−yAl_y]O₄ demonstrates the same rate of capacity reduction with Al/(Al + M) ratio. These studies serve as baseline characterization of samples to be used to determine the impact of Al content on the thermal stability of delithiated Li[Co_1−zAl_z]O₂ in electrolyte.     

A new aluminium-hydrate species in hydrated Portland cements characterized by Al and Si MAS NMR spectroscopy

Recent ²⁷Al MAS NMR studies of hydrated Portland cements and calcium-silicate-hydrate (C-S-H) phases have shown a resonance from Al in octahedral coordination, which cannot be assigned to the well-known aluminate species in hydrated Portland cements. This resonance, which exhibits the isotropic chemical shift δ_iso = 5.0 ppm and the quadrupole product parameter P_Q = 1.2 MHz, has been characterized in detail by ²⁷Al MAS and ²⁷Al{¹H} CP/MAS NMR for different hydrated white Portland cements and C-S-H phases. These experiments demonstrate that the resonance originates from an amorphous or disordered aluminate hydrate which contains Al(OH)₆³⁻ or O_xAl(OH)_6-x^(3+x)− units. The formation of the new aluminate hydrate is related to the formation of C-S-H at ambient temperatures, however, it decomposes by thermal treatment at temperatures of 70-90 °C. From the experiments in this work it is proposed that the new aluminate hydrate is either an amorphous/disordered aluminate hydroxide or a calcium aluminate hydrate, produced as a separate phase or as a nanostructured surface precipitate on the C-S-H phase. Finally, the possibilities of Al³⁺ for Ca²⁺ substitution in the principal layers and interlayers of the C-S-H structure are discussed.     

Adsorption characteristics of sericite for cesium ions from an aqueous solution

To efficiently remove cesium ions from aqueous solution, sericite was used as a novel adsorbent. The silanol (SiO₂) and aluminol (Al₂O₃) groups in sericite are likely to play an important role in adsorption process. The maximum adsorption capacity (q_m) and adsorption constant (K_L) for cesium ions obtained from the Langmuir isotherm model were 6.68 mg/g and 0.227 L/mg, respectively and regression curve fit well with the experimental data as the 0.965 of correlation coefficients (r²). However, when the Freundlich isotherm model was used correlation coefficient (r²) was 0.973. Therefore, it was concluded that Freundlich model fits equilibrium data better than Langmuir model. When the 6.0 g/L of sericie concentration was added to aqueous solution, cesium ions were removed by about 80% and the increase was not happened above 6.0 g/L of sericite concentration any more. The process was determined as exothermic reaction because the removal efficiency of cesium ions decreased as temperature increased. Furthermore, all adsorption was completed in 120 min and comparing the pseudo first and second-order kinetic models indicates that the adsorption of cesium ions using sericite follows well the pseudo-second-order kinetics.     

Pitting corrosion studies on Al and Al-Zn alloys in SCN solutions

Pitting of Al and Al-6%Zn and Al-12%Zn alloys in KSCN solutions was studied by means of potentiodynamic anodic polarization, cyclic voltammetry, potentiostatic and impedance techniques. Measurements were conducted under different experimental conditions, complemented by ex situ scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA). The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al₂O₃ on the anode surface (in case of Al) and the formation of ZnO on the Al₂O₃ matrix, in case of the two Al-Zn alloys (as evidenced from EDXA). The passive region is followed by pitting corrosion as a result of passivity breakdown by the aggressive attack of SCN⁻ anions. SEM images confirmed the existence of pits on the electrode surface. Alloyed Zn was found to enhance pitting attack. The pitting potential (E_pit) decreases with an increase in SCN⁻ concentration and temperature, but increases with increasing potential scan rate. The current/time transients show that the incubation time for passivity breakdown decreases with increasing applied positive potential, SCN⁻ concentration, and temperature. Impedance measurements showed that Nyquist plots are characterized by a depressed charge-transfer semicircle, the diameter of which is a function of SCN⁻ concentration, applied potential, solution temperature and sample composition.     

Spectroscopic study of the electrochemical behaviour of tantalum(V) chloride and oxochloride species in 1-butyl-1-methylpyrrolidinium chloride

FTIR spectroscopy was used to identify the oxochloride species of tantalum(V) in ionic liquids and to confirm the correlations between their presence in electrolytes and the changes in the route of electrochemical reduction of tantalum(V). Electrochemical behaviour of the mixtures (x)1-butyl-1-methyl-pyrrolidinium chloride-(1 − x)TaCl₅ at x = 0.80, 0.65, and 0.40 was investigated over the temperature range 90-160 °C with respect to the electrochemical deposition of tantalum and was discussed in terms of spectroscopic data. The mechanism of electrochemical reduction of tantalum(V) in the basic and acidic electrolytes depends strongly on the structure and composition of the electro active species of tantalum(V) defined by the molar composition of ionic liquids and on the competition between tantalum(V) chloride and oxochloride species. In the basic mixture at x = 0.80, with octahedral [TaCl₆]⁻ ions as the electrochemically active species only the first reduction step Ta⁵⁺ → Ta⁴⁺ at −0.31 V was observed. The competitive reduction of tantalum(V) oxochloride species occurs at more anodic potential (−0.01 V) than the reduction of the chloride complexes and can restrict the further reduction of tantalum(IV). In the basic ionic liquid at x = 0.65, the cyclic voltammograms exhibit reduction peaks at −0.31 V and −0.51 V attributed to the diffusion controlled process as [TaCl₆]⁻ + e⁻ → [TaCl₆]²⁻ and [TaCl₆]²⁻ + e⁻ → [TaCl₆]³⁻. The further irreversible reduction of tantalum(III) to metallic state may occur at −2.1 V. In the acidic ionic liquids, at x = 0.40 the electrochemical reduction of two species occurs, TaCl₆⁻ and Ta₂Cl₁₁⁻ and it is limited by two electron transfer for both of them at −0.3 V and −1.5 V, respectively.     

Initial surface topography changes during divalent dissolution of silicon electrodes

The changes of the surface topography of float zone (FZ) n-Si(1 1 1) upon conditioning of the electrodes at potentials slightly anodic of the rest potential are monitored with atomic force microscopy (AFM) in the contact mode. The influence of the composition of the used 0.1 and 0.2 M NH₄F electrolyte at pH 4, of the potential and of the charge passed on the topography is investigated. The dissolution charges Q_diss ranged from 0.28 to 10.6 mC cm⁻² corresponding to ∼0.5 and ∼21 bilayers (BL), respectively. The root mean square roughness R_q changes from R_q=0.2 nm for the H-terminated surface to 2.9 nm for a charge passed of 10.6 mC cm⁻² at an electrode potential of 0.1 V positive of the rest potential. The evaluation of height, deflection and line scan AFM data shows pitting to originate at edges of oriented steps which separate atomically smooth terraces. Upon increased dissolution charge, island-type smooth and rather circular features form. Only for the highest Q_diss, these islands are beginning to show corrosion. An exponential relation between R_q and Q_diss is found by evaluation of the three-dimensional roughness. The slope, i.e. the increase of ln R_q with Q_diss depends on the composition of the electrolyte and is higher for the 0.1 M NH₄F solution. From these data, a branching of the dissolution reaction between charge going into terrace removal or pit formation is obtained. Synchrotron radiation photoelectron spectroscopy (SRPES) is used to identify chemical products of the dissolution process.Comparison of data obtained at 0.15 V anodic of the electrode rest potential with an elaborate model of Gerischer and coworkers (which, however, only describes terrace dissolution) yields partial agreement with the predictions.     

Preparation and cycling performance of Aland F co-substituted compounds Li4AlxTi5−xFyO12−y

Li₄Al_xTi_5−xF_yO_12−y compounds were prepared by a solid-state reaction method. Phase analyses demonstrated that both Al³⁺ and F⁻ ions entered the structure of spinel-type Li₄Ti₅O₁₂. Charge-discharge cycling results at a constant current density of 0.15 mA cm⁻² between the cut-off voltages of 2.5 and 0.5 V showed that the Al³⁺ and F⁻ substitutions improved the first total discharge capacity of Li₄Ti₅O₁₂. However, Al³⁺ substitution greatly increased the reversible capacity and cycling stability of Li₄Ti₅O₁₂ while F⁻ substitution decreased its reversible capacity and cycling stability slightly. The electrochemical performance of the Al³⁺-F⁻-co-substituted specimen was better than the F⁻-substituted one but worse than the Al³⁺-substituted one.     

Structural and electrochemical behaviors of metastable Li2/3[Ni1/3Mn2/3]O2 modified by metal element substitution

Layered metastable lithium manganese oxides, Li_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ (x = y = 1/36 for M = Al, Co, and Fe and x = 2/36, y = 0 for M = Mg) were prepared by the ion exchange of Li for Na in P2-Na_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ precursors. The Al and Co doping produced the T^#2 structure with the space group Cmca. On the other hand, the Fe and Mg doped samples had the O6 structure with space group R-3m. Electron diffraction revealed the 1:2 type ordering within the Ni_1/3−xMn_2/3−yM_x+yO₂ slab. It was found that the stacking sequence and electrochemical performance of the Li cells containing T^#2-Li_2/3[Ni_1/3Mn_2/3]O₂ were affected by the doping with small amounts of Al, Co, Fe, and Mg. The discharge capacity of the Al doped sample was around 200 mAh g⁻¹ in the voltage range between 2.0 and 4.7 V at the current density of 14.4 mA g⁻¹ along with a good capacity retention. Moreover, for the Al and Co doped and undoped oxides, the irreversible phase transition of the T^#2 into the O2 structure was observed during the initial lithium deintercalation.     

Effects of phosphate species on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 M NaHCO₃ + 0.02 M NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with the phosphate concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [HPO₄²⁻] = 0 to 180 ± 40 mV/SCE for [HPO₄²⁻] = 0.02 mol L⁻¹. During anodic polarisation (E = 50 mV/SCE), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via the scanning vibrating electrode technique (SVET). The addition of a Na₂HPO₄ solution after the beginning of the polarisation did not lead to the repassivation of pre-existing well-grown pits. The corrosion products forming in the pits were identified in situ by micro-Raman spectroscopy. They depended on the phosphate concentration. For [HPO₄²⁻] = 0.004 mol L⁻¹, siderite FeCO₃ was detected first. It was oxidised later into carbonated green rust GR(CO₃²⁻) by dissolved O₂. The beginning of the process is therefore similar to that observed in the absence of phosphate. Finally, GR(CO₃²⁻) was oxidised into ferrihydrite, the most poorly ordered form of Fe(III) oxides and oxyhydroxides. Phosphate species, adsorbing on the nuclei of FeOOH, inhibited their growth and crystallisation. For [HPO₄²⁻] = 0.02 mol L⁻¹, siderite was accompanied by an amorphous precursor of vivianite, Fe₂(PO₄)₃·8H₂O. This shows that, in any case, phosphate species interact strongly with the iron species produced by the dissolution of steel.     

Effect of Al content on the corrosion behavior of Mg-Al alloys in aqueous solutions of different pH

The effect of systematic increase of Al content on the electrochemical behavior of the Mg-Al alloys in aqueous solutions of different pH was investigated. Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy, EIS, were used to investigate the electrochemical behavior of the alloys in aqueous solutions. The results have shown that Mg-5Al is easily corroded due to the microgalvanic effect between α-phase and β-phase, its corrosion rate is even higher than that of Mg itself. The increase of Al content increases the corrosion resistance of the alloy due to the formation of the β-phase (Mg₁₇Al₁₂) together with the Mg α-phase. The ranking of the corrosion rate of these alloys was Mg-5Al > Mg > Mg-10Al ≅ Mg-15Al. The corrosion rates of the alloys in acidic solutions are pronouncedly high compared to those measured in neutral or basic solutions. The impedance measurements are in consistence with the polarization techniques and the impedance data were fitted to theoretical data obtained according to an equivalent circuit model describing the electrode/electrolyte interface.     

Study on phase structure and electrochemical properties of Ml1−xMgxNi2.80Co0.50Mn0.10Al0.10 (x = 0.08, 0.12, 0.20, 0.24, 0.28) hydrogen storage alloys

Phase structure and electrochemical properties of the Ml_1−xMg_xNi_2.80Co_0.50Mn_0.10Al_0.10 (x = 0.08, 0.12, 0.20, 0.24, 0.28) (Ml = La-rich mixed lanthanide) alloys were studied. X-ray diffraction (XRD) analysis and Rietveld refinement show that the alloys consist mainly of LaNi₅ and (La,Mg)Ni₃ phase. Due to variation in phases of the alloys, the maximum discharge capacity, the high rate dischargeability (HRD), and the low temperature dischargeability increase first and then decrease. The maximum discharge capacity increases from 322 mAh g⁻¹ (x = 0.08) to 375 mAh g⁻¹ (x = 0.12), and then decreases to 351 mAh g⁻¹ (x = 0.28) with increasing x. As the case of x = 0.20, HRD at 1200 mA g⁻¹ and discharge capacity at 233 K reaches 41.7% and 256 mAh g⁻¹, respectively. The cycling stability is improved by substituting La with Ml and B-site multi-alloying, and the capacity retention of Ml_0.72Mg_0.28Ni_2.80Co_0.50Mn_0.10Al_0.10 at the 200th cycle is 71%.