Electrochemical hydrogen storage properties of mechanically alloyed Mg0.8Ti0.2-xMnxNi (x = 0, 0.025, 0.05, 0.1) type alloys

Mechanical alloying was used in the synthesis of Mg_0.8Ti_0.2-xMn_xNi (x = 0, 0.025, 0.05, 0.1) quaternary alloys to analyze the effect of Mn substitution for Ti on the electrochemical performance of MgNi alloys. The milling was carried out for 25 h. By adding a small amount of Mn (x = 0.025) to the Mg_0.8Ti_0.2Ni alloy, a completely amorphous structure was obtained. The maximum discharge capacity of the Mg_0.8Ti_0.175Mn_0.025Ni alloy was observed as 543 mAh g^?1 at the initial charge/discharge cycle. When x = 0 and x = 0.05, the discharging performances of Mg_0.8Ti_0.2-xMn_xNi alloys were approximately the same. However, when x = 0.1, the lowest initial discharge capacity (401 mAh g^?1) and discharge capacity performance were observed. The capacity retention rates of Mg_0.8Ti_0.175Mn_0.025Ni, Mg_0.8Ti_0.2Ni, Mg_0.8Ti_0.05Mn_0.05Ni, and Mg_0.8Ti_0.1Mn_0.1Ni alloys were 81%, 68%, %67, and 47%, respectively, at the 20th cycle.     

Structure of the secondary phase and its effects on hydrogen-storage properties in a Ti0.7Zr0.2V0.1Ni alloy

The phase structures and hydrogen storage properties of a Ti_0.7Zr_0.2V_0.1Ni alloy are studied. It is found that this alloy consists of a matrix and a secondary phase. The matrix is a B2-type compound with bcc structure and the secondary phase is a C14-type Laves phase with hexagonal structure. This alloy electrode has a larger charge–discharge capacity than stoichiometric TiNi electrodes but shows poorer activation behaviour. Optimizing the content of secondary phase is necessary for improving the overall hydrogen-storage properties of substituted Ti–Ni system alloys.     

Effect of Ni content on the structure and hydrogenation property of mechanically alloyed TiMgNix ternary alloys

In this study, TiMgNi_x samples (x = 0.2, 0.4, 0.6, 0.8, 1) have been prepared by mechanical alloying using a planetary high-energy ball mill. The structural transformations were characterized by XRD and indicated that all the as-milled TiMgNi_x alloys consist of mixtures of crystalline Mg and amorphous Ti-Ni-(Mg) phase. TEM analyses also show that nanocrystallites and amorphous phases coexist in the as-milled TiMgNi alloy. Electrochemical test shows that the TiMgNi composition yields the highest discharge capacity. The discharge capacities and activation properties of TiMgNi_x alloys linearly increase with increasing Ni content. The MgTiNi_0.8 composition boasts the best cycling property which is consistent with its XRD pattern after the electrochemical test. No decomposition nor crystallization are found after 21 consecutive charge/discharge cycles which implies that the as-milled TiMgNi_x alloys possess a good corrosion resistance in alkaline electrolytes. PCI measurements were carried out at 598 K and 648 K on the TiMgNi composition. The pressure plateau during hydrogen absorption is raised by two steps and its level increases with increasing temperature. The hydrogen absorption capacity of TiMgNi is around 1.3 weight %. XRD analyses show that the dehydrogenated TiMgNi sample consists of TiH₂, Mg, Mg₂Ni and TiNi₃, in which Mg and Mg₂Ni are identified as the hydrogen absorption phases.     

Carbon coating with different carbon sources on rare earth hydrogen storage alloy

The rare earth hydrogen storage alloy was coated with the same contents of carbon particles using sucrose, glucose, pitch, and chitosan as carbon sources, and compared with the samples of uncoated and mechanically mixed with the carbon powder. The results show that the maximum discharge capacity (C_max), high-rate dischargeabilitiy (HRD), and cyclic stability after 500 cycles (S₅₀₀) are improved to various degrees by carbon coating. It is found that the better the fluidity of carbon source in the carbonization process, the more uniform the carbon distributed on the alloy surface, and the longer the cyclic life of the alloy electrode. The less impurity remained after carbonization and the higher degree of graphitization of carbon, the better the electrocatalytic activity and HRD performance of the alloy electrode. The alloy coated with carbon particles using sucrose as carbon source has the best electrochemical properties. Compared with the uncoated sample, the C_max of the alloy electrode increases from 354.5 to 359.0 mAh/g, the HRD₁₂₀₀ increases from 65.84% to 74.39%, and the S₅₀₀ increases from 63.14% to 69.63%.     

Electrochemical properties of ZrO2-coated LiNi0.8Co0.2O2 cathode materials

The effect of a ZrO₂ coating on the structure and electrochemical properties of the cathode material LiNi_0.8Co_0.2O₂ was investigated using EPMA, TEM, XRD, and electrochemical impedance spectroscopy (EIS). In particular, we focused on the distribution of the ZrO₂ on the particle surface to study the relationship between electrochemical properties of the coated cathode and the distribution of the coating materials in the particle. Based on the results from composition analysis and electrochemical tests, it was found that the coating layer consisted of nano-sized ZrO₂ particles attached nonuniformly to the particle surface and the ZrO₂ layer significantly improved the electrochemical properties of the cathode by suppressing the impedance growth at the interface between the electrodes and the electrolyte.     

Electrochemical investigations on cobalt-microencapsulated MmNi2.38Al0.82Co0.66Si0.77Fe0.13Mn0.24 hydrogen storage alloys for Ni–MH batteries

Cobalt coatings were applied over lanthanum process-rich MmNi_2.38Al_0.82Co_0.66Si_0.77Fe_0.13Mn_0.24 alloy particles by an autocatalytic electroless deposition process. Electrode characteristics such as electrochemical capacity and cycle life were studied for the uncoated and coated alloys. The structure and morphology of the surface modified samples were characterized with XRD and SEM/EDAX techniques. The cobalt coating forms a thin layer on the surface of the core material and the coated alloys exhibit a 15% improvement in performance over the bare alloy. A comparison of the electrochemical impedance behaviour of the bare and cobalt-coated metal hydride electrodes at different states-of-charge reveals that the relaxation period is distinct for different SOCs. The cobalt microencapsulations influence the apparent activation energy of the dehydriding process. The calculated equivalent rate constant (k_eq) values confirm the improvement in reversibility for the cobalt-coated alloy as compared to the bare alloy.     

Elaboration and electrochemical characterization of Mg–Ni hydrogen storage alloy electrodes for Ni/MH batteries

Mg–Ni hydrogen storage alloy electrodes with composition of Mg–33, 50, 67 Ni at. % in amorphous phase were prepared by means of mechanical alloying (MA) process using a planetary ball mill. The electrochemical hydrogen storage characteristics and mechanisms of these electrodes were investigated by electrochemical measurements, X–ray diffraction (XRD) and scanning electron microscope (SEM) analyses. The relationship between alloy composition and electrochemical properties was evaluated. In addition, optimum milling time and composition of Mg–Ni hydrogen storage alloy with acceptable electrochemical performance were determined. XRD results show that the alloys exhibit dominatingly amorphous structures after milling of 20 h. The electrochemical measurements revealed that the discharge capacity of Mg₃₃Ni₆₇ and Mg₆₇Ni₃₃ alloy electrodes reached a maximum when alloys were prepared after 20 h of milling time (260 and 381 mAhg^?1, respectively). The maximum discharge capacity of Mg₅₀Ni₅₀ alloy was observable after 40 h milling (525 mAhg^?1). It was also found that the cyclic stability of the alloys increased with increasing Ni content. Among these alloys, the amorphous Mg₅₀Ni₅₀ alloy presents the best overall electrochemical performance. In this paper, electrode process kinetics of Mg₅₀Ni₅₀ alloy electrode was also studied by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. The impedance spectra of electrodes were measured at different depths of discharge (DODs). The observed spectra were fit well with the equivalent circuit model used in the paper. The electrochemical parameters calculated from electrochemical impedance were also compared. The electrochemical discharge and cyclic performance of 20, 40 and 60 h milled Mg₅₀Ni₅₀ alloy electrodes were demonstrated by the fitted charge transfer resistance and Warburg impedance obtained at various DODs. It was further observed that the controlling-step of the discharge process changed from a mixed rate-determining process at lower DODs to a mass-transfer controlled process at higher DODs. The fitted results demonstrated that charge–transfer resistance (R_ct) increased with DOD. The R_ct of 40 h milled Mg₅₀Ni₅₀ alloy (29.27 Ω) was lower than that of 20 h (41.89 Ω) and 60 h milled alloys (92.43 Ω) at fully discharge state.     

Performance studies of solid oxide fuel cell cathodes in the presence of bare and cobalt coated E-brite alloy interconnects

The electrochemical performances of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) electrodes were studied by half-cell measurements in the absence of chromia-forming alloy, in the presence of bare and Co coated E-brite alloy interconnects, respectively. The surface and cross-section properties of the bare and Co coated E-brite alloys, and LSCF electrodes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, and electron probe microanalysis (EPMA). The results showed a rapid degradation in LSCF performance when the bare E-brite alloy was used as interconnect. The growth of chromia scale on the E-brite alloy and the increase of Cr content throughout the LSCF electrode were observed. The uniform and dense Co coating process was developed to coat the E-brite alloy by using both electroless and electrodeposition methods. It was demonstrated that the Co layer effectively mitigates the Cr migration, leading to improved electrochemical stability of LSCF electrodes.     

Effects of surface coating with polyaniline on electrochemical properties of La–Mg–Ni-based electrode alloys

The effects of surface coating with polyaniline on electrochemical properties of La_0.8Mg_0.2Ni_3.4Al_0.1 electrode alloys were studied in this paper. The flake-shaped polyaniline coatings were deposited on the surface of La_0.8Mg_0.2Ni_3.4Al_0.1 alloy powders by electrodeless deposition. Electrochemical studies showed that the discharge capacity increased to 391.8 mAh g⁻¹ after surface modification with polyaniline, compared to 382.5 mAh g⁻¹ for the bare alloys. The cyclic stability over 100 cycles improved from 81.6% to 87.5%. Also, the kinetic properties were investigated in detail.     

Effect of cobalt on the microstructure and hydrogen sorption performances of TiFe0.8Mn0.2 alloy

The microstructures and the hydrogen sorption performances of TiFe_0.8Mn_0.2Co_x (x = 0, 0.05, 0.10, 0.15) and TiFe_0.8Mn_0.2-yCo_y (y = 0.05, 0.10) alloys have been investigated. For TiFe_0.8Mn_0.2Co_x alloys, the lattice parameters of TiFe phase decreased and the Laves phase contents increased with the addition of Co. With the increase of Co content in TiFe_0.8Mn_0.2Co_x alloys, the maximum hydrogen storage capacities of TiFe_0.8Mn_0.2Co_0.05 and TiFe_0.8Mn_0.2Co_0.10 alloys decreased, but the effective hydrogen capacities increased, which is ascribed to the improved flatness of the α-β desorption plateau. Substitution of Co for Mn in TiFe_0.8Mn_0.2-yCo_y alloys can effectively lead to single phase of TiFe alloys. Therefore, TiFe_0.8Mn_0.2-yCo_y alloy showed a deteriorated activation property, but its effective hydrogen capacity increased remarkably due to the obviously improved flatness of the α-β desorption plateau. The addition of Co might adjust the change of the octahedral intersitial environment caused by Mn doping in TiFe phase, which contributes to the improved flatness of the α-β desorption plateau and hence the increased effective hydrogen capacity.     

Investigations on hydrogen storage performances and mechanisms of as-cast TiFe0.8-mNi0.2Com (m=0, 0.03, 0.05 and 0.1) alloys

In order to improve the hydrogen storage performances of TiFe-based alloys, a new type of TiFe_0.8-mNi_0.2Co_m (m = 0, 0.03, 0.05 and 0.1) alloys were prepared through vacuum medium-frequency induction melting. XPS results showed that the composition of surface oxide film contains TiO₂, FeO and NiO for the cobalt-free alloy, and it also includes CoO and Co₃O₄ besides the above oxides for the cobalt-containing alloys. The activation temperature is 523, 403, 383 and 373 K for the TiFe_0.8-mNi_0.2Co_m (m = 0, 0.03, 0.05 and 0.1) alloys, respectively. The changes of the composition and microstructure of the surface oxide film are the root causes of the reduction of the activation temperature. XRD and SEM analyses showed that all the alloys are composed of the majority phase of TiFe phase and non-hydrogenated phase of Ti₂Fe phase. Adding appropriate amount of cobalt is beneficial to inhibiting the generation of Ti₂Fe phase and increasing the cell volume of TiFe phase. The hydrogenation capacity is proportional to the content of TiFe phase, which is 1.11, 1.48, 1.54 and 1.29 wt% for the TiFe_0.8-mNi_0.2Co_m (m = 0, 0.03, 0.05 and 0.1) alloys at 313 K, respectively. The hydrogenation plateau performance also is improved correspondingly.     

Effects of surface modifications of the LMNi3.9Co0.6Mn0.3Al0.2 alloy in a KOH/NaBH4 solution upon its electrode characteristics within a Ni–MH secondary battery

This study examined the effects of surface modifications of the LMNi_3.9Co_0.6Mn_0.3Al_0.2 (LM; Lanthanum rich misch metal) alloy in KOH solutions, both with and without NaBH₄, on its electrode characteristics in a nickel–metal–hydride (Ni–MH) secondary battery at low and ambient temperatures. The activation behavior, discharge capacity, rate capability, and electrochemical catalytic activity of the alloy were significantly improved by surface treatment with a KOH/NaBH₄ solution. This phenomenon is attributable to the alkaline treatment's removal of the oxide film and corrosion products on the surface of the alloy that can hinder surface activation, as well as its formation of a nickel- and cobalt-rich layer that exhibits high catalytic activity on the surface.     

Electrode performance of a new La0.6Sr0.4Co0.2Fe0.8O3 coated cathode for molten carbonate fuel cells

To improve the cathode performance in molten carbonate fuel cells (MCFCs), Lanthanum Strontium Cobalt Ferrite (La_0.6Sr_0.4Co_0.2Fe_0.8O₃, LSCF) of perovskite structure was coated on a porous Ni plate by a vacuum suction method. The electrochemical performance of modified cathode was examined and compared with that of uncoated conventional cathode via single cell operation and electrochemical impedance analysis (EIS). The cell voltage of the single cell using the LSCF coated cathode, measured at 650 °C with current density of 150 mA/cm² is 0.837 V and it is higher than that of the cell with uncoated conventional cathode, 0.805 V. The higher performance and the lower charge transfer resistance were obtained at 600–700 °C after LSCF coating. The lower activation energy of oxygen reduction reaction was also obtained. The lower activation energy of oxygen reduction reaction after LSCF coating shows that LSCF on lithiated NiO cathode plays a role of catalyst on the oxygen reduction reaction in cathode.     

Surface modification of LiNi0.5Co0.2Mn0.3O2 cathode materials with Li2O‐B2O3‐LiBr for lithium‐ion batteries

LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) cathode material suffers from phase transformation and electrochemical performance degradation as its main drawbacks, which are strongly dependent on the surface state of NCM523. Herein, an effective surface modification approach was demonstrated; namely, the fast lithium‐ion conductor (Li₂O‐B₂O₃‐LiBr) was coated on NCM523. The Li₂O‐B₂O₃‐LiBr coating layer as a protecting shell can prevent NCM523 particles from corrosion by the acidic electrolyte, leading to a superior discharge capacity, rate capability, and cycling stability. At room temperature, the Li₂O‐B₂O₃‐LiBr–coated NCM523 exhibited an excellent capacity retention of 87.7% after 100 cycles at the rate of 1 C, which is remarkably better than that (29.8%) without the uncoated layer. Furthermore, the coating layer also increased the discharge capacity of NCM523 cathode material from 68.7 to 117.0 mAh g^?1 at 5 C. Those can be attributed to the reduction in the electrode polarization and improvement in the electrode conductivity, which was supported by electrochemical impedance spectroscopy and cyclic voltammetry measurements.     

Electrochemical performances of the as-melt La0.75−xMxMg0.25Ni3.2Co0.2Al0.1 (M = Pr,Zr; x = 0, 0.2) alloys applied to Ni/metal hydride (MH) battery

The partial replacement of La by M (M = Pr, Zr) has been performed in order to ameliorate the electrochemical hydrogen storage performances of La–Mg–Ni-based A₂B₇-type electrode alloys. For this purpose, we adopt melt spinning technology to prepare the La_0.75−xM_xMg_0.25Ni_3.2Co_0.2Al_0.1 (M = Pr, Zr; x = 0, 0.2) electrode alloys. Then systemically investigate the effects that the preparation methods and M (M = Pr, Zr) substitution have on the structures and electrochemical hydrogen storage characteristics of the alloys. The analysis of XRD and TEM reveals that the as-cast and spun alloys hold a multiphase structure, containing two main phases (La, Mg)₂Ni₇ and LaNi₅ as well as a trace of residual phase LaNi₂. Besides, the as-spun (M = Pr) alloy displays an entire crystalline structure, while an amorphous-like structure is detected in the as-spun (M = Zr) alloy, implying the replacement of La by Zr facilitates forming amorphous phase. Based upon electrochemical measurements, an impact engendered by melt spinning on the electrochemical performances of the alloys appears to be evident. The cycle stabilities monotonously augment with the growing of the spinning rate. The discharge capacity and high rate discharge ability (HRD), however, exhibit difference. For the (M = Pr) alloy, they first mount up and then fall with the rising of the spinning rate, whereas for the (M = Zr) alloy, they always decline as the spinning rate elevates. Furthermore, the replacement of La by M (M = Pr, Zr) considerably enhances the cycle stability of the alloys and the replacement of La by Pr clearly increases the discharge capacity, but the Zr replacement results in an adverse impact.     

Influence of polarization on the electrochemical activity of La2–xCaxNiO4+δ electrodes in contact with Ce0.8Sm0.2O1.9 electrolyte

The effect of electrode polarization on the electrochemical activity of La₂NiO_4+δ and La_1.9Ca_0.1NiO_4+δ electrodes in contact with the Ce_0.8Sm_0.2O_1.9 electrolyte is studied by impedance spectroscopy. It is found that anodic polarization facilitates electrode reaction for both electrodes leading to significant decrease in the polarization resistance. The effect of cathodic polarization differs between the electrodes: the polarization resistance of La₂NiO_4+δ electrode slightly increases, while the polarization resistance of La_1.9Ca_0.1NiO_4+δ electrode strongly decreases with the increase in the applied potential. It is established that in all cases the polarization mostly affects the low-frequency stage of the electrode reaction, connected with oxygen surface exchange and diffusion. The surface state of the samples after exposure under polarization is studied by X-ray photoelectron spectroscopy. Correlations between electrochemical activity of the electrodes and the changes in their surface composition under polarization are discussed.     

Effect of nanosized Mg0.8Cu0.2O on electrochemical properties of Li/S rechargeable batteries

Nanosized Mg_0.8Cu_0.2O powders were prepared by sol–gel method. In order to improve the electrochemical performances of Li/S rechargeable batteries, Mg_0.8Cu_0.2O was used as an additive for crystalline vanadium pentoxide (c-V₂O₅)/S composite cathode. The composite electrodes with and without additive were characterized by scanning electron microscopy, galvanostatic charge–discharge, rate capability and cycle performance. The results showed that not only the cycle life and discharge capacity were improved, but also the rate capability was improved after the addition of Mg_0.8Cu_0.2O. The improvements of electrochemical performances were due to the adsorbing effect on polysulfide of Mg_0.8Cu_0.2O. Furthermore, the additive also had catalytic effect on promoting redox reaction of the Li/S batteries.     

Effects of partial substitution by Fe and Co for Ni in the Mg1.75Al0.25 Ni electrode alloy on their electrochemical performances

In this paper, Co and Fe were selected as the partial substitution elements for Ni to form Mg_1.75Al_0.25Ni_0.9X_0.1 quaternary electrode alloys prepared by means of solid diffusion method (DM) on the basis of the ternary Mg_1.75Al_0.25Ni alloy. The XRD patterns exhibited that quaternary alloys still possessed the main phase of Mg₃AlNi₂ as that of the ternary Mg_1.75Al_0.25Ni alloy when introducing the substitution elements Co or Fe. The electrochemical studies found that Fe- or Co-substituted quaternary alloy possessed larger discharge capacity and higher cycling stability than the Mg_1.75Al_0.25Ni ternary alloy. Cyclic voltammetry (CV) tests demonstrated that the additional Fe and Co could improve the electrocatalytic oxidation activity on the surface and also the discharge capacity of the Mg_1.75Al_0.25Ni electrode alloy. Anodic polarization curves indicated that the additional Fe and Co led to potentials shifting toward position direction and decrease of the corrosion current. Electrochemical impedance spectroscopy (EIS) revealed that the exchange current density decreases considerably with the augmentation of the cycle number.     

Effects of annealing temperature on the structure and properties of the LaY2Ni10Mn0.5 hydrogen storage alloy

The effects of annealing at 1123, 1148, 1173 and 1198 K for 16 h on the structure and properties of the LaY₂Ni₁₀Mn_0.5 hydrogen storage alloy as the active material of the negative electrode in nickel–metal hydride (Ni–MH) batteries were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy linked with an energy dispersive X-ray spectrometer (SEM–EDS), pressure-composition isotherms (PCI) and electrochemical measurements. The quenched and annealed LaY₂Ni₁₀Mn_0.5 alloys primarily consist of Ce₂Ni₇- (2H) and Gd₂Co₇-type (3R) phases. The homogeneity of the composition and plateau characteristics of the annealed alloys are significantly improved, and the lattice strain is effectively reduced. The alloys annealed at 1148 K and 1173 K have distinctly greater hydrogen storage amounts, 1.49 wt% (corresponding to 399 mAh g^?1 in equivalent electrochemical units) and 1.48 wt%, respectively, than the quenched alloy (1.25 wt%, corresponding to 335 mAh g^?1 in equivalent electrochemical units). The alloys annealed at 1148 K and 1173 K have relatively good activation capabilities. The annealing treatment slightly decreases the discharge potentials of the alloy electrodes but markedly increases their discharge capacity. The maximum discharge capacities of the annealed alloy electrodes (372–391 mAh g^?1) are greater than the extreme capacity of the LaNi₅-type alloy (370 mAh g^?1). The cycling stability of the annealed alloy electrodes was improved.