Hydrogen Storage Electrode Alloys Research Articles

Cooperative effect of Co and Al on the microstructure and electrochemical properties of AB 3-type hydrogen storage electrode alloys for advanced MH/Ni secondary battery

The crystal structure and electrochemical properties of the La 2MgMn 0.3Ni 8.7− x (Co 0.5Al 0.5) x ( x = 0, 1.0, 2.0 and 3.0, at%) hydrogen storage alloys are investigated systematically. The results show that all the alloys consist of (La, Mg)Ni 3 and LaNi 5 phases, the cyclic stability S 60 increases from 61.2% ( x = 0) to 78.7% ( x = 3.0) after 60 charge/discharge cycles, and the peak high rate dischargeability (HRD) at the discharge current density of 1200 mA/g appears at the alloy of x = 2.0 with the value of 68.3%. Moreover, the electrochemical kinetic properties of the alloys are also improved at different extent with increasing x. All the results indicate that the substitution of Co and Al for Ni in AB 3-type hydrogen storage alloys is effective to improving the overall electrochemical properties, and the optimum content is x = 2.0.

Mechanisms of improving the cyclic stability of V–Ti-based hydrogen storage electrode alloys

In this work, the mechanisms of improving the cyclic stability of V–Ti-based hydrogen storage electrode alloys were investigated systemically. Several key factors for example corrosion resistance, pulverization resistance and oxidation resistance were evaluated individually. The V-based solid solution phase has much lower anti-corrosion ability than C14 Laves phase in KOH solution, and the addition of Cr in V–Ti-based alloys can suppress the dissolution of the main hydrogen absorption elements of the V-based phase in the alkaline solution. During the charge/discharge cycling, the alloy particles crack or break into several pieces, which accelerates their corrosion/oxidation and increases the contact resistance of the alloy electrodes. Proper decreasing the Vickers hardness and enhancing the fracture toughness can increase the pulverization resistance of the alloy particles. The oxidation layer thickness on the alloy particle surface obviously increases during charge/discharge cycling. This deteriorates their electro-catalyst activation to the electrochemical reaction, and leads to a quick degradation. Therefore, enhancing the oxide resistance can obviously improve the cyclic stability of V–Ti-based hydrogen storage electrode alloys.

Effect of metal oxides on electrochemical performances of La–Mg–Ni-based alloys

Metal oxides (TiO2, Er2O3 and ZnO) were added to La–Mg–Ni-based hydrogen storage alloy electrodes and their effects on the structural and electrochemical properties were studied. The charge efficiency, especially at high charge current density was greatly ameliorated, and the high rate charge capability at 1440 mA g−1 increased from 85.1% (blank) to 94.1% (TiO2), 93.3% (Er2O3) and 90.5% (ZnO). The high temperature dischargeability was also improved in case of TiO2, Er2O3 and ZnO additives. These additives suppressed formation of Mg(OH)2 and La(OH)3 during charge/discharge process and therefore the cycling stability was improved. The discharge capacity retention at the 200th cycle increased from 72.9% (blank) to 79.6% (TiO2), 87.5% (Er2O3) and 77.9% (ZnO).

Influence of additives on the thermal behavior of nickel/metal hydride battery

This study discusses the thermal behavior of the 6.5 Ah cylinder Ni/MH hydride battery with 0.5 wt% ytterbium oxide (Yb2O3) in nickel electrode and 1.0 wt% super absorbent polymer (SAP) in hydrogen-storage alloy (MH) electrode during charging to 150% of its rating capacity. Quantity of heat and heat generation rate of the battery during charging are studied by quartz frequency microcalorimeter. Heat generation curve is fitted into a function, and heat transport equation is solved. Using measured data, the internal temperature profiles at the terminal moment of charging at 1C, 3C, and 5C are simulated by FEM. Influence of Yb2O3 and SAP on the thermal behavior of Ni/MH battery is examined by the two-dimensional thermal model. Results show that addition of Yb2O3 and SAP can achieve substantial improvement for thermal behavior of Ni/MH battery at 1C,3C, and 5C charging.

The correlative effects of Al and Co on the structure and electrochemical properties of a La–Mg–Ni-based hydrogen storage electrode alloy

Replacing Co in the La–Mg–Ni-based alloys by other cheap materials is an expected goal to reduce the cost of the hydrogen storage alloys. The present study aims to partially substitute Co by Al for the La–Mg–Ni-based hydrogen storage alloys and to get the correlative effects of Al and Co on the structure and electrochemical properties of the alloys. Alloys with composition of La 0.7Mg 0.3Ni 2.75Co 0.75− x Al x ( x = 0–0.4) were investigated. The results from X-ray diffraction analyses show that all of the alloys mainly consist of a LaNi 5 phase and a (La, Mg)Ni 3 phase. While LaNi 5 phase increases, the (La, Mg)Ni 3 phase decreases and the cell volumes of both the phases are enlarged with the increasing substitution of Al for Co. Results of the electrochemical studies indicate that the substitution of Al for Co can enhance the cycle stability of the alloy electrode effectively. The capacity retention after 100 cycles is improved from 45.9% ( x = 0) to 82.1% ( x = 0.4). However, the maximum capacity and the electrochemical kinetics decrease. The correlative actions of Al and Co on the corrosion and pulverization resistance of the alloys and hence on the cycle stability and electrochemical kinetics of the alloy electrodes were discussed.

Effects of electrolytes and temperature on high-rate discharge behavior of MmNi 5-based hydrogen storage alloys

A series of experiments have been performed to investigate the effects of electrolyte composition and temperature on the high-rate discharge behaviors of MmNi 5-based AB 5 hydrogen storage alloy electrodes. Two types of AB 5 electrodes have been used using different alloys: Ce-rich alloy V (La 0.26 Ce 0.44Pr 0.1Nd 0.2Ni 3.55Co 0.72Mn 0.43Al 0.3) and La-rich alloy N (La 0.58Ce 0.25Pr 0.06 Nd 0.11Ni 3.66Co 0.74Mn 0.41Al 0.18). Electrolytes EN were obtained by adding a saturated amount of Al 2(SO 4) 3 to the original electrolyte EO (6 M KOH + 1 wt% LiOH). The electrolyte EN has previously been shown to be very effective to stop the self-discharge of the AB 5 electrodes, better charge/discharge cycle life have been observed. The electrochemical properties of the electrodes were measured by two methods: step mode high-rate discharge and continuous mode high-rate discharge. The results indicate that at 298 K and 333 K, high-rate discharge capacity of Ni–MH battery was mostly affected by the chemical composition of the electrolyte, then the type of alloy. Better dischargeabilities in high-rate discharge capacity have been observed in electrolyte EO than in electrolyte EN. The Ce-rich alloy V has a higher high-rate discharge capacity than La-rich alloy N. High-rate discharge capacity decreases in the following order: VEO > NEO > VEN > NEN (VEO denotes the combination of alloy V and electrolyte EO used in the test battery, similarly equivalent representations for NEN, VEO and VEN).

Influences of low-Ti substitution for La and Mg on the electrochemical and kinetic characteristics of AB3-type hydrogen storage alloy electrodes

The (La0.67Mg0.33)1-xTixNi2.75Co0.25 (x = 0, 0.05, 0.10, 0.15 and 0.20, at%) alloys were synthesized by arc melting and subsequent heat solid-liquid diffusing method. The structure, electrochemical properties and kinetic characteristics of the alloys were investigated systematically. The results showed that all the alloys mainly consisted of the (La, Mg)Ni3, LaNi5 phases, and the lattice parameters and the cell volumes of the (La, Mg)Ni3 and LaNi5 phases decreased with increasing Ti content. The alloy electrodes could be activated to reach their maximum discharge capacity within five cycles. The cycle life after 100 charge/discharge cycles (C100/Cmax) and the high-rate dischargeability at a discharge current density of 1200 mA/g first increased and then decreased. All the results showed that low-Ti content in AB3-type hydrogen storage alloys was beneficial to improvements of the overall electrochemical properties, and the optimum overall electrochemical performance of the alloy electrodes was obtained when x = 0.05. AB3-type alloy, metal hydride electrode, microstructure, electrochemical property, kinetic characteristic

Effects of alkaline treatment of hydrogen storage alloy on electrocatalytic activity for NaBH 4 oxidation

Activation of the MmNi 4.03Co 0.42Mn 0.31Al 0.24 hydrogen storage alloy electrode is performed by immersing the electrode in a solution containing 6.0 mol dm −3 NaOH and 0.1 mol dm −3 NaBH 4. The effects of activation on the electrocatalytic activity of the electrode for NaBH 4 oxidation are investigated by cyclic voltammetry and chronoamperometry. Immersion activation greatly improves the electrocatalytic activity of the alloy electrode. Hydrogen was absorbed in the alloy during the immersion activation treatment and its electrooxidation is responsible for the high initial oxidation current. The stabilized current mainly results from the direct oxidation starting from the borohydride species. The effects of activation on structure and surface chemistry of the alloy are also discussed.

Fe2O3‐Modified Hydrogen Storage Alloys as Electrocatalyst for Borohydride Oxidation

AbstractThe effects of hot alkaline treatment and Fe2O3 modification of hydrogen storage alloy on the electrocatalytic activity for oxidation of borohydride have been investigated using linear sweep voltammetry. The performance of borohydride electrochemical oxidation was significantly influenced by the hot alkaline treatment and Fe2O3 modification of the hydrogen storage alloy. The results showed that the current density of the Fe2O3‐modified hot alkaline‐treated hydrogen storage alloy electrode containing 5 wt% Fe2O3 reached 125 mA·cm−2 in 0.10 mol·L−1 NaBH4 and 2 mol·L−1 NaOH solution at −0.55 V vs. saturated Ag/AgCl, KCl electrode.

Pulverization mechanism of the multiphase Ti–V-based hydrogen storage electrode alloy during charge/discharge cycling

Pulverization is an important key factor for the electrochemical cycle stability of many hydrogen storage alloys. In this paper, the pulverization mechanism of the multiphase Ti–V-based hydrogen storage alloy which mainly consists of a V-based solid solution phase with the BCC structure and a C14 Laves phase is studied based on a sample material of the Ti 0 .8 Zr 0 .2 V 2.7Mn 0.5Cr 0.6Ni 1.25Fe 0.2 alloy. The microstructure of the alloy and the morphology change of the alloy electrode during the charge/discharge process were observed by transmission electron microscope, scanning electron microscope and atomic force microscope, etc. The effect of mechanical properties of the V-based phase and the C14 Laves phase on the pulverization behavior of the Ti–V-based alloy is discussed. The results show that microcracks initially occur at the phase boundary of the V-based phase and the C14 Laves phase and then extend to the C14 Laves phase in the charge/discharge process. The phase boundary is composed of a Ti segregated amorphous layer with a thickness of about 90 nm, mismatching with the crystallized V-base phase and C14 Laves phase. The toughness of the C14 Laves phase is much lower and the hardness is higher than that of the V-based phase. The weak bonding strength of the phase boundary, the lower toughness of the C14 Laves phase and the large volume expansion/contraction of the C14 Laves phase during charge/discharge cycling are the main factors that cause the pulverization of the Ti–V-based alloy.

Effect of CuO addition on electrochemical properties of AB 3-type alloy electrodes for nickel/metal hydride batteries

In order to improve overall electrochemical properties of AB 3-type hydrogen storage alloy electrodes, especially the cycling stability, CuO was added to the electrode. Electrochemical properties of the electrodes with and without additives were studied. Cyclic voltammetry and SEM results show that CuO is reduced to Cu during the charging process and the fine Cu particles deposit at surface of the alloy particles. The as-deposited Cu particles form a protective layer to increase electronic and heat conductivity of the electrodes and thus improve maximum discharge capacity, high rate dischargeability, cycling stability and dischargeability at high temperature of the electrodes. The maximum discharge capacity increases from 314 mAh g −1 (blank electrode) to 341 mAh g −1 (3.0 wt.% CuO) and the capacity retention rate at the 200th cycle increases from 71.6% to 77.2% (2.5 wt.% CuO).

Effect of electrolyte on electrochemical characteristics of MmNi3.55Co0.72Al0.3Mn0.43 alloy electrode for hydrogen storage

Abstract A series of experiments have been performed to investigate the effects of three electrolytes of different compositions (EO, EA and EM) on the electrochemical characteristics of MmNi3.55Co0.72Al0.3Mn0.43 hydrogen storage alloy electrode. Electrolytes EA and EM were obtained by adding appropriate amounts of Al2(SO4)3 and MnSO4 to the original electrolyte EO (6 M KOH + 1 wt% LiOH), respectively. Electrode activation, maximum capacity, cycle life, self-discharge and high-rate discharge characteristics have been studied. It was found that a maximum capacity of about 260 mA h/g has been obtained for the alloy electrodes in all these electrolytes after 5–7 cycles of charging/discharging. The alloy electrodes have a good durability in electrolytes EA and EM, especially after 175 cycles. Using the capacity retention as an indication of self-discharge resistance, almost identical degree of capacity retention (82% after 4 days and 45% after 16 days) has been observed at 298 K, regardless of the electrolytes used. When tested at higher temperature, however, a higher capacity retention (46% after 3 days) at 333 K has been observed for electrodes in electrolyte EA, and about 32% for electrodes in both electrolytes EO and EM. As to high-rate discharge behavior of the results of high-rate discharge tests indicated that about 50% of discharge efficiencies were obtained in the three electrolytes at 333 K by continuous-model high-rate discharge method, at a discharge rate of 7C, and 22% in 298 K. The alloy electrode in electrolyte EM has the best durability, in which about 50% of discharge efficiency at DC = 9C was obtained by step-model high-rate discharge method at 333 K, which was even higher than that at 298 K.

Electrochemical impedance study of discharge characteristics of Pd substituted MgNi-based hydrogen storage electrode alloys

Quaternary Pd substituted MgNi-based hydrogen storage alloys Mg 0.9− x Ti 0.1Pd x Ni ( x = 0.04, 0.06, 0.08, 0.1) in amorphous phase were prepared by mechanical alloying (MA). They exhibited enhanced cycle life in our previous studies. However, their electrochemical impedances and its relationship with electrode reaction kinetics are still unclear. In this work, the impedance spectra of the above electrode alloys were measured at different depths of discharge (DODs). The observed spectra were fit well with the equivalent circuit model used in the paper. We further observed that the rate-determining step changed from charge-transfer to hydrogen diffusion with proceeding of discharge according to the fitting analyses of spectra. The fitted results demonstrated that charge-transfer resistance R ct decreased and then increased with the depth of discharge (DOD) and it also increased with Pd content among the studied alloys. Thickness of surface passive film, which is proportional to the reciprocal of double layer capacitance near the electrode alloy surface according to Badawy et al., increased with the DOD and especially with Pd content in the alloys. Meanwhile, the thickened film on the surface of electrode alloys ensured its long cycle life. Through Randles plotting and comparing the variation of Warburg coefficient σ vs. DOD for the electrode alloys, it was found that σ increased with the DOD and substitution of Pd also improved the diffusion performance of the studied electrode alloys.

The effect of partial substitution of Si for Ni on the electrochemical properties of cobalt-free LaNi 4.2− xAl 0.35Mn 0.45Si x ( x = 0–0.35) alloys

The structure and electrochemical properties of a cobalt-free hydrogen storage electrode alloys LaNi 4.2− x Al 0.35Mn 0.45Si x (0 ≤ x ≤ 0.35) have been investigated with small amount of Si in replacement of Ni. With the increase of Si content, the maximum discharge capacity and the high rate discharge (HRD) ability gradually decrease, however the cycle stability is improved correspondingly. The lowest capacity decay rate can remain 19.4% ( x = 0.35) after 300 charge/discharge cycles. The lower corrosion current densities of the alloys with the addition of Si can explain the good cycle life for them. It is found that all of the alloys are single CaCu 5 phase structure through XRD analysis. But some white dot-like LaNi 3 phase separated out in the matrix only for the alloy x = 0.35 through further EDX analysis.

Effects of Boron Addition on Structural and Electrochemical Properties of La-Mg-Ni-Co System Hydrogen Storage Electrode Alloys

Abstract The structural and electrochemical properties of La 0.7 Mg 0.3 Ni 2.65 Co 0.75 Mn 0.1 Al 0.2 B x ( x =0-0.08) hydrogen storage electrode alloys were investigated systematically. The results from XRD show that all of the alloys consist of a (La, Mg)Ni 3 phase and a LaNi 5 phase. With the increase of B content, the abundance of the (La, Mg)Ni 3 phase increases, accordingly that of the LaNi 5 phase decreases, and the cell volumes of the two phases decrease. Electrochemical studies indicate that B addition can effectively improve the high rate dischargeability ( HRD ) of the alloy electrode, and the optimal value of HRD 1000 reaches 57.1% as x being 0.04. With the addition of small amount of B, the cycling stability and activation properties of the alloy electrodes keep almost unchanged, but the maximum discharge capacity ( C max ) decreases. In addition, the electrochemical kinetics investigations including the linear polarization and anodic polarization were also performed in detail.

산업용 밀폐형 니켈수소전지의 전극 제조 및 전기화학적 특성

Electrochemical studies were performed by a half-cell test for the nickel hydroxide (cathode) and hydrogen storage alloy(anode) electrodes for the sealed Ni-MH batteries applicable to industrial use. The electrodes were fabricated and checked a charge efficiency and an internal pressure of the battery during charge-discharge cycling. In order to reduce the internal pressure of the sealed Ni-MH battery, cyclic voltammetry (CV) were performed on the electrodes of nickel hydroxide(cathode) and hydrogen storage alloy(anode), respectively. The results of the test showed clearly the oxidation/reduction and oxygen evolution reaction in a nickel hydroxide electrode and the hydrogenation behavior of a hydrogen storage electrode. The sealed Ni-MH battery of 130Ah was fabricated by using nickel hydroxide of a high over-voltage for an oxygen gas evolution and hydrogen storage alloy of a good performance for activation The battery showed a good characteristics such as a high charge efficiency of 98% at 1 C charge current, a low level internal pressure of 4 atm on a continuous over-charging and a large preservation capacity of 95% at 400 cycle.

The influence of Cu(OH) 2 addition on the low-temperature electrochemical performance of La 0.75Mg 0.25Ni 3.5 hydrogen storage alloy

The influence of Cu(OH) 2 addition in 7 mol/L KOH alkaline electrolyte on the electrochemical properties of La 0.75Mg 0.25Ni 3.5 hydrogen storage alloy electrode was investigated in the testing temperature range of from 25 °C to −40 °C in this paper. XRD Rietveld analyses shows that the La 0.75Mg 0.25Ni 3.5 hydrogen storage alloy consists of LaNi 5 phase, (La, Mg) 2Ni 7 phase and (La,Mg)Ni 3 phase. The maximum discharge capacity and the high-rate dischargeability (HRD) of the La 0.75 Mg 0.25Ni 3.5 alloy electrode both decrease with decreasing testing temperature, which mainly due to the slower hydrogen transfer in the bulk of the alloy and the lower electrocatalytic activity at lower temperatures. The scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDS) analyses indicate that, with the addition of Cu(OH) 2 to the KOH alkaline electrolyte, fine particles of metal Cu can be coated on the La 0.75Mg 0.25Ni 3.5 alloy electrode surface during charging process by electrodeposition. The Cu powders deposited on the La 0.75Mg 0.25Ni 3.5 alloy electrode can improve the alloy’s low-temperature discharge properties by decreasing its charge-transfer resistance and increasing its exchange current density. The cyclic stability of the Cu-deposited La 0.75Mg 0.25Ni 3.5 alloy electrode can be also improved to a certain extent by increasing its corrosive resistance in KOH alkaline electrolyte.

Intrinsic/Extrinsic Degradation of Ti−V-Based Hydrogen Storage Electrode Alloys upon Cycling

The capacity degradation mechanism of the Ti−V-based hydrogen storage electrode alloys was systematically investigated from the viewpoint of intrinsic/extrinsic degradation for the first time. The oxidation/corrosion of the active components and the appearance of the irreversible hydrogen were found to be the two dominating intrinsic factors for the cycling capacity degradation of the Ti−V-based electrode alloys, rather than the segregation and dissolution of the main hydrogen absorbing element V, the loss of which had some effect on the degradation of its discharge capacity but not dominating. The extrinsic degradation of the electrode alloys was mainly caused by the pulverization of the alloy particles, the increase of the contact resistance between particles, and the reaction resistance on surfaces during charge/discharge cycling. In the meantime the pulverization of the alloy particles accelerates the oxidation/corrosion of the active components, which further promotes the formation of the oxide layer and decreases the reaction speed. The decay of the apparent discharge capacity of the Ti−V-based electrode alloys is jointly affected by these factors.

Phase structure and electrochemical properties of laser sintered La 2MgNi 9 hydrogen storage electrode alloys

Phase structure and electrochemical properties of laser sintered La 2MgNi 9 alloys were studied. The sintered alloys contained a main phase, LaNi 5, and a ternary La-Mg-Ni phase, with a PuNi 3 structure and a small amount of LaMgNi 4. The ternary La-Mg-Ni phase with a PuNi 3 structure had the composition of La 1.8Mg 1.2Ni 9 and La 2MgNi 9, for alloys laser sintered at 1000 and 1400 W, respectively. Owing to further reactions between LaNi 5 and LaMgNi 4, the amount of the PuNi 3 phase increased for alloys sintered at 1400 W. Both alloys had good activation property (three charge/discharge cycles). The discharge capacities of the sintered alloys were 321.8 and 344.8 mAh/g, respectively. Compared with the alloy laser sintered at 1000 W, the poor cyclic stability of the alloy sintered at 1400 W was mainly attributed to the lower corrosion resistance of the La 2MgNi 9 phase.

Electrochemical properties of Mg-based hydrogen storage alloys prepared by hydriding combustion synthesis and subsequent mechanical milling (HCS+MM)

In this paper, a novel method, namely hydriding combustion synthesis (HCS) and subsequent mechanical milling (MM) was used to prepare Mg-based hydrogen storage electrode alloy. The phase structures and electrochemical properties of the alloys before and after MM were characterized by X-ray diffraction (XRD) analysis and galvanotactic charge–discharge cycle test, respectively. The XRD results showed that the structure of the as-milled alloys was nanocrystallite or amorphous-like state. Electrochemical measurements showed that the discharge capacity was improved greatly for the products of HCS+MM. The HCS product with only 5h MM showed markedly increased discharge capacity up to 481.5mAh/g for the first cycle, which was 10 times higher than the HCS product (39.4mAh/g). The discharge capacity was further increased to 628.3mAh/g for the HCS product after milling with nickel powder. Besides, the addition of nickel also led to an improved cycling stability of the alloy electrode during cycling in KOH electrolyte. It was indicated that the HCS+MM was promising for preparing Mg-based hydrogen storage electrode alloys.