Effect of Mn on the long-term cycling performance of AB5-type hydrogen storage alloy

Rare-earth AB₅-type hydrogen storage alloys are widely studied due to their extensive application potentials in hydrogen compressors, heat pump, Ni–MH batteries etc. However, their shortcomings such as plateau splitting and capacity degradation during hydrogen absorption/desorption hinder their practical applications. In this paper, we study the effect of Mn partial substitution for Ni on the plateau characteristics and long-term cycling performance of LaNi_5-xMn_x alloys. It is found that Mn addition expands the lattice interstitial for hydrogen accommodation, thus prohibiting the plateau splitting phenomenon. In addition, the substitution of Mn for Ni stabilizes the crystal structure of the alloys against hydrogen absorption/desorption, thus relieving the capacity degradation. The capacity retention of the alloys at the 1000th cycle (S₁₀₀₀) increases from 83.2% (x = 0) to 94.0% (x = 0.75). But when x reaches 1, the hydrogen desorption reversibility is reduced due to the low plateau pressure, resulting in a slight decrease in capacity retention.     

Effect of electrolyte concentration on the electrochemical properties of an AB5-type alloy for Ni/MH batteries

This publication is the first of a series of three that we have undertaken to study the effect of electrolyte concentration on electrode performance. Here, the electrochemical properties of an AB₅-type alloy, namely LaNi_3.6Co_0.7Mn_0.4Al_0.3, are investigated using different KOH electrolyte concentrations (i.e. 2 M, 4 M, 6 M and 8 M). The next two publications will be concerned with an AB₂-type alloy and a Mg-based alloy, respectively.     

Effect of annealing on the hydrogen-storage properties of rapidly quenched AB5-type alloys

The effect of annealing on the hydrogen-storage properties of rapidly quenched AB₅-type alloys is investigated by electrochemical measurements and X-ray diffraction. Annealing affects discharge capacity, voltage and cycle life. Annealing at an appropriate temperature can give a promising alloy with large capacity, high discharge voltage and long cycle life. Annealing at too high a temperature, such as 800 °C. results in the largest discharge capacity but the worst cycle life. Annealing can also flatten the discharge plateau regions. The striking difference in the phase structure between as-quenched and as-annealed alloys is that new phases occur as the product of annealing at high temperatures such as 600 and 800 °C. These phases are responsible for the deterioration in cycle durability.     

Investigations of AB5-type negative electrode for nickel-metal hydride cell with regard to electrochemical and microstructural characteristics

In the present investigation, AB₅-type hydrogen storage alloys with compositions Mm_0.8La_0.2Ni_3.7Al_0.38Co_0.3Mn_0.5Mo_0.02 and Mm_0.75Ti_0.05La_0.2Ni_3.7Al_0.38Co_0.3Mn_0.5Mo_0.02 are synthesized by radio-frequency induction melting. The electrochemical properties are studied through the measurements of discharge capacity, activation process, rate capability, self-discharge rate and cyclic stability of both the electrodes. Pressure-composition isotherms are plotted by converting the electrode potential into the hydrogen pressure following the Nernst equation. The structural and microstructural characterizations are performed by means of X-ray diffraction phase analysis and scanning electron microscopy of as-fabricated and electrochemically tested electrodes. An attempt is made to correlate the observed electrochemical properties with the structural–microstructural characteristics.     

Effect of vanadium content on electrochemical properties of la-based AB5-type metal hydride electrodes

Commercial La–Ni–Al–Co–Mn–V hydrogen storage alloys have been investigated to examine the effect of non-stoichiometry on the microstructure and electrochemical properties. It is found that for the stoichiometric ‘B’-rich compound, single phase with CaCu₅-type exists. However, for B-poor compounds, there is principally a CaCu₅-type phase with a small amount of V-rich type phase and the amount of V-rich phase reduces with vanadium. With the increase of Vy⩽0.1 content, hydrogen storage capacity is enhanced, whereas when y=0.2–0.3 it is decreased. The discharge capacity and cyclability are increased considerably by addition of vanadium in the range 0.02–0.1 with a maximum value at about 0.02%. The decrease of capacity for high V content was also correlated with the amount of V-rich phase. The V-rich phase is consisted of La_0.1Ni_2.6Al_0.2Co_2.0Mn_0.6V_1.3. The improvement of kinetics is due to the catalytic effect, grain boundary diffusion effect or more pronounced alloy pulverization upon cycling. This can be explained because the improvement of capacity for alloys with low V content is due to better kinetics. These alloys have been subjected to analysis by EDS, SEM and XRD. In order to determine the hydrogen storage capacity, the pressure composition isotherms (PCT curves) have been used. The metal hydride electrodes were characterized by galvanostatic cycling test.     

Influence of electrostatic field on the interaction of AB5-type alloy LaNi4.4Al0.3Fe0.3 with hydrogen

The effect of the electrostatic field on hydrogen absorption is experimentally studied for the case of AB₅-type intermetallic compound LaNi_4.4Al_0.3Fe_0.3 with low equilibrium pressure. Experimental facility contained control and measurement system for PCT-isotherms and a non-conductive polymer vessel immersed in a bath of a thermostat with transformer oil. The test sample with 100 g of the activated alloy powder was used. Electrostatic field was created between a copper tube, which simultaneously served as a hydrogen inlet, connected to a high voltage source and a grounded nickel plate rolled in the form of a cylinder around the outer wall of the vessel. The electrodes were arranged coaxially, the maximum voltage on the internal electrode was 15 kV. The high voltage source also allowed changing the polarity on the internal electrode.It was found that the electrostatic field had no effect on the already established equilibrium in the hydrogen-alloy system at a voltage at the electrode up to 15 kV, regardless of the polarity. However, the process of hydrogen absorption is noticeably slowed down when a voltage of up to 15 kV with negative polarity is applied to the internal electrode, and the effect increases with increasing voltage. At a voltage of 15 kV and the positive polarity of the internal electrode, there was no noticeable effect on the hydrogen absorption process.     

Theoretical model on the electronic properties of multi-element AB5-type metal hydride

Nowadays, multi-element alloys are preferred over binary alloys for application point of view. The hydrogenation properties strongly depend on the thermodynamic, structural and electronic properties of the alloys. At present, no model is available which can predict the hydrogen storage properties of the multi-element alloy, before actual synthesis of the alloy. In the present investigation, efforts are made to develop a theoretical mathematical model to predict the hydrogenation properties of multi-element AB₅-type metal hydride. The present investigation deals with the various electronic parameters which may affect the hydrogenation characteristics of the metal hydride. Based on all such parameters, an electronic factor has been proposed for AB₅-type alloys. Electronic factor has been combined with the structural and thermodynamical factor to propose a new combined factor, which was further correlated with the hydrogen storage capacity of the alloy. Atomic radius and electronic configuration of substituted elements in the multi-element AB₅-type hydrogen storage alloy have been found as key players to predict the hydrogenation properties of the alloys before synthesis. It has been shown that in the case of alloy series with multiple substitutions, the combined factor is more relevant in deciding the hydrogen storage capacity in comparison to electronic factor alone. Combined factor is directly proportional to the hydrogen storage capacity. All the three factors thermodynamic, structural and electronic together may lead to the prediction of pressure-composition isotherm of the multi-element AB₅-type hydrogen storage alloy.     

Effect of fluorination on the lanthanum-doped AB2-type metal hydride electrodes

The effect of fluorination on the surface of an AB₂-type hydrogen-storage alloy with the composition Zr_0.7Ti_0.3V_0.4Mn_0.4Ni_1.2 is investigated. Electron probe microanalysis (EPMA), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), and X-ray diffraction (XRD) techniques show that the fluorinated AB₂-type alloys have a unique surface morphology and high reactivity with a protective film. The treatment is found to be effective when lanthanum is incorporated in the alloy. The cycle-life of the metal hydride (MH) electrode increases significantly as a result of a decrease in the overpotential by electrode polarization. The degradation rate of the MH electrode is less than 3% after 100 cycles when lanthanum is added at 1–3 wt.%.     

Effect of rare earth composition on the high-rate capability and low-temperature capacity of AB5-type hydrogen storage alloys

Effects of rare earth composition on the high-rate capability and low-temperature capacity of AB₅-type hydrogen storage alloys have been studied and analyzed with pattern recognition methods. The results show that the increase of Ce and Pr and the decrease of La and Nd concentration improve the high-rate capability and low-temperature capacity of AB₅-type hydrogen storage alloys, Ce exhibiting better favorable influences than Pr. The improvement of both high-rate capability and low-temperature capacity are mainly ascribed to the lower stability of the hydride. The alloy with the rare earth composition of La_0.1645Ce_0.7277Pr_0.0234Nd_0.0845 shows very good high-rate capability and low-temperature capacity.     

Effects of particle size and type of conductive additive on the electrode performances of gas atomized AB5-type hydrogen storage alloy

The phase composition, morphology, structure, and state of the surface of gas atomized LaNi_4.5Al_0.5 alloy powders constituting a fine (≤50 μm), a medium (160–316 μm), and a coarse (630–1000 μm) fraction have been investigated. The electrochemical and storage characteristics of electrodes made from these powders with addition of electrolytic copper powder or a carbon composite (1 wt.% carbon nanotubes + 7 wt.% nanosized carbon black) as a conductive additive have been studied. In the work, X-ray diffraction, scanning electron microscopy, electron-probe microanalysis, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and several electrochemical methods have been used. It has been established that, in the initial state, the coarse-fraction gas atomized powders show a better kinetics of the hydrogen exchange reactions and higher discharge capacity (∼300 mA h/g). It is shown that electrodes made from the powders of all the fractions have a good high-rate discharge capability. Hydrogen diffusion coefficients during discharge of the electrodes made from the alloy powders of all the fractions and conductive additives have been calculated. It is shown that, for LaNi_4.5Al_0.5 alloy electrodes with the composite carbon additive, the hydrogen diffusion coefficients during discharge computed from data obtained by the electrochemical impedance spectroscopy method agree well with those calculated from cyclic current–voltage curves (2–4 × 10⁻⁹ cm²/s).     

Effect of Titanium additive element on the discharging behavior of MgNi Alloy electrode

Mg_0.90Ti_0.10Ni, Mg_0.85Ti_0.15Ni, Mg_0.80Ti_0.20Ni, Mg_0.90Ti_0.15Ni_0.95, Mg_0.90Ti_0.20Ni_0.90 and Mg_0.95Ti_0.15Ni_0.90 ternary alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. Among the Mg-Ti-Ni ternary alloys Mg_0.90Ti_0.20Ni_0.90 alloy showed the best discharge performance. The initial discharge capacity was observed to depend on Mg/Ni atomic ratio rather than Ti/Mg atomic ratio in alloys. As the Ti/Mg atomic ratio increased the alloy charge transfer resistances decreased probably due to the partial selective dissolution of the surface Ti/Ti-oxides and thus the limited enrichment of the surface by the electro-catalytic Ni. The average hydrogen diffusion coefficients in all the Ti-including alloys were higher than that in MgNi alloy. The increase in Ti/Mg atomic ratio, however, did not cause any further increase in the average hydrogen diffusion coefficient.     

Enhanced cycling stability and reduced hysteresis of AB5-type hydrogen storage alloys by partial substitution of Sn for Ni

Rare-earth AB₅-type alloys have been widely studied due to their great application potential in gaseous hydrogen storage, but their overall hydrogen storage properties still need to be further improved for more extensive applications. In this work, the effect of Sn partial substitution for Ni on the plateau characteristics and cycling performance of the LaNi_5-xSn_x (x = 0, 0.25, 0.5, 0.75, 1) alloys are systematically studied. It is found that the segregation effect caused by Sn addition leads to the multi-CaCu₅ phase structure with different cell parameters which may have a positive effect on stabilizing the alloys’ structure during cycling by playing a buffer role. Also, the replacement of Sn element results in a higher anisotropic c/a value, which reduces microstrain and improves the cycle life. The capacity retention after 1000 cycles increases from 83.2% (x = 0) to 95.8% (x = 0.75). Moreover, the addition of Sn significantly reduces the hysteresis of the alloys from 0.212 (x = 0) to 0.023 (x = 0.5) at 383 K, owing to the reduction of the microstrain during hydrogen absorption/desorption.     

The effect of temperature on the electrode properties of the directly prepared Ti3Ni2 alloy

The electrochemical properties of a new hydrogen storage alloy Ti₃Ni₂ have been studied in detail. The results show that the discharge capacity of Ti₃Ni₂ alloy reaches 384.4 mA h/g (corresponding to Ti₃Ni₂H_3.7) at 353 K and 145.5 mA h/g (corresponding to Ti₃Ni₂H_1.47) at 278 K. At the higher temperature (353 K), two plateaus appear on the discharge curve, and at the lower temperature (278 K) only one plateau. The kinetic property of Ti₃Ni₂ alloy electrode is poor. The following properties, namely the retention of charge, the activation of alloy, the kinetic property of Ti₃Ni₂ alloy (including the exchange current, the diffusion coefficient of hydrogen and high-rate dischargeability) can all be improved by increasing the temperature. It can be seen from X-ray diffraction analysis that two phases exist in alloy: namely the cubic Ti₂NiH phase with Fe₃W₃C structure and the cubic TiNiH phase for the alloy cycled in alkaline solution, and the cubic Ti₂NiH_0.5 phase with Fe₃W₃C structure and the cubic TiNiH phase for the alloy hydrogenated with gaseous hydrogen.     

Evaluation of the effects of oxygen evolution on the capacity and cycle life of nickel hydroxide electrode materials

The effect of charge–discharge cycling on the capacity of surface-adhered nickel hydroxide (Ni(OH)₂) micro-particles is investigated in aqueous KOH by cyclic voltammetry, and compared with that for pasted nickel hydroxide electrodes. Cyclic voltammetry on adhered Ni(OH)₂ micro-particles enables rapid screening of four types of commercially available, battery-grade, nickel hydroxide samples and allows the separation of the oxidation process from the oxygen evolution reaction. With large pasted electrodes, due to their high uncompensated resistance (R_u), these processes are poorly resolved. Pasted β-nickel hydroxide electrodes with a specific capacity of between 190 and 210 mAh g⁻¹ are charged and discharged at constant currents greater than 15 C (18 mA cm⁻²). With no voltage limit in the charging profile, excess oxygen evolution occurs and capacity fading is observed within the first 50 cycles. Loss of capacity is attributed to the degradation of the electrode due to excess oxygen evolution at switching potentials greater than 0.55 V versus Hg/HgO (1 M KOH). X-ray diffraction (XRD) measurements confirm the formation of γ-NiOOH in these electrodes. Limiting the cell voltage to 1.5 V, and thereby minimizing oxygen evolution, results in no observed capacity loss within 100 cycles, and only β-Ni(OH)₂ can be detected by XRD phase analysis.     

Microstructures and electrochemical performances of La2Mg(Ni0.85Co0.15)9Crx (x = 0–0.2) electrode alloys prepared by casting and rapid quenching

In order to improve the electrochemical cycle stability of La–Mg–Ni system (PuNi₃-type) hydrogen storage alloy, a trace of Cr was added and rapid quenching techniques were employed. The electrochemical performances and microstructures of the as-cast and -quenched alloys were determined and measured. The effects of Cr content and quenching rate on the microstructures and electrochemical properties of the alloys were investigated in detail. The obtained results show that the as-cast and -quenched alloys are composed of the (La, Mg)Ni₃ phase (PuNi₃-type structure) and the LaNi₅ phase as well as the LaNi₂ phase. The amount of the LaNi₂ phase increases with the increase of Cr content. The addition of Cr enhances the cycle stability of the as-cast and -quenched alloys, but decreases the discharge capacities of the alloys. The cycle lives of the alloys increase with the increase of the quenching rate. The as-cast and -quenched alloys have an excellent activation performance.     

Effect of driving cycle on the performance of PEM fuel cell and microstructure of membrane electrode assembly

Driving cycle effect on proton exchange membrane fuel cell (PEMFC) performance and microstructure of membrane electrode assembly (MEA) is simulated to predict the lifetime of PEMFC. The single cell performance after different simulated cycles (100, 150 and 200 cycles) was evaluated by I–V curves. Electro-chemical impedance spectroscopy (EIS) was used to characterize the electro-chemical properties of fuel cell and the active surface area was characterized by cyclic voltammetry (CV). It was found that after 100, 150 and 200 cycles the electro-chemical surface area (ESA) for electro-chemical reaction decreased about 13.6%, 15.4% and 38.9%, respectively, and the performance of PEMFC declined about 0.1 V at 500 mA cm⁻² after 200 driving cycles. Impedance analysis and equivalent circuit showed that ohmic and charge transfer resistance respectively increased from 2.06 to 2.13, 2.215, 2.435 Ω cm² and from 0.7 to 0.905, 1.26, 1.915 Ω cm² after three kinds of driving cycles, and they all increased with increasing driving cycles, which may be one reason for the decline of PEMFC performance. Besides, scanning electron microscopy (SEM) result revealed that the thickness of catalyst layer after cycle test was weakening much more than the fresh one.     

Effect of the Nafion content in the MPL on the catalytic activity of the Pt/C-Nafion electrode prepared by pulsed electrophoresis deposition

The aim of this work is to study the effect of Nafion content in the microporous layer (MPL) on the electrophoretically deposited Pt/C-Nafion electrode performance. First, the MPLs are prepared with different Nafion ionomer contents (5, 10, 20, 30 and 40 wt%) on the carbon paper substrates. Next, Pt/C-Nafion electrodes are prepared by pulsed electrophoresis deposition (pulsed EPD) from a Pt colloidal solution as a plating bath. The catalytic activities of the prepared Pt/C-Nafion electrodes are evaluated using the cyclic voltammetry (CV) technique for hydrogen oxidation reaction (HOR). Also, a PEMFC single cell test is carried out using the Pt/C-Nafion electrodes prepared with the pulsed EPD method as a cathode. The mass-specific power density for the Pt/C-30wt% Nafion electrode is $1578.8\text{mW}{\text{mg}}_{\text{Pt}}^{?1}$, which is higher than the rest of the Pt/C-Nafion electrodes. The Nafion content in the cathode MPL should affect the performance of the PEMFC, especially at a high current density.     

Effect of electrode structure on performance of Si anode in Li-ion batteries: Si particle size and conductive additive

The effects of Si particle size and the amount of carbon-based conductive additive (CA) on the performance of a Si anode in a Li-ion battery are investigated by adopting combinations of two different Si particle sizes (20 and 3 μm on average) and CA contents (15 and 30 wt.%), respectively. The CA contains graphitic flakes and nano-sized carbon black. Cyclic voltammetry, charge–discharge tests, scanning electron microscopy and X-ray diffraction establish that the CA content has a profound effect on the cycle-life and irreversible capacity of the Si anode. The former increases, while the latter decreases significantly with increasing CA content. Reducing the particle size of Si, on the other hand, facilitates the alloying/de-alloying kinetics. For instance a cycle-life of over 50 cycles with >96% capacity retention at a charge capacity of 600 mAh per g-Si has been demonstrated by adopting of 30 wt.% CA and 3 μm Si particles.     

Effect of Al, B, Ti and Zr additive elements on the electrochemical hydrogen storage performance of MgNi alloy

MgNi, Mg_0.9(M)_0.1Ni and Mg_0.8(M)_0.2Ni (M = Al, B, Ti, Zr) type alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that although 15 h milling was enough to obtain amorphous/nano-crystalline MgNi alloy structure, the dissolution of all Ni in the main phase required at least 25 h milling. The discharge capacities of alloys were observed to increase sharply up to 15 h milling. Further increase up to 25 h did not cause change in the discharge capacities considerably. Titanium improved MgNi alloy discharge performance significantly. Although Al deteriorated the initial discharge capacity of MgNi alloy, it improved the alloy capacity retention rate. Despite the absence of any positive effect of B, Zr had limited positive effect on the alloy cyclic stability. It was estimated that up to 80% discharge level there was no considerable degradation by the anodic reaction in MgNi alloy since uncharged MgNi alloy stayed in the cathodic regime.