Sodium tungstate as electrolyte additive to improve high-temperature performance of nickel–metal hydride batteries

Sodium tungstate (Na₂WO₄) used as new electrolyte additive to enhance the high-temperature performance of Nickel–metal hydride (Ni–MH) battery is investigated in this paper. The effects of Na₂WO₄ on nickel hydroxide electrodes are investigated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and a charge/discharge test. It is found that the Ni–MH cell with the conventional KOH electrolyte containing 1 wt.% Na₂WO₄ additive exhibits higher discharge retention and better cycling performance than the cell without Na₂WO₄ additive at both 25 °C and 70 °C. These performance improvements are ascribed to the enhancement of oxygen evolution overvoltage and lower electrochemical impedance, as indicated by CV and EIS. The results suggest that the proposed approach be an effective way to improve the high temperature performance of Ni–MH batteries.     

A non-isothermal model of a nickel–metal hydride cell

A model for a nickel–metal hydride cell was constructed based on the planar electrode approximation. The mass balances of active species, the kinetics of electrochemical reactions, the ohmic effects of internal resistance, and the energy balance of the whole cell were considered in the model. An empirical approach was utilized to account for the hysteresis potential behavior of the nickel electrode. The model predictions showed favorable agreement with the experimental data.     

Enhancement of the high-temperature performance of advanced nickel–metal hydride batteries with NaOH electrolyte containing NaBO2

In this paper, an alternative approach to improve the high-temperature performance of nickel–metal hydride (Ni–MH) batteries is proposed by introducing NaOH electrolyte with sodium metaborate (NaBO₂) additives. Compared with conventional batteries using KOH electrolyte, the in-house prepared batteries with proposed electrolytes exhibit an enhanced discharge capacity, improved high-rate discharge ability, increased cycle stability and reduced self-discharge rate at an elevated temperature (70 °C). The charge acceptance of these Ni–MH batteries at 70 °C is over 96% at a charge/discharge rate of 1 C. These performance improvements are ascribed to the increased oxygen evolution overpotential, slower oxygen evolution rate and lower electrochemical impedance, as indicated by CV, steady-state polarization measurements and EIS. The results suggest that the proposed approach be an effective way to improve the high-temperature performance of Ni–MH batteries.     

High performance nickel–metal hydride battery in bipolar stack design

The consumption of fuel in cars can be reduced by using hybrid concepts. Even for fuel cell vehicles, a high power battery may cut costs and allow the recovery of energy during retarding. Alkaline batteries, such as nickel–metal hydride batteries, have displayed long cycle life combined with high power ability. In order to improve the power/energy ratio of Ni/MH to even higher values, the cells may be arranged in a bipolar stack design.     

The effect of the discharge rate on the electrochemical properties of AB3-type hydrogen storage alloy as anode in nickel–metal hydride batteries

In this work, the ternary CeY₂Ni₉ alloy was prepared and served as anode materials in the Ni–MH battery system. The effect of the discharge rate on the electrochemical proprieties of CeY₂Ni₉, such as activation capability, polarization, discharge capacities, hydrogen atomic diffusion capability and redox parameters, were also investigated systematically during activation and long cycling. Charge–discharge measurement showed that this alloy is characterized by fast activation and requires only three cycles to be activated regardless of the discharge rate, the maximum discharge capacity was obtained for the medium discharge rate (C/10). An important correlation was observed between the evolution of the electrochemical parameters and that of the kinetic parameters, such as the hydrogen atomic diffusion capability and the exchange current density.The total substitution of La by Ce in LaY₂Ni₉ parent alloy enhanced its activation, polarization and stability despite the decrease of its discharge capacities, especially at high rates.     

Influence of Pd addition on the electrochemical performance of Mg–Ni–Ti–Al-based metal hydride for Ni–MH batteries

Amorphous Mg_0.9Ti_0.1NiAl_0.05 and Mg_0.9Ti_0.1NiAl_0.05Pd_0.1 alloys were prepared by high energy ball milling and evaluated as metal hydride electrodes for Ni–MH batteries. The Mg_0.9Ti_0.1NiAl_0.05Pd_0.1 alloy showed a much higher cycle life with a capacity retention of 72% after 100 cycles (C_100th = 288 mAh g⁻¹) compared to 26% for the Pd-free alloy (C_100th = 117 mAh g⁻¹). This was mainly attributed to the improvement of the alloy oxidation resistance in KOH electrolyte with Pd addition, as confirmed by cyclic voltammetry experiments and X-ray diffraction analyses on cycled electrodes. In addition, in situ acoustic emission (AE) measurements revealed that the energy of the AE signals related to the particle cracking is lower for the Mg_0.9Ti_0.1NiAl_0.05Pd_0.1 electrode, suggesting that the cracks are smaller in size than with the Pd-free alloy. The Mg_0.9Ti_0.1NiAl_0.05Pd_0.1 electrode also displayed a higher discharge rate capability than the Mg_0.9Ti_0.1NiAl_0.05 electrode. On the basis of their respective electrochemical pressure–composition isotherm, it was shown that the presence of Pd in the alloy decreases the thermodynamic stability of the metal hydride. Through a comparative analysis of discharge polarization curves, it was also shown that Pd addition decreases substantially the H-diffusion resistance in the alloy whereas its positive effect on the charge-transfer resistance is limited.     

Modelling of metal hydride hydrogen compressors from thermodynamics of hydrogen – Metal interactions viewpoint: Part II. Assessment of the performance of metal hydride compressors

Performance of the thermally-driven metal hydride hydrogen compressor (MHHC) is defined by (a) its H₂ compression ratio and maximum output H₂ pressure; (b) throughput productivity/average output flow rate; (c) specific thermal energy consumption which determines H₂ compression efficiency. In earlier studies, the focus of the R&D efforts was on the optimisation of the design of the MH containers and heat and mass transfer in the MH storage and compression system aimed at shortening the time of the H₂ compression cycle. This work considers an important but insufficiently studied aspect of the development of the industrial-scale thermally driven MHHC's – selection of the materials and optimisation of the materials performance. Further to the operation in the specified pressure/temperature ranges, materials selection should be based on the estimation of the productivity of the compression cycle, and specific heat consumption required for the H₂ compression, which together determine the process efficiency.The current work presents a model to determine productivity and heat consumption for a single- and multi-stage MHHC's which is based on use of Pressure – Composition – Temperature (PCT) diagrams of the utilized metal hydrides at defined operating conditions – temperatures and hydrogen pressures – and main operational features of the MHHC (number of stages, amounts of the MH materials used, cycle time). In Part I of this work [Lototskyy, Yartys, et al., Int J Hydrogen Energy, DOI: 10.1016/j.ijhydene.2020.10.090], we showed that the calculated cycle productivities significantly vary for the different materials. Analysis of the system performance carried out in this work (Part II) shows that the throughput productivity and efficiency of a multi-stage MHHC also depends on the types and amounts of the used MH materials in the multi-stage compressor layout. This has been analysed for a number of the most practically important AB₅ and Laves type AB₂ hydrogen storage alloys integrated into the MHHC's.A comparison of experimentally measured performances of single-, two- and three-stage industrial-scale MHHC's developed by the authors earlier shows their satisfactory agreement with the modelling results thus demonstrating a high value of the presented method for the proper materials selection during development of the MHHC. As an important future development, the work presents a performance evaluation of a two-stage MHHC for H₂ compression operating in the pressure range from 30 to 500 atm at operating temperatures between 20 and 150 °C.     

Modelling of metal hydride hydrogen compressors from thermodynamics of hydrogen – Metal interactions viewpoint: Part I. Assessment of the performance of metal hydride materials

This work presents a model to determine productivity and heat consumption of hydrogen compression utilising metal hydrides (MH) by using Pressure – Composition – Temperature (PCT) diagrams of the MH materials at defined operating conditions – temperatures and hydrogen pressures. The present Part I is focused on the analysis of hydrogen compression performances of several AB₅- and AB₂-type intermetallic alloys which, when operating between temperatures of 20 and 150 °C, provide H₂ compression up to 500 atm, with a cycle productivity about 100 NL H₂/kg MH and compression ratio of up to 10, at H₂ suction pressure below 10–15 atm, or up to 5 at higher suction pressures.We show that calculated cycle productivities of hydrogen compression are related to the operating conditions and significantly vary for the different MH materials, even though showing similar trends in their changes. The cycle productivity of MH material increases with decrease of the cooling temperature, decrease of the discharge pressure, increase of the heating temperature and increase of the suction pressure. When hydrogen pressure approaches plateau pressures for H₂ absorption at cooling or H₂ desorption at heating, the changes of the cycle productivity become very pronounced. Particularly, the compression productivity becomes very sensitive to the P-T variations when the isotherms show presence of “flat” pressure plateaux which are characteristic for the ideal PCT diagrams of the MH. Thus, in the latter case, even minor changes in P-T result in a dramatic variation of the cycle productivity and when aiming at increased efficiency of the process, a strict P-T control is required.     

Uniform carbon coating drastically enhances the electrochemical performance of a Fe3O4 electrode for alkaline nickel–iron rechargeable batteries

Fe₃O₄@C composites for use in alkaline nickel-iron rechargeable batteries are synthesized via a spray drying method. SEM and TEM confirm that the Fe₃O₄@C composite is a secondary particle microsphere formed by many primary particles uniformly coated with carbon. The particles aggregate with each other, which is equivalent to forming a three-dimensional conductive mesh. The electrochemical properties of the bare Fe₃O₄ and Fe₃O₄@C composites as anode materials for alkaline nickel-iron batteries are investigated. The results show that the Fe₃O₄@C composite synthesized by spray drying exhibits considerably high discharge capacities and an excellent rate capability. The specific discharge capacity of the Fe₃O₄@C composite reaches 693.7 mAh g⁻¹ at a current density of 300 mA g⁻¹, with a charging efficiency of 92.5%. Moreover, the Fe₃O₄@C composite exhibits an admirable cycling stability with a superior capacity retention of 92.0% for 100 cycles at a current density of 300 mA g⁻¹. In addition, discharge capacities of 556.7 and 420.1 mAh g⁻¹ are achieved at high current densities of 1200 and 2400 mA g⁻¹, respectively.     

A novel parameter for evaluation on power performance of Ni–MH rechargeable batteries

In the work, two novel conceptions of “capacity quality” (CQ) and “capacity quality coefficient” (λ) were defined to evaluate cycling power capabilities of Ni–MH rechargeable batteries when considering the effect of the kinetic limitation. For convenient comparison, the capacity quality coefficient (λ) and the efficiency of charge/discharge (η) were in parallel applied to characterize cycling capabilities based on the data from BYD H-3/4AAA800 Ni–MH batteries at 1C–3.5C. The results show that there is an obvious difference between λ and η which served as evaluation indexes for rechargeable batteries, and that the secondary battery with good capacity quality also has a good cycling capability and rate capability, especially at high rate. The introduced capacity quality not only subtly covered kinetic information of the rechargeable batteries but also factually reflected stability of the electrode materials.     

Electrochemical properties of Ti–Ni–H powders prepared by milling titanium hydride and nickel

Mechanical alloying has been carried out to synthesize a hydrogen storage alloy by milling titanium hydride and nickel. The structure and electrochemical properties such as discharge capacity, charge-transfer, and hydrogen diffusion of the milled powders were investigated. The results of X-ray diffraction showed that an amorphous phase was formed after ball milling. The electrode potentials of the milled powders were −0.989, −0.878 and −0.941 V (vs. Hg/HgO) in the electrolyte of 6 M KOH when the milling periods were 20, 40, and 60 h, respectively. The Ti–Ni–H powders milled for 60 h had a maximum discharge capacity of 102.2 mAh/g at a discharge current density of 60 mA/g. The results of the linear polarization showed that the exchange current density decreased as the hydrogen concentration within the powders decreased. The electrochemical impedance spectroscopy (EIS) demonstrated the same consequence and presented that the hydrogen diffusion decreased by decreasing the hydrogen concentration.     

Graphite–FeSi alloy composites as anode materials for rechargeable lithium batteries

Iron–silicon are prepared by annealing elemental mixtures at 1000 °C followed by mechanical milling. Graphite–Fe₂₀Si₈₀ alloy composites have been prepared by ball-milling a mixture of Fe₂₀Si₈₀ alloy and graphite powder. The microstructure and electrochemical performance of the composites are characterized by X-ray diffraction and an electrochemical method. The FeSi₂ matrix is stable for extended cycles and acts as a buffer for the active centre, Si. The Fe₂₀Si₈₀ alloy electrode delivers large initial capacity, but the capacity degrades rapidly with cycling. Fe₂₀Si₈₀ alloy–graphite composite electrodes, however, show good cycleability and a high reversible capacity of about 600 mAh g⁻¹. These composites appear to be promising candidates for negative electrodes in lithium rechargeable batteries.     

Nanocrystalline cobalt–nickel–boron (metal boride) catalysts for efficient hydrogen production from the hydrolysis of sodium borohydride

Innovative metal boride nanocatalysts containing crystalline Co–Ni based binary/ternary boride phases were synthesized and used in the hydrolysis of NaBH₄. All the as-prepared catalysts were in high-purity with average particle sizes ranging between ~51 and 94 nm and consisting of different crystalline phases (e.g. CoB, Co₂B, Co₅B₁₆, NiB, Ni₄B₃, Ni₂Co_0·67B_0.33). The synergetic effect of the different binary/ternary boride phases in the composite catalysts had a positive role on the catalytic performances thus, while the binary boride containing phases of unstable cobalt borides or single Ni₄B₃ were not showing any catalytic activity. The Co–Ni–B based catalyst containing crystalline phases of CoB–Ni₄B₃ exhibited the highest H₂ production rate (500.0 mL H₂ min^?1 g_cat^?1), with an apparent activation energy of 32.7 kJ/mol. The recyclability evaluations showed that the catalyst provides stability even after the 5th cycle. The results suggested that the composite structures demonstrate favorable catalytic properties compared to those of their single components and they can be used as alternative and stable catalysts for efficient hydrogen production from sodium borohydride.     

Effects of the addition of ZnO and Y2O3 on the electrochemical characteristics of a Ni(OH)2 electrode in nickel–metal hydride secondary batteries

This study examined the effects of the addition of ZnO and Y₂O₃ on the electrochemical characteristics of a Ni(OH)₂ electrode in nickel–metal hydride (Ni–MH) secondary batteries. The discharge capacity of the electrode was less affected by the addition of ZnO and Y₂O₃ at a 0.2 C-rate and 25 °C. However, the addition of Y₂O₃ deteriorated the discharge capacity and the cycle life of the electrode by increasing the charge transfer resistance of the electrode at an increased C-rate of 1 C and 25 °C. Under severer conditions at 1 C-rate and 60 °C, the electrode materials were separated from the current collector and, accordingly, the discharge capacity was abruptly degraded with cycling for the electrodes comprising only 4 wt% ZnO or 4 wt% Y₂O₃. In contrast, the electrodes containing both 2 wt% ZnO and 2 wt% Y₂O₃ exhibited stable discharge capacity with cycling and excellent cycle life due to the high overvoltage for oxygen evolution. The present results indicate that the addition of ZnO and Y₂O₃ with an optimum composition suppresses oxygen evolution in the interfaces between active materials and the current collector and improves the cycle life of the electrode.     

On the growth of Li2CO3 dendrites in nickel–cadmium industrial batteries

A large amount of Li₂CO₃ dendrites has been detected on positive electrodes in Ni–Cd industrial pocket plate batteries, intended to work in stationary applications, after 3 years in float charge. The lattice parameters were refined to a=8.353(1) Å, b=4.974(1) Å, c=6.194(1) Å and β=114.6(1)° [monoclinic], which is in complete agreement with structural data reported in the literature. Oxidation of graphite present in the positive active material is enhanced at elevated temperatures, and at high anodic potentials. This results in an extremely high carbonate concentration in the active material, as well as in the electrolyte. The high carbonate content, in combination with the relatively high lithium concentration present in both electrolyte and positive electrode, is very likely to be the reason for the formation of the Li₂CO₃ dendrites. As this process continues, agglomerates of the dendrites in combination with attached β-Cd(OH)₂ and graphite may generate short circuits between the positive and the negative electrodes.     

A paste type negative electrode using a MmNi5 based hydrogen storage alloy for a nickel–metal hydride (Ni–MH) battery

Different conducting materials (nickel, copper, cobalt, graphite) were mixed with a MmNi₅ type hydrogen storage alloy, and negative electrodes for a nickel–metal hydride(Ni–MH) rechargeable battery were prepared and examined with respect to the discharge capacity of the electrodes. The change in the discharge capacity of the electrodes with different conducting materials was measured as a function of the number of electrochemical charge and discharge cycles. From the measurements, the electrodes with cobalt and graphite were found to yield much higher discharge capacities than those with nickel or cobalt. From a comparative discharge measurements for an electrode composed of only cobalt powder without the alloy and an electrode with a mixture of cobalt and the alloy, an appreciable contribution of the cobalt surface to the enhancement of charge and discharge capacities was found.     

Thermal–fluid behavior of the hydriding and dehydriding processes in a metal hydride hydrogen storage canister

A study of the hydrogen absorption and desorption processes using LaNi₅ metal hydride is presented for investigation on the influences of expansion volume and heat convection. The hydrogen storage canister comprises a cylindrical metal bed and a void of expansion volume atop the metal. The expansion volume is considered as a domain of pure hydrogen gas. The gas motion in the metal hydride bed is treated as porous medium flow. Concepts of mass and energy conservation are incorporated in the model to depict the thermally coupled hydrogen absorption and desorption reactions. Simulation results show the expansion volume reduces the reaction rates by increasing thermal resistance to the heat transfer from the outside cooling/heating bath. The assumption usually adopted in simulating heat transfer in a metal hydride tank that heat convection in the reaction bed may be ignored is not valid when expansion volume is used because heat convection dominates the heat transfer through the expansion volume as well as the metal bed. The details of the thermal flow pattern are demonstrated. It is found that, due to the action of thermal buoyancy, circulations are likely to happen in the expansion volume. The hydrogen gas accordingly, instead of going directly between the inlet/outlet and the metal bed, tends to move with the circulation along the boundary of the expansion volume.     

Preparation and characterization of a new nanosized silicon–nickel–graphite composite as anode material for lithium ion batteries

A new type of nanosized silicon–nickel–graphite (Si–Ni–G) composite was prepared by high energy mechanical milling (HEMM) and pyrolysis using SiO as the precursor of Si for the first time. X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM) and scanning electron microscopy (SEM) were used to determine the phases obtained and to observe the microstructure and distribution of the composite. The composite powders consisted of Si, Ni, SiO₂, NiO and a series of Si–Ni alloys. The formation of the inactive SiO₂ and Si–Ni alloy phases could accommodate the large volume changes of the active particles during cycling. In addition, cyclic voltammetry (CV) and galvanostatic discharge/charge tests were carried out to characterize the electrochemical properties of the composite. The composite electrodes exhibited an initial discharge and charge capacity of 1450.3 and 956.4 mAh g⁻¹, respectively, maintaining a reversible capacity of above 900 mAh g⁻¹ for nearly 60 cycles.     

Fabrication of hollow metal oxide–nickel composite spheres and their catalytic activity for hydrolytic dehydrogenation of ammonia borane

In this paper, we report an effective approach to the fabrication of hollow titania–nickel composite spheres, hollow zirconia–nickel composite spheres, and hollow silica–nickel composite spheres. In this approach, metal oxide–nickel composite shells were coated on polystyrene particles by the sol–gel method and the polystyrene templates were dissolved subsequently, or even synchronously, in the same medium to form hollow spheres. Neither additional dissolution nor a calcination process was needed to remove the polystyrene templates. The as-prepared hollow metal oxide–nickel composite spheres were characterized by transmission electron microscopy. The catalytic activities of hollow titania–nickel composite spheres, hollow zirconia–nickel composite spheres, and hollow silica–nickel composite spheres for hydrolytic dehydrogenation of aqueous NaBH₄/NH₃BH₃ solution were compared. The evolutions of 64, 58, and 18 mL hydrogen were finished in about 49, 69, and 162 min in the presence of the hollow titania–nickel composite spheres, hollow zirconia–nickel composite spheres, and hollow silica–nickel composite spheres from aqueous NaBH₄/NH₃BH₃ solution, respectively. The molar ratios of the hydrolytically generated hydrogen to the initial NH₃BH₃ both in the presence of hollow titania–nickel composite spheres, hollow zirconia–nickel composite spheres, and hollow silica–nickel composite spheres are 2.8, 2.4, and 0.1 (the theoretical value of 3.0), respectively, indicating that the hollow titania–nickel composite spheres and hollow zirconia–nickel composite spheres show much higher hydrogen evolution rates and the amount of hydrogen evolution via hydrolytic dehydrogenation of ammonia borane than the hollow silica–nickel composite spheres. From the results of ATR-IR spectra, a certain amount of residual PS templates exists in hollow silica–nickel composite spheres, and the amount of the residual PS templates were able to be reduced by increasing the amount of aqueous ammonia solution used for the preparation. The catalytic activity of hollow silica–nickel composite spheres increases when the amount of residual PS templates decreases.