Recent advances in NiMH battery technology

Nickel-metal hydride (NiMH) is a commercially important rechargeable battery technology for both consumer and industrial applications due to design flexibility, excellent energy and power, environmental acceptability and cost. [1] From the initial product introduction in 1991 of cylindrical cells having an energy of 54 Wh kg⁻¹, today's small consumer cells have a specific energy over 100 Wh kg⁻¹. Numerous licensed manufacturers produce a myriad of NiMH products ranging from 30 mAh button cells to a wide variety of consumer cylindrical products, prismatic cells up to 250 Ah for electric buses and 6 Ah multicell modules for hybrid electric vehicles. Power has increased from under 200 to 1200 W kg⁻¹ commercially and up to 2000 W kg⁻¹ at a development level [2].     

Spent NiMH batteries—The role of selective precipitation in the recovery of valuable metals

The production of electronic equipment, such as computers and cell phones, and, consequently, batteries, has increased dramatically. One of the types of batteries whose production and consumption has increased in recent times is the nickel metal hydride (NiMH) battery. This study evaluated a hydrometallurgical method of recovery of rare earths and a simple method to obtain a solution rich in Ni-Co from spent NiMH batteries. The active materials from both electrodes were manually removed from the accumulators and leached. Several acid and basic solutions for the recovery of rare earths were evaluated. Results showed that more than 98 wt.% of the rare earths were recovered as sulfate salts by dissolution with sulfuric acid, followed by selective precipitation at pH 1.2 using sodium hydroxide. The complete process, precipitation at pH 1.2 followed by precipitation at pH 7, removed about 100 wt.% of iron and 70 wt.% of zinc from the leaching solution. Results were similar to those found in studies that used solvent extraction. This method is easy, economic, and does not pose environmental threats of solvent extraction.     

Studies on rechargeable NiMH batteries

Nickel-metal hydride (NiMH) batteries offer some advantages in the aspects of power, cycle life and environment. However, they are encountering challenge from other rechargeable batteries such as Li-ion batteries. In this paper, the possibility of performance improvements and cost reduction of NiMH batteries were analyzed. Some approaches to improve energy densities and reduce the battery cost were discussed. An attempt to increase the energy density of NiMH batteries was made through improvements in the specific capacity of rare earth-based AB₅${}_5$-type hydride alloys in the negative electrodes and capacity enhancement of nickel hydroxide electrode. A relatively lower cost small cylindrical NiMH cell was constructed and its properties were evaluated.     

Progress in high-power nickel–metal hydride batteries

High demands to power performance, high cycle and calendar life as well can be met by NiMH batteries, making this battery system very suitable for HEV applications. The hydrogen storage alloy plays an important role with respect to power performance and life duration. Power performance and cycle life behaviour are related to each other by the electrochemical and mechanical properties of the alloy, via a more or less reciprocal relationship. In terms of power performance at medium-discharge rates, the charge transfer reaction at the hydrogen storage alloy interface was found to be crucial for the temperature-dependent behaviour of the cell, whereas at discharge rates above about 15C diffusion limitation was found especially at the negative electrode. The alloy corrosion is taking place in alkaline media, leading to the formation of surface films and a change of the chemical composition, especially in near surface regions of the alloy particles. Consecutive electrochemical cycles lead to mechanical stress and finally cracking of the alloy particles. Stability against corrosion and pulverisation on one hand and good electrochemical performance on the other hand both depend on the chemical composition of the alloy, its morphological properties and the cycling regime used.     

Thermal behavior of VRLA battery during closed oxygen cycle operation

This paper presents the results of the investigations of the thermal behavior of a model valve-regulated lead-acid cell at overcharge and saturation level around 90%. The heat capacity and the heat transfer coefficient are experimentally obtained. The quasi-steady state characteristics of the cell at constant charging current are measured. The experimental data show that, when operating in recombination regime, the stationary cell temperature is a linear function of the charging current. When the cell is gassing, part of the heat is dissipated with the released gases, as a result of which the cell temperature is lower than predicted from the linear temperature/current dependence. Introduced dimensionless slope of the temperature/current dependence is a characteristic of the VRLA battery, which could be used for fast and easy estimation of the temperature increase of the battery. The kinetic parameters of the oxygen evolution reaction (OER) proceeding at the positive plates of VRLA cells are experimentally determined. The current/overpotential dependence of the OER obeys the Tafel relation. The temperature/current dependence and the kinetics of the OER are used to estimate the critical charging voltage for safe operation of the battery.     

Safe and fast-charging Li-ion battery with long shelf life for power applications

We report a Li-ion battery that can be charged within few minutes, passes the safety tests, and has a very long shelf life. The active materials are nanoparticles of LiFePO₄ (LFP) and Li₄Ti₅O₁₂ (LTO) for the positive and negative electrodes, respectively. The LiFePO₄ particles are covered with 2 wt.% carbon to optimize the electrical conductivity, but not the Li₄Ti₅O₁₂ particles. The electrolyte is the usual carbonate solvent. The binder is a water-soluble elastomer. The “18650” battery prepared under such conditions delivers a capacity of 800 mAh. It retains full capacity after 20,000 cycles performed at charge rate 10C (6 min), discharge rate 5C (12 min), and retains 95% capacity after 30,000 cycles at charge rate 15C (4 mn) and discharge rate 5C both at 100% DOD and 100% SOC.     

A numerical analysis of cell density effect on oxygen transport in a micro-bioreactor with a tissue engineering scaffold

The effect of cell density on the fluid dynamics and oxygen transport in a micro-bioreactor with a tissue engineering scaffold was investigated. Variations of the permeability and porosity of the scaffold with cell density were estimated by a simple model. Based on the permeability and the porosity obtained, the flow and oxygen transport in a micro-bioreactor were simulated numerically. It was found that the flow rate passing through the scaffold is reduced with an increase in cell density. The oxygen concentration within the scaffold also decreases with an increase in cell density, due to lower oxygen convection caused by lower porous flow rate and higher oxygen consumption. The results suggest that, for high cell density culture, the oxygen concentration within the scaffold may be overestimated if the cell density effect is neglected.     

Combustion with mixed enrichment of oxygen and hydrogen in lean regime

A low global richness of combustion is interesting from an ecological and economic point of view as it helps to limit fuel consumption. In fact, the consequences of the combustion in poor mode are the appearance of local or global flame extinctions, energy loss by radiation and change in flame structure. The flammability limits of the diffusion methane flame can be resolved by the enrichment of the combustion air with oxygen or the use of the pure oxygen as oxidant as well as the addition to hydrogen in natural gas. Moreover, the use of oxygen and hydrogen as previously mentioned allow working in low ranges of richness while maintaining good flame stability. For a constant burner power of 15 kW, the reduction of the richness involves an increase in the oxidizer flow rate injected into the combustion reaction. In this present study, the variation of the richness, the fuel enrichment with hydrogen and the oxidant enrichment with oxygen are shown as major parameters where they have direct influences on the flow dynamic, the flame structure and the pollutant emissions.The Chemiluminescence of OH* radical and the PIV (Particle image velocimetry) are used in this work to characterize the flame structure, the stability and the dynamics of the flame. The measurement of pollutant emissions effected by a gas analyzer. The enrichment in oxygen and hydrogen provides a stable flame, which is well attached to the burner for the following richness values: 0.7, 0.8, 0.9 and 1. The reduction of the richness promotes the mixture quality of the reactants and leads a reduction in CO₂ and CO concentration. By contrast, the decrease of the richness supports the formation of NOx.     

A novel silver oxides oxygen evolving catalyst for water splitting

A novel silver oxides oxygen evolving catalyst (Ag-OEC) for hydrogen production by water splitting was formed in situ on an indium tin oxide anode, in a near-neutral potassium tetraborate (K₂B₄O₇) electrolyte. The catalyst exhibited high activity and low overpotential for O₂ evolution under mild conditions. The main functional composition of the catalyst was a redox couple of Ag₂O/AgO. Catalytic activity during oxygen evolution was evaluated by cyclic voltammetry and Tafel plot. The effects of the concentration, temperature, and pH of K₂B₄O₇ solution on the catalyst, and the Faradaic efficiency of the oxygen evolving reaction were examined. The results show that the Ag-OEC exhibits excellent oxygen evolution properties, with an oxygen evolving overpotential of 318 mV at a current density of 1 mA/cm².     

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.     

Research of coal-direct chemical looping hydrogen generation with iron-based oxygen carrier modified by potassium

Coal-direct chemical looping hydrogen generation (CLHG) is a promising process for hydrogen production with high coal conversion efficiency and low carbon footprint. In this work, experiments on coal-direct CLHG process were carried out using K₂CO₃ modified Fe₂O₃/ZrO₂ as oxygen carrier (OC) and Shenmu (SM) char as fuel in a fixed-bed reactor. The effect of char/OC mass ratio on CO₂/CO volume ratio, H₂ production and phase transformation was investigated. Multicycle tests with SM char and deashed SM char were conducted to investigate the activity stability of OC and the reason for the deactivation of OC. The results confirm the feasibility of coal-direct CLHG process. Higher char/OC mass ratio could enhance the H₂ production and decrease the CO₂/CO volume ratio and oxidation state of iron oxides in OC. In the multicycle tests with SM char, carbon conversion and H₂ production remained almost constant during the first 2 redox cycles and then decreased abruptly in the 3rd cycle. During the 3 cycles, the phases of OC residues remained unchanged and no detectable surface sintering was observed. Furthermore, the K contents of residues decreased slightly. In the multicycle tests with deashed SM char, the carbon conversion and accumulation H₂ production were stable during the first 10 cycles and then decreased slowly. Some morphology features changes appeared during the 11 cycles, but no obvious surface sintering was observed. The K contents of residues declined by 2/3 after the 10th cycle.     

Novel lithium titanate hydrate nanotubes with outstanding rate capabilities and long cycle life

Novel lithium titanate hydrate nanotubes for lithium ion batteries have been easily prepared via a hydrothermal method. This material demonstrates high energy density, outstanding rate capabilities and a very long cycle life comparable to those of supercapacitors. At a rate equivalent to a 10-min total charge/discharge, the as-prepared lithium titanate hydrate nanotubes exhibit a life of over 5000 charge/discharge cycles while still retaining up to 86.3% of its original capacity. The abilities of lithium titanate hydrate nanotubes to fully charge within minutes for thousands of times and still retain a large capacity may find promising applications in hybrid and plug-in hybrid electric vehicles.     

Chemical looping combustion of Victorian brown coal using NiO oxygen carrier

This study is part of a program assessing the suitability of chemical looping for direct combustion of Victorian brown coal. The performance of NiO as an oxygen carrier in presence of a dried Victorian brown coal was assessed during five alternating cycles of reduction and oxidation in a CO₂ environment using a TGA. The experiments indicate a 4.4-7.5% weight loss of the oxygen carrier per cycle. Preliminary SEM-EDX and FACTSAGE predictions also indicate weight loss, but not to the same extent. The percentage of combustion of coal achieved at the 5th cycle was approximately 67%. Cycle 2 showed maximum reactivity (during reduction) with a decreasing trend during the subsequent cycles. These initial experiments did not reveal much agglomeration between ash and NiO although longer duration experiments are required to explore this issue further.     

Gas evolution in a flow-assisted zinc-nickel oxide battery

An experimental study on hydrogen and oxygen evolution was carried out in a sealed zinc-nickel oxide battery. The effects of flowing electrolyte on gas evolution were quantitatively evaluated. The results show that both the hydrogen and oxygen evolution are suppressed by making the electrolyte flow on a regular cycle. This is due to increased cell polarization in the non-flowing case attributed to the concentration boundary layer of zinc (zincate) ion near the anode surfaces. Though the Coulombic efficiency in the flowing case was higher than that in the non-flowing case, the fraction of gas evolution against Coulombic loss was the same for both the cases. When deeply discharging the cell, more hydrogen is evolved in the flowing case than in the non-flowing case.     

Co-Magneli phases electrocatalysts for hydrogen/oxygen evolution

The subject of this work is the use of non-stoichiometric titanium oxides – Magneli phases as support material of Co-based electrocatalysts aimed for hydrogen/oxygen evolution reaction. Commercial micro-scaled Ebonex (Altraverda, UK) was mechanically treated for 4, 8, 12, 16 and 20 h and further Co metallic phase was grafted by sol-gel method. Morphology of Co/Ebonex electrocatalysts was observed by means of TEM and SEM microscopy, while electrochemical behavior by means of cyclic voltammetry and steady-state galvanostatic method.     

An assessment on the quantification of hydrogen releases through oxygen displacement using oxygen sensors

Gas sensors that respond directly to hydrogen are typically used to detect and quantify unintended hydrogen releases. However, alternative means to quantify or mitigate hydrogen releases are sometimes proposed. One recently explored approach has been to use oxygen sensors. This method is based on the assumption that a hydrogen release will displace oxygen, which can be quantified using oxygen sensors. The use of oxygen sensors to monitor ambient hydrogen concentration has drawbacks, which are explored in the current study. It was shown that this approach may not have adequate accuracy for safety applications and may give misleading results under certain conditions for other applications. Despite its shortcomings, the Global Technical Regulation (GTR) for Hydrogen and Fuel Cell Vehicles has explicitly endorsed this method to verify hydrogen vehicles' fuel system integrity. Experimental evaluations designed to impartially assess the ability of oxygen and hydrogen sensors to reliably measure hydrogen concentration changes are presented. Specific limitations on the use of oxygen sensors for hydrogen measurements are identified and alternative sensor technologies that meet the requirements for several applications, including those of the GTR, are proposed.     

Metal-free catalysts with phosphorus and oxygen doped on carbon-based on Chlorella Vulgaris microalgae for hydrogen generation via sodium borohydride methanolysis reaction

Metal-free catalysts (C–KOH–P) containing phosphorus (P) and oxygen (O) prepared by the modification with phosphoric acid (H₃PO₄) of activated carbon (C–KOH) obtained by activation of Chlorella Vulgaris microalgae with potassium hydroxide (KOH) were investigated for the hydrogen (H₂) generation reaction from methanolysis of sodium borohydride (NaBH₄). Elemental analysis, XRD, FTIR, ICP-MS, and nitrogen adsorption were used to analyze the characteristics of metal-free catalysts. The results showed that groups containing O and P were attached to the carbon sample. In the study, the hydrogen production rates (HGR) obtained with metal-free C–KOH and C–KOH–P catalysts were 3250 and 10,263 mL/min/g, respectively. These HGR values are better than most values obtained for many catalysts presented in the literature. Besides, relatively low activation energy (Ea) of 27.9 kJ/mol was obtained for this metal-free catalyst. The C–KOH–P metal-free catalyst showed ideal reusability with 100% conversion and a partial reduction in the H₂ production studies of NaBH₄ methanolysis after five consecutive uses.     

Microcrystalline silicon, grain boundaries and role of oxygen

The authors report on the correlation of the oxygen content for three high growth-rate series of thin Si films crossing the boundary between amorphous and microcrystalline growth together with the evolution of the prefactor and the activation energy of the dark d.c. conductivity. The different roles of oxygen, such as doping, alloying or defect passivation, are discussed in the framework of the model of transport based on the formation of large grain boundaries with an increased band gap due to hydrogen and/or oxygen alloying.     

The cost analysis of hydrogen life cycle in China

Currently, the increasing price of oil and the possibility of global energy crisis demand for substitutive energy to replace fossil energy. Many kinds of renewable energy have been considered, such as hydrogen, solar energy, and wind energy. Many countries including China have their own plan to support the research of hydrogen, because of its premier features. But, at present, the cost of hydrogen energy production, storage and transportation process is higher than that of fossil energy and its commercialization progress is slow. Life cycle cost analysis (LCCA) was used in this paper to evaluate the cost of hydrogen energy throughout the life cycle focused on the stratagem selection, to demonstrate the costs of every step and to discuss their relationship. Finally, the minimum cost program is as follows: natural gas steam reforming – high-pressure hydrogen bottles transported by car to hydrogen filling stations – hydrogen internal-combustion engines.     

Two-step pyrolytic engineering to form porous nitrogen-rich carbons with a 3D network structure for Zn-air battery oxygen reduction electrocatalysis

It is of great urgency to design inexpensive and high-performance oxygen reduction reaction (ORR) electrocatalysts derived from biowastes as substitutes for Pt-based materials in electrochemical energy-conversion devices. Here we propose a strategy to synthesize three-dimensional (3D) porous nitrogen-doped network carbons to catalyze the ORR from two-step pyrolysis engineering of biowaste scale combined with the use of a ZnCl₂ activator and a FeCl₂ promotor. Electrochemical tests show that the synthesized network carbons have exhibited comparable ORR catalytic activity with a half-wave potential (~0.85 V vs. RHE) and outstanding cyclical stability in comparison to the Pt/C catalyst. Beyond that, a high electron transfer number (~3.8) and a low peroxide yield (<7.6%) can be obtained, indicating a four-electron reaction pathway. The maximum power density is ~68 mW cm^?2, but continuous discharge curves (at a constant potential of ~1.30 V) for 12 h are not obviously declined in Zn-air battery tests using synthesized network carbons as the cathodic catalyst. The formation of 3D porous structures with high BET surface area can effectively expose the surface catalytic sites and promote mass transportation to boost the ORR activity. This work may open a new idea to prepare porous carbon-based catalysts for some important reactions in new energy devices.