Wenhua H. Zhu

A study of the electrochemistry of nickel hydroxide electrodes with various additives

Nickel composite electrodes (NCE) with various additives are prepared by a chemical impregnation method from nitrate solutions on sintered porous plaques. The electrochemical properties, such as utilization of active material, swelling and the discharge potential of the nickel oxide electrode (NOE) are determined mainly through the composition of the active material and the characteristics of nickel plaques. Most additives (Mg, Ca, Sr, Ba, Zn, Cd, Co, Li and Al hydroxide) exert effects on the discharge potential and swelling of the NOE. Chemical co-precipitation with the addition of calcium, zinc, magnesium and barium hydroxide increases the discharge potential by more than 20 mV, but that with zinc hydroxide results in an obvious decrease of active-material utilization and that with calcium and magnesium hydroxide produces a larger increase of electrode thickness. The effects of anion additives are also examined. Less than 1% mol of NiS in the active material increases the discharge potential, Cadmium, cobalt and zinc hydroxide are excellent additives for preventing swelling of the NCE. Slow voltammetry (0.2 mV s(-1)) in 6 M KOH is applied to characterize the oxygen-evolving potential of the NCE. The difference between the oxygen-evolution potential and the potential of the oxidation peak for the NCE with additives of calcium, lithium, barium and aluminium hydroxide is at least + 60 mV.

New structures of thin air cathodes for zinc–air batteries

Thin composite cathodes for air reduction were manufactured using microfibre-based papermaking technology. The electrodes have a thin structural design, less than 0.15 mm in thickness. Composite cathode materials for oxygen reduction applications were fabricated by entrapping carbon particles in a sinter-locked network of 2–8 μm diameter metal fibres. The thin structure not only results in electrodes that are 30–75% thinner than those commercially available, but also offers an opportunity for custom-built air cathodes optimized for high-rate pulse applications. Using a thin composite structure for the air cathode in a zinc–air battery that is part of a zinc–air/capacitor hybrid is likely to increase the pulse capability of the hybrid power system. The thin cathode structure provides a better, more efficient three-phase reaction zone. In a half-cell test, the ultrathin air cathode generated more than 1.0 V vs Zn/ZnO for a current of 200 mA cm−2. Half-cell, full-cell and pulse-power tests revealed that thin composite cathodes have a better rate and pulse performance than the air cathodes commonly used.

Diffusion and Gas Conversion Analysis of Solid Oxide Fuel Cells at Loads via AC Impedance

Impedance measurements were conducted under practical load conditions in solid oxide fuel cells of differing sizes. For a 2 cm2 button cell, impedance spectra data were separately measured for the anode, cathode, and total cell. Improved equivalent circuit models are proposed and applied to simulate each of measured impedance data. Circuit elements related to the chemical and physical processes have been added to the total-cell model to account for an extra relaxation process in the spectra not measured at either electrode. The processes to which elements are attributed have been deduced by varying cell temperature, load current, and hydrogen concentration. Spectra data were also obtained for a planar stack of five 61 cm2 cells and the individual cells therein, which were fitted to a simplified equivalent circuit model of the total button cell. Similar to the button cell, the planar cells and stack exhibit a pronounced low-frequency relaxation process, which has been attributed to concentration losses, that is, the combined effects of diffusion and gas conversion. The simplified total-cell model approximates well the dynamic behavior of the SOFC cells and the whole stack.

Gas Recombination in Sealed Ni–MHx Cells

This paper focuses on investigation of gas recombination in a positive-limited-sealed Ni–MHx cell. The positive electrodes were prepared by electrochemical impregnation of fibrous nickel plaques. The metal hydride negative electrodes were made by pasting the mixture of rare-earth hydrogen storage alloy powders, conducting and binding agents on foamed nickel substrates. The measurement of the positive capacity at different charge times was used to estimate the partial current for oxygen evolution at the same time. The effects of charge rate, electrolyte saturation level and initial state of charge of the positive electrodes on the recombination were investigated in sealed Ni–MHx cells. By determining the differential capacity of nickel hydroxide electrodes, an improved mathematical model was used to evaluate the gas recombination parameters during charge, overcharge, rest and discharge of the positive-limited-sealed Ni–MHx cell. The gas recombination during rest, discharge and overdischarge was also examined. The oxygen recombination on the nickel hydroxide electrodes can be neglected due to the consumption of water when the nickel hydroxide electrodes were discharged. The longer overdischarge produced an increase in cell pressure for the sealed Ni–MHx cell at an electrolyte unsaturated level and the evolving gas can be recombined by a following recharge operation.

Electrochemical impregnation and performance of nickel hydroxide electrodes with porous plaques of hollow nickel fibres

Abstract Porous plaques that contain hollow nickel fibres are successfully prepared by a traditional slurry-scraping technology. The fibrous, nickel-plaque materials have high porosity (85–87%) and are for application in high specific-energy nickel—metal hydride batteries. Nickel electrodes are fabricated by cathodic electrodeposition from metal nitrate solutions on to the fibrous plaques. Parameter changes in the concentration of nickel nitrate, current density, pH, temperature and plaque porosity, as well as changes in the impregnation time, are examined with respect to their influence on the weight gain of fibrous nickel plaques. The influence of fibre content on the electrode performance is investigated by determining the capacity, utilization, expansion and cycling characteristics. Both C-type and D-type electrodes with ∼ 85% fibrous plaques reach a specific capacity of 141 mAh g−1. It is found that utilization of active material and electrode swelling both decrease with increasing content of the hollow fibres.

In-Situ Dynamic Characterization of Energy Storage and Conversion Systems

The combustion of fossil fuels predominates in the commercial implementation of energy conversion and power generation. However, it brings severe problems to the environment due to inevitably incomplete combustion. Meanwhile, the price of fossil fuels keeps increasing due to the depletion of natural resources. The growing concerns of global warming, as well as the reducing availability of fossil fuels, require replacements of gasoline and diesel fuels, such as Fischer-Tropsch synthetic fuels [1-3], biofuels [4, 5], and hydrogen fuel [6, 7]. The thermal conversion efficiency of a traditional automobile engine is between 17% and 23% [8], limited by the intrinsic characteristics of Carnot cycle. Energy storage and conversion systems with low/zero emissions, high efficiency, and great durability are required by the development of sustainable energy and power economy.

Sintering preparation for porous plaque containing hollow nickel fiber

The flexible plaque of porous nickel is crucially important for the cycle life of nickel/cadmium, nickel/hydrogen and nickel/metal hydride batteries. The use of hollow fiber can increase the flexibility of porous plaque. The sintered porous plaque containing hollow nickel fiber is fabricated by a traditional. slurry-scraping technology. The pore-creating agent and the pore-strengthening additive are applied to increase the plaque porosity and optimize the pore structure. Nickel hydroxide is used as a pore-strengthening agent and it also assists the gasification of organic pore-creating agent to some extent. The plaque porosity reaches more than 88% when using thinner skeleton and large amount of hollow fiber.

Simulation of Ni-MH Batteries via an Equivalent Circuit Model for Energy Storage Applications

Impedance measurement was conducted at the entire cell level for studying of the Ni-MH rechargeable batteries. An improved equivalent circuit model considering diffusion process is proposed for simulation of battery impedance data at different charge input levels. The cell capacity decay was diagnosed by analyzing the ohmic resistance, activation resistance, and mass transfer resistance of the Ni-MH cells with degraded capacity. The capacity deterioration of this type, Ni-MH cell, is considered in relation to the change of activation resistance of the nickel positive electrodes. Based on the report and surface analysis obtained from the energy dispersive X-ray spectroscopy, the composition formula of metal-hydride electrodes can be closely documented as the AB5 type alloy and the “A” elements are recognized as lanthanum (La) and cerium (Ce). The capacity decay of the Ni-MH cell is potentially initiated due to starved electrolyte for the electrochemical reaction of active materials inside the Ni-MH battery, and the discharge product of Ni(OH)2 at low state-of-charge level is anticipated to have more impeding effects on electrode kinetic process for higher power output and efficient energy delivery.

Electrochemical characteristics of encapsulated metal-hydride-alloy electrodes

Metal hydride electrodes with copper-encapsulated alloys and non-coated alloys were fabricated using suitable conductive and binding agents. The charge-discharge characteristics of three kinds of hydride electrodes were comparatively investigated. The encapsulated alloy electrode is remarkably superior to the non-coated LaNi5-based one, discharging at a high rate and exhibiting a smaller capacity decay at the stage of cycle tests. The hydride alloy quality of hydride electrodes can be effectively determined by measuring rate capability. The results of vented cell experiments confirm that the capacity decay of non-coated alloy electrodes in sealed cells is not due to the oxidation of oxygen from the nickel hydroxide positive electrodes. The relationship between the equilibrium potential of hydride electrode and the equilibrium hydrogen pressure has been deduced by a succinct thermodynamic method, without consideration of the unknown activity of water and fugacity coefficient of hydrogen.