Wei-Kang Hu

Studies on cobalt-free AB5-type hydrogen storage alloys

Abstract Cobalt-free AB 5 -type hydrogen storage alloys were examined for lowering MH raw materials cost. The hydrogen absorption–desorption and electrode properties of these alloys were investigated. For comparison, a cobalt-containing AB 5 -type hydrogen storage alloy was also examined. Electrochemical measurements showed that cobalt-containing La 0.6 Ce 0.4 Ni 3.77 Co 0.6 Mn 0.36 Al 0.27 alloy had a maximum capacity of 295 mAh/g at a rate of 0.2 C. The capacity decay is 26.5% after 300 cycles at 1.0C rate with 100% depth of discharge (DOD). The Cobalt-free La 0.5 Ce 0.4 Ti 0.1 Ni 3.77 Mn 0.36 Al 0.27 (CuFeCr) 0.6 alloy showed very good cycling stability, although the initial capacity (266 mAh/g at 0.2C rate) was lower than that of the cobalt-containing alloy. The capacity decay of the cobalt-free alloy was 23.1% after 300 cycles under the same rate and DOD. The high cycling stability of the cobalt-free alloy was mainly due to a secondary phase present as a three-dimensional network since it played a key role of microencapsulating the main phase grains. The alloy microstructure and chemical composition were examined by scanning electron microscopy (SEM) and electron probe X-ray microanalysis (EPMA). In addition, the mechanism of the hydriding/dehydriding reaction on alloy electrodes was also investigated and is discussed using the electrochemical impedance spectroscopy (EIS) technique.

Effect of microstructure, composition and non-stoichiometry on electrochemical properties of low-Co rare-earth nickel hydrogen storage alloys

Abstract A series of low-Co stoichiometric and non-stoichiometric rare-earth nickel hydrogen storage alloys were examined for the possible application as negative electrodes in NiMH batteries. Partial substitution of Co by one transitional metal such as Cu, Fe, Cr or Zn did not effectively improve the electrochemical cycling stability, while a combined replacement of Co by Cr, Cu and Si was very effective in improving the cycling durability though the initial capacity was slightly low as compared with the parent alloy. In this work, the low-Co non-stoichiometric alloys did not appear superior in the capacity, rate capability and the charge–discharge cycling stability. The discharge capacity seems to be determined by the alloying components and compositional homogeneity. The cycling durability was dominated by the microstructure of alloys. The microstructure and chemical composition of alloys were examined and analyzed by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The relationship between the microstructure and electrochemical properties of alloys was discussed.

Improvement in capacity of cobalt-free Mm-based hydrogen storage alloys with good cycling stability

Abstract A multi-phase and multi-component cobalt-free Mm-based hydrogen storage alloy was examined. The phase composition, alloy microstructure, P – C – T and electrochemical properties were investigated. The physical and electrochemical properties of the cobalt-free alloy were compared with a typical AB 5 alloy containing higher cobalt content and with a low-cobalt Mm-based alloy. The results showed that the cobalt-free alloy had a flatter discharge plateau with electrochemical capacity of 302 mAh/g at 0.2 C rate and good rate capability. The hydrogen storage capacity of the cobalt-free alloy was significantly improved as compared with that of the low-cobalt Mm-based alloy (273 mAh/g). The cobalt-free Mm-based hydrogen storage alloy is a promising candidate for development of low-cost NiMH batteries with high capacity, good rate capability and cycling stability.

Influence of MH electrode thickness and packing density on the electrochemical performance of air-MH batteries

Abstract Thin and thick metal hydride (MH) electrodes used in air–MH batteries were evaluated with respect to electrochemical performances. The influence of electrode thickness on the discharge capacity, rate capability, charge and discharge voltages was investigated as well as the effect of MH packing density within electrodes on the electrode performances was also examined. The mass transport within the porous electrode was found to be the rate limiting step with increasing electrode thickness. The power output could be increased by either using thin electrodes or decreasing electrode density. This was, however, done at the expense of the energy density of the cell.

Rare-earth-based AB5-type hydrogen storage alloys as hydrogen electrode catalysts in alkaline fuel cells

Hydrogen oxidation by noble-metal-free, rare-earth-based, AB5-type catalysts was investigated in a double-layer hydrogen anode where the catalyst layer and the gas-diffusion layer were rolled together. The catalytic activity and operational stability were examined in a half-cell with 30 wt% KOH at 55 °C. The electrode had good catalytic activity for hydrogen oxidation and long-term stability over 1600 h at a current density of 40–50 mA/cm2.

A Low-Cobalt AB 5-Type Hydrogen Storage Alloy Stabilized by an Inert Second Phase for Use in Low-Cost, High-Performance NiMH Cells

Cost reduction of rechargeable nickel/metal hydride (NiMH) batteries is an important and challenging issue. We report here a multicomponent, low-cobalt AB 5 -type hydrogen storage alloy stabilized by an inert second phase for use in low-cost, high-performance NiMH batteries. X-ray diffraction and scanning electron microscopy measurements certify that the second phase is close to a Ni 2 Ti phase composition, while the main phase remains a hexagonal CaCu 5 structure. It is demonstrated that the low-cobalt AB 5 -type hydrogen storage alloy can maintain good cycling stability as well as high hydrogen storage capacity up to 310-315 mAh/g, when stabilized by the secondary phase.