The characterization of new type of alkaline fuel cell based on oxidation of chemical hydride has been studied. The chemical hydride can be used as a new fuel source in a fuel cell system. As a result, we have discovered that the electrochemical reaction rate is higher at a normal temperature compared with cells containing other hydrogen fuels where a hydrogen-releasing agent, NaBH4, is added to an aqueous alkaline solution of electrolyte as hydrogen fuel. That is, the fuel can be supplied very simply for the cell. If air is supplied to the oxygen cathode made of highly dispersed platinum particles supported in high-surface-area carbon paper, and the hydrogen releasing agent is fed to the alkaline solution of electrolyte at the side of metal hydride anode false(ZrCr0.8Ni1.2 alloy), the cell can produce electric current continuously. Also it can be operated at a normal temperature and produce a large amount of energy due to its high energy density of 6,000 Ah/kg or more (for NaBH4 or KBH4false). Therefore, the developed cell has higher electrochemical reaction rate and energy density than the conventional fuel cells using other hydrogen sources. © 2002 The Electrochemical Society. All rights reserved.
The electric vehicle (EV) and the hybrid-electric vehicle (HEV) have attracted significant interest in recent years, and the Ni/MH battery has been investigated as a highly promising power source for these advanced vehicles. However, the energy density of a Ni/MH battery must be improved in order to reach the target EV driving distance. The development of hydrogen storage electrodes with a high discharge capacity is one of the key requirements for improving the energy density of the Ni/MH battery.Recently, efforts have concentrated on Zr-based Laves phase alloys because they exhibit a larger hydrogen storage capacity than the AB 5 -type alloy as well as good cycle life durability. 1-3 However, Zr-based Laves phase alloys require many charge-discharge cycles to achieve activation because dense oxide layers on the surface inhibit the diffusion of hydrogen atoms into the bulk alloy. This slow activation can lead to overcharge of the Ni(OH) 2 electrode and the consequent formation of ␥-NiOOH, which degrades the electrode. In order to avoid overcharge of the Ni(OH) 2 electrode, it is necessary to fully activate the Zr-based alloy electrode within about five cycles.In order to improve the Zr-based alloy activation behavior, several pretreatments have been proposed. Lee et al. reported that Zrbased alloy activation was greatly improved by adding light rareearth elements, attributed to the formation of a porous La oxide film on the surface. 4-6 Beside the standard metallurgical methods, KBH 4 treatment, a pulse-charging and a hot-charging treatment have been suggested. 7-10 However, these proposed treatments are not satisfactory because they do not avoid overcharge of the Ni(OH) 2 electrode during the formation process.It was reported that immersion in a boiling 6 M KOH solution is very effective for improving the activation characteristics of Zrbased alloys. 11 After immersion, the Zr-oxide layer is eliminated and a Ni-rich region is formed at the alloy surface. However, it is difficult to apply this formation process, because the high immersion temperature could damage the separator or the Ni(OH) 2 electrode. To use an immersion treatment to form a Ni/MH cell, the temperature should be lowered.We recently found that the activation characteristics of Zr-based alloys depend on the applied current density. Charging at a low current density leads to more rapid activation, however, the reason for this improved activation is not yet clear.In this work, we introduce a new activation process by combining the hot-immersion treatment with a slow-charging method. It was reported in our previous work that the Zr 0.7 Ti 0.3 Cr 0.3 Mn 0.3 V 0.4 Ni 1.0 alloy (termed "alloy 1" throughout this paper) is one of the most promising materials for the MH electrode. 10 An alloy 1 electrode was treated through two steps as follows: the alloy electrodes were immersed at 80ЊC for 12 h in a KOH solution and then charged at a very low current density. The effects of this treatment on the alloy electrochemical and physicochemical properties are di...
For the purpose of developing a Zr-based Laves phase alloy with higher capacity and better performance for electrochemical application, extensive work has been carried out. After careful alloy design of ZrMn 2 -based hydrogen storage alloys through varying their stoichiometry by means of substituting or adding alloying elements, the Zr 0.9 Ti 0.1 (Mn 0.7 V 0.5 Ni 1.4 ) 0.92 with high capacity (392 mAh/g at the 0.25C) and improved performance (comparable to that of commercialized AB 5 type alloy) was developed. Another endeavor was made to improve the poor activation property and the low rate capability of the developed Zr-based Laves phase alloy for commercialization. The combination method of hot-immersion and slow-charging was introduced. It was found that electrode activation was greatly improved after hot immersion at 80 o C for 12 h followed by charging at 0.05C. The effects of this method are discussed in comparison with other activation methods. The combination method was successfully applied to the formation process of 80 Ah Ni/MH cells. A series of systematic investigations has been rendered to analyze the inner cell pressure characteristics of a sealed type Ni-MH battery. It was found that the increase of inner cell pressure in the sealed type Ni/MH battery of the above-mentioned Zr-Ti-Mn-V-Ni alloy was mainly due to the accumulation of oxygen gas during charge/discharge cycling. The fact identified that the surface catalytic activity was affected more dominantly by the oxygen recombination reaction than the reaction surface area was also identified. In order to improve the surface catalytic activity of a Zr¯Ti¯Mn¯V¯Ni alloy, which is closely related to the inner pressure behavior in a sealed cell, the electrode was fabricated by mixing the alloy with Cu powder and a filamentary type of Ni and replacing 75% of the carbon black with them; thus, the inner cell pressure rarely increases with cycles due to the active gas recombination reaction. Measurements of the surface area of the electrode and the surface catalytic activity showed that the surface catalytic activity for the oxygen recombination reaction was greatly improved by the addition of Cu powder and the filamentary type of Ni. Finally, we have collaborated with Hyundai Motors Company on fabrication of the 80Ah cells for Electric Vehicles and evaluated the cell performance.
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