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Nickel Cells - How they Work | Nickel-based Cells: Now that you have a basic understanding of cells and how they work, I will continue on and detail to you a type of battery that is constantly evolving from one year to the next. The nickel-based rechargeable cell.
Nickel-Iron Cells: Nickel-Iron cells were the first rechargeable Nickel cell, one of many with a voltage around 1.2V, invented in 1890 by Thomas Edison. The cell consisted of a Nickel hydroxide cathode, an Iron anode and an alkaline electrolyte using Potassium Hydroxide (KOH). The invention of this rechargeable nickel battery paved the way for the development of two other types of Nickel-based cells, the common Nickel Cadmium cell (Ni-Cd is the name used by everyone now) and Nickel-Metal Hydride cells (known as Ni-MH cells in the portable computer world). Continue reading to learn more about these constantly developing variations on a cell.
Nickel-Cadmium Cells: Features:
Nickel Cadmium is well suited for motor driven applications, such as power tools and even electric cars, because of it's high energy density. Energy density is a term used to define how much energy can be produced by a cell compared to it's weight, measured in Watt-hours per kilogram (1 Wh = 3600 Joules), so if a battery can supply large amounts of energy and is contained in a small package, it will have a desirably high energy density.
 Design:
Nickel Cadmium batteries were originally designed with a solid Nickel Hydroxide cathode and a solid Cadmium anode. Unfortunately the use of a solid anode and cathode offers very low energy densities, because only a fraction of the electrode components are available to each other. As time passed, a "wound" cell was adopted, giving the Ni-Cd a higher energy density.
Cathode:
The design of the nickel cathode has changed over the years also. One cathode design was called a "sintered" plate, which involved the coating of a steel foil strip with dendritic-type Nickel powder. The coated strip is then passed through a high temperature furnace, causing the dendritic projections of the Nickel powder to fuse together. This forms a strong, highly porous base layer known as Nickel plaque. Nickel Hydroxide, the active material, is impregnated into the pores of the nickel plaque by one of two methods: Chemical impregnation of the plaque with Nickel Nitrate, which is subsequently converted to nickel hydroxide. Several of these cycles are necessary to completely load the pores of the plaque. An electrochemical process can be used to convert an aqueous solution of Nickel Nitrate directly to Nickel Hydroxide, only requiring one cycle to load the pores of the plaque.
Another, more recent development in the creation of the cathode is called a "foam electrode" which it really is. The foam electrode is made by mechanically pasting, compressing, or spraying powdered Nickel Hydroxide, along with other additives, into an "open-celled" foam. The larger amount of active material accommodated by the larger pores increase the energy density of the battery by about 15% or 20%. Because the foam is not a good conductor of electrons, additives such as Cobalt metal and Cobalt Oxide are included with the Nickel Hydroxide.
Anode:
The anode of the Ni-Cd has also changed over the years to increase the overall energy density of the Ni-Cd cell. There are also two different processes to producing Cadmium electrodes, one product is called a "pasted" Cadmium electrode, made by mixing Cadmium Hydroxide with a binder (keeping the Cd(OH)2 in a paste form) and pasting it to a metal foil substrate. Electrically conductive materials are also added to the mixture to enhance it's function. The other type of anode produced is called a sintered anode, produced in a similar way as the cathode.
Reactions:
There are three separate and different reactions that occur in a Ni-Cd cell, the anode reaction and the cathode reaction and the overcharge reaction. The first two reactions are going in the direction of right to left when the cell is discharging, and from the left to the right when the cell is being recharged.
Here is the anode reaction:
Cd(OH)2 + 2e1-  Cd + 2OH1- Ni(OH)2 + OH1- NiOOH + H2O + e1- The overcharge reaction is a reaction that occurs when the Ni-Cd is fully charged and can't replenish any more of the cathode. In this reaction, the Cadmium electrode takes a part:
4OH1- --> O2 + H2O + 2e1- The Oxygen produced in this reaction then finds it's way to the anode, and through a few steps reproduces Hydroxyl ions, OH1- and creates heat. The heat created is not a good thing, but won't kill the battery immediately. Heat damage, when it happens, usually occurs at the separator that is made of one of two types of organic molecules, polyamides (almost like your body's proteins) or polypropylene. Polyamides are usually in consumer, run-of-the-mill batteries not meant to deal with high temperature situations, where the polypropylene separator batteries are more meant for high-heat applications. Also, if the overcharge occurs too fast or for too long, Oxygen will build up and be forced to vent, releasing along with it water, therefore reducing the cell's available working components.
Nickel-Metal Hydride Cells: Features and Design:
Nickel-Metal Hydride cells are the newer cousin of Ni-Cd cells, offer most of the same features, and even appear that they will surpass the Ni-Cd battery in the future as the technology used to create the anode storage metal. Ni-MH cells incorporate a different type of structure at the anode that allows Hydrogen to be stored in a specialized metal structure, instead of using a solid metal or compound as an anode. One drawback to the Ni-MH design is the rapid loss of charge, known as self-discharge, that amounts to a dead cell within a month just sitting in storage.
 Cathode:
Same design concept as a Ni-Cd cell (above).
Anode:
The anode of this type of battery is somewhat unique because it stores Hydrogen, without bonding to it, then releases it. Only a few types of alloy can do this. Some follow the composition of AB5, where A is a metal and B is a different metal, for example: LaNi5. Other Hydrogen storing alloys have a composition of AB2, where A and B are also different metals. The AB5 is preferred over the AB2 alloys for it's ability to efficiently store and release Hydrogen.
Reactions:
There are also three separate and different reactions that occur in a Ni-MH cell, much like the Ni-Cd cell. The three reactions are the anode reaction, the cathode reaction and the overcharge reaction. The first two reactions are going in the direction of right to left when the cell is discharging, and from the left to the right when the cell is being recharged.
Here is the anode reaction:
Alloy + H2O + e1- Alloy[H] + OH1- Here is the cathode reaction:
Ni(OH)2 + OH1- NiOOH + H2O + e1- The overcharge reaction is very similar to the Ni-Cd overcharge reaction. The first step is the same, and also involves the anode. 4OH1- --> O2 + H2O + 2e1- The Oxygen produced in this reaction then finds it's way to the anode, and through a few steps reproduces Hydroxyl ions, OH1- and creates heat. The Oxygen produced will also do the same thing in a Ni-MH cell as in a Ni-Cd cell, also not a good thing with Ni-MH cells.
Summary Overall, Nickel-based cells are excellent for applications requiring the ability to use rechargeable batteries that take very little time to recharge, will survive under less than optimal conditions, and need available density to run high-drain applications. The possibility of advancement in the Ni-MH is inevitable, because a reliable means for Hydrogen storage still needs to be found, and as soon as that hurdle is crossed, the Ni-MH cell has a good chance of beating out Ni-Cd cells in the rechargeable industry. Last, but not least, yet another cousin of Ni-Cds is being developed, the Nickel-Zinc battery, which is said to have an even higher energy density than Ni-Cds. The Ni-Zn battery still has a while to go before it will be commercially acceptable, considering that a Ni-Zn cell lasts less than 500 cycles (because of the formation of Zinc dendrites), compared to about 1000 cycles for a Ni-Cd.
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