Nickel charging-discharging reaction

Edison cell — A nickel-iron (Ni-Fe) secondary (rechargeable) cell independently developed by Edison in USA and W. Jiinger in Sweden in 1900. The cell (-> battery) is based on the use of nickel oxyhydroxide (NiOOH) at the positive electrode and metallic iron for the negative electrode, and a potassium hydroxide (KOH) solution containing lithium hydroxide (LiOH) is the electrolyte. The Ni-Fe cell is represented as ( Fe/KOH/NiOOH. The charge-discharge reactions for the Edison (Ni-Fe) cell are as follows ... 

The electrochemistry of the charge and discharge of the positive electrode is quite complex and not well understood, especially, the role that cobalt plays in the active material. For simplicity, lets consider the role of nickel hydroxide in the charge-discharge reaction. 

Oxygen evolution occurs on nickel oxide electrodes throughout charge, on overcharge, and on standby. It is the anodic process in the self-discharge reaction of the positive electrode in nickel-cadmium cells. Early work in the field has been reviewed [9], No significant new work has been reported in recent years. 

The cell reactions are shown in Fig. 8.21, from which it can be seen that overdischarge and overcharge reactions are possible. Use is made of the overdischarge reaction to allow the assembly of the cell without the addition of elemental sodium. To the products of the discharge reaction - nickel and sodium chloride - is added a small quantity of aluminium which on the first charge generates enough excess sodium to prime the sodium electrode ... 

The cell or battery is enclosed in a stainless steel or Inconel pressure vessel (Fig. 9.23). Hydrogen pressure rises from about 0.5 MPa in the fully discharged state to 3-10 MPa when charged, and the pressure in the vessel can be used to monitor the state of charge. Direct reaction between hydrogen and nickel oxide is relatively slow, but 6-12% of capacity is lost per day. 

The nickel-cadmium secondary battery contains NiOOH/nickel hydroxide as a positive active material, cadmium/cadmium hydroxide as a negative active material, and an aqueous solution containing potassium hydroxide as the main component as an electrolyte. Generally the charge-and-discharge reaction is shown in the following formulas 1, 2 and 3. 

So far, metal hydrides have only been considered for use in nickel-metal hydride batteries where the anode consists of a LaNisH -based metal hydride. During discharge, the hydride ions are oxidized and react with 0H to H2O. At the cathode, NiOOH is reduced to Ni(0H)2. The charge transfer reaction in the system therefore is based on protons. 

Besides the application of solid hydrogen storage, MH alloys also play a very important role in nickel/metal hydride (Ni/MH) rechargeable battery as the negative electrode. The chemical reaction during charge/ discharge is ... 

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