Reactions nickel-hydrogen batteries

The reaction of hydrogen at the nickel electrode determines the rate of selfdischarge in nickel-hydrogen batteries. 

The electrolyte is an important component of the cell. Often it is only the medium for electrode reactions and ionic conductivity and does not appear in the cell reaction (e.g., in nickel/cadmium and nickel/hydrogen batteries), sometimes as in lead-acid batteries, it is also a component of the cell reaction. A certain interaction, however, between the electrolyte and the active material usually cannot be prevented and often influences aging of the battery. 

In normal battery operation several electrochemical reactions occur on the nickel hydroxide electrode. These are the redox reactions of the active material, oxygen evolution, and in the case of nickel-hydrogen and nickel-metal hydride batteries, hydrogen oxidation. In addition there are parasitic reactions such as the corrosion of nickel current collector materials and the oxidation of organic materials from separators. The initial reaction in the corrosion process is the conversion of Ni to Ni(OH)2. 

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. 

Lead-acid, nickel-iron (Ni-Fe), nickel-cadmium (NiCd), and nickel-metal hydride (NiMH) batteries are the most important examples of batteries with aqueous electrolytes. In lead-acid batteries, the overall electrochemical reaction upon discharge consists of a comproportionation of Pb° and Pb4+ to Pb2+. All nickel-containing battery reactions are based on the same cathodic reduction of Ni3+ to Ni2+, but utilize different anodic reactions providing the electrons. Owing to toxicity and environmental concerns, the formerly widely used Cd°/Cd2+ couple (NiCd cells) has been almost entirely replaced by H/H+, with the hydrogen being stored in a special intermetallic compound (NiMH). 

Combining the nickel cadmium and nickel-hydrogen systems technologies has given rise to the nickel-metal hydride rechargeable battery, one of the most advanced rechargeable systems commercially available and an environmentally friendlier alternative to nickel-cadmium batteries. The cell and its reaction may be written ... 

Hydrogen evolution can also be prevented, and thus the unwanted secondary reactions hydrogen evolution and grid corrosion that disturb the internal oxygen cycle in lead-acid batteries, as shown in Fig. 1.25, are not present in nickel/cadmium batteries, which therefore can be hermetically sealed so that neither vapor or gas escapes from the battery. This is the reason for the market success of these batteries in the field of portable applications. 

The situation with respect to secondary reactions is shown in Fig. 1.34. It is similar to that in the nickel/cadmium battery shown in Fig. 1.32 as far as the positive electrode is concerned. Different is the situation at the negative electrode. The electrode potential is nearly the same, since the equilibrium potential of the hydrogen electrode is only about 20 mV below that of the cadmium electrode. But now hydrogen is used as active material instead of cadmium, and hydrogen evolution as well as hydrogen oxidation are fast reactions, since both are catalyzed by the platinum surface of the negative electrode. 

In battery systems based on aqueous electrolyte, water decomposition, which occurs above a cell voltage of 1.23 V, is such an unavoidable secondary reaction. But under certain conditions the resulting water loss can be avoided, and the system is used as a sealed one, as achieved with sealed nickel/cadmium, nickel/hydrogen, and nickel/metal hydride batteries. In lead-acid batteries corrosion is an additional unwanted secondary reaction with the consequence that lead-acid batteries cannot be made virtually sealed, but must have a valve, and a certain water loss cannot be prevented. 

Figure 2.6 shows the typical structural design of a cylindrical nickel-cadmium battery. It has a safety vent, as illustrated in Figure 2.7, which automatically opens and releases excessive pressure when the internal gas pressure increases. Formation of hydrogen is avoided by extra Cd(OH)2 oxygen is removed by reaction with Cd. 

The electrochemical reactions for the normal, overcharge, and overdischarge (cell reversal) of a sealed nickel-hydrogen reversible battery cell or summarized in Table 19.5. 

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