Hydrogen electrode secondary batteries

The nickel-based systems have traditionally included the following systems -nickel-iron (Ni/Fe), nickel-cadmium (NiCd), nickel metal hydrides (NiMH), nickel hydrogen (Ni/H2), and nickel-zinc (Ni/Zn). Of these, the metal hydride chemistry has been the most successful in the secondary battery market. AU nickel systems are based on the use of a nickel oxide active material (undergoing one valence change from charge to discharge or vice-versa). The electrodes can be pocket type, sintered type, fibrous type, foam type, pasted type, or plastic roll-bonded type. All systems use an alkaline electrolyte, KOH. 

The vented-type battery needs to perform water addition in which water consumed by electrolysis is periodically added. However, in recent years, the vented-type nickel-cadmium secondary battery which has reduced the water addition frequency is produced commercially for trains and the like. The battery controls the electrolysis of the water under float charging by using the pasted-type cadmium electrode which has a high hydrogen overpotential characteristic for the negative electrode. 

Nickel/metal hydride (Ni/MH) battery is a secondary battery using hydrogen storage alloy for the negative electrode, Ni(OH)2 for the positive electrode, and alkaline solution for the electrolyte. Polypropylene nonwoven fabric is usually selected for the separator. The theoretical voltage is about 1.32 V, and the operating voltage is about 1.2 V which is almost the same as that of Ni/Cd battery [1]. The Ni/MH battery has been put to practical use for portable electric equipments in 1990 and for HEV (hybrid electric vehicle) in 1997 [2, 3]. 

The potential of an electrode can only be defined in relation to that of another electrode. In chemistry, it is often measured in relation to the NHE (the Normal Hydrogen Electrode, defined in footnote 58). For lithium secondary batteries, we rather take a reference electrode of metal lithium. Therefore we use the notation XV versus Lf/Li, which denotes a difference of potential of X volts versus (versus) a metal lithium electrode. Alternatively, we use YV versus NHE, which means a difference of potential of Y volts versus a normal hydrogen electrode. We also find the notation Y V/NHE. 

Nickel/cadmium batteries (line 8 in Table 1.1) have been in technical use nearly as long as lead-acid batteries. They belong to a whole family of secondary batteries that are based on aqueous, but alkaline electrolyte, usually diluted potassium hydroxide. Nickel/cadmium, nickel/hydrogen, and nickel/metal hydride batteries are the most important members of this group. A further common feature of these battery systems is that they employ the nickel-hydroxide electrode as the positive one. Some of their basic features will be described in the following. 

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. 

We have so far described the research and development for nickel-metal hydride batteries in which a hydrogen storage alloy (metal hydride) is used as a negative electrode material. This type of battery has been attracting a great deal of attention as a new non-military battery because it is characterized by several advantages over the conventional secondary batteries ... 

Briefly describe each of the following ideas, methods, or devices (a) salt bridge (b) standard hydrogen electrode (SHE) (c) cathodic protection (d) fuel cell. Explain the important distinctions between each pair of terms (a) half-cell reaction and overall cell reaction (b) voltaic cell and electrolytic cell (c) primary battery and secondary battery (d) Eceii and E°en. 

Note Actually not the true equilibrium voltage but only the open circuit voltage can be measured with lead-acid batteries. Due to the unavoidable secondary reactions of hydrogen and oxygen evolution and grid corrosion, mixed potentials are established at both electrodes, which are a little different from the true equilibrium potentials (cf Fig. 1.18). But the differences are small and can be ignored. 

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