Nickel Electrode Batteries

Though nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) are the most common and well-known nickel electrode battery technologies, many other materials can be used as the negative electrode to combine with the positive nickel hydroxide (Ni(OH)₂) electrode (Baxter, 2006). With this in mind the next most common nickel electrode batteries are nickel-iron (NiFe), nickel-hydrogen (NiH₂) and nickel-zinc (NiZn). In a nickel electrode battery the positive nickel hydroxide (Ni(OH)₂) electrode, commonly in the form of spongy mass, is placed in an aqueous electrolyte solution of potassium hydroxide, KOH·(H₂O) (Figure 1) (EPRI, 2003).

Figure 1 basic diagram of a nickel electrode battery during discharge.

The approximate chemical equation for charging and discharging on the nickel electrode is:

^{(EPRI, 2003)}

The different battery technologies differ in the chemistry of their negative electrode and by minor differences in the construction of the batteries. Table 1 lists the different types of nickel electrode batteries based on the material used for the negative electrode as well the common usage and concerns for each battery type.

Table 1 summary of the most common types of nickel electrode batteries (EPRI, 2003).

Battery Type	Negative Electrode Material	Usage	Concerns
nickel-cadmium (NiCd)	metallic cadmium, Cd	Higher energy density, longer life, abuse tolerant, require less maintenance than lead-acid batteries. Most common nickel electrode battery in utility industry.	Cadmium is highly toxic.
nickel-hydrogen (NiH2)	gaseous hydrogen, H	Long life, low maintenance, high reliability. Commonly used in aerospace applications.	Most expensive of the nickel electrode batteries.
nickel-metal hydride (NiMH)	hydrogen absorbed in a metal alloy	Higher energy density and better cycle life than NiCd (without using cadmium).	Less tolerant of electrical abuse such as high rate of discharge. Not well suited for power applications. Metal hydride batteries are difficult to manufacture in large sizes.
nickel-iron (NiFe)	iron (Fe)	Extreme durability, tolerant of physical and electrical abuse.	Highly variable with temperature, low power density, poor charge retention (mostly phased out by NiCd and lead-acid batteries).
nickel-zinc (NiZn)	zinc (Zn)	Slightly higher energy density and lower cost than NiCd (also cadmium free).	Least mature technology, Zn electrodes currently have a short life.

In general nickel batteries have a round trip DC to DC efficiency of 65% to 85%. They are prone to self-discharge of between 2% and 5% per month and experience capacity losses through a number of mechanisms (Baxter, 2006). These capacity loses in some cases are reversible through special reconditioning procedures. The life expectancy, both in calendar terms and number of cycles, is highly dependent on the chemistry, the depth of discharge and the rate of discharge. Considering the characteristics of nickel electrode batteries, aside from portable energy supply, the most appropriate energy storage applications for these batteries are applications related to maintaining power quality. Taking advantage of their rapid response time, nickel electrode batteries can correct short term power quality events such as voltage sags and frequency variations. They are not considered appropriate for applications requiring large amounts of power or energy (EPRI, 2003).

Works Cited

Baxter, R. (2006). Energy Storage; A Nontechnical Guide. Tulsa, Oklahoma: PennWell Corporation.

EPRI. (2003). EPRI-DOE Handbook of Energy Storage for Transmission & Distribution Applications. Washington DC: EPRI, Palo Alto, CA, and the U.S. Department of Energy.