Nickel-metal hydride cell

What is a nickel-metal hydride cell?

Before we get to that, “What is a cell?” A cell is a  source of energy that converts chemical energy into electrical energy. There are two types of cells—primary (non-rechargeable) cells and secondary (rechargeable) cells. 

A Nickel-metal hydride cell is a rechargeable cell. It uses a compound of Nickel and a metal hydride as electrodes. It is a basic definition to understand it better. Let us dive more into the nickel-metal hydride cell notes.

How does a nickel-metal hydride cell work?

The anode of a Nickel-metal hydride cell is made from a hydrogen-absorbing alloy (MH) or other inter-metallic compounds, and nickel-oxide hydroxide ( NiO(OH) ) or nickel hydroxide (Ni(OH)2) makes the cathode. It uses alkaline substances (e.g., KOH ) as electrolytes. 

Reaction in anode: M + e-+ H2O →MH +OH- 

Reaction in cathode: Ni(OH)2 + OH- → NiO(OH) + H2O + e-  

Overall reaction : Ni(OH)2 + OH- ⇌ NiO(OH) + H2O +e-

The overall reaction is reversible. While charging, it occurs from the left-hand side to the right-hand side; while discharging, it reverses.

Nickel-metal hydride cell advantages and disadvantages

A Nickel-metal hydride cell has consistently rated voltage and higher capacity than alkaline batteries. It lasts more in high drainage situations than a typical alkaline battery. It has a longer lifespan, it also discharges slowly when kept idle; its retentivity of charge is typically 50% to 80% in a year. In addition, it has a broader range of working temperatures than Ni-Cd batteries. Unfortunately, everything comes at a cost; Nickel-metal hydride cells are no exception. They tend to get damaged while overcharging, short circuits and overheating.

Applications

The most common and popular AA or AAA batteries contain a nickel-metal hydride cell. It replaces Ni-Cd batteries in many aspects, though a Nickel-metal hydride battery usually works at 1.2V more than typical 1.5V Ni-Cd batteries. It is still a popular choice for many consumer electronics, like, digital cameras, remote-controlled cars, drones, etc. But as of the 2020s, Li-ion cells are replacing nickel-metal hydride cells. In the late 1990s, nickel-metal hydride cells were very successful in the automobile industry. First-generation electric vehicles (Toyota Prius was the most iconic one) and hybrid vehicles were produced with Nickel-metal hydride batteries. But nowadays, it is being replaced with Li-ion batteries.

Safety concerns of nickel-metal hydride cells

• Nickel-metal hydride cells are safe to use, as they do not contain hazardous components like cadmium in Ni-Cd cells. But the battery should be made waterproof and leakproof.
• A nickel-metal hydride cell produces H2 gas even in normal charging and discharging scenarios. Accumulated Hydrogen gas pops open battery caps. Hydrogen is a flammable gas, so it is a safety concern in nickel-metal hydride cells.
• During short circuits, a high amount of current flows in the Nickel-metal hydride batteries, which in turn generates high heat and becomes a fire hazard.
• The chemicals used in nickel-metal hydride cells are corrosive in nature. Keeping this in mind, cells should be kept in a leakproof container.

Recycling and disposals of nickel-metal hydride cells

Nickel-metal hydride cells contain high amounts of metals, like Nickel (Ni) and its alloys; low amounts of Aluminium (Al), Cobalt (Co), Manganese (Mn) and some Rare Earth Elements (REE, e.g., Lanthanum). To date, recycling Nickel-metal hydride batteries means recycling only a few battery parts (like steel used in battery cases); other parts, including the Rare Earth Elements (REE), go into waste. Though metallurgical developments have been made to recycle these parts, they are not available commercially. 

Disposals of nickel-metal hydride cells should be according to norms; for instance, they should be fully discharged, are not to be punctured, not ignited or heated.

Conclusion

Nickel-metal hydride cells batteries have now lost their throne to the new OG of batteries, the Li-ion batteries, maybe because it popped too many of its safety caps during overcharging. Lower lifespan (Charging cycle), higher discharge rate during idle state, less energy density than Li-ion batteries, problematic charging conditions have sunk the floating boat of nickel-metal hydride cells. But new research is working on the cons and changing its properties for better performance. 

Hopefully, you understood the definition, applications and concerns regarding these cells. Manufacturers are working to resolve the need for improvement in some areas of its workings.