Batteries and Fuel Cells

Batteries are voltaic cells used as portable electrical sources. They are categorized into primary, or non-rechargeable, and secondary, or rechargeable, batteries.

The common primary battery is the dry cell. It contains a zinc case as the anode and a graphite cathode immersed in a moist, acidic electrolyte paste of manganese(IV) oxide and ammonium chloride.

The zinc oxidizes to zinc chloride, and manganese(IV) oxide reduces to solid manganese(III) oxide, generating a potential of 1.5 volts. When operated, the electrodes get depleted, making dry cells single-use batteries.

Alkaline batteries are similar but use an alkaline electrolyte, extending their shelf and working lives.

Lead-acid batteries are secondary batteries predominantly used in automobiles. These typically have six electrochemical cells connected in series, generating a total potential of 12 volts. Each cell has a lead anode and a lead oxide cathode immersed in sulfuric acid.

During discharge, the redox reactions deposit solid lead sulfate on both electrodes, which are converted back during recharging by using an external power source.

Nickel–cadmium rechargeable batteries contain a nickel oxyhydroxide cathode, a cadmium anode, and a potassium hydroxide electrolyte. When operated, the anode oxidizes to solid cadmium hydroxide, and the cathode reduces to solid nickel hydroxide, generating a cell potential of 1.3 volts.

However, cadmium’s toxicity drives the replacement of nickel–cadmium batteries by eco-friendly nickel–metal hydride batteries. Here, the cathode remains the same, but the anode is a hydrogen-absorbing alloy. The anodic reaction involves the formation of water from hydrogen.

Lithium–ion batteries are the newest secondary batteries. Lightweight lithium metal makes these batteries portable with high energy density.

A lithium–ion battery contains a lithiated graphite anode and a lithium–transition metal oxide cathode. When charged, the transition metal is oxidized, while lithium ions are incorporated between the anodic graphite layers. Upon discharge, lithium ions flow from the anode to the cathode, generating a potential of 3.7 volts.

Lastly, fuel cells are voltaic cells that require a continuous external supply of redox reactants for energy production.

A typical hydrogen fuel cell uses hydrogen and oxygen from the air to generate a current while emitting water. Graphite electrodes contain platinum-based catalysts to accelerate the redox reaction.