Batteries and Fuel Cells

Primary batteries are non-rechargeable and designed for single-use applications, such as dry cells. Secondary batteries are rechargeable and use reversible cell reactions, allowing them to be recharged by an external power source. Examples of secondary batteries include lead-acid, nickel-cadmium, and lithium-ion batteries used in automobiles and portable electronics.

A dry cell contains a zinc anode and graphite cathode immersed in an acidic electrolyte paste of manganese(IV) oxide and ammonium chloride. During discharge, zinc oxidizes to zinc chloride while manganese(IV) oxide reduces to manganese(III) oxide, generating a cell potential of 1.5 volts. This redox reaction produces the electrical current that powers the device.

Lead-acid batteries are inexpensive and capable of producing the high current required by automobile starter motors. Each cell produces 2 volts, and six cells connected in series generate a total potential of 12 volts. Although heavy and containing caustic sulfuric acid electrolyte, their high current density and reliability make them the preferred choice for vehicles despite requiring proper disposal due to lead content.

Lithium-ion batteries are lightweight and provide high energy density, making them portable and efficient. They contain a lithiated graphite anode and lithium-transition metal oxide cathode, generating a cell potential of 3.7 volts. These batteries deliver large amounts of current, produce nearly constant voltage during discharge, and lose charge slowly during storage, making them ideal for smartphones, laptops, and other portable electronics.

Fuel cells are voltaic cells requiring continuous external supply of redox reactants, typically hydrogen and oxygen, for energy production. Unlike batteries with finite stored energy, fuel cells generate electricity through ongoing chemical reactions. A hydrogen fuel cell uses platinum-based catalysts on graphite electrodes to accelerate redox reactions, producing approximately 1.23 volts while emitting only water, with energy efficiency typically exceeding 50 percent compared to internal combustion engines.

Alkaline batteries use an alkaline electrolyte, typically potassium hydroxide, instead of the acidic paste found in dry cells. This design allows alkaline batteries to deliver three to five times more energy than similarly sized dry cells, with a cell potential of 1.43 volts. Alkaline batteries also have extended shelf and working lives, though they are prone to leaking and should be removed from devices during long-term storage.

Nickel-cadmium batteries contain toxic cadmium, which poses environmental and health hazards if batteries rupture or are incinerated. Nickel-metal hydride batteries replace the cadmium anode with a hydrogen-absorbing alloy while maintaining the nickel oxyhydroxide cathode and potassium hydroxide electrolyte. This substitution eliminates cadmium toxicity while preserving the 1.3-volt cell potential and rechargeable capability, making them an eco-friendly alternative.