18.8
Batteries and Fuel Cells
A battery is a galvanic cell that is used as a source of electrical power for specific applications. Modern batteries exist in a multitude of forms to…
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.
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Q1: What is the difference between primary and secondary batteries?
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.
Q2: How does a dry cell battery generate electrical current?
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.
Q3: Why are lead-acid batteries commonly used in automobiles?
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.
Q4: What makes lithium-ion batteries ideal for portable electronic devices?
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.
Q5: How do fuel cells differ from traditional batteries in energy production?
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.
Q6: What are the advantages of alkaline batteries over dry cells?
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.
Q7: Why have nickel-cadmium batteries been replaced by nickel-metal hydride batteries?
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.