10.4 Batteries and fuel cells

Batteries and fuel cells are crucial energy storage and conversion devices in electrochemistry. They harness chemical reactions to produce electrical energy, powering everything from smartphones to vehicles. Understanding their principles is key to grasping the practical applications of electrochemical cells.

These devices differ in their structure and operation. Batteries store energy in chemical form, releasing it as needed, while fuel cells continuously convert chemical energy to electricity as long as fuel is supplied. Both play vital roles in our increasingly electrified world.

Primary vs Secondary Batteries

Rechargability and Energy Density

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• Primary batteries are not rechargeable and are designed for single use, while secondary batteries can be recharged multiple times
• Primary batteries have a higher energy density and longer shelf life compared to secondary batteries
  • Higher energy density means they can store more energy per unit mass or volume
  • Longer shelf life allows for storage without significant self-discharge
• Primary batteries are more expensive in the long run due to their disposable nature

Recharging Secondary Batteries

• Secondary batteries, such as lead-acid and lithium-ion batteries, can be recharged by applying an external electrical current
  • The external current reverses the electrochemical reactions that occur during discharge
  • During recharging, the anode becomes the cathode and vice versa
• Examples of secondary batteries include:
  • Lead-acid batteries (used in automobiles)
  • Lithium-ion batteries (used in portable electronics and electric vehicles)
  • Nickel-cadmium batteries (used in power tools and emergency lighting)

Battery Types and Components

Common Battery Types

• Lead-acid batteries consist of lead and lead dioxide electrodes immersed in a sulfuric acid electrolyte
  • During discharge, lead sulfate forms on both electrodes, and the electrolyte becomes more dilute
  • Rechargeable lead-acid batteries are used in automotive applications and backup power systems
• Lithium-ion batteries use a lithium compound as the positive electrode (cathode) and a carbon-based material as the negative electrode (anode)
  • Lithium ions move from the anode to the cathode during discharge and vice versa during charging
  • Lithium-ion batteries are widely used in portable electronics, electric vehicles, and grid storage
• Alkaline batteries, such as zinc-manganese dioxide batteries, use an alkaline electrolyte (usually potassium hydroxide)
  • They have a zinc anode and a manganese dioxide cathode
  • During discharge, the zinc is oxidized, and the manganese dioxide is reduced
  • Alkaline batteries are commonly used in household devices (remote controls, toys, and flashlights)

Battery Components and Their Functions

• The electrolyte in a battery serves as a medium for ion transfer between the electrodes
  • It helps maintain charge balance by allowing the flow of ions while preventing the flow of electrons
  • The electrolyte can be liquid (aqueous or non-aqueous), gel, or solid
• The separator in a battery prevents direct contact between the electrodes
  • It is a porous material that allows ion flow through the electrolyte while preventing short circuits
  • Common separator materials include polymers, ceramics, and glass fiber
• Current collectors are conductive materials (metal foils or meshes) that facilitate electron transfer from the electrodes to the external circuit
  • The anode current collector is typically made of copper, while the cathode current collector is usually made of aluminum

Fuel Cell Structure and Function

Basic Fuel Cell Components

• A fuel cell consists of an anode, a cathode, and an electrolyte
  • The anode is where the fuel (usually hydrogen) is oxidized
  • The cathode is where the oxidant (usually oxygen) is reduced
  • The electrolyte allows the transfer of ions between the electrodes while preventing direct mixing of the fuel and oxidant
• Common electrolytes in fuel cells include:
  • Polymer electrolyte membranes (PEM) used in low-temperature fuel cells
  • Molten carbonate electrolytes used in high-temperature fuel cells
  • Solid oxide electrolytes used in solid oxide fuel cells (SOFC)

Hydrogen Fuel Cell Operation

• Hydrogen fuel cells produce electricity, water, and heat as byproducts
  • At the anode, hydrogen is oxidized, releasing electrons and producing protons (H+)
  • The protons pass through the electrolyte to the cathode
  • At the cathode, the protons combine with oxygen and the electrons from the external circuit to form water
• The overall reaction in a hydrogen fuel cell is: $2H_2 + O_2 \rightarrow 2H_2O$
• Hydrogen fuel cells are used in various applications, including:
  • Stationary power generation
  • Fuel cell vehicles (FCVs)
  • Portable power devices

Batteries vs Fuel Cells

Energy Density and Capacity

• Batteries generally have a lower energy density compared to fuel cells
  • Energy density is the amount of energy stored per unit mass or volume
  • Lower energy density means batteries are heavier and larger for the same energy output
• Fuel cells have a higher energy density and can provide continuous power as long as fuel is supplied
  • Batteries have a limited capacity and need to be recharged or replaced when depleted
  • The capacity of a battery is determined by the amount of active material in the electrodes

Environmental Impact and Applications

• Some batteries, such as lithium-ion batteries, can have a significant environmental impact
  • Mining of rare earth metals for battery production can lead to environmental degradation
  • Improper disposal of batteries can result in soil and water contamination
• Fuel cells have a lower environmental impact, especially when using renewable hydrogen sources
  • Hydrogen can be produced from water electrolysis using renewable energy (solar, wind)
  • The byproducts of hydrogen fuel cells are water and heat, which have minimal environmental impact
• Batteries are more suitable for small-scale and portable applications
  • Examples include consumer electronics (smartphones, laptops) and electric vehicles
• Fuel cells are more appropriate for larger-scale stationary power generation and some transportation applications
  • Examples include backup power systems, combined heat and power (CHP) plants, and fuel cell buses

Electrochemical Reactions in Batteries and Fuel Cells

Oxidation and Reduction Reactions

• In a battery or fuel cell, the anode is the site of oxidation, where electrons are released
  • Oxidation involves the loss of electrons and an increase in oxidation state
  • The species undergoing oxidation is called the reducing agent or reductant
• The cathode is the site of reduction, where electrons are consumed
  • Reduction involves the gain of electrons and a decrease in oxidation state
  • The species undergoing reduction is called the oxidizing agent or oxidant
• The direction of electron flow in the external circuit is always from the anode to the cathode
  • Electrons move from a higher potential energy (anode) to a lower potential energy (cathode)
  • In the electrolyte, ions flow in the opposite direction to maintain charge balance

Cell Potential and Electrical Energy Production

• The electrical energy produced by a battery or fuel cell is a result of the redox reactions occurring at the electrodes
  • The difference in the reduction potentials of the half-reactions determines the cell potential (voltage)
  • The standard cell potential ($E^°$) can be calculated using the standard reduction potentials of the half-reactions: $E^°_{cell} = E^°_{cathode} - E^°_{anode}$
• The Nernst equation can be used to calculate the cell potential under non-standard conditions
  • It takes into account the concentrations of the reactants and products
  • The Nernst equation for a general redox reaction is: $E = E^° - \frac{RT}{nF} \ln \frac{[Products]}{[Reactants]}$
• Faraday's laws of electrolysis relate the amount of electrical charge passed through an electrochemical cell to the number of moles of substances oxidized or reduced
  • The first law states that the mass of a substance altered at an electrode is directly proportional to the quantity of electricity transferred
  • The second law states that the mass of a substance altered at an electrode is directly proportional to its equivalent weight