Electrochemical Cells and Batteries

Study Notes

Electrochemical Cells

Electrochemical cells are devices that convert chemical energy into electrical energy, or vice versa.
Galvanic cells generate an electrical current from a spontaneous chemical reaction.
Electrolytic cells use electrical current to drive a non-spontaneous chemical reaction.
All electrochemical cells consist of two electronic conductors and an ionic conductor (electrolyte).

Batteries

Batteries are galvanic cells that function as self-contained sources of direct electric current.
Dry Cell and Alkaline Batteries:
- Consist of a zinc anode, graphite cathode, a moist paste electrolyte containing ammonium and zinc chlorides, and manganese dioxide and carbon black.
- Produce approximately 1.5V.
- The electrolyte is a viscous paste rather than a liquid solution.
Lead Acid Battery:
- Consists of a lead anode, lead dioxide cathode, and sulfuric acid electrolyte.
- Made of six identical cells connected in series, producing a total of 12V.
- Commonly used for starting automobiles and are rechargeable.
- The state of charge can be assessed by measuring the solution's density using a hydrometer.
Nickel Cadmium Battery:
- Consists of a nickel cathode, cadmium anode, and potassium hydroxide electrolyte.
- Commonly used in calculators and portable power due to their rechargeable nature.
Lithium Batteries:
- Anode consists of lithium or lithium atoms inserted into a graphite electrode.
- Cathode typically contains a lithium salt (e.g. $LiClO_4$) in an organic solvent.
- Commonly used in cell phones, laptops, and cameras.
- Lightweight and high voltage (~3.0V).

Fuel Cells

Fuel cells generate electricity directly from the continuous supply of fuel and oxidant.
Characterized by:
- High efficiency.
- Water as the only waste product.
- The overall reaction is the conversion of hydrogen and oxygen to water.

Corrosion

Corrosion is the deterioration of a metal through electrochemical processes.
Rusting is the specific term for iron corrosion, occurring in the presence of water and oxygen.
Types of Corrosion:
- Uniform corrosion: Occurs evenly across the exposed surface.
- Galvanic corrosion: Occurs at the junction of two dissimilar metals in an electrolyte.
- Crevice corrosion: Occurs in crevices or under coatings where oxygen concentration is low.
- Pitting corrosion: Forms localized pits in metals that are typically passive.
- Intergranular corrosion: Occurs along grain boundaries of the metal.
- Dealloying corrosion: Selective removal of one element from an alloy.
- Stress corrosion: Occurs when tensile stress is combined with a corrosive environment.
- Concentration cell corrosion: Similar to galvanic corrosion but driven by different oxygen concentrations.
Factors Affecting Corrosion:
- Nature of the metal: Position in the galvanic series, overvoltage, purity, surface films, passivity, etc.
- Nature of the environment: Temperature, humidity, impurities, pH, etc.
Corrosion Control:
- Physical protection: Barriers like paint, grease, oil, plastic coatings, and galvanizing.
- Chemical protection: Cathodic or sacrificial protection, electroplating, anodizing, and tin plating.
Methods to Prevent Corrosion:
- Passivation: Formation of a protective oxide layer.
- Alloying: Creating alloys with improved corrosion resistance.
- Coating with a less active metal: e.g., coating with zinc (galvanizing).