chem

• Electrochemical cells are systems that incorporate a redox reaction to produce or utilize electrical energy
• Types of electrochemical cells

  • Voltaic Cell or Galvanic Cell
  • Electrolytic Cell

  
• Voltaic Cell or Galvanic Cell


  Chemical energy to electrical energy, Spontaneous reaction

  
• Electrolytic Cell


  Electrical energy to chemical energy, Non-spontaneous reaction
• Oxidation is the loss of electrons, and Reduction is the gain of electrons
• Oxidizing agent


  The species that does the oxidation, and itself gets reduced
• Reducing agent


  The species that does the reduction, and itself gets oxidized
• The purpose of the salt bridge is to allow ions to flow through both compartments and complete the circuit
• Cell potential (Ecell)


  The measure of the potential difference between two half cells in an electrochemical cell

  
• For spontaneous cell operation, the cell potential (Ecell) must be positive
• Standard cell potential (E0cell)

  The potential measured at a specified temperature (usually 298 K) with no current flowing and all components in their standard states
• A spontaneous reaction has a negative free energy change (DG < 0), and a spontaneous electrochemical reaction has a positive cell potential (Ecell > 0)
• Energy density is the amount of energy in a given mass (or volume)
• Power density


  The amount of power in a given mass
• A high energy density does not necessarily mean a high power density
• If a system has a high power density, than it can also have very high energy density. However, it can release the energy quickly
• Examples of energy density vs power density

  • Starting battery
  • House battery
• Primary batteries cannot be recharged, while secondary batteries can be recharged
• Alkaline battery


  • Anode is a zinc can, Cathode is an inactive graphite rod, Electrolyte is a basic paste

  
• Mercury and silver batteries

  • Anode is a zinc container, Cathode is HgO or Ag2O, Electrolyte is KOH
• Lead-acid battery


  • Has six cells connected in series, Each cell delivers about 2.1 V, Total voltage is about 12 V
• Silver battery


  • Uses Ag2O
  • Uses a steel can around the cathode
• Mercury battery


  • Uses a steel can around the cathode
• Battery construction


  1. Solid reactants are compacted with KOH

  2. Separated with moist paper
• The half-reactions for silver battery
• The half-reactions for mercury battery
• Secondary (Rechargeable) Batteries


  In contrast to primary batteries, a secondary, or rechargeable, battery is recharged when it runs down by supplying electrical energy to reverse the cell reaction and re-form reactant
• Secondary batteries


  • Voltaic cells are periodically converted to electrolytic cells to restore non-equilibrium concentrations of the cell components
• Widely used secondary batteries


  • Common car battery
  • Nickel–metal hydride battery
  • Lithium-ion battery
• Lead-Acid Battery


  • Six cells connected in series, each delivers about 2.1 V for a total of about 12 V
  • Each cell contains two lead grids loaded with the electrode materials: high-surface-area (spongy) Pb in the anode and high-surface-area PbO2 in the cathode
  • Grids are immersed in an electrolyte solution of ~4.5 M H2SO4
• Lead-Acid Battery Discharge


  1. Generates electrical energy as a voltaic cell

  2. Both half-reactions produce Pb2+ ions, one through the oxidation of Pb, the other through the reduction of PbO2

  3. Pb2+ forms PbSO4(s) at both electrodes by reacting with HSO4-
• Lead-Acid Battery Recharge


  Uses electrical energy as an electrolytic cell, and the half-cell and overall reactions are reversed
• Lead-Acid Battery Advantages


  • Stable, reliable performance and good applicability
  • Dilute sulfuric acid is used as electrolyte without flammability, designed at normal pressure or low pressure with good safety
  • High working voltage, wide operating temperature range, suitable for hybrid electric vehicle (HEV) and other high rate discharge applications
  • Large-capacity battery technology is mature and can be made into batteries with thousands of ampere-hours, providing convenience for large-scale energy storage
• Lead-Acid Battery Cost Advantage

  • Cheapest secondary batteries, with unit energy costs about a third of those of lithium-ion or nickel-hydrogen batteries
• Lead-Acid Battery Recycling Advantage


  • Simple composition, mature regeneration technology and high recovery value, the easiest battery for recycling
• Lead-Acid Battery Disadvantages


  • Low energy density, only about 1/3 of that of lithium ion batteries and 1/2 of that of nickel-hydrogen batteries
  • Short cycle life, about 1/3 of that of lithium-ion batteries
  • Lead pollution risks exist in the industrial chain, lead accounts for more than 60% of the battery quality and more than 80% of the total lead used globally
  • Older batteries had safety concerns with H2 and O2 generation during recharging
• Lithium ion battery


  Secondary lithium-ion battery has an anode of Li atoms that lie between sheets of graphite (designated LixC6), cathode is a lithium metal oxide, such as LiMn2O4 or LiCoO2, electrolyte is 1 M LiPF6 in an organic solvent, such as dimethyl carbonate (often mixed with methylethyl carbonate)
• Lithium-Ion Battery Cell Reactions (discharge)

  Anode: LixC6 → xLi+ + xe- + 6C

  Cathode: LiMO2 + xLi+ + xe- → Li1+xMO2

  Overall: LixC6 + LiMO2 → 6C + Li1+xMO2
• Advantages of Li-ion battery

  • High energy density
  • Low self-discharge rate
  • Low maintenance
  • No requirement for priming
  • Variety of types available
• Disadvantages of Li-ion battery

  • Require protection from overcharging and over-discharging
  • Suffer from ageing
  • Limitations on transportation
  • Higher cost
  • Immature technology