Fuel cells

• modern cells and batteries:

  • key requirement is for the 2 electrodes to have different electrode potentials
• cells can be divided into 3 main types:

  • primary
  • secondary
  • fuel cells
  • modern cells and batteries are part of a fast moving technological field and much research is being carried out to develop more efficient and more effective portable sources of electrical energy
• Primary cells:

  • non-rechargeable
  • designed to be used once only
  • when in use - chemical energy is produced by oxidation and reduction at the electrodes
  • however the reactions cannot be reversed
  • eventually the chemicals will be used up, voltage will fall, battery will go flat, cell will be discarded or recycled
• • primary cells still find use for low-current, long-storage devices such as wall clock or smoke detectors
• • most modern primary cells are alkaline based on zinc and manganese dioxide, Zn/ MnO2, and a potassium hydroxide alkaline electrolyte
• Secondary cells:

  • rechargeable
  • unlike primary cells, the cell reaction producing electrical energy can be reversed
  • the chemicals in the cell are then regenerated and the cell can be used again
• common examples of secondary cells include:

  • lead-acid batteries used in car batteries
  • nickel-cadmium, NiCd, cells and nickel-metal hydroxide, NiMH - the cylindrical batteries used in radios, torches
  • lithium-ion and lithium-ion polymer cells used in modern appliances - laptops, tablets, cameras, phones - also being developed for cars
• Lithium-ion and lithium-ion polymer cells:

  • extremely popular
  • lithium is a light metal - translated into very high energy density when used in lithium-ion batteries
  • can be a regular shape, as in camera cells and also as lithium-ion polymer pouch batteries, with an internal salt bridge made of micro-porous polymer covered in electrolytic gel
  • bc the solid polymer is flexible, flexible batteries can easily be formed into various shapes and sizes, ideal for fitting round other components in a tightly packed and lightweight laptop, tablet, phone
• when a lithium-ion cell charges and discharged, electrons move through the connecting wired to power the appliance, while Li+ ions move between the electrodes within the cell:

  • the neg electrode is made of graphite coated in lithium metal
  • the pos electrode is made out of a metal oxide, typically CoO2
• Simplified equations at each electrode:

  • neg: Li = Li+ + e-
  • pos: Li+ +CoO2 + e- = LiCoO2
• when fully-charged, a lithium cell has a voltage of 4.2V, but this drops with use - typical operating voltage is about 3.7-3.8 V
• Lithium-ion cells do have some limitations:

  • they can become unsustainable at higher temperatures
  • rare occasions have ignited mobiles and laptops
  • care must be taken in their recycling - lithium is very reactive
• The structure of a typical lithium-ion rechargeable battery:

  • the battery consists of a positive electrode and a negative electrode with a layer separating them
  • when in use, electrons flow from the neg electrode to the pos electrode and at the same time, Li+ ions travel from the anode to the cathode - to keep the electrical charge balance in the cell
  • when charging, the process is reversed and lithium ions are transferred back to the negative electrode
• Fuel cells:

  • a fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage
  • the fuel and oxygen flow into the cell and the products flow out - the electrolyte remains in the cell
  • fuel cells can operate continuously provided that the fuel and oxygen are supplied into the cell
  • fuel cells do not have to be recharged
• many different fuels can be used, but hydrogen is the most common

  • Hydrogen fuel cells produce no carbon dioxide during combustion, with water being the only combustion product
  • fuel cells using many other hydrogen-rich fuels, such as methanol are also being developed
• Hydrogen fuel cells have either an alkali or acid electrolyte