galvanic cell Flashcards
galvanic cells
• Cells and batteries can be used as fixed energy storage systems
- Portability of these devices relies on their sources of electrical energy
• Cells and batteries: use spontaneous redox reactions as an energy source
- Primary cells: disposable and designed not to be recharged
› Fixed amount of oxidant and reductant, which once consumed cannot be replaced
- Secondary cells: rechargeable and designed to be reused
› Recharged by applying DC voltage that forces current through cell in opposite direction to that which occurs spontaneously during discharge
› Recharging electrolysis:
- Electrical energy forces reverse of spontaneous discharge reaction
- Regenerate oxidant and reductant originally present in charged cell
› Limit to recharge cycles before electrodes become physically degraded and longer able to be recharged
- Fuel cells: continuously produce electricity for as long as fuel is fed into the cells
› Oxidant and reductant continuously fed into cell
- Reductant typically a fuel (hydrogen)
- Oxidant usually oxygen gas
- Long life: without limit as long as oxidant and reductant supplied into cell
primary cell
Primary cells:
• Non rechargeable
- Products slowly migrate away from the electrodes or are consumed by side reactions occurring in the cell, this prevents cells from being recharged
- E.g., Alkaline cells go “flat” when most reactants are used up. Cells reaction reaches equilibrium and replacement is necessary
• Max voltage: 1.5V, slowly decreases over cell’s life
- Disadvantages: low energy to mass ratio
- Inexpensive: ideal for when low currents are required
dry cell
• Dry cell:
- Materials pose little environmental
- Anode: zinc foil
- Cathode paste: contains oxidant (MnO2)
- Electrolyte: ammonium chloride
- Graphite powder: carbon conducts electrodes through paste
- Carbon rod: conducting path for electrons moving into cathode paste
- Porous fibre allows ion flow between cathode and anode while preventing direct contact with reductant and oxidant
alkaline cell
• Alkaline cells:
- Reductant: powdered zinc
- Oxidant: MnO2
- Electrolyte: KOH
› Improves shelf life by eliminating effect of acidic ammonium ions that ultimately dissolve zinc anode of dry cell
- Non-hazardous waste
- Used for higher current flow:
› Faster RR: sustain high current flow without fall in voltage
› Higher energy density: and longer operating life
- Contain greater mass of oxidant and reductant compared to dry cell
- *energy density: energy available per unit mass of the cell
lithium cell
• Lithium cells: fire hazard
- Very high energy density
- Lithium must be fully discharged before disposal
› Consume all of lithium solid
secondary cell
Secondary cells:
• Rechargeable cells
- To recharge, cell reaction occurs in reverse
- Products converted to original reactants
- Connect cell to charger (electrical energy source)
› Has potential difference that is a little greater than potential difference of the cell
› Positive terminal of charger connected to cell’s positive electrode
› Negative electrode of charger connected to cell’s negative electrode
- Secondary cells can be recharged by connecting to an external source of electricity
- Electrical energy supplied by charger is converted into chemical energy in the cell
› To regenerate reactants: products formed in cell during discharge must remain in contact with electrodes in convertible form
car battery
• Car battery (lead acid battery):
- Relatively cheap, reliable, provide high currents, long lifetime
› Start car engine and operate car’s electrical accessories when engine isn’t running
› Once engine starts: alternator provides electrical energy to operate electrical system and recharge the battery
- Modern lead acid battery: 6 separate cells connected in series
› Positive electrode: lead grid packed with lead oxide
› Negative electrode: lead grid packed with powdered lead
› Sulphuric acid as electrolyte
• Lead acid cell:
• Lead acid cell: - Two lead grid electrodes › Cathode: powdered lead dioxide › Anode: spongy lead › Electrolyte: sulphuric acid - Powdered nature of lead and lead oxide on anode and cathode gives increased surface area (which increase RR) and contributes to ability to produce high currents - High density of lead leads to low energy density - Hazardous waste › Compounds: toxic › Sulphuric acid: corrosive
lithium ion cell
• Lithium ion cell:
- Higher cost
- Technology
- Anode: porous graphite with lithium ions interspersed between graphite layers (intercalation)
- Cathode: porous lithium metal oxide
› Electrodes are in sheet form with porous separator sheet in-between
› Electrode assembly immersed in a nonaqueous organic electrolyte containing mobile lithium ions
- Discharge: lithium ions move from anode to cathode
› To maintain charge balance
- Recharge: reversed by applying sufficient voltage
› Electrons move from cathode to anode
fuel cell
hydrogen fuel cell
Fuel cells:
• Reactants continuously supplied from external source
• Use chemical energy of hydrogen or other fuel to generate electricity
- Electricity produced as long as fuel is supplied to them
- 40-60% efficient
› Transform chemical energy directly into electrical energy
› Some modern fuel cells use waste heat to make steam, steam used to heat or operate turbine, raising efficiency of cell to 85%
- 2 compartments:
› One for hydrogen gas: anode
› Other for oxygen gas: cathode
- Gas compartment separated by two porous electrodes and electrolyte solution (potassium hydroxide)
- Only product is water and heat is given off
› Electrodes: must be conducting and porous to allow redox half equations to occur at surface
- Size of current depends on electrode SA
› Catalysts enhance RR and current produced
- Anode: platinum
- Cathode: nickel powder
- Electrode must be porous to allow reactants to diffuse through to react with ions in electrolyte
- Porous nickel electrodes that incorporate particles of a catalyst (platinum),
› KOH as electrolyte
- In fuel cell: efficient in converting available enthalpy change into electrical energy
› ~70% energy conversion efficiency
• Fuel cells seen as replacement for internal combustion engine
- Hydrogen production: 95% from fossil fuels
- Collecting biogas and converting methane into hydrogen
- Using electrical energy to convert water to hydrogen
• Storage: liquid/compressed provides less energy compared to petrol
- E.g., adsorption to surface of materials (metal hydrides)
• Advantages:
- More efficient as chemical energy is directly converted to electrical energy
› Whereas fossil fuels convert chemical to heat to mechanical to electrical
- No greenhouse gases
- Generate electricity as long as fuel is supplied
- Use variety of fuel
- Waste heat is recycled
• Disadvantaged:
- Require constant fuel supply
- Expensive electrolyte and catalyst
- Hydrogen production mainly sourced from fossil fuels
PEMFC
• PEMFC:
- 99.9999% pure hydrogen and oxygen gas from air to produce DC electrical current, water and heat
- Thin solid polymer PEM as electrolyte and electrode separator
› Allows protons to move from anode to cathode while preventing H2 or O2 moving across thin membrane
- Solid electrolyte: compact, thin cell design
› Anode and cathode consist of nanoparticles of platinum and impregnates onto porous carbon
› PE: catalyst
› Carbon conducts electrons from electrode surface
- ~1.1V each, but stacking increases voltage
phosphoric acid fuel cell
• Phosphoric acid fuel cell:
- H2 from petrol reforming, able to tolerate small carbon monoxide impurities
› O2 from air=oxidant
› DC voltage, water and heat
- Phosphoric acid electrolyte in ceramic matrix of silicon carbide
› High temp increases electrolyte conductivity and decreases catalyst poisoning from gas impurities (CO)
- Electrodes: porous carbon and platinum catalyst particles
