Chapter 9 - Electrochemical Cells Flashcards
What are the advantages of Fuel Cells?
- MORE EFFICIENT
- Converts chemical energy DIRECTLY to electrical energy (70%)
- No need for a series of energy conversions that take place in power stations.
eg. chemical energy, heat energy, mechanical energy, electrical energy.
- 25-30% - Produces Water and Heat as BYPRODUCTS
- NO greenhouse gases (CO2, SO2) RELEASED - Will Generate electricity for as LONG AS THE FUEL IS SUPPLIED
- conventional batteries need to be recharged or replaced - Fuel cells can use a VARIETY OF FUELS
- Electricity can be GENERATED ON-SITE and USERSare NOT RELIANT ON CONNECTION TO AN ELECTRICTY GRID.
- WASTE HEAT can be USED to HEAT WATER for a HOT WATER SYSTEM or PROVIDE HEATING for a HOME DURING WINTER.
What are the disadvantages of Fuel Cells?
- REQUIRES a CONSTANT SUPPLY OF FUEL
- EXPENSIVE - still DEVELOPING TECHNOLOGY, and are NOT BEING MADE IN LARGE NUMBERS
- so no economies of scale as there are in other industries - Use of FUEL CELLS IN TRANSPORT WILL REQUIRE AN EXTENSIVE NETWORK OF HYDROGEN FILLING STATION BEFORE IT CAN BECOME WIDESPREAD.
- HYDROGEN USED IN FUEL CELLS are SOURCED FROM FOSSIL FUELS.
- involves ENERGY LOSSES AND GENERATES GREENHOUSE GASES. - SIGNIFICANT ISSUES ASSOCIATED WITH THE STORAGE OF HYDROGEN FUEL.
- as compressed or liqoiud H
- provides less energy per L stored
- Highly flammable
What is a Fuel Cell?
What?
Red. nd Oxd.
Life?
POROUS ELECTRODES?
Type of Galvanic Cell
- In these cells, the oxidant and reductant are CONTINUOUSLY SUPPLIED to the cell.
REACTANTS ARE NOT STORED IN THE FUEL CELL.
REDUCTANT typically is a FUEL (Combustible substance)
- EG. H, CH4, METHANOL
OXIDANT usually OXYGEN GAS
Has a very LONG LIFE
THEORETICALLY can OPERATE WITHOUT LIMIT as long as oxidant and reductant are supplied.
CONTAINS POROUS ELECTRODES - that allow reactants to diffuse through them to react with ions in the electrolyte.
- OFTEN CONTAIN CATALYSTS TO INCREASE THE RATE OF REACTION.
Example of a Fuel Cell
How does it work?
ALKALINE Hydrogen-Oxygen Fuel Cell
Used in Apollo program
How does it work?
- Has 2 compartments
- one for H2 gas
- one for O2 gas - 2 half cells are Separated from each other BY 2 POROUS ELECTRODES AND AN ELECTROLYTE SOLUTION
- Electrode at H compartment is the ANODE
Electrode at the O compartment is the CATHODE - H and O circulate under pressure OVER POROUS NICKLE ELECTRODES that incorporate particles of a Catalyst (Pt)
- Potassium Hydroxide solution ACTS as the ELECTROLYTE SOLUTION, operated at 250 degrees Celcius.
- Each cell produces 1V
- higher voltages are obtained by CONNECTING A NUMBER OF FUEL CELLS IN SERIES to form A BATTERY or FUEL CELL STACK. - THE ONLY BYPRODUCT IS WATER, HEAT IS GIVEN OFF.
- expelled during operation of the cell - Water formed in the Anode can be seen leaving the cell as steam mixed with the circulating H2 gas.
Why is the type of electrode important for a fuel cell?
The efficiency of the cell is determined by the type of electrode.
- size of current drawn from the fuel cell depends on the surface area of the electrodes.
- Must be CONDUCTING
- Must be Porous
- to allow H and O to come in contact with the ions in the electrolyte solution
- to allow redox half equations to occur at their surface
What is a Primary Cell? What? Why not rechargeable? Negative? Found where
A non-rechargeable, disposable, single-use = galvanic cell.
- Contains a Fixed amount of OXIDANT AND REACTANT which once CONSUMED and Reached Equilibrium Cannot be Be replaced.
- goes ‘flat’
Prevented from being RECHARGED
- Because the PRODUCTS SLOWLY MIGRATE AWAY FROM FROM THE ELECTRODES
- Or are CONSUMED by SIDE REACTIONS occurring IN THE CELL
Negative
- Unrechargeable
- Max Volt 1.5
- Pose negligible environmental impact due to the material used (Zn, MnO2, NH4Cl and C)
Inexpensive and ideal for portable use such as in remotes, calculators ect.
Example of Primary Cell and how it works
Dry Cell, Lithium Cell
Alkaline cell
How it works
1. Zinc is Reducing Agent
Manganese Dioxide is Oxidising agent (MnO2)
- Ammonium Hydroxide = KOH paste as an Electrolyte solution
(improves shelf life eliminating effects of acidic ammonium ions that ultimately dissolve zinc anode of a dry cell) - Zinc Anode = powdered zinc form, rather than foil form than dry cell
- faster reaction rates - Typically Greater mass of reductant, Zn and Oxidant, MnO2 than dry cell
- higher density and longer operating life
What is a Secondary Cell?
Rechargeable galvanic cells
AKA ACCUMULATORS
found in car batteries, laptops, phones and cameras.
- Can be Rechargeable by APPLY a DC Voltage (Through a Charger) that FORCES CURRENT THROUGH THE CELL in an OPPOSITE DIRECTION to that which OCCURS SPONTANEOUSLY During its Discharge.
How are Secondary cells Recharged?
Recharging is a Process of Electrolysis where electrical energy is USED TO FORCE SPONTANEOUS DISCHARGE REACTIONS to be REVERSED and in the PROCESS REGERNATE THE OXIDANT AND REDUCTANT ORIGINALLY PRESENT IN A CHARGED CELL.
The charger’s potential difference is a little greater than the potential difference of the cell.
- Positive terminal of the charger is connected to the cell’s positive electrode and a negative electrode is connected to the negative end of the charger.
Although rechargeable there is a limit to the no. of recharge cycles they will undergo before electrodes become physically degenerated and no longer able to be recharged.
Examples of Secondary cells and how it works
Lithium-ion cell,
LEAD-ACID CELL
How it works
1. A single lead-acid cell has 2 GRID electrodes, immersed in an Electrolyte of approx. 4.5 molL-1 of H2SO4
- ANODE grid contains SPONGY LEAD
CATHODE GRID is PACKED with POWDERED LEAD DIOXIDE, PbO2 - During Discharge spongy, Pb at the anode is oxidised to Pb+2 which immediately precipitates onto the electrode as INSOLUBLE PbSO2(s)
- Simltaneoulsy at the cathode, PbO2 is reduced to Pb+2 which also precipitates onto electrode as insoluble PbSO4 solid.
- Powdered nature of the Pb and PbO2 on the anode and cathode gives these reagents high SA (increases reaction rate)
- hence cell is able to produce high currents
How is a Lead-acid cell Recharged?
- Apply a Direct Current of slightly greater than 2 V to each cell.
- The recharging current is made to flow in a direction opposite to the discharge current.
- This has the effect of reversing the anode and cathode half reactions that occur during discharge
hence regenerated reducing agent and oxidizing agnet.
What is the drawback of lead-acid cells?
The high density of lead
(11.3kg L-1)
Which results in the cells having a LOW ENERGY DENSITY OF 150Kg-1.
Is an issue where the cells are used in TRANSPORT SITUATIONS.
These batteries must be handled with CARE AS lead and its compounds are acutely and chronically toxic, whilst sulfuric acid is corrosive.
This battery is classified as HAZARDOUS WASTE.
Explain Lead-acid cells in cars.
Car battery = contains 6 secondary cells connected in a series to produce 12V (2 V from each cell)
It can undergo many DISCHARGE-RECHARGE CYCLES over a period of several years before the internal components eventually lose their structure and the cell no longer functions.
Used to Start the car’s engine and to operate the car’s electrical systems when the engine is not running.
- ONCE THE ENGINE STARTS, AN ALTERNATOR WHICH IS RUN BY THE ENGINE PROVIDES ELECTRICAL ENERGY TO OPERATE THE CARS’ ELECTRICAL SYSTEM AND RECHARGE THE BATTERY.
- WHEN RECHARGED IT ACTSVAS AN ELECTROLYTIC CELL (electric energy transformed into chemical)
The ability to be recharged and to deliver VERY LARGE CURRENTS makes it ideal for running the electric starter of motor cars and trucks.
Lifts, solar power storage systems, energy power systems also use this.
