Cells & Fuel Cells PDF - A Level Chemistry Past Paper

A LEVEL CHEMISTRY 3.1.11 ELECTRODE POTENTIALS THE LITHIUM ION CELL Lithium ion cells (batteries) are used in my battery powered devices. The benefit of these is that, unlike regular cells, the reactions are revers...

A LEVEL CHEMISTRY 3.1.11 ELECTRODE POTENTIALS THE LITHIUM ION CELL Lithium ion cells (batteries) are used in my battery powered devices. The benefit of these is that, unlike regular cells, the reactions are reversible so they can be recharged. The basic principle behind these is the same as any cell. Two different half cells produce a potential difference and the flow of electrons (current) is used to power the device. HOW IT WORKS ⊖ Electrolyte ⊕ C(s) Cu Li Li+ Li+[CoO2-] Al(s) Graphite e- e- e- DEVICE e- When the cell is in use, at the negative electrode, the Li is oxidised to Li+. The graphite acts as the electrode and the electrons transfer through the device to power it. Ignore the copper. It just acts as a contact for wiring the battery in. The Li+ ions then migrate through an electrolyte to the positive electrode, where they combine with CoO2 and an electron to form Li+[CoO2]-. The opposite is true when the cell is recharging. Learn these reactions as they do come up in exam questions! When in Use When Recharging ⊖ Li → Li+ + e- Li+ + e- → Li ⊕ Li+ + e- + CoO2 → Li+[CoO2]- Li+[CoO2]- → Li+ + e- + CoO2 AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.11 ELECTRODE POTENTIALS FUEL CELLS Fuel cells work in a similar way to chemical cells in that they produce a current which powers the device. However, a “fuel” used to generate that flow of electrons. e.g. an hydrocarbon or hydrogen. So long as fuel is constantly supplied, the cell will produce a current. This provides an advantage over chemical fuel cells as they don’t need to be recharged. HYDROCARBON FUEL CELLS e- DEVICE e- e- e- C2H5OH Electrolyte + H + O2 H2O ⊖ H+ ⊕ H+ CO2 At the negative electrode: C2H5OH + 3H2O → 12H+ + 12e- + 2CO2 The electrons flow through the device to power it. The H+ ions travel through the electrolyte to the other half cell, where they react with oxygen… At the positive electrode: 12H+ + 12e- + 3O2 → 6H2O When you combine the two half equations you get the same equation as you would expect for the combustion of the fuel… C2H5OH + 3O2 → 3H2O + 2CO2 AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.11 ELECTRODE POTENTIALS THE HYDROGEN FUEL CELL This “alkaline” hydrogen fuel cell works in the same way as a hydrocarbon fuel cell, except Hydrogen gas is the “fuel”. e- DEVICE e- e- e- Electrolyte H2 OH- O2 ⊖ OH- ⊕ OH- H2O At the negative electrode: 2H2 + 4OH- → 6H2O + 4e- The electrons flow through the device to power it. Meanwhile, OH- ions travel through the electrolyte from the other half cell, where… At the positive electrode: O2 + 2H2O + 4e- → 4OH- When you combine the two half equations you get the same equation as you would expect for the combustion of hydrogen… 2H2 + O2 → 2H2O AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.1.11 ELECTRODE POTENTIALS BENEFITS & RISKS OF CELLS CELL TYPE BENEFITS RISKS - Rechargeable, so less - Lithium mining has a waste compared to non- negative impact on rechargeable batteries ecosystems LITHIUM ION - Recharge quickly - Lithium is very reactive - Produce a high voltage and toxic - High amounts of energy - Expensive to produce stored - Do not need to be - Produce CO2, a recharged greenhouse gas - Work for as long as fuel is - Use a non-renewable supplied resource (fossil fuels) - Relatively cheap to produce compared to Li HYDROCARBON ion batteries FUEL CELL - Maintain a constant voltage (compared to chemical cells) - Less heat energy is lost compared to the standard alkane combustion engine - Use a renewable fuel - Expensive to produce source (hydrogen) - Toxic chemical used in - Only by-product formed is production H2O - H2(g) is expensive to - Maintain a constant produce HYDROGEN voltage (compared to - One source of H2(g) is FUEL CELL chemical cells) fossil fuels! Non- - Less heat energy is lost renewable compared to the standard - Storing H2(g) can be hydrogen combustion difficult & dangerous engine AQA www.chemistrycoach.co.uk © scidekick ltd 2024