module 5- fuel and storage cells Flashcards
What are the key characteristics of fuel cells
- Uses energy from the reaction between a fuel and oxygen to produce a voltage
- can keep operating providing the fuel and oxygen are supplied as gases continuously (don’t need recharging)
Whats the key difference between a primary storage cell and secondary storage cell
a secondary cell can be recharged
How do electrode potentials apply to storage cells
the bigger the difference between the two electrode potentials of the two chemicals, the bigger the voltage created (anti-clockwise rule and flow of electrons from high conc to low conc)
How do storage cells differ to fuel cells
In fuel cells, the fuels are supplied externally as gases
Equations for ACIDIC fuel cell
Redox systems
2H+ + 2e-> H2. e=0.00v
0.5O2 + 2H+ + 2e->H20. e=+1.23v
Oxidation at anode H2 -> 2H+ + 2e-
reduction at cathode 0.5O2 + 2H+ + 2e- -> H20
overall H2 + 0.5O2 -> H20
Equations for ALKALINE fuel cell
Redox
2H20 + 2e- -> H2 + 2OH- e= -0.83v
0.5 O2 + H20 + 2e- ->2OH- e= +0.40v
oxidation at anode
H2 + 2OH- -> 2H20 + 2e-
Reduction at cathode
0.5O2 + H20 + 2e- -> 2OH-
overall H2 + 0.5 02 -> H20
Benefits of electrochemical cells
- No moving parts so more efficient, safer
- H2-02 cells offer alternative to the use of fossil fuels
- Avoide producing pollutants like CO2
Risks of cells
- Lithium ion batteries have such a high energy density that they can become unstable at high temperatures and catch fire (from impurities causing them to short circuit)
- Lithium v reactive, must be disposed of carefully
- some chemicals used eg cadmium is very toxic and must be disposed of carefully
what are the standard electrode potentials of the 4 equations in a hydrogen oxygen cell (acid and alkaline)
acid-
oxidation at anode = 0.00v
reduction at cathode= +1.23v
alkaline
oxidation at anode= -0.83v
reduction at cathode = +0.40v
