Fuel Cells Flashcards
What does hydrogen fuel cell use
Continuous supply of hydrogen and oxygen from the air to generate a continuous current
Waste product of hydrogen fuel cell
Water- seen as relatively environmentally friendly
Downsides of hydrogen fuel cells
High flammability of hydrogen, expensive to produce, alkaline uses expensive platinum electrode
Overall equation for hydrogen fuel cell
H2 + 1/2O2 reversible to H2O
Half equations for acidic conditions hydrogen fuel cell
Anode: H2 reversible to 2H+ + 2e-
Cathode: 1/2O2 + 2H+ + 2e- reversible to H2O
What is emf of all hydrogen fuel cells
1.23V
Half equations for hydrogen fuel cell in alkaline electrolyte
Anode: H2 + 2OH- reversible to 2H2O + 2e-
Cathode: O2 + 2H2O + 4e- reversible to 4OH-
What is a storage cell
An electrochemical cell that is based on reversible chemical changes so it can be recharged by an external electricity supply
What is a battery
Two or more electrochemical cells connected in series
Why is the formation of insoluble lead (II) sulfate a problem for lead-acid cells
If cells discharged for long periods the precipitate becomes coarser and thicker and the process cannot be reversed when the cells are recharged
What occurs at negative terminal in lead-acid cell
Lead gives up electrons forming Li(II) ions, in presence of sulfate ions they precipitate to form lead (II) sulfate
What occurs at positive terminal of lead-acid cell
Is lead coated with lead(IV) oxide, during discharge the lead (IV) oxide reacts with H+ ions in sulfuric acid electrolyte and takes electrons and lead (II) ions precipitate as lead (II) sulfate
Benefits of lithium cells
The metal has a low density so cells based on lithium electrodes can be relatively light, lithium also very reactive meaning the electrode potential of lithium half cell is relatively high and each cell has a large emf
Limitations of lithium cells
Lithium so reactive it readily combines with oxygen in the air forming layer of non-conducting oxide on surface and reacts rapidly with water
What is a fuel cell
Electrochemical cell continuously supplied with fuel and oxidising agent, produces electric power from a fuel directly without having to burn it
Where are hydrogen and oxygen pumped in
Hydrogen pumped into side with negative electrode, oxygen into side with positive electrode
Where do electrons and hydrogen ions flow in hydrogen fuel cell
Into the external circuit as an electric current whereas hydrogen ions migrate through the electrolyte
Process of how hydrogen fuel cell works
Hydrogen gas flows into negative terminal where H2 split into single H atoms in presence of catalyst, H atoms then lose electrons and form H+ ions
-oxygen flows onto positive terminal where nickel catalysed the splitting into single oxygen atoms which combine with H+ from electrolyte and electrons to form water
Are hydrogen fuel cells 100% efficient
No, usually around 70%
How is storage cell recharged
Electric current passes through it in the opposite direction to the current that the cell produces, example of electrolysis
