Questions

[Destiny]

Ok, so I have absolutely no idea how on earth I'm meant to derive formulae for fuel cells. 
Guys, please tell me how I am supposed to approach these questions?
Types of fuel cell are usually characterised by their electrolyte, temperature of operation,
transported ion and fuel. In the March 2011 edition of ‘Technology Review’ published by
MIT the following description was given of Solid Oxide Fuel Cells (SOFC).
Solid-oxide fuel cells, which can run a variety of fuels including diesel or natural gas, bring
in oxygen from the air to be reduced at the cathode, and then pass the oxygen [oxide] ions
through a solid-oxide electrolyte membrane to the anode, where the fuel is oxidized to
produce electrons that are drawn out of the device. Their high operating temperatures are
dictated by the fact that the ions move more quickly through the electrolyte at higher
temperatures (500-1000 degrees C).

d. Assuming the fuel used in an SOFC is methane, write balanced half-equations for the
reactions at
i. the (-) electrode (1 mark)

ii. the (+) electrode (1 mark)

[Nobby]

So you know the fuel is methane, and the combustion equation for this is:
CH₄ + 2O₂ --> CO₂ + 2H₂O

The text says that "oxygen from the air to be reduced at the cathode", so you know that O₂ will be reduced to O^2-:
O₂ + 4e^- --> 2O^2-

Now all you need to do to ascertain the other half equation is subtract this reduction half equation (multiplied by 2 to balance electrons) from the overall (combustion) equation, and you get:
CH₄ + 4O^2- --> CO₂ + 2H₂O + 8e^-

And, since the cell is discharging, the anode is negative and the cathode positive. Since reduction occurs at the cathode and oxidation at the anode, you can now place each reaction at the appropriate electrode.


...I think that's right

[Destiny]

Ah, so do we just pretend it's combustion? Because my crappy teacher says fuel cells are redox rather than combustion reactions e_e

[Nobby]

Combustion reactions ARE redox reactions.

[Destiny]

Oh.

[charmanderp]

Yeah during combustion there's a transfer of electrons. Where did you find this question, if you don't mind me asking?

[Destiny]

LCATT