Re: Fuel Cells

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

Since, regarding fuel cells, James Marsh asked:
> ....
> Because of the publicity, I would like to bring these up in my general
> physics classes.  The problem is I don't know how they work.  How do you
> use the heat or the chemistry to produce electricity?  Help.

and Rick Swanson added:
> On behalf of the original poster of the question on Fuel Cells and the
> rest of us who don't really know how they work, could someone give a
> nice explanation as if you were explaining it to a student in an
> introductory physics class.

I'll give it a shot from my rapidly fading memory.

Fuel cells work, essentially, just like a battery cell.  Both battery cells
and fuel cells involve electrochemistry where a half-cell oxidation
reaction (electrons are removed from a molecular species) takes place at
one electrode and a half-cell reduction reaction (electrons are added to a
molecular species) takes place at the other electrode.  The electrons
liberated by the oxidation reaction flow through the external electrical
circuit and supply the reduction reaction at the other electrode.  Overall
electric charge neutrality is maintained (preventing an excess charge build
up at each electrode) via the diffusion of ions through the interior of the
cell between the cathode and the anode.  The main difference between a
battery cell and a fuel cell is that a battery cell is usually* a sealed
self-contained cell which contains a fixed amount of original chemical
reactants.  As the battery cell discharges the concentrations of the
reactant chemicals is depleted and the concentration of product chemicals
increases increases to the point that the emf (and maximum current)
delivered by the cell falls to an unacceptable level, at which point the
battery is declared 'dead' and is thrown away if it is not a rechargable
cell, and is recharged if it is rechargable.  When a battery is recharged
an external driving current reverses the electro-chemical redox reactions
and restores the initial concentrations of the reactant chemical species.

OTOH, in a fuel cell the system is continually flushed of the chemical
products and resupplyed with a fresh batch of chemical reactants at the
corresponding electrodes at the same rate that the redox reaction proceeds.
This keeps the cell in a 'charged' condition as long as the external supply
of reactants holds out.  The reactant chemicals are the 'fuel' and the
'oxidizer' and the reaction products are then discarded as 'waste exhaust'.

*  I said 'usually' here since some battery cells allow a steady
throughput of one -- but not both -- of the reactant (oxidizer & reducing
agent) species.  An example of such a cell are the zinc-air battery cells
that power hearing aids.  Here a fresh batch of O2 oxidizer is supplyed to
the cell but a fixed initial amount of Zn reducer is sealed in the cell.

To a very good approximation one can think of the electrolysis of water into
H2 at one electrode and O2 at the other one by passing an electric current
between them as a H2/O2 fuel cell operating in *reverse*.  In the fuel cell
case H2 is supplied to one electrode; O2 is supplied to the other one;
electricity is generated doing work on an external circuit; and water is
produced in the cell.  The electrolysis is not the exact reverse of the
fuel cell because the microscopic chemical reactions that occur at the 
electrodes are not the exact reverses of each other in the two cases, but the
overall thermodynamic result of all the processes taken together *is*
reversed between the fuel cell and the electrolysis.

BTW, I assumed in my explanation above that our hypothetical introductory
physics student had already had a year of general chemistry.

David Bowman
dbowman@gtc.georgetown.ky.us