• Total reaction: 2H2O → 2H2 + O2
(twice H2 versus O2)
Figure 2. Electrolysis Mode.
Figure 3. Fuel Cell Mode.
Figure 4. Horizon Hydrocar.
In Fuel Cell Mode (Figure 3),
the process is reversed along with
the polarities of the anode and
cathode. As hydrogen flows into the
fuel cell on the anode (negative) side
of the MEA, the platinum catalyst
facilitates the separation of the
hydrogen gas into electrons and
protons (hydrogen ions). The
hydrogen ions pass through the
membrane and combine with oxygen
and electrons on the cathode
(positive) side producing water. The
electrons — which cannot pass
through the membrane — flow from
the anode to the cathode through an
external load which consumes the
power generated by the cell. The
overall electrochemical process of a
fuel cell is called “reverse
electrolysis,” or the opposite of
electrolyzing water to form hydrogen
and oxygen. Once again, here are
the chemical reactions:
Fuel Cell Mode Reactions
• Anode reaction: 2H2 → 4H+ + 4e-
• Cathode reaction: 4H+ + 4e- + O2 → 2H2O
• Total reaction: 2H2 + O2 → 2H2O
(back to water again)
Experiment Setup
There’s obviously a lot more to
say about fuel cell technology, but
the best way to learn about it is
through experimenting with it. For
our reversible fuel cell, I’ve chosen
one manufactured by Horizon Fuel
Cell Technologies that comes in a
neat little package called the
Hydrocar (Figure 4). It has everything
you’ll need to do all of the
experiments, plus it’s a car and is one
of the least expensive and most
versatile ones on the market without
sacrificing quality by any means.
We’ll use it for this and the following
two fuel cell articles in this series. If
you have
another kind
of hydrogen reversible fuel cell you’re
of course free to use it.
Electrolysis Mode
We’ll start the experiment by
Figure 6. PICAXE 28X2
Electrolysis Mode Schematic.
Figure 5. Parallax BS2 Electrolysis Mode Schematic.
May 2010 49
