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Edison cell to you in two ways. First, I’d like to briefly
cover the historical construction of the cell. I think this will
spark some ideas and even possible improvements to the
cell. Then, I’d like to present the details of my own
homemade cell experiments from the last couple years so
you can construct one, as well.
Principles of
Construction
In many ways, the remarkable Edison cell is
functionally the opposite of batteries we use today. Edison
used simple iron (anode) and nickel (cathode) screens for
the electrodes, submerged in a potassium hydroxide
electrolyte. Next, he bucked the popular methodology
and rather than a strong acid, used an alkaline electrolyte
(potassium hydroxide) for his cell. The basic chemical
reaction can be written as:
2 NiOOH + 2 H2O + 2 e¯ ↔ 2 Ni(OH) 2 + 2 OH¯
Fe + 2 OH¯ ↔ Fe(OH) 2 + 2 e¯
(Discharging is read left to right; charging is from right
to left.)
FIGURE 1. Edison cell cut-away.
(Photo courtesy of John DeArmond.)
negative plate. In later designs, Edison used perforated
tubes for his anode (as seen in this particular cell), instead
of packets to hold nickel oxide for the positive plate.
Nickel II hydroxide is a very poor conductor, so in
order to make electrical contact with it Edison used 32
layers per centimeter of alternating nickel flake and nickel
II hydroxide inside the nickel tubes (as shown in Figure 2).
So, the positive electrode works by allowing electrolyte in
through the perforations in the nickel tube when
submerged, and the nickel tubing and nickel flake make
contact with the nickel II hydroxide to pass the charges
back and forth. Each 10 cm of 6. 3 mm (1/4 in) tubing will
produce a full 1.25 Ah capacity!
Forming the negative electrode requires only filling
tubes (or nickel plated metal packets) with the pure iron
oxide (Fe3 O4) under pressure. (This essentially forms little
iron oxide briquettes held inside the packets.) Gassing in
the cell increases when the negative electrode is charged.
So, the negative electrode is typically 30% larger than the
positive electrode. This serves to minimize gassing inside
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