There’s a lot of talk about Nickel II and Nickel lII oxides and related hydroxides in the information offered online. I found it extremely difficult to understand them and their differences. Which ones are actually used in NiFe batteries? Why? How do we know? The oxides of iron are similarly treated and just as confusing.

For instance, there is a naturally occurring mineral called magnetite. It is a black oxide of iron. At least one experimenter made the claim that you should build your battery with magnetite as the negative terminal then promptly followed up with an excuse why he didn’t want to do it. He proceeded instead with another oxide. Wow. There’s nothing quite like leading by example!

Let’s take a minute to review all the possible oxide/hydroxides involved in NiFe batteries. I’ve created a table of useful information below. The first 2 items listed represent the electrolyte and play no role in the electrochemistry of the battery.

valence	oxide	chemical name	color	mineral	hydroxide	chemical name
I	K₂O	potassium oxide	white	none	KOH	potassium hydroxide
I	Li₂O	lithium oxide	white	none	LiOH	lithium hydroxide
* * *	Fe	iron	grey	iron	several	several
* * II	FeO	ferrous oxide	greenish or blue	wüstite	Fe(OH)₂	ferrous hydroxide
* * II,III	Fe₃O₄	ferrouso-ferric oxide	black	magnetite
* III	Fe₂O₃	ferric oxide	reddish (rust)	hematite	FeO(OH) · H₂O	ferrihydrite
					or Fe(OH)₃	ferric oxyhydroxide
* II	NiO	nickelous oxide	green	bunsenite	Ni(OH)₂	nickelous hydroxide
* * II,III	Ni₃O₄	nickelo-nickelic oxide	grey
* * * III	Ni₂O₃	nickelic oxide	black	none	NiO(OH) · H₂O	nickel peroxide
					or Ni(OH)₃	nickelic oxyhydroxide

Note:
  *    Discharged State
 * *   Partially Charged State
* * * Fully Charged State

TABLE 1.

For those who are not familiar with ideas of chemistry, a chemical compound’s electrical charges must always balance to zero. The term valence refers to the number of electrons an element would like to share in order to balance its electrical charge condition. Iron and nickel are known as ‘transition’ metals. They can and do form compounds with more than one valence, namely II (+2) and III (+3). Oxygen always has a valence of -2 and hydrogen, +1 so Table 1 doesn’t list them.

You may notice that the formula for magnetite looks a bit odd. It doesn’t seem to balance the total charges to zero as one expects. This dilemma is resolved however when you come to understand that the three iron atoms don’t all carry the same valences. That compound contains Iron II and Iron III in the same molecule!

As it turns out, this is essential for the operation of a NiFe battery. Magnetite (black iron oxide) plays the part of the middleman between the charged and discharged nickel hydroxide states. It is itself alternately converted between the two other iron hydroxides at the negative terminal as the battery charges and discharges.

The negative electrode of a NiFe battery is made of Fe₃O₄, ferrouso ferric oxide, the black oxide known as magnetite or lodestone, not a simple iron plate. This is what the former experimenter didn’t understand while the latter had the correct idea but chose not to follow his own advice.

Forming the negative cell requires only filling tubes (or any other nickel plated metal packets) with the pure iron oxide (Fe₃O₄) under pressure.

Here’s an interesting side note. Hydrogen out-gassing usually increases when the negative electrode has reached a full charge. If its surface larger area is made up to 30% larger than the positive electrode, the cell may be fully charged before the negative electrode is completely reduced, thus nearly eliminating out-gassing.

“That’s all well and good”, you say. “So what?” But think about it for a minute, hydrogen out-gassing? Where do you suppose the hydrogen might have come from? If you guessed electrolysis of the electrolyte’s water, you would be a winner.

Now, you know of course, that electrolysis breaks down water into hydrogen and oxygen, right? Do you want a chance to double your winnings? OK. Tell me. Where did the oxygen go? Yep, I bet you guessed it. It made more iron oxides!

Ah, now the extra bonus question. Where did the iron for the oxides come from?

Oh, you ARE good. It came from the iron plates. Those plates we’ve been trying to protect. But guess what? More iron oxides means more capacity! Over-charge your NiFe batteries and THEY GET BETTER! You heard it before, now you know why. Just don’t forget to maintain the proper water levels.